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DIY Stompboxes => Building your own stompbox => Topic started by: dano12 on May 29, 2006, 06:06:01 PM

Title: Big bag of Ruby mods [renamed]
Post by: dano12 on May 29, 2006, 06:06:01 PM
So I finally figured out the wildly complex Big Muff tone control :) I've also mapped out the Ruby amp and actually spent time with the 386 datasheet in an attempt to understand what each part is doing. I searched this forum to figure out how to insert the BMP tone control into the Ruby, and came up with a couple general ideas but nothing concrete. What I've learned:

1. The tone control will result in a loss of signal--that's the nature of the beast.
2. A post here suggested the following order for adding a tone control to the ruby: buffer->tonestack->buffer->volume->386->output
3. The first transistor and related components in the Ruby represent an input buffer.

So I've drawn my own version of the Ruby schematic, inserted the tone control, added another buffer stage, and added a switch and cap in the feedback loop of the opamp.

Here it is so far:

(http://www.beavisaudio.com/Projects/ruby/ac.gif)

Questions:
1. Am I close?
2. Will my second buffer stage make of for the signal loss of the tone control.
3. The LM386 datasheet says that a 10uf cap placed between pins 1 and 8 will yield a gain of 200. Will my switch/cap (in red) accomplish this, or is the 1kohm pot doing essentially the same thing when it is fully open?

Thanks very much in advance for any help with my first attempt at modifying an actual schematic!
Title: Re: Ruby amp with a tone control--Am I even close
Post by: B Tremblay on May 29, 2006, 06:30:28 PM
Questions:
1. Am I close?

Very close!  The only issues are an extra power supply filter cap (you only need one 100uF from +V to ground), the redundant gain switch  (which you suspected was unnecessary in question 3), and there should be coupling caps before and after the tonestack.

Quote
2. Will my second buffer stage make of for the signal loss of the tone control.

No.  A buffer has at most, a gain of 1.  The buffers are in place to prevent the loading of the guitar and amplifier.  When builders have asked me about adding a tonestack to the Ruby amp, I usually suggest adding a small gain stage before the tonestack and then the stock Ruby.  You could use a Fetzer Valve, the NPN Boost, or the output stage of a BMP.

Quote
3. The LM386 datasheet says that a 10uf cap placed between pins 1 and 8 will yield a gain of 200. Will my switch/cap (in red) accomplish this, or is the 1kohm pot doing essentially the same thing when it is fully open?

Correct.  When the Gain pot is at maximum, pins 1 and 8 are connected, providing full gain.

Your schematic is very attractive, as is your website.  Nicely done!
Title: Re: Ruby amp with a tone control--Am I even close
Post by: bancika on May 29, 2006, 06:33:39 PM
Nice looking schematic, which software are you using?
Title: Re: Ruby amp with a tone control--Am I even close
Post by: dano12 on May 30, 2006, 09:35:42 AM
Very close!  The only issues are an extra power supply filter cap (you only need one 100uF from +V to ground), the redundant gain switch  (which you suspected was unnecessary in question 3), and there should be coupling caps before and after the tonestack.

Wow, I'm close! This is exciting stuff. Regarding coupling caps, would the existing 47 nf in the Ruby schematic be the coupling cap? So I could just add another 47 nf?

Quote
No.  A buffer has at most, a gain of 1.  The buffers are in place to prevent the loading of the guitar and amplifier.  When builders have asked me about adding a tonestack to the Ruby amp, I usually suggest adding a small gain stage before the tonestack and then the stock Ruby.  You could use a Fetzer Valve, the NPN Boost, or the output stage of a BMP.

Thanks much, this cleared up a large mystery for me. I'll look at the options and redraw schematic. Hmmm....maybe this is a great place for your awesome ToneMender circuit....

Quote
Your schematic is very attractive, as is your website.  Nicely done!

And thanks to you--I've built quite a few things from runoffgroove.com.
Title: Re: Ruby amp with a tone control--Am I even close
Post by: dano12 on May 30, 2006, 09:37:22 AM
Nice looking schematic, which software are you using?

I use Visio for pretty much everything. The electronics schematics library is a bit weak, so I've created my own templates for various things like FETs and opamps. But Visio covers just about everything I've thrown at it. I also use to do all my wiring diagrams so I've built up a library of graphical representations of pots, switches, jacks, etc. Highly recommended.
Title: Re: Ruby amp with a tone control--Am I even close
Post by: Mark Hammer on May 30, 2006, 10:09:44 AM
Not meant as a criticism, but it seems a little more complicated than necessary to me, and at the same time insufficient in some ways.  Your FET input stage provides all the buffering needed, and a second one is overkill.  What the input stage needs to do, though, is provide some gain to offset the passive loss of the tone control.  Keep in mind the BMP-style tone control works by bleeding signal to ground at both the high end and low end, and what comes out the wiper of the tone pot is what's left.  That's why the BMP needs a gain recovery section after it to be able to output something hot enough to achieve noticeable boost and produce additional overload in subsequent sections.

So, what you need here is a hotter input stage that gives 9db or more of gain. Then, what comes off the wiper of the tone pot can simply be fed to the 47nf cap and the volume pot directly.

Alternatively, the 386 provides a number of ways to get boost at the top and bottom ends, and these may well be more of what you want.  Keep in mind that the BMP tone control is really more oriented at "revoicing" a fuzz, then EQ in the classic sense of compensating for speaker or signal source.  That MAY be what you intended...but maybe not.

There is a feedback loop between pins 5 and 1 that can be exploited for bass boost, and the more familiar feedback loop between pins 8 and 1 can be modified to use different capacitor/resistor feedback combinations in parallel (just like the Rat) to yield differential gain for different frequency ranges.  I was tuned onto this a couple years ago by a Nuts & Volts article/project that used much smaller value caps than 10uf to provide a big boost in the upper treble.  The project was a pair of powered extension speakers that older people with high-end hearing loss could plug into the earphone jack of their TV and park beside them on the couch/sofa.  By sticking a smaller than normal cap in that loop, the higher gain that comes from having a low-impedance path in the 8->1 path was applied only to high end, with more modest gain applied to the rest of the spectrum. 

Another thing to consider is the use of a "bright" cap on the volume control so that top-end is preserved as you turn down.

But again, all of this assumes your goal for the tone control is to provide compensatory sonic adjustment, rather than tonal overhaul.
Title: Re: Ruby amp with a tone control--Am I even close
Post by: B Tremblay on May 30, 2006, 10:15:03 AM
Wow, I'm close! This is exciting stuff. Regarding coupling caps, would the existing 47 nf in the Ruby schematic be the coupling cap? So I could just add another 47 nf?

Yes, though you could probably go a little bigger than that after that first stage so you're providing the tonestack with a nice full-range signal, maybe 100n or 220n.  Though, like Mark suggests, that first stage should have some gain.
Title: Re: Ruby amp with a tone control--Am I even close
Post by: dano12 on May 30, 2006, 09:07:26 PM
Thanks very much for the pointers. I chose the BMP tone control simply because it was simple. However, given the feedback, I decided to go back to the drawing board and do some more research. I've found a JFET based boost, tacked on (what I think is an accurate rendition of) the Stupidly Wonderful Tone Control, and sent it all of to the input of the stock Ruby.

Does this look better? Is the stock Ruby buffer in the correct place? Can I use an MPF102 instead of the J201 for a more managable boost? Why do I ask so many questions?


(http://www.beavisaudio.com/Projects/ruby/ac2.gif)
Title: Re: Ruby amp with a tone control--Am I even close
Post by: jrc4558 on May 30, 2006, 09:48:35 PM
You dont really need a buffer in your second attempt. just a booster, a tone control, and LM386
Title: Re: Ruby amp with a tone control--Am I even close
Post by: cab42 on May 31, 2006, 06:32:21 AM
Why do I ask so many questions?

Because they are very relevant questions!! I have followed this thread with great interest as I have been thinking in the same lines.

BTW: I use Visio on a regular basis, but I have never thought of using it for drawing schematics.

Regards

Carsten
Title: Re: Ruby amp with a tone control--Am I even close
Post by: Mark Hammer on May 31, 2006, 08:14:34 AM
The SWTC is what you might call a "something for nothing" control, in that it gives some tonal adjustment without having to pay too huge a price.  Note that it will only trim top end, and not provide any boost.  If all you want is to mellow things out, it'll work fine.  If you want some brightness added, you need to look elsewhere.  Similarly, if you want some bottom boosted, you'll need to look elsewhere.

One of the clever things Joe Gagan packed into a number of his excellent Fuzz Face extrapolations was a variable bottom control on the input.  Here, the signal is faced with two possible capacitors to pass through.  One of the caps is fairly small in value, giving a thinner input signal.  The other is a larger value with a variable resistance in series to adjust how "easy" it is for the bass to pass through.  When the resistance is turned up, the signal "prefers" to pass through the smaller cap and there is less bass content.  When the resistance is turned down, the higher-value cap now becomes an easier path for low end to proceed along and more bass content is introduced.

If you decide to stick with the SWTC in some form, here's a suggestion:
These series caps will provide the needed DC blocking, sao the 47nf cap you show becomes unnecessary.  The SWTC will roll off high end, and the parallel pot/cap will let you dial in low end.  These two should permit you to achieve thinner brighter tones, bright full-range tones, and darker tones, as well as other combinations.  You may well need to tinker with cap values to suit whatever speaker you settle on, but you'll have a nice range of possibilities, a simple gain structure, and a fairly small board.
Title: Re: Ruby amp with a tone control--Am I even close
Post by: dano12 on May 31, 2006, 08:50:17 PM
If you decide to stick with the SWTC in some form, here's a suggestion:
  • First, wire up the SWTC pot directly to the volume pot
  • Second, replace the .15 cap ahead of the SWTC with a smaller (say, .0033-.0068uf) cap
  • Third, wire up the .15uf cap in series with a 100k pot set as variable resistor and put it in parallel with the small cap
These series caps will provide the needed DC blocking, sao the 47nf cap you show becomes unnecessary.  The SWTC will roll off high end, and the parallel pot/cap will let you dial in low end.  These two should permit you to achieve thinner brighter tones, bright full-range tones, and darker tones, as well as other combinations.  You may well need to tinker with cap values to suit whatever speaker you settle on, but you'll have a nice range of possibilities, a simple gain structure, and a fairly small board.

This is most helpful. Thanks for the the lead on Joe's circuits--I'll have a look.

I followed your suggestions and although I was a wee bit lost on step three, I think I have it right. Think its ready for the breadboard?

(http://www.beavisaudio.com/Projects/ruby/ac3.gif)
Title: Re: Ruby amp with a tone control--Am I even close
Post by: B Tremblay on May 31, 2006, 09:18:13 PM
Looks good!  I'm still a bit slow when it comes to impedance issues, so I tend to err on the side of caution; hence my suggestion of the keeping the stock Ruby input buffer.  However, I'll gladly defer to those with a better grasp!

My only suggestion is to remove the 1M resistor from the first stage's gate to ground.  It's unnecessary since you have that 1M pot there.
Title: Re: Ruby amp with a tone control--Am I even close
Post by: Mark Hammer on May 31, 2006, 10:22:24 PM
First, since I haven't said it yet, these are really nice drawings.  Kudos.

Second, BT is bang on about the redundant 1M resistor.  The only question mark for me is whether you need the pot there or can simply do with a 1M fixed resistor.  I take it that the input stage doesn't add THAT much gain, so the input pot and volume control don't really fulfill any real sort of gain/master function, like they might with a hotter gain structure.  On the other hand if you anticipate feeding it potentially overloading signals, then the input pot can be used to pad down such signals and make them more manageable.

Looks just about ready.  Keep in mind that the assorted tone-cap values are merely starting points and you may find other values more suitable for your needs.  Also, keep in mind that 12v worth of penlights (an 8-pack) can make 386s peppy little things with a nice punch and longer battery life than with a 9v.  You obviously can't stick it in an Altoids container, but you'll appreciate what a beefier supply can do.  If you go with 12v, make sure to use an 8 ohm speaker.  386s don't mind higher supply voltages, but they'd rather encounter low-impedance speakers/loads when using more modest supply voltages.
Title: Re: Ruby amp with a tone control--Am I even close
Post by: dano12 on June 01, 2006, 09:04:00 AM
My only suggestion is to remove the 1M resistor from the first stage's gate to ground.  It's unnecessary since you have that 1M pot there.

Good catch. Thx!
Title: Re: Ruby amp with a tone control--Am I even close
Post by: dano12 on June 01, 2006, 09:11:29 AM
First, since I haven't said it yet, these are really nice drawings.  Kudos.

Second, BT is bang on about the redundant 1M resistor.  The only question mark for me is whether you need the pot there or can simply do with a 1M fixed resistor.  I take it that the input stage doesn't add THAT much gain, so the input pot and volume control don't really fulfill any real sort of gain/master function, like they might with a hotter gain structure.  On the other hand if you anticipate feeding it potentially overloading signals, then the input pot can be used to pad down such signals and make them more manageable.

Looks just about ready.  Keep in mind that the assorted tone-cap values are merely starting points and you may find other values more suitable for your needs.  Also, keep in mind that 12v worth of penlights (an 8-pack) can make 386s peppy little things with a nice punch and longer battery life than with a 9v.  You obviously can't stick it in an Altoids container, but you'll appreciate what a beefier supply can do.  If you go with 12v, make sure to use an 8 ohm speaker.  386s don't mind higher supply voltages, but they'd rather encounter low-impedance speakers/loads when using more modest supply voltages.

Thanks for the kudos and the help! This is going on the board tonight sans the redundant 1M.

I'll play around with cap values: we are talking about the .0033uf and .15uf parts right? I'm thinking that if I find a nice range of values, I may put several off them on a rotary switch.

I've run my existing stock Ruby on both a 9v battery and a 12v wall wart. You're right--the higher voltage makes it even more interesting.

Have you seen the movie "Men in Black"? I've decided that the name for this circuit will be the "Noisy Cricket", as in:

(http://beavisaudio.com/Projects/NoisyCricket/NoisyCricket254x173.jpg)
Title: Big bag of Ruby mods [renamed]
Post by: dano12 on June 03, 2006, 11:39:55 AM
As I've been breadboarding this, I happened upon an interesting little mod:

- Remove the 100nf cap that goes from pin 7 to ground
- Add a 56nf cap across pins 5 and 7

This adds a *nice* amount of extra gain. A bit more noise too, but it sounds great.

(http://www.beavisaudio.com/Projects/NoisyCricket/GainyMod1.gif)
Title: Re: Ruby amp with a tone control--Am I even close
Post by: Ardric on June 03, 2006, 07:25:37 PM
The resistor in the Zobel network, R5 in the last schematic, should be 10 ohms, not 10k ohms.  I think this is scaled to an 8 ohm speaker load, but it'll probably be fine with 4 or 16.  It's purpose is usually to protect the chip amp, but it may also help take some of the top end fizz off.

I think some series resistance on the MPF102's gate may help with RF rejection.  To be really complete you could include a dc-blocking capacitor and protection diodes.

I'd also embellish the power supply a little.  I'd put C1 as close to the 386 as possible, and supply the battery V+ to C1's + terminal.  I'd use C1's - terminal as a star ground point for IC1 pin 4 (gnd), the gnd side of the speaker cable, and the gnd for the rest of the circuit.  I'd then use a small RC filter like 100 ohms/22uF to filter the power before Q1's drain, because I'd be worried that IC1 will be pulling enough current out of the power supply to overwhelm C1 and modulate the signal on to V+, which will feed back through the PS to Q1.  Oh, and it wouldn't hurt to make C1 bigger, like say 1000uF, and include a polarity protection diode.

My last suggestion is to "valvestate" the 386 by using negative feedback, as discussed here: http://www.diystompboxes.com/smfforum/index.php?topic=38331.0 (http://www.diystompboxes.com/smfforum/index.php?topic=38331.0)

Anyways, hope that's of some help.
Title: Re: Ruby amp with a tone control--Am I even close
Post by: dano12 on June 04, 2006, 08:30:00 PM
The resistor in the Zobel network, R5 in the last schematic, should be 10 ohms, not 10k ohms.  I think this is scaled to an 8 ohm speaker load, but it'll probably be fine with 4 or 16.  It's purpose is usually to protect the chip amp, but it may also help take some of the top end fizz off.

I think some series resistance on the MPF102's gate may help with RF rejection.  To be really complete you could include a dc-blocking capacitor and protection diodes.

I'd also embellish the power supply a little.  I'd put C1 as close to the 386 as possible, and supply the battery V+ to C1's + terminal.  I'd use C1's - terminal as a star ground point for IC1 pin 4 (gnd), the gnd side of the speaker cable, and the gnd for the rest of the circuit.  I'd then use a small RC filter like 100 ohms/22uF to filter the power before Q1's drain, because I'd be worried that IC1 will be pulling enough current out of the power supply to overwhelm C1 and modulate the signal on to V+, which will feed back through the PS to Q1.  Oh, and it wouldn't hurt to make C1 bigger, like say 1000uF, and include a polarity protection diode.

My last suggestion is to "valvestate" the 386 by using negative feedback, as discussed here: http://www.diystompboxes.com/smfforum/index.php?topic=38331.0 (http://www.diystompboxes.com/smfforum/index.php?topic=38331.0)

Anyways, hope that's of some help.


Much help. Thanks for pointing out the 10 ohms. I've updated the schematic.

Thanks for the additional ideas--I'll start playing with them.
Title: Re: Big bag of Ruby mods [renamed]
Post by: DEZREJECT on June 04, 2006, 09:48:24 PM
wow... dude looking at ur site... great site, but beyond that.. wow.. amazing pedals... that tubescreamer thing.. Thats effing amazing... where do you get those look meters?!!?! i loooove those... people at gigs would be like why the hell do you nede that many switches.. i love it.. total control!!!! :-D!!!!!!!!! i deff am gunna have to try to do something like that.. isaw your site a good long time ago.. but i couldnt find it again.. i remembered the kick ass enclousers.... sorry to be off topic.. but effing amazing bro
Title: Re: Big bag of Ruby mods [renamed]
Post by: dano12 on June 06, 2006, 08:38:26 AM
wow... dude looking at ur site... great site, but beyond that.. wow.. amazing pedals... that tubescreamer thing.. Thats effing amazing... where do you get those look meters?!!?! i loooove those... people at gigs would be like why the hell do you nede that many switches.. i love it.. total control!!!! :-D!!!!!!!!! i deff am gunna have to try to do something like that.. isaw your site a good long time ago.. but i couldnt find it again.. i remembered the kick ass enclousers.... sorry to be off topic.. but effing amazing bro

Thanks for the kind words. The meters were from Mouser--search for analog panel meters. They are not cheap though.

All the switches for mods are not terribly practical for live use, but then again I design for the 80% of folks who don't gig.
Title: Re: Big bag of Ruby mods [renamed]
Post by: DEZREJECT on June 06, 2006, 10:11:53 AM
hmm and you know when it comes down to it i think theres two schools of rock.. theres the guitarist that just plays, then the modder... usually the great guitarists didnt mod their pedals... thats where i guess im the crossbreed.. not claiming im the next amazing guitarist dont take that the wrong way... but more so over that i like to have total control, ESPECIALLY live because everything changes live... in a studio it doesnt need to be on the mark exact at that exact second.. you can redo the take if its shitty... but live... especially in a touring situation... most of hooking of your intial fan base is how good the show is... then say compare it to a release... if you can nail exactly how it sounds in the studio where as far as the fans know you spent a year tweaking EVERYTHING... if you can recreate that perfect... that is in my mind as a musician my job when playing a show... yeah sometimes a little different solo is cool.. but to a point you get what im saying? but also beyond that with a pedal like yours... you can play with different combinations...

i always read about people breadboarding, and all that... i just cant do it... if im gunna build a pedal im gunna build it, done totally... and then if i dont like it.. i might play around a little to mod this or that... but beyond its not gunna go anywhere... usually ill only build something i want though so its not a total shot in the dark... but the point is... with say that Lab-esque pedal idea you NAILED dead on... with that you can find which combination of different things work good... and for me its not so much sitting at my work bench fiddling around to find that perfect sound.. its spur of the moment.. at a practice jam or whatever have you... and then.. WAIT ive nevre tried the bridge of this song with the deep, bassy, double germanium diode clipping, etc etc etc (fill in mods here...).. then you do.. and now you have that perfect sound for that song that you may never have thought to use there.. and then say you could rebuild the pedal with that mod switchable or permanent, but the point is its like a prototype to dial in exactly what you want....

id compare it to my amp.. i have a peavey XXX half stack... i love it.. i have about 3 different sounds i love out of every channle.. ranging from all influence styles... classic rock, metal, and then my own unique blend of i guess all those influences if you will.. the point is... if i were to now rebuild this head.. which i have considered doing for a custom enclouser type deal... now i would just make the different channles with those 3 settings switchable... no need for the 5 controls per each 3 channles... especially in a live situation because ive tested this all out so many times.. i have what i like, what i want... and i can have the "lab" deal for in the studio for the one off ideas.. but for live i like it stupid simple once u get it dialed in right you know???

honestly when i first started building i built a lil headphone amp...i wanted to build a pedal like yours that you could switch everything around and basically have the control of a head but pocket sized.... but obviously experience limited what i could do or was intimidated to do... im not exactly sure but i think it was ur site that kind of inspired me to get into modding per se... so in that sense dont thank me for thanking you, just let me thank you again haha

yeah lost post... i had a lot of coffee... love me, hate me, deal with it haha
Title: Re: Big bag of Ruby mods [renamed]
Post by: dano12 on June 20, 2006, 06:13:17 PM
Ok I have the Noisy Cricket Mark I completed and tested. I added a very simple tone control, a boost switch, and I couple of LED indicators. I also added the headphone jack suggested on the generalguitargadgets.com site. All in all, I'm very happy with this little amp and the progress I'm making actually learning a bit about how these circuits work.

Here's the schematic:

(http://www.beavisaudio.com/Projects/NoisyCricket/NoisyCricket_Mark1_Rev1_Schematic.gif)
Title: Re: Big bag of Ruby mods [renamed]
Post by: Peter Snowberg on June 20, 2006, 06:40:37 PM
Wow! How did I miss this thread before?

First off, NICE JOB! 8)

There are a couple of things that may be mentioned above but still need attending to. First, C7 is unnecessary and should be omitted. There should also be a some series resistance in the input. I would use the same 33K value found in many tube amps and place the resistor between the input jack and the 1.5M. That will help with RF rejection and it will provide a little protection for the JFET. A small cap (47-100pf) should also be installed in parallel with the 1.5M. The 33K and little cap form a lowpass filter to keep RFI out. Finally, R4 should have a cap in series with it (10uF). That cap preserves the DC balance between the + and - input transistors. The chip obviously operates without it but it sounds better with it in. (check out the internal schematic on the datasheet)

Again, nice job!
Title: Re: Big bag of Ruby mods [renamed]
Post by: dano12 on June 20, 2006, 07:40:33 PM
Edit: I've updated the schematic with your suggested changes.

Wow! How did I miss this thread before?
First off, NICE JOB! 8)

Thanks very much. It is really gratifying to be making forward progress against what seems to be a huge learning curve :)

Quote
First, C7 is unnecessary and should be omitted.

From my understanding of the headphone mod on general guitar gadgets, C7 and R9 form a Zobel network for the headphones. Is the fact that C7 should be omitted because I already have a 220uf cap before the output jack?

Quote
There should also be a some series resistance in the input. I would use the same 33K value found in many tube amps and place the resistor between the input jack and the 1.5M. That will help with RF rejection and it will provide a little protection for the JFET. A small cap (47-100pf) should also be installed in parallel with the 1.5M. The 33K and little cap form a lowpass filter to keep RFI out.


So these changes are basically geared towards reducing RFI? Even on my breadboard with long nasty wires all over the place it is quiet as a mouse.  I wonder if I am just in a good, low-RFI area...

Quote
Finally, R4 should have a cap in series with it (10uF). That cap preserves the DC balance between the + and - input transistors. The chip obviously operates without it but it sounds better with it in. (check out the internal schematic on the datasheet)

Very nice catch. I read the datasheet and it talked about putting a small cap between the gain pins. I assumed that since I was using a pot there was no reason for the cap. I just tried it on the breadboard it does clean things up a bit.

Thanks very much for the pointers!
Title: Re: Big bag of Ruby mods [renamed]
Post by: dano12 on June 27, 2006, 11:39:09 AM
I've updated my Cricket page to include the Pico Cricket and the Tube Cricket.

http://beavisaudio.com/Projects/NoisyCricket/index.htm (http://beavisaudio.com/Projects/NoisyCricket/index.htm)
Title: Re: Big bag of Ruby mods [renamed]
Post by: vfr800fiman on June 27, 2006, 04:55:06 PM
I love the "The Noisy Cricket Mark I" on your web site.
I built a Ruby for my Niece, and now I think I'd like one for
myself as a practice amp.

Anyone ever think about putting a CD jack in the circuit to practice
along with your favorite band? I think you'd need 2 pots to mix the
2 signals for your own taste (or sound  :P)

I've been saving an article from "Silicon Chip" called the "Guitar Jammer" that
has this nice feature.

Glenn