I/O transformer proximity

[warpie]

I was wondering whether there are any general rules of thumb regarding placing an input and an output transformer in an enclosure. By "rules of thumb" I mean things such as distance, orientation, shielding, etc.. like in the case of two (or more) output transformers. Speaking of shielding, the input transformer is a line transformer and it's shielded.

Is there any risk of coupling between the two transformers? I'm not even sure whether I should call it crosstalk or not And although, in the case of two output transformers, you can actually test it, I'm not sure how to do this with I/O transformers.

 

[Philbo King]

You could test the shielding by holding a tape head demag or tape bulk eraser near it. As always the devil is in the details...

 

[abbey road d enfer]

Indeed, there's a risk of mahnetic coupling between xfmrs. Between output xfmrs it will result in x-talk. Between an output and an input xfmr, the coupling may be sufficient to create feedback, which could be positive or negative. In both cases, it's not welcome, because it's seldom linear.
The two dominant factors are distance and orientation. I try as much as possible allowing to rotate xfmrs.

 

[JohnRoberts]

Do what works... Back last century when I was designing fixed install amps for commercial background music applications it was oddly a bigger problem in the smaller amps.

I recall one small 5W amp that had 70V and 100V output transformer windings only inches away from microphone input transformers. The input transformers were shielded but it was rough teaching the Chinese vendors what mu metal was.

JR

 

[warpie]

Thank you. So, what would be considered a "safe" distance? 1 inch? 2 inches? , 5 inches... ?

The "problem" is that I'm working on a new PCB and it's not quite practical (and efficient) to apply trial and error methods by re-designing and re-ordering boards over and over again.

 

[JohnRoberts]

Can you breadbord the critical parts of the circuit without a PCB?

The original breadboards were literally wooden boards.

JR

 

[warpie]

Can you breadbord the critical parts of the circuit without a PCB?

The original breadboards were literally wooden boards.

JR

I guess I could but since there is a bit of room on the PCB I thought I would ask for some advice about what would be considered a safe distance.

On a similar note, how can I actually test if there's any coupling between the I/O xfmrs? In the case of two input or output xfmrs, it's pretty simple to check for x-talk but what about this case? FWIW, I do have a couple of 'scopes, software audio analysers, etc...

 

[JohnRoberts]

In my case the issue was crosstalk from a 100v or 70v audio output coupling into an open mic input and causing oscillation. Terminating the mic input with at least the nominal 1.5-2k helps. Look for misbehavior in typical circuits.

If you have the room there is no benefit from placing them closer together. Sometimes transformer orientation can affect magnetic coupling.

JR

 

[dmp]

As to distance, coupling follows the inverse square law. So increasing the distance is very effective.
Assuming these are EI transformers, the emitted flux is different in different directions. So aligning the transformers so they will have minimal coupling to each other is very effective. The proper alignment with EI is at right angles not pointing at each other (look at a picture of a Fender amp with the power transformer lying down and the output transformer upright).

 

[dmp]

On a similar note, how can I actually test if there's any coupling between the I/O xfmrs?

It isn't an yes/no question, it is a question of level. (i.e. dB of feedback). You really want to know if the coupling is insignificant or not.
It is easier to measure and comprehend with a fixed signal fed to the output transformer, separated electrically from the circuit of the input transformer. Then measure the level induced in the input transformer. Then compare the fixed signal level in the output transformer to the induced signal level in the input. If you fed 10v and measured 0.1v, for instance, the coupling would be -40 dB

 

[abbey road d enfer]

Thank you. So, what would be considered a "safe" distance? 1 inch? 2 inches? , 5 inches... ?

There's no "magic number". As far a possible.

 

[warpie]

So, if I understood well, I can totally skip the whole circuit and simply feed the OPT primaries with a sinusoidal from my sig gen and measure the IPT secondaries with the scope?

Am I on the right track?

 

[KIDVOX]

i think its a case of having the trannies at 90 degrees to each other ie one lies flat to the chassis and one stands up if you get my drift. if you have a choke as well that also lies at 90 degees to the other two trannies. that's only based on what i have seen in commercially built stuff. please understand im no expert

 

[Tubetec]

I tried a pair of 1:1 10kohm bridging transformers at the output end of a preamp recently ,
there housed in a small wood and perspex box , there was a clear interaction the moment both transformers were fed , and a low level motorboating thing started to happen ,
Just looking at how I physically arranged the transformers its likely the way I placed them was wrong and maximises interaction
I can easily re-orientate them in the enclosure.

 

[abbey road d enfer]

i think its a case of having the trannies at 90 degrees to each other ie one lies flat to the chassis and one stands up if you get my drift. if you have a choke as well that also lies at 90 degees to the other two trannies.

There are not an infinity of solutions for orienting the xfmrs at 90°, so, in practice they all look similar.
Actually, the line forces are not strictly at 90°, particularly when the xfmrs are not coplanar (typical of xfmrs with different stack height). That's why minimum x-talk can be optimized by orienting/moving them.

that's only based on what i have seen in commercially built stuff. please understand im no expert

Your observation is quite pertinent.

 

[GussyLoveridge]

What about having a look at some 500 series stuff to see what is commonly done?

 

[terry setter]

Max distance available, plus core orientation at right angles (which means there are X, Y, and Z axes) is your best starting place. If you can move the units around while sending signal through them, you can find the "magic" spot/best placement available.

TIP: take and old single coil guitar pickup and mount it on a short wand. Run it's wires into a guitar amp (portable and battery powered is the most convenient). You now have a "HUM SNIFFER". Turn on any circuit and wave that thing in and around near it and you will hear - coming out of the guitar amp - what ever field noise the pickup is capturing and sending back to the amp. I've even been able to help folks find interference in their homes that was coming from a large power pole transformer that was located just outside the wall of their house. The solution in that case, was to move their audio gear to the other side of the room. Worked swell. You can also pick up digital junk that comes from unsuspected components in some circuits. In cases like that, you can actually shield the little fellas right on the board, though it looks funny and is not much fun to do.

 

[Audio1Man]

What you have is a FLUX TRANSMITTER and a FLUX RECEIVER. You want to have is that the FLUX RECEIVER should not to hear the FLUX TRANSMITTER.

[D1]

I use a PLASTIC DYNAMIC MIKE as a pickup and connect its output to a speaker and spectrum analyzer or an Audio Precision System Two Cascade. Excite the transformer with a low frequency high level and move the microphone around the transformer while measuring audio level/FFT

I also plot Excitation Current vs Freq/Voltage. I have attached a typical way I do power transformers. Use the same basic tests for audio transformers.

Checking a power transformer for flux density using basic setup with an AP or similar Audio Analyzer.

Set the AP for 60Hz and drive a power amplifier/programable AC power source. Sweep output for 60 to 140 or 120 to 280 volts and plot the current trough 1 Ohm resistor. Then repeat using 50Hz source.

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[D1]

 

[KIDVOX]

TIP: take and old single coil guitar pickup and mount it on a short wand. Run it's wires into a guitar amp (portable and battery powered is the most convenient). You now have a "HUM SNIFFER". Turn on any circuit and wave that thing in and around near it and you will hear - coming out of the guitar amp - what ever field noise the pickup is capturing and sending back to the amp.

i like this idea.