I've been toying with the idea of building one 1176 with switchable topologies in certain parts of the circuit (I/P = Xformer vs. IC, O/P = Class A vs. Class AB, and maybe even Zener vs. 7824 for 30V regulation). Maybe this has been done before, but I wasn't able to find anything on the board here.

I'm trying to think of a possibility to use _one_ I/P control for both topolgies (Xformer / IC). The problem obviously is that the values for the respective pots are quite different (and probably the tapers as well): 600R double pot used in a T config. before the Xformer, as opposed to a single 10K shunt after the IC stage. Which would mean that you would have to have two I/P controls, one for each topology... would look a bit odd, you couldn't use the standard frontpanel layout etc. etc.
Now if I took that 3-deck I/P pot as offered by Hairball (500R/500R/1K), used the two 500R decks for the T attenuator in the Xformer stage and used the 3rd deck (1K) as the shunt after the IC input stage, would that work?

- The original shunt behind the IC's is a 10K pot, forming a pad with 27K in series with the following FET that performs the gain reduction. I figue that 5532 wouldn't have a problem driving just 1K, but maybe it would affect the characteristic of the gain reduction? On the other hand, in the original Xformer I/P, the secondary is terminated with just 270R, and then the 27K and FET in series...
- What about the tapers of that triple pot? Would the two 500R-decks (linear) match the taper of the original T600 attenuator if used in the same configuration (incl. 2 fixed resistors, forming a bridget T)?
- and would the remaining 1K deck ("CW modified log taper") be right at all? If the "CW" indication that the changes in value get less rapid as the pot is turned clockwise, I would be in the ballpark... but if it's just the other way round, it wouldn't really be usable as I/P control...

While I'm at it, here's another question regarding sonics of the different original/replica Xformer models that may be used for each topology:
- in what way do the Magnetika O-12 and Cinemag 3837-1 differ from the original UTC O-12, regarding construction (core material etc.) and resulting sonics?
- and what about the versions of the 5002 O/P Xformer by UA, Ed Anderson and Cinemag? Also, for the Class AB-Stage (B11148 Xformer), are there different properties with different manufacturers?

Thanks a bunch, my appreciation in advance for any comments!


one way to do what you're talking about would be with a very custom 4 deck pot, basically the current t-pad available from hairball with an extra deck on it to use as the 10k shunt after the IC input.  you probably aren't going to be able to get such a pot.

i don't think you understand that the t-pad sold at hairball uses all three decks to function correctly.  it is not the same as the two deck pots (which are not 600 ohm pots, they use 600 ohm FIXED RESISTORS to set the impedance) which have two very unusual and dissimilar taper decks.

i understand that you want some options, and maybe for your purposes, having the high impedance IC input AND the 600 ohm transformer input would be useful.  but it will be difficult to implement.  maybe you could use the t-pad for both, but you'd need a lot of switching to make that work; could probably be done with a handful of relays, but i'm not sure it is worth it.

as far as the C-3837, it uses the same core material, turns, winding, etc. as the original utc O-12... that's the point of making the transformer in the first place.  it was originally a project i did for purple audio so that they could stop having to buy vintage utc O-12s.  i'm reasonably sure the magnetika O-12 is a good copy of the utc part, since they own all of the utc files.

the 5002s made by cinemag, UA (they've had a few different versions by different winders), and the version i make are all slightly different in terms of how they are wound.  the materials are all the same, though i suppose it is possible that they are treated differently by different manufacturers.

the b11148s that i make are copies of originals (which were made by a few different companies over the years), but i'm reasonably sure that i'm actually using the same transformer company as UA.

good luck,



Hi Ed, thanks for your thorough answer.

i don't think you understand that the t-pad sold at hairball uses all three decks to function correctly.  it is not the same as the two deck pots (which are not 600 ohm pots, they use 600 ohm FIXED RESISTORS to set the impedance) which have two very unusual and dissimilar taper decks.

Thanks for clearing that up, in the back of my mind I was thinking that I might be missing something...!
So what's the story with the original double deck, which pot values and tapers are in there?
About the 3-deck pot, searched the thread but haven't come across any wiring info really. Could make an educated guess but would prefer the real info...!

So if I were to go with the 3-deck pots and maybe some overkill relay switching  ;D , would that 1K deck be good for the IC-stage I/P control? And would the taper and sense of rotation be good for that in the first place?
Or (when I'll be given the lowdown about the Hairball switch) I might decide to use the whole T-Pad for the IC input as well.

Has anyone done a PSU with the original Zener regulation? Chances are that the audio circuit might respond differently to the passive regulation (at least when sourcing a lot of current, as for high signal levels)...? Or am I splitting hairs here.

THanks again!


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