Popular electronics 22 watt amp

[cbg Rick]

I found the plans for this amp in the December 1967 Popular electronics. It is rated at 22 watts with a 45 volt power supply. I'm sure the transistors are long out of production and I plan on using a 24 volt power supply. I'm using this amp as a project to dust off 30 years of cobwebs that accumulated on the knowledge I used to have.
I did build this amp on a breadboard using 2n3904 and 2n3906 for the first two stages and I used TIP 31 and TIP 32 as the output transistors. I used the resistor values called for in the schematic. I powered it up with 12 volts and it sort of worked with a very distorted output. Next step, I built a 24 volt power supply and powered it up again and the magic smoke escaped from a couple transistors  I had rebuilt the circuit and may have made a mistake in wiring.
I decided to regroup and start over fresh. Since this is a learning project I'm going to recalculate the bias resistor values and see what I come up with.

[cbg Rick]

I have a couple questions about calculating the resistor values for this circuit.

1. How does one decide what Ic (collector current) value to use for each stage of an amp? The information I'm using says to use an Ic of 5ma for the first transistor and an Ic of 10 ma for the second transistor. Just wondering how these values were arrived at 

2. In the schematic above I was able to come up with values for the collector and emitter resistors for the first transistor and for the resistor from base to ground..... but then I noticed there is no direct DC current path from the base to the positive rail. How do I calculate the value of that resistance?

[Roly]

Found a cleaner print;




Sadly the thread on tdpri.com doesn't have much to offer, only a long rambling debate about transistors, distortion, and negative feedback.



SWTPC - Southwest Technical Products Corp. (a.k.a. "sweat pack") had a good reputation in the day, did a lot of very early personal microcomputer stuff, S-100 buss.




The "22 watts" is IHFM or "music power", the actual rating of the amp is 18 watts RMS (which is typical of modern small gigging combos and generally more than enough).

It claims a very high bandwidth, 20Hz to 100kHz, which is fine but would need to be confined for guitar work. (or excessive hiss)

The raw input sensitivity is 1.5V for full output.  A guitar will make a noise but not drive it fully.

The raw input impedance is more of a problem at only 5k.  This is fine for a power amp but far too low for an unbuffered (passive) guitar.

So you are going to need a preamp of some sort to provide;

- an input impedance of 1Meg or more
- voltage gain for sensitivity
- EQ/tonestack for sound colouration
- a volume control

The very simplest thing you can do is put a FET source-follower buffer in front of it, but it really needs a preamp that is a bit more full-blooded.  But later.


On closer consideration...

Uh oh ... "one silicon and one germanium output transistor" (and no emitter resistors either)

That does not bode well.

Sorry, but I don't think this circuit makes a good starting point.


PS.

1. How does one decide what Ic (collector current) value to use for each stage of an amp? The information I'm using says to use an Ic of 5ma for the first transistor and an Ic of 10 ma for the second transistor. Just wondering how these values were arrived at 

I go about it backwards.  For 50W in 8 ohms we need about 5 amps peak.  Assuming a venerable 2N3055 output transistor, it has a current gain, Hfe, of as low as 20 at this current.  This means that it will require;

5/20 = 0.25A into the base.

Now the driver has to deliver 250mA peak, and assuming an equally venerable BD139 as the driver it has a minimum Hfe of 40, so it will in turn require;

250/40 = 6.25mA of drive at its base.

And so it goes.

For preamp and small signal stages one can assume a current of about 1mA which is about where most small signal transistor parameters give their best, gain, noise, but this is a massive generalisation and the datasheet always rules.

2. In the schematic above I was able to come up with values for the collector and emitter resistors for the first transistor and for the resistor from base to ground..... but then I noticed there is no direct DC current path from the base to the positive rail. How do I calculate the value of that resistance?

The base current for the first transistor is derived from the output "half-rail" (emitters of Q4 and Q5).

At power-on Q3 and Q5 start to conduct until the half-rail rises to where Q1 brings the rise under control.

This bias is also therefore a DC (and AC) negative feedback path from the output.  The pot R5 is used to adjust the half-rail so it idles at half the voltage of the supply (and the AC gain ends up being whatever it is at that setting - a runcible arrangement IMO).

[cbg Rick]

Working backwards makes perfect sense to me. It seems simple now. 

The mention of germanium transistors was referring to other amplifiers, this amp uses all silicone transistors. I did a quick search and data sheets are available for all the transistors in the parts list.

I realized at about 3:30 a.m. this morning that the first transistor doesn't use a typical voltage divider to bias the transistor. Thanks for explaining how it actually works.

I'm looking at a couple FET preamp schematics and will start with building one and get it working while I study the lil tiger amp.

Thanks for the help, things are getting clearer 

[cbg Rick]

The base current for the first transistor is derived from the output "half-rail" (emitters of Q4 and Q5).

At power-on Q3 and Q5 start to conduct until the half-rail rises to where Q1 brings the rise under control.

This bias is also therefore a DC (and AC) negative feedback path from the output.  The pot R5 is used to adjust the half-rail so it idles at half the voltage of the supply (and the AC gain ends up being whatever it is at that setting - a runcible arrangement IMO).

I'm  seeing how this might not be the best circuit to learn on. I'm going to forge ahead with it for now.
In the mean time I have built a FET preamp on my breadboard and gotten it to work it's funny how little accomplishments like that can be so inspiring.

[g1]

a runcible arrangement IMO).

Sorry but I will sidetrack here a moment.
I'd never seen that word "runcible" before and had to look it up.  Now I know how Roly meant it, but I also see the word has been hijacked for a very interesting product.  They call it an "anti-smartphone" so I guess that makes it a "dumb-phone"  .
 The ideas and philosophy behind it are quite interesting and inspire a little bit of optimism for me as far as tech for future generations go.  Of course it may just be a way for someone to make lots of money, but "dreaming is free" as she said. 
http://mono.hm/runcible.html

[Roly]

"Runcible" is a nonsense word invented by poet Edward Lear and first used in "The Owl And The Pussy Cat", but Lear never defined what it meant and used it in other places where the meanings were inconsistent, just for the sound.

In my use it means an arrangement that could be better and more refined, not impressive, fairly ordinary or lacking.

[Loudthud]

I have found that the more runcible a circuit, the more it sounds like a tube amp.

In the Poptronics circuit, the AC gain is not affected by the bias Voltage set pot R5 because that pot is bypassed by capacitor C3. One little quirk of the circuit is that setting R5 doesn't track the main power supply. As the power supply sags, the Voltage at the + side of C5 doesn't move much. This works to your advantage if you use a soft power supply. You get more compression and touch sensitivity.

I have attached my version of a similar circuit. No global AC feedback puts the gain up around 100. The output impedance is not as low as you might think, but it depends on the beta of the output transistors you use. In the one I built, it was around 5 Ohms. Power the preamp from a decoupled supply from the main Voltage rail. Add enough capacitance to stop the low frequency motorboating.

[Roly]

AC gain is not affected by the bias Voltage set pot R5 because that pot is bypassed by capacitor C3.

Yep, sorry, overlooked that.   

[cbg Rick]

I have attached my version of a similar circuit. No global AC feedback puts the gain up around 100. The output impedance is not as low as you might think, but it depends on the beta of the output transistors you use. In the one I built, it was around 5 Ohms. Power the preamp from a decoupled supply from the main Voltage rail. Add enough capacitance to stop the low frequency motorboating.

I appreciate you taking the time to do this. I will breadboard this amp and see what happens.
I got a bit sidetracked from this project, I built a Fender Blackface FET preamp  https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&cad=rja&uact=8&ved=0CCsQFjAE&url=http%3A%2F%2Fwww.redcircuits.com%2FPage120.htm&ei=ztpHVfmmGMKNNuWVgNAL&usg=AFQjCNG6A16LMLO7h7v6qNx0MGwYvWLn3g&sig2=aGc3cA9fmjTxmGkb-utV_Q&bvm=bv.92291466,d.eXY and in a rush to hear it work used it in front of a TDA 2030 amp I had built from a kit.
The Runcible Lil Tiger or "Loudthud" is moving back to the top of the todo list now. Thanks again.

[Loudthud]

A little over a year ago that preamp was dissected here.

Link: http://www.ssguitar.com/index.php?topic=3373.0 

It should work great with this power amp, but it will want to oscillate (low frequency motorboating)through the power supply. Resist the temptation to use a Voltage regulator or zener diode. You will have to experiment to get the right Voltage and enough bypass capacitance to make it behave.

[cbg Rick]

I had issues with motorboating when I ran it through the TDA2030 also. I'm in the process of soldering the preamp onto perfboard now. I think I read the thread you linked to, but I'll read through it again.

[Roly]

I have to say again that the redesign required to turn the original Ge+Si "Lil Tiger" into a workable Si-Si "Runcible Lil Tiger" is not inconsiderable the child will be a hardly recognisable child of the parent.  This is about as far from a kit as you could get.

As an amp designer I would, and I suggest you also, seek out a design that is closer to your desired end point to reduce the amount of modification required.

I have been searching for a suitable circuit on line and I'm a bit surprised that I can't find what I'm looking for; a bit of refinement without too much complexity.

My biases lead me to;
- Long-Tiled Pair, LTP, (diff amp) input stage, say 2x BD139
- Voltage Amplifier Stage, VAS, say a BD140
- quasi-comp output stage, say BD139+2N3055, BD140+2N3055
- simple split-rail supply.

This basic transistor selection could be improved but mainly depends on what is locally available.

Many amp designs are scalable, mainly by changing the supply rails, and may work over a wide range of supply voltages and thus powers without modification (e.g. a "50 watt" design should scale down to 15 watts).  At the same time you can scale/adjust the transistor and heatsink specs.

[teemuk]

Ok, I'll take a bite...

The PE magazine design undoubtedly works to some degree. To some that might be enough and they may be the same people who appreciate the seeming simplicity.

But then there are inherent issues...

Bias. Well, the bias string probably tracks thermal changes to some degree if diodes are thermally coupled to output devices. But it probably has to be somewhat "cold" bias, though, because... well... in all its simplicity it's a sub-optimal design and more susceptible to thermal overruns. This issue is enhanced by the lack of emitter resistors in the output stage. So we can expect more crossover distortion and compromised reliability.

Gain. As is, the stage is configured as inverting amplifier. Very similarly to opamps (this is a generic NFB circuit after all), gain is determined by feedback impedance and total input impedance. So, consistent performance will require an additional buffering stage to keep source impedance to power amp stage constant. Added complexity.

Reliability. Obviously this amplifier is not protected against exessive load currents in any efficient manner. A momentary short circuit or a complex load may destroy the entire amplifier in less than an eyeblink.

Overload performance. If this was a small signal stage, operating at low currents, similar circuit might provide rather musical, though somewhat hard and asymmetric, clipping. Being a high current stage it's much more susceptible to transistor "deep saturation", which leads to much less musical momentary "rail sticking", which will sound nasty. In worst cases rail sticking may become permanent and destroy the amplifier instanteniously.

Stability. As is, it looks poor and is highly affected by layout and overall design. A practical circuit may require local high frequency negative feedback in VAS (and possibly drivers), and the output probably should have a Zobel filter (remember those complex loads?). Added complexity.

I agree with the opinion that addressing these issues will require a complete redesign of the amplifier (I built a similar circuit myself and vouch for that). In the end, picking a more adequate design as a starting point may prove more effective. Unless you want to use this as a learning experience of course.

Anyway, as I said, to some degree it works. It's a moderately powerful power amp, makes noise and all. I would just be constantly worried about its reliability. No thanks.

[cbg Rick]

I have to say again that the redesign required to turn the original Ge+Si "Lil Tiger" into a workable Si-Si "Runcible Lil Tiger"

I'm sorry but, you keep referring to this amp as having Ge transistors, I checked the transistors in the parts list and I'm not seeing any Ge transistors listed. Am I missing something?