Phono Hum

We all start somewhere
User avatar
IslandPink
Social Sevices have been notified
Posts: 8830
Joined: Tue May 29, 2007 7:01 pm
Location: Denbigh, N.Wales

#1 Phono Hum

Post by IslandPink »

I have been putting off dealing with this for a while, too much going on with the speaker work.
Finally I started having a look during the week.
I suspected it was arm wiring or earthing , but taking the phono wires from the Mayware and checking continuity, all checked-out.
So I then took Nick's advice and used some phono connecters earthed with small-values resistors ( actually 169R ) as inputs, to take input signals out of the picture. Switched things on and loads of hum.
This was without the turntable motor supply , which does radiate some hum, so clearly that's a lesser problem.
Turned off the phono amp, and almost everything went away.

So, has to be something internal to the phono amp.
I got it upside down on the kitchen table and had a good look. Nothing obvious, nothing disconnected, all earths in place.
Got the voltmeter on, and also everything seemed more or less as noted on the picture below ( red ) when built.

So today I got the scope out and started a more detailed investigation.
Results are below (blue) - pretty surprising. There are some things I don't understand. Why is the mixed signal of 0.5mV above the anode resistor turning into 1.7mV of mostly 50Hz at the D3a anode, then ending up as 60mV of quite clean 50Hz after the 2nd D3a ?
Islandpink_phono_hum.jpg
I hope I was using the scope correctly. I did turn a couple of knobs to 'Cal' ( ibration ) to get the frequency measurements - that looked correct given you'd expect 100Hz from most passive smoothed supplies. It was on 'Auto' triggering.
Ps. the main PS is now cLCLC before the CCS/Vr , hence the small value of 2.5mV there. Seems the CCS isn't doing its job properly, unless the 60mV hum demand is affecting it somehow ?
"Peridots and periwinkle, blue medallions Gilded galleons spilled across the ocean floor" ( Mitchell )
izzy wizzy
Old Hand
Posts: 1367
Joined: Fri Nov 02, 2007 7:02 pm
Location: Auckland NZ
Contact:

#2 Re: Phono Hum

Post by izzy wizzy »

Have you earthed the frame of the input tx?

I normally have a binding post that has mains earth on it and then take all other metal things to this post including 0V. I have tx out so isolated earth wise from other components. Some peeps float the 0V connection on a resistor or b2b diodes or some such to reduce earth loop issues.

You might also have interaction with other components. This is where I have lots of croc clips and muck around earthing things in various combinations to see what happens and hopefully some combo sorts it out.

Cheers,
Stephen
User avatar
IslandPink
Social Sevices have been notified
Posts: 8830
Joined: Tue May 29, 2007 7:01 pm
Location: Denbigh, N.Wales

#3 Re: Phono Hum

Post by IslandPink »

Good point. I checked and the step-ups each have two M4 studs that come through the top-plate and attach with nuts. So I think there's earthing of the cases there.. except that I can't see that the outer part of my top-plate is actually earthed anywhere :oops:
There's a job for some time.
However, all this comes to naught , because I checked using a clip-lead connected to main ground and touched this to the bolt heads and the actual mu-metal casework of each step-up, and there was no change at all on the 60mv.
ps. the two channels are behaving almost exactly the same, maybe the gain is a bit different in the L channel as there's 70mV, but it all looks the same. So it's very unlikely there's a fault in one D3a with the heater insulation, and the fil supply affecting the signal. In any case, heater hum is 100Hz, not 50Hz ....
"Peridots and periwinkle, blue medallions Gilded galleons spilled across the ocean floor" ( Mitchell )
izzy wizzy
Old Hand
Posts: 1367
Joined: Fri Nov 02, 2007 7:02 pm
Location: Auckland NZ
Contact:

#4 Re: Phono Hum

Post by izzy wizzy »

Have you lifted 0V to ensure you haven't got an earth loop? Or are you doing all this in isolation of it being connected to anything downstream?
User avatar
IslandPink
Social Sevices have been notified
Posts: 8830
Joined: Tue May 29, 2007 7:01 pm
Location: Denbigh, N.Wales

#5 Re: Phono Hum

Post by IslandPink »

This is on the kitchen table, just the phono and scope.
I'm just going to go upstairs for some mu-metal to see if there's any chance the input D3a's are just picking up 50Hz from the mains transformer or chokes - although they are at the other end of the unit to the PS.
"Peridots and periwinkle, blue medallions Gilded galleons spilled across the ocean floor" ( Mitchell )
User avatar
IslandPink
Social Sevices have been notified
Posts: 8830
Joined: Tue May 29, 2007 7:01 pm
Location: Denbigh, N.Wales

#6 Re: Phono Hum

Post by IslandPink »

IslandPink wrote: Sun Jun 23, 2019 4:46 pm I'm just going to go upstairs for some mu-metal to see if there's any chance the input D3a's are just picking up 50Hz from the mains transformer or chokes - although they are at the other end of the unit to the PS.
No effect at all. Surprising.
One thing I did notice, checking scope earth lead , there's a measurement limit around 0.2 to 0.3mV that could be just due to the probe earth picking up fields or something scope-internal. Some of the non-50Hz components on the HT feed to the 1st valve ( ~0.5mV ) may not be real.
In that case, the question is how the 1.7mV of 50Hz is getting onto the first valve anode. After that, the gain is probably about right to deliver 60mV out. There's not much RIAA attenuation at 50Hz.
"Peridots and periwinkle, blue medallions Gilded galleons spilled across the ocean floor" ( Mitchell )
User avatar
Nick
Site Admin
Posts: 13656
Joined: Sun May 06, 2007 10:20 am
Location: West Yorkshire

#7 Re: Phono Hum

Post by Nick »

Being 50Hz instead of 100Hz makes me think its pick up instead of supply related. I would try removing the input transformer and just leave the 10k grid to ground and see if that makes any difference.
Little known fact, coherent thought can destructively interfere with itself leaving no thought at all, that’s why I prefer incoherent thought.
User avatar
IslandPink
Social Sevices have been notified
Posts: 8830
Joined: Tue May 29, 2007 7:01 pm
Location: Denbigh, N.Wales

#8 Re: Phono Hum

Post by IslandPink »

Yes, good hunch.
I got about 10x reduction in output hum.
Perhaps it's just a few inches of unshielded ( but twisted ) litz from the connectors to the step-ups, then ?
The wire from Txs to circuit is shielded.
Phono_hum_test.JPG
"Peridots and periwinkle, blue medallions Gilded galleons spilled across the ocean floor" ( Mitchell )
User avatar
Nick
Site Admin
Posts: 13656
Joined: Sun May 06, 2007 10:20 am
Location: West Yorkshire

#9 Re: Phono Hum

Post by Nick »

You might want to try isolating the step up cases from the chassis. There is a lot of potential for hum loops there. I will take a look in the hammond case my pair of those stepus are in how I did the earthing.
Little known fact, coherent thought can destructively interfere with itself leaving no thought at all, that’s why I prefer incoherent thought.
User avatar
IslandPink
Social Sevices have been notified
Posts: 8830
Joined: Tue May 29, 2007 7:01 pm
Location: Denbigh, N.Wales

#10 Re: Phono Hum

Post by IslandPink »

Ah, righto. Interesting ...
"Peridots and periwinkle, blue medallions Gilded galleons spilled across the ocean floor" ( Mitchell )
User avatar
IslandPink
Social Sevices have been notified
Posts: 8830
Joined: Tue May 29, 2007 7:01 pm
Location: Denbigh, N.Wales

#11 Re: Phono Hum

Post by IslandPink »

Nick : "I would try shorting after the 12k resistor to ground and see if the 60mv hum goes away. That way you are tracking it down to the first stage and transformers"

Yes, this did the trick. Hum at output much less , < 5mV.
I also checked the ripple on the B+ after the CCS and this was halved. So that must have been ripple from AC demand on the VR tubes, which are not very 'stiff' regulators.
"Peridots and periwinkle, blue medallions Gilded galleons spilled across the ocean floor" ( Mitchell )
User avatar
IslandPink
Social Sevices have been notified
Posts: 8830
Joined: Tue May 29, 2007 7:01 pm
Location: Denbigh, N.Wales

#12 Re: Phono Hum

Post by IslandPink »

Been a bit sidetracked but managed to try a few things and this evening with success.
So we left-off with the rogue signal coming in to the signal inputs, nothing wrong with the power supply or the signal circuit picking up anything on its own.
Step-ups are under suspicion.
Found my attempt to earth the top-plate had failed. This was an interesting one -
Earth wire is soldered to the clip holding one of the VR tubes, mounted on the top-plate. This (gold ) clip is screwed to the top-plate with washers and stainless-steel screws. Checked continuity to the plate, and there was none - at least up to the kOhm region. remarkable !
I unscrewed one of the screws and used copper grease on the head and under the washer underneath to achieve something below 1 ohm.
This did nothing for the hum.
Touched earthed clip-lead to the actual casing of the step-up transformer. Also no improvement.

So, this evening, finally ... connections on step-up.
The secondary has screened wire from the step-up output to the circuit and the screen (neg tag) is earthed at the circuit star-earth near the input valve.
The primary, on the other hand, is made of a few inches of twisted litz wire, and comes in from the RCAs to the primary tags without any earthing - ie. floating. I checked this with an earthed clip-lead and bridged across from the primary 'neg' to the secondary 'neg'. Hum on the output went down by a factor of 10 .
:D

I might also get some screened wire prepared, to replace the litz on the primary side. I'll see how it sounds first.
"Peridots and periwinkle, blue medallions Gilded galleons spilled across the ocean floor" ( Mitchell )
User avatar
Ali Tait
Eternally single
Posts: 4151
Joined: Fri Jun 08, 2007 8:10 pm
Location: Galashiels

#13 Re: Phono Hum

Post by Ali Tait »

Result! Well done.
izzy wizzy
Old Hand
Posts: 1367
Joined: Fri Nov 02, 2007 7:02 pm
Location: Auckland NZ
Contact:

#14 Re: Phono Hum

Post by izzy wizzy »

Nice one. Is it possible you've unearthed a turntable and/or arm earthing/screening issue? Thinking along the lines of keeping it configurable rather than permanent by using earth post or sumfink.

Cheers,
Stephen
User avatar
IslandPink
Social Sevices have been notified
Posts: 8830
Joined: Tue May 29, 2007 7:01 pm
Location: Denbigh, N.Wales

#15 Re: Phono Hum

Post by IslandPink »

I don't know for sure. Hopefully it isn't the arm, but I do have a proper earth post for the arm , & that connects straight in to the signal star earth.
I have played around a bit with the arm earth, at some stage I think it sounded good with that attached to the top-plate rather than the ground connection. It may be that I was just playing off this hum signal against the arm hum signal. At least this way (now) I should have the arm, the top-plate, the step-up housings and the step-up primary and secondary negatives all connected to the same star earth. Fingers crossed, will try later.

I have a suspicion things went wrong when I changed from the circular Tribute step-ups to a square pair that Pieter had made with more primary inductance. I think on the older pair I had the neg connections bridged so they shared the signal earth.
"Peridots and periwinkle, blue medallions Gilded galleons spilled across the ocean floor" ( Mitchell )
Post Reply