Audio Level Meter
Build this versatile LED PPM/VU meter project.
The opinion of the recording world remains firmly divided as to whether the Peak Programme Meter (PPM) or the Volume Unit (VU) meter provides the most useful level information. PPMs are usually only to be found in professional equipment mainly due to their high cost. A good proportion of the PPMs cost is in the meter movement itself because of its exacting ballistic requirements. This is where an LED-based meter really scores, since it has no inertia, and can thus exhibit truly instantaneous response.
A feature which PPM users miss is the visually 'immediate' response of the VU meter on both rising and falling signal amplitudes. PPMs have a heavily slugged decay characteristic, intended to hold overload transients long enough for them to be visible.
Recognising that neither one system or the other will ever be universally accepted, some equipment manufacturers provide a switch to select either VU or PPM response. The HSR Audio Level Meter not only does this, but also allows both responses to be displayed simultaneously, brightness modulation being used to distinguish between them. To make the unit even more useful, a peak hold switch has been included, which is invaluable for initial level setting, and for keeping tabs on an unwatched level.
Some LED level meters lack resolution, but the project described here uses 20 LEDs to achieve a resolution of just 1.5dB. Close fitting rectangular LEDs are used to yield an attractive unbroken bar of light which expands and contracts in a thermometer-like fashion. The project is described as a PCB assembly which can be installed horizontally or vertically either as a free-standing cased unit, or in some existing piece of equipment such as a mixer or tape recorder. The assembly is extra compact and slim so that several units can be installed close together.
Figure 1 shows that the circuit is based around the LM3915 logarithmic 10-segment LED driver, IC4. This is in fact operating in its dot mode, but driving the LEDs in series to achieve bar operation. This configuration results in much lower supply current. IC4 is able to drive 20 LEDs by multiplexing 2 sets of 10 using TR1 and TR2. Flip-flop IC3, which is toggled by the astable multivibrator formed by IC1d, drives not only the multiplexing transistors TR1 and TR2, but also part of the analogue switch, IC2. This shifts IC4's reference voltage by 1.5dB in synchronism with the multiplexing transistors. The result of this multiplexing is that an additional step is placed half way between each of IC4's natural 3dB steps.
The audio signal to be measured is presented via C1 to the buffer/amplifier, IC1a. RV1 allows the 0dB calibration of the unit to be varied for input levels in the range -18 to +15dBm. IC1b & c form a precision full-wave rectifier and peak detector which simultaneously charges C2 and C3 via D5, R9 and D6, R10. R9 and R10 provide an attack time sufficient to eliminate very short transient peaks that would not be audible. The VU signal on C2 has a fairly short decay time constant governed by R8, whereas the PPM signal on C3 decays much more slowly via R11. Opening the Peak Hold switch, SW1 disconnects the discharge circuit so that only leakage currents cause any discharge from the PPM voltage on C3.
Further multiplexing of the remaining analogue switches in IC2 chop between the VU and PPM signals under control of the astable multivibrator IC1d. The duty cycle of this 400Hz clock is varied by RV2 so that the visual contrast between the VU and PPM readings can be adjusted. Operating the mode select switch SW2 either way overrides the 400Hz clock, forcing the analogue switches to select either VU or PPM continuously.
It may seem a little strange that the LEDs below 0dB are red while the 'overload' LEDs are yellow. This is because red LEDs have by far the lowest forward voltage drop of all LED colours. If any other colour had been used for D19-26, then a higher supply voltage would have been needed for IC4, which the device dissipation constraints prevent. In use though, the colours seem quite natural; it is the change in colour at the 0dB point that is more important.
To built this project, you will need a soldering iron with a fairly fine, clean tip, used with a high quality fine solder, such as 22 SWG. Construction begins with inserting the 7 Veropins from the component side of the PCB, so that the tails protrude from the track side, leaving the component side clear.
Next insert the 2 wire links and solder these in place. Refer to the component overlay, Figure 2 for component positioning. Insert and solder D1-24 and D45, noting that D1-6 are mounted vertically with the cathode band uppermost. Do not fit the LEDs at this stage.
The resistors are best inserted and soldered a few at a time to prevent crowding. Fit and solder the 2 transistors, and the IC sockets next but not the ICs. Now continue with the 2 presets and the capacitors, taking care with the polarity of the electrolytics. Also make sure that the lead of C2 does not touch the nearby veropin head.
Now for the crucial bit! Fitting the LEDs to the PCB is not really difficult, as long as you make the right preparations, and don't rush it. Unevenly fitted LEDs could ruin the good looks of the unit. First though, you must determine how high the LEDs must sit from the PCB, which is obviously dependent on the mounting method you decide on.
Insert all the LEDs in the PCB, noting that the anodes (longest leads) face towards IC1. The four yellow LEDs are at the IC4 end of the PCB. Now suspend the PCB, LEDs downwards, onto a flat surface using blocks (or plasticine) to set the required height. The LEDs can now be arranged evenly using a straight edge to keep them aligned. A strip of double-sided tape underneath will help to keep them in place while you carefully solder each one.
Having completed the PCB assembly it must be checked carefully, first for correct component orientation, and second on the track side for any dry joints or bridged tracks. You must use an eyeglass, or at least a magnifying glass, since some of the tracks are quite fine and close together.
If all is well, then the ICs can be fitted into their sockets, taking care with orientation. Anti-static precautions must be taken with the CMOS type IC2 and 3, which should be left in their anti-static packing until the last moment.
One of three methods can be used to present the LEDs to the outside world:
1. A slot can be cut and carefully filed in the front panel itself so that the faces of the LEDs engage and become flush with the front of the panel. This method requires the constructor to be quite proficient with a saw and file, especially if the meter is to be installed in existing equipment.
2. Make a slotted escutcheon, preferably from an easy to work plastic, which then attaches to the front of the panel, which itself has a less carefully cut hole covered by the escutcheon.
3. The method preferred by the author, and used for the prototype shown in the photograph, is to make a masked escutcheon from an unslotted piece of special filter material. Although a neutral density filter, or even clear perspex can be used, by far the best results are obtained using uncoloured circularly polarised sheet, if you can get it. It has the unique ability to allow light to escape from the LEDs, but it does not allow ambient light to reflect back from anything behind the sheet.
This results in a quite black screen which 'magically' lights up when the LEDs are energised. You must make sure that you get the sheet around the right way, otherwise it won't work! RS Components stock no. 586-677 should do the job nicely. Although RS do not sell to the general public, you should find that an obliging local TV repair shop will make the order on your behalf.
After carefully determining the LED positions, mask the active area with tape and spray the rear surface with black paint to form a light mask with a window (after removing the tape), which is slightly larger than the bar of light from the LEDs. Letraset will then make a nice professional job of the scale, which ranges from infinity below the bottom segment, then -21.5dB at the first transition to +6dB above the top segment.
Refer to Figure 3 for the wiring details. If the peak hold facility is not needed, then SW1 can be omitted, and the 2 relevant pins on the PCB linked together. Similarly, SW2 can be omitted, leaving its PCB pin open for 'simultaneous', tied to +15V for 'VU', or to 0V for 'PPM' mode. The +/-15V power supply must be regulated, and capable of providing +/-45mA. This could possibly be the supply of the existing equipment if it has sufficient spare capacity.
If the meter is to be a separate stand-alone unit, then the PCB assembly slots very nicely into a plastic verobox no. 75-1237J, as used for the prototype. For a stereo unit, 2 PCB assemblies will stack in a verobox type 75-1239K. Both of these cases match well with the series of signal processors published in E&MM and HSR, such as DNF, Comp-lim, Sweep EQ and Twinpak, the latter making an ideal power supply for this project.
Calibration is best done with the mode switch set to VU. With both presets initially set to mid position, apply a steady medium frequency tone at the required 0dB level, and adjust RV1 so that all the red LEDs light, just below the point where the first yellow LED lights.
Now with the simultaneous mode selected connect a source of 'punchy' music to the input. The PPM reading should be seen to drag behind the VU after signal peaks have passed. The intensity of the background PPM reading is adjusted by means of RV2. It will probably be found best to keep the PPM reading fairly dim so that a reasonable contrast ratio is maintained.
The brightness returns to maximum when either the VU or PPM modes are selected. The peak hold function only acts on the PPM indication, although it will invisibly store the peak amplitude when in the VU mode. Thus, VU could normally be used, but switching to PPM displays the stored maximum peak amplitude.
Another use for this function is in the simultaneous mode, so that VU is used for instantaneous level, and the background PPM continuously displays the maximum attained peak level. The prototype exhibited a peak hold decay rate of less than 1.5dB in 20 seconds when the simultaneous mode is selected, due to the stray capacitances being switched to and fro. The peak hold function is very useful for initial level setting as it eliminates the need for constantly watching the reading.
Apart from being extremely attractive, the HSR Audio Level Meter is very functional, retaining or bettering the accuracy you would expect from an analogue instrument.
Feature by Paul Williams
Previous article in this issue:
Next article in this issue:
mu:zines is the result of thousands of hours of effort, and will require many thousands more going forward to reach our goals of getting all this content online.
If you value this resource, you can support this project - it really helps!