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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
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sean
sean OP
6,231 posts
 
The parts to modify this meter are quite inexepensive, making it a good deal even after investing the money. The fact that RS is convenient and many people already have these units makes such an approach even more acceptable. In comparison to many other SPL meters that cost more money ( $70 - $100 ), the modified RS meter is more accurate. One can find a very good quality SPL meter on Ebay for not that much money, but the calibration of such a unit may be questionable due to having a "rough life".

As to why they haven't built it right, maybe they have. I know the original analogue meter circuitry remained pretty much the same for a couple different incarnations of the meter and the digital meter basically copied the existing circuitry ( from what i've been told ). Like El stated above though, "if it works, don't fix it". Their circuit worked, it just wasn't as accurate as it could have been.

For more info on this and other RS SPL meters, try taking a look at Eric Wallin's website. He's got mods for a few different RS SPL meters along with several other audio related projects. Sean
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sogood51
2,157 posts
 
Thanks Sean, I guess that would explain why my subs measure down 10db or more at the extream low freq's yet still shake the room.

I did raise their output level at one time to bring it into line with the higher bass freq's and found this to excessive to blend smoothly.

Your chart confirms what I was hearing (and feeling) in my room.

Dave
bobf31e
1 posts
 
I just purchased a 33-4050 model (7 scale selector switch, Tandy catalog Q1461 here in Australia), and I have not seen any other models. I opened it up, and just from the description of the component values, I'd say it is different from the model described above. The original values I see are:

C1, C2, C3: 1 uF
C4: 100uF
C7, C12: 22uF
C15: 100 uF

C5, 13, 14, 16, 17 are electrolytics on the component side of the board but I forgot to get their values before putting the case on again, mainly because they were not mentioned in the ones to modify.

C6, 8, 9, 10, 11, 18, 19 are surface mounted tiny capacitors on the rear side of the board.

So it would be goot to know if anyone has information on the calibration of this model. It does have a trim-pot called "cal" for calibration, but no instructions other than to take it to an expert for that.

Bob Long
sean OP
6,231 posts
 
Bob: The calibration pot simply allows one to adjust for factory tolerance at one reference frequency and spl. Once that is adjusted, the non-linearities of the circuitry / mic still come into play at other frequencies and spl's.

As to whether or not these mods apply to the newer digital meters, i have no idea. I always prefer to use "analogue" ( i.e. "ye olde school" ) meters as they tend to be a little more revealing of trends, etc... They might not give the absolute resolution of a digital read-out, but at the same time, we are basically looking at the average frequency response here, not correction down to the .1 dB level. One could only hope to achieve anywhere near that type of linearity in-room and if they did, i will guarantee that there would be no sense of "ambience" what-so-ever. Sean
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ack's avatar
ack
115 posts
 
I have a RadioShack 33-2055 digital meter and the values of the caps are completely different. Anyone know if the mods will still work?
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