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Post by drumdrym on Feb 17, 2015 22:32:35 GMT
Hi Frans, need your help here with this small project, having need of a small ECM preamp and having most of the parts to build in stock I decided to try this circuit. Module built, EMC captures sound and passes it through circuit and out small 8 ohm speaker but has a put! put! put! akin to motor-boating on the output. Also when I blow harder across the EMC the sound crackles as if it's clipping, the setup with speaker is only to confirm that it works as ultimately I would like to record spoken word over USB to Audacity on PC. Power supply is from 9V PP3. Your observations would be most welcome. Regards, Alan
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Post by drumdrym on Feb 17, 2015 22:58:27 GMT
Here are a couple of pics of built module.............
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Post by drumdrym on Feb 17, 2015 23:05:21 GMT
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solderdude
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Post by solderdude on Feb 18, 2015 7:15:50 GMT
The motor boating could be caused by the 8 Ohm speaker load. I suggest to connect the output of the amp to one of your amplifier inputs and have a listen again.
When you remove the mic and load you can measure the DC voltages in the circuit. The voltage on the emitter of Q2 should be stable and about half the battery voltage
Why not plug a mic in the 'mic input' of the PC/laptop soundcard ?
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Post by drumdrym on Feb 18, 2015 21:00:38 GMT
The motor boating could be caused by the 8 Ohm speaker load. I suggest to connect the output of the amp to one of your amplifier inputs and have a listen again. When you remove the mic and load you can measure the DC voltages in the circuit. The voltage on the emitter of Q2 should be stable and about half the battery voltage Why not plug a mic in the 'mic input' of the PC/laptop soundcard ? Hi Frans, thanks for your reply, I connected output to amp as suggested, motor boating still purring away. Q2 emitter voltage 4.56v Q1 "" "" "" 7.7v These cheap ECM capsules lend themselves to experimenting with multiple close miking of acoustic guitar, clarinet and sax which is the ultimate aim of the present project. Also if I can marry an ECM and preamp with one of those cheap USB sound cards see here..................... www.ebay.co.uk/itm/PORTABLE-USB-2-0-3D-VIRTUAL-5-1CHANNEL-PC-SOUND-CARD-FREE-POSTAGE-IN-UK-/220918920706?pt=UK_Computing_ComputerComponents_SoundCards&hash=item336fcb3602then I have a nice little gadget to hand out to my elderly lady readers to use at home. Just give 'em a mic to plug into the back of their computer........nah! too easy. I'd end up twiddling my thumbs.
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solderdude
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Post by solderdude on Feb 18, 2015 22:07:31 GMT
Q2 emitter voltage 4.56v Q1 "" "" "" 7.7v
Those voltages make no sense at all.
What happens when you remove the 10uF between the mic and base Q1 ? E Q1 should be around 2V
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Post by drumdrym on Feb 18, 2015 23:44:24 GMT
Sorry Frans, retook both voltages this time correctly.........with 10uf mic to base Q1 Q1.....1.96v Q2.....5.08v PP3....9.91v
with 10uf mic to base Q1 removed.... Q1....1.895v Q2....4.9v
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Post by drumdrym on Feb 20, 2015 9:04:36 GMT
Removed 10uf at Mic connected to amp no motor boating no sound.
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solderdude
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Post by solderdude on Feb 20, 2015 9:13:06 GMT
It would appear as though the battery voltage variations are amplified. There is a simple solution for this and requires an extra resistor + extra capacitor. I suspect with these mods it'll probably work correctly. You should replace the 1k resistor (which is the wrong value anyway) with a 6.8k + 2.2k + 100uF circuit described below. Your 10uF cap should remain 10uF (it is 2.2uF in my schematic)
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Post by drumdrym on Feb 20, 2015 12:18:04 GMT
Thanks Frans your solution has sorted the problem.
Regards,
Alan
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Post by drumdrym on Feb 21, 2015 1:46:10 GMT
I wish to add some control on the output would adding a trimmer in the manner shown below be suitable and can you advise on value of resistance, I was considering 50K.
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solderdude
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Post by solderdude on Feb 21, 2015 20:12:45 GMT
I wish to add some control on the output would adding a trimmer in the manner shown below be suitable and can you advise on value of resistance, I was considering 50K. 10k, 20k or 50k will do. Would leave out C5 or if you really want a capacitor in there would use 1uF or 2.2uF non-polar instead.
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Post by drumdrym on Feb 22, 2015 0:00:43 GMT
Many thanks for your help Frans much appreciated.
Alan
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