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?
Also recently seen a schematic at Harmonic Appliances with a rectifier diode in parallel with the bypass caps.
Can anyone educate me?
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
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
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
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
I would suggest they do absolutely nothing?.
The diodes are reverse biased, so will never conduct.
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,
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,
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.
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.
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
jan
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.
Quite.
As drawn, those diodes don't look like they're going to have to get out of bed too often. I'd guess they're meant to be the other way up too, and I'm sure Ricky will add it to his already enormous list of distortion possibilities!
It's a horrible schematic to look at though - pink, brown, red, blue and black? Wonky symbols that look as though they've been scratched into a beer can with a nail?
Bleugh!
haha, diode biasing has been done, though in a mic pre/EQ. For early stages in distortion, they give far too much bass, and you need tiny coupling caps which start to void the whole point of it... if feedback was involved however, that could change everything
Apart from the diodes across the cathode bypass caps it is quite apparent that someone has been reading Kevin O'C TUT Series. That preamp circuit is so London Power Standard Preamp its heritage is obvious.
I don't see what those diodes do either. The usual reason for adding diodes like this is to protect electrolytic bypass caps from reverse bias at startup WHEN the cathode resistor is returned to a negative rail rather than to 0V. Since there is neither a negative rail, nor electrolytic bypass caps then I think that they make a rodents hind quarters difference.
If I were to be cynical, and given the "copy cat" nature of the circuits used, I would guess they are probably a legacy from one of the ripped off designs "blended" into this amp, the "designer" just did'nt know enough to realise they were no longer required.
Cheers,
Ian
I don't see what those diodes do either. The usual reason for adding diodes like this is to protect electrolytic bypass caps from reverse bias at startup WHEN the cathode resistor is returned to a negative rail rather than to 0V. Since there is neither a negative rail, nor electrolytic bypass caps then I think that they make a rodents hind quarters difference.
If I were to be cynical, and given the "copy cat" nature of the circuits used, I would guess they are probably a legacy from one of the ripped off designs "blended" into this amp, the "designer" just did'nt know enough to realise they were no longer required.
Cheers,
Ian
OK just my guess here. That when the bypass cap is switched in circuit the diode will do nothing. The diode's cathode is at AC ground then.
But if the bypass cap is out of circuit, then R28 has some of the ac signal across it. Now the diode would , I think, clip part of the ac signal. The small cap is getting rid of some high frequency switching noise. Same for next stage but for the other half of the signal, as it is inverted. Again just a guess here
But if the bypass cap is out of circuit, then R28 has some of the ac signal across it. Now the diode would , I think, clip part of the ac signal. The small cap is getting rid of some high frequency switching noise. Same for next stage but for the other half of the signal, as it is inverted. Again just a guess here
shanx etc.
I thought about this some more. The voltage on the cathode of the tube can only be affected by current through the tube, that is by voltage dropped across the cathode bias resistor. The cathode will accordingly never go below zero volts and that diode will never conduct. The diode capacitance will be so low and will vary so little with the change in REVERSE voltage across it the we can dismiss that as having any influence as well. The more I think about it the more sure I am that my post #13 above had it right.
Cheers,
Ian
I thought about this some more. The voltage on the cathode of the tube can only be affected by current through the tube, that is by voltage dropped across the cathode bias resistor. The cathode will accordingly never go below zero volts and that diode will never conduct. The diode capacitance will be so low and will vary so little with the change in REVERSE voltage across it the we can dismiss that as having any influence as well. The more I think about it the more sure I am that my post #13 above had it right.
Cheers,
Ian
^agreed, this diode is one of those "feel good thing" that the owner thinks he need those diodes in there....
Me too, I guess there was an intent to have some sort of clipping function there, but I checked it as well and that diode will not go into conduction placed like that in the circuit.
The circuit is apparantly a somewhat famous design by Kevin O'Connor. Anyone familiar enough with his books to comment what he claims the diodes are doing?
Teemuk,
Yes the circuit is a copy of the London Power Standard Preamp (LPSP), with a couple of minor component value changes AND those diodes added, they are not in Kevins's LPSP design.
Cheers,
Ian
Yes the circuit is a copy of the London Power Standard Preamp (LPSP), with a couple of minor component value changes AND those diodes added, they are not in Kevins's LPSP design.
Cheers,
Ian
I found reference to this apparently it is in TUT, maybe someone can check that..even better perhaps Kevin O can comment. If the diode was flipped over then I could see some play between cathode resistor and small bypass and maybe diode opening up bypass at higher ac levels across all frequencies. Still not sure but it would have to have an ac peak across cathode resistor beyond -0.7v the way it is set up right now to do anything.
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