Output level attenuator for API 312 amp circuit

[ruffrecords]

Thanks you so much Ian.

So in this application is there any reason to use an expensive 600ohm T PAD over the 1K pot or practically they will both achieve the exactly same results?

All a 600 ohm T pad will do for you is ensure the load on the transformer stays close to 600 ohms no matter what its setting.  A 1K pot connected to a 10K bridging input will load the transformer with 909ohms when fully up and 1K when fully down. Most preamp's distortion will be load dependent so there may some small benefit in making the 1K pot look closer to a 600 ohm load. You can do this by adding a 1K8 resistor across the ends of the pot. Other than that the expensive T-pad has nothing going for it.

Cheers

Ian

 

[Whoops]

Thanks you so much Ian.

So in this application is there any reason to use an expensive 600ohm T PAD over the 1K pot or practically they will both achieve the exactly same results?

All a 600 ohm T pad will do for you is ensure the load on the transformer stays close to 600 ohms no matter what its setting.  A 1K pot connected to a 10K bridging input will load the transformer with 909ohms when fully up and 1K when fully down. Most preamp's distortion will be load dependent so there may some small benefit in making the 1K pot look closer to a 600 ohm load. You can do this by adding a 1K8 resistor across the ends of the pot. Other than that the expensive T-pad has nothing going for it.

Cheers

Ian

Thank you Ian

 

[Humner]

I like the look of this 12 stepped version. Do you think it is worth going with shorting or non-shorting switch? AML Audio have both 3 deck grayhill variants.

I'm planning on making a PCB referencing the same scheme as I'm keen for an attenuation box for myself and a friend.

If anyone else is interested let me know and I can add extra boards to be made.

Ive done three 6db steps after the output transformer with great success. just a bridged-t on a breadboard with a lorlin 4-position, 3-pole, rotary switch. 0, -6, -12, -18db. I used this schematic (thanks to whoever made this - I think it was someone on this forum):

also check out this link for basic info on pads - http://www.uneeda-audio.com/pads/

edit: I reposted this part in another thread:
On a related note - I've been thinking about a basic attenuator pcb that could be stuffed for any amount of attenuation in up to twelve steps (or maybe even 24) at whatever input and output impedance you chose and would fit a greyhill or lorlin pcb-mount switch. It would be a bridged-t attenuator. It seems like it would be really handy for a number of applications like this one or even a (way-less-cool-and-useful-than-Igor's) rudimentary monitor controller (obviously this would require a pair or a 2 deck switch wired to two boards) you could also experiment with driving your consoles two-buss a bit or whatever else. I'd like to have a rack of 6 or so attenuators that i could put in front of my converters to experiment with driving everything, just patch them in. chime in with your thoughts - i'll start a new thread if it gets cluttered. -Ben