Few questions about Linkwitz' mic amp circuit - Page 3 - diyAudio
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Old 11th February 2009, 01:25 AM   #21
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I would've laid it out nicer but I wasn't planning on using a virtual ground in the beginning.

The big caps are Rubycon 50V 220uF, paralleled. The small medium-blue ceramic caps in the middle are 50V 0.47uF, also paralleled.
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Old 11th February 2009, 10:18 PM   #22
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Only thing I can't work out is why there's 4 of those yellow film caps? I can only see a place for two, one from the opamps V+ to vGnd, and one from the opamps V- to vGnd. What are the other two connected to?

Actually, they don't look like film caps, either, they look like tants to me, which the definately shouldn't be.

The input caps (2.2uF on the schematic), which I'm assuming are the blue caps in the top and bottom left corners, shouldn't be tants either, they're really the worst cap for this particular job, they should be at least polyester box caps (though they're shown with polarisation markings, if you use them with an unpolarised cap it doesn't matter), and preferably polypropylene.
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Old 12th February 2009, 12:12 AM   #23
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Quote:
Originally posted by TheSeekerr
Only thing I can't work out is why there's 4 of those yellow film caps? I can only see a place for two, one from the opamps V+ to vGnd, and one from the opamps V- to vGnd. What are the other two connected to?
The rounder ones which you refer to are the tantalum input caps.

Quote:
Originally posted by S. Linkwitz
The input blocking capacitor can be a small size polar capacitor (Tantalum) because of the applied 9 V bias voltage.


Quote:
Originally posted by TheSeekerr
The input caps (2.2uF on the schematic), which I'm assuming are the blue caps in the top and bottom left corners, shouldn't be tants either, they're really the worst cap for this particular job, they should be at least polyester box caps (though they're shown with polarisation markings, if you use them with an unpolarised cap it doesn't matter), and preferably polypropylene.
Those are the ceramic output caps, not the input caps. Why are tants bad as input caps? And, if I really should replace the tants with poly caps, do I still need to reach 2.2 uF?
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Old 12th February 2009, 12:27 AM   #24
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I've cleaned up some of the signal routing, but I can't for the life of me figure out how to get that 5K resistor hanging over pin 5 of the OPA to sit flush against the board without using a jumper.
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Old 12th February 2009, 12:29 AM   #25
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closer side is the "left" channel

If you look at pin 1 of the TLE and go straight down, that's the column that splits the V- (on the left) and V+ nodes at the bottom two rows
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Old 12th February 2009, 06:01 AM   #26
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Damn, I hooked it up and I didn't get any sound out of it. Would the WM-61a work with ~0.9V bias? For some reason a 9V battery hooked up only delivers ~0.951V, and yet the bias is ~0.9V. V+ doesn't get much at all. Also, the battery gets warm after a few minutes. It's a crappy no-name but I don't think that's normal.

So, it looks like I've hooked something up wrong, but the power supply was working correctly before I connected it to the preamp section. It would deliver +/- 1.6V from two AAs in series.

How would I go about diagnosing this? All the solder joints look good and there are no shorts. My best guess is that I might have overheated the ceramic output caps when I was trying to reposition them - possible? And then there's the thing with the power supply...
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Old 12th February 2009, 09:40 AM   #27
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Definitely something weird going on, I'll have a closer look at it in the morning, right now I'm not thinking brilliantly.

Tants are fine for measurement preamps, but rubbish for recording preamps. And yes, the 2.2uF value is a minimum, it has been selected in order set the -3dB point of the low pass filter formed by it and the input impedance to a value < 20hz, I think it's about 9hz on that schematic.
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Old 13th February 2009, 10:00 PM   #28
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Some huge pics (~4MB each)

http://leonli.com/howard/IMG_0506.JPG
http://leonli.com/howard/IMG_0508.JPG
http://leonli.com/howard/IMG_0509.JPG
http://leonli.com/howard/IMG_0510.JPG
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Old 13th February 2009, 10:15 PM   #29
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I disconnected the OPA from the power supply and tested the power supply with a Panasonic 1.5V battery which provided ~1.65V.

The positive rail is 0.9V while the negative rail is -.7V. Something's up...
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Old 14th February 2009, 06:19 AM   #30
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Quote:
Originally posted by 454Casull
I disconnected the OPA from the power supply and tested the power supply with a Panasonic 1.5V battery which provided ~1.65V.

The positive rail is 0.9V while the negative rail is -.7V. Something's up...
Looks like I'd better pick up a pair of decent 9V batteries. The nominal minimum voltage for the TLE is 4V, while the op-amp needs at least +/- 2.5V.

EDIT: I found an unused 6V lantern battery putting out ~6.4V. The rails are +3.203V and -3.198V on my crappy RS multimeter, so I think the power supply works. Maybe the other side will work now too?

EDIT2: Nope. Is the op-amp output voltage supposed to be anywhere near the voltage of its rails?
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