Stereo amplifier- a new? approach?

[bardkin]

I know there is a long thread on this dating back to 2006 but I read several instances of what i believe are misunderstandings between the difference in a stereo effect and a stereo amplifier. This will be a bit of a long post, so grab a drink, smoke em if ya got em, i'll keep it as short as I can.

I want to build a tube stereo amplifier, but there are many complications and what ifs and all. So after long long time, I have what I think is the plan. Please let me know what your thoughts are as I am not really an electronics guy so far, but am learning. I have been a musician for over 40 years now so I do know a little bit. Okay, Here goes.

Misconceptions from another thread- Stereo effects are not really stereo.

Take for instance a chorus pedal. This effect takes an original signal, then makes a duplicate. this second signal is then processed for delay and detuning. both signals are modulated back and forth between two output paths. The physical outputs of the pedal are tip and sleeve such that the first signal is passed through one jack, and the second signal is passed through the second jack. at any instance in time you will see a completely different wave form between the two, so what you really have are two distinct signals being sent. The "rate" sets the speed at which the effect modulates. The depth sets how intensive the effect is.

A stereo delay has the ability to repeat a signal on each side at the same time, or change side to side in a "ping pong" way. It too, uses a tip and sleeve for its input and output.

A stereo amplifier uses two audio signals, one panned to the left, and one panned to the right. On car or home audio systems you have a balance and even a fade control to change the center point of the listening area. You can hear a bass guitar to one side and an acoustic guitar to the other. You could do this with a stereo guitar amplifier as well if you wanted. The important thing to understand is that if 2 speakers are right next to each other you might have stereo output, but you are hearing it in mono. The further the speakers are from each other, the better the stereo sound can be.

Following that line of thinking I believe the best coarse of action to building a true stereo guitar amp that can take advantage of "stereo" effects is this:

Build it modularly.

The simplest idea is to build a stand alone preamp circuit. "patch" in an effects loop that splits the signal into two separate outputs. Then build 2 power amplifiers to drive the speakers. The build should be made as a head, and the speakers should be cabinets that can be spread out.

In doing so, it is easy to see having 1 or more separate preamps so that you can have either a clean or dirty sound. By adding a second, you get the best of both worlds. I could see building a 3rd or even 4th preamp, modelling after Fender, Vox, Peavy etc..These preamps are controlled at the input jack of the amplifier. Just switch to whichever one you want at will. But there does need to be 2 separate power amps- 1 for left and one for right. The stereo splitting happens in the effects loop system which is actually more of a crossover circuit that "inserts" than a send/return loop. Now I realize we are talking about a lot of extra tubes and work here, but there are plenty of cheap solid state stereo amplifiers out there so...

In order for the effects loop to work properly, it needs 3 distinct paths. One path should go directly to to the left power amp. the second path should split between the left and right, and the third should be dedicated right. Here is where the "crossover" can work with the "Stereo" effects.

Because a stereo effect has an output A with a tip and sleeve, and the output B does as well, you cant actually connect using stereo (T-R-S-) cables. To do so may cause several issues including ground faults and imbalances due to different voltages being used in the effects chain. Therefor, use 2 pole jacks and cables. The "Crossover" part of this loop is to always keep a signal to both power amps. I would hate to turn off my chorus pedal and have only 1 side. The path that splits should be used for mono effects such as an analog delay or a reverb. The left and right should be used for devices having 2 outputs.

Another interesting thing here is that now in your loop design you find an opportunity to build in another module circuit to:
A) bypass the loop
B) incorporate in a potentiometer to control how much of the loop is in the mix
C) build in your left and right line outs for recording or to PA
D) turn several effects on and off at will while still maintaining the signal integrity.

Effects such as distortion and Wha's should happen between the instrument and the preamp so they are not accounted for, but could be used..

Each of the two power amps should have mono output- so that your left stays left and your right stays right. The preamp is all mono. The loop that splits actually sends a mono signal to each power amp, as do the left and right loops respectively. so, each power amp puts out a mono signal, and you can actually follow a single signal path from start to finish. But both sides can be modulated between the two, or "Panned" to one side or the other at any given time.

So again, it needs to be built as a head, without a speaker. There should be 2 speaker cabinets, spread out as far away from each other as they can be, and dont use stereo cables at all.

Mono Preamp out to effects chain, then out to left and right power amps, each driving their own speaker cab. I know you could buy the parts and hook it all together, but the effects chain wouldn't work right, and I hate multi effects units anyway. So I want to build this from scratch. Have the sound I want and not what Fender and Marshal think I want.

Okay- so what are your thoughts on this? I know the loop sounds complicated, but if you think about how a PA mixer handles individual and grouped signals, it becomes much easier to identify with.

Thanks for reading

[Enzo]

You have spent time covering connectors, but conceptually they don't matter. Make any sort of connections you like. the overall product you describe is more or less what a Peavey Stereo Chorus 400 was, or one of their other chorusing amps. A chorus pedal may have stereo outputs, but is really intended to use before a guitar, so any chorus effect is mixed into mono before you use it. But a chorusing amp like the Peavey or a Fender Princeton Stereo Chorus is a stereo amp - two power amps - that both are fed the same mono signal usually, but when the chorus is on, one channel gets the dry signal, and the other gets the chorused signal. That is the delayed/shifted signal. The dry and processed signals swirl about one another, each from its own speaker. The preamps are mono, the stereo separation coming only after the chorusing circuit.

The fender Princeton Stereo CHorus has a mono FX loop after the preamp. It also has a stereo FX loop. Two jacks, one is FX send with left and right on tip and ring, then stereo return to the two power amps, also left and right on tip and ring. They could just as easily gave made them two distinct pairs of mono jacks, but the TRS jack saves panel space. All you need to do is use a breakout Y-cord, otherwise called an insert cord. The peavey lacks the mono FX loop in the preamp, but does have insert style FX loop jacks before the two power amps.

So in either case, you can rely on their built in chorus, or you can send the mono preamp output to whatever stereo processing unit you care to and feed the resulting two stereo signals to the power amps. As to adding panning and FX mix, most of that is included within any stereo processing unit.

[bardkin]

um, I am pretty sure both the peavy and the fender are solid state amps..

[Enzo]

yes, of course they are. My point was that what you proposed has already been on the market. No reason those solid state circuits couldn;t be replaced by tube amps. Think of an amp as a black box. It can be tube or solid state, you can connect either kind to either kind. You will have a hard time making a tube based chorus unit, but that didn;t seem to be your goal. I was pointing out that it has already been done, you can look to those examples for system organization. A block diagram as it were. The loop/jack circuits don;t care if it is tube or SS.

Peavey made the Classic 50/50, as well as other Classic series amps like the 60/60 and 120/120. All are tube stereo power amps. Marshall made some as well, I forget the model names. Same with your preamps, LOTS of tube preamps to choose from. NO problem to clone all that in one head. Heavy as hell, it would be.

[The Dude]

NO problem to clone all that in one head. Heavy as hell, it would be.

Indeed! Rivera made some stereo amps. They may still. I stopped investigating as price and weight were restricting. I recall some of them did not have a published weight. They didn't want you to know.

Whatever the case, the concept of a true stereo guitar rig is nothing new. As Enzo says, most achieve this with seperate pre and power amps. I used to have an all tube pre and a Classic 50/50 in a rack myself. Also in the rack- a power conditioner and a multi-effect unit. The rack was much heavier than the 4x12 cabinet it powered. I've since gotten older, lazier, or smarter (pick one). The 50/50 was replaced with a much lighter cabinet emulator and the cabinet stays home. The only person potentially unhappy about this was my chiropractor.

[bardkin]

I cant say as Ive heard of an all tube rack mounted system. but then again, I dont know why there shouldn't be one. Being that I first started playing in 1974 I have seen tons of amplifiers come and go, and at least 90% of them were mono amps. Until stereo chorus and stereo flangers came out, there was never really much need to having one as the PA always took care of that. What I do know is that I never hear a stereo sound from anyone who isnt either a big name like Alex Lifeson or Trevor Rabin.

So, the approach you say is the same, sorry, I havent seen it. and in all examples, all I see are opamps all over a PCB. and setting up the effects chain in this manner is certainly a new idea as chains almost always are a simple in and out. Even the stereo inputs- the ones you push in to the first click to run a mono effect-they are confined to the preamp and dont bridge the two amps together.

At any rate, I did ask for opinions, and the so far it is all about solid state instead of tube. I think I should push on with my idea and see how well I can make it work.

Thanks for your thoughts!

[Enzo]

Please understand, your basic premise applies to either tube or solid state. The only practical difference will be the amount of space and weight a tube version would add. If you set up a system with solid state amps, then replaced each power amp with a tube unit, and the preamp with a tube unit, then your connections and stuff all remain exactly the same. If you want to know how to do it, the example shows you. If you are looking for a ready schematic for a tube version, then I don;t have it.


I may be wrong, but when you talk about first-clicking the TRS jack on something, I think you are worried too much about using some exact product. You are making your own amp, you don;t need to use a TRS jack, put full service jacks side by side on your panel. Your FX unit just needs left and right outputs. If it has limits because it is all crammed into a little pedal, then use a different FX unit.

A guitar is a mono device. The preamp is a mono device. Only the effects unit makes a guitar stereo. The preamp out runs to the FX in. That's mono, and you can have a mono FX loop between them of you want, to add compression or noise gate or whatever. Once you come out of the FX unit in stereo, you have left and right signals, and they are fed to your power amps. AT this point you suggested adding some panning and bland controls. IN a guitar amp this usually has limited use. Panning anywhere but L/R just gets us closer to mono again. And I can;t think of many FX units that don;t already have dry/wet mix controls. But one could add panning if one wanted.

But none of that cares whether the preamp and power amps are solid state or tube. SO you see a sea of op amps on a board. OK< exchange it for a sea of 12AX7s on a metal box. All the other stuff doesn;t change. And that other stuff if the stuff you are wanting to invent.

If you want to cook a beef steak, and I am cooking a pork chop, it doesn't mean you can;t learn something about the process watching my pork chop cook. Just apply the technique to your steak.

An all tube rack mounted system? A Peavey Classic 50-50 (or Marshall) and a Peavey all tube Rockmaster preamp (or Mesa or whatever) If you are looking for a stereo guitar head with the two power amps and a preamp, then you will find very limited choices. Mainly, it will weigh a ton, because you will need all those transformers.


One problem with stereo guitar amps is that if you separate the speakers on stage, YOU can hear the effect, but the audience will not. Even live, most of us play in dance clubs, and no one is sitting in the sweet center spot just listening to the stereo. They are sitting on one side or the other drinking and socializing. I am not saying you shouldn;t have stereo, if you like it, then have it. But I say it because that is why there is not going to be a lot of selection in the market of amps. ALl the stero effect in recorded music took place in the recording process. And they can make you sound any way you like.

[silverfox]

A good use for stereo outputs is for tone construction. From what I've observed some of the best sounding artists play through two different amps or different EQ settings on dual amps then combine the tones for a composted sound-effect. The latest version of this uses 3 disparate sources combined to produce a unique sound.

Silverfox.

[kg]

I cant say as Ive heard of an all tube rack system

interesting. I first cobbled mine together in 91 or so, using the aforementioned peavey 60/60.

[silverfox]

Here is What you Want!

Try this link for an exceptional build. Scroll down about 2/3 of the page and you will find everything you need.

All tube Stereo guitar pre amp - Hoffman Ampifiers Guitar Tube amp forum

All tube stereo guitar power amp - Hoffman Ampifiers Guitar Tube amp forum

That's what the Fox Says,

Silverfox.