Spark Plug: Biasing Op Amps and Hard Clipping Change

[BrownMagicMurad]

This is an open question to Bean, that I figured could benefit everyone. Alternatively, I would appreciate anyone else's input. I will be using the links below as references to my questions.

Original Sparkplug: http://www.madbeanpedals.com/projects/Sparkplug/Sparkplug.pdf
Sparkplug Rev.1: http://madbeanpedals.com/projects/Sparkplug/Sparkplug_rev1.pdf

I notice that after the output of Op amp one of the Original Sparkplug, there is no cap and the diodes are referenced to VB for hard clipping. Makes sense. We're biased at 4.5V, so biasing the diodes at that makes sense. In Revision 1, there is a 1UF cap (removing that 4.5V DC bias) and subsequently the diodes are referenced to ground. Also makes sense. Why the change? The only seeming disadvantages of the additional capacitor are the addition of a new part and the inherent filter of the cap in the signal path (even if 1UF is more than enough to pass all the signals). Can you please comment on the reason for the change?

Secondly, the method of bypassing the hard clipping has changed. Previously, you would only make/break the ground/VB connection with an SPDT. Now, you're shorting the 1M resistor to provide a reference. What is the added benefit of this arrangement? It should be noted that this is somewhat similar to how you introduce the bypass capacitor to the gain stage in your Lavache Design (I believe a deviation from the original design), by shorting a large 3.3M Resistor.

Thank you for the help, and I hope this offers some insight to others.

[madbean]

Explanation is simple: C6 was used on rev.1 to eliminate DC on the Cut pot. Although the original design didn't have an scratchy type noise on the Cut pot when you turned it I thought it was better to just remove it altogether. The added 1M resistor is to provide a small discharge path for C7 just in case any voltage sat on it when the Sat switch is flipped to ground (unlikely to happen in this case though). This section is probably a little over-engineered but I thought that the couple extra parts was worth the peace of mind of no one having any possibility of a noisy Cut control or pop when the switch is engaged.

[BrownMagicMurad]

Explanation is simple: C6 was used on rev.1 to eliminate DC on the Cut pot. Although the original design didn't have an scratchy type noise on the Cut pot when you turned it I thought it was better to just remove it altogether. The added 1M resistor is to provide a small discharge path for C7 just in case any voltage sat on it when the Sat switch is flipped to ground (unlikely to happen in this case though). This section is probably a little over-engineered but I thought that the couple extra parts was worth the peace of mind of no one having any possibility of a noisy Cut control or pop when the switch is engaged.

Brilliant! Thanks for the description. That pretty much answered everything.

Now if we were to look at the Green Bean:
http://www.madbeanpedals.com/projects/GreenBean/docs/GreenBean2014.pdf

I'm wondering why R9 is needed to re-establish the bias voltage, which is already set to VB by the previous stage.

[madbean]

That is a good question. Of course, it is directly from the TS design. But, why? Looking through the Tech. of the Tube Screamer on geofex, I don't see any mention of that resistor but I did not re-read the whole thing: http://www.geofex.com/Article_Folders/TStech/tsxtech.htm

My guess is that it raises the input impedance of the active tone control circuit. Keeping in mind that the output impedance of the previous gain stage is likely very small, the 10k resistors pulls it up a bit higher. It doesn't need to be very large...I doubt changing the value to 15k or 22k would offer much tonal difference, if any. Or, possibly it is to ensure that the bias voltages on both ends of the Tone pot are equal. Or even still, it is acts as a current limiter to keep the circuit load a bit smaller. I don't have a definitive answer here...just a few guesses. It will work without that resistor, but obviously there is a reason for it being there in the mind of whomever designed it.