SOLVED. Odd behavior with a current transformer as an audio input source

Thread Starter

JohnnyC1951

Joined Jun 27, 2020
22
Audio source is a via a shielded cable from a 1000:1 current amplifier transformer similar to the one shown below.current-transformer.jpg
If I feed this into a PC sound card line-in, a hi-fi line-in or cheap audio pre-amp modules it works great with very low hum and good signal to noise.
If, however, I connect it to cheap Hi-Z guitar amps of several allegedly different manufacturers (may really be the same) the hum is HUGE and swamps the signal.
Here is the weird part: If I put the transformer into a closed earthed steel biscuit tin (Faraday Cage) even with no primary IT STILL HUMS like a beehive. the hum can be seen on a scope at the input. I suspect that the very high inductance is somehow upsetting a positive feedback impedance raiser in the input stage of the amp.
I have tried with a coupling cap and without and various values of 'damping resistor' with no effect.
My product does need to be connected and work with guitar amps.
Any input welcomed. I do not get why the Faraday Cage does not work. When used with a hi-fi amp the cage does quieten the already low hum.
 

Yaakov

Joined Jan 27, 2019
5,494
That transformer has a winding resistance of 93Ω according to its datasheet. What haapens if you substitute a 100Ω composition resistor for the transformer?
 

Thread Starter

JohnnyC1951

Joined Jun 27, 2020
22
The recommended load for the particular transformer I am using is 1kΩ. I tried that and the audio signal practically vanished but the hum remained in the case of the guitar amp. It the case of a hi-fi amp, it just reduced the signal (and residual hum). I am using it to get a usable signal from a very low-Z source (<4Ω).
 

Thread Starter

JohnnyC1951

Joined Jun 27, 2020
22
I have two guitar amps and they do the same.
I think I will try a simple IC buffer. My guess is that it will behave.
Had it not been exactly 50Hz, I would be investigating parasitic oscillation.
The amps work perfectly with Stratocaster guitars that do have inductive sources.
 

Yaakov

Joined Jan 27, 2019
5,494
I’m sorry, I don’t understand your response. Are you saying you‘ve removed the transformer and substituted a resistor? If so, what was “the same behavior” you observed.

Sorry, I just can’t work out if you‘ve done this,
 

LesJones

Joined Jan 8, 2017
3,701
You could try adding different values of load resistor. I would suggest trying values of between 10K and a few hundred K. This idea is based on the fact that the sound card probably has a lower input impedance than the Hi-Z guitar amp.
I assume that you are threading a wire that goes between a power amplifier an speaker through the current transformer. If so you could try having a number of turns threaded through the current transformer to increase the output from the secondary of the current transformer.

Les.
 

Thread Starter

JohnnyC1951

Joined Jun 27, 2020
22
The primary goes to my special low-Z pickup. But it does not matter if there is even no primary at all, the hum issue persists.
I have an old electric guitar somewhere buried. I will try running this through it's very high value volume and tone pots. Into the hi-fi the quality is really excellent.
 

Alec_t

Joined Sep 17, 2013
12,733
The amp input impedance is the transformer burden resistor. The higher that is, the greater the transformer output voltage for a given input. Doesn't that explain the greater hum level with the guitar amps?
Even with no input wire through the core, methinks the core plus output winding would make a pretty good pickup for stray magnetic fields. The biscuit tin may have offered electrical shielding but not much magnetic shielding.
 

Thread Starter

JohnnyC1951

Joined Jun 27, 2020
22
Thanks everyone for replying.
I would have expected the signal and hum to go up proportionally, but the hum is overwhelming on the guitar amp - >10x larger than the signal. The hum is barely noticeable when connected through the hi-fi but a nice strong signal.
The Biscuit tin is magnetic mild steel and noticeably cuts the residual hum when connected to the Hi-Fi, so that works even if it is nowhere near optimal. I can see no cut in hum at all when using the guitar amp.
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.
 
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LowQCab

Joined Nov 6, 2012
1,915
The Guitar-Amp expects a "Microphone-Level-Signal", so it has massive Gain, expecting a couple of millivolts.
All the other Pre-Amps you mentioned have "Line-Level" Inputs, with relatively low Gain.
Line-Inputs are usually up to 1-Volt or so, ~100-times higher level.
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.
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strantor

Joined Oct 3, 2010
5,907
Are you just going to ignore the question repeatedly asked by @Yaakov ? You still haven't coherently answered it. It's a great question and the answer will steer the discussion in the correct direction toward solving your problem rather than a bunch of unproductive cogitating over all the things you've already tried that didn't work.
 

Thread Starter

JohnnyC1951

Joined Jun 27, 2020
22
>> "I’m sorry, I don’t understand your response. Are you saying you‘ve removed the transformer and substituted a resistor? If so, what was “the same behavior” you observed."
Sorry, I didn't see this earlier, no idea why, probably I had not refreshed the page.

On BOTH amplifiers: If I remove the transformer and it is open circuit there is a very feint hum. With a 1k ohm load there is silence. So logically one would think that it is pickup from the toroid.

However, with the coil fitted inside an earthed steel biscuit tin which is now inside a bigger earthed steel biscuit tin and all the leads screened, I get the overwhelming hum.
A real pickup has 15,000 turns and 1500 ohms and hardly hums at all.

I shall now try inserting 1500 ohms in series with the toroid.
 
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Yaakov

Joined Jan 27, 2019
5,494
I am going to make a suggestion, mostly for the information it might provide. If oyu have two transformers, wire them together in opposite phase and place them as closely as you can. This should cancel anything they are both picking up. If that works, then it is certainly induced noise from the environment.

It might also be possible to create a common mode filtering scheme that way if only one of the transformers is used as a live pickup while the other is used as an ambient pickup.
 

Thread Starter

JohnnyC1951

Joined Jun 27, 2020
22
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.
 

LowQCab

Joined Nov 6, 2012
1,915
It's not a "product".
What You are doing might be an interesting or thought provoking experiment,
but it won't be anything practical that some one would actually pay Money for.

What do You think that You have invented ?,
that no genius in the last 200-years couldn't figure out ?

A "Current-Transformer" will never be a practical Audio-Device.
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.
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Thread Starter

JohnnyC1951

Joined Jun 27, 2020
22
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.
 
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