Xiang Sheng 728A Hiss / HUM

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yaka24

Member
2011-12-11 8:28 am
Hi All - I have a Xiang Sheng 728A preamp that I hadn't had hooked up in a while. When I place it into my system now, attach 1 input (tried 2 different sources) to my Krell KAV-500 SS amp I'm getting a fairly audible hum / hiss from my speakers (both woofer and tweeter). Measured from an app on my phone it appears around 150hz.


The hiss is fairly audible with no music playing. It does not change with the volume control or on any other input / with inputs unplugged.


Replacing the preamp with a passive pre I have, completely eliminates the noise. Everything is plugged into a Furman Power Conditioner PST-8



Any ideas where to start probing? Here's a link to the schematic:


http://www.g4cnh.com/public/overall_circuit.bmp
 
Looks like you've got a ground loop there. Both of these devices appear to be protective earth referenced (IEC Class I). A high impedance line isolation transformer will be required.

BTW, that preamp is a pretty clever construction in that it makes use of the level handling of vacuum tube circuitry and puts the volume control last for minimum noise output. The only thing I'd consider critical is its potentially rather high output impedance (roughly 25 kOhms worst-case)... it could have used an extra buffer at the output if you want to be driving a solid state power amp (where a 20 kOhm input impedance is not uncommon and performance may suffer with excessive source impedance), not to mention that cable shielding and capacitance become moderately critical like that. It may still be possible to hack in a high voltage MOSFET source follower.
 
Last edited:

yaka24

Member
2011-12-11 8:28 am
Looks like you've got a ground loop there. Both of these devices appear to be protective earth referenced (IEC Class I). A high impedance line isolation transformer will be required.

BTW, that preamp is a pretty clever construction in that it makes use of the level handling of vacuum tube circuitry and puts the volume control last for minimum noise output. The only thing I'd consider critical is its potentially rather high output impedance (roughly 25 kOhms worst-case)... it could have used an extra buffer at the output if you want to be driving a solid state power amp (where a 20 kOhm input impedance is not uncommon and performance may suffer with excessive source impedance), not to mention that cable shielding and capacitance become moderately critical like that. It may still be possible to hack in a high voltage MOSFET source follower.


Any link to one I can purchase? thank you..
 
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