22 watt per channel stereo amp project

[doug7410]

Hi Everyone,

I have posted some threads about the SS amp I'm making and it's really starting to come together now (circuit wise). I wanted to post my progress on this in case anyone is interested, or if you have any comments, questions, or constructive criticism. Here is what I have so far.

The plan is to make a stereo multiple channel combo amp with the following features:

• mono and stereo inputs, the left jack will be mono, left and right are stereo
• Multiple channels - So far I know there will be a clean, and a Dr Boogie channel
• effects loop
• extention speaker cap out
• whatever I can think of next....

Power Amp - this is the power amp circuit I'm using http://www.aaroncake.net/Circuits/amp20w.asp
It's 22 watts per chanel. I made a PCB layout in Eagle cad and built it allready. It works!

Pre Amp - This is made out of several circuits combined
I'm using stomp box amp emulators to get the distortion channels. I'm using a Don Tillman pre amp circuit with an eq
for the "Clean" channel.

• Clean Chanel- Don Tillman pre amp - http://www.till.com/articles/GuitarPreamp/index.html
   that goes into a Baxandall EQ - http://amps.zugster.net/articles/tone-stacks#Baxandall
   then it goes into a LM386 amp I found here  http://www.elecfree.com/electronic/simple-lm386-audio-amplifier/
   I'm adding a gain pot between the cap and pin 1 of the chip.

• Metal Chanel - Dr. Boogie  - http://gaussmarkov.net/wordpress/circuits/dr-boogey/
   Then into a Tillman pre amp then into a LM386 amp with no cap between pin 1 & 8, so there won't be any additional gain boost.



As of now I have only tested with one side of the amp chip. I've tried out an 8" 25 watt/50 watt musical( I still don't know what that means)  4 ohm Jensen. I also ran it thrugh my Marshall 4X12 cab.

I havn't figred out the effects loop, or stereo parts yet. I think I'm only going to do stereo through the clean channel. I'll just double the clean preamp. All the other channels will just be mono.

I'm thinking about adding a Run Off Groove UNO and a Thor. I might be getting carried away.

I'll post some sound samples and pictures soon. Let me know any ideas you have and what you think.

Doug

[phatt]

Hello Doug,
Are you intending to breadboard all this first or are you just jumping straight into the build stage,,, and prey for a miracle? :loco

I don't mean to hurt your feelings but I strongly advise you to think about it as there is a minefield of problems that I can see already.

Just one issue for starters ; Tillman circuit is only for outboard preamp
ala Acoustic guitar peizo PU's,, Not really intended as a main Preamp.
Phil.

[doug7410]

Hi Phil,

Yes, I am breadboardig this. The experimental parts at least. I built the power amp section and the Dr. Boogie. On the breadboard is the Tillman, the Baxandall EQ, and The LM386 circuit. So far I've been able to get a really nice clean chanel with the Tillman in front. I'm going to test that with some stomp boxes in front to see what happens.

Don't worry about hurting my feelings. I'll be the first to admit I'm learning as I go. Please let me know if you see anything wrong with my ideas so far.

[Zappacat]

Just wondering how you decided on the 22 watt figure?  Are you going to utilize 2 LM386's for the stereo channel?  I'm anxious to hear about your progress.  Good luck!

[phatt]

Hi Doug,
Well if your going to use fets be warned that tillman preamp will just distort very quick on such a low supply.

I've built fet circuits with 35VDC supply and no matter what you do they just hate really big signals.
Lets face it there are 100's of circuits that will distort but try and find one that does really good clean?
This is where valves shine as they can handle really big signals.
The best way round this often over looked issue that I've come across was to make the preamp passive this way the input to your amp will always be clean.

Then you insert the other OD devices if you so wish.
Read what JM Fahey has to say about those fet preamps as they are way to over rated sure they work but you can do a lot better with other tricks.

Fets sound and react much like a pentode on a staved voltage,,
masses of gain but very little headroom.
Some like that   but don't ask for clean.
Just my two cents,, Phil.

[doug7410]

I decided on the 22 watt figure simply because I had a power adapter that put out 12V and 5 Amps. A local electronic store had the chip in stock so I decided to give it a try. It can get pretty loud with the LM386 pushing it. As for the stereo channel, I am going to use 2 LM386's.

Phil,
I agree most of the fets circuits will distort, but when you push a LM386 with the Tillman pre amp, you gets lots of clean headroom. I think this is because the gain on the fet never changes. I've tried a couple other fet circuits (fetzer valve), and they all have at least a little distortion. This one just doesn't seem to have any.

I'm still just experimenting, and so far I've been way happier with this than anything else I found for a clean channel.

I have even juged this against my 100 watt Peavey XXL solid state head, and this clearly sounds better.

[doug7410]

I came up with a way to switch between 4 preamp channels with led status indicators. There will be 2 sets of 2 channels. The 2 sets will be switched between with a DPDT toggle. That switch will go to another DPDT switch to choose one of the channels in the set. I have the diagram in the link below switched to the Dr. Boogie channel.

http://www.duskyonline.com/wiring.jpg

Now I need to figure out how to ad a footswitch option to this. Any sugestions?

[Zappacat]

I came up with a way to switch between 4 preamp channels with led status indicators. There will be 2 sets of 2 channels. The 2 sets will be switched between with a DPDT toggle. That switch will go to another DPDT switch to choose one of the channels in the set. I have the diagram in the link below switched to the Dr. Boogie channel.

I think it would be really cool to have the footswitch setup like this :

5 buttons total.  All have LED indicators when active.  4 of the buttons to switch between the independent preamps and the 5th button the toggle on/off effects loop.

[doug7410]

That does sound cool. But I have no idea how it would work

[Zappacat]

That does sound cool. But I have no idea how it would work.

I'm not really sure how it would break down switch-wise but wouldn't it be simpler than breaking the preamps into 2 different sections with 2 channels like you were talking about above?

[Zappacat]

I was trying to find an example of this in a modern solid state amp.  http://tech21nyc.com/support/index.html Check out the foot controller on the Trademark 120.  For my personal taste this footswitch design is awesome.  3 channel selectable with toggles on the effects loop, reverb, and boost elements.  Why are you grouping the different preamps in to 2 groups?  Is it because of a hardware issue with certain switches you are trying to utilize?

[doug7410]

Wow! That footswitch would be awesome. And the really amazing thing is that all you need is a regular 1/4" guitar cable to connect it. I think it must be some kind of digital controller telling the amp what to do. The reason I'm breaking the channels down into two's is, in my tiny brain it's the only way I can see how I can have 4 selectable channels. And it can only let one preamp run at a time. So you don't accidently have more that one on at once. I could wire it so all the preamps run in series, and just use dpdt's to activate or bypass, but then you might end up with more than one on at a time. The wiring I did in the diagram would only let one run at a time.

Any ideas how to give them each a seperate switch?

[Zappacat]

I really like the idea of what you are trying to do here with the different switchable preamps.  Maybe the way to approach this switching problem is to use a microcontroller in the footswitch that triggers another microcontroller on the main circuit board.  I'm sure  there are some controllable switches out there that would suit this task.  Programming micros is "no big deal" these days.  I've seen people using them in their own "home brew" stereo audio amps in some amazing ways.  The big deal here is that if you get it to work you're really unlimited on what you can do with it in terms of switching channels, mirroring tone stack controls, tone stack memory presets, etc...  Let me look into this a little bit more and I'll get back to you on this.

Anyone got any ideas about the multiple channel switching dilemma with respect to switch specifics?

I could be oversimplifying this in a great way but it sure sounds like fun.

[doug7410]

Microcontroller... I'm gonna have to google them, because I know nothing about them. Sounds good though. I think I saw a kit at Radio Shack called "My first microcontroller. I also think I saw a litte microcontroller board there for $50. I hope I don't have to spend that much for my switching. I really wanted to keep the cost of this thing as low as possible.

Thanks for keeping the ideas comming  

[Zappacat]

FPGA http://en.wikipedia.org/wiki/Field-programmable_gate_array is what I'm leaning towards in terms of switching.  Kind of funny if we could get this to work.  In the late 80's I was working with EPROMS at one point.  These are just the evolution of PROMs and EPROMs.  Lots of tools available to program them and they are cheap.  I've been away from electronics for so long that I'm unfamiliar with MOST of the current digital logic terminology.  I'm trying to find one (microcontroller)with BIG pins(ha, ha).  Just think you could really just pick which preamps/tone circuits you wanted and join them together with something like this.  The ability to control them from the footswitch using a device similar to this opens up a LOT of ground.

I'm sure a lot of people familiar with digital logic circuits would consider the use of FPGA overkill for utilizing them in a guitar preamp switching circuit but this is really intriguing to me.  I think it leaves the door open for the future and it gets me interested in what I like to call "low level programming" again.  For the time being I'll just breadboard it using typical logic gates.

Sideline info about programming microcontrollers http://www.sparkfun.com/commerce/tutorial_info.php?tutorials_id=93

Can you guys give me recommendations about breadboards.  I threw all that stuff out 20 years ago.  I need to buy two or three of them and I want them big and of good quality.  Thanks!