I looked the schematic more closely and it seems to have at least three identical gain circuits that have no controls. If you only switch between preamp A and B (and do not, for example, blend them) you don't have to build gain stages separately for each preamp. (Ie. Sw1 pole also to sweeper of both 1M gain set potentiometers and to "input" of both 10k gain set potentiometers. The other gain stage looks like it could use a very similar arrangement). If you figure out the switching and signal routing more thoroughly you simplify the circuit a lot. Trust me. You probably have to implement switches with more poles to succeed. Also, i already mentioned about the "master volume" potentiometer.

Edit: I'd use a mono potentiometer and put sw1 before the series 10k resistor and 33n capacitor at the sweepers of 50k potentiometers: You now now need only one 10k resistor and 33n capacitor instead of two.

The volume potentiometer(s) trouble me. If it's not ganged then you have a serious imbalance between the two power amps. On the other hand, if it's ganged then why not use just one?

Rod Elliott explains it quite well in his article about variable amplifier impedances (http://sound.westhost.com/project56.htm). The following quote is an excerpt from it:

"Voltage drive maintains a constant voltage across the load, while a current drive circuit will produce exactly the same current into the load - in both cases regardless of impedance."

If you think of the quote in a context of gain it makes more sense. If an impedance at certain frequency is R and P=UI we get: P=U^2/R or P=I^2 * R. You see the R is a dominant factor. In the first formula, large R means smaller power and in the second formula large R means higher power - and opposites. Now think what happens in both cases (voltage and current drive) when the speaker impedance might swing anywhere between 1 and 150 ohms - according to the model of course. Here's a link to a datasheet of one Eminence model speaker. Look at the impedance graph, it's a quite good example of any guitar speaker:

http://www.eminence.com/pdf/legend-1028k.pdf

If you read the thread i posted a link to (http://www.ssguitar.com/index.php?topic=75.msg785#msg785), you already seen the effects of both ways to drive a load: The frequency response of the power amplifier will change from linear a great deal into enhancing bass and treble. Some people regard this as "warm" and "punchy" tone. For high-fidelity systems any colorization is of course a bad thing, but for guitar... well, it's a matter of taste. Mixed-mode drive is just a mix between the two - current and voltage - a compromise between two unideal approaches. If you look at Rod Elliott's article you can see a simple schematic how it is achieved by combining two feedback paths: One for current, one for voltage.

Note that the introduction of mixed-mode output to these designs is no rocket science. Just compare the schematics.  

Be aware: This circuit is conceptual and i haven't been able to test it in real-life. Simulation shows it works but the real circuit might experience some instability problems - also i suspect the gain might be too high. On the other hand, the introduction of protection diodes and Zobel network might even make it more stabile than the example application circuit.

"A good poweramp will not color the sound."

I have to disagree on this since it's a statement mainly reserved for High Fidelity reproduction of sound, not production that is going on in guitar amps. A good guitar power amp may, or may not, color the sound - depending on how it reacts to the varying load impedance.

I second using the Peavey pre and building a new power amplifier circuit and a matching supply. But there is a but: You want to retain the similar sound of your amplifier, in this case you have to figure out what IS the sound that you like and what makes it: Do you have the "sound" already in the signal that you get from the preamp (I guess you should get a signal out from the CD in jack) or is the speaker and the power amplifier an important factor to it. If you play your guitar "dry" to power amp (from the CD in jack) do you get the "sound" this way instead. (Note that there will most definitely be a terrible impedance mismatch and you should try playing a guitar signal from a tape or computer instead). Is the preamp tone a lesser factor than power amp tone or do they equal. What aspects of the sound you appreciate most.

It might help to point out that all the difference you hear in headphone out vs. speaker is due to either speaker's frequency response or power amplifier reacting with the load impedance. All the difference you hear from the CD in is due to power amp - speaker interaction too. Does the headphone/preamp signal seem to lack bass and highs?

Building a new amplifier will not sound identical, it may sound "similar" though. If you want an easy way to build a high power (100W) amp i suggest you avoid discrete designs. Two parallel chip amps and a rugged supply should be OK but if a huge part of the "sound" lies within the interaction of poweramp and the speaker then you have to match the new and more powerful speaker system to the old. This means at least matching up both resonant frequency and HF roll-off of speakers. Then you probably have to design a mixed-mode power stage that drives your speaker setup properly too, otherwise you will have less effect of speaker's impedance.

I wrote something about this earlier. It might explain what i'm talking about better:

http://www.ssguitar.com/index.php?topic=75.msg785#msg785

Well, my first suggestion for getting more power would be using a higher power chip from the TDA series since the amp already seems to have one. This definitely also includes changing the power transformer to a proper one; it has to have a higher supply voltage and a higher VA rating. You probably have to redo the whole power supply. Extra bad is that the circuit is single supply, it will undoubtedly cause problems. You shouldn't forget heat sinking either - more power means more iron. Also, if you change the speaker to match up with more power the amp will not have the same tone - even ignoring the speakers frequency response: The power stage runs in mixed-mode (note that feedback path after the speaker) and is very much dominated by the load impedance. Put a new speaker in and the carefully designed circuit becomes... well, not so carefully designed. You have a 50-50 chance of getting a good tone from it.

Taking this path will quite surely mean a complete rebuild and redesign of some parts. The result will not be the same low power amplifier, thus the tone will not be the same either. However, this should be easier than it first seems - at least if you find a high power TDA chip that shares the same pinout and works on single supply. It also could happen that PCB traces are too thin for higher currents and there is insufficient room for a bigger heatsink. Anyway, i'm quite sure that properly done the end result would be far more better than the original circuit though.

Another option is to replicate the power supply and power amp and run the amp in stereo. You have to find a matching speaker though - or at least one that suits you. You could built as many of these as you want, say for example four: That would equal power of 60 watts - which you could get by building only one new power amp and power supply circuit. Probably the same amount of work. I guess parallel configuration wouldwork as well but once again you need a power transformer which is capable for feeding enough power to the amp. Probably you need a higher power speaker too, thus changing the tone. Frankly, i see no point of using two 15 W amps in parallel or in stereo - if this is for loudness reasons just mic the amp or find a more powerful speaker system.

By the way, thanks for the schematic. It's the first "TransTube" amplifier in my collection. 

My first feeling is that I prefer simple setups but in the end I always find myself being a control freak: If it has a tonestack I prefer options to bypass it and have controls at least for mids, highs and bass. Same goes with everything: Gain has to have a control, so does the master volume. Switchable distortion/clean channel, as well as reverb/FX loop is a must. Everything preferably with tone/blend controls. In the end my ideal amplifier would have numerous knobs and switches but the good thing is that I wouldn´t have to use all of them. Sometimes i just love the easy front-end of digital pedals equipped with patches of tone. I guess the next amplifier I will build will have knobs mounted so that they will not turn by accident.

By the way, that Dr. Boogey design looks somewhat familiar, is it a design from the Runoffgroove.com? Higher supply voltage? I think there should be no major problems. It should be fairly easy - at least if you used those trimmer resistors at drains. However, raising the supply voltage will probably raise the headroom too and as a result the circuit would need a higher input amplitude to be driven into clipping.

As mentioned earlier, the constant voltage gain -circuit can be replaced with a current driven circuit or better yet, with a circuit that mixes both of them. This so called mixed-mode-feedback topology can actually provide very nice results, especially if the gain is matched to a specific speaker impedance. In a mixed-mode amplifier the other speaker node is not directly connected to ground, instead it is in series with a small value "current sensing" resistor. The load current over the resistor is fed back, thus effecting the output impedance of the amplifier. As a result the circuit operates almost like a constant current gain amplifier that provides more power to high impedance loads.

While the previous might have sounded difficult it's actually not and the amount of extra components required is only a handfull - in minimum two resistors. I've seen this topology used in various guitar amplifiers (both chip and discrete) but it's quite rare in PA- and especially bass amplifiers - i guess for obvious reasons. I will not go into further details of how to design a mixed-mode amplifier since everyone can find that out by themselves from the link i that provided earlier, however i will demonstrate how it compares with a transformer coupled circuit.

1. The expensive?, tricky-to-use and very often custom-type transformer can be omitted and still comparable results can be yield from the circuit.
2. The circuit is a combination of constant voltage -and current drive so it's features are compromises of both topologies.
3. It is difficult to match up the "punchy" bass and emphasized highs of a transformer coupled circuit but the mixed-mode amplifier definitely comes a lot closer than a constant voltage gain circuit. Differences between the tone of these circuits is not so big anymore. It's easy to distinct which topology is used only during bass-heavy content.
4. What is important, at least for Hi-Fi, monitor and PA amplifier designers, is that the topology can be used to gain a very linear frequency response.
5. Setting the gain of higher impedances too high will weaken the low frequency response.
6. Mixed-mode circuit's work as well as intented only with speaker loads for which they were designed to.

I included a plot where the "modern" circuit's output impedance is as closely matched to the transformer coupled circuit's as possible without affecting the low frequency response too much. Note that the circuit's frequency response could be tweaked easily to provide a very linear response too. Once again, the plots are dependant of the used speaker's impedance. For comparison the "vintage" plot is identical with the previous one.

Quote from: RDV on April 30, 2006, 08:33:21 AM
It seems that you(I) are not allowed to download or view attachments on this board.

I think there´s some sort of an cookie error troubling this forum system: I get the same message now and then. If you were logged in and got the message (as i suppose you were) do not try to log in again,  it will not work very likely. Instead, clear the web browser´s cache and cookies and restart it. Now login again and everything should work. This works at least with IE.

The other line is the phase shift. There's an error in naming of the plots too that might cause confusion: The second plots does not represent a function V(x) but a function of V(x)*I(Z), which is power.

Here's some plots. Note that the impedance would vary according to specific speaker load. A speaker model with a free air resonance peak @ 88Hz was used in these simulations.

I expanded the simulation to compare between transformer coupled circuits...

The direct coupling of the voltage amplifier stage can be replaced with an interstage push-pull configuration as follows: Direct coupled load on collector is replaced with the primary of the interstage transformer. Gates are biased with a network of four resistors and secondaries control gate voltages in opposite polarity. PNP transistor "side" can be replaced with another NPN side. "Bootstrapping" is done by bypassing one gate to the output and the other gate to Vee. Here's one example of the output stage configuration:

http://www.univox.org/pics/schematics/pa960_column.gif

1. As expected, even order harmonics decrease due to push-pull drive. However, odd harmonics increase since the circuit is not as linear as the "modern" circuit.
2. As expected, the transformer forms a high- and low-pass filter which decreases both low and high frequency response.
3. Voltage swing ability is weakened and the circuit needs higher supply voltages to match up the "modern" circuit. For the same reason the circuit will also clip earlier. The described bootstrap circuitry helps a lot but is still insufficient.
4. The supply voltages of an interstage coupled circuit "sag" much more.
5. Between 20-80 Hz the interstage coupled circuit delivers more power than the "modern" one.
6. Although differences were much more clearly visible when waveforms were compared i couldn't detect any of them by listening.

Things really change when also the output is transformer coupled. First of all, the circuit after the voltage amplifier stage is no more a "buffer" like in other power amplifier topologies so far but also provides voltage gain since power transistors operate in a common emitter configuration seeing primary inductance as collector load. See example of configuration in here:

http://www.vintage-radio.com/images/figs/transistor/class-b-output-1.gif

As expected, the collector loading reflects the impedance of the speaker meaning that the output stage no longer operates at a constant voltage gain: More power is fed to high impedance loads and less to low. When P = U^2 / R or I^2 * R, a current driven circuit will put out more power with a  larger value of R - assuming it is provided with enough voltage, which is also the major drawback of current drive. As a result the amplifier is "less powerful" on small loads but feeds a lot more power to, for example, speaker in resonance. This is the "punch" that solid state amps are reputed to lack.

Here's few things that i noticed:
1. As expected, the transformer forms a low-pass filter and highs roll-off faster.
2. Surprisingly, the "modern" circuit has a much weaker low frequency response and the bass begins to roll-off much steeper.
3. The frequency response of the "modern" circuit is quite flat when compared to transformer coupled circuit which shows more gain on higher speaker impedances. If voltage conditions are met, the amplifier is definitely more powerful on speaker's resonance and higher freqs. There was a figure showing the same result in W. Stephen Bussey's and Robert M. Haigler's article Tubes versus Transistors in Electric Guitar Amplifiers.

http://milbert.com/articles/TvsT/tvtiega.html

As we can see, this is not actually a valve character. Also, there are ways to get the same results on "modern" circuits but that's another story. For those who are interested about it see Rod Elliott´s excellent article about variable amplifier impedances:

http://sound.westhost.com/project56.htm

The tone of both amplifiers is indeed very different and it's definitely very easy to distinct the tones from each other. For the sake of truth i should say that neither one of the circuits reproducts the original signal perfectly: The "modern" can not provide enough power for high impedance loads and the "vintage" one provides too much of it. The power vs. frequency plot is not flat on either one.

A word of warning: The transformer coupled circuits are highly more unstabile and a lot more "unpredictable". The result of having "wrong" component values may not be just distortion, it can also be crazy oscillations or very high voltage peaks. These circuits are definitely much more difficult to design and tweak.

I like Eminence too. They make really good products and on top of that they publish better specs of them than i've seen any other instrument speaker manufacturer do. They really make life easier for people who wish to design their own cabinets. Jensen might also be a good choice. They also publish good specs but i guess their product variety isn't as big as Eminence's though.

You should also think whether you need a cabinet that is loud or a cabinet that has a certain tone. If you prefer loudness use speaker(s) with highest SPL rating as possible - if tone, concetrate on specs about frequency response and free air resonance. The later one is quite important since a resonant speaker can really rattle an inadequate cabinet. Tuning the resonance up and down with the cabinet has a huge effect. Tone - loudness is in most cases a trade-off: You can always shape the tone a certain amount with the cabinet design but the more you focus on the tone of the driver(s) the smaller the choice of drivers with high SPL ratings get.

One thing you should also consider is that acoustic amps are usually equipped with some kind of an "anti-feedbacking" circuit. Those hollow-body guitars tend to get really crazy when you stand too close to an amp - even at very low volumes.

I wouldn't trust the "Cheap 100 to 150 Watt Amp" at http://europa.spaceports.com/~fishbake/amp/ca100.htm to work reliably enough. Why:
1. Bias adjust and thermal coupling is done with only 2 x 1N4007 diodes. I sense a serious threat of thermal breakdown. I'd also suspect that the circuit has a high amount of crossover distortion since it has a darlington output.
2. High supply voltage and only one pair of output transistors. With 150 watts even the maximum ratings of TIP142/147 are way exceeded. For watt ratings such as these i'd recommend at least another pair of output transistors in parallel.

Yeah it's simple and i'm quite sure it would work, but it could destroy itself in a matter of milliseconds as well. With decreased rail voltages, i would trust it to run a maximum of 40W amp or so. With slight modifications to improve stability maybe an amp of 50 to 60 watts. But 100 to 150? No way!!!! The designer has not been on the level with the output transistor specs or demands required for stable enough high power applications. I'd recommend anybody to stay away from this design as it is.

The Rod Elliot's project looks a lot more like a design that i would choose from the two. Actually, i think it could be quite educative for anyone planning to build an high power amp to compare both circuits and find out why the ESP design is so much more complex. Even if they choose to build a chipamp afterwards.