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Thread: Modding a cheap solid state amp for more clean headroom.

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    Modding a cheap solid state amp for more clean headroom.

    So, I have this cheap little solid-state amp that was my first guitar amp ever. I've never really liked it. Take the gain knob or volume knob up at all, and the amp quickly goes into a very ratty, nasty distortion. Clean, I don't have much complaints, but getting the amp to stay clean, or at least transitioning more smoothly and gracefully into a dirt that isn't disgusting is the hard part.

    Yeah, yeah, I know, get a tube amp. I have a tube amp I'm very happy with, but a 100W Mesa half-stack can't escape being loud. It would be nice to be able to use the cheapie solid state amp to practice while still respecting the neighbor's right to sleep.

    So, I opened it up and started tracing out the circuit, hoping to find some things I could change to improve the headroom of the amp. I actually haven't finished tracing the circuit beyond the first gain stage, but I think I've found part of the reason why the amp sounds so nasty when turned up past 2.

    I've attached a schematic. LTSpice didn't have a symbol for a for a jack, so I used an AC voltage source instead, since that's what a guitar signal is. I didn't put a ground symbol on the schematic, because as far as I can tell nothing in the circuit is grounded, it's all just tied to the negative terminal of the DC adaptor input.

    I found a calculator for opamp gain, and if the math is right, with the gain pot cranked, the gain in this circuit is 370. That's way too much gain. Even a vintage single coil will send the op amp into hard clipping against it's meager 14V power rails.

    The solution seems simple enough, change the value of R2 to a high enough one to reduce the gain. Problem is, how do I pick a value short of plenty of trial & error? I'd think I'd want the gain low enough that the opamp never clips, regardless of how hot a signal it's fed, and let the clipping diodes make the dirt. But what's a reasonable upper limit voltage wise to design for? Some pickups are pretty damn hot. I'd suck to have to have things that the amp was usless with a neck single coil but just right for a high output bridge humbucker, or vice versa. The other concern is screwing up the rest of the circuit (since I haven't traced it out, and don't know what the rest of it does). For all I know the rest of the amp circuit may need the first opamp stage running at max voltage all the time to get any output at all.

    Am I over thinking things again?
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    Noodle of Reality Steve Conner's Avatar
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    Yes, you are overthinking things. If there's too much gain with the gain pot cranked, then don't turn it up so far!

    Turning the gain pot down has the same effect as changing the resistors.

    If you're finding that the gain pot has no useful range at all, try increasing R2 by a factor of 10. (2.7k.)

    Also try changing the 10uF cap for something smaller (1uf if you didn't change R2, 0.1 if you did.) This will focus the distortion more into the treble and upper mids.

    It can be fun to replace the clipping diodes with various kinds of LEDs. You can also put another pair of diodes across the gain pot, then the op-amp can never hit its rails at all.

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    Hi,
    the gain of the circuit would be 370 only if there were no frequency compensating capacitors around. You have to take into account the different capacitive reactance the two capacitors have at the various frequencies to properly apply the "non-inverting stage" gain formula.

    Just to give you an idea, and neglecting the other RC networks:

    The 330 pf capacitor in parallel with the feedback resistor limits the gain at the highest frequencies, while the 10 uF capacitor does the same at the lower end of the frequency range.

    1st case - Fin = 100 Hz.

    Capacitive reactance, Xc, is 1/(2pi*F*C), so at this frequency the 330 pF cap behaves like a 4.8 MOhm resistor, and the equivalent (parallel) feedback resistor can be approximated to 100KOhm.

    At the same frequency, the 10 uF capacitor behaves like a 159 Ohm resistor, and the equivalent (series) resistor is : 159+270=429 Ohm.

    The gain at 100 Hz therefore is : 1 + 100E3/429=234

    2nd case - Fin = 1000 Hz.

    The 330 cap in this case acts like a 480 KOhm resistor, so the equivalent feedback resistor is 83 KOhm.

    The 10 uF cap, OTOH, behaves like a 16 Ohm resistor, so the equivalent resistor is : 16+270=286 Ohm.

    The gain at 1000 Hz therefore is : 1 + 83E3/286 = 291

    3rd case - Fin = 10000 Hz.

    The 330 pF cap now acts like a 48 KOhm resistor, so the feedback resistor now is 32 KOhm.

    The 10 uF cap at 10KHz has a negligible Xc, 1,6 Ohm, so the equivalent resistor is 270+1,6=271.6 Ohm.

    The gain at 10 Khz therefore is: 1 + 32E3/271.6 = 119.

    4th case, - Fin = 20000 Hz.( Just for the record, my ears don't go this high, neither does a guitar )

    The 330 pF cap now acts like a 24 KOhm resistor, so the equivalent feedback resistor is 19 KOhm.

    The 10 uF capacitor is practically a short circuit at this frequency, so the equation divider will be 270 Ohm.

    The gain will therefore be 1 + 19E3/270 = 71

    You can clearly see that at the lower end the 10 uF cap acts as a gain reducer, while the same task at the higher frequencies is accomplished by the feedback cap.

    You can play with these caps to "tonally shape" the stage, or try to alter (reduce) the gain, by altering the two resistors ( and the capacitors accordingly, to get the same frequency response ) but the waveform you'll get out of it will always resemble a square wave, due either to the op-amp clipping ( close to the rail voltages ), or to the two diodes on the output, that act as clippers, limiting the p-p voltage and clipping it at their forward voltage (0.6V Si - 0.3 Ge ). The harder the clipping, the higher the odd-order harmonic content, and the harsher the sound you'll get. You might then want to try with different clipping arrangements ( asymmetric clipping, LEDs etc ). Asymmetric clipping is less aggressive ( one half of the waveform being "less clipped" than the other ) and it softens the sound somehow as it tends to introduce even order harmonics.

    At this point, I'd ask myself "is this amp really worth the effort?" - If not, I'd take the other ( simpler ) way around - get yourself ( or better yet build it yourself ) a small tube amp, you'll be glad you did it!

    Hope this helps

    Best regards

    Bob

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    Last edited by Robert M. Martinelli; 12-02-2009 at 10:58 AM.
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    Quote Originally Posted by Koreth View Post
    ...
    Yeah, yeah, I know, get a tube amp.
    Maybe. The issue there is that even a small tube amp is going to be plenty loud. When used in the clean range, SS amps do a good job of reproducing the signal fed to them by distortion pedals, etc. Think about it - the PA at gigs is feeding good sound to the crowd and it's almost certainly solid state.

    So, I opened it up and started tracing out the circuit, hoping to find some things I could change to improve the headroom of the amp. I actually haven't finished tracing the circuit beyond the first gain stage, but I think I've found part of the reason why the amp sounds so nasty when turned up past 2.
    ...
    I found a calculator for opamp gain, and if the math is right, with the gain pot cranked, the gain in this circuit is 370. That's way too much gain. Even a vintage single coil will send the op amp into hard clipping against it's meager 14V power rails.

    The solution seems simple enough, change the value of R2 to a high enough one to reduce the gain. Problem is, how do I pick a value short of plenty of trial & error? I'd think I'd want the gain low enough that the opamp never clips, regardless of how hot a signal it's fed, and let the clipping diodes make the dirt. But what's a reasonable upper limit voltage wise to design for? Some pickups are pretty damn hot. I'd suck to have to have things that the amp was usless with a neck single coil but just right for a high output bridge humbucker, or vice versa. The other concern is screwing up the rest of the circuit (since I haven't traced it out, and don't know what the rest of it does). For all I know the rest of the amp circuit may need the first opamp stage running at max voltage all the time to get any output at all.

    Am I over thinking things again?
    Maybe. That stage you've drawn is a not-too-unreasonable imitation of the old MXR Distortion Plus. It's intended to do what it's doing. The designer may have had uninformed intents, though.

    Steve and Robert have it right. I would do a somewhat simpler thing, though. I would pull up one end of R2 and solder in a pot of about 10K to 100K in series with it, connected like the gain pot so it's a two-terminal variable resistor. 20K would be the most useful value, but they're harder to find. Then play through it, and diddle the pot until you like it. When you get there, take a meter, measure the pot+R2 value and solder in the nearest 5% resistor value instead of R2. Once you've done that, the overall gain will be within bounds to your ear, and you can start tinkering with gain structure and clipping.

    One thing that sounds very nice in setups like this is to change R5 to about 2K to 10K to taste, then to place a resistor in series with the two diodes, taking the output from the end of R5 as it is now, and with the added resistance in series with the diodes only.

    The R2 machinations will get your gain down, the tinkering with cap values and such will tame the rattyness of the distortion to some degree.

    On a similar vein, I picked up a "Rogue" brand 100W 2-12 combo from craigslist for $40. The complaint was "it's too loud and too ratty/SS sounding". It's got a very workable 100W SS power amp and OK if not great speakers in it. I pulled out the preamps and subbed in the preamp from an old UK Vox SS amp. Worked just fine. I'm still messing with the SS preamp, the the "soul transplant" part of the operation was a success. You can tame the grack from the amp.

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    Quote Originally Posted by Steve Conner View Post
    Yes, you are overthinking things...
    Good to know. I do that sometimes. (Okay, maybe a lot.)

    Quote Originally Posted by Steve Conner View Post
    If you're finding that the gain pot has no useful range at all, try increasing R2 by a factor of 10. (2.7k.)
    I'll give this a shot. It also ocurred to me to check out schematics of other amps and dirt boxes that make similar use of a non-inverting opamp stage for resistor values to try in the loop to play with the gain structure.

    Quote Originally Posted by Steve Conner View Post
    Also try changing the 10uF cap for something smaller (1uf if you didn't change R2, 0.1 if you did.) This will focus the distortion more into the treble and upper mids.
    Quote Originally Posted by Robert M. Martinelli View Post
    the gain of the circuit would be 370 only if there were no frequency compensating capacitors around. You have to take into account the different capacitive reactance the two capacitors have at the various frequencies to properly apply the "non-inverting stage" gain formula.

    (lots of useful math)

    You can clearly see that at the lower end the 10 uF cap acts as a gain reducer, while the same task at the higher frequencies is accomplished by the feedback cap.

    You can play with these caps to "tonally shape" the stage, or try to alter (reduce) the gain, by altering the two resistors ( and the capacitors accordingly, to get the same frequency response )...
    Good to know, I'll give that a shot to tune the flavor of the dirt once I've got the the overall headroom vs dirt in this stage to my liking.


    Quote Originally Posted by Robert M. Martinelli View Post
    ...the waveform you'll get out of it will always resemble a square wave, due either to the op-amp clipping ( close to the rail voltages ), or to the two diodes on the output, that act as clippers, limiting the p-p voltage and clipping it at their forward voltage (0.6V Si - 0.3 Ge ). The harder the clipping, the higher the odd-order harmonic content, and the harsher the sound you'll get. You might then want to try with different clipping arrangements ( asymmetric clipping, LEDs etc ). Asymmetric clipping is less aggressive ( one half of the waveform being "less clipped" than the other ) and it softens the sound somehow as it tends to introduce even order harmonics.
    Quote Originally Posted by Steve Conner View Post
    It can be fun to replace the clipping diodes with various kinds of LEDs. You can also put another pair of diodes across the gain pot, then the op-amp can never hit its rails at all.
    I'd been thinking about playing with the clipping diodes, both the hard clippers at the output and putting soft clippers in the negative feedback loop. I did the asymmetrical clipping thing in my Boss DS-1 with an LED and some 1N00x rectifier diodes and I really liked the result. Said pedal now is great for adding some dirt to my tone, the level of which I can control with my guitar's volume knob. I'll probably look into playing with the clippers once I've got the gain and tone of the stage to my liking.

    Quote Originally Posted by R.G. View Post
    One thing that sounds very nice in setups like this is to change R5 to about 2K to 10K to taste, then to place a resistor in series with the two diodes, taking the output from the end of R5 as it is now, and with the added resistance in series with the diodes only.
    Are you suggesting some kind of voltage divider to tweak with how hard the hard clipper diodes clip? That sounds like a cool idea.

    Quote Originally Posted by Robert M. Martinelli View Post
    At this point, I'd ask myself "is this amp really worth the effort?" - If not, I'd take the other ( simpler ) way around - get yourself ( or better yet build it yourself ) a small tube amp, you'll be glad you did it!
    Asking myself honestly? No, this little First Act ME104 is not worth the effort. It is a cheap disposable amp that sells at Wal-Mart for $30 bucks. I'm doing this more sh!ts and giggles, and for the fun of experimenting. I never thought I'd become addicted to huffing flux fumes, but I guess I have. I simply figured I'd have more fun tweaking something useless into something more likely to be useful, and even if I don't I'll probably learn something useful in my future music electronics adventures.

    I'd love to build myself a little mini tube amp, like a champ or princeton clone of some kind, but that's just not in the cards right now. My main source of income right now part time delivering Chinese food for almost enough money to keep gas in the tank and keep my creditors from taking me to court. Even if I pour as much as $30 dollars of parts into this little amp to try to make it sound better with negligible results, that's more results in the more immediate future than trying to build a tube amp. $30 bucks can get me plenty of parts to tweak this cheapie SS amp if I shop right. $30 bucks won't even cover the cost of tubes, never mind the iron or speakers. I look at it as having fun with what I have and can readily get right now.

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    Quote Originally Posted by Koreth View Post
    I'm doing this more sh!ts and giggles, and for the fun of experimenting. I never thought I'd become addicted to huffing flux fumes, but I guess I have. I simply figured I'd have more fun tweaking something useless into something more likely to be useful, and even if I don't I'll probably learn something useful in my future music electronics adventures.
    Welcome to the "solder fume breathers" club! ( credits ----> Enzo ).

    Be careful, as solder fume IS addictive!

    All the best

    Bob

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    Lifetime Member Enzo's Avatar
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    Watch out, that stuff will flux you up.

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    Don't forget to try this amp connected to a different [bigger] speaker. You may be surprised at how much better the modded circuit sounds when you're not using the stock speaker.

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    Quote Originally Posted by Gunny View Post
    Don't forget to try this amp connected to a different [bigger] speaker. You may be surprised at how much better the modded circuit sounds when you're not using the stock speaker.
    That was actually the very first mod I did to this amp. I had a couple 1x10s left over from a 20w SS amp that died on me, so I grafted some speaker wire and a jack onto the ME104. The 1x10 definately fills out the lows and low mids that ME104 was so severely lacking with that raspy little 4" speaker.

    We'll see what the modified circuit sounds like.

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    So, I decided it would be a good idea to trace out the rest of the amplifier circuit before making and modifications. My rationale was that I could mod more intelligently, and hopefully avoid any gotchas had from not understanding from how the different parts of the circuit interact with each other.

    After spending about an hour tracing what lies beyond C6, and doubling back multiple times to retrace something that didn't make sense at first glance (like a 1M/1M voltage divider from the 2nd stage non inverting input to the 1st stage non-inverting input, WTF?), I have more respect now for the guys who took the time to trace out the now readily available circuit schematics of various FX pedals and tube amps. I'm taking a break for now. I'll have an updated schematic posted later.

    An aside, some of the solder joints in this little thing are really sloppy. I'm surprised the amp works with the potential cold joints on the circuit board. Maybe that contributes to the less than ideal sound of the amp? Either way, when I start modding, I'll be reflowing all the bad joints.

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    The more I trace this amp's circuit out, the more complex it becomes and the less it makes sense to me.

    I've attached an updated schematic. If anyone's good with LTSpice and has some pointers on how to re-draw the schematic so its easier to follow or is willing to redraw it for me, lemme know.
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    Quote Originally Posted by Koreth View Post
    ME104 was so severely lacking with that raspy little 4" speaker.
    I think it's funny that I am hooking a $200 processor up to this thing at it sounds good at living room volume. I struggle trying to suppress my 65w Peavey with a 12" in it.

    What do you guys think a higher quality speaker would do? Same size, maybe a 2 or 3 way car stereo speaker. Seems like if it had a $30 speaker the thing would scream just fine. I like this thing because it was free, small, and cheaper than a microcube.

    Here is what mine looks like. I sanded off all the silk screened junk by the knobs and scribbled my own (junk) "Orange" style symbols in sharpie.


    I have faith in this thing that it will turn into SOMETHING someday. Some kind of Frankenstein amp with all sorts of diy boxes hanging off of it.

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    Noodle of Reality Steve Conner's Avatar
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    Quote Originally Posted by Koreth View Post
    The more I trace this amp's circuit out, the more complex it becomes and the less it makes sense to me.
    As far as I can tell, R18 and R6 are a potential divider that creates a reference of half the supply voltage. This is a common trick to run op-amps off a single supply, you can find more details at RG Keen's geofex.com site.

    So the node at the junction of R18 and R6 doesn't handle any signal, and they play no part in the amp's tone. Also you should find that the bottom ends of R6 and C6 are connected to ground, as are the left-hand ends of C16 and C20. Hence these are all bypass caps and not really part of the signal path either.

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    Okay, finally got around to making the suggested mods. I changed R2 from 270 to 2.7K, and C4 from 10F to 1F. The result? Much better. The gain knob has a much more usable range now. It can go from totally clean, to a light crunchy breakup.

    Though now I think I have the opposite problem from before. There's not *enough* gain. With the gain cranked to ten and using my axes high output bridge pickup, the amp only goes into a light crunch. That by itself is fine, but I prefer the sound of LED hard clippers to that of 1N4148s. LED's have a much higher forward voltage than 1n4148s do, so I'll probably have to up the gain on the opamp to get a get a decent clip out of LEDs if right now the 1n4148s are only lightly clipping. I was thinking red LEDs and changing R2 to either 1K or 1.5K. 1.5K is will be closer to 2x the current gain, and 1k, will be closer to 3x the current gain.

    Maybe I'll try both and let you guys know.

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    Just so we're on the same page, is this what the guts of yours looks like?




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    Close. Mine looks a bit different, but it has skilk screenings on the board for the jack and pots to be mounted to the board like they are in your pic. Mine ar mounted to the chassis with leads running back to the board. However I do see a lot of the same parts in the same locations (or close) with the same component values, R1 10K, C1, 223, etc. Using one of the schematics, above, I'd trace out the circuit first before making any mods since yours seems to have a different revision of the board. If you find R2, one leg of it will be going to C4, a 10F film cap, which goes to pin 2 of the opamp chip if it a JRC48558D like in mine. The part numbers and values might have been changed between your and my versions of the amp, but if it has too much gain, the resistor from the feedback loop to ground is the one you want to change.

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    Update:

    I settled on the 1k resistor at R2. I get a much fuller sweep across the gain pot now, from fully clean, to rather distorted. Mission accomplished in that regard. Now

    With the 2k7 ground resistor, the amp's highs seemed a bit muffled making for a bit of a muddy sound. Using the useful math that Robert posted, and a few other references graciously linked to by Google, I put together a spreadsheet to calculate gain dependent on frequency with ten steps per decade and the values of the capacitors in the feedback loop voltage divider. With R2 at 2k7, gain at 10khz was a bit more than half of what it was at 100hz. Gain peaks at or near 600hz, and remains fairly flat till about 1000hz, at which point it drops off shortly, to just over half the gain at 10khz than what is had at 100hz. With R2 at 1kOhm, the gain peak shifts to around 900hz, and doesn't fall off as fast as the frequency goes up, leaving only a little less gain at 10khz than is had at 100hz. My ears confirm the brighter sound, both clean and at max distortion.

    However, clean, it still isn't enough. I want the amps clean tone needs to be brighter still. I have two thoughts on this. One: lower the vale of the bright cap in the feedback loop till I have the highs I want. Two: Since the Tone controls seems to be a simple high-cut filter, play with the values there till I have the highs I want. On one hand, if I make the opamp stage bright enough clean, it is probably going to be nasty when distorted, but I could always turn the tone knob down. On the other hand, tweaking the tone knob won't make the distortion unbearably bright, but might not get back enough highs, as you can't add back what isn't there with a passive filter. That Fender clean we all know and love has is rather mid-scooped, and since the current design is effectively mid-boosted, I'm not sure I'm going how much I can accomplish either way before I have to reconfigure the tone knob to something like the Fender/Marshall TMB tone stack, which isn't worth the effort at this stage, IMO.

    Another thing I noticed when testing the resistor values was that even when the gain pot is all the way down, there's still a bit of breakup on attack, even with vintage output single coils pickups. Sweep the volume past 4, and the volume stops rising as much, and the breakup on attack becomes more prominent. This makes me think the amp is still being over driven somewhere, either in the second opamp stage which drives the output chip, or the output chip itself. I think I'll want to finish tracing the amp's circuit, (or at least the signal path) before starting to play with the clipper diodes. When the clipper diodes aren't clipping, the amp should be completely clean, IMO.

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    Junior Member Robert8192's Avatar
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    Lots of good ideas for this little circuit.

    Just one thing I found when I rebuild some
    preamps. The Ibanez TK999ht uses Zener
    diodes on this first stage to prevent it from
    overdriving the second stage.
    I added the same thing to another preamp
    and it limitted the weird sounding distortion
    out. Maybe can be used in your design.
    They are 5.1V limiters.
    Robert.

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    So that design uses zenners in the feedback loop to limit how hard the next stage is hit. Don't the hard clipper 1N4148s in my amp effectively do the same thing? If I'm understanding correctly, they clip by sending any signal above their forward threshold voltage to ground, in which case, the signal into the 2nd opamp stage would never be greater than .65-.7Vp, with the gain and volume controls all the way up. Is this correct?

    Assuming that's the case, then I suppose I should look at the 2nd opamp stage. It has R11 at 22k in the feedback loop and R10 at 4.7K hanging off that. R10 doesn't go to ground, but it is is tied to the same line which feeds the opamps's positive power rails, which I suppose could act as a ground if the negative power rail of the previous opamp stage could. If that's the case, then the gain in the 2nd stage would peak at 5.47 at 2000hz. So then the 2nd opamp stage would never hit more than 3.8Vp, which less than the 7Vp limit of the power supply. Does my reasoning make sense, or did I get confused again somewhere?

    I should trace out and examine at the output stage, shouldn't I?

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    Junior Member Robert8192's Avatar
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    Yes, the diodes do almost the same thing.

    The setup for your first stage is so different from
    the one I posted, Zeners wont work anyway.
    Tube King uses a tube in the second stage anyway.
    Maybe add a 2nd stage in that is a tube, Say the easy to build
    sub-mini tube stage involving a 6021, and we're in
    business. LOL. Anyway, hope you acheive what
    you're trying to do.

    Robert.

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    Okay, finally finished tracing out the rest of the circuit. Turns out the voltage divider on the negative feedback loop of the 2nd op amp stage does go to ground, and not the positive supply as I initially thought. The output chip had "UTC2003" silk screened on it. Google didn't seem to find anything relevant about a UTC2003, but it did find a TDA2003 made by UTC. The ratings on the TDA2003's datasheet seems to fall in line with First Act's claimed power output for this amp, as does the resultant arrangement of supporting components when using the TDA2003's pinout. Now that the amp's circuit is fully traced, I think it will be easier to go at modding this thing.

    1. More highs. When I run the amp clean, I dime the tone knob. When I play dirty, the tone knob goes down to tame the nastier high freq components of the distortion. That's all well and good, but when playing clean, diming the tone control isn't enough. Looking at the tone stack, it seems to me that C8 is responsible for rolling off the highs, and if I want more, I should change it for a lower value. Thoughts? Yes? No? Right know the amount of highs is short of "pleasantly warm." I have a .01, .0047, and .0022 to hand. Which should I try first?

    2. More clean headroom.There's still a bit of dirt when the gain knob gets turned all the way down that appears as the volume knob gets turned up. Since the Volume knob is before the 2nd opamp stage, I've a few thoughts here:
      1. Reduce the gain of the 2nd stage. Not sure how useful that'd be as the 2nd stage's gain is only a bit over 5. With such a low gain, I'm not even sure what the 2nd stage's purpose is, except perhaps to provide the output stage with enough current maybe? I'm guessing. I may want to reduce the 2nd stage gain when I start playing with clipper diodes with higher forward threshhold voltage.
      2. Tweak the voltage divider to ground on the 2nd stage's output. Obviously, this will attenuate the signal going into the output stage, but as is, it is shaving so little off that, again I'm not sure why it's even there. If I don't understand a portion of the circuit, I'm loathe to change it.
      3. Lower the gain of the output stage. This seems simple enough. Though the resistor values currently in the negative feedback loop are so low, I'm not sure if I can go lower with what I currently have.


    Other thoughts: The C1815 transistor going from the positive supply to the op amp's positive rails is an emitter follower, right? What the heck is it doing there? Why is C16 so huge a value and why is it in series with the output stage's inverting input? Is it there to keep the output from frying the input or something? Since it is on the other side of the voltage divider, what effect does C16 have on calculating the gain of the output stage?
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    Supporting Member Jazz P Bass's Avatar
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    Mod Mod Mod

    Q1. Is it not a constant current source?
    TDA2003. Did you read the datasheet? Page 8.
    R15 & C16 are there for ripple rejection on the supply voltage.
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    If Q1 is constant current source, why is it there? Why would you feed an opamp's positive supply from a constant current source?

    Okay, rejection of ripple on your supply voltage is good, but if it is a concern on the negative feedback loop, then why wouldn't all opamps have a huge series capcaitor on the opamp input?

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    Hello again. I'm still occasionally making some tweaks to this little SS toy amp, and have a new question.

    Quote Originally Posted by R.G. View Post
    One thing that sounds very nice in setups like this is to change R5 to about 2K to 10K to taste, then to place a resistor in series with the two diodes, taking the output from the end of R5 as it is now, and with the added resistance in series with the diodes only.
    I'm trying to visualize what RG is describing here and made a couple circuit diagrams on what I think he means. Which is the correct implementation of what RG is describing? Either way I look at it, it seems to me both circuits would have the same effect: the input of the clipper diodes is taken from the middle of a voltage divider. Are both circuits going to have the same effect on the amp's sound or is the former going to sound different from the latter?
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    Supporting Member Alex R's Avatar
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    I've heard the resistor-in-series-with-diodes version touted as a way of smoothing out diode clipping. I have tried it and in practice I can tell you it works, it sounds less harsh and more 'natural'. I look forward to struggling with the theory...

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    Old Timer J M Fahey's Avatar
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    Hi koreth.
    They are *not* the same.
    The left one is the right one (resistor in series with diodes)
    The right one is resistors in parallel with diodes, a very different beast.

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    Do they both still act as voltage dividers into the clipping diodes though?

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    Quote Originally Posted by Koreth View Post
    I'm trying to visualize what RG is describing here and made a couple circuit diagrams on what I think he means. Which is the correct implementation of what RG is describing?
    Neither of them is what I meant. Try this:
    Take two resistors. Connect them in series. Call the one you'll connect to the signal source R1. Call the one you'll connect to the diodes R2. Now connect a back-to-back pair of clipping diodes from the unused end of R2 to ground. Hook the unused end of R1 to your signal source.

    Now imagine that Both R1 and R2 are 10K variable resistances. We can turn the knobs and set them to any value between 0 and 10K.

    Re-capping, from the signal source that will be clipped, the signal goes through the series connection of R1, then R2, then the pair of diodes to ground. We take the signal out at the junction of the two resistors R1 and R2, NOT at the diodes.

    First test: Dial R1 to 10K and R2 to zero. How does this sound? It sounds *exactly* like R2 is not there. And it's not. R2 is pretending to be a wire. The signal is simply the original voltage for voltages less than a diode drop.

    When either diode starts conducting, current flows in the diode and that's how the diode limits the voltage that appears on the output. The current through the diode is **at all times** either zero for signal voltages too low to turn on a diode, or signal voltage minus a diode voltage divided by the total of all the resistance in series with the diode. And the diode has complete control of the output signal.

    Test 2: dial R1 to zero, and R2 to 10K. The signal out is **completely** unaffected by the diodes. No clipping at all (this is a simplification, but a valid one). Effectively, we're listening to the signal source, and the diodes can't do anything to it through that 10K R2.

    Test 3. Change R1 to 10K again, but now make R2 be 1K. As the signal voltage increases, there is no clipping up to the voltage which would turn on a diode again. But now, R2 is causing a voltage of (1K/11K)times the signal voltage to be added on top of the diode voltages when they're clipping. With some of the orignal signal there, the diode clipping can't be as harsh.

    Test 4: R.G., R.G.! I know! Replace R1 and R2 with a 10K pot!! Yes, that works. But the signal level varies wildly, because the diode signal is only +/-0.7V (about) and the drive signal at the top of the pot may be many volts. So you not only dial out distortion, you dial IN a lot more signal. The two-pots setup helps you tame that. Try it that way if you like.

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    Quote Originally Posted by Koreth View Post
    Do they both still act as voltage dividers into the clipping diodes though?
    No.

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    Drew up a new diagram. Here's hoping I got it right this time.
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    Old Timer J M Fahey's Avatar
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    This is *exactly* the same as the left one you drew before.

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    Supporting Member Alex R's Avatar
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    Quote Originally Posted by J M Fahey View Post
    This is *exactly* the same as the left one you drew before.
    This one has R2 to ground.

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    Old Timer J M Fahey's Avatar
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    The first one, on the left, was accurately described by:
    The left one is the right one (resistor in series with diodes)
    The last drawing can be accurately described by:
    resistor in series with diodes
    In a series circuit, the same current runs through all elements or paths so connected, the exact order is irrelevant.
    If the current that goes through R1 is shunted to ground through R2 and an antiparallel set of diodes or through an antiparallel set of diodes and R2 it's the same.

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    Quote Originally Posted by J M Fahey View Post
    In a series circuit, the same current runs through all elements or paths so connected, the exact order is irrelevant.
    If the current that goes through R1 is shunted to ground through R2 and an antiparallel set of diodes or through an antiparallel set of diodes and R2 it's the same.
    This is correct. All series connections of parts may be freely interchanged in order with no observable effect at the ends of the series strings. The voltages at the points inside the series string vary from arrangement to arrangement, but there is and can be no change in the external voltages and currents.

    I insisted on R1 and R2 in series above the diodes to ground to make the description of what happens as R2 varies from zero up be a little more intuitive. But R2 has the same effect if it's on the ground side of the diodes.

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    Color me confused.

    The one on the left is not correct. The one in the middle is functionally equivalent, and therefore, also incorrect.

    Quote Originally Posted by R.G.
    Take two resistors. Connect them in series. Call the one you'll connect to the signal source R1. Call the one you'll connect to the diodes R2. Now connect a back-to-back pair of clipping diodes from the unused end of R2 to ground. Hook the unused end of R1 to your signal source.

    Now imagine that Both R1 and R2 are 10K variable resistances. We can turn the knobs and set them to any value between 0 and 10K.

    Re-capping, from the signal source that will be clipped, the signal goes through the series connection of R1, then R2, then the pair of diodes to ground. We take the signal out at the junction of the two resistors R1 and R2, NOT at the diodes.
    Done. That's the one on the right. What's confusing me is that is the same as the middle one as far as I can tell, which would make it also incorrect. There's only so many ways you can draw two resistors in series, input at the unused leg of R1, output at the junction of R1 and R2, and clipper diodes at the unused leg of R2 going to ground.

    I'll assume that I'm splitting hairs and set that aside for a second. There's another thing confusing me.
    Quote Originally Posted by R.G.
    Test 3. Change R1 to 10K again, but now make R2 be 1K. As the signal voltage increases, there is no clipping up to the voltage which would turn on a diode again. But now, R2 is causing a voltage of (1K/11K)times the signal voltage to be added on top of the diode voltages when they're clipping. With some of the original signal there, the diode clipping can't be as harsh.
    (emphasis mine)

    So, are we increasing the threshold voltage at which the diodes clip, or achieving some blend of the unclipped and clipped signals? I'm trying to visualize the effect it would have on the waveform coming through the clipper, and the levels coming out of the clipper diodes so I can set the 2nd and output stage gains appropriately so as to avoid overdriving them. An originally 2Vpp signal clipped to 1.4Vpp by the diodes is easy to visualize, as is an unmolested 2Vpp signal. A signal clipped to oh, 1.8Vpp by way of increasing the clipping threshold, is likewise easy to visualize.

    And as I type this, another idea occurs to me. The normal 1.4Vpp clipped signal is a hard clip, everything above 1.4V is gone, and the top of the waveform is shaved off. Does the voltage divider with the diodes cause the clipping to transition from a hard clip to a softer one, where the clipping still starts at the 1.4Vpp caused by the diodes, but the top of the waveform is compressed instead of shaved off, peaking at some value higher than the 1.4Vpp hard clip?

    Am I on the right track here or am I off in the weeds again?
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    Last edited by Koreth; 02-05-2010 at 04:09 PM.

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