Sean, you've obviously gone to a lot of trouble to research this mod and to write it all up. Sharing this with us via this medium is what being an audiophile is all about. I, for one, deeply appreciate it and also try to assist others whenever possible. Kudos to you!
Radio Shack SPL Meter mods corrections
I know that others here were interested in the specs to correct the non-linearities of the factory stock RS SPL meter. The figures below are to be either added to (+) or subtracted from (-) the readings that you obtain from your factory stock meter. In other words, the meter in stock form is EXTREMELY deficient when it comes to taking low frequency measurements. Keep in mind that these corrections are only valid under the following conditions. The meter must be set to C weighting, using 1/3 octave pink noise (easily available from various CDs), with the mic pointed at the speaker. These measurements were verified on both the RS analogue and digital meters using laboratory grade test equipment. Baseline testing was done using the 80 dB scale for reference purposes. While on the low side, this should give you a good baseline as to what your actually getting out of your system. Needless to say, if you had BIG peaks in the bass region with the stock meter and you weren't calculating in these correction figures, you're in even worse shape than you thought.
10Hz +20.5
12.5Hz +16.5
16Hz +11.5
20Hz +7.5
25Hz +5
31.5Hz +3
40Hz +2.5
50Hz +1.5
63Hz +1.5
80Hz +1.5
100Hz +2
125Hz +0.5
160Hz -0.5
200Hz -0.5
250Hz +0.5
315Hz -0.5
400Hz 0
500Hz -0.5
630Hz 0
800Hz 0
1KHz 0
1.25Khz 0
1.6KHz -0.5
2Khz -1.5
2.5Khz -1.5
3.15Khz -1.5
4KHz -2
5KHz -2
6.3KHz -2
8KHz -2
10Khz -1
12.5KHz +0.5
16KHz 0
20KHz +1
In order to correct the low frequency roll-off, you can do the following modifications to your meter. This will make the meter FAR more sensitive to low frequencies and allow measurements with very good accuracy to well below 20 Hz. Due to the increased sensitivity at very low frequencies, it is possible for low frequency "thumps" to slam the meter if using a very low SPL setting. In order to prevent meter movement damage, take precaution not to peg the meter off scale on a regular basis. If this is occuring regularly, you either need to move up to a higher SPL range on the meter or take more caution as to how your performing your tests.
All of the following capacitors must be rated for AT LEAST 15 volts or so. Size does matter, so try to use the smallest package possible.
C1 & C2 are changed from 1 uF to 10 uF
C3 & C4 are changed from 1 uF to 47 uF
C7* is changed from 10 uF to 220 uF
C8 is changed from 100 uF to 470 uF
C9 is changed from 22 uF to 220 uF
C15 is changed from 100 uF 220 uF
While all of the above parts are spec'd as microfarads, the following is in picofarads. Do not confuse the two values or the meter will not work very well at all. This last change helps minimize high frequency roll-off that is inherit in the stock microphone
C12 is changed from 33 pF to 12 pF.
Please note that all of the above parts can be soldered directly in place of the originals EXCEPT C7. Due to its location, a "normal sized" 220 uF cap will be too big to allow the case to close correctly. In order to get around this, simply solder it on the "solder side" of the board instead of on the "parts side" of the board. Pay special attention to the POLARITY of ALL of the caps as you pull them out to replace them. For this reason, i recommend pulling and replacing the caps one at a time to minimize confusion. Once all of these mods are done, the meter is more than accurate enough for anything that a home audio enthusiast would ever need use of. If you really want to "get crazy", you can remove the factory installed mic and either remotely mount it on a "wand" or make use of a calibrated mic like those available from Old Colony.
I hope this helps some of you out and sorry it took so long to dig all of this up. Sean
>
10Hz +20.5
12.5Hz +16.5
16Hz +11.5
20Hz +7.5
25Hz +5
31.5Hz +3
40Hz +2.5
50Hz +1.5
63Hz +1.5
80Hz +1.5
100Hz +2
125Hz +0.5
160Hz -0.5
200Hz -0.5
250Hz +0.5
315Hz -0.5
400Hz 0
500Hz -0.5
630Hz 0
800Hz 0
1KHz 0
1.25Khz 0
1.6KHz -0.5
2Khz -1.5
2.5Khz -1.5
3.15Khz -1.5
4KHz -2
5KHz -2
6.3KHz -2
8KHz -2
10Khz -1
12.5KHz +0.5
16KHz 0
20KHz +1
In order to correct the low frequency roll-off, you can do the following modifications to your meter. This will make the meter FAR more sensitive to low frequencies and allow measurements with very good accuracy to well below 20 Hz. Due to the increased sensitivity at very low frequencies, it is possible for low frequency "thumps" to slam the meter if using a very low SPL setting. In order to prevent meter movement damage, take precaution not to peg the meter off scale on a regular basis. If this is occuring regularly, you either need to move up to a higher SPL range on the meter or take more caution as to how your performing your tests.
All of the following capacitors must be rated for AT LEAST 15 volts or so. Size does matter, so try to use the smallest package possible.
C1 & C2 are changed from 1 uF to 10 uF
C3 & C4 are changed from 1 uF to 47 uF
C7* is changed from 10 uF to 220 uF
C8 is changed from 100 uF to 470 uF
C9 is changed from 22 uF to 220 uF
C15 is changed from 100 uF 220 uF
While all of the above parts are spec'd as microfarads, the following is in picofarads. Do not confuse the two values or the meter will not work very well at all. This last change helps minimize high frequency roll-off that is inherit in the stock microphone
C12 is changed from 33 pF to 12 pF.
Please note that all of the above parts can be soldered directly in place of the originals EXCEPT C7. Due to its location, a "normal sized" 220 uF cap will be too big to allow the case to close correctly. In order to get around this, simply solder it on the "solder side" of the board instead of on the "parts side" of the board. Pay special attention to the POLARITY of ALL of the caps as you pull them out to replace them. For this reason, i recommend pulling and replacing the caps one at a time to minimize confusion. Once all of these mods are done, the meter is more than accurate enough for anything that a home audio enthusiast would ever need use of. If you really want to "get crazy", you can remove the factory installed mic and either remotely mount it on a "wand" or make use of a calibrated mic like those available from Old Colony.
I hope this helps some of you out and sorry it took so long to dig all of this up. Sean
>