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Maybe Nelson Pass can explain it

Posted by 1973shovel (A) on July 17, 2021 at 16:28:41

In Reply to: Tube Buffers ? posted by mark.korda@myfairpoint.net on July 17, 2021 at 12:32:06:

Everything below the line is Nelson Pass explaing, among other things, buffers, with regard to his B1 buffered preamp. Link to the full article at the bottom.

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"Is impedance matching an issue? Passive volume controls do have to make a trade-off between input impedance and output impedance. If the input impedance is high, making the input to the volume control easy for the source to drive, then the output impedance is also high, possibly creating difficulty with the input impedance of the power amplifier. And vice versa: If your amplifier prefers low source impedance, then your signal source might have to look at low impedance in the volume control.


This suggests the possibility of using a high quality buffer in conjunction with a volume control. A buffer is still an active circuit using tubes or transistors, but it has no voltage gain - it only interposes itself to make a low impedance into a high impedance, or vice versa.

If you put a buffer in front of a volume control, the control's low impedance looks like high impedance. If you put a buffer after a volume control, it makes the output impedance much lower. You can put buffers before and after a volume control if you want.

The thing here is to try to make a buffer that is very neutral. Given the simple task, it's pretty easy to construct simple buffers with very low distortion and noise and very wide bandwidth, all without negative feedback."

• First Watt B1   (Open in New Window)

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