passive preamp with transformer output - which parts to use?

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krishu

krishu

Member
Joined 2003
krishu.de
Hi,

I'd like to build a simple passive pream using a pot (or stepped attenuator) and an output transformer to get low output impedance.

Which values and which suppliers do you recommend? My only thought is Lundahl;

Which pot size is best? I want to go for best sound quality, but price must be reasonable.

Thanks in avance,
Christian.
 
analog_sa

analog_sa

Member
Joined 2002
I'd like to build a simple passive pream using a pot (or stepped attenuator) and an output transformer to get low output impedance.
Click to expand...


Rather controversial requirements. An output transformer will aid low output impedance only if the transformer is stepping down so your preamp will not even have unity gain at full volume. Driving the transformer from a pot may be possible but the pot will have to be of very low resistance which probably means extreme load for the preceding active stage.

If, hypothetically, you use a capable line stage and drive a 1k pot it will probably be able to drive the primary of a 1660 Lundahl connected as 4:1. Theoretically, it seems, there will be a worthwhile improvement in output impedance (1k/16) but in practice the dc resistance of the windings will have to be accounted for and very possibly you'll get back where you started. And you'll get 4 times smaller voltage.

In short this project will provide the worst of both worlds and it's kind of obvious no one uses such a topology.
 
krishu

krishu

Member
Joined 2003
krishu.de
Hello,

thanks for your explaination;

in the recent stereophile issue as well as in some of the more serious German hifi mags there were some passive pre amplifiers with output transformers, e.g. the Placette Audio Remote Volume Control.

So you say a simple pot and short wires to the power amp would be better choice?

Cheers
Christian
 
Steve Eddy

Steve Eddy

Member
Joined 2002
theaudioguild.com
krishu said:
in the recent stereophile issue as well as in some of the more serious German hifi mags there were some passive pre amplifiers with output transformers, e.g. the Placette Audio Remote Volume Control.
Click to expand...

I just read the Stereophile review of the Placette remote volume control as well as what Placette had to say about it on their website, and I can't find any mention of it using an output transformer.

And I'm not aware of any passives which do use an output transformer, except for those that use a transformer for the volume control itself (i.e. it's a transformer with multiple taps on its secondary which are selected with a switch).

So you say a simple pot and short wires to the power amp would be better choice?
Click to expand...

Better than using a pot with an output transformer. :)

If you want to use some iron, check out the volume control transformers from Sowter and Stevens & Billington.

se
 
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