Modding a cheap solid state amp for more clean headroom.

[J M Fahey]

This is *exactly* the same as the left one you drew before.

[Alex R]

This is *exactly* the same as the left one you drew before.

This one has R2 to ground.

[J M Fahey]

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.

[R.G.]

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.

[Koreth]

Color me confused.

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

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.

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?

[J M Fahey]

All three drawings are exactly the same, with only the obvious typo on the third that you drew both diodes pointing downwards, they should point opposite ways.
Understand it as this:
the diodes clip the signal hard, squaring it and killing all harmonics (the dreaded buzzy/muffled/overcompressed diode clipping sound)
If you put a resistor in series with them, either above o below ( it's the same), *some* original signal remains, the sound is softer, more natural, retains *some* of the original harmonics.
Tubescreamers retain some of the original guitar sound, Distortion+ or Rats don't, now you know why TS are loved.

[Dimi Pana]

I recently made the mistake to purchase a First Act small amp as well.

It is the MA1248, exactly the same like "theWicked" posted, and it is shown here: First Act Guitar Amplifier - Brown - First Act - Toys "R" Us

Yes, I was fooled by the reduction in price. I am guilty as charged, should have known better...

--
I though it would be usable but it is not. I was wondering if I posted some info about it you guys who all seem very knowledgeable with electronics could help me mod it to the point that I can:

1. Have a usable -full range- volume control. Now I cannot turn it up more than 50% if I do even with clean sound the guitar's bass strings produce a "farty" sound.

2. Have a usable -full range- gain control. Now I cannot turn it up more that 0%. Yes, zero percent, more produces noise and unpleasant distortion.

3. Have a usable -more noticeable- tone control. Now it ONLY makes the sound apparently brighter by cutting bass, how can I make it to REALLY add more treble for clarity.

4. Also, with all pots turned all the way to the left (e.g. down) I get noticeable noise (something between a hum and a buzz) when I turn ON the amp.

5. May be get a little more power out of it -say- 5 Watts (total) in order to try it with a bigger cabinet I 'm thinking to build (for fun) (lol).

--

I opened it up and it is using a JRC4558D, a TDA 2030A, and a transistor(?) which I believe is a 9013. Of course there's also a bunch of caps, resistors, diodes, etc. but even to a novice like me this circuit seems simple enough. Still, I can solder or de-solder components, I understand how to read (not by heart) resistors with a chart, I know some caps have polarity, I know diodes allow current flowing only one direction, I know about proper grounding techniques, etc. So if some one tells me remove this and put that in its place I am comfortable enough to do it. I would n't know why but if it sounds OK, I am happy.

So, is there a consensus what I should do to improve on this little amp. I'd really appreciate any input!


If you are interested please respond,

Thank you very much in advance.

[Jazz P Bass]

I would hook the amp up to a decent speaker cabinet.

[olddawg]

Buy a trashed Epiphone Valve Jr chassis (or whatever) off of ebay, repair it and mount it in your box.

[Dimi Pana]

Yes, I intend to build a cabinet from scratch, should I go for a 1x12 or a 2x8 or what combination, for this amp? I want it to be a decent home practicing, small venue cabinet. Also, if this AMP is rated 4W at 4Ω, this means that the cabinet should also be 4Ω, right? So in the case of -say- 2x8 what should the Ω of each speaker be and what should the rated Watts. Sorry, these are probably trivial questions, but I am new to all this. I was actually also hoping to hear from those who did mod this amp and have (through trial and error) come to some useful conclusions. Thanks to all again.

[J M Fahey]

I recently made the mistake to purchase a First Act small amp as well.

No mistake, you paid $4.98, you got a $4.98 amp.

1. Have a usable -full range- volume control. Now I cannot turn it up more than 50% if I do even with clean sound the guitar's bass strings produce a "farty" sound.

Most amps do so, specially noticeable in SS amps.
Your amp behaves like any other amp.
The only difference is that "5" on, say, a Peavey Bandit makes the neighbours very angry, while yours makes only you angry.

2. Have a usable -full range- gain control. Now I cannot turn it up more that 0%. Yes, zero percent, more produces noise and unpleasant distortion.

3. Have a usable -more noticeable- tone control. Now it ONLY makes the sound apparently brighter by cutting bass, how can I make it to REALLY add more treble for clarity.

4. Also, with all pots turned all the way to the left (e.g. down) I get noticeable noise (something between a hum and a buzz) when I turn ON the amp.

5. May be get a little more power out of it -say- 5 Watts (total) in order to try it with a bigger cabinet I 'm thinking to build (for fun) (lol).

Don't waste your time and money.
The only "mod" that will provide instant upgrading will be fitting an external cabnet with a good speaker to it, as JPBass suggested, but for the same price and less hassle you can purchase any "real" starter amp, say any 15W Fender/Marshall/Laney/Crate/Peavey at a garage sale, EBay or Craigslist.
Worlds apart.

Good luck.

PS: popular sayings usually have more than a grain of truth in them:

"don't throw good money on bad money"

"don't beat a dead horse"

PS2: you find it a toy ?
Well, you bought it at

"Toysīr Us"

[Dimi Pana]

Thanks for your post -

You are 100% right nothing to disagree with here, on the contrary. I know buying a $5 amp at ToysRUs would not buy me a marshal or a fender. I was just curious. This thing is almost useless. Funny thing it originally sold for $24.99, then 2-3 weeks ago they slashed it at $12.98 and this past Monday they brought it down even more at $4.98 (!!!!!!!). Which naturally makes you wonder how much c*r*a*p (that is manufactured in China) is sold in the US at unrealistic prices. If they can sell this amp under 5 bucks how much do you think they are paying each, at bulk rates. And you know the worst thing is that maybe they are still making some profit even at $5.

I am only trying to spend some time toying around with some basic electronics. Replacing a cap here or a transistor or a diode there is not going to break the bank. Maybe in the process I 'll learn a bit something more about diy electronics. If Koreth (who started the thread) says this amp is "improve-able" I think I'll give it a try.

I have a small 15W Bugera tube amp I like a lot, I'm not a professional musician, just playing on my free time and at friends parties.

I have an old DOD effects unit with stereo input and output and just for kicks I am thinking of buying another MA1248, modify both to sound better and hook it up in "true" stereo via the DOD unit. This is going to be a blast. Most fun I ever bought in my life with around 10 bucks!

Thanks again for your post!

Cheers!

[J M Fahey]

For U$ 5 itīs a heck of an amp.
I can't build it for that price even by scavenging my junkbox for parts.
In fact, it must cost around $10 each by the container, so the original around $20 price was right, around $12 is selling at cost and $5 means just "we need that warehouse space .... NOW!!"
I'd buy 5 or 10 of them, just as gifts for kids or simply to re-amplify my MP3 player when earphones become uncomfortable or tiring.
Just do not waste $$$ on them.
As of experimenting: great !!!!
If you get a cigar box or similar and mount there an "old style" car radio speaker, meaning no foam edge but a plain old thin cardboard integral cone, and install the FA as the chassis, youīll get quite an usable bedroom amp.
Those speakers can still be bought new for around $4 or 5, or scavenged from car graveyards and similar places.
As a similar example, Iīve seen THD selling 4x10" sets on EBay for around $40 or $50 total price, for just the same reason.
I would have bought "all" of them if I were in the USA
Even though I make my own speakers, couldnīt match that price for sure.

[Dimi Pana]

Hi there (again), J M Fahey !

Really funny how you say, they must really need the warehouse space, I agree 100%.

So, the way I read your post is try to scavenge a better speaker (or speakers) use some scrap wood (I have some MDF laying around) build a basic cabinet, and try it AS IS without any mods, right?

Actually, the metalic enclosure is removable and pretty decent, I can use it and attach it to the (new) cabinet.

Now, a few quick questions.

1. With no instrument plugged in, all three pots turned all the way to the left (down) turning on this amp produces a noise, something between a hum and a buzz. Any ideas why, and if this is fixable?

2. I have posted some pics of the actual PCB, and the amp itself. I was wondering if you'd be interested in pointing some quick and obvious mods. Nothing major or expensive(!) just a cap here or a resistor there, etc., that might improve on this design based on your experience. One thing that could really make sense is the gain pot which currently is unusable since it produces lots of noise and non-musical distortion at even the lowest setting.

FYI, this thread was started by Koreth in another forum Modifying a First Act MA104 as well, they have posted some interesting ideas there as well, if you care to comment.

Thank again, I do not want to impose, just in case you (or anyone else) has a free moment.

Cheers!

[Koreth]

Wow, I'd forgotten about this thread and the one over at ssguitar.com. It's been a while since I've tweaked the MA104. The speaker is now a refrigerator magnet, and I'm using a leftover 10" speaker from another old SS amp as the main speaker. I'd have to go over my circuit diagrams and trace out a few parts in the chassis again, but IIRC, the main changes I made were thus:

• Lower the gain of the 1st opamp stage. - Stock, it had way more gain than it needed.
• Add a pair of clipper LEDs in the negative feedback loop of the 1st stage. - This makes it similar to the gain stage in a tube screamer, which should make it clip a bit softer than it would in a distortion pedal. I had some green LED's in my parts bin. Green LED's have a forward voltage of about 2V. This, combined with the regluar diodes strapped across this stage's output, allows for the clipping to come from multiple sources, and allows for a slightly different flavor of dirt.
• Change the 2nd opamp stage from a gain stage to a buffer. - Stock, this stage adds more gain. I don't remember if the 1.4Vpp signal from the prior stage was enough to overdrive this opamp stage itself, but this stage definitely adds enough gain to overdrive the output chip.
• Change the gain on the output chip. - If I remember correctly the stock configuration had the TDA2003 configured with a gain of 100, exactly like the example given in the datasheet. That's enough gain that you could overdrive the chip plugging your guitar straight into it and strumming hard. With all the gain from the previous to opamp stages in the previous configuration, you're guaranteed to overdrive the output chip. With a power supply of 14V, and a gain of ten, you could theoretically put a 1.4Vpp signal into it before overdriving. Well, not exactly. There's a caveat to that which I'll detail below, but I did set the gain on the TDA2003 to a much saner value of 10.
• Tweak the voltage divider on the output of the 2nd stage to trim down the signal feeding the output chip. - This was basically added insurance that the output chip would not be overdrive. With the 1N4148's across the 1st stage's output, the maximum signal out of that stage is 1.4Vpp. With the 2nd stage changed to a a buffer, it is still only 1.4Vpp max. With this voltage divider, we could trim it down further to say...12Vpp, and thus ensure we never overdrive the TDA2003. The only downside of this is that you can never turn up the amp volume so the cleans will be as loud as the distortion. However, you can turn down the distortion so it is about as loud as the cleans.
• Splice some speaker cable and a 1/4" plug onto the output wires so I can plug the amp into other speaker cabinets.

I recalled from previous simulations in LT spice that an opamp usually can't swing its output voltage all the way to the same level as it's power supply. I assumed that the same was true of the TDA2003. Looking on the datasheet, there was an output vs harmonic distortion graph. The lines on that graph were very flat at a very low percent distortion up until about 4 or 5 watts or so, and then shot up sharply. I figured the beginning of that curve was the point where the chip was starting to clip, and with a little bit of math (Yay Ohm's law!), was able to calculate what that output voltage was. I don't' remember what said voltage is off the top of my head anymore, I'd have to look at the datasheet and bust out a calculator again. Another caveat is the power filters in the amp's circuit. IIRC, the wall wart that comes with this amp puts out 14V. After the filtering, voltage is down to 13V or so if I remember my simulations correctly. You can't configure the filters in the amps power supply to give you more voltage. A small voltage drop is the price you pay for being able to filter the noise out.

If the circuit in your amp is similar to the one in mine, then that transistor does nothing as far as signal shaping goes - it is part of the power supply. The transistor in my MA104 is configured to be a constant current source to the JRC4558D - I would assume to cut down on noise from this stage that leaked in from the power supply. It is best to leave it alone.

I do have a final circuit schematic of the MA104, either in its stock form, or with my modifications (maybe both!)...somewhere. I'll see if I can find it.