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Old 13th June 2009, 02:51 AM   #1
rman is offline rman  Canada
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Default Interesting circuit, filter question.

Hi.

I found this rather interesting tone circuit. It looks to me like the filters are each two first order filters cascading into each other, not Butterworth alignments or some such. My filter theory is pretty weak, I just don't have the math skills, but here is my guess how this works. Two simple rc filters with the same corner frequency, but the impedance scaling somehow keeps them from throwing each other out of whack. Is this right?

Kind of neat isn't it? With some clever design of the control knobs, you could have volume and tone in one. Turning both together would make a volume control, while separately they would form the bass and treble. Also gain and tone stage all in one. with a bypass cap on the second stage to lower output impedance, maybe a complete line stage?
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Old 13th June 2009, 04:14 PM   #2
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There are a few things I do not like about this circuit.

I dont like the output of the high pass filter shorted to the output of the low pass filter at the grid of the output tube. My intuition tells me they should be isolated withe a couple of 100K resistors.

With 100K plate resistors the 6SN7s wont have much current through them, I think 30 K or thereabouts would be better.

But the main problem is the unequal values of the treble and the bass pots. With the values as shown the treble filter will be putting out 20 or 30 times the voltage of the bass filter. You wont get a flat response except at the extreme range of the pots.
May be the original designer did not want a flat response ?

It would be better to use equal values of pots, say 10K or 20K each for a 6SN7, then the input is like a concertina phase splitter. Equal outputs for equal pot positions, which are then filtered and summed in the output tube.
As you say, move the pots together to adjust gain, move seperately to shape the frequency response.
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Old 13th June 2009, 09:34 PM   #3
rman is offline rman  Canada
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Hi Robert.

I totally agree with your assessment. This circuit definitely needs some tweaking! Another improvement might be a higher corner frequency for the treble, and a band pass filter for the midrange. I might just whip up a mono version with a 6dj8 or a 12bh7 which I have laying around.

Cheers.
Rolf.
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Old 17th June 2009, 11:51 AM   #4
Merlinb is offline Merlinb  United Kingdom
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That circuit is straight out of RDH4! http://home.comcast.net/%7Estphkeri/tone_control.html
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Old 17th June 2009, 11:57 AM   #5
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... and with the treble and bass controls set to minimum, there will be no output at all.
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Old 17th June 2009, 11:25 PM   #6
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Quote:
That circuit is straight out of RDH4!
I took a look at RDH4 page 665, and the schematic there shows the Treble pot as 1k ohm, not 100K ohm as shown in the website.
This makes sense, if one pot was 100 times the other you would never be able to adjust for flat response.
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Old 18th June 2009, 02:41 AM   #7
rman is offline rman  Canada
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Merlinb,

The link you attached is indeed where I found this circuit.

Frank,

Quote:
.. and with the treble and bass controls set to minimum, there will be no output at all.
I was thinking this quirk could be used to advantage combining volume and tone controls in one stage with gain.

As I have some 6dj8's, I was thinking 15k pots, and 250 volt B+ without the extra Filter just to try it out. I don't have enough parts in my junk box so this will have to wait a while. Finding decent pots for a keeper version, should it come to that might be the hard part. stepped attenteunators maybe?

Rolf.
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