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I recently purchased two mystery VU meters from Amazon that I’d like to install in my F6 (I’d link to the ones I bought but they are no longer available). I’d like to drive these meters off the amplifier inputs rather than across the speaker outputs. The meter looks like a Sifam presentor meter, but it doesn’t react to an AC signal w/ a 3.6K resistor in-line, as the Sifam meter claims it should. It seems the meter does not have internal rectification.

So I purchased a JLM audio VU2 stereo buffer PCB kit. I don’t have a schematic for the kit but it includes a dual op-amp, adjustable potentiometers, and diodes to rectify the AC signal to DC. Conveniently, it also has headers to provide 12VDC to the lamps in the meters.

The meter works with the buffer kit (I did have to replace two 3k3 ohm resistors with 680 ohm resistors on the buffer PCB to provide more gain, per Note 3 here in the build instructions) but it appears to respond in a linear, rather than a logarithmic fashion.

The problem of course is that the meters react barely – or not at all – at normal listening levels and peg during loud peaks at medium-to-load listening levels. What I’d like to do is to create a logarithmic circuit in-line between the buffer board and the meter that will compress the range of the meter across a wider range of input signals so that the both quiet and load signals read on the meter. Something similar to what is proposed here, but tailored to the specifications of the meters in question and designed for a (much) lower-level signal rather than the speaker-output signal.

I’m hoping that folks here might be able to let me know whether it is even possible to create a similar circuit w/ diodes for such a significantly lower range of input voltages and run it in-line with the buffer kit. I’m still relatively new to this hobby and currently working my way through the Art of Electronics but I haven’t learned enough yet to solve this problem on my own.

I took a bunch of measurements of the meter being fed a 1KHz signal from my DAC/preamp through the buffer board. I measured the current being fed into the meter, the average voltage across the meter terminals, and the RMS voltage of the input signal into the buffer board. (I took a few other measurements, all of which can be seen in this Google spreadsheet).

It seems the meter has an internal resistance of around 50-60 ohms. It measures ~51 ohms with a multimeter. Based on the measured Vavg values across the terminals correlated with the measured current the impedance at 1KHz is ~59 ohms. It appears to reach full deflection with 1.15mA of current and a +3 reading with exactly 1mA.

I’ve attached a table of measurements and some charts (the scale is a bit weird given the VU measurements, forgive me) as well as an image of the front face of the meter.

I’ve also attached two screenshots from the scope. One shows the meter at VU reading 0 – yellow trace is the signal across the meter terminals after rectification by the buffer board. The blue trace is the input signal to the buffer board. Note the different scales and readings. The second shows the same at reading +3 (nearly full deflection).

Attached Images

	table_of_data.JPG (34.3 KB)
	current_vs_reading.JPG (29.3 KB)
	input_v_vs_reading.JPG (25.7 KB)
	v_terminals_vs_reading.JPG (28.7 KB)
	VU_meter_0_position_1.png (81.6 KB)
	VU_meter_3_position.png (88.9 KB)
	meter_face.JPG (23.4 KB)