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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)
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.
PPM and VU measurements differ a lot from each other.
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.
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:
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):
Last update: May 28th, 2024 | Questions? Suggestions? Mail Me! | Uwe Beis |