Hello,
I have an Audio Research SP9 MKII that has spontaneously developed a hum in the left channel phono stage. Schematics, etc. are here: http://www.audioresearch.com/ContentsFiles/SP9MKII Manual.pdf
The preamp still functions normally and sounds fine, except for the hum which is very noticeable. Removing C27 from the inputs to both channels causes a big reduction in the hum, but it is still present.
I have physically examined the preamp, and made a bunch of measurements, but nothing obvious has been found. I'm not sure where to look next and any help and suggestions would be much appreciated!
Take care,
Doug
I have an Audio Research SP9 MKII that has spontaneously developed a hum in the left channel phono stage. Schematics, etc. are here: http://www.audioresearch.com/ContentsFiles/SP9MKII Manual.pdf
The preamp still functions normally and sounds fine, except for the hum which is very noticeable. Removing C27 from the inputs to both channels causes a big reduction in the hum, but it is still present.
I have physically examined the preamp, and made a bunch of measurements, but nothing obvious has been found. I'm not sure where to look next and any help and suggestions would be much appreciated!
Take care,
Doug
While I realize that the SP9 was not one of ARs best efforts, this is my only phono preamp, I am happy with it's sound, and I would really like to get it working properly. I suppose that I could always gut it and use the chassis to build a new preamp, but I would prefer not to do that, and it seems like a shame to destroy something that works well in every respect other than the (relatively minor I hope) hum problem.
Here are some more details and troubleshooting steps that I have taken, hopefully somebody can give me some hints or tips that might point me in the right direction for solving this.
The preamp worked just fine until one day I went to listen to some records and noticed the humming, loud enough to be immediately noticeable, only when using the phono inputs. It sounds just like any other ground loop problem, except that it only really affects the left channel. Since the problem spontaneously appeared, it seems unlikely to be a wiring problem, and I suspect some type of component failure.
My first step was to swap out the tubes in both the phono and line stage, but this made no difference. Disconnecting C27 (1000pF capacitor between the phono input ground and chassis) on both channels significantly reduces the hum, although it is still slightly audible at normal listening levels. Installing shorting plugs into the phono inputs makes the hum a lot louder, and in both channels instead of only the left. All RCA jacks have been unscrewed from the back panel to confirm that there are no shorts between their ground connections and chassis, as have the front panel potentiometers and selector switches. All electrolytic capacitors in the preamp have been replaced with no effect. All coupling capacitors in the phono section (C6/28) and the line section (C9/29 and C12/30) have also been replaced. Disconnecting the main chassis ground from the preamp circuitry ground or the AC outlet ground does not significantly affect the problem. Removing C25 and C26 from the mains inputs makes no difference. I have checked continuity and secured all fasteners between every chassis point and ground connections.
Power supply voltages measure reasonably closely to specification and there is no significant AC ripple. DC voltages between both channels are reasonably close to each other. Adding new capacitors in parallel with C18 and C20 in the power supply section made no difference.
I can post actual voltage readings if anyone would like me to provide them. One thing that I notice is that touching the voltmeter leads to various points in the phono preamp circuitry produces significant "thumps" and causes loud buzzing/humming sounds in the output of the channel being measured. I'm not sure if this is normal.
After closely examining the schematics I can see no component failure that I could imagine causing this problem without affecting the sound or output level, yet the preamp still sounds just fine to me.
Any and all help and suggestions are very much appreciated, hopefully with a little help I can get to the bottom of this strange failure.
Take care,
Doug
Here are some more details and troubleshooting steps that I have taken, hopefully somebody can give me some hints or tips that might point me in the right direction for solving this.
The preamp worked just fine until one day I went to listen to some records and noticed the humming, loud enough to be immediately noticeable, only when using the phono inputs. It sounds just like any other ground loop problem, except that it only really affects the left channel. Since the problem spontaneously appeared, it seems unlikely to be a wiring problem, and I suspect some type of component failure.
My first step was to swap out the tubes in both the phono and line stage, but this made no difference. Disconnecting C27 (1000pF capacitor between the phono input ground and chassis) on both channels significantly reduces the hum, although it is still slightly audible at normal listening levels. Installing shorting plugs into the phono inputs makes the hum a lot louder, and in both channels instead of only the left. All RCA jacks have been unscrewed from the back panel to confirm that there are no shorts between their ground connections and chassis, as have the front panel potentiometers and selector switches. All electrolytic capacitors in the preamp have been replaced with no effect. All coupling capacitors in the phono section (C6/28) and the line section (C9/29 and C12/30) have also been replaced. Disconnecting the main chassis ground from the preamp circuitry ground or the AC outlet ground does not significantly affect the problem. Removing C25 and C26 from the mains inputs makes no difference. I have checked continuity and secured all fasteners between every chassis point and ground connections.
Power supply voltages measure reasonably closely to specification and there is no significant AC ripple. DC voltages between both channels are reasonably close to each other. Adding new capacitors in parallel with C18 and C20 in the power supply section made no difference.
I can post actual voltage readings if anyone would like me to provide them. One thing that I notice is that touching the voltmeter leads to various points in the phono preamp circuitry produces significant "thumps" and causes loud buzzing/humming sounds in the output of the channel being measured. I'm not sure if this is normal.
After closely examining the schematics I can see no component failure that I could imagine causing this problem without affecting the sound or output level, yet the preamp still sounds just fine to me.
Any and all help and suggestions are very much appreciated, hopefully with a little help I can get to the bottom of this strange failure.
Take care,
Doug
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Seems like an aging capacitor, like C13, C21 etc. that is injecting noise into the system ground,
but it could be anywhere in the preamp. The C27 (RF filter) is just a local chassis ground path
in the phono stage for the noise, and when removed the path is opened.
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Thank you so much for your reply! I am starting to make some headway with this problem now thanks to your input.
I had already replaced both C13 and C21 with no change. Disconnecting C31, C32, C33 made no difference.
I then removed C23 which is a 0.22uF capacitor across the 15 volt rail where it goes to power the muting circuit. The hum immediately vanished and even the right channel is quieter now. Thinking that I had found the problem I installed a new capacitor, and the hum returned just like before. A second capacitor of a different brand did the same. When I measured the original capacitor I could find no sign of any leakage.
When I first bought this preamp about 12 years ago the mute circuit was inoperative. I ended up replacing both of the MC1455 timer chips to fix the problem. Now I am wondering if there is a strange problem with one of the timer chips or with the 15 volt regulator. The tube heaters are on this same supply, but I can't imagine how they could cause this issue.
I suppose that I could always leave this capacitor out, but I would prefer to fix this properly. Any further suggestions?
Take care,
Doug
Could be oscillation in the 15V regulator. Try adding a 10uF 25V electrolytic at the regulator's output,
very close to pin 3, with a short lead to ground. This should be stable with any type of 15V regulator.
It's unlikely to be the timers.
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OK, I replaced the regulator, no change.
I will try a 10uF capacitor next.
Take care,
Doug
Yes, I have gone over the whole preamp multiple times looking for bad connections, shorts, etc. and found nothing. I definitely seems like a grounding issue to me too.
Take care,
Doug
Yes, they all have been replaced actually. I will check them again, however.
Take care,
Doug
This covers the most likely parts. Now it's down to (more or less) trial and error.
Do look for bad solder joints first.
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Unfortunately this URL is death. From where I can download schematics of all versions ?
A friend of me have sometimes (temporarely) hum effects while turn on the attenuator knob resp. for volume control and balance (Audio Research part No resp. order code 45100528 and 45100529) - maybe through internal contact issues at the variable resistor for attenuation.
Accordingly of various images on the web this parts look like Alps RK27112 - go to
https://export.rsdelivers.com/produ...tric-rotary-carbon-film-potentiometer/2199130
My friend want to replace it by an ELMA 04 rotary switch like
Elma 2 pole 24 way switch, 04-2133 | Hifi Collective
or include already exist resistors - go to
Classi-Audio Elma 24 Position Stepped SHUNT Attenuator, 79,95 €
Are there any experiences ? Most interesting question is, whether there is enough space therefore or not.
In case of too small space, other versions of rotary switches are better suited for this application. in post #135 under
Potentiometer Overview - Alps, Penny+Giles, TKD and Vishay and more, what's your fav?
I have listed various other.
Thanks for advices.
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Thank you very much. For the Owner's Manuals of the versions MK-II and MK-III there are even schematics including. Unfortunately not for the first version.
This may be the first version schematic.
AUDIO-RESEARCH SP9 PREAMPLIFIER Service Manual download, schematics, eeprom, repair info for electronics experts
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