Fender rhodes peterson vibrato preamp - diyAudio
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Old 9th May 2013, 02:52 AM   #1
cancon is offline cancon  Canada
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Default Fender rhodes peterson vibrato preamp

Hello all,

I've got a 1967 peterson era rhodes preamp on hand.

Schematic: http://www.fenderrhodes.com/org/ch11/fig11-8.jpg

There is considerable hiss brought on by the circuit. Cutting the treble knob helps greatly (but no fun with treble cut!). When the vibrato circuit is active, the his is worse. Following these discoveries, I re-capped the board given the schematic values as well as those shown on this picture:

http://www.vintagevibe.com/misc/rhod...oard%20pic.jpg

At this point I've only changed out the electrolytics. Swapping them had little effect on overall hiss, but the swap caused the vibrato to thump in the attack portion of waveform. There was no thumping before the re-cap. When I received this preamp I noticed that someone had altered the cap values. If you follow that picture, the 4x 47uf caps and 1x 16uf cap were all installed as 100uf.

I know there is a quick-fix for thumping vibrato by adding some RC networks to the oscillators, but I want to avoid this if possible. This changes the characteristics of the vibrato wave to a triangle and I think it will negatively impact the sound.

Before I go re-installing all those caps I was wondering if anyone had any input on the matter.

thanks!
adam

Last edited by cancon; 9th May 2013 at 02:54 AM.
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Old 9th May 2013, 05:00 AM   #2
Enzo is offline Enzo  United States
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Hiss is not generally caused by filter caps. They fail and cause hum or unwanted DC leakage. Usually in a low voltage circuit like this my first suspects for hiss would be semiconductors. Changing the e-caps is probably a good idea though.

If the treble control drops the hiss, then the source is before that control, so the input stage. It only has two transistors. SCope it to see, or just replace them. Drawing says 2N3392, but most any nice low noise signal transistor should work fine. The circuit only runs on +25v, so most transistors cover that.
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Old 9th May 2013, 04:23 PM   #3
cancon is offline cancon  Canada
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Thanks enzo.
I've got a few 3392 kicking around - but is there a better low-noise substitute?
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Old 9th May 2013, 07:28 PM   #4
Enzo is offline Enzo  United States
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Low noise NPN signal transistors? My own go-to choice would probably be an MPS8097 or maybe 2N5088. And if there are any left in my drawer some MPSA18. I forget now what I used them in a lot, but I have a bunch of 2N5210 too. Those are old school numbers, there may be some more recent favorites, but I have large numbers of those in stock.

I suspect if you went to On Semi and pulled up the selector guide page for low noise TO92 you would find some other possibilities. It really isn't critical.

If transistor XYZ is quiet enough in a circuit, I see no reason to sub something else just because it has a lower noise figure. This isn't hifi.
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Old 10th May 2013, 02:23 AM   #5
cancon is offline cancon  Canada
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Ok, I picked up some NTE123AP which cross-referenced to both numbers you listed first.

Any thoughts to which caps would most likely influence thumping?

thanks again. and again. and again.
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Old 10th May 2013, 08:57 AM   #6
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Fender Rhodes do hiss. Always did, always will; we just didn't notice so much before digital. Its a function of being high impedance (all those series coils, like lots of guitar pickups) and inductive; the theoretical noise generated by a resistor at a given temperature goes up with the value of the resistance, and the amount of high frequencies goes up because the impedance rises with frequency.

My trick was a dynamic noise filter; a sort of noise gate with a very fast attack speed and quite fast delay, operating only on the high frequencies. It opens with the "ting" of the tine attack, and drops the hiss as soon as the sustain comes in, leaving a relatively sinusoidal decay (which is what a Rhodes produces anyway, unless you hit it hard enough for major nonlinearities in the tine/pickup position)

Using a ring of three circuit as a crossover you can easily sweep the frequency; invert the polarity of the bandpass output before mixing it with the LP and the gated HP (incidentally, if you put pots on the three signals this is a very interesting instrument equaliser).

I'm afraid I'm in the middle of moving and all my circuits and paperwork are sealed in cardboard boxes, but I'm sure noise gate circuits will have improved in the twenty years since I built it, anyway.
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Old 11th May 2013, 12:47 AM   #7
Enzo is offline Enzo  United States
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Nothing prevents you from trying those NTE parts, but always remember the NTE cross refernce tells you which one of THEIR transistors they think is closest to the original, the NTE parts are NOT the originals. That is why one NTE number crosses to hundreds of industry parts, and not just one.


Yes they are noisy, but I think transistors have improved in the last 40 years, and for the whole dollar a pair of the newer types would cost, it is worthwhile to try to reduce it. But I don;t disagree, a noise gate might be the ticket.
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Old 11th May 2013, 09:31 AM   #8
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Not noise gate; that chops off all the sustain. Dynamic filter; only the high frequencies brought down, Mine was a copy of the Audio and Design Scamp module, with a LED/photocell active device, thus there was a touch of delay on the attack; I always felt it would have been better with a totally solid state solution, but (having installed the unit into the Rhodes 88) never got round to it.

And we sold on the Rhodes over ten years ago (new proprietor loved the sound, but was quite upset about me having made the unit phantom powered. Had to add an external power supply).
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Old 14th May 2013, 07:03 PM   #9
cancon is offline cancon  Canada
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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.
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Old 14th May 2013, 11:13 PM   #10
Enzo is offline Enzo  United States
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Hey, it would be a cheap experiment, either way.

I suspect the most likely place for a noisy transistor would be the input stage.
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