Tube buffered transformer volume control (TVC)

[adason]

When i was going through attic stuff, i found musical fidelity buffer, don't remember much more, they called it missing link. Anyway, its a tube buffer, just like aikido. So i tested it by rightmark. Works fine, 0.008% distortion at 1Vpkpk, flat response, all good. I plugged small radio shack mic step up transformer on output, an got the same terrible results as with aikido buffer.
Rolling off low frequencies, plus distortion.

 

[adason]

So i used dats to measure resistance on input (600 ohm side). Obviously, at 1kHz it measures close to 1k ohm, but it progressively drops resistance towards low frequencies. Reaching 50 ohm and lower. That is too low for poor tube buffers to drive. Even if i would increase cap on outputs of tube buffers. Which i have not done so far. Solid state, i use opa604 for now, of acp+, to drive the step up tranformer, has no issue.
Anyone driving these transformers should keep this in mind. Here is what i got from dats. Not pretty.

 

[adason]

Ed,

Do you mind doing an additional experiment when time allows? 🤓

With your Edcor XS4400, connect it conventionally:

1. Connect the input signal to Pin 1 (+) and Pin 3 (-), the primary of the transformer.
2. Eliminate the connection from Pin 1 to Pin 12.
3. Connect Pin 6 to Pin 7, and Pin 9 to Pin 10 (i.e. series connect the secondaries).
4. Make Output Pin 4(+) and Pin 12 (-).

I’m curious with regards to the gain…I believe the turns ratio would be 1:3 and also see if the frequency response is flat or not. It bothers me that you said “Its no problem for me, since i use eq to flat in listening position”! That doesn’t eliminate the problem that we see, i.e. a transformer [as previously wired] that is -10dB at 20khz! 😉

BTW, are you loading these transformers with anything but the input impedance of the soundcard? What is the input impedance of the soundcard?

Best,
Anand.

P.S. I would be very curious how this Edcor transformer would perform in an M2 amplifier_it is available in a 1:5 turns ratio.

Received that MX5 mic step up transformers, 1:5 ratio. Briefly measured 1Vpkpk out to 14Vpkpk out. Distortion ~0.01%, all the way, not bad. Flat fr response, just a dB down at low fr. Don't have time to post data, nor listen. But looks like great replacement for M2 edcor.

 

[adason]

Since that big edcor xs4400 showed high fr rolloff, I improvized and used it in very simple crossover for low pass filter.
Btw, i have thread 'simple active crossover' somewhere, but this is even simpler.
Volume pot, then just B1 buffer, with two outputs, small cap and resistor as CR upper pass filter. Then normal big cap on output to block dc from transformer, arranged as step up to boost the low level signal, then RC as low pass. Works fine as shown. It did not work when i put RC before the xs4400.
Low pass is still going to plate amps, so the filter is adequate. Upper pass does not need anything steeper.
Now to finish it into the box. Transformers need shielding too.
Attached schematics has B1 simplified, there are some gate resistors...
Cheers.