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Old 16th June 2007, 11:54 PM   #1
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Default Input attenuation on Leak amp

Hi, first post... I'm hoping someone can help me, I'm banging my head against a wall.

I have a Leak Stereo 20 which drives my Quads beautifully, which I've fully restored... the only problem is that its 2.87V input sensitivity is much too high - I get virtually no action on my preamp volume control.

So, taking a tip from Japanese audiophiles, I have been trying to pot down the input using a 250K carbon pot. No attenuation (and the pots check out OK on my DMM). I don't know if I'm wiring them wrong or what.

My latest wiring for the pots was RCA positive (center) pin to terminal 3, output (to first 1/2 12AX7) to terminal 2, ground to terminal 1. Terminals are numbered starting from max attenuation (all the way CCW) on the pot, when the pot is held with terminals sticking down.

No attenuation... no variance when the pot is adjusted either.

Any hints?

Thanks in advance,
Patrick
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Old 17th June 2007, 03:20 AM   #2
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Patrick,

You tackle the problem from 2 different perspectives. At the amp's I/Ps a 12AX7 is shared between the 2 channels. Replace that 'X7 with a 12AY7. You go from a mu 100 twin triode to a mu 40 twin triode. Both the 'X7 and the 'Y7 are high Rp/low gm types. The reduction in gain will be beneficial, but insufficient.

You attenuate the signal at the 20's I/Ps by inserting resistive voltage dividers into the RCA plugs. Use tiny 1/8 W. metal film parts. A 68 KOhm part in series with the wire going to the center pin and a 33 KOhm part from center pin to outer shell will provide almost 10 dB. of attenuation.



Edit: fixed typo
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Old 17th June 2007, 03:22 AM   #3
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Firstly, your wiring plan wounds okay, provided that you remembered to disconnected the lead from the RCA jack to the input tube grid. If not, the 250k pot would just be in parallel with the grid resistor and it wouldn't work.

Secondly, is the 250k pot linear or log law (audio taper)? If it's linear, all the 'action' will be in the first few degrees of rotation from minimum.
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Old 17th June 2007, 04:16 AM   #4
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Eli - thanks for the suggestion. I'm not familiar with the 12AY7 but will look into it. I'm fond of the Mullard 12AX7 that I have in the V1 position. Presumably the voltage divider would have to be 2 resistors in series to ground, with the output coming from the join between the 2?

Ray - I didn't remove the 22K grid resistor, but kept it in series from the output of the pot. Also, there's a 1M resistor between the center pin of the RCA jack to the ground of the jack - I don't know what that does, but I kept that as well. Should both be removed?

The pot is linear taper... but it didn't provide any attenuation during any portion of its rotation. I didn't think I needed audio taper for this application... right? I hope?

Patrick
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Old 17th June 2007, 04:57 AM   #5
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Patrick, you don't really need an audio taper pot, it just makes it easier to adjust. What you DO need, however, is to disconnect the 1Meg resistor (which acts as a grid resistor) from the RCA jack. Reconnect it to the center terminal of the pot, together with the lead that goes to ther 22k (grid stopper) resistor. See if it works like that.
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Old 17th June 2007, 07:19 AM   #6
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Ah, thanks Ray. I need to learn the difference between grid resistor and a grid stopper resistor. I shall try this first thing tomorrow.

Patrick
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Old 17th June 2007, 04:16 PM   #7
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Sorry, I'm slow... I actually had the 1M resistor going from terminal 1 of the pot to ground.

So it should go in series with the 22K resistor from the center terminal on the pot to the V1 grid?
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Old 17th June 2007, 09:16 PM   #8
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No, Patrick, the 1Meg resistor should go from the center terminal (wiper) of the pot to ground.
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Old 17th June 2007, 11:14 PM   #9
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That worked, thanks Ray!

Patrick
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Old 17th June 2007, 11:36 PM   #10
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It's not too often that I hear someone complain that 2.87V input sensitivity is too much. Have you thought about lowering the gain of your preamp? How about eliminating it altogether and going with a passive attenuator?

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