Fully ballanced valve preamps

I was obviously referring to maximum power transfer theorem, though I admit maximum power transfer may not always lead to a great sound.

 

Hi,

The output impedance of the TX-102 is the source's output Impedance plus around 400 Ohm and then divided by the square of the attenuation (as pure voltage ratio, not as db).

To illustrate, my Phono Stage has an output impedance of around 1KOhm. If I attenuate the signal by 12db (normal in my system) or a ratio of 4:1 the actual output impedance after the TVC is:

(1000+400) / 4 * 4 or 1400/16 or around 100 Ohm. Any more attenuation and the output impedance starts to approximate 0 Ohm ever more closely.

Conversely, the TVC's input impedance is again the load impedance transformed, if I use my 3D Sonics "Foundation" monoblocks I have a 20KOhm balanced input impedance, which is in effect multiplied by a factor of 16 to around 320K load on the source if 12db attenuation is selected.

Again, as attenuation increases the input impedance of the TX-102 quickly approaches infinity in parallel to around 600 Henry inductance and around 100pF which limits the practical input impedance to 75KOhm @ 20Hz & 20KHz and around 1100KOhm @ 1KHz.

The minimum "look-up" factor is around 10, but it should include the capacitive part of the input impedance, which is often not specified.

The S&B TX-102 initial design was derived from a 10K:10K line transformer, in other words it is designed for a load of 10KOhm or greater.

Ciao T

 

Hi,

ATC of great Britain manufactures amplifiers and preamplifiers AND speakers, Air Tight of Japan manufacturers amplifiers and preamplifiers only.

Ciao T

 

Dev,

Maximum power transfer is relevant only where a load requires a lot of power (eg. Speakers). Using normal length line cables be it in studio or domestic use the terminating impedance on the receiving end is around 10K or higher and is meant to be a "bridging load", in other words a load that consumes virtually no power (referenced to a 600 Ohm "matched" load).

The 600 Ohm termination is needed if balanced lines are made several hundert yards to miles long. In this case we are dealing with a fully terminated transmission line where things are different.

If you raise the output impedance of your preamplifier above the neccesary minimum all you do is to lower the corner frequency of the lowpass formed by the preamplifiers output impedance and the cable capacitance (plus amplifier input capacitance which may be non-linear signal modulated) and you leave yourself open to problems from the high impedance interacting with grid-emission currents (or their transistor equivalents of base/gate current) and such currents are related to the signal in a strongly nonlinear fashion.

Hence all else being equal the lower the output impedance of a preamplifier the better.

Ciao T

 

Hi,

If there is a Law (Torah, Shariat et al) what is "right" and "wrong" is pre-determined for you.

If there is no law, there is no "right" or "wrong" and you need to again take full responsibility for your actions.

As it has been said "All things are lawful, but not all things are advantageous".

Ciao T

 

Is that the Robert Anton Wilson hippy drivel thing from the late 60s? I was hoping for something more, um, contemporary.

No there isn't any evidence, nobody knows one way or another. The wording that is generally ascribed to him is, however, "Nothing is true, everything is permitted", and not "Everything is true, everything is permitted".

Stick to the soldering, logic isn't your forte.

-- Ian

 

Hi,

There is very little contemporary on HbS. And IIRC the PD was not written by RAW, but I may be wrong on that count.

"In this world there are two kinds of people, those who get it and those who don't. If the meaning of this is not immediatly obvious count yourself among the latter...." (Robert Anton Wilson)

Ciao T

 

That mirrors my findings with passives. With a 10k pot you ideally want the input on the power amp to be 100k or higher. If not you'll find things sound a tad flat and dynamically squashed. When I ran my WAD power amp as an integrated I used a 20k pot to drive the very high 1m Ohm input of the power amp circuits and it worked beautifully. Take the pot too low though and some sources, particularly tube based or those with poor OP amps in the output stage will struggle. As ever in hifi it's necessary to apply a little theory and common sense.

 

going (partly) back to the theme, has anyone ever tried unison research mistery one. the looks shocked me and it has some kind of battery supply that kills the hum. i am wondering about reliability and sound character?

 

Hi,

Not particulary. But IIRC it uses ECC83, which means it is very much current starved design that will not work well into any significant loads, meaning either long cables or "pro-audio" impedance levels (like ATC active speakers) are out.

I rather doubt it. It uses a very generic AC supply.

Ciao T

 

I tried one several years ago. Beautiful looking bit of kit I must agree. Surprisingly large though.

Same sort of sound as my PV-10 was, very smooth. Would have been tempted on looks alone but sonically didn't seem worth the extra.

It does indeed have a standard lithium battery in it. Must confess can't exactly remember what it's function is though

Both absolutley trounced in my system by the MF Passive though.

Jeff

 

Hi,

That would be biasing of (one of) the valves.

Ciao T

 

as some of the TVCs has been mentioned here, anyone knows why bent NOH is discontinued? it seemed pretty popular. any reliability (or similar) problems?

 

Hi,

Simple. John Chapman (aka "BentAudio") was building them himself and lacked the time to continue doing so.

The margins he had in this originally where insufficient to pay someone else to do it and he did not want to turn this into a major commercial operation with much increased pricing.

And yes, I know so from John himeself....

Ciao T

 

Thanks T, meanwhile I read the whole story on the Bent forum.... I was just confused as he has a new product - TAP.

Anyway would you foresee any troubles matching my active ATCs, with 1V sensitivity and >10Kohm impedance via 5m (I can cut them to 3m) Kimber PBJ i/c's (I know I need better, don't tell me.

Also what kind of input parameters should a matching CDP have?

I understood from your writings that matching sensibilities and impedances is critical with TVCs.

 

Hi,

Nope, no problems at all. I used driver longer cables into 10K loads in my own setup.

The key is your source, not the load so much.

Something like 200 Ohm output im pedance, 2V full scale output. Typhcially all CD Players will be much like that, but some unusual esotherica will be different and might cause issues.

Which CD Player do you have?

Ciao T

 

i currently use a borrowed quad cdp 99-2

 

Hi,

That will do fine.

I think the only ones to worry about are extremist Valve based designs and some other exotica, I'm unaware of any mainstream or even semi-mainstream player or DAC that would be problematic, even the Zanden DAC should be fine, if slightly borderline....

Ciao T

 

whatever came out of your big preamp battle session?

 

Hi,

I was actually waiting for Dev, Simon or Rob to comment, due to my own (very modest) commercial involvement with one of the products.

So guys, spill it! :MILD:

Ciao T