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Pre-Amp Kit 2.0. DC in Output, Help

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Posted on January 13, 2014 at 17:27:53
J Corner
Audiophile

Posts: 26
Location: Caracas
Joined: May 4, 2005
 I am having some problems with my pre-amp.
Some years ago I assembled the Audio Note Pre-amp KIT 2.0, it was initially connected to a pair of Jeff Rowland Model 1 amplifiers, and worked fine. Later, I bought a BAT VK-55SE amp and it has been working fine for several months.
Today I replaced the BAT for one of the Rowland and every time I turn the amp on it instantly turn itself off, some kind of protection circuit I guess. When I connected directly my DAC (it has a volume control) to the Rowland, it works perfectly.
So, I disconnected the AN pre-amp from everything but the AC cable and I measured up to 2.0 DC volts from the outputs. I think it is triggering the Rowland protection circuit ???
Is it normal to have DC current from a Pre-amp output ? Why the BAT amplifier works ?

I appreciate any help.

Thanks

 

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RE: Pre-Amp Kit 2.0. DC in Output, Help, posted on January 14, 2014 at 01:36:26
Vincin
Audiophile

Posts: 200
Joined: October 14, 2005
 You have a bad output capacitor, that is leaking DC. Replace it.

 

RE: Pre-Amp Kit 2.0. DC in Output, Help, posted on January 14, 2014 at 06:56:28
J Corner
Audiophile

Posts: 26
Location: Caracas
Joined: May 4, 2005
 Thank you very much Vincin,

I do not know too much about these things.

The Line Stage Board is quite simple, it has several resistors, the two ECC82 valves and three capacitors per channel:

1- 0.47 uF - 400V MKT big golden AN Copper Foil paper in oil and connected directly to the valve's plate.

2- 470 uF - 16V Electrolytic connected directly to the valve's cathode.

3- 100 uF - 450 V Electrolytic connected to ground.

I guess the bad ones are the No. 1 ???

One channel has ~2 DCV output the other one ~10 DCV output!!!.

I measured the capacitors with my capacitor tester and seems okey. Are these DC leakings detectable with these tester ?


Thank you again.

 

RE: a little more Help, posted on January 15, 2014 at 01:39:37
J Corner
Audiophile

Posts: 26
Location: Caracas
Joined: May 4, 2005
 Thank you very much Vincin.
,I do not know too much about these things.The Line Stage Board is quite simple, it has several resistors, the two ECC82 valves and three capacitors per channel:
1- 0.47 uF - 400V MKT big golden AN Copper Foil paper in oil and connected directly to the valve's plate.
.2- 470 uF - 16V Electrolytic connected directly to the valve's cathode.
3- 100 uF - 450 V Electrolytic connected to ground.
I guess the bad ones are the No. 1 ???
One channel has ~2 DCV output the other one ~10 DCV output!!!.
I measured the capacitors with my capacitor tester and seems okey. Are these DC leakings detectable with these tester ?
Thank you again.

 

RE: Pre-Amp Kit 2.0. DC in Output, Help, posted on January 15, 2014 at 15:24:22
xaudiomanx
Audiophile

Posts: 3647
Joined: August 16, 2004
 If you have any DC coming out of the RCA positive to ground with your test leads then they would trigger a protective circuit before it does more damage. DC into an amp is a definite NO! NO! That is the what the coupling caps going to the RCA outputs does. It blocks any DC and just lets AC pass. Same as in a power supply. AC goes to ground and dumps the AC ripple which causes hum.

Why it does it to the Rowland and not to the BAT is a good question. It could be the Rowland protective circuit is very sensitive to avoid a real meltdown but the fact that when you take out the preamp and go directly to the CD player with no issue tells you there is something not good with your preamp.

Try another preamp if you have the availability!

 

RE: Pre-Amp Kit 2.0. DC in Output, Help, posted on January 16, 2014 at 13:31:48
garymuffley
Dealer

Posts: 445
Location: Maryland
Joined: March 29, 2006
 Vincin is right. The .47 MFD coupling caps are bad. Older versions of the Audionote paper in oil caps were notorious for developing DC leaks.

 

RE: Pre-Amp Kit 2.0. DC in Output, Thank, posted on January 17, 2014 at 15:45:15
J Corner
Audiophile

Posts: 26
Location: Caracas
Joined: May 4, 2005
 Thank you xaudiomanx, I already ordered a pair of Jensen capacitors.

 

RE: Pre-Amp Kit 2.0. DC in Output, Help, posted on January 17, 2014 at 15:46:27
J Corner
Audiophile

Posts: 26
Location: Caracas
Joined: May 4, 2005
 Thank you Vicin, I already ordered a pair of Jensen caps.

 

RE: Pre-Amp Kit 2.0. DC in Output, Help, posted on January 18, 2014 at 08:34:27
Wojciech
Audiophile

Posts: 4124
Joined: June 23, 2009
 I guess Bat amp works because I has DC blocking cap on the input which Rowland amp doesn't have.

 

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