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The Design of the upcoming ALM77

- A PPM and VU-Meter with multi-colour LED bars for Revox B77 -

Confidential - For your eyes only!

(Photomontage - not the final layout)

Content

Preface

Status at the end of May: Finally the ALM77 is available as a complete, ready-to-install replacement module by Heroms.com.

This article here used to be a preview on the ALM77 before it became finally available, based on the manufacturing sample that arrived up to then:

I have developed this ALM77 (Analog Level Meter for Revox B77) from the AMBM, which, unlike the ALM77, is only available as a kit. It is functionally identical, but many modifications have been made to meet the requirements for use in the B77. The AMBM is again based on the ALM, a development I made a long time ago for our former company SSB Audio.

Properties

• The ALM77 is specially designed to replace the B77 moving coil meters in the B77 VU meter frame.
• A DIP switch with ribbon cable is included for setting the 6 options. It can be easily concealed and operated from under the flap of the B77 front panel, on the left of the variable speed adjuster (if available), fixed with 2-sided adhesive tape. The 6 switchable display options are (opened switch mentioned first):
  
  1. PPM or VU measurements
  2. Bar display or dot display
  3. Bar display: With or without peak hold or
     Dot display: With yellow or green-yellow-red dot
  4. Background LEDs on or off
  5. Color pattern green-yellow-red or green-red
  6. Display intensity high or low
  Detailed information about the options can be found at the AMBM article.
  
• There is a clear indication for the VU meter or PPM operating modes.
• ALM77 is supplied as an out-of-the-box and ready-to-connect module: Ready-made cables, nothing to solder, just screws and plugging the cables (2 wires: +21 V / Ground, polarity protected, and 3 wires: Left In, Ground Reference, Right In).
• It comes with a front panel (resp. scale) as a layered structure made of scratch-resistant clear glass and contrast filter screen, with the scale printed in between the layers(!).
• The sensitivity is calibrated to the nominal 1.55 V full modulation level of the B77 with measuring device accuracy (±0.2 dB). In other words, unlike with the original B77 moving-coil meters, no adjustment is necessary or even possible with the ALM77.
  → When the leftmost red LED on the scale lights up, the signal is above 1,55 V, and when it does not light up, it is below 1,55 V.
• A special power filter, which is not available in other LED bar solutions, ensures that very little interference from the strongly and rapidly changing current consumption of the multiplexed LED bars has a reverse effect on the B77's internal supply voltage. These rapid changes in current consumption might have a negative effect on the audio signal. With the filter, only slow (low-frequency) and harmless changes remain, if at all, see below.

A Note on PPM and VU measurements

PPM and VU measurements differ a lot from each other.

• VU (Volume Unit) measurements were very early introduced when hardly anything else but electromechanical instruments were available to indicate measured values. Thus somebody took a simple rectifier and an available moving coil instrument and called it a "Volume Unit" meter. (Ok, that's simplified.) In order to make comparable VU measurements, the dynamic characteristics of that instrument was important, of course. Thus all other VU meters ought to have the same dynamic characteristics. And so the VU meter was standardized in ANSI C 16.5 / IEC 268-17:
  
  
• The attack time, i.e., the time it takes for the display to reach 90% of the actual value, is 300 ms.
• The overshoot is within 1 to 1.5%.
• The decay time is the same as the rise time, 300 ms.

The VU meter response time is high so that short peaks are widely invisible. This is how the AMBM in VU mode behaves, too. But in contrast to original VU meters, which have a linear scale from 0 to +3 VU (+3 VU is 3 dB above 0 VU), the ALM77 uses the same logarithmic scale as for the PPM scale. Anyway, the measured values are identical.


• PPMs (Peak Program Meter) were developed and standardized much later for significantly higher professional and studio requirements. These standards are not exactly identical worldwide. Here I adopted the IEC 60268-10:
  
  
• The attack time, i.e., the time it takes for the display of a sine wave to reach 90% of the actual value, is 10 ms.
• The decay time is 20 dB per 1.5 s.
  

PPMs respond much faster to peaks so that any overdrive is much better visible. As compensation for the disadvantage of VU meters, the VU meters are usually adjusted with a so-called "lead". This means that a continuous tone of a few dB less than the actual full level will be indicated as full level. How large this lead should be is naturally more of a "gut feeling" decision.

In contrast to that the scale of the ALM77 shows 0.0 dB (start of the red range) in both PPM mode and VU mode when a continuous tone is measured at 1,55 V. In VU mode with short peaks, a display of 0.0 dB would mean overmodulation. That's why I recommend professional measurements with PPMs. Otherwise, for VU, you could regard the maximum level for music recordings to be the beginning of the yellow range, i.e. at -5 dB, so that clipping at peak values is less likely.

By the way: In the B77, the VU meters are set so that 0 dB is displayed at a signal voltage of 0.775 V. This is 6 dB less than the 1,55 V full modulation at which the clipping LEDs lights up. The lead of the the B77 VU meters is therefore 6 dB.

One more note: Even if the level meters of almost(?) all consumer electronics devices indicate "VU", they are by no means measuring devices that comply with the VU standard. I don't know how near Revox comes close to the standard. At least in the B77 the VU peak display problem has been taken into account a little with the peak LEDs.

The Supply Current Ripple

There ist an effect that can easily lead to problems when designing or using a level meter with multiplexed LED bars is the rapidly changing current consumption caused by multiplexing. This can lead to fluctuations in the operating voltage, which in turn can become measurable or even audible in the audio signal. In the ALM77, a special and rather unusual measure has been taken to counter this effect. An active part of the circuit, which behaves like a filter choke, smoothes out the fluctuations in power consumption so that they are not only reduced, but in particular their higher-frequency (and more audible) components are also significantly lower.

I have documented the effect here using an oscilloscope. Supplied by the laboratory power supply unit, the current consumption curve, measured across a 1 Ω resistor in series, is shown. The scaling is therefore 10 mA per scale division:

• Upper: Direct connection, except with a 470 µF buffer electrolytic capacitor without further measures: The current ripple is 50 mA_pp.
• Middle: With an additional resistor of 20 Ω in the supply line: the current ripple is reduced to 8 mA_pp, but still contains the high-frequency harmonics.
• Bottom: With the additional circuit. The current ripple is reduced to approx. 2 mA_pp, the high-frequency harmonics are widely eliminated.

Photos and Simulations

Up to the end of March 2024, there was only the handmade lab sample of the ALM77, but no front panel resp. scale. Anyway, I would like to show you what I wrote up to then:

The following picture shows the ALM77 lab sample built into a B77 VU meter frame:

Here is a photo of a working ALM77:

And this shows a simulation of the ALM77 with the front panel/scale we designed, reminiscent of the LM128 front panel:

The installation of the switch for the display options located on the B77 front panel under the front flap might look like this photomontage:

I don't have a B77 myself and this photo (not mine) shows the lab sample with a kind of mask instead of the scale, covering the LED bars:

Finally another photomontage of how the ALM77 would look installed in the B77 (click to enlarge):