Attenuation with a passive pre !
- Thread starter Ianp
- Start date
les24preludes
pfm Member
The Leak Stereo 20 was designed for around half a volt input. So there's already too much gain with a passive preamp since normal line level input these days is 2V. So any preamp with even more gain is just giving you loads of gain you don't know what to do with. Simples.
So from the above, you shouldn't need a preamp with a Stereo 20, just a really good stepped attenuator and something to switch inputs, with or without a zero gain buffer. If you have to have a preamp it should be very low gain. Typical 9 pin tubes e.g. like 12AX7, 12AU7 etc have a gain of 20 or more up to 100 so not what you would choose for a preamp with low gain. Add the numbers up...........
You don't find the excellent low gain tubes like 6AH4, 6V6 in triode, type 37, in commercial preamps. And certainly not the stars of the show like 26 and 01A DHTs. You may get a 6SN7 but again that's a gain of 20. So there we are. A step-down output transformer works with valves but £££. A really good solid state preamp might offer something more interesting, but the sound won't necessarily have the features of a valve sound. That's commercial preamps for you.
So from the above, you shouldn't need a preamp with a Stereo 20, just a really good stepped attenuator and something to switch inputs, with or without a zero gain buffer. If you have to have a preamp it should be very low gain. Typical 9 pin tubes e.g. like 12AX7, 12AU7 etc have a gain of 20 or more up to 100 so not what you would choose for a preamp with low gain. Add the numbers up...........
You don't find the excellent low gain tubes like 6AH4, 6V6 in triode, type 37, in commercial preamps. And certainly not the stars of the show like 26 and 01A DHTs. You may get a 6SN7 but again that's a gain of 20. So there we are. A step-down output transformer works with valves but £££. A really good solid state preamp might offer something more interesting, but the sound won't necessarily have the features of a valve sound. That's commercial preamps for you.
Alex S
carbon based lifeform
Not only that, it’s going up. 3, 4, 5 even 6V is now pretty common.
Ianp
pfm Member
It certainly is a winning combination. I've probably said it before, as have many, the Leak is a world class performer. And a passive simply lets it do what it does best. I'm a fool for not trying one previously, my only defence being that there are so many boxes to try !
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vacuum_tubes
pfm Member
I know, bonkers. Creek OBH-22 into a Leak 20 into Falcon LS3/5a. -10db Rothwells required to achieve a reasonable turn on the volume knob. The only conclusion I can arrive at is that the Leak is an utter thug with anger issues !
Many old valve amps are very sensitive.
Get someone to add a dropper resistor / voltage divider to the input of the Leak
Alex S
carbon based lifeform
I challenge anyone to find a better £1k amplifier if you have an Allegri in a barn and vaguely sensible speakers. Also, it can be kept running and will gain value.
les24preludes
pfm Member
if you mean the Leak Stereo 20 I am taking up the challenge as we speak. My Stereo 20 has been rebuilt many times - I learned a lot of my valve knowledge from working on it over the years. When I dropped the global feedback loop and wired the EL84s in triode it immediately turned into a much nicer sound. This then meant rebuilding the front end, so I just used the long tailed pair without the input valve and substituted lower gain tubes - I used a pair of ECC40 for a while, which was a nice sound. I'm currently rebuilding it with a simple type 37 or 76 front end or maybe 6P5GT. Same valve with different sockets, and with a gain of 9 or 14 it's all the gain needed. Phase splitter will be a mosfet splitter.
Upgraded parts right through - 8H choke (2x 100mA in // underneath) and DC Link caps in the PSU, FT-3 teflon coupling caps, AudioNote 100uF Kaisei cathode bypasses in parallel with 100uF Elna Silmic II. The bypass caps took me a week of AB testing to figure out and I went through multiple recommended choices. The OPTs are rewired according to Morgan Jones for optimum performance. It will be another week or two's work but I'm expecting something very nice when it's all done. It's a personal thing - I was with my father when he bought it new many years ago. So it's a kind of homage to my dad. He was always experimenting and redesigned everything.
Tony L
Administrator
I know, bonkers
I guess it depends on the specific taper of the pot in question. I use an Audio Synthesis stepped attenuator pre and the position of the steps is fine with a standard 2V digital source and LS3/5As, I end up with the knob about ‘quarter to’ to ‘ten to’, which isn’t many steps, but it is enough to not jump crazily in level. The AS stuff has very well chosen steps though so these little passive pres work well in most scenarios.
The TL12 Plus are the same gain as a Stereo 20. The source, out of shot, is a Marantz SA-8005. It is plenty usable, no issues with gain at all. The Tannoys downstairs would be more problematic as they are 10db more efficient than the LS3/5As or JR149s. I’d likely have too little adjustment there.
I use an Allegri with a Leak Stereo 20. I'm very happy with this arrangement from a hifi prospective. No remote but I don't find the need change the volume very often. With the Allegri 6 on the clock face is 0. I'm usually at 7 or 8 for most of my listening so quite low on the volume scale. I don't know whether the channel matching is better than a potentiometer at that level but I've no complaints ....
Oh, I should add my speakers are Proac db1's at 88dB.
Some photos of the internals posted here when completed would be interesting to see.
Tim Jones
pfm Member
This is getting a bit spooky - one of my reasons for putting this off is that I'm not sure how my speakers would work with a Leak. They're ProAc DB1s...
Mike1965
pfm Member
Seems like the stars are aligning?
That's about where most of my use was on the Allegri as well, it used to slightly annoy me that one more was too loud but sometimes too quiet on the 7/8 setting. Down sides of sensitive speakers and not needing high SPLs, maybe normal people and systems don't have these issues.
TonyScarlett
pfm Member
Will have an ST20 for sale soon...
FWIW it seems, from my experience, to be a lottery which speakers will or won't work with the ST20. I think it has much more to do with the impedance curve than sensitivity, otherwise how would the they ever work with 83dB LS3/5As?
The only speakers I've owned that definitely didn't work were Amphion Argon 1's & Argon 3S's. Just sounded flat & lifeless which was a shame because otherwise the Argon 1's in particular were superb with the SS amps I owned, one of my favourite speakers.
TS