passive preamp with transformer output - which parts to use?

[krishu]

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.

[sreten]

Winding your own transformer and using stepped
taps is the route to low output impedance.

sreten.

[analog_sa]

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

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]

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]

Originally posted by krishu
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.

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?

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

[krishu]

Hi,

i am not too sure it was the Placette stuff where i read about the transformers;

I just wanted something without power supply and with good impedance behaviour ;o)

I should put a pot directly in my power amp...

Cheers
Christian.

[Bas Horneman]

Like Steve said...something like the TX-102 should do the trick...

http://www.stevens-billington.co.uk/page102.htm

[krishu]

Hi,

thanks, this looks fairly good but I expect it to be out of my budget. How much is such a "stepped transformer"?

Kind regards
Christian.

[Bas Horneman]

The S&B's are quite expensive...not sure how much..but the Django that uses them are $649
http://www.diyhifisupply.com/diyhs_django.htm



The Sowter's are more affordable, around 66 to 88 Uk pounds..

[krishu]

Hi,

looks as if the sowther one is well within my budget; I'll check more deeply later on. Thanks so far!

Cheers
Christian.