Hi,
I have quite a large hiss on 1 channel of an old audio research valve pre amp. This is whilst the amp is on and irrespective of selection channel or volume, it is not any specific valve (swapped them all) and is not the power amp. I have been told to try and swap out the anode resistors, and the optocoupler that creates an unusual auto mute circuit on this amp. No joy so far in either approach - should I carry on swapping out resistors on the cathode and grid or are there any other thoughts?
Hoping someone might have some insight. apart from the hiss on the left hand channel it sounds very nice...so quite annoying.
I have quite a large hiss on 1 channel of an old audio research valve pre amp. This is whilst the amp is on and irrespective of selection channel or volume, it is not any specific valve (swapped them all) and is not the power amp. I have been told to try and swap out the anode resistors, and the optocoupler that creates an unusual auto mute circuit on this amp. No joy so far in either approach - should I carry on swapping out resistors on the cathode and grid or are there any other thoughts?
Hoping someone might have some insight. apart from the hiss on the left hand channel it sounds very nice...so quite annoying.
The pre amp is SP8 Mark II. It would be great to isolate the problem if I can 
The circuit diagram is here it's revision E. the last one SP8_Manual_Schem.pdf - Audio Research
Great!
The circuit diagram is here it's revision E. the last one SP8_Manual_Schem.pdf - Audio Research
Great!
ok so I have tested the DC voltages and got some strange results.
B+1 and B+2 are pretty much on spec at 401v and 390v measured on both valves
Something weird is going on the LH channel as you can see / well beyond my logic+knowledge
180v V4 RH 300K across anode resistor (socket 1)
180v V5 RH 300K across anode resistor (socket 6)
180v V4 LH 301K across anode resistor (socket 6)
345v V5 LH 144k across anode resistor (socket 1)
single leg connection on q13 RH 190v
single leg connection on q13 LH 350v
any help much appreciated, and have a lovely musical weekend,
Rich
B+1 and B+2 are pretty much on spec at 401v and 390v measured on both valves
Something weird is going on the LH channel as you can see / well beyond my logic+knowledge
180v V4 RH 300K across anode resistor (socket 1)
180v V5 RH 300K across anode resistor (socket 6)
180v V4 LH 301K across anode resistor (socket 6)
345v V5 LH 144k across anode resistor (socket 1)
single leg connection on q13 RH 190v
single leg connection on q13 LH 350v
any help much appreciated, and have a lovely musical weekend,
Rich
I have swapped all tubes (every one int he pre amp) a number of times as I have a few in store so it's not the tubes.
I just can't understand how one side seems to have nearly twice the voltage yet the supply B+ are the same, almost everywhere I test a voltage point on the LH channel it is nearly 2x the RHS. I am unfortunately not an electronics expert (at all....) I am just using basic logic. Resistors drive voltage and somehow the LH channel must be different to the RH....etc. They are a symmetrical layout mostly using one side of the triod for each channel.
I just can't understand how one side seems to have nearly twice the voltage yet the supply B+ are the same, almost everywhere I test a voltage point on the LH channel it is nearly 2x the RHS. I am unfortunately not an electronics expert (at all....) I am just using basic logic. Resistors drive voltage and somehow the LH channel must be different to the RH....etc. They are a symmetrical layout mostly using one side of the triod for each channel.
Just make resistance measurements.
Power off and discharge.
Remove V5, V6 from sockets.
Remove left channel Q13 from board.
Remove left channel R73 from board.
Measure across left channel R26 (the part connected to V5 pin 1).
A) If not 301k, there is a path that should not be there.
Check board around V5, V6 carefully, both sides, and under sockets.
Something is connecting the pin 1 trace to some other circuit node.
Remove bad circuit path (solder, wire, etc.).
Reinstall R73 and Q13.
B) If 301k, reinstall Q13. Remeasure R26.
If not 301k, replace Q13. Remeasure R26.
If 301k, reinstall R73.
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That's how most all ARC preamps have been designed.
Did the hiss problem just now happen, or was it there since you bough it?
If it was there when you bought it, where did you get the preamp, and was it
worked on by someone before you got it? Has it been "modified" by someone?
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You may as well remove all the tubes when doing this. Then they won't be damaged during the work.
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Probably a hail mary.
Did the hiss start right after his work? Were the parts bought from ARC?
The fets are selected by ARC for proper operation in the circuit.
The power supply is common to both channels, so it can be ruled out.
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FETs not from ARC, I will see if I can get some - is this necessary do you think?
Hiss probably started after his work, but difficult to tell as the amp had lots of hum before due to a bad noisy transformer and the voltages on the circuit were all over the place. I would have given up with it years ago, but it just sounds to damn nice....even with hiss and hum!!
Hiss probably started after his work, but difficult to tell as the amp had lots of hum before due to a bad noisy transformer and the voltages on the circuit were all over the place. I would have given up with it years ago, but it just sounds to damn nice....even with hiss and hum!!
ok - so the repair guy has put in 2N 5460 JFETs not 2N5462, in all 4 locations.....Guessing I should swap all of these back into the original specifications.
I have also measured resistance across the JFET in circuit and it measures 250 ohm on the bad LHS and 447Kohm on the good RHS.....so it looks like the FET could be bad I guess.
I am almost certain to spoil the 1k grid resistor on removal, any suggestions of good quality replacements? I live in the UK and cannot buy directly from ARC in the US, I did with a very long process get a friend to buy 4 300k resistors when I though it was one of these as the ohmeter said 140k etc. They were just vishay(roederstein) mk8.
Super thanks for all the help
I have also measured resistance across the JFET in circuit and it measures 250 ohm on the bad LHS and 447Kohm on the good RHS.....so it looks like the FET could be bad I guess.
I am almost certain to spoil the 1k grid resistor on removal, any suggestions of good quality replacements? I live in the UK and cannot buy directly from ARC in the US, I did with a very long process get a friend to buy 4 300k resistors when I though it was one of these as the ohmeter said 140k etc. They were just vishay(roederstein) mk8.
Super thanks for all the help
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