Cheaper T pad?

[JW]

Is the Bourns 51 series 600 ohm T pad the only game in town? Imagine you were doing a 4 channel API with these. Like I am. That's around$100 just for that part. 1 T pad = more than the power transformer. Thanks Antek @$17.00

Anyway, anyone come across an alternative part? Or is it just too custom?

BTW, this part, the Bourns 51 T pad for the 1176 input I noticed went scratchy very quickly. I was like, cool I have a "vintage" 1176 now. Not. It's got new caps, I dunno. Maybe I should measure for DC on it or something. . . .

 

[shabtek]

One k log pot works in a pinch

 

[Rob Flinn]

If you must have a t pad speak to brett at blore edwards.  He has cheaper ones.

 

[abbey road d enfer]

Anyway, anyone come across an alternative part? Or is it just too custom?

the only notable feature ofa T-pad is that its input and output impedances are constant, so they can be inserted between lines that nned to be matched. It's a sequel of the POTS, where lines of several hundred miles had to be perfectly matched.
When audio equipment was designed by telephone engineers, the practice persisted, although only the side that connected to the world needed to match.
A long time ago matching was abandoned at the profit of bridging, where a low impedance source meets a higher (>10x) impedance receiver.
There is no objective reason for using a T-pad in a 1176, except vintage-correctness.
Shabtek's suggestion is wise.

 

[mjrippe]

I believe group member Avdo was making them for a reasonable price.

 

[emrr]

I don't know that any of them these days are a true T, none disconnect the shunt at full on so there's some loading still, but close enough for most things. 

Several other reports of going scratchy in 1176's, I think almost always shown to be DC on the pot, not a problem with the pot. 

 

[Whoops]

...Removed my post because I'm not supporting any Russian products or companies due to the recent aggression in Ukraine

 

[JW]

Ah nice. Thanks for that!

 

[JW]

Bumping this one again as avd audio doesn't stock these anymore and the Bourns pots are so expensive. Gotta be a better alternative. Probably find a nice 6 pos. switch from Mouser and a handful of resistors for around 6 bucks. What about switched output attenuation? Maintaining the same in/out impedance (Tpad) like these pots? Where should I look for a schemo?

P.S. I'm assuming the only reason for a T pad as output attenuator is to include the output transformer in the "juicing up" Otherwise seems a 1K trimmer on the output of 2520 opamp is a very cheap alternative.

 

[abbey road d enfer]

I'm assuming the only reason for a T pad as output attenuator is to include the output transformer in the "juicing up" Otherwise seems a 1K trimmer on the output of 2520 opamp is a very cheap alternative.

It's really a bad idea driving a transformer from a variable resistor, since it results in poor LF response and excessive LF distortion.
However, using a simple pot at the output often produces good results. Typically, if the piece is designed for about 600r load and the actual load is 10-20k, a 1k pot works quite well.
Constant-impedance attenuators were a requisite for impedance-matching systems. Now, almost everyone agrees on impedance-bridging (low-Z output driving a high-Z inpuy).

 

[JW]

Well, okay, so it'd be a dual 1k pot on the output then, for balanced attenuation. But then I guess we're wondering do the two potentiometer tracks line up?

 

[Whoops]

Well, okay, so it'd be a dual 1k pot on the output then, for balanced attenuation. But then I guess we're wondering do the two potentiometer tracks line up?

It's not a dual Pot, it's a single 1K LOG pot.
You do it like this:

https://groupdiy.com/threads/output-level-attenuator-for-api-312-amp-circuit.31472/post-385293

 

[sonicmess]

I have some available in the white market.
https://groupdiy.com/threads/600-ohm-t-pad-attenuators-in-stock-europe.80050/

 

[Rob Flinn]

It's not a dual Pot, it's a single 1K LOG pot.
You do it like this:

https://groupdiy.com/threads/output-level-attenuator-for-api-312-amp-circuit.31472/post-385293

That's not a good way to do it, because when you have the pot turned completely down the output is driving into a short. You need to feed the pot with a pair of 300R resistors, one from either side of the output transformer. That way when the pot is turned all the way down the output is still driving 600R (or whatever resistance you choose).

 

[Whoops]

That's not a good way to do it, because when you have the pot turned completely down the output is driving into a short. You need to feed the pot with a pair of 300R resistors, one from either side of the output transformer. That way when the pot is turned all the way down the output is still driving 600R (or whatever resistance you choose).

well,
it seems to be done like this in API 312 preamps...

 

[Rob Flinn]

well,
this seems to be done like this in API 312 preamps...

I won't be copying them ...

 

[abbey road d enfer]

That's not a good way to do it, because when you have the pot turned completely down the output is driving into a short.

Wrong. Think it over.

 

[Rob Flinn]

Wrong. Think it over.

Gotcha, misread diagram. I still won't be copying them though.

 

[Rob Flinn]

I usually use this method

 

[innercityman]

Nice thread ! Need a pair of 600OHM T-Pad for my dual REDD47 preamp.