Ron,
Yes the WM61A is spec'd at -35 dB. But I calc that at 64 dB SPL the output will be more like 50 mV PP than 35 mV PP.
In any event, I am not building this mic pre for the 61A. I am using the WM64PCT that is spec'd at -44 dB. Data sheet attached.
I have been all over the net trying to get a handle on what the input overload is on soundcard line in. All I get is confusion. The manufacturers do not spec that cause the typical customer has no clue what the spec might be for. There appear to be no standards at all on that parameter. I have emailed M-Audio to see if they even know the spec for their own product. I am waiting for a reply.
So, for now, with no target at the receive end, I propose a thought experiment.
To use a laptop or equivalent for measurement you must use an outboard (USB) sound card cause the internal sound cards are usually crap and sometimes not even duplex which is necessary for proper measurement. Often they do not have line-in so you would have to use mic in which is usually crap and usually mono and not suitable for proper measurement. So we are now talking about a USB stereo duplex sound card with line-in.
We know the USB port provides no more than 5V to the sound card. If we assume the sound card will use all the room 5V provides and that it will accept a rail to rail signal, then we can hypothesize that the max input possible is 5V PP or 2.5 V P or 1.78 V RMS.
Using that as an underlying assumption for this thought experiment, let's compare the output of the WM61A against the WM64PCT and see if we can figure out why Linkwitz did what he did when he set the gain on his preamp at about 10 dB and see if that helps us with the WM64PCT.
The next image is a Table I put together that shows the output of the WM61A and WM64PCT at 1 Pa (94 dB SPL) and at 125 dB SPL. Then I show the result with +10 dB gain a la Linkwitz, then at +20 dB and finally at +18 dB. You will note I left out the 61A after + 10 dB cause it blows up the 5V PP limit at and above +10 dB. Note I used +10 dB for Linkwitz because that is what he calls it . Actually his gain is X3 which is +9.54 dB but 10 is close enough.
On my table I highlighted where Linkwitz crosses the thought experiment peak to peak and where the 64PCT crosses it as well. This suggests to me that the circuit gain should be around 17 dB. Now this does not deal with the reality of sound card overload and presumes Linkwitz understood what he was doing. But its better than nothing.
What do you think?
Last I attach some spec snips I took from the LinearX site re: their mics. The variations are not much different from other companies. It makes me think all the mics are buffered but some are amped and others are not. Hmmm.
Thanks