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Old 11th February 2005, 06:17 PM   #1
Bill F. is offline Bill F.  United States
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Default Poor man's transformer volume control?

What do you guys think about this transformer/pot hybrid volume control scheme? (Scroll down the page to the bright yellow box and description.)

How would something like this compare to the pure-transformer solutions like the S&B tx102? Respective strengths and weaknesses? Aside from voltage gain, what does the transformer bring this otherwise simple shunt system? If you need <unity gain, would a commercial 1:1 600ohm line transformer work ok/better, or would the whole point of the transformer then be moot?

I'd like to do a 6-channel volume control for triamping, and I can't afford 6 tx102 units. Is there anything to recommend the above system over a ladder-resistor attenuator?
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Old 11th February 2005, 06:38 PM   #2
tiroth is offline tiroth  United States
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Well, if we're both talking about that yellow box, then I'd say it really has nothing at all to do with a TVC. That doesn't mean it isn't a good idea though; using a line input transformer can lower the common mode noise of even an unbalanced connection by as much as 60-80dB at 60Hz. I think a 150-10k stepup transformer is a poor choice though, which results in that particular device being so finicky.

Jensen Transformers has an excellant whitepaper on unbalanced input line transformers.
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Old 11th February 2005, 06:42 PM   #3
tiroth is offline tiroth  United States
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I tried to gather a list of transformers with statements about quality and cost. I also have some measurements on the second page of this thread.

Input transformers

I would say the Edcor's are usable (barely). If you are really going for high end I would either take a gamble with Cinemag or get a Jensen, but for anything other than subwoofer bass the distortion is probably lower than that of the speakers themselves.

Talk to me if you go the Cinemag route, I might be interested in piggybacking if we'd have enough for a discount.
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Old 17th February 2005, 02:52 PM   #4
Previously: Kuei Yang Wang
 
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Default Re: Poor man's transformer volume control?

Konnichiwa,

Quote:
Originally posted by Bill F.
What do you guys think about this transformer/pot hybrid volume control scheme? (Scroll down the page to the bright yellow box and description.)
Well, I think it is time to give soem TVC background....

Some time in the dark days last millenium rumours of a Product that was hideously expensive and hideously looking but sounded great reached the Joenet. Not much was information was available except the thing was a transformer and called Silver Rock (but cost it's weight in 22 Carat Gold!), in fact they had resurrected a level control approach first shown by WE in the 1920's.

Click the image to open in full size.

This originally got me thinking and the benefits of using this principle for a passive line controller where immediatly obvious. No specifications where available on the SR, so I had to think of my own.

I first approached Sowter in the UK about arranging a one-off production run of such devices for a group order of the Joe-Net and the London Live DIY Circle. I decided to specify the nominal impedance of the line controller transformer as 10K, based on a number of considerations (I'll come back to them below).

For a number of reasons (mainly Sowters refusal to make what we wanted and our refusal to buy what Sowter wanted to make). The group buy blew up. At any extent, a year later or so Sowter made a Volume Control transformer available, with the exact specifications I had originally asked for. This one was soon being "improved" by Sowter, based on listening to the "improvements" they seemed more related to manufacturing economics and measured performance than good sound.

Once I got in contact with Stevens & Billington Transformers I got them to make a "no compromise" design to the same specs I had given Sowter and the TX-102 was born.

Now to the specs.

I first of all considered (and still do) the following basic truisms:

1) The ideal linestage has an infinite Input impedance
2) The ideal linestage has a zero output impedance

The aim of the TVC's which I specified was to approach this ideal as much as possible, especially in the midrange.

If you use any significant level of attenuation on the typhical TVC your load on the source will become very high. If for arhguments sake you use a TX-102 into a 10K load and you apply 12db attenuation, your input impedance of the TVC will become around 160KOhm in the midband rising further as attenuation increases.

At 20Hz the inductive reactance limits your input impedance to around 50KOhm, but even that is quite high by anyones accounts. However, importantly the Midband where the ear is most sensitive sees very little load on the source and thus low distortion.

Now let us look at the output side of things. If we have a source with 3k Output Impedance (not ususual for many Valve Circuits) and we attenuate 12db, what will our output impedance be? Around 200 Ohm, this will drop further as attenuation goes up.

So what we have created is in effect a "gearbox" that attempts to load the source as little as possible while making ALL the power drawn from the source (minus a small amount of loss) to the load. The low load on the source minimises distortion there and the low output impedance allows a TVC to "drive" their loads and cables.

Much of what makes typhical TVC's sound the way they do is (IMHO) a result of this principle.

Now let us look what EP does. Note that I cannot comment how their approach sounds, I have not heard any of their stuff, I can only make a comparison on electrical grounds and based on the discussed specs....

The EP "Passive Preamp" takjes a transformer with a 1:8 Stepup and apply the result to a 10K Pot. Therefore, on the output side of things the whole thing looks like a classic "passive preamp" with a 10K Pot.

The output impedance can be low but may reach 2.5KOhm if the source has zero ohm source impedance, higher if there is a significant output impedance. We come back to this.

This 10K Pot loads the transformer which transformers the impedance by a factor of 64 (square of the ratio). In other words the 10K will appear to the source driving the EP arrangement as a 150 Ohm Load. You may wish to peruse the measurements in many stereophile reviews to see how the output stages of much equipment react to being loaded with a 600 Ohm load, which is in line level circuits considered a severe ,oad, 150 is four times as severe!

So, in electrical terms at least the EP arrangement achieves the exact and polar opposite of what the classic TVC achieves. Instead of loading down the source minally while keeping output impedance low as the classic TVC does the EP arrangement maximally loads down the source and creates a fairly high output impedance.

Sayonara
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Old 17th February 2005, 03:24 PM   #5
Bill F. is offline Bill F.  United States
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Thanks KYW. I'm a transducer wonk, and my electronics understanding is weak, so I appreciate the explanation.

A couple more questions:

1) In terms of input/output impedance, etc., how do autoformer volume controls (AVCs?) compare to TVCs?

2) TVCs like the Silver Rock, S&B, etc. typically have a single primary winding and multiple secondaries, correct? Could even better loading and bandwidth characteristics be had from multiple primary and secondary windings connected by a ladder switch?

Bill
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Old 17th February 2005, 03:56 PM   #6
Previously: Kuei Yang Wang
 
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Konnichiwa,

Quote:
Originally posted by Bill F.
1) In terms of input/output impedance, etc., how do autoformer volume controls (AVCs?) compare to TVCs?
Exactly the same, assuming the (primary) inductance and ratios are the same.

Quote:
Originally posted by Bill F.
2) TVCs like the Silver Rock, S&B, etc. typically have a single primary winding and multiple secondaries, correct?
I cannot talk about others, but the current S&B TX-102 uses quite complex layering and sectioning for both primary and tapped secondary. The bandwidth of this transformer is as it is as a deliberate choice, t could have been made anything one likes.

Quote:
Originally posted by Bill F.
Could even better loading and bandwidth characteristics be had from multiple primary and secondary windings connected by a ladder switch?
Not neccesarily. There are many ways of doing things. There are winding structures that would allow a frequency response well into the MHz range. I do not feel however that using these offers benefits.

Sayonara
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Old 17th February 2005, 04:08 PM   #7
Bill F. is offline Bill F.  United States
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Quote:
quote:
Originally posted by Bill F.
1) In terms of input/output impedance, etc., how do autoformer volume controls (AVCs?) compare to TVCs?

KYW: Exactly the same, assuming the (primary) inductance and ratios are the same.
Does one have any performance advantage at all over the other? It seems to me that an AVC could be produced more cheaply than a TVC, or does it introduce some type of compromise?
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Old 17th February 2005, 05:01 PM   #8
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Konnichiwa,

Quote:
Originally posted by Bill F.
Does one have any performance advantage at all over the other? It seems to me that an AVC could be produced more cheaply than a TVC, or does it introduce some type of compromise?
There are no real advantages either way if you aim at high sonic quality, the seperate input and output current loops of the true transformer makes it more versitale and can in some situations lead to better sound.

At some point in time a large customer of S&B was looking into having the "slagleformer" made by S&B (under licence and all as I understand). Calculations suggeste that the cost savings over a 102 would be minimal but the 102 is a much more generally compatible design.

If you want to save money it is easiest to replace the expensive large nickel permalloy transformer core with a smaller Steel core and to forgo potting the transformer in shielding cans (as some makers of commercial TVC's have done).

The price to pay is comparably poor sound and high sensitivity to external hum fields.

Sayonara
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Old 17th February 2005, 06:06 PM   #9
Bill F. is offline Bill F.  United States
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Thanks, evil KYW--I'm learning a lot here.
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Old 17th February 2005, 06:13 PM   #10
Previously: Kuei Yang Wang
 
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Konnichiwa,

Quote:
Originally posted by Bill F.
Thanks, evil KYW--I'm learning a lot here.
Well, here a final one bit to note....

My favourite "traditional" linestage is the line part of the Arthur Loesch Preamp.... Look here to see what it looks like:

http://www.fortunecity.com/rivendell...a/Toccata.html

Sayonara
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