Hum issues on transformer output preamp

[bikerhifinut]

This is a strange one.

I eventually got round to putting together a preamp based on the transformers and PCB I bought a while ago. I've kept it pretty much as standard that way with the only mod being JC's one to attenuate the output to give a gain of 3x, i did this by splitting the grid leak to second stage grid into 2 x 470k and connecting the 0.1uF cap to the top of the divider. this is done by putting a resistor in place of the wire link as per Johns info.
There's also a Third party remote control board in place but this can be isolated which i did as part of the diagnosis, and a straight feed to the Volume control inserted. (Alps RK271 50k).
So everything connected up, fed with 270V from the regulated PSU. Voltages check out, it's drawing around 28mA HT which I think is about right? The voltages are a little bit lower than the book but that seems to require 300V HT although the Circuit diagram shows the Ht as 250V. Heaters are of course 6.3V Regulated DC.
Ok so regulated HT therefore I expect no PSU hum. PSU ground lifted from mains /chassis earth by use of 100R lift resistor.
The issue seems to be a Hum loop related to the mains earth. On a WD88VA power amp there is audible if quite low level hum as soon as the pre is plugged in to it.
On my normally silent hum wise SE EL84 amp it goes into orbit. None of my other preamps do this.
The SE hums really bad even with the power off, but as soon as I disconnect the umbilical power lead it reverts back to silence. I noticed that touching any of the metal chassis or earth points also causes a bit of hum, this surely shouldnt happen. I checked the input phonos, they arent the insulated sort so to eliminate them I rigged an input socket that was isolated from the chassis, thinking if that fixed it I'd simply go and buy a set of insulated phonos. But its just the same, as soon as the umbilical is connected, even powered down it starts to hum like a demented bee.
Checked and rechecked the wiring. All measures well and looks ok. No obvious hum loops. I tried an earth lift resistor here too, no effect.
So it brings me to the point of scratching my head. There HAS to be an answer, it HAS to be something fairly obvious. At least I would think so.
Now, of the few things I havent checked out or altered, one is the isolated transformer secondaries I'm wondering if contrary to logic, the hum loop is to do with the cold ie earthy end of the output not being referenced to ground? Much is made of the Pre3 having an isolated output to eliminate Hum loops and I was rather hoping that would be the case. It has to be a loop.
I didnt use screened cable from the input to the volume pot and from the volume pot to the input on the board but if it was hum pickup caused by that then disconnecting the umbilical wouldnt affect it and it does. At line level and with no mains transformer fields in the vicinity, screening doesnt have any real effect, certainly on the CF and other preamps it hasnt caused hum. I'll give it a go but I know it'll still show the fault.
I have heard in the past of people having all manner of problems with transformer output preamps and I'm wondering if it's going to be worth the bother. However I want to get it right.
For the record its built in a modified early WD HD3 case which I believe was one of the original prototypes from back in the day.
There's got to be a logical reason thats staring me in the face.
Ah well its going away for a month or 3 this weekend as on Monday the builders start and the audio kit will be boxed/bagged up and in protective custody.
A.

[bob orbell]

Andy, I had this very issue with Pre II, what happens if you remove just one interconnecting plug from your pre to power amp, left or right dose it go silent ?, and as soon as you reconnect the hum returns. BOB

[snowman_al]

^^^ +1
And what happens if you remove the earth wire from the mains plug?

[bob orbell]

You could get a shock or worse, you die . Bob

[bikerhifinut]

I did think about removing the mains plug earth, but it's not a fix really and it seems my IEC leads have moulded plugs anyway. There is obviously a fault inside the new build and I just have to trace it.
I tried altering the run of the signal ground as one run right back to the main star at the earth terminal as I would on a hard wire build but none of it makes any difference.
Bob I tried that trick removing one plug but it makes not a jot of difference. I subbed my CF pre using the same PSU and everything went back to its normal silent background.
It must be something fundamental that I have missed as if there was an inherent issue with the transformer output design we'd know of it here.
I'm going to leave it be for a while. Lifes going to get very busy at home.
If I can't get to the bottom of it. I'll swap the aikido CF board over into the big case as a known quantity and use the full remote facilities. I had wondered about the remote control board and input relay board but after disconnecting the power (6.3V off the heaters) and also bypassing the relay input board, the hum was still very present. It's got to be a hum loop, just a question of tracing it.
Ok packing it all away, i may have a word with JC at Tonbridge for some pointers.
A.

[bikerhifinut]

Another look and I tried the solid state mitsubishi power amp, this is double insulated so has no mains earth. The hum is very low on this, but still there and I think theres a bit of mileage now in new input sockets and better screening of the input signal wires as I suspect a small internal loop.
Now heres a thing, I disconnected the mains earth temporarily at the power plug of the PSU and the hum got way worse! I'd previously noticed that putting a finger on any earthed points in the preamp caused a hum, again this is an anomaly I think. Finally i did a ear to the speaker check on the WD88VA which is as good as silent on the RH channel and the residual hum on LH that I never got rid of is usually so low as to be only just discernible with lug ole to speaker. I usually do this with a shorting plug or the quiet preamp in the inputs. Well, blow me if there isn't a raised level of hum on that too. I have a hunch that the ring main upstairs where the kit now resides may have a fault on it.
So on monday at 9am the electrician is coming to isolate all the first floor electrics prior to the work starting in march on ripping the roof and floors out. I shall have a word with him as I wonder when we had the consumer unit replaced and moved to a more convenient location, if the sparks that did that bodged it. I found out a few months later when we were getting a new kitchen in that he'd done an illegal install with insufficient paperwork and we had to shell out for a new set of tails in from the meter as he'd put undersized ones in, no doubt to save effort threading them under the floorboards. I now wonder if the reconnection wasnt done right and theres an earth fault on the upstairs ring main. We have 4 separate rings in this house which is historical due to previous extensions and moving kitchens etc. That and a couple of spurs for odd appliances needing their own fused supply. Anyway I will have a word with the lad, he can do a full check as well as the new mains upstairs post refurb. We'd decided on running a completely new ring and spurs anyway as it will work out cheaper than trying to trace the old wiring and expend it round etc. So I hope it will all check out.
Do any forumistas who have a working knowledge of domestic wiring think my suspicions may be correct in that I'm chasing a fault that lies outside the equipment in the household wiring?
It'll be a good 3 months before I get back up there so I'll not know till thats done, but I will at least have a nice supply for the A/V gear with a bank of sockets to supply all the bits n bobs along with concealed ethernet cables for the digital stuff and a co ax run for a bigger FM Aerial so the troughie will come out of hiding.
I feel a bit better in that the hum issues are probably easily sorted out. I may still ditch the transformer output but a quick hour or so suggests that theres something to the sound, it seems more gutsy yet full of the HF and midrange detail that I like. Worth persevering with, I may even get Matthews control board just because its probably a bit more smoother in operation than the ebay job. I cant use the WD relay board because the holes dont match up but thats no big deal wiring a relay board up isnt that hard to do all said and done.
A.

[bikerhifinut]

I had another look at this today.
I replaced the Phono sockets with proper insulated ones that isolate their outers from the chassis to eliminate any risk of a ground loop that way.
Plugged it into the lounge audio system via its aux socket and turned the wick up full to use it like a power amp.
If anything the hum is even worse, very loud 50Hz hum.
Wiring checked, and I can not see where I may have fouled up. Transformer wiring just like the manual shows.
Checked the aerth/ground runs, I cant see any deviation from the manual in terms of the Pcb grounding and signal grounding.
Switched the psu off and left it connected to the mains, the hum actually got louder
Disconnected Umbilical.................. Silence.
So I thinks maybe my PSU is faulty. But all checks out.
I pulled out the ACF Cathode follower preamp/buffer stage. Plugged it all in with the same PSU. Total silence in the background.
So the issue is somewhere in the transformer output preamp, but I can't see how hum is coupling in.
I am at the stage of ripping the transformers out and hoying them in the bin and reverting to the old faithful CF output job. I want a bit of gain but that is dead easy to achieve and I have plenty of circuit choices to choose from.
I have eliminated the possibility of the remote control electronics being the culprit by the simple expedient of disconnecting it fully and hardwiring the volume control to an input. And still it hums.
It's either something simple, or the transformers are coupling into an external hum field somewhere, in which case the whole thing is a waste of time.
I'm very disappointed right now but at a complete loss as to where to start.
I did do the gain mod of inserting a potential divider after the interstage coupling capacitor, using a resistor in place of the link wire, I simply split the 1meg grid leak into 2 x 470k to give a halving of the signal into the output valve, this gives 3x gain which is about right for my needs.
If thats where the problem lies, maybe due to an impedance mismatch? it's a relatively easy fix.
Otherwise I cant see where to look.

A

[bob orbell]

Andy, I may have said before but, do the cables from the transformers to the output sockets connect to ground, the output should float, IE, "NO" connection to ground, that is as far as my thinking goes. BOB

[bikerhifinut]

yes Bob, completely isolated outputs floating and meter checks that out.
The Fault lies either in the internal circuitry or is being somehow coupled directly into the output transformers themselves.
There's a heck of a lot of gain in the 2 stage amp and any slight amount of hum would be boosted accordingly but would be attenuated in the output transformer along with the rest of the signal.
I havent screened the wiring to and from the Volume control, but again that shouldn't make a blind bit of difference at line levels. I've never really needed screened wiring and the hum disappears as soon as the connection to the PSU and I assume mains earth is unplugged, this happens whether the PSU is switched on or off, i.e the Preamp is switched off but still connected up. In fact the hum increases when the pre is turned off.
Disconnecting the remote control and bypassing the input relays to make a direct hard wired connection to the volume control has no impact either. This because its an after market job off ebay and that set of boards hasn't given me problems in the past anyway.
Which boils it down to earth loops as far as I can see.
I tried a chassis earth lift inside the case, an adapted HD3 case used for one of the early prototypes and bought off Nigel a couple of years ago so I am following the Pre 4 layout. That made no difference either and I didnt expect it to. I tried moving the signal earth from the input sockets from the earthy end of the Volume control to the star earth point on the HT decoupling capacitor, this is a 60 +250uF job and I used it as an extra CRC filter on top of the already ripple free regulated Ht supply. There are no reasons to suspect that.
The PCB does seem to have a lot of earthing points on it but I followed the instructions on this anyway, If there is a loop there it's designed in and surely would have been spotted in the development stage.
The transformer screens are all earthed as per the diagrams, and the transformer frames make a good chassis contact. Primary and secondary windings measure correctly and are isolated from each other and the frames.
It leaves me with only 1 option which is to use some screened wire from input to pot and down to the board. I've got a length of the blue stuff that was supplied with my KT88 amp and loadsa other high quality 2 core screened microphone cable which is what the blue stuff is. If that's the issue then sorted but if thats the case, then those transformers must be Radiating an induced field as if they were picking it up and coupling into the secondary, the truns ratio which must be someweher between 100:1 and 200:1 assuming a gain between 10 and 15 on each ECC82 stage would push any induced noise right down into the background.
Obviously the kits must work and work noiselessly or there'd be a rash of threads on here to that effect.
I bought the the boards and transformers from a member here, I forget who now and they were all brand new unused so we can eliminate damage I would think.
I'm Baffled.
If I don't get to the bottom of it soon, I'm Scrapping the transformer circuit and reverting to the known high performing circuits, basically a decent CF, probably based on the Broskie active load designs as theres room for the extra valve, with a low gain front end based on something like a 6CG7 or 6SN7 which I have in the parts bin.
I think a complete rework is in order with attention to earth runs and if that fails, then there is something awry else where.


Andy.

[bikerhifinut]

I think I sorted it.
However i departed from the WD build instructions.

I knew it had something to do with Earthing and signal ground and there was a hum loop of some sort.
Now you would think that the "floating" output from the transformers would isolate this problem and that somehow the hum was induced elsewhere.
I noticed the hum got really bad when the PSU was connected and even worse when the PSU was plugged in to the mains but not switched on. This had been baffling me for weeks and in truth still does.
Anyway I got curious and thought I would try a short wire link from the outers of the output RCA's i.e. the CT of the output transformer to signal ground at the star earth point. Nothing to lose.
Well all the nasty 50hz hums disappeared and its as quiet in the background as you'd like it to be.
So I wonder why this is and why as a standard build, following the WD build instructions, the preamp was a nasty noisy thing?
It's fine fixing something, but if I don't really know the reasons why then I will probably go out and make the same errors again as for all I know I am merely masking an existing issue?

A.