The "Forum Amp" design thread

[Taylor]

I've decided that a cool project would be a solid state amp. I will be designing this, getting professionally manufactured PCBs as with my previous projects, and creating a detailed PDF to help you build it.

Design considerations:

1) Approximately 20 watts.

2) About as easy to build as an effects pedal. This is the big thing for me. I know that many people aren't confident doing mains wiring, or else are more confident than they should be and end up doing unsafe wiring. So this project will be powerable using a pre-built and easily available power supply. No high voltages, no transformer to track down.

For the rest of the details, I turn to you guys, the forumites, to ask what you'd want in a DIY solid state amp.

Questions for you:

1) Do you want this to have an integrated preamp, or just act as a power amp to drive a speaker to decent volumes?

2) If yes to 1, do you want EQ?

3) If yes to 1, do you want switchable channels? Clean/OD? We could even do a relay-based footswitch for channel switching.

In other words, do you want this to just be a power amp, to be married with whatever preamp, overdive, reverb etc. of your personal choosing, or would you like to see this as more of a full-featured standalone amp? I'd probably just want a power amp for myself, but it's not too difficult to add a preamp with overdrive to the PCB, so if enough people want that, we can do it, and if you don't want to use it, it will be easy to bypass.

Let me know your thoughts.

[Taylor]

I made a poll for the questions, let's see if this works:

http://www.ballot-box.net/service3/poll.view.php?pollid=23271

[jkokura]

Cool ballot idea. I think there's a way to do survey's here, but I think you're limited in choice.

Also, along with this project, it would be cool if we could get someone who's handy with the amps (I'm looking at those guys over in the amp picture thread) to draft plans for either a 1x10 or a 1x12 combo we can fit this kind of thing into. I don't know how to source parts for the chassis and stuff needed for putting a whole amp together, but perhaps putting links to where you can get everything to build yourself a solid state amp from scratch and plans to do it would be something even the Kit companies out there haven't been able to do yet!

Jacob

[Taylor]

That's an interesting idea. I'm just going to work on the amp itself, but perhaps info will spring up along the way on how to build a cab or combo.

[Taylor]

Hmm, from the poll, some definite trends are beginning to emerge. Looks like everybody wants a preamp.

I didn't intend for the poll to preclude discussion here, only to give a clear view of the numbers. But I do want to encourage people to discuss these things in the thread as well.

[Nasse]

If they are kept on separate boards people can have their own special preamps

[Taylor]

That was my thinking too, but it looks to me like everybody wants an integrated preamp. If it continues to be true that most people want that even if there are dissenters, it would still be easy to bypass an integrated preamp for those who want to do so.

[hday]

A preamp input would be an excellent excuse for me to save up some cash for a Murder One.

[Taylor]

By preamp input, do you just mean an input directly to the power amp, bypassing the integrated preamp? That will definitely be possible.

[davidallancole]

My vote is a power amp with a high input impedance and volume knob!

High Impedance Buffer -> Volume Knob -> Power Amp

[tuckster]

By preamp input, do you just mean an input directly to the power amp, bypassing the integrated preamp? That will definitely be possible.

This would be a great idea. As I wrote in your other thread I try to build a practise amp based on a TDA2040 (circuit from data sheet) & Sansamp GT2. With send & return.
Maybe I can finish this thing this month....

[petemoore]

  Dave's vote certainly makes his post a more simple project, same thing would go for the amp, driving it with even a boost probably would 'kick it into high gear', that or a Fuzz, will sound a bit different of course with a preamp' or high headroom boost driving it.
 k..here's ways to do it..I always think it's best to start at the far end of the chain when designing any audio production system.
 Starting with the speaker, choosing it for:
 Effeciency [how loud per certain watt range
 Eq [what frequencies does it make the loudest
 Resonant peak [which ones make it resonate
 Compression [has to do with cabinet maybe
 ...anything else like cost, weight, size.
 That^ said, immediate availablity [ie I own it now] has often been the #1 consideration for speaker choices [especially around the time of affording a 1rst power supply and amplifier build]. I've been lucky and scrounged some decent speakers [old stereo stuff has occasionally been a good source], if the operator understands the limitations of a limited speaker and stays within it..great, reliable sound can be the result, if the speaker is near limit and clipping is presented to it...not so much. Basically to have a sweet relationship between a SS output and a speaker requires proper matching or proper volume control management, over-driving a too small speaker or amp clipping is never recommended.
 Then the next thing [besides good speaker wire] is the amp.
 Have others help you choose a chipamp, I have a 3875 @18v I like a lot, 18 was chosen because it was there [clean and portable power] bigger voltage certainly adds and doesnt really subtract anything from the SS amp performance until it nears the max. So this really depends on the speaker-loudness and how loud you want to get, the idea is run the amp clean [no peaks clipping] at all times, I've peaked the amp and it was only horrid, not super-ungraceful to seeming dangerous with the 12'' GB speaker at the time.
 Reading the data sheets on the chip features:
 What kind of protection features, what are the power supply options/demands..anything else you can dig up. Plenty of chip experiences to draw from around here.
 Once the data sheets are understood the amp instructions are also available at Chipamp.com and other places, choosing the filter capacitors and whatever else [snubbing caps?] for the design[s..a lot of options sorting.
 The amp/supply should be capable of at least getting the speaker 'going', [or having a loudness type control?] so lower volumes have the frequencies / tone 'filled in' sounding...yet another option, however.
 That would be in the category of Tone Control, I would say since the tone control introduces passive losses, it might be a nice feature to have a preamp drive it. By preamp I mean something that drives the TC [so it works right] and enough to drive the amp past 1/2 volume also comes in handy right in the package with 1 handle. Most ampsters agree that TC is a nice feature, allowing frequency adjustments for speaker volumes, room textures and sizes, where the speaker is located etc. And one unit that gets the speaker[s fairly loud includes the preamp.
 Since I'm not using that many effects with my 18v, I used a 3/2 Jfetzer preamp, and that generally gets the amp up to or over what the 18v supply imposes as a headroom level. The supply [battery] uses a polarized jack/plug [2 conductors worth of a computer supply bundle] allows various supplies to safely be used, I would have preferred a chassis mounted jack, mounting the heavy wires seems workable though, I used pressure-cooker pots for my..pressure cooker amps ! Chassis choice of course is a necessary evil.
 Otherwise, a hefty output from a booster or fuzz generally produces enough input voltage to outdo the 18v supply and <18v output before clipping. Even a well voiced booster on the same supply can produce enough to satisfy to some extent what the amp [and speaker] expect as input...the amps really need 'something/anything' as a boost up, a hot output guitar gets them strong enough to know it's working, but won't 'fill in' the tone.
 So...it could be a plain amp, and working with various preamp designs [ie to get the voltage up enough to drive the chip input] will be needed, choosing 'the one' [could be two I suppose...why not? it complicates discussion.]..anyway, I've done that, and having a clean and slightly dirty side...got amende-chopped to a slightly dirty /cleanish Jfetzer [I think it's a decent choice, it goes with dirtpedals pretty good, and cause although super-clean is cool, I prefer it with a slightly dirty boost 90% of the time..rolling back the G-volume of course a cleanup option.
 2,000 types of speakers to choose from [or more often just the one that is ready available]. I like the little alnico's or a 10'' or 12'' speaker with the chipamps @ lower voltages, getting up to around 35v or so [since that's what i've tried] pumps an old EV-15B ok. If super frugal about amp building, a 'more suitable' new speaker may constitute about 1/2 the cost of the project, figureing this part out [and the cabinet] can be a little tricky, depending on how much the used/free market is searched...and how much is actually known about x, y, and z speaker.
 For the criterion you've established, around 30v supply [if it can be figured out without mains wiring is suggested, suitable current capability] would be able to get more out of the chip and still be relatively innocuous as far as safety.  
 >18v chip supply recommended though, and begins to make the amp capable of 'starting up' most speaker types enough to get 'going', and not be an Eq problem.

[zambo]

I have built the vellman ss amps of the 3 watt and 7 watt type and both sound reallly good for what they are. You can fit either of them into a smaller hammond box easily. Both are simple and sound decent when driven by a ts or other pedal. Both sound good when driven by valvecasters as well. I think something like a ts or even a lm386 based preamp/distortion channel thast bypassable to just a power amp with a hi impedenced master volume input would be sweet amd easey to do. A tube pre would be an easey option as well. Just my thoughts. I think the tda2003 chip amp is simple and can run on a 9 ,12 or 15 volt supply and does around 5 to 7 watts.

[Cliff Schecht]

I've looked into SS power amps quite a bit. The cheap route to go is to use either a 2N3055 pair or a 2N3055/MJ2955 compliment. They're still readily available and a lot of amps have used these in the past (meaning lots of reference designs). Two of either flavor (NPN or PNP/NPN) will put out well over 20W though, more like 100-150W if you get the heat out right. But keeping them well under their rated dissipation would work in your favor, you could simplify some of the requisite protection circuitry and could perhaps simplify or completely eliminate the heatsinking as well (or use the PCB for heatsinking). Just a thought as most high power amplifier IC's are relatively expensive.

Another route to go would be to look into some of the TI class D power amp IC's. They are easy to design with, offer cheap and efficient power and have good enough linearity for anything we are concerned with. Something like the TPA3112 will do 20W into 4 Ohms and I'm sure would sound fine as a power amp (just don't distort it!!).

[Johan]

for powersupply, both for your own convinience and to possibly to get around liabillity issiues, I'd suggest you use a ready made, store bought 24volt/60watt Laptop powersupply
a few extra caps and resistors could sort and powersupply whining..
J

[Nasse]

So something like that  i suggested 40 watts tda2030 beefed with two medium power transistors and 12...44 volts supply circuit might be nice, seems easy and cheap and low parts count too, might be loud nough with drummer if the speaker cabinet is efficient

just bought two supercheap 10" guitar loudspeakers, someone said they are "fenderish" but that might be sales talk...

[Taylor]

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.

[maarten]

I like the idea very much, Taylor! Why not have a PCB with lots of space in the pre-amp section for modding, adding extra channels etc.?
Also, a chip like the 2030 (and other ic's prabably as well) will give you many options for different poweramps: by varying the power supply you could have versions ranging from like 6 tot 18 watts, by adding some  transistors, like Nasse said, you can go up to 40 wats. Two poweramps and  2 spaekers, sharing the same preamp channel(s) would double this output...
Kind of like the AX84 module idea, always liked that.
Maarten

[MarcoMike]

I have a couple of tda1514 at home, not sure they are good for guitar, and I heard it's not produced anymore... so this is probably not a good suggestion for the present project... well...
I was just planning to try and build something with it.. I'll let you know in case..

but I think the whole automotive TDA range of amplifier is the easy way to go...

[Joe Hart]

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