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 Post subject: Volume control position
PostPosted: 16 Sep 2009, 12:30 
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Joined: 11 Sep 2009, 04:24
Posts: 9
Location: Portugal
Hello,
I started to look at tube amp schematics, reading some stuff and trying to understand the magic beneath the hood.
One of the thinks that I could not understand was the volume control positioning. Some circuits have it on the input and others after the preamp.
Does it make a difference? Which one is better?

I don't even know if this is a pertinent question. :)
Regards,
TVeiga.


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PostPosted: 16 Sep 2009, 14:51 
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Joined: 02 Mar 2009, 12:41
Posts: 1099
Location: Vänersborg, Sweden
I'd say that we try hard to obtain low OP impedance (So You think You Can Impedance) from borth power AND control (pre) amplifiers. But you sort of mess with that good intention by hooking up a potentiometer on the output of an amplifier, so ...

... either let another cascode follow the volume control. With cascode I mead a cathode- or an anode follower or whatever.

.... skip the control (pre) amplifier and add enough inputs to your power amp. Most amps have enough gain to allow direct connect to a standard music source.

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“If you don't know where you are going, any road will get you there.”
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PostPosted: 17 Sep 2009, 04:38 
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Joined: 11 Sep 2009, 04:24
Posts: 9
Location: Portugal
Ok, as I see the volume control can be in too places.
- A pot in ha place of the grid-leak resistor (I'm thinking of a triode preamp in common cathode)
- Or a pot after the preamp stage and before the pentode power gain stage.
I think the output impedance is not a problem, because input impedance of the pentode is higher, nevertheless I was thinking of using a cathode follower stage to lower the impedance.

For my point of view the first option is the best one.
The capacitor in the preamp output and the resistor form a high pass filter, so using the second option, the resistance value will change thus changing the cut frequency of the filter.

But this is all theoretically speaking. In reality does it make a difference in sound quality?


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PostPosted: 17 Sep 2009, 05:28 
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Joined: 02 Mar 2009, 12:41
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Location: Vänersborg, Sweden
My concern was that a potentiometer on the output could possible sometime be followed by an amplifier with low Rin (tr@n$i$t0r) ...

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“If you don't know where you are going, any road will get you there.”
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