Does anyone know what/how to do an analog meter similar to the ones on the front of pass labs XA 200 amps (like the 1st picture on his home page)
In class we called these D' Arsonval movements, but I haven't been able to find anything like this from digikey etc. I assume that I can buy just that part, or would I have to disect an old multimeter?
BTW, I want this to add to the front of my leach amp (I'm not sure what it would measure yet... maybe temperature)
Re: analog indicator
They are generically called analog meters. Usually available surplus. Good looking and well made ones are not cheap.
Ive used a lot of the Sifam AL29B meters. They are a good movement, very versatile and reasonably priced. Many quality broadcast and recording manufactures use them, however I dont think this is the same device as in the Pass Labs products.
The Vu and PPM scales (and ballistics) available on these meters refer to specific calibrations designed to optimize SNR with respect to peak headroom. Consequently they may not be the ideal indicator for an amplifier.
Sifam does however offer a number of other movements with alternative ballistics and scales.
I’m not sure what the meter on the Pass Labs products indicates. Amplifiers often have meters indicating the peak voltage at input roughly calibrated to Vu levels and ballistics. I used to be a fan of the meter on the SWTP Tigersaurous (darn, how’d I get so old). It was a measure of the current drawn by the amplifier at the power supply output.
Thanks guys, these are closer to what I was looking for.
Would I be able to take the things apart and use a the movement with my own custom backing? If you have experience with these I would be interested.
BTW what is the current drawn at the power supply related to? Output levels?
The VU meters look fairly easy to set-up. I'm guessing you run a line out to a buffer with a 600R impedance and right into the meter. And that's assuming that the system won't start to distort at 0 dB.
The PPM puzzle me a bit. If these are voltage meters or ammeters, how do you measure power (VA), especially since the speaker load may not always be the same?
Random thoughts about meters:
Metering for an amplifier presents interesting issues.
Monitoring the input tells you little about the health or performance of the amplifier. I suppose it would be possible to calibrate the amplifier in such a way that its peak headroom before distortion would correspond to a VU or PPM scale. Perhaps as useful would be having a full scale reading that indicates the point were the amplifier first reaches significant distortion.
I’m not sure what the Pass Labs meters are showing.
I liked the SWTPc approach because it was not simply modulated to the audio waveform but by watching for abnormalities, would also indicate the health of the amplifier. Of course this wasn’t a class A amplifier.
Bridging the output with a metering device likely makes the most sense. Monitoring the peak voltage should make it possible to scale the meter to show the actual output in watts. Will the impedance load of the speakers make it possible to accurately scale the meter to show watts by reading the peak voltage? That could take a bit of head scratching.
I loved the Youngs Surplus site. It has lots of fun stuff. I saw an aluminum Hoffman box that would be great to put a GC in for patio speakers…
Poking around in he Sifam site meters can be found with all sorts of scales. I seem to recall that the AL29 can be disassembled with a certain amount of effort. Might be easier to inquire about a unit that will allow for scale changes.
One of the reasons I settled in on the Sifam meter was the influence of small budgets dictating poor engineering practices. With some time with a distortion analyzer I found that the rectifier inside the Sifams created the lowest amount of distortion when directly bridging a +4dB balanced audio line. Trim resistors were used for calibration.
Vu and PPM ballistics have their roots in telephony broadcasting and recording industries. They represent different attempts at giving the operator the best instantaneous view of the audio waveform so that SNR can be optimized without distortion of over modulation. The characteristics are defined stringently as voltages across certain loads, how fast the meter movement rises, and how slowly it decays. Simply stated the PPM rises faster but decays slower. I found that the Dorrough meter was a great compromise. Dorrough is a high resolution that simultaneously displays peak and average levels. A solid bar approximates Vu performance while a floating dot indicates the peak level above average. Showing both simultaneously seems to minimize the controversy between operators as how to “properly interpret the levels.
I noticed that my copy of the Audio Cyclopedia has a few articles about output level metering. I have however run out of time and energy today. Given a little prompting, I might be able to find the time to go back and try to relate some of the more salient points. Do not however expect me to divulge the rational behind the Goldilocks audio reference meter standard. This was developed while working for a certain television network in order to facilitate the proper calibration of DBX noise reduction on satellite and microwave links. The details however are highly proprietary and cannot be divulged.
The analogue types used in Europe have a log response of 4dB per step.
As Da5id4Vz points out, they have a very fast attack, but slow decay. This makes them useful (as the name states) at displaying peak levels, but also general volume levels.
The drive circuitry for these is not trivial. Until recently, it has been common practice to use a precision rectifier followed by a log amp, followed by a non-linear amp for meter scaling. About 10 op-amps per channel does the job. More recently, micro-controllers are used with look-up tables, but although they conform to spec, they're not as nice to use.
Re: Random thoughts about meters:
The PASS amplifiers meter showing current:
"The meter on the face plate of the X1000 measures the current drawn by the output stage. At idle, it sits slightly less than half full scale, reflecting the standing bias current of Class A operation. As the amplifier exceeds its class "A" rating the needle on the amplifier will register the increased bias being drawn by the output stage. "
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