the "lost" post. Enzo........if you please, and have time !

[tonequester]

tonequester here.

Enzo, either i'm loosing my mind(a possibility) or I somehow "erased " that post while I was having keyboard problems. Anyway, I,m sure it's about using
my Ht-1R as a "pre-amp for a s.s. 20W bridged power amp I,ve decided to buy from a kit dealer. The power amp uses 2 TDA 2003 monolithic I.C.'s. Once again I'm lacking some specs which would be helpful. I could take the signal from the HT-1R's speaker out(8 Ohm) or from the emulated speaker/headphone jack( Z unknown ). So I have an obvious impedance matching problem. The input Z of the s.s. amp is listed as min.-47kOhm, max.-150kOhm. I even have doubts here as they refer to it as input R, so I'm afraid thats not the same. This comes from the TDA 2003 spec sheet. Perhaps my biggest hurdel is that neither amp, nor the TDA sheet gives a specification for max. input
voltage, or current. I don't believe that I can measure output Z of the HT-1R. Correct me if wrong, but that would vary with frequency, wouldn't it. I realize that the output signal voltage of the HT-1R could also vary. I fear that too much signal input to the TDA 2003 would most likely be catastrophic. I know that a tube amp can be used this way
with a s.s. power amp. If you have the time, let me know how you would go about doing this if YOU had to. If I need to contact the makers of the TDA, and the kit folks by phone to get more info, I'll do it. If I need some sort of impedance matching I'll do that as well. If I'm asking the impossible here in your opinion, then I'll scrap the idea and
proceed to making my first all tube amp. I had hoped for a little more building experience first, but what the heck. This wouldn't be the only screwy idea i've had, and I'm sure
there are more to come. Quote : "Conformity is the jailer of freedom and the enemy of growth." Churchill.

P.S. Thanks in advance for your trouble and time. I pretty much consider your word to be the last word. tonequester.

[Chuck H]

I don't have much time this morning. But maybe I can help. First, the input resistance is not the same as the input impedance. I'm not sure why the SS amp data would include that spec and not the actual impedance. But it's almost certain to be a high impedance input. The reason is... It's always ideal with voltage amplifiers to have a low impedance output feeding a high impedance input. This serves two purposes. It protects the input from current and it avoids the loading effect on the input signal, keeping it more accurate. I know that "high" and "low" are vague. But the relative high-ness and low-ness ARE sometimes. Still, it's virtually always a compatible situation. Since this has been standard practice since the beginning of audio amplifier history it's highly unlikely that your intended slave amp has a low impedance input. Now...

Since the HT-1r puts out a watt across an 8r load the AC voltage will be about 1.25VAC. You can look up an on line "ohm's law calculator" or just read about ohms law and learn to do the fairly simple equations (I used an on line calculator out of lazyness ) The input on your slave amp will want either a low level signal like 100mV to 250mV (typically called a -10dB input) or a line level signal at just under 1V (0dB input). Using the parameters above for the HT-1r and assuming reasonable design on the intended slave amp I used ohms law and the diagram I drew in your other thread to make the circuit below.

If you take what has been discussed here and in your other threads and study the diagram I think things will become a little more clear. Also, keep in mind that not all things need to be exactly ideal. Especially WRT guitar amps. Some "loading" effect can be a good thing sometimes. Or some clipping. People find great tones by breaking electronic rules on a regular basis.

This circuit should allow a safe load for the HT-1r and 0V to .7VAC for driving the input of a slave amp. The output variable so that you can adjust it for any type of input. The capacitor in the design only affects very high frequencies and is there to reduce HF voltage spikes that are common in overdriven tube amps. This just makes the design more input friendly.

EDIT: Just a couple more notes. This circuit is specifically for a 1W amp that wants an 8 ohm load. So it's not ideal for use with amps that are very different from your HT-1r. I chose the 500R pot because it provides a very low output impedance. But the 10k value Steve mentions is certainly low enough and would offer better control and more range. Many guitars have an output impedance over 10k. You can make this circuit in a project box for much less than the cost of off the shelf devices to accomplish the same goal. And with an EQ in line it should be much more versitile.

[Steve Conner]

The TDA2003 is essentially a power op-amp. That means that the input voltage required for full output is set by the feedback circuit used with the chip, not the chip itself.

The datasheet application circuit has a gain of 40dB, which is 100x. So, you'll need less than 100mV of input for full output. The datasheet says 50mV. However, if your kit dealer used a different circuit, the sensitivity could be different. He should be able to provide you with information.

I believe the input resistance figure quoted on the datasheet is actually the input impedance, because it says "Measured at 1kHz". I guess it just happens to be resistive. Again the kit vendor may have done something to change it.

Chuck's little circuit should do fine to get you started, but good luck finding a 500 ohm audio taper pot. It is essentially your master volume, so you'll want audio taper. A 10k pot would probably do, since the TDA2003's input impedance is much higher than 10k.

[tonequester]

Yhings are "clicking" now.

I don't have much time this morning. But maybe I can help. First, the input resistance is not the same as the input impedance. I'm not sure why the SS amp data would include that spec and not the actual impedance. But it's almost certain to be a high impedance input. The reason is... It's always ideal with voltage amplifiers to have a low impedance output feeding a high impedance input. This serves two purposes. It protects the input from current and it avoids the loading effect on the input signal, keeping it more accurate. I know that "high" and "low" are vague. But the relative high-ness and low-ness ARE sometimes. Still, it's virtually always a compatible situation. Since this has been standard practice since the beginning of audio amplifier history it's highly unlikely that your intended slave amp has a low impedance input. Now...

Since the HT-1r puts out a watt across an 8r load the AC voltage will be about 1.25VAC. You can look up an on line "ohm's law calculator" or just read about ohms law and learn to do the fairly simple equations (I used an on line calculator out of lazyness ) The input on your slave amp will want either a low level signal like 100mV to 250mV (typically called a -10dB input) or a line level signal at just under 1V (0dB input). Using the parameters above for the HT-1r and assuming reasonable design on the intended slave amp I used ohms law and the diagram I drew in your other thread to make the circuit below.

If you take what has been discussed here and in your other threads and study the diagram I think things will become a little more clear. Also, keep in mind that not all things need to be exactly ideal. Especially WRT guitar amps. Some "loading" effect can be a good thing sometimes. Or some clipping. People find great tones by breaking electronic rules on a regular basis.

This circuit should allow a safe load for the HT-1r and 0V to .7VAC for driving the input of a slave amp. The output variable so that you can adjust it for any type of input. The capacitor in the design only affects very high frequencies and is there to reduce HF voltage spikes that are common in overdriven tube amps. This just makes the design more input friendly.

EDIT: Just a couple more notes. This circuit is specifically for a 1W amp that wants an 8 ohm load. So it's not ideal for use with amps that are very different from your HT-1r. I chose the 500R pot because it provides a very low output impedance. But the 10k value Steve mentions is certainly low enough and would offer better control and more range. Many guitars have an output impedance over 10k. You can make this circuit in a project box for much less than the cost of off the shelf devices to accomplish the same goal. And with an EQ in line it should be much more versitile.

tonequester here, Hey Chuck !

Thanks my friend for all of your help. Especially the last 2 posts, which I have copied in their entirity, including the circuits provided. I don't want to take up too much of yout time. Suffice to say that reading the 2 posts together, really helped me to better grasp the situatioin at hand. The circuits are great for study purposes or to actually use . I have found more specs for the chip. the Ri-input resistance is 150kOhms @ 1kHz. In agreement with you and Steve Conner, I myself don't understand why they site input R., instead of input Z, anyway Ri(150K) is probably high enough for my purpose, with proper matching. The 100mv inout is in line with another spec that I did find. the Vi(rms)-input saturation voltage is 300mv. I believe at this point that the terminology-input sensitivity, is the only thing remaining that I don't fully inderstand, nor do I know if ity is anything that I have to be concerned with. This I will Google. The s.s. amp has an input sensitivity of 100mv @ 47kOhms. I can probably even get a decent idea of just what it means to me in this situation from Wiki ! Thanks again for your time and talent at lowering yourself in explanation to my level. you would have made a good teacher.
I truly appreciate you helping me get this thing figured out beforehand. have a great day Chuck H. tonequester.

Quote : "Always carry a flagon of whiskey incase of snake-bite, furthermore.......always carry a small snake." W.C.Fields

[tonequester]

The TDA2003 is essentially a power op-amp. That means that the input voltage required for full output is set by the feedback circuit used with the chip, not the chip itself.

The datasheet application circuit has a gain of 40dB, which is 100x. So, you'll need less than 100mV of input for full output. The datasheet says 50mV. However, if your kit dealer used a different circuit, the sensitivity could be different. He should be able to provide you with information.

I believe the input resistance figure quoted on the datasheet is actually the input impedance, because it says "Measured at 1kHz". I guess it just happens to be resistive. Again the kit vendor may have done something to change it.

Chuck's little circuit should do fine to get you started, but good luck finding a 500 ohm audio taper pot. It is essentially your master volume, so you'll want audio taper. A 10k pot would probably do, since the TDA2003's input impedance is much higher than 10k.

tonequester here.

Thanks for the input Steve. I found another data-sheet/different website for the TDA 2003. It also gave the input R figure,with no mention of input Z. Ri is 150kOhms(typically). neither data-sheet for the chip, or the poor excuse of a one for the amp itself, gave any indication of what max./ Vi would be under any conditions. Chuck H's explanation and his derivation of the 100mv signal seems to be a good approximation to me. As I told Chuick, the only thing left that concerns me is input sensitivity. I intend to Google it presently.
The amps specs list input sensitivity at : 100mv/47kOhms. Amp and chip data are in agrement with each other as to input R.(why not Z ?) being 150kOhms @ 1kHz, typ. So the kit builder, I don't think changed anything there. I'm thankful to Chuck for the circuit. I don't even have to worry about the 500 Ohm audio taper pot. I'm like Enzo in one respect. I've been collecting
parts and components since about 1980. I not only have that value of pot but you ought to see how way back. The 500ohm pots that I have are wire-wound, and probably weigh 4x what a new one would. If you ever need the oddball part for one of your projects, keep me in mind. The brand on those pots is Ohmite, if memory serves me. Now all I have to do is do a little
"search and seize" in my "shop'. Thanks again for being on the tonequester Board of Advisors, tonequester inc. would sink without guys like you and Chuck H. Have a great day in Glasgow !
tonequester. Quote ; "Greetings, and death to our enemies" ! Dan Aykroyd.

[Chuck H]

The point Steve was making about the pot was that it may not adjust smoothly in an intuitive manner since the "taper" of most 500 ohm pots is incorrect for the application. Pots come in several tapers. But the most common are "linear" and "audio". A linear taper pot is, well, linear. It will present 50% of it's resistance at 50% rotation. Audio taper pots are more gradual. Since the human ear (brain) percieves loudness and volume differently than math does, 50% voltage division doesn't equate to a 50% volume perception. Audio taper pots, as the name implies, are tapered to compensate for this difference in perception. An audio taper pot typically presents between 10% and 25% of it's resistance at 50% rotation. So, regardless of the quality of a pot, the "taper" must be correct for it to operate intuitively. This may, or may not be important to you. I personally don't mind if a pots taper isn't intuitive. The result is the same regardless of an odd pot setting. For example. With a linear taper pot that requires a setting of two, if you replaced that pot with an audio taper it may require a setting of five or six. This seems pedantic, but it's important to consider if your designing products that will be marketed at non tech musicians who think that the treble setting should be nominal at five. So that's it. The "taper" is relative to the pot setting for a given adjustment. The circuit can be adjusted the same with any taper pot, though the actual number setting may seem odd.

On another note... The circuit I drew above should work fine for your intended purpose. And, because it has an output that is adjustable up to near line level, you could also take your HT-1R to a club gig and plug into a channel on the PA mixer with an appropriate drive signal. Or use the "effects return" or "power amp in" on another amp that wants to see something closer to "line level". So it could prove to be a very useful device for future aspirations as well as this one. But to be sure, it will be a low output pot setting for use with the slave amp you've described. But the pot still offers enough attenuation that setting it incorrectly should only result in unwanted distortion with no danger to your slave amp.

[tonequester]

Lession well taken and preserved !

The point Steve was making about the pot was that it may not adjust smoothly in an intuitive manner since the "taper" of most 500 ohm pots is incorrect for the application. Pots come in several tapers. But the most common are "linear" and "audio". A linear taper pot is, well, linear. It will present 50% of it's resistance at 50% rotation. Audio taper pots are more gradual. Since the human ear (brain) percieves loudness and volume differently than math does, 50% voltage division doesn't equate to a 50% volume perception. Audio taper pots, as the name implies, are tapered to compensate for this difference in perception. An audio taper pot typically presents between 10% and 25% of it's resistance at 50% rotation. So, regardless of the quality of a pot, the "taper" must be correct for it to operate intuitively. This may, or may not be important to you. I personally don't mind if a pots taper isn't intuitive. The result is the same regardless of an odd pot setting. For example. With a linear taper pot that requires a setting of two, if you replaced that pot with an audio taper it may require a setting of five or six. This seems pedantic, but it's important to consider if your designing products that will be marketed at non tech musicians who think that the treble setting should be nominal at five. So that's it. The "taper" is relative to the pot setting for a given adjustment. The circuit can be adjusted the same with any taper pot, though the actual number setting may seem odd.

On another note... The circuit I drew above should work fine for your intended purpose. And, because it has an output that is adjustable up to near line level, you could also take your HT-1R to a club gig and plug into a channel on the PA mixer with an appropriate drive signal. Or use the "effects return" or "power amp in" on another amp that wants to see something closer to "line level". So it could prove to be a very useful device for future aspirations as well as this one. But to be sure, it will be a low output pot setting for use with the slave amp you've described. But the pot still offers enough attenuation that setting it incorrectly should only result in unwanted distortion with no danger to your slave amp.

tonequester here.

Awesome, Chuck H. I,ve copied your recent, and extermely helpful posts to paper( I know. Old school ) for the forum notebook that I" ve started. My memory
isn't so great now a days, so I'm going to keep a lot of the good stuff that I find especially useful and educational. I know that a lot of the info that you took the tome to bring
down to my level is pretty basic stuff. However, I'm kind of a freak. I have been very lucky in my time to be fairly successful at fixing and modifying guitars, amps, and even a stereo or two. My luck was helped along with a little common sense and a talent for finding the information that I needed to succeed. I've haunted every library within 50 square miles of my home, even spending hours at the Linda hall Library of Science and Technology(U.M.K.C.) just to overcome some problem that most forum members that I've had contact with would have resolved in their sleep. This forum, and corresponding with folks like you, Enzo, Steve Conner, big---tee, Austin, and a dozen others, has been one of the best "moves" that I've ever made. You have all been so thoughtful to have not only offered advice and opinions, but most who have replied to my posts seem to have a
talent for simplifying(sometimes the obvious) the issue at hand, as few teachers I've had in my school days. This forum can be a humbling experience. I was actually a little
big headed when I registered. I thought that I would probably help as many as would help me. That hasn't been the case at all. So pride has been put aside by me and I now
look forward to learning something every time I log on. I still have the hope that at some point in time I will be able to share something of value to somebody else.
My projects are slowly, but surely coming together with the help of the forum. I wish that I was more computer friendly so that I could share some pictures that
I would like to share.I had never given this aspect of forums any thought until big---tee expressed some curiosity at my guitar upgrade work. You see. I am trying to learn at
the same basic level the many "how to" lessons one needs to be proficient at doing things on my computer besides Googling and e-mail. I'm not even expert at Googling and
e-mail. You may think that I just crawled out from under a rock. In a way, I guess that I did. Anyway, I wish to thank all who have responded to my queries, and gave of their time and knowledge, to a perfect stranger. The old tonequester firmly believes "what goes around,...comes around". I believe that, and I pray that you will all be rewarded
for taking time, and giving effort, to help me out. I didn't realize that I had 'em, but I can feel the cobwebs of my mind falling away. One needs to stay active in their "retirement", if they wan't it to be a long one. You gentlemen are surely exercising my mind. Thanks again Chuck, and everybody else as well. Here's to your success in
all of YOUR endeavors. Tonequester out.

Quote : "If you wasnt to get laid, go to college. If you want an education , go to the library. Frank Zappa. (may I add....go to the forum !)

[DrGonz78]

Hey Tonequester-

You nailed the head of it right on that last post... I was so impressed by the help I received from the cast and crew on MEF that I have even donated to the site. Now I am telling this only because I have never ever done that before on any site anywhere. The valuable information that was passed onto me was instantly realized and appreciated by my donation to help keep this site going...! What's the greatest part about this site...? They would help you either way and it is karma based giving of knowledge. It's like the purists of electronics and their love of music that congregates on the MEF site to pass on this torch of knowledge.