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 Post subject: Re: Baxandall circuit
PostPosted: 29 May 2015, 02:34 
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Suncalc wrote:
....place a 10MΩ resistor in the defeat line between the poles of the switch.


Might want to add a freq comp cap across it, lest Mr. Miller step in and clean out the highs ;)

Cheers!

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 Post subject: Re: Baxandall circuit
PostPosted: 29 May 2015, 07:11 
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Geek wrote:
Might want to add a freq comp cap across it, lest Mr. Miller step in and clean out the highs
Yup. My bad! :blush:

Don't do that. I'll come up with a more reasonable gain compensation scheme.

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 Post subject: Re: Baxandall circuit
PostPosted: 29 May 2015, 08:37 
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What value would the cap be?

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 Post subject: Re: Baxandall circuit
PostPosted: 29 May 2015, 11:23 
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I'm very sorri if this is a silly question but is this circuit a passive or active type tone control. I have never built one using tubes but i've made plenty using IC's.

I would like to install it into my headphone amplifier.


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 Post subject: Re: Baxandall circuit
PostPosted: 29 May 2015, 16:38 
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Just a thought. With relation to the defeat switch what if it also lifts the bypass cap, Ck, from the second stage thus lowering the gain. When the defeat is operated it open ccts. Ck on the second tube. Therefore no 10M resistor required with associated compensation cap.

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 Post subject: Re: Baxandall circuit
PostPosted: 29 May 2015, 18:22 
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mwhouston wrote:
With relation to the defeat switch what if it also lifts the bypass cap, Ck, from the second stage thus lowering the gain. When the defeat is operated it open ccts. Ck on the second tube. Therefore no 10M resistor required with associated compensation cap.
Basically lifting Ck will only give about 6dBv of attenuation; not nearly enough. I dislike the compensation cap because it messes with the mid band phase shift.

I have a simple -21dBv resistive pad option that I've worked out but I need to run a simulation to validate my calculations at the upper rolloff frequency. Give me a day and I'll post what I have.

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 Post subject: Re: Baxandall circuit
PostPosted: 29 May 2015, 18:32 
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philip davies wrote:
... is this circuit a passive or active type tone control.
It depends what you mean by "active". This circuit has no bandpass compensation in either tube stage. As such, I would classify the tone control part as "passive".

However, the circuit has three parts: an active buffer on the input, a passive tone control circuit (James-Baxandall type), and a 4S universal preamp on the output. The input assures a very high input impedance and isolates the tone circuit from any input loading, the passive tone control is well balanced with a 1000Hz primary pole position with a nominal 21dBv of attenuation, and the 4S provides a way to make up the lost signal so input level can be made essentially the same as output level (or significantly greater with the right tube in the 4S).

Does this answer your question?

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 Post subject: Re: Baxandall circuit
PostPosted: 29 May 2015, 19:00 
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I should have explained myself better.

What I mean by "active" is the ability to cut and boost the bass and treble frequencies.

I do think you have answered my question. I will have to post up a schematic that I had found which was labeled as an active tone controll circuit.


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 Post subject: Re: Baxandall circuit
PostPosted: 29 May 2015, 20:02 
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Matt, yes -6db loss of gain is far less than 20db of gain or the difference there of.

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 Post subject: Re: Baxandall circuit
PostPosted: 29 May 2015, 23:24 
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Suncalc wrote:
Geek wrote:
Might want to add a freq comp cap across it, lest Mr. Miller step in and clean out the highs
Yup. My bad! :blush:

Don't do that. I'll come up with a more reasonable gain compensation scheme.


I've just used a voltage divider around 100K in impedance :)

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