maxon od808 / ts808

[bukas]

here is an idea. build a pedal around ts808 circuit with switch that would change from ts808 tranies to maxon od808 IC and therefore change the effect. so, can i use IC1 for distorsion and tone like in ts808 and IC2 for tranies ? and if its possible should i kill signal with the switch or power to these components ?

[Pojo]

Not sure if I'm following what you want to do....are you looking to just switch between 2 different op amps? If so, I don't know if you will accomplish exactly what you're looking for doing that. The actual op amp used has an incredibly small influence on the tone of the circuit.

See this video:

But if that's really what you wanna do, check out this link for an idea of how to accomplish op amp switching. Scroll down to the header labled 'Switching and Stacking'.
http://www.beavisaudio.com/projects/ScreamerLab/

[bukas]

nah...

i wana switch between JRC4558 (or MC1458) and 2N3904 transistors to get maxon od808 sound and ts808 sound in one pedal(circuit) switchable

[Mark Hammer]

Shouldn't really make any audible difference of any musical value.  A lot of effort for very little value...really.

The 1458 can have some sonic value, as I recently found swapping one in for the stock chip in a buddy's Timmy pedal.  But in that case the 1458 was used in a gain stage, whereas in the Maxon pedal is it simply used as a unity-gain buffer stage.  If you were pounding the daylights out of it with a super-hot input signal, maybe it would have some sort of "sound"...but then you wouldn't really be getting an OD808 sound either.

There are a zillion mods one could do to a Tube Screamer - most of them described in relentless detail on this forum - that would be easier and yield far more of value for the effort expended.  Trust me.

[bukas]

thanx for reply

so i noticed output cap is 47uF in ts808 and 4.7uF in maxon od. is this then the only deference ?

[Mark Hammer]

To the best of my knowledge, the output cap in a stock TS808 is 10uf.  There is a 47uf cap in the reference/bias voltage sub-circuit, but that's it.  If there WAS a 47uf cap in the output path, its role would be to allow bass (that doesn't really exist) to pass.

[bukas]

sry, my mistake.
this is what had in mind
                  in               out

od808      1uF+/-        4.7uF+/-

ts808       0.027uF      10uF+/-

[Mark Hammer]

Still wouldn't make much difference.  The .05uf cap that goes to Vref/Bias has far more impact.

[bukas]

you mean the 0.05 cap between -IN and vref ?

ts808 value is 0.047 and od808 is 0.05. how much can that be a change?

[Mark Hammer]

.05 is what they call .047 in Japan.   

I'm talking about the one between the 4k7 resistor and Bias on the inverting (-) pin of the op-amp stage with the diodes.  Those two components establish a low end rolloff starting around 720hz.  The difference between 10uf and 4u7 on the output of the pedal changes the rolloff point in the sub-audio range, but since there IS no subaudio once that .047/.05 cap has its way, changes to the output cap don't really matter much.  You could probably have 1uf in there and not hear any difference.

The traditional "fix" for the low-end rolloff is to increase the value of that cap to something in the range of .22uf or so to move the rolloff point further down.  Alternatively, since the rolloff point is a joint function of both the .047 cap and 4k7 resistor, one of the clever tricks used by some builders, and used in the Timmy pedal, is to make the 4k7 resistance variable instead of fixed.

For example, by using a 3k3 resistor in series with a 10k pot (wired as variable resistance), you can go from a 3k3 resistance to 13k3.  With 3k3 and a .047uf cap, the rolloff point can be moved from 1026hz (using 3k3) to 254hz (with 13k3).  It's not a bass "boost" so much as a bass preservation adjustment.  NOTE, however, that since the gain is a function of that resistance and the feedback resistance in that stage, you will lose some gain as you adjust for more bass.  Max gain, with no other changes than the resistance substitution, will be 168x with R=3k3 and 42x with R=13k3. 

Although retaining more bass will produce a little more clipping, it doesn't fully compensate for the drop in max gain.  A more suitable adjustment would be to use .1uf instead of .047/.05, 2k2 instead of 4k7, and a 5k pot.  That will give a max gain of 251x when R=2k2 (rolloff around 720hz), and 78x when R=7k2 (rolloff around 220hz).  That will make for a respectable grind when max bass is used, and more "emphatic" clipping when set for stock rolloff.