842 Meter Bridge
Parts Cost Guide £28
Optional Meter Bridge for the ElectroMix 842
In most studio set-ups tape recorders are mounted remote from the mixer, so metering on the mixer is essential. When correctly calibrated these act as Record level meters for the tape machine. The unit described here was designed as an option for the Electro-Mix 842 with five VU type meters; four to monitor the group outputs and one which is switchable between Aux 1 and 2. The meters are mounted on a 19" panel which can be rack mounted with the mixer. High input impedance and adjustable sensitivity should make the circuit usable in other audio set-ups.
The meter drive circuitry is shown in Figure 1. Since the five meter drive circuits are identical only one is shown, along with the calibration circuitry.
IC1 is a non-inverting amplifier with a gain set by
(R4 + R5 + RV1) / (R5 + RV1)
With the component values shown and R1 in circuit, 0VU reading corresponds to 775mV rms (0dBm). If R1 is replaced with a link the 0VU reading is for 330mV rms (-10dBV). C2 blocks any DC at the output of IC1 which would offset the meter. The signal is rectified by D1 and D2, providing the meter with positive going signals. Reading positive signals only is common amongst most equipment and appears to be adequate. The meters used do not have a very linear movement but this characteristic is fairly constant for each meter. R6 and R7 help to linearise the meter to give a useful and reasonably accurate range on the VU scale.
Provision is made on the PCB for a simple calibration circuit. This relies on the forward voltage drop across a particular type of diode being the same - in this case the 1N4148.
In the calibration circuit D3-6 clip the incoming 15V AC to approximately 1.2V peak to peak. R8 limits the current through the diodes but its value ensures the diodes are fully biased on. The potential divider R9 and R10 gives the required output. For -10dBV, R9 is 18k + 2k7 and 3k9 for 0dBm, R10 is 10k in both cases. Note the output of the calibrate circuit is not exactly -10dB or 0dB but slightly higher level to compensate for the lower sensitivity of the meters at 50Hz.
Construction is very straightforward as each meter drive circuit is identical (see overlay in Figure 2). Check if R1 is to be used or linked out (earlier text - signal levels). If the calibrate circuit is not required leave out R8-10 and D3-6, C4 and C5 should be included to provide supply decoupling. The meter polarity is marked just above the solder tags and should be noted when connecting to the PCB. Single unscreened wire is suitable for this.
Screened wire is used to connect the meter PCB to the output sockets of Groups 1-4 and Aux 1-2 on the mixer. Connect both screen and conductor (the screens for the Aux meter can be commoned on a spare tag of S1) at the meter PCB, but only the conductor at the mixer end. This is to prevent earth loops which could cause hum. A return 0V path is made via the power supply.
Cut back the screen and insulate with sleeving before connecting to the mixer sockets. Note the 15V AC to the calibrate circuit is temporary and is disconnected after calibration to avoid possible hum pick-up.
If the meter bridge is likely to be separated from the mixer at a later date, it can be connected via a 9-pin multiway connector, as shown in the photo.
The meters can be fixed to the panel using Super Glue or an epoxy type adhesive. It is best to first position the meter and then run a small amount of glue around it. This ensures that you don't get glue on the meter face.
Dimensions for the meter panel are given in Figure 3 although a panel is available through E&MM. The scale of the meter is semi-transparent and therefore ideal for back lighting. Alternatively, an LES type bulb can be fixed to the top edge of the meter, which is clear. It is advisable to power these bulbs from a DC supply to minimise the chances of hum pick-up. Always run bulbs at less than their rated voltage as they don't need to be very bright and they will last a lot longer.
S1 is mounted on a bracket which needs to be spaced off the panel. This can be done with long bolts using nuts for spacers.
Calibration is done before connecting the unit to the mixer. If using the on board calibrate circuit, single unscreened wire is sufficient to feed each meter drive circuit in turn. When using the power supply described in November '82 issue the 15V AC can be taken off the power supply PCB. If an external transformer is to be used one side of the secondary is connected to 0V and the other to 15V AC input on the meter PCB.
Alternatively an oscilloscope and oscillator (set at 1kHz) can be used, in which case a signal level of 2.2V peak to peak on the scope will give 0VU at 0dBm and 880mV peak to peak for -10dBV (see table in November '82 for other signal levels).
If the mixer/meter unit is to be used with a tape recorder of unknown input sensitivity set up as follows: Firstly, build the calibrate circuit with R9=18k + 2k7 then feed this signal to a line input on the recorder and using the Input/Record level control adjust to get an 0Vu reading on its meter. Now feed the calibrate signal to a line input on the mixer and with the tape recorder connected to the appropriate Group output adjust Gain control and Faders on the mixer to achieve an 0Vu reading on the tape recorder's meter. Trim the meter unit to give a similar reading, repeat for each meter and track of tape machine. Remember not to adjust the level controls on the tape machine after setting up.
An aluminium panel, punched, drilled and finished in matt black is available from Electronics and Music Maker, (Contact Details). The price is £7.95 including VAT and P&P.
The Meter Bridge PCB is also available at £2.75 including VAT and P&P.
Please order as: Meter Bridge Panel and Meter Bridge PCB.
A set of 5 meters for the project is available from Ambit International, (Contact Details). These are at a special price of £12.10 including VAT and P&P. Order as: Meter Set for E&MM Meter Bridge project (5 off 37-09600).
Please allow 28 days for delivery.
The component overlay for the power supply PCB given in the November '82 issue — P71, Figure 6 was incorrect, the track layout having been shown from the wrong side.
A corrected component overlay is shown above. This does not affect assembly as the component overlay has not changed.