So I built this kit (http://tpadevices.com/hifiproducts.html), price seemed fair & I thought it would be a good baseline to compare my own sorry phono-pre efforts against.
Although the preamp is working fine I am experiencing a hum (10mV) on the output. Obviously the first place I suspected was my PSU which is in a seperate chassis connected with a 4-pair cable & 4-pin XLRs.
My B+ is 320VDC and my heaters are at 6.8VDC. Both supplies look very clean on the 'scope. I have experimented with both tube & solid-state supplies for the B+ and with both full wave and bridge supplies for the heaters. All supply designs were checked with PSU designer and performed to the programs' specifications. None of the designs made a difference to the hum level.
I contacted T-PAD but haven't heard back yet. Any suggestions from the brain trust?
Thanks in advance...
PB
Although the preamp is working fine I am experiencing a hum (10mV) on the output. Obviously the first place I suspected was my PSU which is in a seperate chassis connected with a 4-pair cable & 4-pin XLRs.
My B+ is 320VDC and my heaters are at 6.8VDC. Both supplies look very clean on the 'scope. I have experimented with both tube & solid-state supplies for the B+ and with both full wave and bridge supplies for the heaters. All supply designs were checked with PSU designer and performed to the programs' specifications. None of the designs made a difference to the hum level.
I contacted T-PAD but haven't heard back yet. Any suggestions from the brain trust?
Thanks in advance...
PB
What about your layout? There's lots of ways to get hum into a phono stage, most commenly from grounding and AC magnetic fields.
PSU is physically 2 feet from the preamp, audio connections are via shielded (one end tied to ground) audio cable. I tried locating a completely seperate B+ supply away from the heater supply, also bypassed my umbelical cable and ran direct wires to the pre, no change.
B+ secondary CT is tied to chassis ground as is incoming AC ground. I tried lifting the AC ground, no change.
Thanks for the ideas...
PB
B+ secondary CT is tied to chassis ground as is incoming AC ground. I tried lifting the AC ground, no change.
Thanks for the ideas...
PB
It'll be the earthing (it's always the earthing).
The 0V should be connected to the chassis really close to the input valve, not at the power supply. Really close means mm, not inches.
The 0V should be connected to the chassis really close to the input valve, not at the power supply. Really close means mm, not inches.
Most XLRs are only rated for around 80V !
now I can't wait to get off work to try that, aaah they've got me here until 10pm...
Thanks, I was suspecting grounding but thought I had tried most everything (except that of course).
PB
Thanks, I was suspecting grounding but thought I had tried most everything (except that of course).
PB
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