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Post by Ned Ward on Jul 26, 2020 16:27:05 GMT -6
I have a Farfisa Compact Duo organ - and it's great. The F/AR preamp, however is noisy and low output with a good hum that appears to be grounding related; when I plug the 3-prong cord into an Ebtech Hum-X, it decreases. But signal:noise is still bad even with volume cranked. Wondering if it's easier to build a simple version of this preamp without accordion input, reverb or tone controls using more modern components, and with that, what a cost estimate would be? farfisa.org/wp-content/uploads/2009/08/F-AR.jpg |
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Post by johneppstein on Jul 26, 2020 17:53:45 GMT -6
Has the organ been serviced regularly? The reason I ask is that the caps in those are pretty crappy and could probably stand replacing.
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Post by EmRR on Jul 26, 2020 18:09:29 GMT -6
Germanium transistors. Many types absorb moisture from the air and grow noisy. Since it's only a 2 stage amp (turning the reverb off) it should be pretty quiet.
There are probably a ton of things that'll sub in perfectly, off the shelf, as far as a replacement amp goes.
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Post by Ned Ward on Jul 27, 2020 9:57:20 GMT -6
Has the organ been serviced regularly? The reason I ask is that the caps in those are pretty crappy and could probably stand replacing. John - I took it into Advanced Musical Electronics down here in Venice back in 2010-11 and they did recap it. It's been a while since, but I remember it was expensive to do but the right thing to do. |
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Post by Ned Ward on Jul 27, 2020 9:59:43 GMT -6
Germanium transistors. Many types absorb moisture from the air and grow noisy. Since it's only a 2 stage amp (turning the reverb off) it should be pretty quiet. There are probably a ton of things that'll sub in perfectly, off the shelf, as far as a replacement amp goes. Hmm - that's helpful as I live 1.5 miles from the beach and would make sense; the F/AR unit was already fairly rusted on the exterior when I got it. So it may be easier to mod the existing preamp to get the hum out and improve signal:noise? My electronics experience is zip, so even if I bought a soldering station and multimeter I wouldn't want to mess with this since without the preamp the organ is useless. |
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Post by EmRR on Jul 27, 2020 10:34:04 GMT -6
IT really looks like you can plug it into almost ANY hi-Z preamp and get something. Have you tried an active DI box into a preamp?
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Post by Ned Ward on Jul 27, 2020 11:43:49 GMT -6
IT really looks like you can plug it into almost ANY hi-Z preamp and get something. Have you tried an active DI box into a preamp? Doug - The Farfisa Duo has a 7-pin Amphenol plug that goes into the preamp, and then the external preamp has the 1/4" output. While I do get sound, there's a lot of hum, and signal to noise has gotten worse. Without the Farfisa preamp (which also supplies power to the organ), no power and no sound. Here's a link to 2 photos of the preamp unit: |
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Post by svart on Jul 27, 2020 12:09:06 GMT -6
I think you already pointed out that it might be grounding, and I agree that it sounds like a bad ground somewhere, mostly likely in that 7 pin cable. The 9VAC could be a significant source for hum if one of the leads were grounded by accident or wear as well.
As EMRR pointed out, you could simply feed this with a signal directly and then use a high impedance input like a FET DI input on a preamp and see what you get directly. The output of the "LF output" is very high impedance due to the PNP common emitter and the series pots for the tonestack.
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Post by EmRR on Jul 27, 2020 12:16:05 GMT -6
IT really looks like you can plug it into almost ANY hi-Z preamp and get something. Have you tried an active DI box into a preamp? Doug - The Farfisa Duo has a 7-pin Amphenol plug that goes into the preamp, and then the external preamp has the 1/4" output. While I do get sound, there's a lot of hum, and signal to noise has gotten worse. Without the Farfisa preamp (which also supplies power to the organ), no power and no sound. Here's a link to 2 photos of the preamp unit:O, that's all in a separate box. Which you can keep for powering the organ, but you need to break out the audio from the organ into a DI box separately, see how that goes. May not kill the hum, but should kill the noise if its from the preamp in that box. And maybe it's noise from within the organ instead? Have to determine that. |
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Post by Ned Ward on Jul 27, 2020 14:08:50 GMT -6
I think you already pointed out that it might be grounding, and I agree that it sounds like a bad ground somewhere, mostly likely in that 7 pin cable. The 9VAC could be a significant source for hum if one of the leads were grounded by accident or wear as well. As EMRR pointed out, you could simply feed this with a signal directly and then use a high impedance input like a FET DI input on a preamp and see what you get directly. The output of the "LF output" is very high impedance due to the PNP common emitter and the series pots for the tonestack. Thanks - in looking at the schematic, it seems like having a box that can supply power to the organ (it's my understanding 30V are for the organ, 9V are for lights on the front) and have a no-tone stack preamp or some way to get audio out would be ideal to then put into a DI. Right now it's going into a KC500 keyboard amp and the gain has to be 3/4 full to get any volume - same level on my K2000 is deafening. And for fun, the 1/4" output is 1/2" from the AC cord (grounded) output on the side. While a 3-prong was installed, the grounding point may not be the best - I did notice reduced hum plugging it into a Hum-X isolation transformer. Part of me wants to keep the F/AR stock and build a streamlined modern unit that delivers clean power, no grounding issues and the sound. I know it can sound good as 9 years ago it sounded great without a lot of noise. |
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Post by svart on Jul 27, 2020 14:42:13 GMT -6
I think you already pointed out that it might be grounding, and I agree that it sounds like a bad ground somewhere, mostly likely in that 7 pin cable. The 9VAC could be a significant source for hum if one of the leads were grounded by accident or wear as well. As EMRR pointed out, you could simply feed this with a signal directly and then use a high impedance input like a FET DI input on a preamp and see what you get directly. The output of the "LF output" is very high impedance due to the PNP common emitter and the series pots for the tonestack. Thanks - in looking at the schematic, it seems like having a box that can supply power to the organ (it's my understanding 30V are for the organ, 9V are for lights on the front) and have a no-tone stack preamp or some way to get audio out would be ideal to then put into a DI. Right now it's going into a KC500 keyboard amp and the gain has to be 3/4 full to get any volume - same level on my K2000 is deafening. And for fun, the 1/4" output is 1/2" from the AC cord (grounded) output on the side. While a 3-prong was installed, the grounding point may not be the best - I did notice reduced hum plugging it into a Hum-X isolation transformer. Part of me wants to keep the F/AR stock and build a streamlined modern unit that delivers clean power, no grounding issues and the sound. I know it can sound good as 9 years ago it sounded great without a lot of noise. The issue is that it's probably an easy fix, but it takes some gear and some know-how to do it. it should be easier to fix this one than build a new amp by far. |
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Post by Ned Ward on Jul 27, 2020 16:11:54 GMT -6
Thanks svart - will wait on this and research who in LA could do this (or within a 2-hour drive). Appreciate everyone's input.
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