Replacement Transformer for Studer A80VU repro preamp

The preamp cards I have for my 1/2" A80VU are transformerless. The cards used for 1/4" playback have an input transformer. I think I may be hearing some rectified RF/EMI in the 1/2" machine. I'd like to try the preamp cards with the input transformer to see if it helps.

The input transformers are  a 1:10 stepup. That is too much gain for the 1/2"heads. I'd like to find a 1:1 or a 1:2 transformer to try.  Ideally I'd like to find a transformer in the same form factor.

Here are some of the vital statistics as measured with a Peak LCR meter. Any help in sourcing suitable transformers would be greatly appreciated.

Ratio 1:10
Pin Spacing = 2.54mm
Primary:
DCR= 526Ω
L= 27.13H
Impedance at 1K= 187.2k +j 1.169

The Secondary is centertapped. Measurements are the whole coil
DCR = 22.9Ω
L=313.6mH
Impedance at 1K = 2.10k + j 1.169k

Why not look at the way the wiring is done first? The normal trick is just twisting the two wires from the head, maybe some shielding on those wire would help? Does the machine produce noise with the heads off? Put a short on the wires and see if its still there. You could add small chokes in series at the head. Lots of things to try.

I don't hear any more noise or excessive RF/EMI than the 1/4' machine. I think more is getting rectified into the audio and contributing to the edginess I hear in the 1/2" machine compared to the 1/4" machine. The main difference is the preamp card.

I could post the schematic. I don't know if the 1/2" head will saturate the transformer. It may be possible to reduce the gain of the card.

Thinking it is one thing, measuring it is another. The heads are very unlikely to saturate the transformers, signals there are very low level. Dont forget the tape eq following is high gain at low frequencies, so not much level comes off the heads at lf.
If the card is a direct swap I would try it.
Or maybe the transformerless cards are too clean for your taste?
This is a Studer, the engineering is to a very high standard. Are you in a strong rf field?

radardoug said:

Thinking it is one thing, measuring it is another. The heads are very unlikely to saturate the transformers, signals there are very low level. Dont forget the tape eq following is high gain at low frequencies, so not much level comes off the heads at lf.
If the card is a direct swap I would try it.
Or maybe the transformerless cards are too clean for your taste?
This is a Studer, the engineering is to a very high standard. Are you in a strong rf field?

Click to expand...

Since CMRR is frequency dependent and intermittent I don't think it's really possible to measure it. Too much of a moving target.

If the heads are unlikely to saturate the transformer then if I reduce gain by 20 - 30dB I should be good.

The card is a direct swap. I need the machine to work everyday so I can't do too many experiments.

I generally like squeaky clean. I like IC op amps. I don't think that's it. I'm in Brooklyn NY.  Right down the block from Transmitter  Park. It's heinous RF around here.

maybe like a Neve/Carnhill line or mic in,

Is that the 1.081.652 pcb? That is single ended preamp. So no advantage from balancing. Why not just try the transformer cards? The 1/2 inch will have no more than 6 dB extra level, should be well within the adjustment range.

The 1/2" machine  has 1.080.652 transformerless preamp cards with the 1.317.205 heads.

The 1/4"machine  has 1.080.804.11 preamp cards with transformer with the 1.317.125 heads.

There must be much more gain on the 1.080.804.11 cards because there is way too much gain for the 1/2" heads.

I think you will find one is a low impedance head, the other is a high impedance head. The inductances are markedly different.
The transformer preamp has a 1:10 stepup transformer. Thats where the different gain comes from, plus I would think the frequency response will be drastically different, because of matching. I would say look at what is getting in on the head wiring.
Because the 1/2 inch heads are probably high impedance, the wiring at the head is where stuff might be getting in. The first thing to check is what gets in with the headstack removed, and either short or open at the head connector. But I would be looking to replace the standard twisted wiring with shielded wiring. You can strip the sheilding of standard cable and fit it to the cabling. It will be fiddly but is possible. Or try wrapping aluminium foil around the existing wiring. This would be the better option, if you have a lot of vhf rf in your area.  Another option is small chokes, preferably bifilar, fitted to the wiring. Or a Faraday cage around the machine.

Another thing, there is no capacitor right on the front end of the transformerless card. You could experiment with 100 pF caps right on the card input terminals.

I'll take a peek at the wiring. I have braided shield of a few different sizes I can use if the run isn't shielded. I could try 100pf at the input without too much disruption.

The cable from the head plug down is shielded, its just the bit up by the heads that is not. Perfect 100MHz antenna! Try the 100 pFs, you could go a bit higher but not much, check the repro hf response after fitting.