Because I am impressed with the SILK (SAC Thailand) grid chokes in my Kaiju, I picked up a pair of SILK's supermalloy TVCs (transformer volume controls). The manufacturer's reported specs are -1 dB @ 10Hz and 35kHz referenced to 1KHz, and greater than 1,100H. Most folks used these things to build passive pre-amps (which I did this wknd - just to see). But I bought them to test whether they might sound different/better than a resistor-based attenuator. But before getting to that: a quick report on the results of making a simple passive preamp out of them - b/c it informs why I want an "easy" way of testing the TVCs with my BeePre 1.
The passive pre I put together is super simple: bascially a pair of rca inputs -> 23 step TVCs -> rca outputs. Nothing fancy. (I'll be testing it with XLR inputs and various switchable grounding configurations when I get the correct switches from Mouser this week).
I tried the passive pre this evening in place of the BP1 feeding my Kaiju. It actually sounds ok - vocals are very nice. But the bass lacks punch and doesn't go as deep, and the edges of everything sound a little too rounded over. It sounds "musical": pleasant, smooth, nothing to annoy. But nothing to excite either. No jump, attack, etc.
That's not a problem - to the contrary I'm pleased by the results: they confirm my already tested belief that the BP1 is pretty extraordinary. Plus, I never intended to replace the BP1 with a TVC-based passive pre (or, indeed any other thing, except maybe the BP2 - but I consider that a rejigged BP1). However, having read people rave about TVC and AVC passive preamps, and having the makings for one on hand, I figured I should throw one together and listen for myself before trying the TVCs as attenuators in the BP1.
Which brings me to where I am at now. I think the TVCs *might* hold some promise of sounding good as replacements for the BeeQuiet attenuators -- but I don't have enough faith that they will to justify pulling parts out of, and rewiring, my rebuilt BP1. So I'm looking for insight, conjecture or even wild guesses as to whether and how I can get away with testing the passive pre I built as an external replacement for the BeeQuiet attenuators. IOW: disconnect the BeeQuiet and have the BP rca input directly feed the grid of the 300B, with the TVC attenuator box sitting between my DAC and the BP. Not ideal, b/c the attenuator will have to drive 2 ft of interconnect cable instead of only a few inches of wire - so there will be some additional capacitance to drive.
What I haven't been able to wrap my head around is whether I need/want an input grid resistor that would load the TVCs and provide a purely resistive path to ground for any tiny amounts of grid current that might arise. My first thought was "Yes, obviously", but given the +1,100H inductance, I'm wondering whether I need a reflected load. That's a ton of inductance - nearly 70K of impedance at 10Hz, and tons more at higher frequencies. So I'm thinking of simply connecting one leg of the TVC secondary to the grid (but leaving the grid stopper resistor in place) and the other leg to 0V/chassis. Since the 7000H SILK grid chokes seem to do a good job of replacing the 300B grid resistors in my Kaiju, maybe the 1100H is enough at the input of the BP?
What do people think? Any risk of harm to the gear in doing this?
I'd also try loading the TVCs via a grid resistor as well. And I'd appreciate suggestions as to values. Since SILK reports that the TVCs have "very low output impedance", maybe I should try lower load values - e.g., maybe as low as 25K or 10K?
As always, all thoughts, speculations, etc. welcome.
Edit: And I should add that if the TVC box sounds good as a replacement for the BeeQuiet, I'd proceed to try them properly installed in the BP.
MTIA, Derek