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-   -   Attenuator for passive pre (LEAK ST 20) (http://www.diyaudio.com/forums/tubes-valves/221322-attenuator-passive-pre-leak-st-20-a.html)

vincula 10th October 2012 09:35 PM

Attenuator for passive pre (LEAK ST 20)
 
Hi guys,

I'd like to build a passive pre for a Leak stereo 20. Would like to use a stepped attenuator, the question is which value do I have to pick? 10k, 25k, 50k? Which input selector do you recommend?

Any ideas regarding this humble DIY project are very much welcome:)

Regards,

Vincla

andyjevans 10th October 2012 10:17 PM

It's nice to make a stepped attenuator and switch and put them in an old Leak preamp case. I used to use an old mono one - looked really nice!

H.J.Leak and Co. Ltd. Shows the range.

The value depends on what's going into it - solid state or tube equipment. 100K is usually pretty safe for both.

I made stepped attenuators from 12 way switches - enough range for me since I'm not too fussy.

enzoastro 10th October 2012 10:30 PM

you can get 22-23 position 2 channel stepped attenuators on *bay for 17-18 quid. sm or leaded resisters. saves time and very possibly money!

enzo

vincula 11th October 2012 11:47 AM

Thanks for the tips, guys. I have read that 100k would be much of a good thing. My friend has a problem with his pre, as he has got way too much gain. Add sensitive speakers, and that's a bad combination. I cannot find the Leak's input sensitivity anywere, thus my quiestion...

Regards,

Vincula

andyjevans 11th October 2012 11:54 AM

Frankly, the Leak circuit is crying out for modification. You will get a big increase in quality of you use the EL84s in triode and replace the whole front end with a simpler one with no global feedback. Less sensitivity and you can ditch those ECC83s for something a lot more sensible.

barrymagrec 11th October 2012 05:11 PM

Quote:

Originally Posted by andyjevans (Post 3197504)
Frankly, the Leak circuit is crying out for modification. You will get a big increase in quality of you use the EL84s in triode and replace the whole front end with a simpler one with no global feedback. Less sensitivity and you can ditch those ECC83s for something a lot more sensible.

In which case sell the Leak and get / make something else, why destroy a valuable old amp?

The value of the input attenuator is more dependent on the source, anything up to 250k or so will be fine for the Leak.

The Stereo 20 is very sensitive by modern standards, 125mV for full output if I remember correctly

andyjevans 11th October 2012 05:29 PM

Well, yes, there's always an argument for keeping vintage amps intact.

On the other hand, it could sound a LOT better, and you still have the vintage look in your hi-fi which a lot of us cherish. Unless you're going to sell the amp, there's an argument for discreet modification. By which I mean leaving the top side pretty intact and just working under the bonnet.

Mind, my Leak Stereo 20 is now all triode - PPP 6S4 with an ECC40 input, so I'm biased....

barrymagrec 11th October 2012 05:35 PM

ECC40, that`s a valve from the past, I assume you must have a stash.

andyjevans 11th October 2012 05:40 PM

The ECC40 is the antecedent of the E80CC which is now very expensive. It's nice.

But I'm all DHT these days, no more indirectly heated tubes. I'd use 4P1L for inputs and outputs. I'm just trying those out in another amp, but I'm using them as drivers for 300b and as preamp tubes too. In triode with filament bias. Another input tube you could use is 3a5 - that's a much nicer sound than indirectly heated tubes, cheap and lowly as it is. If you have gain in the preamp that could be fine with no global feedback.

rongon 11th October 2012 06:03 PM

Maybe not what you had in mind, but have you seen these?


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