Are TVC/Autoformers slow?

[Andrew]

-- Gents,

I have heard Greg going on about the music being slow several times, I find its hard to spot in isolation but at one EggFest(2 I think) he produced a CD that showed up the problem on one or two amps. Then announced he was impressed with Dave Dove's 300B as it was the first SET he had heard that was speedy enough for him.

I found it a very enlightening moment as I had never really encountered this before.

So....it's interesting...that's two reports I have read that have shown the magnetic volume control to be described by Greg, and heard by others, at the same time, in the same place, as slow.

Now niether test with the S&Bs. Perhaps the magnetics (the core) reduce the slew rate of the amplification chain? Making it sound slow. But if I slew-rate is also related to bandwidth, which I think it is (am I right?) then the S&B is flat to 100K with a 1K source 50K load conditions (and assuming S&B are to be believed).

Question is....is 100K enough? What else could it be?

If it's the magnetics, rather than the design, then the S&Bs will be slow as well. But if not then S&B could be OK. Why am I asking this? Well I have a vested interest as I own a pair of S&Bs and wonder what is the cause of the slowness...

cheers,

-- Andrew

[James D]

No it's synergy...

If you want to use a TVC then you need a seriously low output impedance with real current capability - same as driving an IT properly only more so as you have variable impedance and variable reactance to drive -

AVC are easier to drive than TVC which frankly are very difficult to drive properly..

Do it wrongly and they will sound 'slow' and/or 'soft'...

And, no you won't find this just by doing the impedance and reaftance calculations... its a hard won by experience thing

James

[Andrew]

Hi James,

On the ball as usual...

How low is seriously low?

cheers,

-- Andrew

[Clive]

It seems like Thorsten's phono stage that Andrew modified is what we need (and what we both have in different forms), this has a 6922 output valve running at 14ma, if memory serves me correctly.

I suppose even CD players with good opamp outputs can be pushed too hard.

Also, it's good to know Andrew and myself aren't necessarily deluding ourselves.

[Greg.]

Trust your ears

[Andrew]

Hi Gents,

The phono output stage Clive mentioned has a 3K Zout.....this is essentially the same output stage as TL designed in his 'simple phono' an ECC88/6922 in grid leak bias with 14-15mA standing current - I added a CCS to his output stage. The input stage is almost completely different.

If we need lower, then the Aikido, I think, could be you man.

cheers,

-- Andrew

[Andrew]

Originally Posted by Greg.

Trust your ears

Well exactly Greg, but as I said I can't spot it in isolation, I don't think. I can't walk in and say that's slow, tho perhaps my perceptive ability is improving...

cheers,

-- Andrew

[Clive]

I had several email exchanges with T re which phono stage would work well with the S&Bs. He said Zout was a factor but it wasn't the primary factor, he designed the simple phono with the S&Bs in mind, it was originally intended for MFA to produce alongside their TVC (MFA are the ratil part of S&B).

Of course T could have got this wrong but I'm trusting my ears!

[Clive]

Originally Posted by Andrew

Well exactly Greg, but as I said I can't spot it in isolation, I don't think. I can't walk in and say that's slow, tho perhaps my perceptive ability is improving...

cheers,

-- Andrew

I'm a little wary of dismissing a sound in direct comparision to another. Faster or brighter will normally sound "better". I can make a quick conclusion but I find listening for a long period more in isolation is better. I then get an idea for what moves my heart and not my head. Of course this isn't viable for the fests. What I do know is that the S&Bs have lasted much longer in my system than the Passion or active preamps (valve and SS).

[Greg.]

Hi Andrew,

Yes, it's fair to say that generally this element of dynamics is not readily obvious unless you are conducting an A/B comparison. I personally do seem to be sensitive to this 'speed' thing and it is particularly noticable to me if the music being played is naturally pacey and dynamic. I can imagine as Clive has posted elsewhere, not being disatisfied with a TVC or AVC if I majored on fairly simple jazz and folk type sounds. I suppose we could extend this arguement and it explains why I also still remain in the PP camp although I recognise it is possible to get SE to perform very dynamically as Dave Dove and clearly James D (a former PP man) have achieved. I'd also love to experiment in this area as well but at the moment, finance is limiting, the right channel on my CDP has just packed up so I'm looking at Squeezebox and I've just ordered a Slatedeck ......eventually I'll get around to building that phono stage.

Best wishes,

Greg