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Old 15th May 2013, 12:25 PM   #11
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Join Date: Aug 2006
Location: W.Sussex
Originally Posted by cancon View Post
I think the NTE123ap made a modest improvement. Today I noticed that one channel has more hiss and gain than the other. I can get a handful of MPSA18 so perhaps it would be wise to replace the lot?

Chris - I should mention that my preamp is built into a separate enclosure for any input source.
And a very good idea, too, as long as you go straight from the RCA and avoid all the fender passive controls. I was too recently back from touring, and wanted minimum wiring, rather than maximum flexibility.

More gain? What kind of circuit makes the gain dependent on the actives rather than the passives? (I can't access the schematics page from your first post, gives me a frowny face, perhaps because my ip address isn't in the states.
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Old 15th May 2013, 01:29 PM   #12
cancon is offline cancon  Canada
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Here's a fresh copy.

Also, I'm not even going straight RCA - that cable is gone and a fresh shielded wire is in its place leading to a 1/4" input. I'll assess the hiss and gain again - I'm not certain if it is present during non-vibrato operation. Definitely uneven during vibrato.
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Old 15th May 2013, 04:34 PM   #13
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The only thing you've got after the volume control is emitter followers; they shouldn't hiss (no gain) and should give same signal in or out. And, when you switch the vibrato in, you're not adding any extra components, and you're not even starting the oscillator up (I'd be ready to accept the astable multivibrator moving the ground a bit, although I'd expect LF noise rather than hiss), and you're reducing the gain of one side or the other, never boosting it. When you switch the vibrato out of circuit all you're doing is turning the little light bulbs on full. I could understand a mismatched photocell, or light bulb for that matter, unbalancing the gain, but generating hiss?
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