Author Topic: Preamp beat  (Read 388 times)

Offline fugo

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Preamp beat
« on: February 18, 2019, 03:49:20 PM »
I have a '75 piano w/ satellite speakers.  Everything sounds great, but when the vibrato is turned on there is a low-level beat heard through the speakers that varies with the vibrato speed.  I also have a cheek-block power supply for the preamp so I can use a different amp.  The beat is audible through the different amp as well.  Any thoughts on how to fix this annoyance?

Offline pnoboy

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Re: Preamp beat
« Reply #1 on: February 18, 2019, 04:23:45 PM »
Someone with a knowledge of electronics need to trace the signal through your preamp.  No doubt some component(s) in the tremolo circuit have problems.

Offline sean

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Re: Preamp beat
« Reply #2 on: February 27, 2019, 02:41:14 PM »

Fugo,

You report your problem occurs when the tremolo is ON, so I assume that is the turn-on click from the lamp driver transistors.

The only clear solution to CLICKING that I have seen posted is from 2008, where user Jordan attributed this solution to Sir Steve Woodyard:  Install a 50uf capacitor between the base of the lamp driver 2N3053 transistors and ground.  Positive leg of the cap to the transistor base, and negative leg to ground.  See https://ep-forum.com/smf/index.php?topic=4355.msg20992#msg20992

I am surprised about the value of the capacitor being so large, and I have never seen anybody post information about implementing this fix.  In fact, I haven't seen any mention of this fix since 2008.  I would expect that adding these capacitors would delay the turn-on of the lamp driver transistors, and possibly change the precious waveform of the ping-pong tremolo.  It might also slightly delay the turn-off of the lamp driver when the tremolo is turned off, as the capacitor has to discharge through the nearby diode to bring the base voltage down to equal the emitter voltage.
I wish I had a Peterson preamp to test with.  Oh well.

In 2006, Fred D posted a comment about bad cable connections contributing to thump:
https://ep-forum.com/smf/index.php?topic=1616.msg6599#msg6599



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You have the Super Satellite speakers, not Janus speakers? Right?

Super Satellite would mean you have the Peterson preamp (4-pin) with lamps.  And your power amp is FR-7054 (not FR-7710)? Right?  Your preamp has concentric knobs and is like http://www.fenderrhodes.com/org/ch11/fig11-8.jpg.

Sean


Offline fugo

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Re: Preamp beat
« Reply #3 on: March 04, 2019, 09:56:02 AM »
Sean: the preamp is the Peterson 4-pin with Satellite speakers, not the Janus. The preamp has not been rebuilt or otherwise modified - I am the original owner of the piano. The annoying thump/beat has been there for a long time.  The thread you referenced seems to describe the exact problem I'm hearing.  Hopefully the solution will work. Thanks.