Tube reamer diodes to ground insert

[Xavier]

I have a populated board already for the Tube Reamer (UTR version as per Torchy's layout). I want to insert a couple of back to back 1N4148 diodes to ground, just to have one more choice. Where should I insert them, between R4 and C7, or another location would be better?

Has anybody tried it? Barber's DDSS and the Nobels ODR1 have them in addition to the usual feedback loop clippers and both sound great !!

[stm]

In order to do what you mention you would need to do some surgery on the output network as follows:

1) Place the 0.47u cap just after the opamp's output.

2) Place a new 1k resistor in series.  Left side goes to the 0.47u cap, right side goes with your back-to-back diodes to ground

3) Connect now the 1k and 33n filter to the point where the diodes and the new 1k resistor connect

4) Place the 100k volume pot in parallel with the 33n filter cap (no need to use a decoupling capacitor now since at this point the signal is already referred to ground)


Finally, for best results, I would suggest using a diode combo with higher threshold than your diodes in the feedback loop in order to retain some of the dynamics. For instance, you could try with 2 diodes in series for each side of the ground clipper, or 1N4001+1N34 to each side, or even a pair of red LEDs.

Regards.

[Xavier]

In order to do what you mention you would need to do some surgery on the output network as follows:

1) Place the 0.47u cap just after the opamp's output.

2) Place a new 1k resistor in series.  Left side goes to the 0.47u cap, right side goes with your back-to-back diodes to ground

3) Connect now the 1k and 33n filter to the point where the diodes and the new 1k resistor connect

4) Place the 100k volume pot in parallel with the 33n filter cap (no need to use a decoupling capacitor now since at this point the signal is already referred to ground)


Finally, for best results, I would suggest using a diode combo with higher threshold than your diodes in the feedback loop in order to retain some of the dynamics. For instance, you could try with 2 diodes in series for each side of the ground clipper, or 1N4001+1N34 to each side, or even a pair of red LEDs.

Regards.

Thanks stm, I didn't know there were so many things to keep in mind !!!! I'll give it a try... but this way it wouldn't be a switchable feature, right? as long as it sounds good...

[stm]

Yes, you can make it switchable. Just put a SPST switch that lifts the new diodes from ground and you are set.

I didn't notice before, but this mod will alter the cutoff frequency of the output filter.  In order to avoid this, change the 1k/33n couple to 10k/3.3n and you'll be set.

So the chain will be as follows:

OpAmp_OUT -> 0.47u series -> 1k series -> diodes with switch to GND -> 10k series -> 3.3n to GND -> 100k level pot in //.

[JHS]

For even more mojo: Add a switch for shunting the diodes to ground or to Vref and try different diode-combinations.

JHS