DIY FET mic with very low output.

I've completed a DIY version of the U87ai, schematic here:
http://www.omnipressor.com/MicSchLib/u87ai.gif

I'm using a cinemag 10.5:1 microphone output transformer (David Green recommended the model, not on the website - it's designed as a mic output and is humbucking. Nice and small, which made packing the circuit into a Nady SCM900 body much easier)

I wound my own auto-transformer and I'm generating required +/- voltages from the DC-DC board (see http://www.groupdiy.com/index.php?topic=1651)

Anyway, I'm getting some output though it's extremely low. For example I put my preamp up to close to full and more or less yelled into the mic from 5-6 inches and it only justs registers a signal. When recorded you can hardly hear it - sorry, I deleted the file before noting down actual level measurments. Either way though, it's way too low to be functioning correctly. As a side note, there doesn't appear to be any noise (hum, hiss etc), so that's something at least.

At first I thought I had wired up the transformer incorrectly, but swapping the transformer round the other way yielded no output at all.

I've biased the FET and it's giving somewhere between 10-11 volts, depending on what phantom supply I'm hooking it up to. It's biased with an 11K resistor to get this voltage. I'm using a 2n3819 FET. I remember a thread a while ago (going back several months) where Gus mentioned building a circuit with this Fet and finding that very few biased well for mic use. I'm not sure if that's the problem, but perhaps a consideration?

Low cut and -10b pad are working fine (best I can tell from extremely low output).

I'm using a cardiod only chinese capsule, so I haven't tested the pattern switch yet.

Suggestions?

thanks

tim

Are you sure that your home-made DC/DC converter puts out the right voltages to the capsule?

Are you sure that your home-made DC/DC converter puts out the right voltages to the capsule?

Click to expand...

Reasonably - before the two 1 gig resistors at the pattern switch on the schematic, I'm getting aroun +/-44V - I'm assuming that my meter may be loading these down a bit, I think they may be between 50-60V. I don't have a scope to test the actual voltage. The original runs of 47V to the capsule (all be it with less output), so I'm sure that I should have enough for reasonable output, even if my meter isn't loading it down and I only have +/-44V.

Currently I've got the front diaphragm connected to the + supply, backplate connected as per schematic and no connection to the rear diaphragm (cardiod capsule)

thanks

tim

You can test condenser microphone preamps by injecting a signal through a ~47pF capacitor into the input.

Jakob E.

You can test condenser microphone preamps by injecting a signal through a ~47pF capacitor into the input.

Click to expand...

thanks jacob - I'll give that a go and get back.

thanks
tim

Yeea, the U87 power supply gets loaded down quickly, I get similar voltages to yours, about 40 or 45 volts with a meter in place, maybe 60 if I use a 'scope to measure it.

Check the values of the feedback circuitry (drain back to the capsule). If you're missing a resistor or have a wrong value here you'll get that even if the amplifier is working correctly.

-Dale

how much of the circuit did you copy? did you build the filter network around the capsule? If so disconnect the .47uf from the drain does the output go up ALOT?. The gain will go up with that .47uf removed in a stock u87. You could have your charge leaking away via the filter network.

I have been inside an older u87 that had a 10K source to ground that seemed to work fine. I did not test it at loud levels. The 10K looked like is was done at the factory.

Have you been inside a U87 head? The plastic parts and the way the whole head is built seems to be designed for as little leakage as possable.

What voltage you have drain to ground and source to ground?

is the path from the PS 33 v zener, 10K, 47K drain ,source 10K to ground and what size cap is used as the source bypass.

This might sound strange coming from me I think I like a tant source bypass and a tant at the 10k 47K node in circuit like this. I have built a few circuit like this and tried different parts.

IMHO, a 10k source resistor on the 2N3819 is HUGE. It is too much feedback and not enough current to get gain. I would try:
First: Lowering source resistor value to 5k
Second: Increase drain resistor value to 15k

The trouble with this single transistor scheme is that you are so much dependent on individual device charateristics.
Shurely, Neumann measured and sorter their transistors much like Gus does. They also selected the source resistor for each transistor to get enough gain out of more devices than one or two per hundred.
Have you sorted the FETs for gain?

Thanks for all the suggestions so far.

I haven't had a great deal of time to check things over, but I'm pretty sure everything in the feedback circuitry is correct.

how much of the circuit did you copy?

Click to expand...

The lot

did you build the filter network around the capsule? If so disconnect the .47uf from the drain does the output go up ALOT?

Click to expand...

Disconnecting the .47uf does increase gain, though I'm still getting nowhere near enough.

What voltage you have drain to ground and source to ground?

S-Gr 3.3V
D-Gr 9.4 ish (I've changed my 11k source resistor after rechecking the bias and found it was getting a little to high above 10. I've now got an 8k2 in there - I really need something in between these two, but my parts draws are bare in that area, I need to by a 20k trimpot.)

I'm starting to doubt I have actually got the transformer in the right way you know. I've got a draft data sheet of the transformer from cinemag, but it doesn't (for my simple, inexperience mind) clearly state which is primary and which is secondary. It's got a test circuit showing what I think is the primary. When I tried it the opposite way I've got now, I didn't get any output at all. I'll try it the other way again, and carefully check that everything else is connected right - maybe last time when I switched the transformer around I accidently screwed something else up :s Would explain low output.

I'll have another play with it tonight/tomorrow and investigate all these possibilities in more detail

thanks

tim[/quote]

Hi Saxim,

Check if you have right gain in the FET amp, ref to your U87Ai schematic, with out the capsule, connect a oscilator in series with a capasitor aprox. 100 to 1000 pF to "capsule connector" pin 2 and the ground to pin 7.
Apply a sine tone 1 kHz, 30 mV (-30 dB) from the oscilator and then measure with a AC mV meter what you have after the C10 capasitor to ground, you shall have approx. 1,2 to 1,3 volts (+4dB)
This is approx. 35 dB gain in the FET amp.

The orginal output transformer have aprox. 20 dB loss with no load, (only phantom resistor load) so if you measure on the secondary you shall have aprox. 120 mV (-16 dB)

If you compare your DIY mike with a orginal U87, observe that the U87Ai have approx. 8 - 10 dB higher output compare with the older U87i.
(not when you measure them with a oscillator from FET input to transformer output, but when you compare them acoustical with the capcule)
The U87Ai have more output because it have built in voltage step up for the capsule polarization that not the older have, and maybe the capsule can also change the level a bit ???, because U87 have the old U67 capsule with three connecting leads. (not the old U87 capsule with four connecting leads)

--Bo

Ok,

I've never really used a signal generator before, or measured AC voltage in a circuit (other than measuring mains voltages once).

So, I was probably going about it all wrong. I had a lot of trouble getting any AC voltage readings within the circuit (basically I couldn't get any - probably me, but my meter seem to be doing lots of weird things, like reading voltages when it wasn't connected to anything.) I did get a reading at the point where I'd inserted the signal generator (and confirmed i was getting 30mv)

Anyway, with a 1khz sine wave a 30mv going in, when I recorded the output I was getting a much more respectable level. I could clip my mic preamp if I pushed it towards the end of the dial. The sine wave came through sounding as a sine wave, with no extraneous noise or any other signs of problems.

Can someone check the transformer data sheet for me and confirm I've got it hooked up right? I'm still not sure as I'm getting confused with primary and secondarys and how you'd reverse a mic input transformer to get a step down ratio, where as this one I think is represented with the primary to secondary being step down already. I've got it hooked up with orange/yellow as the primary. I think that's wrong but as I mentioned above when I hooked it up the other way i got no output at all (when I was testing with the capsule, not with the signal generator.)

I've uploaded the draft datasheet for the transformer to here:

http://www.geocities.com/mejafu/CM-2480.pdf

(copy link and paste in new window to open)

I'll leave it there a couple of days then take it down as it's only a draft from cinemag.

If I have got it round the wrong way - will that explain the problems I'm seeing? I figured if anything if I had it wired as a step up it would give much more output rather than less output?

thanks everyone for the help so far

tim

I did a couple more tests with the oscilator running through the head amp, but I don't have enough knowledge to figure out what's wrong.

Firstly the transformer. The more I read the data sheet, the more sure I am that the red/brown is supposed to be the primary. I've tested it both ways though:

With orange/yellow hooked up as primary:

-Normal output from head amp with oscilator input, no extraneous noise
-Very low output with capsule hooked up (low to the point of need preamp to be on full to get a signal you can just hear) Still no extraneous noise though

With red/brown hooked up as primary:

-Higher output from head amp with oscilator input (higher than orange/yellow primary), no extraneous noise.
-0 output from with capsule hooked up of any kind - no noise, no signal, no nothing (even with preamp on full).

This is a standard chinese u67/87 body copy - the grill is attached by screws to the frame, whilst the lower part of the body that houses the electronics can slide off. I tested the microphone with capsule with the entire body assembled. When I did the oscilator, I had the bottom part of the body off so I had access to the electronics. (This may or may not be important. I cant see anywhere where the circuit could short to ground when the body is attached)

I've checked continuity from capsule diaphragm and backplate to circuit - both are fine.

Could someone tell me what the above means in terms of how the transformer needs to be connected? (see post above for link to data sheet). The transformer may or may not be the problem, but I figure I better get that right first.

Thanks

tim

I just can´t open the archive.

> I've got it hooked up with orange/yellow as the primary.

Good for your use. (Backward from mike-input use.)

> If I have got it round the wrong way - will that explain the problems I'm seeing? I figured if anything if I had it wired as a step up it would give much more output rather than less output?

Higher voltage BUT lower current. What happens depends on source and load impedances. Here you have an FET running a very small current, and some kind of mike-input with phantom which may be very current hungry. A 2K input refected through a 1:10 transformer puts a 20Ω load on the FET. Its gain will be very low, so low that the voltage step-up can't fix it. Also the "bass fall-off" may start at around 2KHz, sapping all the body out of the sound.

Good for your use. (Backward from mike-input use.)

Click to expand...

When looking at the data sheet though, the transformer is described as a microphone output with a 10.5:1 ratio. Also in the test circuit, the red/brown side which is shown as the primary has a bigger squiggly line (schematic symbol) relative to the other side of the transformer- I assume this is representative of the turns ratio showing it to be the 10.5 side and the yellow/orange side by the 1 side.

Higher voltage BUT lower current. What happens depends on source and load impedances. Here you have an FET running a very small current, and some kind of mike-input with phantom which may be very current hungry. A 2K input refected through a 1:10 transformer puts a 20Ω load on the FET. Its gain will be very low, so low that the voltage step-up can't fix it. Also the "bass fall-off" may start at around 2KHz, sapping all the body out of the sound.

Click to expand...

This infact seems to be describing what I was having with the orange/yellow as primary (ie the transformer wired 1:10.5, not 10.5:1 as it should be).

This of course doesn't explain why I get no signal from the capsule when I have the transformer hooked up with the red/brown being primary (which I'm pretty sure now is the correct way)

thanks

tim

anyone?

tim

The way I see, the RED and BROWN leads are the primary.

is the amp working the way it should? If you touch a screwdriver tip to the FET gate you should get huge amounts of hum on the output.

Do you have an oscilloscope? This is going to be bloody difficult to troubleshoot without one. You can get one for $50 in a surplus outfit.

Unhook the transformer and set it aside until this is resolved.
Things to try:
- Measure the gain of the FET amp. Input magnitude versus output.
- Power the capsule with the 48V phantom power instead of the DC-DC converter.
- Use a different/known good capsule

The way I see, the RED and BROWN leads are the primary.

Click to expand...

Yep - I'm sure of that now. Amp seems to function best with signal generator with transformer hooked up that way.

Do you have an oscilloscope? This is going to be bloody difficult to troubleshoot without one. You can get one for $50 in a surplus outfit.

Click to expand...

unfortunately no. I might be able to make do for the moment by downloading some oscilloscope software and using a simple audio probe.

Power the capsule with the 48V phantom power instead of the DC-DC converter

Click to expand...

Whats the best way to do this? I unhook the DC-DC converter, but then how do I connect the capsule to the +48V, do I simply run a connection from one of the 48V rails, or should it have some other circuitry first?


thanks
tim