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Old 17th October 2005, 12:00 AM   #1
chuck55 is offline chuck55  United States
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Default Variable resistance transformer preamp

Hi, maybe this question has been asked but I searched and couldn't find any information. Instead of a gainclone with a passive pre using a 10k Alps pot or ladder-type attenuator, would a variable resistance transformer work? I found this as an example called a vernistat:
http://www.paelectronics.com/vernistats.htm

Also variac.com sells variable output voltage transformers. Could they be adapted to variable resistance?

I just don't see why that silver rock(?) variable transformer has to cost $6,000 and why it couldn't be done DIY for cheap.
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Old 17th October 2005, 02:56 AM   #2
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Other than being hideously expense, what would be the advantage of using this to control the level of an audio signal?

I_F
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Old 18th October 2005, 12:51 AM   #3
chuck55 is offline chuck55  United States
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Hi, only on computer for a minute so don't have time to find the link. I think it has to do with constant impedance, no coloration like resistors (carbon vs. tantalum etc), no popping like you can get from ladder type.
http://www.goldpt.com/compare.html
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Old 18th October 2005, 10:02 PM   #4
chuck55 is offline chuck55  United States
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Okay here we go:
http://db.audioasylum.com/scripts/t.pl?f=amp&m=73673

Resistors=more transparent while scaraficing some bass, warmth, and dynamics
TVC=better dynamics, bass and warmth but gives up some transparency.

and:
http://db.audioasylum.com/cgi/search...sformer+volume

Supposedly transformer pre-amps are better at low volumes and add some body as compared to resistors. I've decided the LM3875 chip amp is better than any "T" amp and will post Stereophile's specs on both in my other thread on this subject. But the LM3875 amp is said to need a tube pre-amp to "warm up" which I don't want to use. So the TVC would give a similar sound quality to a tube pre-amp. Installing in the same enclosure so don't think active pre is necessary.
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Old 18th October 2005, 10:25 PM   #5
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Default TVC or variac?

When you talk about transformer based volume control I understand a Tx with one (or two) primaries and multiple secondaries with different attenuation to achieve stepped voltage attenuation, like the famous Stevens & Billington's TX102 (info page not available for the moment). They are very good but expensive and benefit from high Vout sources to blow your hat.

See, for instance:
http://www.bentaudio.com/index2.html

http://www.6moons.com/audioreviews/a...4/pretone.html

When you talk about variac I understand infinitelly variable Vout, but that must be noisy to achieve, I imagine.
Wich one are we talking about?

I have not tried resistor based passives, sorry.

Regards
Mauricio
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Old 19th October 2005, 12:45 AM   #6
chuck55 is offline chuck55  United States
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Hi Mauricio,
I would like to use a variable resistance transformer but can't find one. I did easily find Variac which varies voltage output but like you say, even if it can be used to vary resistance, may be noisy.
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Old 19th October 2005, 01:14 PM   #7
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Default Vernistat

The vernistat uses a resistive element to provide the voltages between the tap points on the transformer. Think about it- a transformer provides x volts per turn. Lets say x=2V. If you tap it at the fourth tun, you get 8V. If you tap it at the 5th turn you get 10V. How do you get 9.5V? That's where the resistive element comes in.

The vernistat is a means of making a transformer look like it can be tapped anywhere, even in between secondary turns.

It still has a resistive element, so if you believe such things are audible, then it won't be very good. It also has a bunch of wipers that you'll probably be able to hear also.

Maybe the price tag will make the resistive element and the wipers harder to hear. That seems to work with all sorts of audio parts.

I_F
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Old 19th October 2005, 01:50 PM   #8
Nordic is offline Nordic  South Africa
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I don't think the 3875 need anything to "warm up", actualy quite a warm sound to me allready.
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Old 19th October 2005, 02:40 PM   #9
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Dear Chukky

Quote:
I would like to use a variable resistance transformer but can't find one.
http://www.bentaudio.com/index2.html (goto "DIY parts")
http://www.diyhifisupply.com/diyhs_sb_transformers.htm
http://www.diyhifisupply.com/diyhs_django.htm
http://www.mfaudio.co.uk/

See also Peter Daniel's thread about his gorgeous TVC (I couldn't find it)

Good luck
Mauricio
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Old 7th November 2005, 02:32 AM   #10
chuck55 is offline chuck55  United States
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Here is a transformer pre-amp technical description -when the page is back up
mfaudio
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