evil grill

Dual gang log pot as bridged t-attenuator?
« on: May 10, 2020, 10:29:50 AM »
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? 



abbey road d enfer

Re: Dual gang log pot as bridged t-attenuator?
« Reply #1 on: May 10, 2020, 02:44:34 PM »
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.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

evil grill

Re: Dual gang log pot as bridged t-attenuator?
« Reply #2 on: May 11, 2020, 06:48:05 AM »
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.
« Last Edit: May 11, 2020, 06:52:19 AM by evil grill »

ruffrecords

Re: Dual gang log pot as bridged t-attenuator?
« Reply #3 on: May 11, 2020, 07:06:13 AM »
If you put different attenuation values into this calculator it will tell you the values of resistor needed . That way you can see what sort of the law the pots will need.

https://www.allaboutcircuits.com/tools/bridged-tee-attenuator-calculator/

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

abbey road d enfer

Re: Dual gang log pot as bridged t-attenuator?
« Reply #4 on: May 11, 2020, 07:23:56 AM »
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.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

evil grill

Re: Dual gang log pot as bridged t-attenuator?
« Reply #5 on: May 11, 2020, 08:03:42 AM »
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

Re: Dual gang log pot as bridged t-attenuator?
« Reply #6 on: May 11, 2020, 08:05:31 AM »
If you put different attenuation values into this calculator it will tell you the values of resistor needed . That way you can see what sort of the law the pots will need.

https://www.allaboutcircuits.com/tools/bridged-tee-attenuator-calculator/

Cheers

Ian

Thanks! Will look into it.

abbey road d enfer

Re: Dual gang log pot as bridged t-attenuator?
« Reply #7 on: May 11, 2020, 08:17:51 AM »
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.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

evil grill

Re: Dual gang log pot as bridged t-attenuator?
« Reply #8 on: May 11, 2020, 08:38:11 AM »
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.

Script

Re: Dual gang log pot as bridged t-attenuator?
« Reply #9 on: May 11, 2020, 09:18:01 AM »
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.


evil grill

Re: Dual gang log pot as bridged t-attenuator?
« Reply #10 on: May 11, 2020, 09:34:34 AM »
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!

Script

Re: Dual gang log pot as bridged t-attenuator?
« Reply #11 on: May 11, 2020, 11:03:10 AM »
The thanks don't go to me...

abbey road d enfer

Re: Dual gang log pot as bridged t-attenuator?
« Reply #12 on: May 11, 2020, 11:50:40 AM »
Thanks but the other side of the transformer is also balanced. See schematic.
And then? A dual pot can take care of both sides. Look how it's done in the Fairchild 670.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

evil grill

Re: Dual gang log pot as bridged t-attenuator?
« Reply #13 on: May 11, 2020, 03:51:56 PM »
And then? A dual pot can take care of both sides. Look how it's done in the Fairchild 670.

Yes. Thanks!


 

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