I need help with a U47 clone build

[Henrizzle]

Hey everyone!

I need some help with a DIY U47 build. The symptom is, the mic has tiny gain unless I disconnect pin 3 from the XLR connector.

I have traced the 7 pin cable connections from mic side to the power supply side, it all checks out with logic probe.

As I understand it, pin 3 is the negative side of the balanced signal. So if pin 3 is connected and the gain is tiny the signal must be inverted and cancels out the signal on pin 2 (mostly) to produce tiny gain. I did some tracing from the mic thru cable to power supply.

- Pin 2 is + side of the transformer output
- Pin 3 is - side of the transformer output

I suspect then, maybe the pattern selector is not installed correctly on the power supply / switching side or the mic side some how.

Has anyone had any issues like this that got found and fixed?

 

[Khron]

I hope you're aware that you haven't provided us with any meaningful information, apart from the mic cable being potentially ok.

What circuit are you using? What power supply? What tube?

 

[Henrizzle]

Hi Khron,

This is from a kit I purchased a long time ago, I don't have the specs on hand. I guess what I'm trying to figure out is where the inversion happens for a given balanced signal and maybe it will point to the problem.

I suppose on XLR pins 2 and 3 there should be a positive and and inverted negative signal. If pin 3 is not inverted, the preamp ends up inverting pin 3 and the sum is nothing. So I'm trying to figure out where this inversion typically takes places in condenser mic?

 

[RuudNL]

You still didn't give any useful information.

 

[Winston OBoogie]

I suppose on XLR pins 2 and 3 there should be a positive and and inverted negative signal. If pin 3 is not inverted, the preamp ends up inverting pin 3 and the sum is nothing. So I'm trying to figure out where this inversion typically takes places in condenser mic?

In a U47 circuit that place would be the secondary of the output transformer.

Reading through your other older posts regarding this issue, my £0.02 is that you either have a faulty output transformer, or it's wired incorrectly.

 

[Henrizzle]

In a U47 circuit that place would be the secondary of the output transformer.

Reading through your other older posts regarding this issue, my £0.02 is that you either have a faulty output transformer, or it's wired incorrectly.

Yes I was suspecting something like that. So in essence there is no "inverter" inverting a signal just the positive and negative ends of the transformer output is wired to the XLR pins 2 and 3. Very good I check it out at the transformer wiring.

 

[Henrizzle]

In a U47 circuit that place would be the secondary of the output transformer.

Reading through your other older posts regarding this issue, my £0.02 is that you either have a faulty output transformer, or it's wired incorrectly.

Question, is there a way to verify output transformer signal? Logic probe? Oscilloscope?

 

[RuudNL]

I would start to measure the resistance of the primary and secundary (pin 2 and 3 on the XLR) windings.

(Usually the primary winding is in the 300 ohms range, the secundary is much lower, 15 ohms or so.)

 

[Henrizzle]

Thanks everyone for the information! The secondary shows OL on the meter. So I got a AMI T8 transformer installed into the mic. It works now but I feel like there could be a little more gain. The T8 has same winding ratios as the BV8 but is much smaller compared to the T48 which I think is the size of the BV8. What is the theory behind transformer windings / size? Will more iron produce more electrons?

 

[gyraf]

no

but transformers are much deeper than that - you need to read

/Jakob E.

 

[Moby]

Core size does not change gain or "electrons" as you call it. It can be related to the core saturation point and maybe to the primary inductance.... Ratio of the tx does the job .Standard Bv.08 has to be 6.5:1 ratio, but there are more variations on the market. I'm not sure about spec's of the transformers you use but you can check it on the manufacturer site.

 

[Henrizzle]

I installed a AMI T47 into the mic. Basically 2 transformers in one... bridged. Wow, loud and clear and boomy now, this mic sounds amazing! Now my next project is to put it in a new body. I managed to find one of those U47 Tuchel connectors and a bell housing to fit it into. I have to drill a few holes for the posts in the right place to fit. Thanks again for all the information guys! I think this is going to become one of my favorite mics in the studio.