Audio and Video Talk > Valves / Vacuum Tubes

Line Level Tube Preamp

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Mars:
Hi All, this is a follow to the thread done in October 2013.  See original here https://www.avforums.co.za/index.php/topic,28051.0.html

So what more can be said about the design? If you have some patience, then let's look at some circuit ideas.
Geco's suggestion in the original thread is more or less where we are now:



I have been wondering about the wisdom of putting the volume right in front of the circuit. It has the advantage of only one capacitor in the signal path, but the volume pot could be seen as a difficult load to drive.

Another way of doing things would be to place the volume pot after the first triode, followed by the cathode follower. The first triode could arguably be a better driver for the volume pot, and the cathode follower gives a low output impedance. Something like this:



This way the output of the source drives a triode directly, and the volume pot is buffered from source and amplifier. Could this work?







DeonC:
I like. I will be curious to hear the answers. :rubhands:

gavinbirss:
Karel,

It could be disastrous when slider disconnects from volume pot?

it would have to be a very reliable pot.

handsome:
The 'rule of thumb' is that input impedance should be at least 10x the output impedance of the device driving it plus your output impedance should not be so high that it causes a treble roll-off with the following device/stage's input capacitance, plus cable capacitance of course.

The output impedance of modern components is <1k (<100R most of the time) your volume pot is 100k so easy to drive. Input impedance of most modern devices is 10k, your volume pot is 10x that so easier to drive than most amp inputs. The output impedance of a grounded cathode 6SN7 with a bypassed cathode resistor is about 10k. That's a x100 to x1000 worse than driving directly from your CD player, streamer, what-have-you.

You could put your volume pot at the output of the circuit driven by the cathode follower, but then worst case output impedance (it varies with volume control position) is 25k. Although yo do get a noise advantage. But now you have lost the benefit of a low output impedance so not going to be great to drive long or capacitive cables. But your real problem here is your cathode follower; it now has a minute amount of bias - so you should grid bias it but this will add another capacitor to your signal path....

I would drop the value of the pot to 50k (small increase in bandwidth feeding the 6SN7 input capacitance) and leave it up front. Plus as Gavinbirss recommended run a 1Meg resistor from wiper to earth to mitigate any pot discontinuities (large thumps). Oh and unbypass the first 6SN7's cathode you don't need all that gain in a modern preamplifier.

1200GXman:
When will this be available in kit form and what will price estimate be?

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