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Author Topic: Blown up something- using a pad on a battery box (M10, Sony ECM 88)  (Read 4168 times)

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Offline ben_m

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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:


So, I build a 9V 3 wire battery box, as below, but without the pad on the right:



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.

Offline ben_m

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Re: Blown up something- using a pad on a battery box (M10, Sony ECM 88)
« Reply #1 on: August 11, 2014, 02:43:40 PM »
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.

Offline Chuck

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Re: Blown up something- using a pad on a battery box (M10, Sony ECM 88)
« Reply #2 on: August 11, 2014, 04:53:35 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.  ;)
Reality is merely an illusion, albeit a very persistent one.

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Offline ben_m

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Re: Blown up something- using a pad on a battery box (M10, Sony ECM 88)
« Reply #3 on: August 11, 2014, 10:50:27 PM »
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.

Offline ben_m

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Re: Blown up something- using a pad on a battery box (M10, Sony ECM 88)
« Reply #4 on: August 12, 2014, 11:49:29 AM »
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!

Offline ben_m

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Re: Blown up something- using a pad on a battery box (M10, Sony ECM 88)
« Reply #5 on: August 12, 2014, 05:20:32 PM »
 :( 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.

Offline ben_m

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Re: Blown up something- using a pad on a battery box (M10, Sony ECM 88)
« Reply #6 on: August 13, 2014, 03:38:31 AM »
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?

Offline ben_m

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Re: Blown up something- using a pad on a battery box (M10, Sony ECM 88)
« Reply #7 on: August 13, 2014, 05:44:40 PM »
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.

Offline ben_m

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Re: Blown up something- using a pad on a battery box (M10, Sony ECM 88)
« Reply #8 on: August 15, 2014, 03:58:55 PM »
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.

 

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