I was wondering if someone could tell me what went wrong, and if I have destroyed the mics internal pre-amp, or if I have just killed the capcitor in the battery box:
I have a Sony M10 recorder and was unhappy with recordings from a Audio-Technica ATR 3350 lavalier (too much self noise). So I bought a Sony ECM 88, after checking specs- it says 1 to 10 V on the web site specs. However, after buying it, the detailed manual says that it needs 5 to 10 V if used with a capacitor and resistor 3-wire battery box power, and the 1 V or more is when used with a transformer. And trying using just the 3 V plug in power, indeed the levels are very low, blowing on the mic is the only way to see a response (-35 dB or so) at full gain.
From manuals:
(http://micklem.com/ECM88-into-M10.jpg)
So, I build a 9V 3 wire battery box, as below, but without the pad on the right:
(http://micklem.com/bad-circuit.jpg)
This works really well, low noise, perfect.
Then I try to be clever, and install a pad of around 8 dB so I can record a back-up track on the second channel at a lower level.
This obviously was not a good idea. For around 1 second, I get a signal like I expected, with the 2nd channel lower than the first, but then it very quickly started to generate a high level of noise. So I disconnect everything, and remove the pad resistors, now both channels come straight out of the DC-removing film capacitor. However, on trying this (effectively the same when as I had the battery box working), I get very high noise, and no response to sound (even gentle tapping). I was sure I had destroyed the microphone, but what gave me a little hope is that when using just the plug-in power of the M10, no battery box, the mic behaves just as it did before I built the battery box (blowing/tapping gives very weak signal at full gain).
But now my hope has been dashed, as I think it is probably the pre-amp, not the capsule I destroyed.
So I swapped the capacitor in the battery box, now I am certain I have destroyed the mic. :(
So the 'great' ideal of the pad making a 1 channel output into 2 channels at different levels is not so great. Hopefully someone can learn from my mistake.
Quote from: Jon Stoppable on August 11, 2014, 03:31:38 PM
I don't see anything in those schematics as drawn that should have caused a failure. The pad is just a load, it was certainly not that. It's no different than simply plugging into the M10 from the mic's perspective. I would suspect that the wiring to the mic was reversed or some other fault.
ben-M, this guy knows his stuff. Hopefully you just got some wiring mixed up. ;)
Thanks very much for the replies. I shall check all wiring very thoughly. Is there a possibility I have not destroyed anything? Is the fact that I get very low signal with PIP straight into the M10 (just like before) a good sign for the health of the capsule and jfet?
Does the fact that I am seeing strong white noise point to any particular connection o should check?
Was my pad idea OK then? Were the resistor values reasonable? I thought maybe the mic was seeing too low an impedance via one channel from the pad, while the other channel still saw the high impedance of the M10.
Thank you very much for your opinion Jon!
I have discovered a defective contact in the mini-jack socket- I was unplugging the mic to solder things, so when I added the pad and re-inserted the microphone jack- that must have been the time when the connection failed. The connection is intermittent, and on the clear cable (signal line, and also has the 21k resistor off it), so I guess the intermittent connection to the resistor is giving wildly varying current in the mic's transistor, and hence this noise I am seeing.
I have sourced resistors to have 3x what I used originally (roughly 10 k series and 7 k shunt) and 10x (36 k series and 24 k shunt). Knowing the impediance of the mic is 2.5 k and the recorder 22k, and other details of the circuit above, which would give the best results?
Very relieved. Hopefully have a working mic tonight!
:( seems the 6k of pad resistors has destroyed the JFET. The jack malfunction must have been coincidental- I swapped sockets, then soldiered directly from the microphone cable into the battery box: still this high level of white noise, and no response to sound.
This has been a bit of a roller coaster of hope.
Now I can draw a line under this, and although I will be considerably out of pocket, the benefit will be that I can now select a lavalier that will work straight into the M10 with PIP, and I wouldn't have to carry a battery box that is half the size of the recorder.
OK. Hmmm. The circuit is not exactly complicated, but I've checked and double and triple checked and can't see anything wrong.
I've tested the mic cable without the capsule attached- the shield has continuity to the capsule connection, and there is no connection between the shield and source nor drain wires.
When I started testing the JFET, it was closed and I measured a resistance of 1.1 M in both polarities. I shorted the source and drain, then afterwards the resistance dropped to 115 k in both polarities. Is this open gate reading too high?
Hi again Jon, and thanks for the continued support.
When I do as you say (and use PIP setting 'on' M10), I get absolutely no signal at all.
Just to check, I re-wired the jack as before (clear to tip, red to ring, shield to sleeve), and I was still able to get minute responses at gain 10 to tapping, etc. This response is unaffected by the PIP being on or off.
Strangely, after the chop and re-termination onto a jack in a 2-wire configuration (2 and 3 to sleeve and 1 to tip), I can now hear the white noise, just like when going through the battery box in 3 wire configuration. I also tested it through the battery box, and get the same white noise with no response to sound or even tapping.
Another strange thing is I now cannot get the tiny responses I used to get when I didn't supply any power to the mic, but turned up the gain to 10.