DIY guitar amp

[PBE6]

I've been looking to build a cheap solid-state guitar amp, and rather than build the power section myself I'm thinking a cheap hi-fi amp might do just fine. Amazon and eBay have loads of simple ones with decent wattage for under $30. Just wondering, what should I be considering if I go this route? I plan on building a preamp for the front end, so hopefully I can address any issues there. Is this actually a workable idea, or is there a reason why these amps are not well-suited to guitar use?

[bluebunny]

Go for it!  Just remember to hook it all up to a proper guitar speaker, otherwise it'll sound all tinny and horrible.

[PBE6]

Lol, very true! I plan on using my bass cabinet, an Ampeg 210AV (which may or may not be a good choice) that can handle 200W RMS. Speaking of power, a lot of these cheap amps are stereo kits - is bridging the output relatively simple and/or safe thing to do? Maximum power would be 100W so it shouldn't annihilate the cabinet...

[PBE6]

I found a cheap deal - $20 for a 120 W+120 W dual-channel power amp, takes 12-26V DC in.  Question: if I were to bridge these channels, I could potentially get something close to 240W.  To get that power at 24V, it would draw 10A, and at 12V it would draw 20A, right?  That sounds...higher than expected to me. Am I evaluating the current requirements correctly? If so, are there power supplies (or power supply kits) that would handle that kind of current for a reasonable price?  The ones I'm looking at seem to be $70 and up. 

[ElectricDruid]

Watch out that the "120W+120W" quoted isn't "peak power" or some other such marketing thing to make the numbers look good.

Aside from that, I think your numbers are sound. If it's outputting 120W, it's got to get that power from somewhere, and if the input is 12V, then that implies 20A drawn. And that's the best case, assuming there are no losses. You'd really want a power supply with some safety margin above that.

But 20A is scary-big power supplies and I've never done anything like that, so take this with a pinch of salt.

Tom

[vigilante397]

But 20A is scary-big power supplies

Seriously, that's a big deal. Anytime you're looking at building an amp, be it solid state or tube, the first thing you should think about it power supply. 20A is not a trivial amount of power, and it could cause some serious damage (pronounced "death") if you're not careful.

So if you have a 20A power supply lying around I would say absolutely go for it, if not I would say maybe still go for it, but do some research on power supplies first.

[printer2]

What speaker impedance will you need to use to draw 120W from 24V? Is the amp module already bridged?

[PRR]

> a cheap deal - $20 for a 120 W+120 W dual-channel power amp, takes 12-26V DC in.

That does not make sense. I can't guess if you read it wrong or they "wrote it wrong".

Straight 24V DC supply can make 8Vrms Sine. That's 8 Watts in 8 Ohms.

If they already Bridged it (many do), it makes 16Vrms and 32 Watts in 8 Ohms, 64 Watts in 4 Ohms.

"120+120W" either needs a super low impedance or very careless (deceptive) rating. Or voltage booster or transformer, which is something else altogether.

Anyway, why would you wish for 240 Watts??

[tonyharker]

If it's this amplifier or a similar one, the outputs are already bridged, so cannot be bridged again and you will only get 120 watts into a 2 ohm load.

[Rob Strand]

That does not make sense. I can't guess if you read it wrong or they "wrote it wrong".

Unless it has a switch mode like a car stereo.  It would be obvious from a pic of the board.  The price looking more like the dodgy rating ElectricDruid mentioned.     I think one of my PC speaker was rated 80W + 80W but in terms of honest ratings it was probably only 2 W + 2W!

[tonyharker]

These are Class D amplifiers.

[printer2]

Not to say that a class D amp module at 24V is a bad thing, I have some for just a project. But I am not looking for 100W.

[Ben N]

There are plenty of lower power modules readily available at reasonable prices, some class D (much more efficient and therefore less power hungry) some class AB (needing beefier power supplies). My kitchen hifi amp is a "50+50" watt (with a big grain of salt--probably closer to 20+20) class D, runs just fine on a 19v 3.16A laptop type power supply. Same for my Vox Nutube amp with a 50 watt class D power amp. In this day and age of Kempers and the like, hardly anyone even needs a 100 watt amp; nobody ever needed 240w for guitar. (Bass is a different matter.) BTW, if you are going for high power, why not keep it modular?

[PBE6]

Agree with most/all of the above. I think the board I was looking at was really reporting the peak power under ideal conditions to get the numbers up, so I'm guessing the average power is not 120W and something more like 30W per channel (or maybe even less!!). That's partly why I was looking at high power, because I know the real power will be significantly less. The other reason I was interested in high power was to make sure there would be enough headroom so the transients don't get squashed, but that may have more to do with slew rate than power.

What do SS guitar amps usually require in terms of current, say for 35W RMS? I have no experience with the power side of things, so I have no frame of reference. That's why I prefer to get a standalone power supply board as well, to keep the mortality rate to a minimum (BTW I will always have to supply the transformer for this separately, correct?).

Further to the above, I do see what appears to be a rectifier section and filter cap on several of these Class D boards, would it be the norm to include that on the power amp board itself?

[PBE6]

Also, does anyone have any suggestions for info on power supply design for guitar amps (preferably SS, I'll blow up some tubes later in life).

[vigilante397]

Class-D amplifiers are tricky. I designed one a while ago that ran on 12V and put out 50W. It was fine, but I didn't love the tone so I scrapped the project. My PA speakers are class D, but I feel like there's a pretty good reason most SS guitar amps aren't.

[Ben N]

My very limited experience with the Vox is that Class D is fine as long as you run entirely within its limits--i.e. not as a guitar amp, but rather as a PA amp reproducing guitar. If you expect it to contribute nothing tonally, you won't be disappointed. Of course, YMMV.

[PBE6]

 One quick question, are switch-mode power supplies a bad idea for guitar amps in general and/or Class D power amps specifically? Trying to get my head around them, but would appreciate any practical experience, especially with regard to noise and transient response (the two main complaints I seem to see...)

[Ben N]

I don't know the answer to your question, but you might want to search here for Tiny Giant, the so-called forum amp project, designed by Taylor from the ground up for a switch mode supply. It's a lot lower power than what you are talking about, but there might be some wisdom there relevant to your question, about, say, suitable filtering.

[vigilante397]

Switch mode power supplies are fine for amplifiers (or anything else frankly) as long as your design as solid. I used a switcher to power my class D build. Use proper filtering, keep clock signals away from audio signals, etc. The majority of switchers will have a switching frequency well above 20kHz so it should be above the audio range, but you still need to make sure you filter properly to keep harmonics of the clock out of your audio lines.