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RE: Poor Design for a Preamp

"OK, so in the case of this 01A preamp, he could use a voltage divider with 68k in series and 22k to ground. This would reduce the voltage about the same as the 330k/100k combo he's using and it would present a load of 90k to the tube and be no problem."

Yes, you are getting this!

"The value of 68k || 22k is 16.62k. If we plug that into the low pass calculator then the -3db point would be 54.75k. That should also not be an issue, right?"

Well, almost. With a -3db point of 54.75kHz the -1db point will be 27.375kHz. That is out of the audio band but the phase is still being shifted by the filter. To get the filter completely out of the audio band the -3db point of the filter needs to be a full decade above 20kHZ.

***Also remember that these calculations do not take into account the Miller of the first stage of the power amp and that could be more capacitance than the cable.***

So to keep the filter totally out of the picture it needs a -3db point at 200kHz.

There are other things in a circuit that will not allow 200kHz bandwidth and there is not much we can do about it (200kHz would be almost impossible for an output transformer) but I don't see that as an excuse to not solve a problem that can be solved. These things stack up so there is no reason to exacerbate the problem.

My question would be, why does he need this gain reduction in the first place? Doesn't the preamp have a volume control in front of the tube stage?

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"



Edits: 02/16/25 02/16/25

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