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mrkva 11th November 2012 01:29 AM

WM 61a advice for non-measurement use
 
Hello!

Just finished 2 wm 61a linkwitz mods and are now considering dual pre-amp box based on this scheme http://www.linkwitzlab.com/images/photos/mic-amp.jpg . Their main purpose is not measurement but binaural recording. I wanted to ask few details:

1. Is electrolytic polarized 2.2uF/63V cap ok for the 7Khz filter?
2. In the text, there is a mention of "The input blocking capacitor", that it "can be a small size polar capacitor (Tantalum) because of the applied 9 V bias voltage" but I don't see in this schematic - where should it be placed?: http://www.linkwitzlab.com/images/photos/mic-amp.jpg
3. Did anyone tried making own windshields for these?

Regards,
Jonas

mickeymoose 11th November 2012 02:06 PM

The filter is a 7 Hz (not 7 kHz) HP formed by the 2.2 uF and the 10 kOhm resistor connected to pin 3 of the IC. This cap is the same as the input blocking capacitor mentioned. The 2.2 uF value was chosen for a measurement mic, but should be reduced to 0.47 uF for other purposes. This would increase the low cut-off to 34 Hz and reduce (not eliminate) the need for a windscreen. E

mrkva 11th November 2012 02:08 PM

Quote:

Originally Posted by mickeymoose (Post 3237328)
The filter is a 7 Hz (not 7 kHz) HP formed by the 2.2 uF and the 10 kOhm resistor connected to pin 3 of the IC. This cap is the same as the input blocking capacitor mentioned. The 2.2 uF value was chosen for a measurement mic, but should be reduced to 0.47 uF for other purposes. This would increase the low cut-off to 34 Hz and reduce (not eliminate) the need for a windscreen. E

Great! Thanks for the response. What kind of cap would you recommend?
Also, do you maybe know, if it is ok to use single OPA2134 as preamp for two mics? The schematic is made just for one, but I see one unused opamp there at the bottom.

Regards

mickeymoose 13th November 2012 03:28 PM

Any inexpensive metalized film or ceramic cap should do
Yes you can use the other opamp in this package for the other mic
Cheers, E

mrkva 22nd November 2012 02:40 PM

Hey guys, I redrew the linkwitz mod with preamp to stereo version in Eagle (to make it less confusing), can you please have a look if it looks good? I've added pot for the gain control in series with resistor (to be sure there is some resistance always).

http://i.imgur.com/B93FY.png

mickeymoose 23rd November 2012 03:22 PM

It will work, but.....
- I would install a NP cap on the output of the amp with a high-R resistor to ground at pin 3 of the XLR
- Why R7 & R14?
-The max. gain (about 15 dB) is propably not enough to drive the signal to line level. You also need to DC isolate the pot. When you rotate it as is the DC on the output of the IC will be all-over
E

mrkva 23rd November 2012 03:34 PM

Quote:

Originally Posted by mickeymoose (Post 3253157)
It will work, but.....
- I would install a NP cap on the output of the amp with a high-R resistor to ground at pin 3 of the XLR
- Why R7 & R14?
-The max. gain (about 15 dB) is propably not enough to drive the signal to line level. You also need to DC isolate the pot. When you rotate it as is the DC on the output of the IC will be all-over
E

Thank you for your answers. Since I am just a beginner in this field, I have to ask some additional questions. NP cap - what values - 0.47uF? high-R resistor - 10k? or 1M. Cannot really tell what is high.

Regarding DC isolation - I am also not very sure how to do that. Is that about connecting 3rd pin to the ground?

Pot - 100k should be better, right?

Thank you again! :rolleyes:

mrkva 23rd November 2012 03:37 PM

Quote:

Originally Posted by mickeymoose (Post 3253157)
It will work, but.....
- Why R7 & R14?

I found it in this schematic
http://www.linkwitzlab.com/images/photos/mic-amp.jpg

mickeymoose 23rd November 2012 06:36 PM

OK, m..., what is it that you want to record? What are you feeding the signal into (next piece of equipment)? The output here says 40mV@100dB SPL (not much for 100dB!). Yours would produce about 110mV. Look at the specs of your gear to see how much level you need and also how loud your source is and how far the mics are away from it
I have seen that pseudo balanced-line setup (the 200 Ohm res) before. I have never tried it and, as I said before, for an extra Euro you can get a proper balanced line drive circuit. E

Speedskater 23rd November 2012 10:47 PM

The XLR connectors pin #1 should connect directly to the chassis. Then the circuit ground connects to pin #1 through R7 & R14. The XLR shell connection is no longer attached in cables.


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