Asking for help trouble shooting a mic circuit?

Hi,
I had some time to work on a circuit I am hoping to use in a DIY mic. I have the circuit built on a bench top test rig and I finally finished the soldering and powered it up to see what happened.

Not much happened.

I have all the parts laid out and even have an XLR output that I am routing into a mixer just to see what I can hear. I expected a bunch of noise and hum because the parts aren't shielded, they are just laying on the test rig.

I never seemed to actually get a signal off of the capsule. It's an inexpensive K67 capsule I bought from Studio 939 just for making mistakes with.

I think some of my voltage readings show where the problem may lie and I am wondering if someone can offer a second opinion. I have attached a schematic with the voltage readings shown.

The circuit is an adaptation of the U47 alternate tube schematic found at AMI-Tab. I drew the pin outs for an EF86 tube and I referred to the Neumann U48 schematic while adding the extra parts to copy a U48 figure 8 option rather than the U47's omni option.

I double, triple checked the layout and I think I have everything soldered where it's supposed to go.

Looking at the voltage going to the back plate makes me think that I may have ruined the 1Gohm R1 resistor.

The voltage going to the rear capsule seems bad too. The voltages on either sense of the 150Mohm R5 seems to indicate a problem.

I can't measure either the 150Mohm or the 1Gohm resistors with my VOM... they just read open.

Are these super high resistors more fragile than others? I've never ruined a resistor while soldering, but I've never worked with these values either.

I used a jumper wire to bridge over R1 and I got 52vDC to the back plate but I still could not hear any response from the capsule... just the noise and hum.

Does anyone have any thoughts about the voltage readings?

Thank You!

You can't measure those high-impedance nodes with a DVM.

Start from the output of the tube and work "backwards" towards the capsule.

Hi Matador,
Thanks for the reply. Can you elaborate a bit?

Are you saying that the locations I highlighted with yellow can not be measured accurately with a Digital VOM and that may be why the readings seem goofy?

When I used a jumper to bypass the R1 I was able to measure the expected voltage at the capsule back plate screws.

What did you mean by start at the tube and work towards the capsule? The voltage measurements at the tube all seem to be as I expected. The wiring looks correct. I started with the EF86 because I had some cheap Svetlena tubes. My plan is to wire the pinout for some E80Fs I have after I have everything figured out. Is it time to pop another tube in there?

Thank You.

Most DVM's have an input impedance (when set to measure voltages) of somewhere in the neighborhood of 1-10M.  If you try to measure source impedances much larger (like over 100M), then the meter will clobber the voltage readings (set up a simple resistor divider with the source resistance of 150M and a shunt/meter resistance of 10M:  the math says that the meter will drag down the measured voltage by 94%).

By working backwards:  I would start by lifting C4 from R7, and injecting a 1kHz, 10V PTP sine wave directly into the output transformer through the C4 coupling cap.  You should be able to hear this clearly in you monitoring of your preamp.  If you can't hear it, then start by looking at the transformer and C4.

Then work "backwards" more:  I would disconnect the capsule, and inject a 1kHz sine wave of about 100mV PTP directly into the grid of the tube (where the grid and R3 meet).  If you can't hear a sine wave out of your preamp, then you are looking in the wrong place by trying to debug the capsule.

So start at the output, and work backwards in the circuit towards the capsule.

Hi Matador,
Thank you so much for the detailed info and the clear explanation.

Hi Matador,
I followed the steps as you advised.

I lifted C4 and sent a AC signal through the transformer. It is a Cinemag CM-2461NiCo witha 6.5:1 ratio. My DVM is a "true RMS" type unit and my signal generator only puts out 500mv AC RMS. I also misread your post and so I adjusted a 1kHz signal to 100mv AC RMS and hooked it up to C4 and ground. The signal passed through the transformer cleanly and sounded ok going through my mixer. I measured the output of the transformer and it seemed to very closely match what I would expect from the 6.5:1 ratio: 14.5mv AC RMS.

Having done that, I reconnected C4 to the circuit, and desoldered the capsule from the circuit.

I injected a 1kHz sine wave at 35.0mv AC RMS (what I think is the equal of 100mv AC P to P) onto the grid of the tube. The very fist thing I noticed was that the output was over driving the input of my mixer. (A Mackie 1402 that I had laying around) so I set the trim accordingly. "Unity" gain seem like a good sweet spot. When I did that the test tone sounded very clean and it was well above the noise and hum, which made me very optimistic as the circuit is just laid out on a sheet of wood as a test rig and as such has no shielding.

I guess my next step is to reconnect the capsule. I have no reason to think that the capsule isn't fine, but I also don't think it was ever opened and tested "state side" as it is one of the very, very affordable K 67s that Chunger sells to us for getting started economically. It looks clean and I handled it with white cotton gloves. When I have it soldered in to the circuit it is also just sitting in the open without any shielding "screen". I've been keeping a upturned glass jar over it in between tests to protect it from dust.

I had tried to adjust the trim on the Mackie several times before I asked for help, but now I have a clear idea of where the trim setting should be. Previously I had noticed that I could hear a slight and very distorted sound coming from that capsule but it seemed to be buried in the hum and noise. Maybe it was actually buried in distortion happening at the input of the Mackie. Maybe it will sound ok now that I know for sure where the trim level setting should NOT be.

At this point all I can think of to do is to reconnect the capsule and hope for the best.

I'm wondering what the next step might be if I don't get any better results.

I"ll appreciate any thoughts or ideas anyone can share.

Thank you!

Ok, sounds like you are good so far.

(First off, the capsule cannot work if you jumper R1.  Although you'll have a proper 52V on the backplate, the source impedance is too low and the capsule won't transfer an output signal any longer).

I would start by:

1) Disconnecting the RC (not needed for cardioid mode)
2) Remove C2 (it's for noise filtering)

If R1 and R3 are good, you should be able to get a signal from the capsule and hear it in your mixer.

If you still hear nothing, I would rotate the capsule 180 degrees and make the old RC the new FC.  If you had one bad side, it would be unlikely you have the exact same problem on both sides.

You can also check the capacitance of the capsule:  if your meter is capable of measuring capacitance, you should read somewhere between 50pF and 100pF between the backplate and the FC (and also between the BP and RC).  You can also check resistance between the BP and FC and RC:  both should read "infinite".

If all of this fails I would replace R1 and R3:  although resistors are pretty sturdy it's possible you have a bum one (and as you said, it's almost impossible to measure these with DVM's that mere mortals possess).

Hi Matador,
Thanks again for your suggestions.

I re connected the front capsule and back plate to the circuit. I lifted C2 as suggested.

I didn't get any discernible signal from the capsule. I have tried snapping my fingers right in front of the capsule and I can't hear anything through the mixer and the mixer's meters don't seem to show any change in levels.

I reconnected C2 and I noticed that the noise level was reduced compared to when it was lifted.

I disconnected the front capsule and connected the other side to the circuit as if it was the front capsule. No change.

I measured the capsule for shorts and front to back plate and rear to back plate both seem to be completely open.

My DVM only goes down to 1nF so I can't measure the capacitance with the tools I have at present, so I'm looking into some options to be able to make small measurements like that.

I guess I'll order some more resistors and see if that helps.



I had not realized or appreciated that the 1GOhm resistors need to be handled so carefully but I've been reading about it today. I didn't exercise any particular special care when handling them and so now I wonder if I contaminated their surfaces with my fingers and if that's the source of my problems. I used some rubbing alcohol and a cotton swab to give them a slight cleaning tonight after trying the suggestions and nothing seemed to change.

My test rig has both the power supply and the mic circuit sitting on the same board. The power supply is using a 5 watt wire wound potentiometer as the B+ trim adjustment. I'm wondering if that is spraying out EMI and swamping the capsule. I can see that the power supply layout is working as I hoped so I guess it's time to stick it in it's case.

Finally, I removed the capsule from the circuit and once again injected a sine wave into the tube grid. It sounds good and there is very little hum.


I have to switch gears for a few days as I have band coming in. I'll appreciate any further comments or suggestions and I'll get back to this project in a few days.

Thank You.

I think you should just build the mic and the PSU in proper enclosures. The noise from a unshielded circuit can mask the signal from the capsule. Other thing, I just see double amount of work breadboarding a mic circuit. When you get the circuit working, you still have to build it in the mic and start troubleshooting everything all over again if there's a problem