Dual gang log pot as bridged t-attenuator?

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evil grill

Well-known member
Joined
Jul 15, 2004
Messages
128
Location
Sweden
Howdy!

Bought a PRR-176 Rev2 PCB and I want to skip the op-amps on the input and use an attenuator for input control. I have a large amount of LL1527 that I want to use if possible.  https://www.lundahltransformers.com/wp-content/uploads/datasheets/1527_7xl.pdf

I drew this attenuator up after studying some alternatives and the prices for rotary switches.
I know that it would be incorrect impedance at 0 dB but I believe the problems will be neglible. 

Would this work? If not, how can I make it work? 

 

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evil grill said:
Howdy!

Bought a PRR-176 Rev2 PCB and I want to skip the op-amps on the input and use an attenuator for input control. I have a large amount of LL1527 that I want to use if possible.  https://www.lundahltransformers.com/wp-content/uploads/datasheets/1527_7xl.pdf

I drew this attenuator up after studying some alternatives and the prices for rotary switches.
I know that it would be incorrect impedance at 0 dB but I believe the problems will be neglible. 

Would this work? If not, how can I make it work?
As I see it, it does only about 7.5dB of maximum attenuation.
 
abbey road d enfer said:
As I see it, it does only about 7.5dB of maximum attenuation.
Thanks, and embarrasing now that I see it too...
So would dual gang log potentiometers work in bridged t-attenuators or does the resistance vary too much?
Drew another version.
 

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evil grill said:
So would dual gang log potentiometers work in bridged t-attenuators or does the resistance vary too much?
I don't think it would. In order to function properly, a bridged-T attenuator requires the two variable resistors to vary differently, one with a log taper (actually exponential), the other with a reverse log taper.
Some manufacturers of such attenuators cheat by using linear taper pots, which results in variable input and output impedances.
IMO it doesn't matter much for most audio applications.
But what is the point of using a bridged T attenuator if it doesn't result in constant I/O Z?
I think you are making things too complicated. If you have these dual 5k "Log" pots, why don't you use them in a basic potentiometer configuration? I don't know what's the input impedance of the PRR176, but I think you can find a way of using these pots, either in balanced mode if the input Z is >5k, or in parallels mode if the input Z is lower.
 
abbey road d enfer said:
I don't think it would. In order to function properly, a bridged-T attenuator requires the two variable resistors to vary differently, one with a log taper (actually exponential), the other with a reverse log taper.
Some manufacturers of such attenuators cheat by using linear taper pots, which results in variable input and output impedances.
IMO it doesn't matter much for most audio applications.
But what is the point of using a bridged T attenuator if it doesn't result in constant I/O Z?
I think you are making things too complicated. If you have these dual 5k "Log" pots, why don't you use them in a basic potentiometer configuration? I don't know what's the input impedance of the PRR176, but I think you can find a way of using these pots, either in balanced mode if the input Z is >5k, or in parallels mode if the input Z is lower.

I have the transformers and they are 800 ohms. I have'nt got the potentiometers. My idea is that potentiometers are cheaper than rotary switches. Maybe there is no cheap way to do this.

Thanks for replying.

 
evil grill said:
I have the transformers and they are 800 ohms.
They're 800:800, right? What's the load on the secondary? I suspect it's connected to the grids.
Then why don't you put the potentiometer between the secondary and the grids? You could use a 2x50k Log pot.
Help me to help you with a piece of schemo.
 
abbey road d enfer said:
They're 800:800, right? What's the load on the secondary? I suspect it's connected to the grids.
Then why don't you put the potentiometer between the secondary and the grids? You could use a 2x50k Log pot.
Help me to help you with a piece of schemo.

Thanks but the other side of the transformer is also balanced. See schematic.
 

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Something like in the attached schematic should work (not PRR schematic).

I think I used a dual 50k pot (maybe) on the original PRR, but with resistors to ground to limit the attenuation range.
 

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Script said:
Something like in the attached schematic should work (not PRR schematic).

I think I used a dual 50k pot (maybe) on the original PRR, but with resistors to ground to limit the attenuation range.

This is a solution I can use. Very nice! Thanks!
 
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