Choosing transformers for RM58 compressor a la Record Plant

soapfoot said:

If there was ringing from mis-loading, is this something someone might hypothetically bodge on a Zobel network to tame?

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yeah but there won't be THAT much ringing, not in a 600:600

While we're here... does anyone have insight into the difference between the A20 and A21?

The A20 boasts wider bandwidth in the UTC catalog (10-50k compared to 30-20k) but seems to lack the higher magnetic shielding (relevant catalog bit attached).

However, it also has a >2x primary resistance. Does that hint at more turns and higher primary inductance, and therefore better (if still not ideal) for an input application?

response tells us it's higher inductance, resistance tells us it's a finer gauge of wire with more turns.  20 is also humbucking where 21 is probably not.  I think CJ has posted tear downs of both here.

What EMRR said. Plus, it's pretty clear to me (and I've always let this guide my use of these two transformer models) that the A-20 was designed to be a general problem solver. Very wide BW, a lot of lugs to allow for a wide variety of impedance configurations on each side, etc. Use it as an input, output, or matching transformer. Kind of a baby LS-33.

The A-21 on the other hand seems to have been made specifically as an input transformer. Extra shielding and terminal options limited to common mic to line scenarios (50, 200 or 500 (600) ohms). While you could still use it as an output in some limited cases, it seems clear to me that it wasn't intended for that use.

Thanks, I'm really starting to get a better grasp I think.

EmRR said:

I think CJ has posted tear downs of both here.

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You know, this crossed my mind, and I did a few searches! But I haven't found the right search terms yet.

rackmonkey said:

What EMRR said. Plus, it's pretty clear to me (and I've always let this guide my use of these two transformer models) that the A-20 was designed to be a general problem solver. Very wide BW, a lot of lugs to allow for a wide variety of impedance configurations on each side, etc. Use it as an input, output, or matching transformer. Kind of a baby LS-33.

The A-21 on the other hand seems to have been made specifically as an input transformer. Extra shielding and terminal options limited to common mic to line scenarios (50, 200 or 500 (600) ohms). While you could still use it as an output in some limited cases, it seems clear to me that it wasn't intended for that use.

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Very interesting, makes sense.

I just have to figure out my way forward now. This is obviously an "effect device" sort of compressor... most likely to be used to smash drums in parallel, destroy boring things to make them more interesting, etc.

The Raspberries notwithstanding, it's unlikely to be strapped across a mix bus anytime soon in our room--but it will be installed in a rack with many feet/meters of install cable to-and-from the patchbay, so leaving it unbalanced is not my aim. With that context:

I already have a pair of A21s laying around, so I'd like to use those if I can. It would be fairly easy for me to grab another pair, or a pair of A20s (much easier than acquiring a nice new 3575 10k:10k from Sowter--their lead times and shipping are untenable at the moment due to the COVID crisis, etc.)

I want it balanced for lowest noise, and could appreciate any added character from the transformers, but am not completely preoccupied with everything being "textbook." In all likelihood, "good enough for Record Plant Studio A, good enough for me."

So... A-21 on the input and A-20 on the output? Vice-versa? A-21s all around? Is there another affordable-ish classic (or new transformer) that I could get (without long lead times or shipping from EU which is just impossible at the moment) that would do as good or better job?