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Old 28th May 2010, 03:39 AM   #1
glenz75 is offline glenz75  New Zealand
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Default Question regarding Leak Stereo 20 and 6SN7 line stage preamp

Hi everyone I'm a newbie sort of to the forum, joined up a couple of years ago but haven't been that active over here.

A couple of years ago I built up Frank's Line Stage preamp using 6SN7's to sue with my Stereo 20. While it sounded really nice and better than I'd expected, I found the gain was far too much for the Stereo 20, and using my Sony CDP with variable output I'd only have it up about a few notches on the CD volume control and the preamp control before it was almost too loud.
The Stereo 20 is sensitive in the gain department as we all know, so what would be the best course of action to take here?

1. Modify the preamp to have lower gain?

2. Change the ECC83's to ECC82's on the Stereo 20 input stage and have the driver tubes as ECC81's to reduce overall gain or modify the Stereo 20's input stage?

Its been a while since I've used this combination together and would like to use it again on my newly aquired Tannoy 15 inch Silvers which sound amazing by the way and because they are so dam sensitive I will probably just have to breathe on the volume control of the preamp and it'll be to much volume!

Any ideas or suggestions would be greatly appreciated.

Preamp circuit as shown below.

Many Thanks

Glen
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Old 28th May 2010, 06:24 AM   #2
fredex is offline fredex  New Zealand
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Suggest you get rid of the stage before the cathode follower first (un-needed gain),
just connect .033 to wiper. Leave Leak as is with orig tubes, but a pot mounted in place of the octal socket (preamp power) to control vol does not seem to affect resale value of this sought after amp.
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Old 28th May 2010, 06:34 AM   #3
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The ECC81 for splitter is a good idea. However, it's only a start, because it won't drop the gain by very much. You should rebias the splitter (i.e reduce the shared cathode bias redsistor) to get about 3mA per plate and reduce the tail and plate load resistors, too, to give similar plate and cathode voltages as with the ECC83. Ideally, you would replace the "long tail" with a CCS, for good balance.

For the first stage, I would not like to use an ECC82, which is not a very linear tube. A 6CG7 or 6FQ7 would be better, but you'd have to rewire the tube socket because the pinouts are different. Again, you'd need to adjust the plate current and voltage for about 5 - 8 mA.

You could then get a real decrease in overall gain by stepping up the NFB (i.e. reducing the NFB resistor). However, this would require some trial-and-error to get the right time constants to avoid LF and HF instabilities. Not a simple job, unless you're lucky!
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Old 28th May 2010, 10:12 AM   #4
SY is offline SY  United States
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As Ray says, unless you're going to change the feedback, making any tube subs will not alter the gain significantly. And changing the feedback will not be a trivial exercise.

Three choices:

1. Rebuild the Stereo 20 with a different input stage. A very simple input stage, such as Morgan Jones's Bevois Valley, will greatly increase the performance of the amp.
2. Use a unity gain preamp (e.g., just a cathode follower)
3. Put an attenuator at the power amp input. This can be a simple resistive divider, but take care in selecting values so that you don't have high frequency losses because of the amp's input impedance.
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Old 28th May 2010, 11:46 AM   #5
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How about lowering the CD output even more?
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