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Aphex 107 'Tubessence' pre amp
« on: December 05, 2011, 12:59:51 PM »
Hi again guys,

I have an Aphex 107 'Tubessence' pre amp that is giving my friend problems.

The gain pots on both channels are very scratchy, and the 'scratchiness' increases for a period of time when phantom power is applied.

I measure .59VDC from ground to any leg of the gain pots.  When phantom is applied this spikes to around 1VDC, before ramping dwn to .59v again. I am under the impression that no DC should be present on a pot. 

When measuring pin 2 of the mic input with phantom power engaged, I get 44.3VDC.

Does anyone have a schematic I could use to troubleshoot?  I guess I am looking for a leaky capacitor somewhere close to the potentiometer. 

Advice is appreciated.



Re: Aphex 107 'Tubessence' pre amp
« Reply #1 on: December 05, 2011, 05:30:10 PM »
> 0.59VDC from ground to any leg of the gain pots

If all legs are the same, this is not a problem.

Re: Aphex 107 'Tubessence' pre amp
« Reply #2 on: December 07, 2011, 02:41:03 PM »
Oh...  Thanks PRR.  I think the voltage is the same on each leg, but will double check.

Any ideas on why the scratching would get worse when phantom is applied?

I think I will replace all the electrolytics for a start, there aren't very many of them.  I will give the pots a good clean as well.


andre tchmil

Re: Aphex 107 'Tubessence' pre amp
« Reply #3 on: December 07, 2011, 03:55:22 PM »
I love this preamp , especially on overheads and such.
Seems simple enough for a 500 redo  8)


Re: Aphex 107 'Tubessence' pre amp
« Reply #4 on: December 07, 2011, 08:06:11 PM »
Sounds like dc voltage , no ? check or subsitute caps before the pot
GKB Audio / Greg Boboski

Re: Aphex 107 'Tubessence' pre amp
« Reply #5 on: January 11, 2012, 10:33:01 AM »
Hi guys,

I swapped the ceramic and electrolytic caps before the pot of one channel.  Its still making noise!  :(

When I hit the phantom power switch and measure between the 2 legs of the pot (2 of the 3 legs are tied together), there is a voltage difference of 20mV for a few seconds before it slowly ramps down to 0.

The noise is only present for approximately 10 seconds after the phantom power switch is engaged/disengaged.

Any ideas?  I'm stumped, but I'll still have a poke around...


Re: Aphex 107 'Tubessence' pre amp
« Reply #6 on: January 12, 2012, 06:43:52 AM »
I've replaced all the electrolytic caps and ceramic caps in one channel of the preamp.  I've also replaced 2 large electrolytic caps on the 48V line, but its still making the same noise.  If it helps, I could upload a sample of the noise?

Thanks guys,


Re: Aphex 107 'Tubessence' pre amp
« Reply #7 on: January 12, 2012, 09:47:19 AM »
why don't you just change the pots?
It solved the problem on mine


Re: Aphex 107 'Tubessence' pre amp
« Reply #8 on: January 12, 2012, 12:55:18 PM »
Are they sealed pots, or just metal bodies? If you can open them you might be able to clean loose debris off the tracks with some alcohol before tossing them. That's worked for me in the past.
- Rodney

" better call Kenny Loggins, 'cause you're in the danger zone."

Re: Aphex 107 'Tubessence' pre amp
« Reply #9 on: January 12, 2012, 07:19:50 PM »
Hi guys,

Thank you for the replies.

I'm not sure if they are sealed or not...  They look like an average pot, plastic shaft metal body.

Hi Pedro planet.  Did you have the same problem with the loud burst of noise when engaging/disengaging phantom power?  I have tried cleaning them with electrical cleaner, but it had no effect on the noise...


Re: Aphex 107 'Tubessence' pre amp
« Reply #10 on: January 18, 2012, 10:52:09 AM »
I replaced the pot on channel 1.  Unfortunately this hasn't made a difference in the sound.  Good to be able to narrow it down further though!

I'm starting to think that maybe we can live with the noise...  If I don't track it down today I'll give it back to my friend as it is now.


Re: Aphex 107 'Tubessence' pre amp
« Reply #11 on: November 10, 2012, 07:08:42 AM »
Just come accross a's in the tech docs.
Love the canbus differential choke in the emitters of the input tr's  a nice practical twist to a classic circuit.
anyone seen the 207 mic lim circuit?
Onward and upward


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