[Steve Eddy]

source is a benchmark dac1

By the way, was just looking at the specs for the DAC1. Says its maximum output level (0dB FS) is +29dBu. That's not 12.28 volts, that's 21.83 volts.

It has various attenuation ragnes, and if you're thinking of using a TVC, what I would do is set the DAC1's attenuation so that you're feeding the TVC with the lowest level you can while still getting sufficient volume at the higher volume settings of the TVC.

se

[choariwap]

thats what the guy at benchmark replied to me when i asked what the unattenuated voltage is 

anyway, this means the dac1 will feed too much power into the tvc unattenuated... some folks say the SQ is degraded when using the attenuators on this thing.

[Steve Eddy]

thats what the guy at benchmark replied to me when i asked what the unattenuated voltage is 

Dunno. Their website says +29dBu. dBu is basically the voltage equialent of dBm, which is referenced to 1mW into 600 ohms which comes to 0.7746 volts, so dBu is referenced to 0.7746 volts. 21.83/0.7746 is 28.18 volts. 20 x log(28.18) comes to 28.999 or, essentially 29dBu. So the math checks out. Either the specs on the website or the guy who replied to you is wrong. 

anyway, this means the dac1 will feed too much power into the tvc unattenuated...

Pretty much, yup. Don't know what the TVC in the Prometheus is rated at, but S&B recommends no more than +20dBu (7.75V) above 40Hz and no more than +14dBu (3.875V) at 20Hz for their TX-102.

some folks say the SQ is degraded when using the attenuators on this thing.

Yeah, but were these folks feeding their DAC1s into TVCs?

With a resistive attenuator it's not a problem. With magnetic devices, whether TVC or AVC, core saturation at low frequencies becomes a limiting factor.

se

[NewBuyer]

Thank you Steve for helping me to understand this better. I now wonder how TVC designers must handle this trade-off between galvanic isolation and EMI/RFI. For it seems that if the input/output grounds were not tied together, then EMI/RFI would dump into the transformers, and contaminate the signal. But if the input/output grounds are tied together, then no galvanic isolation, and thus ground loop opportunities that can also contaminate the signal. Does this reasoning/question seem valid?

Well, usually high impedance transformers will use a Faraday shield between the primary and secondary windings to help keep RF from coupling across it via the interwinding capacitance. Not sure if the various makers of TVCs are doing this or not...

Thanks Steve for the info and the Jensen link, I put some reading time there. Let's see if I have this correct: If the input/output grounds of a TVC were not tied together, then its input jacks and cables (which act like antenna) will pick up RF, direct it right through the transformers, and onward to the amplifier as noise. But if Faraday shields were used between the windings, this problem would be eliminated. Is this correct?

...Don't know what the TVC in the Prometheus is rated at, but S&B recommends no more than +20dBu (7.75V) above 40Hz and no more than +14dBu (3.875V) at 20Hz for their TX-102... With magnetic devices, whether TVC or AVC, core saturation at low frequencies becomes a limiting factor...

From the FAQ section of the Promitheus Audio website:

Maximum input Voltage of our TVC

All our TVC are tested in house at the voltage of 7.5vrms. They are design to take up to 20Vrms. Why do we need such a big factor when it used for a line level of only 2Vrms. Music is not just made out of continuous sine waves instead they are made up of complex sine wave and rather sudden transients like the triangle. This transients can output very high output for a short period of time. We design our TVC in a way that they do not limit any transients. Another reason for over sizing our TVC is allow full body bass just like an active preamp would have with no apparent bass thinning out.


This info is not frequency-referenced, and I don't know if this info pertains to the new balanced version of the Promitheus transformers, or only to the previous unbalanced version. 

choariwap: If this info does apply to the balanced transformers, then it seems you could simply set your Benchmark into non-variable mode (disabling the front volume dial), dial-back the fixed trim-pots (on the back of your Benchmark) just a tiny amount, and this should work very well with the balanced Promitheus TVC...

[Steve Eddy]

Thanks Steve for the info and the Jensen link, I put some reading time there. Let's see if I have this correct: If the input/output grounds of a TVC were not tied together, then its input jacks and cables (which act like antenna) will pick up RF, direct it right through the transformers, and onward to the amplifier as noise. But if Faraday shields were used between the windings, this problem would be eliminated. Is this correct?

Well, presupposing that RF is in fact a problem to begin with, the Faraday shields can help reduce it. I'd hesitate to say eliminate.

And even if the primary and secondary grounds are tied together, you can still get RF coupling via the interwinding capacitance.

From the FAQ section of the Promitheus Audio website:

Maximum input Voltage of our TVC

All our TVC are tested in house at the voltage of 7.5vrms. They are design to take up to 20Vrms. Why do we need such a big factor when it used for a line level of only 2Vrms. Music is not just made out of continuous sine waves instead they are made up of complex sine wave and rather sudden transients like the triangle. This transients can output very high output for a short period of time. We design our TVC in a way that they do not limit any transients. Another reason for over sizing our TVC is allow full body bass just like an active preamp would have with no apparent bass thinning out.

If the 2Vrms figure they're giving here is intended to refer to the output of a Red Book CD player, they're rather off base with their comment that transients can output very high output for a short period of time. The transients can be no higher than 2Vrms either. A digital system has a hard limit and the output can never be any higher than whatever the full scale limit of the system is.

Anyway...

This info is not frequency-referenced, and I don't know if this info pertains to the new balanced version of the Promitheus transformers, or to the previous unbalanced version.  If it does apply to the balanced transformers Choariwap, then it seems to me that you could simply set your Benchmark into non-variable mode (so the front volume dial is disabled), dial-back the fixed trim-pots (on the back of your Benchmark) just a tiny amount, and you would be fine with the balanced version of the Promitheus - yes?

That would certainly work with regard to not exceeding the maximum input level of the TVC, but I'd also suggest trying even more attenuation ahead of the TVC and go with what sounds best.

se

[NewBuyer]

Very interesting information Steve - thanks!

I've got one on order - one with a metal case.  After bugging Nick about it for a bit, he decided to try it.  I'm paying a bit more for this...I have high expectations based on what I've heard.  It will be a couple weeks at least, I'm sure, before I see it (paid him a week or so ago I think).

Paul

I wonder if Paul has received his TVC yet? I hope pics will be posted when it arrives! 

[anubisgrau]

pardon my silly question but...

how's about some good news for us couch potatos...... REMOTE CONTROL?

[PaulFolbrecht]

I've got one on order - one with a metal case.  After bugging Nick about it for a bit, he decided to try it.  I'm paying a bit more for this...I have high expectations based on what I've heard.  It will be a couple weeks at least, I'm sure, before I see it (paid him a week or so ago I think).

Paul

I wonder if Paul has received his TVC yet? I hope pics will be posted when it arrives! 

Nick mailed me last week to say that he was having trouble getting the Al cases built well and he had sent me a wood one in the interim.

While this was very gracious, I'm in no hurry, as the Welborne 45 SET monoblocks I'm going to build to go with this linestage haven't arrived yet.

I do hope he gets the bugs worked out with the Al case as I'm pretty sure I'd prefer that.

[Steve Eddy]

pardon my silly question but...

how's about some good news for us couch potatos...... REMOTE CONTROL?

Remote control? Geez, you want someone to listen for you too? 

se

[lcrim]

The TVC is unpowered.  A remote control which needs power, is a very remote possibility.

[PaulFolbrecht]

The TVC is unpowered.  A remote control which needs power, is a very remote possibility.

NO PUN INTENDED, RIGHT?!  aa

[anubisgrau]

pardon my silly question but...

how's about some good news for us couch potatos...... REMOTE CONTROL?

Remote control? Geez, you want someone to listen for you too? 

se

i will never understand why enjoyment in good sound with some audiophiles has to be linked with physical sacrifice and self mutilation.

anyway, thanks for the info. i thought that existance of RC with other respected TVC models should be inspriring enough, regardless of cost.

[NewBuyer]

  Nick mailed me last week to say that he was having trouble getting the Al cases built well and he had sent me a wood one in the interim...

Did you receive the wooden one yet? 

[GHM]

<sub>i will never understand why enjoyment in good sound with some audiophiles has to be linked with physical sacrifice and self mutilation.

anyway, thanks for the info. i thought that existance of RC with other respected TVC models should be inspriring enough, regardless of cost.</sub>


  You make it sound really tough to get up and turn a knob.
For $280..what do you expect? I'm sure Nick may offer something like the remote down the road. I've become so accustom to not using one..it isn't a problem for me as I haven't used one in two years. Hell ..I have the Bent NOH here with the remote and I don't use its RC either!   Call me crazy..


Good listening

[lonewolfny42]

I have the Bent NOH here with the remote and I don't use its RC either!   Call me crazy..

Did you try the remote ? 

[GHM]

I have the Bent NOH here with the remote and I don't use its RC either!   Call me crazy..

Did you try the remote ? 

Nope Chris ..I never pulled it out of the box.

[lonewolfny42]

I have the Bent NOH here with the remote and I don't use its RC either!   Call me crazy..

Did you try the remote ? 

Nope Chris ..I never pulled it out of the box.

Try it....it should work....

[GHM]

I have the Bent NOH here with the remote and I don't use its RC either!   Call me crazy..

Did you try the remote ? 

Nope Chris ..I never pulled it out of the box.

Try it....it should work....

I'll give it a go and let you know.

[lonewolfny42]

I have the Bent NOH here with the remote and I don't use its RC either!   Call me crazy..

Did you try the remote ? 

Nope Chris ..I never pulled it out of the box.

Try it....it should work....

I'll give it a go and let you know.

Thanks Gymane....

                         Chris

[GHM]

Hey Chris..I dropped in some fresh batteries. The remote works fine.