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GB for DC coupled B1 buffer with shunt PSUs

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#341
Salas said:
The input interconnects will have 100-150pF easily, add average source impedance to the 220R stopper and its already been taken care of. Haven't seen any oscillation on 100kHz squarewaves. If there was an issue, Nelson Pass would include such a filter in the original B1. Don't bother IMHO. 1k series, is audible. Adds too much source impedance. The dynamics are softened, and the resistor quality becomes more obvious. That is why I got down to minimum 220R stoppers.

100kHz close up.
Click to expand...

Not for oscillation of the FETs but to prevent RF creeping in. Can easily be jumpered for those that do not want this.

Maybe oldfashioned but I was taught long ago that input filters are good design practice. Agian, this was long ago.

My experiences with RF creeping in are that an amp sounds harsh with high tones even when it does not oscillate at all.

1 k compared to the let's say 25 or 50 k of the volume control are negligable. The benefits could be better than the negative side effects.
 
#345
xaudiox said:
what do you think of adding RC circuit at the input of the pot? i believe the b1 have bandwidth of more than 10mhz, the rc filter can keep the RF away.. Let's say 1 kOhm and 100 pF polystyrene or MKP = 150 kHz -3 dB.. what do you think?
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1k0 + 100pF = F-3dB @ 1.6MHz.
1k0 + 470pF = F-3dB @ 340kHz.
Aim for this to be about one octave above the filter fitted to the input of the Power Amplifier.
Similarly, if the Power amp is 2Hz to 160kHz, it is usually a good idea for the pre-amp to be just slightly wider than 1Hz to 300kHz.
 
#346
jean-paul said:
1 k compared to the let's say 25 or 50 k of the volume control are negligable. The benefits could be better than the negative side effects.
Click to expand...

Only thing I can tell is that when I got down from 1k stopper to 220R, it was subjectively more dynamic, and nothing got harsher. My pot is 22k log, shunted with the 220k from wiper to ground. No oscillations were evident on 100kHz squarewaves. I know that my amps have rather constricted bandwidth, both my KT88 due to Miller and my JLHMOS82 due to deliberate input filter, so there is no point, RFI it won't pass. Now if somewhere there is much RFI and a very HF open main amp, have the option of an input filter it won't hurt. At least someone could listen with and without it, and decide. Only put a cap option after the gate stopper to ground. So by sizing the gate stopper to 1k and using a 470p polystyrene, the filter is done without extra resistor and pads in series with the signal. Just by taking out the shunt filter cap and exchanging the 1k for a 220R, the comparison to my original can be made.
 
#347
Planning a filter behind the potentiometer isn't a good idea. It should be in front of the potentiometer otherwise the filter will have a variable slope depending how much the potentiometer is turned. Sorry, old school electronics keep coming up in my brain....

It is an option for RF polluted areas. Who does not own a cell phone nowadays ? When it can't come in it can't harm either !

I also would use polystyrene caps as a first choice or otherwise polypropylene caps for this purpose.
 
#349
I always do but that is not the point.

AndrewT said:
1k0 + 100pF = F-3dB @ 1.6MHz.
1k0 + 470pF = F-3dB @ 340kHz.
Aim for this to be about one octave above the filter fitted to the input of the Power Amplifier.
Similarly, if the Power amp is 2Hz to 160kHz, it is usually a good idea for the pre-amp to be just slightly wider than 1Hz to 300kHz.
Click to expand...

This is the right information.
 
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