Does anyone have a diagram for the Radio Shack analogue SPL meter? I've done the mods to flatten the electronics but now I want to interpose a (switchable) +40dB low-noise amplifier between the microphone and existing preamplifier so as to be able to use the meter for cheaply comparing environmental noise between locations. It would help if I knew what the existing diagram was...
You can find one here: http://mysite.verizon.net/tammie_eric/audio/rsmeter/33-2050/rs_spla.gif
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Didn't need a meter for that. My neighbors let us know. I can still sleep in the bed.
I use SPL meter in industrial settings for use in paging requirements. I use the digital thermometer to show my wife it's too cold in the winter.
I use an Audio Control SA-3050A for flat audio readings.
-Chris
EC8010 said:
You want an extra 40 dB gain after the microphone? The microphone capsule itself is not very low noise. I believe, the nosie floor is about 30-35 dB and even higher at low frequencies. Small diameter capsules have high noise levels, even the expensive ones from Brüel&Kjaer or Earthworks. I am using a Panasonic WM-60 in place of the original capusle. The Panasonic capsule is mounted externally. I had to increase the gain of the Radio Shack by 4dB for the same reading. Eric Wallin's article is now to be found here: http://mysite.verizon.net/tammie_eric
Agreed, small capsules have higher noise floors, but the RS capsule is 3/8" compared to the 1/4" of the Panasonic so that should give it about a 6dB advantage (double the area doubles the capacitance). Granted, that doesn't put it in the same league as a large capsule Neumann.
Having spent some time scribbling on the backs of envelopes, it looks as though that single transistor amplifier could be converted into a two transistor amplifier rather than adding another amplifier. Also, the calibration tweak could move to be after the amplifier (helps with noise). I calculate that the existing single transistor amplifier has a gain of about 16.5dB, and that it would be feasible to make a new two transistor amplifier with a gain switchable between 46.5dB and 16.5dB - adding 30dB, rather than 40dB. If your thoughts are correct, even this will put me right on the edge of capsule noise. There's only one sure way to find out...
Having spent some time scribbling on the backs of envelopes, it looks as though that single transistor amplifier could be converted into a two transistor amplifier rather than adding another amplifier. Also, the calibration tweak could move to be after the amplifier (helps with noise). I calculate that the existing single transistor amplifier has a gain of about 16.5dB, and that it would be feasible to make a new two transistor amplifier with a gain switchable between 46.5dB and 16.5dB - adding 30dB, rather than 40dB. If your thoughts are correct, even this will put me right on the edge of capsule noise. There's only one sure way to find out...
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