SS Pultec OP transformer suggestions

[JW]

Hey folks,

I have a couple Solid State pultecs that I built (using a 2520 makeup amp) They have the Sowter Pultec input transformers.

I have Cinemag CMOQ-2S on the outputs, which are steel core Cinemags, cheap and pretty good, but I wonder if someone would recommend another 600:600 output? Not looking to spend the bucks on the original Triad, just want something that's perhaps a little mellower? The eq's (even in bypass) have just a tad too much presence, for lack of a better word.

Anybody ever try Carnhill on SS pultec outs?

Any older mojo model number recommendations? UTC, Stancor, Chicago?

 

[nielsk]

It may be the input... have you tried tuning out any ringing with a zobel?

 

[JW]

I don't have a zobel on anything no.

But I make sure and feed it a 600 ohm output from my board or the outs of my preamps. I can optionally select a 600 ohm termination switch on the pultec's output as well for feeding other stuff. It helps and I can definitely dial in a very good sound. I don't think what I'm hearing is transformer ringing. They just have a bit more presence/bite than I would like, compared to without them in the signal chain. I think this is just the last 5 percent or so of the sound that I'm wanting to dial in.

I don't think it's the Sowters. I think I've built the input well. It's just like the eqp-1A3 schematic, but w/ the sowter pultec input transformers (sowter recommended a 620 ohm resistor in parallel with the secondary as well, I believe, if I remember correctly, which I've included) I just make sure to feed the pultecs with a 600 ohm output.

The rest of the unit is the Gyraf filter board, Max (i/oaudio's) inductors specifically for the Gyraf filter, then the Eqp-1A3 circuit w/ 2520 opamp, and Cinemag CMOQ-2S output.

They sound great and I can totally live with them, just wanted a wee bit more softness/mellowness.
I would replace the 2520's w/ a Melcor 1731 but I mistakenly soldered them in there instead of waiting and getting the mounts to swap opamps out. Couldn't get the solder to melt and I figured I'd better quit heating those opamp pins so hot and just live with the 2520's, because after all, it's what the original used.

 

[MatthisD]

Have you tried running it without the transformers or with just one or the other to compare, transformerless input with the Sowter at the output could also be experimented with.

 

[mitsos]

I'm curious what makes you think it's the output transformer?  CJ once said those trafos are flat from DC - 1 MHZ.

Have you swept the EQs and if so are they acting normal?

FWIW, the SS pultecs did not use the HS56 as 600:600 on output, it was wired 250:600 with the secondary towards the opamp, and the primary to the outside (backwards).  Maybe try running the CMOQ in 1:2 and dialing down the opamp gain, see if that makes a difference.

Also, maybe try a different output cap (IMO, Elna Silmic has nicer mids than a nichicon KZ for example).

But first, sweep them if you haven't done so already.

cheers!

 

[ruairioflaherty]

But first, sweep them if you haven't done so already.

Definitely worth doing.  You may have an anomaly in the filter section that's giving you a presence rise.

 

[JW]

Thanks guys,

Interesting. Yeah, I haven't done a sweep. I don't have a scope. I suppose I could download some sort of sweep file and look at it on VU meters.

Now I can't remember how I wired up the Cinemag output. I think I may have actually wired it 1:2 because now I'm remembering my friend recommending that. I'll have to check that and report back.

The output cap is a cheapo Nichicon PW. Always meant to find a nicer one at some point.

 

[gemini86]

Just run it through Right Mark Audio Analyzer. (RMAA) Either that or run a white noise through it and look at the output with an FFT plugin, or download audacity, which has an FFT function.

 

[ruairioflaherty]

Interesting. Yeah, I haven't done a sweep. I don't have a scope. I suppose I could download some sort of sweep file and look at it on VU meters.

If you're on a Mac download the demo of Fuzzmeasure, it's perfect for quick and easy frequency response sweeps if you have a half decent DA/AD loop.  If you're on a PC RMAA gets the love as Gemini posted above.