Hum /Hiss Problem

[diviy]

I have the following pre amp http://www.ebay.co.uk/itm/Douk-Audio-Affordable-12AX7-Valve-Tube-Preamp-HiFi-Stereo-Pre-Amplifier-4-input-/271995526354?hash=item3f54325cd2
It was rebuilt by Fire Bottle I have had a hum/hiss from the phono stage since I have had it finally getting round to sorting it .
The Pre amp has two Arcam 8p power amps acting as mono blocks after much playing about I have found with the pre amp off and the power amps on only when amp is set to phono even with the pre amp off I am getting a hum /hiis when I turn pre amp on it gets worse .
I am going to change the power supply leads tomorrow but any ideas what else I could look to do ?

Cheers Gerry

[RothwellAudio]

What phonostage? As far as I can see, that pre-amp doesn't have a phonostage.
BTW, a valve pre-amp will usually be noisier when it is switched off than when it is on. Or at least it should be. Having it switched on lowers the source impedance seen by the power amps, hence lower noise.
Having said that it could be designed completely differently and the source impedance seen by the power amps remains high either way.

[diviy]

Firebottle used one of the tubes to add a phono stage plus power mods and a general re design of the amp its is based on the Marantz 7 design

[Firebottle]

What phonostage? As far as I can see, that pre-amp doesn't have a phonostage.

The original circuit design was fairly dire, the low impedance cathode follower output went straight to the volume pot :doh:

As a project I rebuilt it using two of the 12AX7 vaves to give an MM phono stage with 40dB gain, then volume pot into the third 12AX7 as a line stage with 9dB gain and an emmiter follower output.

The finished pre-amp was quiet and hum free so I'm guessing at an earth loop.

What speakers are you using Gerald, are they high sensitivity?

[RothwellAudio]

I rebuilt it using two of the 12AX7 vaves to give an MM phono stage with 40dB gain, then volume pot into the third 12AX7 as a line stage with 9dB gain and an emmiter follower output.
Sounds like a sensible design and not too much gain.
If the problem is an earth loop it would be made worse if the monoblocks are plugged into different mains outlets. Maybe try powering all three components (preamp + both monoblocks) from the same mains socket.
Alan, is the preamp's 0V connected directly to mains earth?

[Firebottle]

There is a 10 ohm earth lift resistor.

[diviy]

I have ditto 15 xrs speaker's