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Old 17th May 2020, 4:53 pm   #1
Alan_G3XAQ
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Default Hum pickup on inter-stage coupling transformer

My lock-down receiver project uses valves. It's a hobby so I don't have to justify that. The product detector output impedance is a few kilohms and the input to the audio amplifier is around 500K. To make the most of the available signal I thought I would try a "600+600 to 47K microphone transformer" similar to T1 here

http://www.antentop.org/020html/020_p74.htm

The actual transformer I have is a Monacor DIB-110. FWIW the primaries are 92mH and the secondary is 38H.

I'm a bit astonished that when the high impedance winding is connected to the audio amplifier there is somewhat distorted mains hum picked up at an amplitude of around 5mV peak to peak. I put the transformer in a mu-metal can to no avail. Using a separate power supply makes no difference either. I've switched off everything in the shack and the hum is still there. It definitely seems to be the transformer: disconnecting it from the audio amplifier stops the hum pickup.

Is this a well known issue with interstage coupling transformers? Although it's somewhat academic can anyone suggest where or what the hum radiator might be?

73, Alan G3XAQ
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Old 17th May 2020, 6:15 pm   #2
kalee20
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Default Re: Hum pickup on inter-stage coupling transformer

Seems odd, especially for a microphone transformer! Plenty of mu-metal should have screened it very well.

With the hi-Z winding connected to the amplifier, what happens if you short-circuit the lo-Z winding?
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Old 17th May 2020, 7:30 pm   #3
Alan_G3XAQ
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Default Re: Hum pickup on inter-stage coupling transformer

Shorting the lo-Z winding makes no difference. The hum remains.
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Old 17th May 2020, 7:38 pm   #4
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Default Re: Hum pickup on inter-stage coupling transformer

And shorting the high-Z winding?

David
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Old 17th May 2020, 8:46 pm   #5
Alan_G3XAQ
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Default Re: Hum pickup on inter-stage coupling transformer

Shorting the hi-Z winding stops the hum. This shorts the grid of the audio preamp to chassis. Disconnecting (only) the grid wire at the hi-Z terminal of the transformer from the preamp also stops the hum. So the amplifier itself doesn't hum.

If I switch off the on-chassis mains PSU the hum remains for several seconds until the amplifier valve's heater cools. So I don't think it's the field from the mains transformer or HT choke.

I've tried switching off everything in the workshop but to no avail. If it is external it doesn't seem to be nearby.

Alan
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Old 17th May 2020, 10:00 pm   #6
GrimJosef
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Default Re: Hum pickup on inter-stage coupling transformer

Does re-orienting the transformer make a difference ? Standard E-I cores are more prone to flux leakage (in and out) in some orientations than others.

If the receiver's chassis is steel and the transformer is mounted directly onto it then another thing which may help is putting a thin (a millimetre might be enough) non-magnetic spacer between the transformer and the chassis.

Cheers,

GJ
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Old 17th May 2020, 10:22 pm   #7
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Default Re: Hum pickup on inter-stage coupling transformer

Some current in a common path impedance, maybe an earth loop from outside the unit bringing current through it?

David
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Old 17th May 2020, 11:03 pm   #8
Alan_G3XAQ
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Default Re: Hum pickup on inter-stage coupling transformer

Yes, changing the orientation of the transformer in the chassis does seem to have an effect. But I'm not sure how that can lead to a cure. I can't be living with a radio that only works when it's pointing towards the local chip shop. If flux leakage is at the root of the problem I fear I'll be ditching the idea of using a transformer.

The chassis is Aluminium.

Is there a likely layout that leads to the common path impedance? I only have a partially wired up chassis with a 2" twisted pair going to the transformer hi-Z winding and terminating at the grid and close to the earthy end of the grid leak and cathode resistors.

I am being "clever" and have a sheet of PCB material filling the underside of the chassis as an earth plane so I can solder component leads to earth with very short leads. Is my cleverness not so clever after all?

Alan
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Old 18th May 2020, 7:55 am   #9
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Default Re: Hum pickup on inter-stage coupling transformer

Try insulating the transformer mountings and have a single ground path to its core.

You can have circulating ground currents coupled into clamp screws etc creating a little bit of flux in the core.

It's the opposite direction of the problem of clamp screws through holes in lams creating transformer losses as a shorted part-turn because they intersect part of the flux.

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Old 18th May 2020, 8:27 am   #10
Alan_G3XAQ
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Default Re: Hum pickup on inter-stage coupling transformer

I've tried both insulating and grounding the core clamp. It makes no difference.

With all this mucking about I've managed to rotate one of the connection pins and break the wire from one end of the hi-Z winding. I'm meeting my chum from Medway tomorrow to receive a gift of some B9A screening cans and he says he will also donate a tiny 3VA 12+12v mains transformer from his extensive junk box for me to try before I order another "proper" microphone transformer.

Alan
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Old 18th May 2020, 9:17 am   #11
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Default Re: Hum pickup on inter-stage coupling transformer

What happens shorting the output with a small capacitor?

I am thinking that it could be an RF pickup problem.
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Old 18th May 2020, 10:09 am   #12
Alan_G3XAQ
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Default Re: Hum pickup on inter-stage coupling transformer

Which output? My original circuit had 2200pF across the hi-Z winding with the intention of resonating it at 550Hz although that capacitor got removed during this hum investigation. It made no difference to the hum.

73, Alan
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Old 19th May 2020, 12:34 pm   #13
Alan_G3XAQ
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Default Re: Hum pickup on inter-stage coupling transformer

I've been loaned a small 6VA 12+12:220V mains transformer to try in place of my damaged microphone transformer. It is the common unshrouded split bobbin type. Unsurprisingly, if anything the hum is even worse.

However, when I said "Using a separate power supply makes no difference either" I lied. When I did that test I placed the external HT supply downstream of the onboard rectifier and paralleled the external 6.3V AC heater supply across the audio amplifier valve. What I failed to realise is that this allows the external heater supply to energise the onboard mains transformer via its 6.3v AC winding. Doh! Disconnecting the 6.3V wiring at the onboard mains transformer cures the hum. I'm still a bit puzzled why with onboard power the hum persists for a few seconds after I switch off the mains but perhaps the big power transformer is coupling to general electrical smog within in the house?

So now the problem becomes how to avoid coupling between the mains transformer and the interstage audio transformer. Maybe a steel screen around the mains transformer made from a baked bean can is the next thing to try? If all else fails I suppose I can use an external PSU but that would be a last resort.

73, Alan G3XAQ
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Old 19th May 2020, 4:05 pm   #14
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Default Re: Hum pickup on inter-stage coupling transformer

Looking at skin depths it would need 1mm thick steel or more to shield a 50Hz field, unless I invest in a biggish piece of mu metal from which to make an enclosure for the mains transformer.

I've asked a local company to look into winding me a toroidal mains transformer to the same 125-0-125+6.3V spec as the existing EI cored unit. I wonder how much external field reduction it is likely to offer?

I see RS and others sell tiny toroidal mains transformers which might also be a good choice for the interstage audio transformer. I don't need hifi. I only need it to work at 550Hz for CW.

Alan
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Old 21st May 2020, 12:24 am   #15
JohnBG8JMB
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Default Re: Hum pickup on inter-stage coupling transformer

I'm sure you've done this, but is the audio wired in screened cable, with due attendance to ground loops?

73
John
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Old 21st May 2020, 10:15 am   #16
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Default Re: Hum pickup on inter-stage coupling transformer

5mV of hum output from a shielded mic transformer is totally exceptional and implies a very high 50Hz field. In a standard mic transformer application, it would be very loud!

As you say, it’s strange that you’ve observed the 50Hz signal continue when the mains transformer is disconnected. There shouldn’t be ‘general electrical smog’ at 50Hz in your house unless you’ve got a weird earthing issue in the mains wiring. Does the hum disappear if you move the input transformer well away from the amp? If so, then your suspicions regarding the mains transformer are confirmed. That’s not normal - think of all those valve tape recorders, relatively hum free, with inbuilt mains transformer near to tape heads, mic transformers etc

You’re sensible to try a different mains transformer.

Martin
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Old 21st May 2020, 2:31 pm   #17
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Default Re: Hum pickup on inter-stage coupling transformer

In post 5, you described the hum remaining, for a few seconds, after switch off. I had a similar problem a while back. That was caused by the live and neutral being wired the wrong way round, so that the on/off switch was in the neutral feed. Reversing it cured that problem. Not sure if it's the same as yours though.

Cheers

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Old 29th May 2020, 6:12 pm   #18
Alan_G3XAQ
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Default Re: Hum pickup on inter-stage coupling transformer

I've (mostly) cured the hum pickup on the audio stages of my lockdown receiver project by replacing the EI-cored mains transformer with a toroidal unit. I was quoted over 100 for a bespoke unit so I took a 16 6+6V 30VA transformer from RS and added an HT winding. I've attached some photos. The construction is described in a bit more detail in a thread on here

https://www.vintage-radio.net/forum/...d.php?t=166961

I am using another cheap 1.6VA 12+12V toroidal mains transformer to replace the broken microphone transformer. For minimum hum pickup I have to carefully orient this transformer so its core is at right angles to the core of the mains transformer.

The 5mV hum pickup referenced to the input of the audio amplifier that I had before is now reduced to about 0.1mV peak to peak, a 34dB improvement. I will try adding some tinplate shielding to see if this can be improved but even at this level the receiver will be usable.

Although it is now hard to hear I can still see the slowly decaying hum signal on a scope immediately after switching off. I am at a loss to explain it but it is no longer of any great importance.

73, Alan G3XAQ
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Old 29th May 2020, 7:12 pm   #19
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Default Re: Hum pickup on inter-stage coupling transformer

One thing to be aware of is that to minimise currents induced into the chassis you need to make sure that both 'sides' of any transformer-winding pass through the *same* hole in the chassis. This is particularly relevant to heater-windings - if you've got 2 or 3 Amps of heater-current going on, and you feed the transformer secondary-windings through different-though-adjacent holes in the chassis you can induce some horrible circulating currents in the chassis.

[My approach to this sort of issue - in a 'communications'-receiver - is to introduce aggressive audio roll-off below 300Hz. Things like not including cathode-decoupling capacitors, or if you do use them use low values like 0.25uF for the output-valve and 10nF for anything earlier in the audio-chain]
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Old 29th May 2020, 10:42 pm   #20
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Default Re: Hum pickup on inter-stage coupling transformer

Hi!

It might be worth while seeing if you can cancel it out by means of a 220 ohm preset across the heater supply of the microphone preamplifier, connecting the wiper to the supressor–grid (G3) of your preamplifier valve with a small capacitor of about 10n added from G3 to chassis.

It is possible to cancel out hum with a specialist "Wheatstone Bridge" technique in the output stage, but it involves alterations to the G.B. arrangements and needs careful adjustments!

Chris Williams
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