please help with ts808 clone ic stacking

[panterica83]

hi, im new to all of the ic stacking and im just not sure which dpdt switches to buy and how they are wired, http://www.beavisaudio.com/Projects/ScreamerLab/ say's and shows some info but not enough, since dpdt switches come in different forms...on,on or on,on,on or on off etc and just wanted to know which to get.

thanks,sean

[armdnrdy]

You want On/On so you can choose between op amps.

This has been discussed a million times on this and other forums.....you would get more out of the circuit if you switch between different clipping diodes or tone caps.

The difference between op amps is very subtle.....IMO not different enough to mess with switching between them.

Before you venture off on this path....might I recommend trying different op amps in a socket to see if stacking and switching them is  worth the trouble.

[Mark Hammer]

One of the things that stacking of op-amps does is modify how much current is available at the output of the stacked devices.  If there is something that comes after those devices that will behave differently due to that current, such as clipping diodes, then stacking might have a useful impact.  In the case of a TS type arrangement, though, the diodes are in the feedback loop and not after the output.

In short, I would expect some sort of audible difference, but not necessarily a pleasing or musically useful one.

Larry is correct in declaring that changes in diode configurations will likely make more of an audible difference than frittering around with chip-choices.  Use of a 2+1 (as in the Boss SD-1), a 2+2 (as in the Timmy and Xotic boosters) or the various MosFet+diode configurations found in other boutique pedals, will yield a wider palette of tonal choices.

To that, I will also add that the ground leg of the clipping stage, made up of the .047uf cap and 4k7 resistor, is also used by many to vary the bass content, and change the character a fair amount.  Stock, it rolls off the bass starting around 720hz, so as to provide relatively consistent clipping across the fingerboard.

One strategy for increasing the bass content is to simply increase the value of the cap from .047uf to .1 or .22.  A different strategy is to insert a pot in series with those two components, so as to vary the rolloff and the gain.  Increasing the total resistance on that leg will reduce the gain, but since the amplitude of the bass end is higher, you'll be able to get similar amounts of clipping - maybe more - even with reduced gain.  Remember, clipping is a function of the amount of gain AND the input signal, not just the amount of gain applied.

Using a .068uf cap, 10k linear pot, and 3k3 fixed resistor instead of .047uf and 4k7 will do the following:
1) At min pot resistance, your max gain (Drive control dimed) will be 168x, instead of the stock 118x, with a bass rolloff starting around 710hz.
2) At half pot resistance, max gain will be 67x, with a bass rolloff starting around 282hz.
3) At max pot resistance, the max gain will be about 42x, with a bass rolloff starting around 176hz.

If you don't need the added bass, then just use a 5k pot instead of 10k.  That added control, in tandem with a couple of diode choices to pick from, will deliver enough variation to keep you happy.

[R.G.]

Larry is correct in declaring that changes in diode configurations will likely make more of an audible difference than frittering around with chip-choices. 

Let me add a philosophical point to that.

The very reason that opamps are designed the way they are is to make the insides of the opamp not matter.

Again - the whole point of opamps is to make the stuff around the opamp matter, not the insides of the opamp.

The entire content of the electronics industry's messing about with opamps is to make opamps that do this better, and to remove limitations that let the insides show through in ever-wider ranges of operating conditions.

[panterica83]

wow, thanks guys for all the info! maybe just modding it with some silicon diodes and what not, would be alot more fun!

hmm

[R.G.]

It'll probably be a lot more immediately rewarding.

[panterica83]

any way of adding a small 12au7 tube to the ts808 circuit? i figure that could be a cool tone!

thanks, s

[JustinFun]

The Spark Gap project is essentially that (although eb91 rather than 12au7) - using valves as diodes in the feedback loop.

http://www.diystompboxes.com/smfforum/index.php?topic=89363.0

If you wanted to replace the op-amp, I wonder if you could put clippers between pins 1 and 2 of a valve caster, and fiddle with coupling caps and tone stack to get something vaguely tubescreamerish? Disclaimer - this may be rubbish.