The "Forum Amp" design thread

[phector2004]

I have a gut feeling my amp buzzing may be related to my 2-prong power supply (although it seems weird to me that it should even matter, cause normally the ground prong shouldn't be doing anything  Huh)

Actually it is doing something, but not like what you're thinking.  It's grounded to the chassis which is a huge shield for your circuit.  And guess where your guitar string ground ends up after it goes through the cable/preamp/etc - the chassis ground.  Somewhere your circuit needs to be Earthed, for safety and noise reasons. 

hmmm well its just a populated + wired circuit board as of now, with +24V DC and Ground connections... No mains voltage, transformers, or filter caps to worry about, but I figure whatever (I / the board designer) did wrong should be looked into. For new builders not to lose sleep over, at least.

I don't mean to hijack this thread, but the buzz slightly spoils an otherwise perfectly working amp 

[Top Top]

Taylor whatever you do, just make sure it is more elegant than this:

[DougH]

I haven't read this whole thread but these guys sell SS power amp kits for less than it would cost me to even think about a design, let alone coming up with a PCB for it. I would go the kit route for the power amp and then have fun coming up with a preamp (or not- just plug pedals into it or etc...).

Yep, somebody posted that a few pages back, and it has definitely given me pause about whether to continue with the project, especially given all of the dissent about what the project should be. I thought it would be cool, but I'm not sure now if enough other people agree to make it worthwhile.

Getting everyone on this forum to agree on anything is like herding cats. IOW- it won't happen. I wouldn't necessarily give up on your idea though. Given that there are so many kits available for SS power amps like this, it might make part of this project a lot easier. Using the XYZ power amp kit you could adapt it by adding a limiter circuit to the preamp, etc. And of course you need to figure out your preamp, tone controls, etc anyway. Unless there is something you are trying to do that is not available in an inexpensive kit form, it may make sense to build your project around an existing piece of hardware. No sense in re-inventing the wheel, IMO.

[B Tremblay]

Getting everyone on this forum to agree on anything is like herding cats. IOW- it won't happen.

It can be done, but it helps to have a well-defined path for them and a skilled cat-herder (like my wife). 

[Nasse]

I planned to dive in black art of makin solid state guitar amp anyway so decided if I build a combo case first so perhaps that pushes and forces me to build it ready, have few chipamps ready and some transformers and more than enough speakers. Went to lumber yard I had find good plywood some year ago, new owner and same import junk that everybody has and not right thickness

Sometimes you can see a spiffy pcb design, such that you can cut/saw it in two or more parts, like power and pre amp and just use what you need, or populate it all

[zambo]

I think making one that is small enough ( like the noisey cricket etc.) to fit in a guitar case compartment and loud enough to use at a reasonable gig volume ( 20 watts ) with a small walwart style power supply and a simple preamp with mid bass treble and fx loop and enough umf to break up just a touch would still be a sweet forum amp and worth developing for sure. Everyone likes the idea and the principal and they will all tweak it to their own taste anyway. its half the fun and it would be a nice back up to have on hand when your tubes go out at a gig and you left the spare parts bag 100 miles away. Does anyone on this forum play without an overdrive or distortion pedal....really..?

[Taylor]

Yeah, that's basically my approach. Don't worry, it'll happen. Some days I get a little pessimistic about things in general, but I think it's a useful project and I'm working on it right now.

[Taylor]

Taylor whatever you do, just make sure it is more elegant than this:

Well, I'm not sure I can beat that unless I can get my fab house to do a custom argyle-pattern soldermask.

[petemoore]

  Here's the how I used:
  Cut off the chips unused legs.
  Install the chip across perfchunk, diagonally [some slight lead bending required].
  Bind 3 capacitor-barrels with strapping to form a triangle, drop a screw down the center, make sure the screw has threads that are tight-turning so they lock, adjust the screw so the 3 'feet' and the screw pin-down forms a solid platform.
  That turns the 3 heavy-large-floating items into a solid place to work from.
  The rest of the circuit is either hanging garden, soldered to copper pads or on the chipleads atop the little perfchunk.
  The legs of the heat-sink-mounted chip holds the perfchunk, thick wires stabilize the 'floating' end.

            C   h   I   P
l HEAT  SINK   /Perf/  Filter-Lugs
l Block-table        3 Filter Capacitors
l------------------------------------------------chassis

  The caps and heatsink are a solid-form ie in a chassis or on a board, so the little bits inbetween and everything is stable.
  PCB is only 11 or 14 buxx? and the kits are bulk...maybe a slight cost decrease to buying the kit ?
  I did it this way because I had the chips/caps and most of the 'etceteras' handy already.
 

[zambo]

I am just posting this because its in the spirit of this project. The amp is just a power amp only. No volume knob or anything, just an input jack. Tube pre ( i know you dont want to mess with those ) . I am surprised at how LOUD and punchy this thing is. It sounds good with solid state preamps as well like a tube screamer. 3 watts is hecka loud......anyway....the tda7267a seems to work well at 9volts and there are only 7 or 8 parts to the amp...for what its worth.   http://www.youtube.com/watch?v=T-sNT_xjJ9E

[Taylor]

I just built up the first prototype, on perf board. Currently I'm using an opamp buffer, into the TDA7240a-based amp, using a 19.5v power supply, regulated down to around 14v with an LM338. Huge thanks to waltk for his suggestions so far.

This amp sounds great! It really couldn't be simpler, and yet it puts out really nice, clean low end, and very rich and natural mids and highs. I am currently playing bass into it, but I will be testing it with guitar and keyboards soon. For guitar, this thing is going to be loud. Even for bass, where our old nemeses  Fletcher and Munson have stacked the deck against us in the relationship between frequency and loudness, this is a totally usable amp. I'm plugged into an EVM 15L 15" cabinet, but pairing 2 of these amps (which would still be quite simple and small) with 2 speaker cabinets would easily keep up with a drummer when used with a bass. For guitar, one of these should be enough to be irritatingly loud.

I mean, I know it should be no revelation that this kind of thing puts out nice, clean power across a full range of frequencies, but having used cheap solid state amps in the past and finding them to sound terrible, I wasn't sure what to expect.

What I have going on right now has fewer parts than a Tube Screamer.   Even once I add an optional clipping stage in the preamp, and 3-band EQ, this should be comparable to a stompbox in terms of build complexity.

Thermal dissipation does not seem to be too difficult. I am currently using two 125b enclosures for the heatsinks - one for the regulator, one for the power amp chip. They did not get particularly warm, and this is with no thermal paste. More intense and prolonged usage will no doubt heat them up more, and I always prefer to err on the side of overkill w.r.t. safety, but that said, I think it should be very possible to build this into a fairly small enclosure, and use the enclosure as the heat sink. Some tricky mechanical issues to be dealt with there, in terms of mounting the chips and the pots, but nothing I can't sort out.


The one real bummer is that the LM338's heat sink tab is not tied to ground, but to V_out. Why the F did they do that!?   So that means that a mica spacer or similar, to conduct thermally but not electrically, will be needed when mounting that tab to the heat sink/enclosure.

[R.G.]

The good thing about low powered amps is that the human ear response works for you because you don't need massive power to get up to too loud for bedrooms.

Speakers are (almost all) rated in sound pressure level at 1W drive. For guitar speakers this is in the range of 94db to 108db at 1W of drive. This is LOUD if you're not trying to be heard over a band or an audience. A 10W amp is just noticeably louder than a 1W, and it takes 100W (roughly) to be subjectively twice as loud as a 1W. As long as the 1W amp is NEVER clipping and doing ugly solid state artifacts on its own, the sound should be good if the preamp is a distortion/limiting device on its own and the speaker is a guitar type speaker for the final filtering it gives.

The acme* of this apoproach is the Thomas Organ Vox amps and UK Vox Supreme/Conqueror/Defiant, which IMHO are the best sounding solid state amps ever made. Others may have other opinions.

*Acme (pronounced /ˈækmiː/; from Greek: ακμή, the peak, zenith, prime) denotes the best of something.

[DougH]

I tried my pentode driver with this the other day and it sounded fantastic. It could get very LOUD too but didn't have to in order to sound good. One of the nice things about this setup is the pentode driver has 30V p-p available in the power supply and the power amp is a 50V circuit. So the pedal could never overdrive the input of the power amp. Turning the output level of it up all the way just made it loud and clear. I think there's a lot to be said for pairing a good sounding "distortion engine" with a SS power stage. The only caveat I can think of is that (in this case anyway) sometimes we have to roll off the highs in the pedal to deal with a typical guitar amp preamp. With the power amp you don't need that, and this setup was slightly dark. (I thought it might be an impedance issue, but I tried buffering it which didn't help.) I'm going to disconnect the LPF in the pedal and see how that sounds. This is a great approach though, IMO.

[brett]

Hi all
interesting ideas +1 for RG's comments, especially about speaker efficiency.  Any speaker with ok bass response and about 100dB/W will sound very big.  Probably need to be at least 8 inch with a v good magnet.  My 18 watter had a single Calestion Heritage and it sounded like a 747 landing  .

As long as the 1W amp is NEVER clipping and doing ugly solid state artifacts on its own....

What about using a small (approx 1W) output transformer (ie push-pull Class AB - and you can add negative feedback if you want to clean it up even more).  This filters high-frequency nasties and takes away any advantage from extra volume (it doesn't get any louder because it saturates).  

Two examples of the push-pull tranformer approach are the Pignose 9 volter (approx 3W), and the "Deacy"/Mullard/shelf radio amps (about 1W).  My personal opionion is that these are outstanding amps.  Like RG suggests, it would be good to start with a classic amp that sounds great.  Then update and mod it for low noise, effects loops, alternate pre-amps, etc.  Recently, a correct version of the Pignose schematic finally appeared on the web, and was referenced in the forum.
cheers

[JKowalski]

I just built up the first prototype, on perf board. Currently I'm using an opamp buffer, into the TDA7240a-based amp, using a 19.5v power supply, regulated down to around 14v with an LM338. Huge thanks to waltk for his suggestions so far.

This amp sounds great! It really couldn't be simpler, and yet it puts out really nice, clean low end, and very rich and natural mids and highs. I am currently playing bass into it, but I will be testing it with guitar and keyboards soon. For guitar, this thing is going to be loud. Even for bass, where our old nemeses  Fletcher and Munson have stacked the deck against us in the relationship between frequency and loudness, this is a totally usable amp. I'm plugged into an EVM 15L 15" cabinet, but pairing 2 of these amps (which would still be quite simple and small) with 2 speaker cabinets would easily keep up with a drummer when used with a bass. For guitar, one of these should be enough to be irritatingly loud.

I mean, I know it should be no revelation that this kind of thing puts out nice, clean power across a full range of frequencies, but having used cheap solid state amps in the past and finding them to sound terrible, I wasn't sure what to expect.

What I have going on right now has fewer parts than a Tube Screamer.   Even once I add an optional clipping stage in the preamp, and 3-band EQ, this should be comparable to a stompbox in terms of build complexity.

Thermal dissipation does not seem to be too difficult. I am currently using two 125b enclosures for the heatsinks - one for the regulator, one for the power amp chip. They did not get particularly warm, and this is with no thermal paste. More intense and prolonged usage will no doubt heat them up more, and I always prefer to err on the side of overkill w.r.t. safety, but that said, I think it should be very possible to build this into a fairly small enclosure, and use the enclosure as the heat sink. Some tricky mechanical issues to be dealt with there, in terms of mounting the chips and the pots, but nothing I can't sort out.


The one real bummer is that the LM338's heat sink tab is not tied to ground, but to V_out. Why the F did they do that!?   So that means that a mica spacer or similar, to conduct thermally but not electrically, will be needed when mounting that tab to the heat sink/enclosure.

I still think the regulator is a needless waste of space & power. It probably isn't helping much as far as filtering out HF ripple, and even if it was a simple reactive filter would do it better & wouldn't dissipate the heat which is definitely something to minimize. From what I have seen nearly all laptop supplies are 19.5 volts or very close anyways, so I don't think universality is a problem.

[Taylor]

The regulator is there to drop 19.5v to a safer voltage, not for filtering. The amp chip cannot take over 18v, and I don't think it's wise to hit it with the max voltage it can handle. What would you suggest instead of a regulator for dropping 19.5v down to 14 or 16v?

[markeebee]

Not so much a derail as a small kink in the track.....

When you push a chip amp into overload, it never sounds quite right to me.  It's never mushy enough.  Always sounds like a sore throat.  So I was thinking about non-linearity and compression, and other nice stuff that happens in a valve output stage, and I found this (apologies if it's old news):

http://www.freepatentsonline.com/5734725.pdf

After about reading about four pages and scrolling ten billion times between the text and diagrams (should've printed it out!!) I got distracted by the woodgrain pattern on my desk, I realised that I wasn't taking it in, and probably didn't care.

I will find it hard to make enough time to read, digest and act on this info.  So I'm posting this in case somebody else who doesn't have cat food for brains can understand and implement it for the common good.  Maybe not for this amp, but maybe for something.

[DougH]

The SS amp in my post above actually doesn't sound bad when you push it a little. I was getting "Jon Lord" with a keyboard with it clipping a little. Past a certain point it got nasty but I wouldn't 'avoid SS clipping at all costs' either.

[petemoore]

  In the days of my youth I was shown what it's like to have an amp !
 Then I reached that age where I too got to feel a volume ramp.
 No matter how I tried all my notes just came out sounding damp.
 Good Amps Bad Amps..you know I've had my share, with all the HV's they kept inside I was just too shy to dare...
 Still 16 I got an amp with a sound as sweet as could be...
 It only took a couple days...my speakers blow on me.
 I swore I'd find the driver..would last me 'till the end.
 Then I found some other places for money I could spend...ohwaoughh !
 Chorus.

[DougH]

Pete, you need to get out more...