TPAD Devices Kit

philipbarrett · 9th January 2006, 07:02 PM

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

ErikdeBest · 9th January 2006, 07:28 PM

Can be a problem of the layout, therefore it would be great if you can post some pictures

Erik

philipbarrett · 9th January 2006, 07:46 PM

Check the website, it's a fairly substantial PCB board, nice fat traces...

SY · 9th January 2006, 07:53 PM

What about your layout? There's lots of ways to get hum into a phono stage, most commenly from grounding and AC magnetic fields.

philipbarrett · 9th January 2006, 08:04 PM

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

EC8010 · 9th January 2006, 08:12 PM

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.

pinkmouse · 9th January 2006, 08:21 PM

Most XLRs are only rated for around 80V !

philipbarrett · 9th January 2006, 08:22 PM

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

9th January 2006, 07:02 PM	#1
philipbarrett diyAudio Member Join Date: Feb 2005 Location: Dallas, Texas	TPAD Devices Kit 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

9th January 2006, 07:53 PM	#4
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	What about your layout? There's lots of ways to get hum into a phono stage, most commenly from grounding and AC magnetic fields. __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

9th January 2006, 08:12 PM	#6
EC8010 diyAudio Moderator Join Date: Jan 2003 Location: Near London. UK	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 loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...

9th January 2006, 08:21 PM	#7
pinkmouse diyAudio Moderator Join Date: Apr 2002 Location: Chatham, England	Most XLRs are only rated for around 80V ! __________________ Al I conceive of nothing, in religion, science or philosophy, that is more than the proper thing to wear, for a while. Charles Fort

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9th January 2006, 07:28 PM	#2
ErikdeBest diyAudio Member Join Date: Mar 2003 Location: Itoculo	Can be a problem of the layout, therefore it would be great if you can post some pictures Erik

9th January 2006, 07:46 PM	#3
philipbarrett diyAudio Member Join Date: Feb 2005 Location: Dallas, Texas	Check the website, it's a fairly substantial PCB board, nice fat traces...

9th January 2006, 08:04 PM	#5
philipbarrett diyAudio Member Join Date: Feb 2005 Location: Dallas, Texas	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

9th January 2006, 08:22 PM	#8
philipbarrett diyAudio Member Join Date: Feb 2005 Location: Dallas, Texas	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

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