The "Forum Amp" design thread

[John Lyons]

Here's another thought for the "amp requirements": make is play well with effects! Some amps (especially SS ones) tend to have issues with distortion effects. I agree with making it easy to build, but this is a stompbox forum and I think the compatibility issue should be of equal importance.
-Joe Hart

Yes, I agree. Everyone builds there own FX, especially here.
An amp that takes pedals well is the way to go.
Just something simple with a volume knob would please
most of us. Making a preamp module to go with it is another thing
but an amp that takes pedals well is 90% of the battle.

[Cliff Schecht]

Interesting options.

The 2n3055-based amps seem to want something around 60v. But the TPA3112 seems to run on a lower voltage.

For me, the big thing here is to make this project easy. It should hopefully be about as easy and safe to build as a guitar pedal, and not require any difficult-to-source parts. There are already tons of great DIY amp projects put together by people infinitely more knowledgeable than I, so my intent here is not to outdo what has been done from an engineering perspective, but to make something that relative novices can build and enjoy. So to that end, I want to do something that can be powered by an easy-to-find laptop power supply. I was planning to use the TDA7240A chip, but I will be looking into all of the suggestions here.

I can definitely say that a tube preamp will not be a part of this project. However, part of the idea of this was to make an amp PCB that's small enough to be paired with whatever preamp and other stuff you want. It's looking like most people want an integrated preamp and EQ, however this will of course be bypass-able, so it would be easy to pair it up with a tube pre of your choosing, parametric EQ, spring reverb driver with reverb tank, etc. I guess maybe down the road, if people were interested, I could do add-on boards to provide this kind of functionality - so this could be a kind of modular amplifier where you just build the sections you want, so you can have an insane monster with 3 tube channels, each with their own EQ, etc. But for now I'd like to keep it simple.

Going discrete or class D is definitely not the simple solution. The TDA7240 looks like a nice option for what you are trying to do. Good luck!

[davidallancole]

That looks good to me.  The TDA7240 seems like a good chip to go with.  You would have an easy time with the power supply then.  I would still suggest an amp that has a chain as follows:

High Impedance Buffer -> Equalizer -> Volume Control -> TDA7240

I would like to see no pre-amp as far as distortion type of effects.  Mostly I say this since the majority of people here I would think have lots of effects boxes that do this type of thing.  I would like to see a power amp that basically takes what your effects puts out and amplifies it for the speakers without any thing else happening in the amp.  Kind of like a poor mans Tech 21 Power Engine.

[R.G.]

I take bus drivers' holidays, obviously. I design effects all day, and then for some relaxation to get away from it all on nights and weekends, I design audio gear for myself (well, and you guys, obviously).

I just finished up a set of PCBs which replicate, in detail, the circuitry inside the Thomas Vox Super Beatle. This includes the normal channel with midrange boosting, the brilliant channel with the embedded distortion effect, the tremolo, the reverb, the mixer/limiter (which I think is the key to the sound these things have) and the repeat percussion. Well, OK, the power amp too. It's a complete set of boards less the power supply for the amp.

This got started when I got frustrated at the yards of overlapping wiring inside the Thomas boxes. The problem is that they use one big PCB and run wires to all the controls. What I did was to separate the circuits by which control they were wired to, and made small PCBs that did one thing and stuck to the backs of the pots with adhesive standoffs, so the wires to the pots, jacks, switches, etc. are only a couple of inches long. The wires into and out of each module are power, ground, signal(s) in and signals out. This approach of putting the circuits near the controls dramatically cuts down on the wiring from module to module and makes the wiring easy to troubleshoot if it does happen to break.

I also reworked the power supply setup from the old cap-resistor-cap-resistor-cap-resistor... to putting a three terminal regulator on each board. The TO-92 regulators will do fine for the local power needed by one board, and mean that you don't need all the power supply junk, just a feed from the main power supply to run the boards; it's converted to the voltage needed local to the module/PCB, and also regulated so it does not have ripple and interaction issues.

The power supply for all these amps is pretty much what you'd use for an LM3886 power amp chip, that being +/-31V at no load, and all but one of the modules run from +31V only. This is a 42Vct power transformer, a diode bridge, and two 4700uF filter caps. As noted, the 2N3055 is the power transistor the original circuit used, and I have reverse engineered the driver transformer that the original circuit used, in addition to making a PCB for the power amp. So you could either use one LM3886 per speaker to get as much power as you want - literally unlimited except by the power supply(s) you want to build and the number of heatsinks and LM3886s you want to hook up to the preamp out. You get 30W per speaker per LM3886 with no particular strain on the LM3886.

A big part of the sound of these amps is the mixer-limiter, which pre-limits the audio getting to the power amp so the power amp never clips. That means the characteristic overdrive sound of the amp is more the sound of the mixer limiter than the sound of overdriving the power amp.

Anyway, that's how I spend my fun, get away from it all time. I had to order five board sets to get the one I'm putting together for myself.

[Tonemonger]

Hey, Taylor I like where this is going.
I have been considering either buying or building another small amp just this week.
If I may offer one suggestion , it would be a row of pads along one edge of Your PCB that could be used for EQ or a loop or reverb or whatever, and if not required then they are just bridged with links.
Good luck !

[R.G.]

It's well to remember that the electronics are the easy part. What's HARD is building an amp enclosure, speaker cabinet, sheet metal chassis, getting the AC wiring right, all the stuff that's NOT putting resistors and chips onto a PCB. It's like Effects Economics 101 from the Guitar Effects FAQ at GEO, but on steriods.

There is a neat solution you can work if you're motivated and patient.

I bought a fully working 100Wrms guitar amp combo with 2-12" speakers in it for $40 a while back on Craigslist. It's a "Rogue" brand - which is a Musician's Friend house brand. It sold for $400 new, and in general has a dreadful sound unless you like squeeky clean and not working well with effects.

However, all the hard part is already done: there's a speaker enclosure with speakers, already wired up, a power supply, a sheet metal chassis, and a generously sized space inside for adding in parts. I intend to use it for a "soul transplant" of a repro of the circuits of a JMI Vox Supreme amplifier. I've seen these on craigslist for $40 to $100. If you have ever looked at the amount of work involved in building an amp and speaker cab out of boards, glue and tolex, this is a major bargain.

[Taylor]

It's well to remember that the electronics are the easy part. What's HARD is building an amp enclosure, speaker cabinet, sheet metal chassis, getting the AC wiring right, all the stuff that's NOT putting resistors and chips onto a PCB.

Right, this amp is intended to obviate most of that. It will fit in a standard Hammond box. No AC wiring.

As far as building a speaker cabinet, I see that as entirely separate from building an amp. The kinds of speaker cabs people use will all be drastically different (for example, as a bass player, I have no use for an open back little 12" cab, nor for a Marshall-style shallow-backed 4x12), so I'm not going to get involved with speaker cabs. This will just be an amp.

[Gus]

If one makes a PCB set to sell and the circuit plugs into the wall,      What is the liability?

9VDC battery powered effects are a bit different than a plug in the wall build, built by someone maybe not aware of how to build and operate the circuit safely.  Say the person builds the circuit wrong and a house burns down and/or people are hurt or worse.   Who is going to get blamed/sued?

Then you have ROHS.

[John Lyons]

I see what you mean gus but if I sell you a fork and you stick it in a wall outlet then what?
Taylor would just be selling a piece of fiberglass with holes in it.  (I think)

[R.G.]

Right, this amp is intended to obviate most of that. It will fit in a standard Hammond box. No AC wiring.

That's good, and simple. Obviating issues is good. But Mother Nature is waiting for you at the end of the assembly line in the form of waste heat.

If something dissipates 1W how hot does it get? The correct answer is "the temperature is unlimited; it will rise in temperature until the 1W gets out and it comes into balance between power out and power in." There is no limit to temperature, only a balance between power in and power out. Temperature is just the "pressure" which drives the transfer. A 300mW incandescent bulb filament gets to over 1000C because it has no way to get the heat out other than getting hot enough to radiate it away as photons. It has a high thermal resistance from the heat source (filament) to the rest of the universe, so the temperature rises until the power gets out.

A standard Hammond box is at least theoretically a heat sink, but not an ideal one. I have not done the necessary test, but I heavily recommend that you do. Get a Hammond box of the size you intend to use, stick a power resistor down on the inside of it with some heat sink goo where you think you're going to put the power transistors or IC from the amp, and run DC into it to make it dissipate 1W, 5W, 10W, etc. while you measure the temperature of the hot spot on the outside of the box where the resistor is. The box is accepting heat from the heat source, and spreading it out while radiating and conducting it to the surrounding air. So the box gets hottest under the heat source, and cools as it spreads heat out and transfers it to air along the way. There is a per-unit thermal resistance from each unit of area of the box to the air, and also a spreading resistance caused by the flow of heat from the heat source through the metal. That impedes heat flow too.

Heating it and measuring the hot spot temperature lets you directly measure how much power you can put out. All transistors and power amp chips have a maximum case temperature you can let them run to. For the TDA2030, the limit is 20W of dissipation at Tcase=90C. This is because the chip has an internal temperature limit system which keeps the chip from going over 150C, and the thermal resistance from the chip to the outside heat tab of the case is 3C/watt. So with 20W dissipated inside, the chip gets to 150C when the tab is 90C. The heat has to get from the tab to the outside air. A mica insulator and heat sink goo will resist by about 0.5 to 1 C/watt, so the temperature of the surface the chip plus insulator is mounted on will have to be less than about 70-80C. Otherwise, the chip will simply go silent without warning until it cools back down. With outside air at maybe 30C, that gives you as little as 40C rise for 20W, and that means you have to have a heat sink which has a resistance of not more than about 2C/W.

Will a Hammond box do that? I don't know. I haven't measured one. But I HAVE measured a U-channel of 1/8" thick aluminum three inches wide and seven inches long oriented so air could flow over it vertically, which is the best condition for heat dissipation. That used both sides of the aluminum to get rid of heat. It measured 2.7C/W. Hammond boxes are cast aluminum-zinc alloy, which conducts heat less well than pure aluminum.

Another way of looking at this is the power you can get out of the chip is limited by how much the thermal resistance of the heat sink is. You can't get any more out than the chip will withstand without shutting down. So one way out of this is to back down the power supply voltage until the waste heat fits within the Hammond box limits for heat removal with the box temp under the chip at 90C. That's the real limit to how much power you can put out.

Interestingly, 90C surfaces will give you an unpleasant burn if you touch them.

One reason this is all familiar to me is that Rick Vance followed exactly this trail when he got into building solid state guitar amps with an LM3886. This was the origin of the ssguitar.com web site. He thought he had done a defective build but he was really just overtemping his chip with too small a heat sink.

As far as building a speaker cabinet, I see that as entirely separate from building an amp. The kinds of speaker cabs people use will all be drastically different (for example, as a bass player, I have no use for an open back little 12" cab, nor for a Marshall-style shallow-backed 4x12), so I'm not going to get involved with speaker cabs. This will just be an amp.

That's OK. It's easy to find speakers in a cab. But you'd be doing the builders a real service if you also told them something about cabs and where to get them or how to build them. I was always very frustrated when I tried to hear what I was playing by feeling of the output wires.  

[Taylor]

If one makes a PCB set to sell and the circuit plugs into the wall,      What is the liability?

9VDC battery powered effects are a bit different than a plug in the wall build, built by someone maybe not aware of how to build and operate the circuit safely.  Say the person builds the circuit wrong and a house burns down and/or people are hurt or worse.   Who is going to get blamed/sued?

Then you have ROHS.

Once again, there will be no AC wiring, no high voltages. This amp will be powered by a pre-built laptop power supply.

[Taylor]

A standard Hammond box is at least theoretically a heat sink, but not an ideal one.

Sorry, I guess I should have put a lot more info about what I was thinking in the first post. I don't intend to use the box as the only heatsink; there will be a real heatsink. Of course, there are mechanical considerations w.r.t. how to attach the heatsink to the chip while the board is in the box, etc.

That's OK. It's easy to find speakers in a cab. But you'd be doing the builders a real service if you also told them something about cabs and where to get them or how to build them. I was always very frustrated when I tried to hear what I was playing by feeling of the output wires.  

I see what you're saying, but to me amp and cab have always been separate things, so I don't really feel that it's necessary to mix this project with a speaker. It's just that everybody will have drastically different ideas about speakers, all of which are mutually incompatible. So for example, I'm ok with having an integrated preamp in this amp, as it's only a few more parts and a little bit of board space, and can easily be defeated for people who won't use it. But designing a speaker cabinet to go with this amp would be lots of work, and would be useless for all the people out there who don't play guitar (including myself), or guitarists who don't like 12" speakers, or people who prefer sealed cabinets, or people who want a horn-loaded design, or people who want a 3-way crossover with mid speaker and tweeter, etc. This amp will be useful (I think) to a wide variety of players, but any particular speaker design would be inherently limited to a very small subset of players.

To me, a solid state amp, being small and electronic, is more at home being integrated into a pedalboard than into a speaker cabinet. My idea here was to make an amp that could be put on your pedalboard if you want, and then you can just run a speaker cable to the cab. This facilitates simpler, easier patching from effects loop to pedal, etc, and makes unnecessary the need to run power to your amp on top of your cab.

But, of course there's no reason that I can't link to a bunch of info on building guitar cabs.

[Taylor]

Here's another thought for the "amp requirements": make is play well with effects! Some amps (especially SS ones) tend to have issues with distortion effects. I agree with making it easy to build, but this is a stompbox forum and I think the compatibility issue should be of equal importance.
-Joe Hart

Yes, I agree. Everyone builds there own FX, especially here.
An amp that takes pedals well is the way to go.
Just something simple with a volume knob would please
most of us. Making a preamp module to go with it is another thing
but an amp that takes pedals well is 90% of the battle.

I think I've read a bit about this topic, but can you guys explain a little about what makes an amp take pedals well? Or maybe more to the point, what makes an amp play poorly with pedals? I've never really experienced this mismatch, but it might be a guitar thing which would explain why I never noticed.

[jkokura]

The classic word I've read about that gets thrown out there for amps that 'take pedals well' is "headroom". I assume this means, to those who use it at least, that the amp has lots of room to be 'pushed' with a hot signal, and handles that without distorting or becoming unpleasant to listen to.

At first when I heard you say, "this is meant to be put in a hammond box" I was turned off. I want to be able to build this into an actual amp cab and have it work well for me. I think if I can manage to do that, or find someone to help me figure out how to do that, we can easily link the neccessary files to the project, as you can the files for a hammond box as a pedal board amp. I like that idea too, and will likely build one of each.

I had a question for you though Taylor: Could you design this to accept a variety of power supplies? Obviously you have one idea, but perhaps I wanted to build an internal power supply with your usual power cord that I could plug into my PP2+ power supply on my pedal board. I like RG's idea of having a seperate board for all sorts of functions in an amp - perhaps we could have 3 boards: Preamp, Poweramp and Powersupply. I also really like the idea of having a bunch of pads for things like a Reverb output and an effects loop and such.

What are we thinking for the preamp? What about something in the style of ROG that emulates a good amp? I was thinking a Blackface Fender Twin myself, clean and pristine with 3 band tone control. I'd also love to see a Bassman preamp, which might be good for the Bassists out there along with the guitarists (half serious). If we were to have channel switching (My vote), an 18 watt marshall type could be good.

Jacob

[Taylor]

The classic word I've read about that gets thrown out there for amps that 'take pedals well' is "headroom". I assume this means, to those who use it at least, that the amp has lots of room to be 'pushed' with a hot signal, and handles that without distorting or becoming unpleasant to listen to.

At first when I heard you say, "this is meant to be put in a hammond box" I was turned off. I want to be able to build this into an actual amp cab and have it work well for me. I think if I can manage to do that, or find someone to help me figure out how to do that, we can easily link the neccessary files to the project, as you can the files for a hammond box as a pedal board amp. I like that idea too, and will likely build one of each.

Well, if it fits in a Hammond box, it would easily fit into a guitar cab. The opposite is not true - if I design it to fit into a particular wooden cab, it wouldn't fit into a small metal box for people who want to put it on their pedalboard.

I had a question for you though Taylor: Could you design this to accept a variety of power supplies? Obviously you have one idea, but perhaps I wanted to build an internal power supply with your usual power cord that I could plug into my PP2+ power supply on my pedal board.

No, the idea is that I want to avoid making people do mains wiring. It's dangerous and it turns a lot of people off. There are already plenty of amps with power supply boards available, so there's not much use in another project like that. But you could easily build the laptop supply into the same box if you just want them physically connected.

I like RG's idea of having a seperate board for all sorts of functions in an amp - perhaps we could have 3 boards: Preamp, Poweramp and Powersupply. I also really like the idea of having a bunch of pads for things like a Reverb output and an effects loop and such.

I would like to have patchability, but there definitely will not be separate PCBs. Doing that increases the cost a lot without much payoff. The preamp section and power amp section will be separate unless you patch them together, so you don't have to build the pre if you won't use it, or you can bypass it easily.

What are we thinking for the preamp? What about something in the style of ROG that emulates a good amp? I was thinking a Blackface Fender Twin myself, clean and pristine with 3 band tone control. I'd also love to see a Bassman preamp, which might be good for the Bassists out there along with the guitarists (half serious). If we were to have channel switching (My vote), an 18 watt marshall type could be good.

Jacob

Something like that is a good idea. I wouldn't want to yoink anything from ROG but that's a good direction to go in.

[jkokura]

No, the schems for the amps are out there. The hard part will be take the tubes and finding out how to make them solid state circtuits instead. I don't know how to do that, but obviously people do with all the Amp emulating pedals out there, both DIY and commercial (Tech 21)...

Jacob

[Taylor]

The Tech21 stuff, at least the modern stuff, is all opamp based, no JFETs as far as I know. Those have fairly large parts count, so I think for the preamp I'd like to stick to something a little simpler, I guess in the vein of the ROG JFET things, or an opamp with diodes in the feedback loop kind of thing. I'm going to wait a few days to see how the poll goes, but since it looks like nearly everyone does want a preamp and a 3-band EQ, I'll be doing another poll to decide what kind of preamp (and also try to figure out the presence or absence of switchable channels, since that question does not look as unanimous on the poll so far).

[Perrow]

I would like to have patchability, but there definitely will not be separate PCBs. Doing that increases the cost a lot without much payoff. The preamp section and power amp section will be separate unless you patch them together, so you don't have to build the pre if you won't use it, or you can bypass it easily.

This I like, if I'd like to include a mixer to mix in an aux (mp3) signal all I'd have to do is "take" the signal between the pre and power amps and do whatever I want with it. Make sure you consider all points where someone would want to insert there own processing (though I can only think of the intersection between pre and power right now and maybe bypass the tone stack or replace it with a custom). I think what my tired and slightly drunk mind is ambling towards is a mod-able design for those inclined.

I also like the fact that I suggested this type of project in your other thread

While I do like this project, I also like the fact that there'll probably be a while before I actually start building one myself as I got an Orange Crush Pix CR20LDX for my 40'th birthday (sept 11, hence tired and drunk, 2 am here now and I'm off to bed now) 

[Brymus]

I would like to know what power supply is gonna be used.
You mentioned a laptop supply but arent most of those switching power supliers?
I just dont see where you are gonna get 20 watts at the spkr,with a small Hammond box and a laptop supply and still have it fit on a pedal board.
I will be watching with interest,even though I have enough amps both SS and tube.
And several that need to be finished.
But your design goals are whats peaking my interest,how you solve them should be very enlightning to me.
Good Luck with this Taylor!

20 watts ,a small enclosure that fits on a pedal board, and no mains wiring or internal transformer.
Sounds like the EH .22 practice amp.

[Taylor]

I would like to know what power supply is gonna be used.
You mentioned a laptop supply but arent most of those switching power supliers?
I just dont see where you are gonna get 20 watts at the spkr,with a small Hammond box and a laptop supply and still have it fit on a pedal board.
But your design goals are whats peaking my interest,how you solve them should be very enlightning to me.
Good Luck with this Taylor!

Waltk has basically already done it:

http://www.diystompboxes.com/smfforum/index.php?topic=86887.msg729085#msg729085