The upper schematic is for the analog version(30-2050); it clearly shows an analog meter. The lower hand drawn one is for the digital display unit (30-2055). Larger value coupling capacitors will improve the low frequency response of the circuit; 1 uF coupling caps would never be used in any microphone preamp considered "Hi-Fi". However, microphone capsule variances may exceed this deficiency. For $1.50 worth of parts, it's worth it, IMHO.
Gotcha. If I had bothered to study them I'd have noticed the dial gauge symbol on the top one. :T
But I’m not sure how making the pre-amp more “hi-fi” would do anything for its basic function of generating SPL readings, unless perhaps it would perform better at low-level readings due to a quieter internal noise floor?
As far as better low frequency response, I can’t see any reason for improving that unless the meter in its current state can’t accurately track the low end of C-weighted response. This meter doesn’t have a Z-weighted option...
I’m no expert with these things but always looking to know more about them, so please educate me!
I have a link to a very good explanation of the modifications to the Radio Shack meters: It explains that the stock meter has some serious limitations at both ends of the spectrum which can be addressed by capacitor replacement, and/or mic capsule replacement, and/or IC replacement.
Unfortunately, I am not allowed to post links on this forum.
You have to have a five-post minimum before being able to put in links. It’s a protection against “spambots.” Go to our post padding thread and burn another post, then you’ll be able to give us the link. I for one would like to see it. :T
There is a significant collection of meter modifications (for the Radio Shack 30-2055), created by Eric Wallin, who says:
The electronics of the stock digital RS SPL meter rolls off the bass (many poles) somewhere around 28Hz (-3dB), and rolls off the treble (two poles) at 12kHz (-3dB). The frequency response mod as described below improves the frequency response of the electronics for the "C" setting of the A/C switch. After performing the mod, direct meter readings should be accurate for bass SPL measurements down to 10Hz or so using the internal mic without using any correction factors.
The treble response of the stock meter is another matter. While the designer dispensed with the skanky stone-age BA 301 op-amp found in the analog meter, he inexplicably settled on using the (in some ways even skankier, particularly for this application) LM324 quad op-amp for most of the electronics, and this is the main cause for the high frequency response of the electronics to be down 3 dB @ 17 kHz even after the mod is performed. You could try replacing the op amp with a higher quality quad unit with the same pinout (a good choice might be a Texas Instruments LMC660: TI says "The LMC660 is pin-for-pin
compatible with the LM324 and offers greater bandwidth and input resistance over the LM324. These features
will improve the performance of many existing single-supply applications") The specs to look for would be near rail-to-rail input and output, along with reduced supply voltage operation and high slew rate. Note also this mod should not affect the calibration of the meter, since only the poles in the circuits have changed, not the ac gain in the pass band. Use electrolytic caps (except where noted), observing polarity: C1, C3 (100uF original) add 47uF or 100uF (no difference, your choice). C2 (1uF original) add 10uF. C5 (1uF original) add 4.7uF. C6 (1uF original) add 22uF. C7, C17 (1uF original) add 1uF. C11 (1uF non-polar original) add 22uF polar, the (-) terminal should go nearest to the output jack. C18 (0.022uF original) add 0.1uF (polyester or mylar). To extend the HF response: CHANGE (replace, use ceramic or NPO cap): C9, C12 (27pF original) change to 10pF. REMOVE (and don't jumper the holes or anything, just take it out of circuit): C4 (100pF original) REMOVE! The SPL meter electronics (including the digital meter itself and the signal at the output jack, but not including the built-in mic capsule) now respond in the following way (regardless of the range switch): "MAX" response switch position, "C" weighting: +/-1dB: 8Hz to 11kHz +/-2dB: 5Hz to 14kHz +/-3dB: 3Hz to 17kHz Flat otherwise.
Comments in italics are mine, and lifted from Linear Technology's website
Since you’ve already posted a question about using the ECM8000 mic, I assume you’re going to use the meter for SPL rather than taking measurements. So I don’t know what you mean by “will it work with REW.” Assuming it is in functioning condition, it will work with or without REW. However, it’s like all cheap meters in that they’re accurate +/- 2 dB at best. Fortunately, dead-on accuracy is not necessary for our purposes.