snake oil and bypass caps

[djgibson51]

I've recently seen some cathode resistor electrolytic bypass caps having a small poly cap added in parallel.

Also recently seen a schematic at Harmonic Appliances with a rectifier diode in parallel with the bypass caps.

Can anyone educate me?

[jan.didden]

I believe those are two different concepts.
The cap in parallel of the elctrolytic is there to keep up the 'capacitive' character of the combo at higher frequencies where the electrolytic cap gets inductive, as well as to bypass any ESR of the electrolytic.

The cap in parallel with the rectifier might be a snubber in parallel with a rectifier diode to reduce switching noise. Do you have a schematic fragment for that?

jan

[djgibson51]

Hi Jan. Thanks for the comment on the poly cap.

Now have a look at page 2, bottom channel of the attached pdf from harmonicappliances.com (a site I really like). See what I mean about the diode in parallel with a bypass cap? This is done on two stages in this schematic.

Cheers
JimG

[Nigel Goodwin]

Originally Posted by djgibson51

Hi Jan. Thanks for the comment on the poly cap.

Now have a look at page 2, bottom channel of the attached pdf from harmonicappliances.com (a site I really like). See what I mean about the diode in parallel with a bypass cap? This is done on two stages in this schematic.

I would suggest they do absolutely nothing?.

The diodes are reverse biased, so will never conduct.

[razorrick1293]

id love to know what whoever designed this was thinking... the only time that would do anything is if you were feeding something back into the cathode for feedback, which could make some really odd distortion.

If the diodes were forward biased, then the diodes serve the same purpose as the resistor, the resistor would just pull a bit more current,

[DF96]

As drawn the diodes do nothing.

If reversed they might add peak extension on strong bass signals, where the decoupling cap won't do much because it is too small. This is a guitar FX box? Or they might prevent strong second-order distortion from shifting the bias too much.

[jan.didden]

The name of the unit is 'tonerator'. Its a guitar amp. So my bet is these diodes indeed create strong nonlinearities which seems to be required for good guitar sound

jan

[Nigel Goodwin]

Originally Posted by janneman

The name of the unit is 'tonerator'. Its a guitar amp. So my bet is these diodes indeed create strong nonlinearities which seems to be required for good guitar sound

And how do you imagine they might do that?.

[Printer2]

Originally Posted by Nigel Goodwin

And how do you imagine they might do that?.

You know how some people being close to you just makes you feel good....

[Brion55]

I noticed they are only used on the overdrive/distortion stages. Since they are indeed reverse biased, It's not clear to me what they are for. It would be interesting to take them in and out of the circuit - A/B test fashion just to see if there is any discernable difference.