I would like to thank everyone for their useful inputs.

 

The other half of the device is a pickup based on Faradays work and comprises a neodynium bar magnet with single turn coil which works very well indeed and is extremely compact. I have registered this novel design. There are another products out there doing very well, generically called 'flat-pups' but this is much smaller, cheaper and faster to make and can sit on the surface of guitars, especially cigar box guitars. Here is one i made encased in wood veneer.

View attachment 257639

So the to idea is to redesign the pickup to be a low-impedance device, then step the voltage up with the current transformer, electrically sort of a bit like a ribbon microphone?
Some of those really titchy current transformers are only intended for high frequency use in switched-mode power supplies and don’t have a response that goes down to 50Hz.

 

The current transformer cores I am using are designed to measure current from 50/60Hz power line cables. I had a batch custom wound to the ratio required, and not the usual 1000:1 that are commonly used, although those do work, but not optimally. The sound is very clear and smooth across
whole guitar's whole range, apart from the hum with certain guitar amplifiers (see above).

 

And You really think you've proved something .........
Go back to YouTube.
.

Thank you, I will. They are polite there.

 

The current transformer cores I am using are designed to measure current from 50/60Hz power line cables. I had a batch custom wound to the ratio required, and not the usual 1000:1 that are commonly used, although those do work, but not optimally. The sound is very clear and smooth across
whole guitar's whole range, apart from the hum with certain guitar amplifiers (see above).

View attachment 257671

It seems much like the distinction between moving coil and moving magnet cartridges. Moving coil is about 100μV @ 10Ω output impedance, moving magnet is 5mV @ 47k. A normal guitar pickup is about 30mV, so yours is probably about 30μV.
Moving coil is stepped up using a transformer, which generally costs a fortune and is encased in a mu-metal screening can.
https://uk.rs-online.com/web/p/audio-transformers/2106469
Putting your transformer in a mu-metal can might be worth a try, but I think I'd be attempting an electronic solution, with a preamp made with ultra-low noise transistors.

 

Those transformers cost 52p each delivered, but I did buy quite a few of them to get that price. If I can figure some passive way to make it behave on Guitar amps I will be happy. Grounding it all did work but I had to get into the cabinet to do it.
I will see if I can buy a mu-metal pot for a test. Thanks for the thought. These builders are generally adverse to putting batteries or phantom power into instruments, although this is changing.

 

Is it perhaps a bit of RF breakthrough? Have you tried a small capacitor across the secondary?
I had always thought that the guitar had always been crying out to be phantom powered and balanced line. A 6H+6kΩ source impedance is a real pain to deal with !
I must admit that I'd never thought of the possibility of winding the pickup for low impedance. The inductance is a pain to deal with and it goes with the square of the number of turns.
My friend with whom I collaborate for weird experimental guitar projects has a Japanese bass guitar from the 1980s (I forget the make) which has a built in battery-powered preamp - unfortunately, it's not a very good preamp!
Have you tried a few more turns on the primary of your current transformer? The wire-through-the-middle corresponds to half a turn. I think the coupling may be better with 1½ or 2½ turns.

This is a link from the DIYAudio forum

 

I did mess around with numbers of turns. I found the best results were with a single turn of thick gauge wire. Thin wire does not work as well either. The gauge and cross section of it can make the actual pickup harder to construct and keep compact. Double the turns through the transformer and you get half the output voltage (confirmed from the scope).
I think tonight I might try lowering the Q of the transformer secondary with a pot and see if that helps, as was suggested earlier.
I will also try an RF shunt as you suggested. The scope shows the typical 50Hz sinusoid we all know and love (not).

 

I would like to thank everyone for their useful inputs but I am leaving this group for some that are less judgemental before having all the facts. Bye

The current transformer cores I am using are designed to measure current from 50/60Hz power line cables. I had a batch custom wound to the ratio required, and not the usual 1000:1 that are commonly used, although those do work, but not optimally. The sound is very clear and smooth across
whole guitar's whole range, apart from the hum with certain guitar amplifiers (see above).

View attachment 257671

 

That is a good thought and I had previously tried that with no change in result.

But I think I am getting closer to nailing the cause:
By design, the guitar amp is not earthed and has no earth wire connected. I am on the 1st floor.
The 'ground' of the input socket was not directly connected to chassis earth. There is no consumer access to chassis earth.
I connected the 'ground' of the input socket to chassis earth and that to mains earth - and IT WORKED!!!
Which is good and bad. I cannot ship product like this.
Ongoing.

Hi Johnny,

Most probably, the question of earthing through the 'third pin' of the mains plug would not have been raised because that's a given for reasons of electrical safety.

The wire from the third pin would be connected to the amplifier chassis ground. The DC power supply 'common' and amplifier 'common' would also be connected to chassis ground. Likewise with the braids of shielded cables (to prevent hum pick-up) and the common of the amplifier input jack through which the guitar 'common' would get earthed via the guitar cable braid.

With the bridge also connected to guitar 'common', the guitar player would be earthed through the strings, resulting in the hum picked up by him / her being effectively shunted to earth.

Nandu.

 

The scope shows the typical 50Hz sinusoid we all know and love (not).

In that case it's not RF breakthrough.
Does the hum change as you move the guitar? What is the geometry of the piece of thick wire that connects your pickup to the transformer? Does it form a loop? Can you minimise the area of the loop? or make it a twisted pair?

 

Most probably, the question of earthing through the 'third pin' of the mains plug would not have been raised because that's a given for reasons of electrical safety.

The wire from the third pin would be connected to the amplifier chassis ground. The DC power supply 'common' and amplifier 'common' would also be connected to chassis ground. Likewise with the braids of shielded cables (to prevent hum pick-up) and the common of the amplifier input jack through which the guitar 'common' would get earthed via the guitar cable braid.

With the bridge also connected to guitar 'common', the guitar player would be earthed through the strings, resulting in the hum picked up by him / her being effectively shunted to earth.

Nandu.

Hi Nandu,

There is no earth wire on either of these guitar practice amps. They both have two-conductor cables only and the double isolated safety symbol is on the molded plugs and amplifiers. It was only when I ran my own earth cable to the amplifier chassis that it stopped humming.

The bridge, and therefore strings, are already connected to the amp signal ground and the wiring cavity is also completely lined with the usual copper foil.

UPDATE
I took the whole setup to another office that is on the ground floor directly below mine and the hum was absent
The hum resumed when I returned it back upstairs to my bench, even with all other electrics switched off including lighting. We have no elevators or other heavy equipment.

Just a reminder, a factory produced guitar does not hum.

I will now try the other tests that were suggested.

 

Hi Johnny,

Thank you for very much for the details you have provided.

If possible, you may try reversing the orientation of the two-pin plug.

Nandu.

 

That is an excellent idea and an interesting test. I do not think consumers would be wanting to cut off the molded plug and rewire with a new one.
Just to clarify, it is a non reversible UK 3 pin molded plug, but only two pins are connected.

 

I have just noticed that the ground of the guitar amp input connector is 2.3v dc above it's chassis ground :\
EDIT: Just tried the other guitar amp and its the same.

That seems unusual. Can you link to a schematic? I did a quick google and the guitar amp circuits I found all had the input referenced to chassis ground, with no DC offset.

 

That seems unusual. Can you link to a schematic? I did a quick google and the guitar amp circuits I found all had the input referenced to chassis ground, with no DC offset.

I thought it odd too, however, BOTH amplifiers have this offset, but I suspect they are the same, or very similar, designs and incorporating positive feedback to raise the impedance. I have no schematic and did try to contact the 'manufacturers' for one, with no response. Remember that chassis is not earthed to the ground pin of the supply.

 

I lowered the transformer's burden by changing the volume control from the standard 500k ohm to 50k and now it all behaves as expected. I did try adding another transformer as a humbucker, which made no noticeable difference, nor did using rf bypass capacitors. Thanks everybody.

 

Do you place the current transformer inside the guitar, with the connection between guitar and amp at a high impedance, or do you place the current transformer inside the amp and connect at a low impedance?
The output DC resistance of a guitar is about 6kΩ.

 

Current transformer hidden inside guitar -> Volume Control -> Jack Socket.