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Old 17th March 2009, 01:02 PM   #1
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Exclamation Question on passive preamplifier - transformer section.

Hi,

I am planning to build a passive pre amplifier. The design I am having in my mind is the one with a potentiometer and 2 transformers. I am planning to use an ALPS 100K log potentiometer. But, I am not so sure on the transformer configuration. Can some one contribute/suggest ?

Regards,
Bins
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Old 6th July 2009, 01:24 PM   #2
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Hi,

Please suggest me a suitable transformer that can be used for the passive pre-amp that I am designing. The input will be provided from a DAC/CD player.

My power amplifier has the following ratings:
* Input impedance (unbalanced): 10 K Ohms
* Input sensitivity: 100 mV to 775 mV.

Best regards,
Bins.
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Old 7th July 2009, 04:52 PM   #3
jonners is offline jonners  United Kingdom
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Hi Bins,

I'm wondering why you want to use a pot with transformers. Why not use tapped transformers by themselves?

John
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Old 8th July 2009, 03:52 AM   #4
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Hi John,

I am not sure on how to design that. Can you give me a circuit/model that I can look at ?

Best regards,
Bins.
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Old 18th September 2009, 11:58 PM   #5
rjm is offline rjm  Japan
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A passive preamplifier ... okay, but idea from the start, but lets see where this goes...

A 100k log pot as a volume the volume control. So far so good. The input impedance of your "line attenuator" is about 100k. The output impedance is 0-25k. If the interconnects are kept really short, your amplifier, with an input impedance of 100k, will be a decent match.

Now, for some reason you want to use output transformers. You can use them as step-up transformers or step-down transformers. Funnily enough, neither configuration makes any sense.

If you need more gain (step up) it would be more efficient to turn up the volume control. (The output impedance increases either way.)

If you need lower output impedance (step down) it would be more efficient to turn down the volume control! (The output signal level decreases either way.)

You could use the transformers as 1:1 isolation units, converting the signal to balanced for the output. That gives you some immunity from ground loops and noise, but you won't find any that have input impedances much above 10k, so it's a really lousy idea to run them directly from the 100k volume pot, your frequency response tanks and, to add insult to injury, varies with the volume position. And you still can't drive any reasonable length of interconnect!

To paraphrase Steve Jobs, passive line stages are a a bag of hurt.

The only way to do it properly would be to drill a hole in your amplifier faceplate and retrofit in the volume control. Then build a passive line selector box.

Amplifiers should be driven by competently designed active line stages. The end.
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Old 19th September 2009, 10:15 AM   #6
danzup is offline danzup  Romania
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Quote:
Originally Posted by binspaul View Post
Hi,

I am planning to build a passive pre amplifier. The design I am having in my mind is the one with a potentiometer and 2 transformers. I am planning to use an ALPS 100K log potentiometer. But, I am not so sure on the transformer configuration. Can some one contribute/suggest ?

Regards,
Bins
Very bad idea !
Trust me on that , been there done that = a lot of crap !

But if you consider : 1 transformer and then the Alps 100k it is ok .
Done also that ! Good configuration .

You must to know when to much is tooo much .
Or as suggested by fellows just one transformer with a lot of taps and a good switch and discard the Alps .

Last edited by danzup; 19th September 2009 at 10:18 AM.
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Old 5th February 2010, 08:07 AM   #7
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Default The end is the new beginning....

Hi,

The original idea to develop a passive preamplifier using transformers has been called off in favor of George's light speed attenuator.....

Best regards,
Bins.
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Old 28th February 2010, 09:35 PM   #8
Guild is offline Guild  United States
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Quote:
Originally Posted by rjm View Post
A passive preamplifier ... okay, but idea from the start, but lets see where this goes...

A 100k log pot as a volume the volume control. So far so good. The input impedance of your "line attenuator" is about 100k. The output impedance is 0-25k. If the interconnects are kept really short, your amplifier, with an input impedance of 100k, will be a decent match.
I think he specified 10k (not 100k) for the amplifier input Z.
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Old 1st March 2010, 07:28 PM   #9
tvrgeek is offline tvrgeek  United States
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Now you have gone another direction, the Jensen isomax transformers seem well received, especially following a CD player.
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Old 7th March 2010, 07:10 PM   #10
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Audio Transformer Overview

Auto transformer for attenuators:
Silverrock (commercial preamp from Audio Consulting)
http://www.audio-consulting.ch/?Prod...ilver_Rock_TVC
http://web.archive.org/web/200710281...c_passive.html
http://web.archive.org/web/*/http://...c_passive.html
Stevens & Billington Limited Audio Transformers
Stevens & Billington Limited Audio Transformers
Stevens & Billington Limited Audio transformers TX-102
inside in finished commercial preamp
Your Page Title
SOWTER
SOWTER AUDIO TRANSFORMERS
http://web.archive.org/web/*/http://....uk/attens.htm
Various informations about auto transformer for volume control
symasym5 - Phono - Restauration und Selbstbau - Analog-Forum go to post from 15. April 2007, 13:53
Euro Stevens and Billington TX102 Group Purchase - diyAudio

All kind of audio transformers:
http://www.cinemag.biz/
http://www.jensen-transformers.com/db.html
Palmer | PLI03
Lundahl Transformers, audio transformer and tube amplifier transformer manufacturer
PLITRON - audio transformers - toroidal transformers - toroids - output transformers - current transformers - power transformers - medical isolation transformers - power toroids

Output transformer
JAMES transformer from the factory in TAIWAN (single ended power amps)
http://www.bella-audioparts.com/transformer.htm
http://www.tubeaudiokits.com/PARTS/Transformers.htm
James Audio Output Transformers
Glockenklang (matching impedance)
uno
GLOCKENKLANG OUTPUT TRANSFORMER 600 W.COVER

Last edited by tiefbassuebertr; 7th March 2010 at 07:23 PM.
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