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Old 13th April 2012, 06:19 PM   #1
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Default Tone controls

Hello me again,

I have a 3 pole 4 position rotary switch and a 470k pot, is there anyone out there that might know a good way to get a variable mid control (maybe bass and treble cut), with the rotary switch changing the frequency range, and the pot adjusting the amount? Ive been playing with some different arrangements but cant seem to find anything but a low pass/ high pass, or is this not possible? I als have a 470k dual gang pot but it is needed for another project, also i would rather avoid inductors.

All help would be greatly appreciated!
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Old 13th April 2012, 09:03 PM   #2
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You could make a version of this (attached). Use the four pole to select one of four different gyrators, and the 470K pot for the adjustment. You may have to parallel the pot with a smaller resistor to bring the total resistance to 10K.

This basically allows you to use a series resonant LC circuit as a frequency selective suck-out filter. By connecting the pot between the feedback node and the input node, you get boost/cut performance, with the mid-point of the pot a null.

This avoids the actual use of wire-wound coils, as the active elements "fake" the most critical property of inductance: opposing changes of current. These "gyrators" perform like an actual series resosnant network made from a capacitor and wire-wound inductor.
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Old 13th April 2012, 09:48 PM   #3
Simon B is offline Simon B  United Kingdom
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Ah! A valve gyrator 3 band EQ!

I think you migh have to actually buy some components to achieve what you're wanting, though I sympathise entirely with wanting to avoid that....

Having four gyrators would certainly be one way of doing it, though you'd only need to use one of the poles of your switch to select which one, and it would mean that you had four triodes running, only one of which would be in use at any given time.

The circuit Miles has put up looks like a good starting point to me, but I do note that the centre frequencies specified are rather extreme - 28Hz, 2KHz, 20KHz ! Not specified is their 'Q', aka bandwidth. On the downside, it needs a -150v supply - hmmm......

As Miles says, each of these gyrator sections acts like a series LCR filter. The cap connected to the pot wiper is the 'C', the other cap and the resistors determine the 'R' and 'L'

Given that you've got a 3 pole switch, and that the basic circuit needs 3 elements switched....if you can decide what shape characteristics you want and you like playing with equations you might be able to get away with one triode ......btw, see Gyrator - Wikipedia, the free encyclopedia if you're not familiar with the principle.

Alternatively, and perhaps less expensively, you could use the upper part of Miles circuit, the boost/cut section, replacing the gyrators with passive RC sections to ground.
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Old 13th April 2012, 11:50 PM   #4
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Hmm looking at it i may need inductors, not enough space for more valves or any SS parts on the power supply...
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Old 14th April 2012, 05:56 AM   #5
nazaroo is offline nazaroo  Canada
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Quote:
Originally Posted by Miles Prower View Post
You could make a version of this (attached). Use the four pole to select one of four different gyrators, and the 470K pot for the adjustment. You may have to parallel the pot with a smaller resistor to bring the total resistance to 10K.

This basically allows you to use a series resonant LC circuit as a frequency selective suck-out filter. By connecting the pot between the feedback node and the input node, you get boost/cut performance, with the mid-point of the pot a null.

This avoids the actual use of wire-wound coils, as the active elements "fake" the most critical property of inductance: opposing changes of current. These "gyrators" perform like an actual series resosnant network made from a capacitor and wire-wound inductor.
I don't understand how the tubes, especially the first,
are properly biased. I'm thinking a 12AX7 only needs a few volts of bias, but if the grid is at "ground", it looks more like +150!

What am I doing wrong, reading this circuit?
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Old 14th April 2012, 09:36 AM   #6
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if you look at the -150v supply as if it is ground, make the +150v 300v, then put 2 resistors (HT to grid and grid to ground) and you have a simple ac coupled cathode follower.
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Old 14th April 2012, 01:51 PM   #7
Simon B is offline Simon B  United Kingdom
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Quote:
Originally Posted by nazaroo View Post
I don't understand how the tubes, especially the first,
are properly biased. I'm thinking a 12AX7 only needs a few volts of bias, but if the grid is at "ground", it looks more like +150!

What am I doing wrong, reading this circuit?


The first pair of triodes are wired as a differential pair, a common sub-circuit in both tube and solid-state design.

A positive input causes current in the left triode to increase and it's cathode voltage rises. This reduces current in the right triode and thus its anode voltage rises. Signals on the right triode grid work the other way round.

The 110k resistor connecting the two cathodes to -150v works as a (fairly) constant current source. In action, a bit more than 150v will be dropped across it, so the two cathodes will be a few volts +ve wrt ground, and thus their grids. The triodes allow sufficient current to flow to bias themselves.

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