RS SPL METER (33-2050) MODIFICATIONS
AbstractThe analog RS SPL meter is an inexpensive instrument that can be employed to measure the frequency response of DIY speaker systems. Two modifications are described that improve its accuracy at the upper and lower extremes of the audio spectrum, and make it more convenient to use. IntroductionThe analog RS SPL meter (cat.no.33-2050) is an effective, useful, and cheap tool that should be in every DIY speaker builder's arsenal. When coupled with a test CD or other signal source, accurate anechoic-like bass measurements can be performed using this meter in a "close-miked" setup; and in-room response can also be measured easily. In many ways the analog RS meter is better than the digital meter RS offers: The resolution of the readout is higher (fractions of a dB, while the digital meter only reads to the nearest dB), the analog meter movement provides more information when analyzing dynamic signals, and finally it is less expensive than the digital meter. If the digital meter dispensed with the range selector switch and read fractional dBs, I would consider purchasing one, but with it, I consider the analog meter more useful and the better value. The electronics of the stock analog RS SPL meter rolls off the bass (many poles) somewhere around 35Hz, and rolls off the treble (one pole) at 10kHz. This is roughly comparable to the "C" weighting curve (from a post to the bass list by Thomas Danley): 20HZ =6.2DB 25HZ =4.4DB 31.5HZ =3DB 40HZ =2DB 50 =1.3DBThe frequency response mod described makes the *electronics* flat over the entire audible range for the "C" setting of the A/C switch. After perfoming the mod, direct meter readings should be accurate for bass SPL measurements down to 20Hz or so using the internal mic *without* using any correction factors like those listed above -- in other words, pure WYSIWYG for bass readings! The treble response of the stock meter is another matter. The poor "roller coaster" response of the meter in the treble region (as seen in the instruction manual for the meter) is caused (this is in fact informed speculation, but speculation nonetheless) partly by the physical mounting of the internal mic and partly by the mic module itself. The external mic mod (coupled with the frequency response mod) *should* correct for this if you use a Panasonic mic module mounted in the end of some tubing ala the MMII. I have no way of checking the frequency response of the mic element, but I have used the added external mic input to check the frequency response of the modified electronics. Basically, if the external mic cartridge you use has a flat frequency response, then the modified meter electronics will be able to keep up with it and not introduce any additional response irregularities. Recommended external mic cartridge (excerpt from an email I got from Mike Feldman, thanks Mike): Panasonic electret mic capsules from Digi-Key; 1-800-344-4539. They carry several, but I think you want WM-60AY, Digi-Key part P9959-ND, $2.24 in quantity 1-9; $19.88/10.This is the same cartridge used in the Mity Mike II, and typically has a +1dB hump around 10kHz, but is very flat otherwise. Both mods are simple and may be free if you have some caps in your junk box and some plumbing supplies in your closet / garage. Mod your meter today and you'll be able to train a monkey to do your sub testing tomorrow!
FREQUENCY RESPONSE MODClick here for the schematic. Instructions are as follows: - To extend the LF response: - CHANGE (either replace or solder in parallel to the old value, your choice, use electrolytic caps):
- To extend the HF response: - CHANGE (replace, use ceramic or NPO cap):
Case Disassembly:
After this mod, I (or the meter, rather) exhibited some instability (hf oscillations). Making C12 12pF cured this (it was 10pF in the inital mod). If you experience similar instability (needle jumps around, stays pegged on the lower ranges, is influenced by the position of your hand over the needle area) increasing C12 is the way to fix this. Be aware that the larger this cap is, the lower the hf rolloff will be, though of course the meter is useless if it is not stable. Regardless of whether you do the external mic mod or not, after the basic electronics mod is done, you will notice that the meter needle pegs a couple of times at turn-on before settling down. This is normal since the circuit now has many very long time constants. As I stated before, on the most sensitive settings, the meter will seem somewhat jumpier. This is due to the amplification of low frequency data such as wind and case thumps. In fact, this is an indication that the mod is working correctly. My meter, on the 60dB setting, registers my moving hand from several feet away. And this isn't the breeze from my hand "blowing" on the mic, this is my hand moving in a rythmic closer-farther-closer motion relative to the mic at a couple of Hz rate, Xmax ~3" p-p (sorry, ~1.5" peak :-). It is the sensitivity of the mic cartridge to very low bass in this test that lead me to believe the cartridge is probably pretty flat to a few Hz. I just want everyone who is contemplating performing this mod on their meter (the frequency response correction mod, with or without the external mic mod) to be well aware of the behavior afterward. I am quite happy with the results, but if this sort of behavior (i.e. the double pegging and settling at turn-on) drives you crazy, perhaps you should decide against surgery and go with the old therapy, i.e. equalization charts. The SPL meter electronics (including the meter needle itself) now respond in the following way (regardless of the range switch, external mic input, C12=12pF):
"Fast" response switch position:
"Slow" response switch position: The "Slow" response setting looks like the way to go for accurate low bass measurements. Based on the -0.2dB @ 20Hz figure, we can now pretty much ignore the frequency response of the electronics for most bass measurements.
EXTERNAL MIC MODI have performed this mod and recommend it highly, as it makes the meter more versatile. Basically, the internal mic element is removed and placed on the end of 3 meters of coaxial cable. This places the element itself in a less compromising position acoustically, and has the added benefit of placing the meter needle far away from the measurement location and closer to the eye where it is much easier to read. If you decide to plug in a better mic cartridge, the HF response should be flat up to 20kHz or so. Instructions are as follows:
That finishes up the mods to the case. Next for the mounting of the mic element:
FAQ SECTION> A Quick Question > > You indicated you left the caps on & attached caps to back of board meaning > you, in case of first cap change, added 9uF to back inparallel or cust lead > on one on front & added a 10uF > > also, I assume I would match the type of caps ie. electrolytic, tantulum, > mica etc.... > > I have a 'basic' understanding of electronics at best The idea is only to lower the poles so that they do not significantly alter the meter response at the frequencies of interest (>10Hz or so). So it is really just a matter of increasing the capacitance of C1, C2, C3, C4, C7, C8, C9, and C15 until this condition is true. There is no need to remove the original caps, and no need to find special values to make the resulting parallel capacitance values exact. For example, soldering a 10uF in parallel with a 1uF only makes the resulting value off by 10%, and this is usually within the tolerance of the 10uF in the first place! The resulting pole may be 10% lower, but all the better. If you use values that are significantly larger than those that I specifiy for the signal path, the meter will take a lot longer to settle down at turn-on, which might drive your crazy if you are sensitive to such things. Mine takes about 3-5 seconds to calm down after turn-on, and is stable during and after changing the range switch. It is very sensitive to wind and case shock after the mod, which makes sense if you think about it. C12 is the only capacitor that needs to be replaced, since its value has to be reduced. If you find that your meter is unstable after the high frequency mod (needle moves erratically, pegs or stays somewhere in the midrange even with no sound input), try adding small values of capacitance (2pF or so) across C12 until the meter stabilizes. This will make your meter less accurate in the high-end (the high-end response will start rolling off sooner). Better to err on the side of stability, since the meter is useless if it is not stable. Use electrolytics in parallel with the electrolytics (which is every modified cap except C12). I used some that I scrounged from an old VCR. These caps were very small for their size, making them an easy fit. Any voltage level at or above 10V is fine. Observe polarity when installing them (s/b same as the originals). A ceramic or NPO will do for C12. R/S sells an assortment of small value caps (1-30pF) that will do nicely for C12, and will allow you to find the critical value that will yield both stability and maximum flat hf response. As I stated above, starting with 12pF would probably be wise. > You had previously written : > > > The easiest way to change these caps (except for c12, which > > must be replaced) is to leave the originals on the PC board, > > and solder the new caps to the back of the board, where there > > is plenty of room. > > I assume from the above that you meant to add the new parts > to the old ones (in parallel). So C1 goes from 1 uF to 11 uF > not to 10 uF. This looks safe enough since you're just lowering > the pole frequency even more than if the old cap were removed. Right. The tolerance on electrolytics is usually rather wide, and all we are trying to do is lower the poles below some (rather arbitrary) frequency (I picked ~1Hz). > FYI, I made the meter mods last night - just the 9 caps. > So far as i can tell, it's acting as you described. The needle > jumps a lot on power-up and when changing ranges, especially in > "fast" mode. Hand motion is detectable. Next step is to > run some tests on it and compare it to previous results. > > Also, I think I'll keep it as an internal mic device only if > the existing mic module seems to be pretty sensitive. It's probably > not flat all the way to DC, but there's a decent chance it's > flat to at least 20 (the Panasonic module I bought from DigiKey is > supposed to be rated 20-20k; I wonder how far off it is below > that ?) and that's good enough for me. Then I'll use the other mic > for something else I guess. Mine pegs twice on power-up, but then settles down. Switching ranges doesn't affect it very much when in the "slow" mode, which is where mine stays now. Note that the "slow" setting will give much better low frequency readings, -0.2dB @ 20Hz vs. the "fast" setting response of -1.2dB @ 20Hz. This also keeps the needle from jumping around during range switching. I guess you changed high frequency cap C12 also. Did you make it 10pF or 12pF? I had to go to 12pF for stability reasons, outlined in my second post. If *holding* your hand near the meter area causes readings, you have oscillation and should increase C12 until they go away. Adding the external mic option is a snap. Have you considered this? I think that one of the reasons the HF response sucks so bad (in the RS manual, the roller coaster action in the highs) is due to the funky mounting of the internal mic. That huge plastic flange (I thought it was aluminum until I took the thing apart; some things I would rather not know :-) and that funky "PC board" screen *have* to be altering the response curve in a major way. An external mic (like the wand-type MMII cartridge at the end of a tube type thing - without all the fancy electronics) would make this unit accurate all the way up to 20kHz, where the response of the electronics is down only -0.8dB (with C12=12pF). Getting my body out of the measurement area and the meter nearer to my eye would be added bonuses (boni?). HF response aside, I bet dollars to donuts that the mic cartridge itself is, as you say, flat to at least 20Hz based on the "waving ~3Hz hand" experiment. Be nice to have some way to check this. All we need is a calibrated sub good to 3Hz (next project...just kidding) or a specral noise source that doesn't employ speakers (maybe released compressed air?). Till then, I'll trust that the bass readings I get are fairly accurate. > Just to clarify, am I correct in assuming that this mod will > correct the deficiencies of the internal mike, but that the > impact of C/A weighting will still have to be cranked in the > the readings? Just the opposite! After the mod, the electronics will be flat (including the meter needle itself) but the internal mic will continue to contribute its poor hf response. The internal mic seems to be fine for bass readings, and continues to work well for those situations when you need calibrated "A" weighting readings (rock concerts and jet planes and such). The external mic mod should easily get around the high-end response problem, and also will get your body out of the measurement field and place the meter nearer to your eyes where you can more closely scrutinize the trembling little needle. > I've just found your instructions for modifying the meter's response and > was wondering if this also affects the signal sent to the output jack? This mod should correct for the output jack response also, since the meter amplifier ic actually feeds the output jack. So you should do both the hf and lf mods if you want the frequency response of the signals at the output jack to be flat. > Have you done any tests on the output? I was wondering if it was constant > if a part of the signal drops off the current range (eg range = 90dB, but > at say, 500Hz the signal is only 80dB)? I have checked the fr of the output jack, and the response of the electronics after the mod follows the actual meter needle response over the entire 20Hz - 20kHz range. Everything *after* the built-in mic in the signal chain is flat over this range, including the meter response and the output jack response. > Will the output jack give a sensible (linear) reading if I'm measuring a > signal where 1kHz reads 90dB, 10kHz reads 80dB and 100Hz reads 100dB with > the range set to 90dB? > > I know the meter scales the reading (ie switching the 80dB range in this > example would make the output higher) but would signals that are out of the > meter's current range be output at the correct relative value? Since the single transistor pre-amp circuit is class A (and I assume the op-amp used for meter drive / output jack drive is class A as well) then signals at the output jack should be linear for input signals outside of the meter range. The caveat here would be for signals that are so large that they cause clipping, and thus overload the op-amp. Smaller signals should be no problem as long as they are not so attenuated that they are buried in noise. |
