BV08 copy transformer from AliExpress low end cut and hum?

[Spencerleehorton]

I have bought a BV08 style transformer for the U47 build I’m doing.
Specs are all as they should be with this transformer and build wise it’s neat and the correct size
PRI windings are each 300R, 600R in series and 122H in series but I can only get the full inductance if I connected it up wrong as per the diagram which it came with?
If I connect the start winds or the finish winds I get full inductance, if I connect up either finish to start winds I get no inductance?
Same on SEC side?

Anyone got any ideas what an earth is going on here?

 

[Gus]

I would drive one of the primary coils with an oscillator and check the POLARITY of the other 3 winds against the driven coil with an oscilloscope
maybe it has the wrong color wires to the start and finish of the coils

 

[Spencerleehorton]

Have done what you suggested and phase aligned all windings then connected in series and test with inductance meter, inductances look good.
Put in mic and still get hum!!!!

Driving me mad now!!!

 

[MagnetoSound]

But the low end is good?

Hum is probably a grounding problem or PSU-related.

What tube are you using now?

 

[Spencerleehorton]

Yes the low end is good but the mic hums even with the power supply off?
I’ve put the txf in the proctop U47EF with the ef800 in it.
Does the lams need to go to gnd?

 

[Spencerleehorton]

When I use another txf this mic is fine?
Only hums when I use this bv08 copy?

 

[MagnetoSound]

I’d be surprised if the lams need to be grounded, but you can try it. If the PSU is off, it must be picking up 50Hz mains field somehow. Assuming the casing is on the mic and the circuitry is shielded, that’s rather odd.

Could it be a ground loop between the PSU and the preamp?

 

[alphasnk]

I had the same problem you're having with this very transformer, i even talked with the store owner about it, telling her that it didn't work proper..

Put it in the trash and use something else

 

[kingkorg]

Could it be one of the coils is physically flipped 180° and you get kind of humbucking arangement+phase flip? Kind of like Strat bridge+mid pickup where mid is flipped, you get the twang but no low end. If you get low cut it has to do with some kind of phase issue. You could try to use just one half of the transformer both primary and secondary, and then the other half, and see what happens.

Inductance meter measures at fixed low frequency, and it would make sense you get no reading because of the flipped phase, coils cancel each other.

 

[ElSmurf]

Put it in the trash and use something else

That would be my suggestion as well. Just get a Moby BV.08.

 

[MagnetoSound]

@kingkorg he got the low end back. Now there’s a mystery hum.

 

[Spencerleehorton]

I can only think that there is some internal shield which is causing the problem as on a scope aligning them so they are all in phase and testing them on a inductance meter so it has inductance and then still getting a hum, something has to be wrong internally to get hum?
Going to try some paint thinner on the lams and take it apart and rewind it as I have a bv08 winding sheet.

 

[MagnetoSound]

Is hum getting in even if the body tube is on and the transformer is shielded by that?

 

[Spencerleehorton]

I have had this before when ive wound a txf wrong and the PSU even being off, when you plug the mic in you get the dreaded hum!!
im going to get some paint thinner in a minute and will take it apart and find out whats going on!!
its probably some internal shielding interfereing with a winding i bet.
the other txf i have that i have tried have to mumetal shielding and are fine, so i know the mic works fine and its this BV08 copy thats at fault and the fact that another member here has bought the exact one and had the same problem.

 

[Spencerleehorton]

have also tried it with just one PRI and one SEC and still hum, so apart it comes!!

 

[Spencerleehorton]

It’s all apart now and I’ve rewound one bobbin, have to get ready for night shift so will do the other tomorrow then test.
I found a couple of odd bits of copper wire within the windings and the windings we not very neat.
Hopefully the rewind will sort it?

 

[soliloqueen]

The company that makes this transformer, deshang, Is a fully automated factory that does a ridiculous volume of transformers (they are essentially the main precision audio transformer manufacturer in all of China) which is probably why there's not much care put into it. A human doesn't even touch it. They have this nuts system there that can make anything. I had them make an entire historically accurate BV12 for me for fun once to test this out kind of as a joke. It was meh. It worked, but it required a lot of feedback calibration for each unit. The sound was fine? But the heat treatment wasn't right

For the curious:


They did actually manage to do it, from a historical sample. I'll give them that. It's definitely a BV12 and a real one. But without human interaction in the manufacturing process the result was strange. The design was already in their list of specialty designs so clearly someone else got curious before me.

Deshang has a good value proposition... if you're buying a thousand of them and selecting one at a time and have some stock to blow through. I know tons of companies that use deshang transformers even in made in the US products, but they are definitely not a company that I would buy single units from off of AliExpress. Especially not if they might be rejects or something (who knows where these stores get these single units)

 

[Spencerleehorton]

total head scratcher!! i reused the copper enammled wire and rewound and still get hum?
tests absolutley fine resistance and inductance wise so must be either the wire or the lams?
i might try and use my own copper wire and see if the wire is shot.

found the culpret!!! it was the GND lug on the XLR!! had cracked somehow!!
have changed the capsule as that M7 copy from aliexpress is awful, at least i know my windings good!!
txf works a treat so far, just need to fine tune and get a better capsule.

so sorry for the run around but got there in the end!!!

 

[soliloqueen]

total head scratcher!! i reused the copper enammled wire and rewound and still get hum?
tests absolutley fine resistance and inductance wise so must be either the wire or the lams?
i might try and use my own copper wire and see if the wire is shot.

Maybe it just sucks. I've actually got a sample transformer here I can send you. That way you can see whether it's a circuit or transformer issue

 

[Spencerleehorton]

i still think the rewind of the transformer worked better than it was as it had at least a couple of little bits in there and the winds were pretty scuffy.
Im happy to rewind the transformer for you and send it back, or if you want to donate im happy for that as well!!!