Mellotron preamp schematic - diyAudio
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Old 23rd March 2011, 08:38 PM   #1
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Default Mellotron preamp schematic

My Mellotron is a noisy beast. It works fine, but the hiss it makes is prodigious. I know that with the head arrangement, its not going to be all that quiet, but OTOH, I've really been thinking a lot about updating the playback electronics with something a bit newer.

Right now it has the stock circuit, although I did go through it and replaced all the electrolytics and a good number of the more critical resistors, and that did seem to help but I think I could do better.

But before I get started, it would be nice to know what I have, without going through the effort of working out the schematic from the boards themselves. Anyone have a schematic for this thing?
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Old 23rd March 2011, 11:37 PM   #2
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Have you considered replacing the tape loops with a better quality low noise tape? jer
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Old 24th March 2011, 03:50 PM   #3
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Mellotrons don't use loops, FWIW. They have about 7-8 seconds of play time, and when you release the key, a spring rewinds the tape.

However, the noise that is bothering me is not so obvious during playback. So I am talking about the noise floor of the electronics.
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Old 24th March 2011, 03:56 PM   #4
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Understood. jer
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Old 25th March 2011, 05:20 PM   #5
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Playback preamps made out of some low niose opamps woudn't be very hard to do.
But on the record side things can get a bit tricky.
However it can be done. jer
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Old 25th March 2011, 06:08 PM   #6
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Quote:
Originally Posted by geraldfryjr View Post
Playback preamps made out of some low niose opamps woudn't be very hard to do.
But on the record side things can get a bit tricky.
However it can be done. jer
Not a problem, because (as far as I know) Mellotron's are play only.

Interesting fact - the 'flute' at the start of 'A Stairway To Heaven' was actually a Mellotron and not a flute.
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Old 25th March 2011, 06:37 PM   #7
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There was a project in Guitar Player a long time ago that was an Echoplex "de-hisser". It was basically 2 circuits, an EQ to boost highs on the input side and and an EQ with the opposite effect for the output side, sort of the same concept as using RIAA for records. Probably would work equally well on your Mellotron, although I'm not sure how the input side would have to be rigged if it's play-only.
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Last edited by leadbelly; 25th March 2011 at 06:40 PM.
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Old 25th March 2011, 07:34 PM   #8
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I was not aware that there is not a record function, very interesting. jer
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Old 25th March 2011, 09:22 PM   #9
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There is no record function. You buy tape sets from the manufacturer, Streetly Electronics.

The hiss is entirely from the EQ circuit, and I suspect that the transistors have become noisy. An opamp circuit is not out of the question, but I have to sort out the EQ first- that is why a schematic would be handy.
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Old 27th March 2011, 05:45 AM   #10
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Other concerns aside for a moment, if you can identify which transistors are in the noisy EQ circuit, it should be easy enough to ID them, and just replace them with low noise equivalents. A bit of a shotgun approach, but bypasses the lack of a schematic.
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