Saturday, 3 August 2013

AUX Send and Return Panel Layouts and the Crosstalk Issue

It is time to do the front panel layouts for the AUX send and return controls. The AUX return controls are fitted to a 3U panel that is attached to the main L & R bus amplifier. There are two reasons for this. First, the AUX returns are fed to the L & R buses and this is an easy connection to make inside the L & R bus amplifier. The other reason is the L & R bus amplifier is a Twin Line Amp  (TLA) board. This has provision for a pair of input transformers (not needed by the bus amplifier) which can be used for the AUX return input transformers. Two AUX return inputs are provided which could be connected to either two mono or a single stereo source. Since they could be mono it makes sense to include a pan pot on each return. If the two are used with a stereo source then they can simply be panned hard right and left. None of this is a problem from the font panel layout point of view but it could lead to a worsening of crosstalk. The reason for this is that a simple passive input has a source impedance that is determined by the position of its level control. If we use a 10K pot for the AUX return level control then the worst case source impedance it presents to the pan pot circuit is 10/4 K = 2.5K. The reason this can affect crosstalk is shown in the sketch below.

The circuit at the top left shows an AUX return channel. The input goes through a 10K:10K transformer and is fed to the level pot. The wiper of the level pot feeds both halves of the pan pot which in turn feed the left and right buses via 100K resistors. Each pan pot is 20K and is slugged with a 10K resistor. The circuit below it shows its equivalent circuit for crosstalk calculation purposes. The transformer and pot are replaced by a 2.5K resistor to ground. Each pan pot is replaced by a 5K/10K pot divider representing the pan pot at the mid position.  Assume we have 0dBu on the bus at the right of the 100K resistor. By the time this reaches the pan pot is is reduced by the pan pot 10K to ground by about 20dB. This signal is then reduced by the pan pot 5K acting with the 2.5K of the fader in parallel with the other pan pot leading to a further loss of about 10dB. The the left hand pan pot itself drops the level by about 3dB resulting in a total loss of 33dB.

To get the crosstak we have to add the bus loss. We have four channels in this mixer so there are three more 100K resistors between the bus and ground plus the 10K master pot for the bus. The total parallel resistance of all these is close to 7.7K so the bus loss is 107.7/7.7 which is about 23dB. Hence the crosstalk is 23dB + 33dB = 56dB. What does this mean? If we send a 0dBu signal from a channel and pan it fully right, and we have the AUX return set so the fader presents exactly 2.5K source impedance and the AUX pan is centred, then that 0dBu signal will appear on the left bus at -56dBu. Note that this is the absolute worst case. If the AUX return pot is at any other position the crosstalk will be better and it will also be better an any other pan position.

In 99.99% of cases this will not cause a problem as the stereo separation between channels in a mix is rarely better than about 30dB. The sole reason for the increased crosstalk is the source impedance of the AUX return level pot. In normal channels the pan pot is fed from the output of a TLA type amplifier which has an output impedance of about 150 ohms. This reduces the crosstalk by a further 24dB to about -80dB. An alternative therefore is to feed the AUX returns through a TLA amplifier which would give us a very low source impedance and we could also have some gain in hand on the return level pot, but this does increase cost. Note also that crosstalk improves if you have more channels because the bus loss increases. If there were 16 channels instead of four, the crosstalk would improve by about 5dB. You could achieve this artificially by slugging the bus to increase the bus loss but we have already partially done this with the 10K master bus pot.

The circuit at the top right shows the equivalent cicuit if the pan pots are 50K instead of 20K. I expected this to give worse crosstalk but it doesn't. The loss from the bus to the pan pot is less (14dB instead of 20dB) because the pan pot has a higher value resistance but the loss from the pan pot to the fader is higher (16dB instead of 10dB) for exactly the same reason. The result is the loss from the bus to the other pan pot is exactly the same which is a very interesting result. It appears that cross talk is independent of the value of the pan pot.

An interesting aside is that when I was at Neve in the 70s, the nominal bus send level was about -8dBu. The reason for this was they wanted to maintain a 26dB headroom and the 24volt rail powering the class A amplifiers meant their maximum output was about +20dBu. At that time, AUX returns were fed in through a 10K:600 transformer which dropped the level by 12dB from a nominal +4dBu to the magic -8dBu. The other benefit is that you can now use a 600 ohm level pot and the worst case source impedance of a 600 ohm pot is 600/4 or 150 ohms. What a coincidence!!

So, having gone through all that I decided pan pots are OK of the AUX returns. The sketch below shows the probable layout of the controls for both AUX send and return. The AUX send front panel is attached to another TLA which acts as the bus amplifier for the AUX sends.

I have kept the send and return level pots at the same height. There is plenty of room on the panels for these controls. All you need to do is allow 10mm top and bottom for the support rails of the sub-rack and 10mm on the left for the fixing of the PCB to the front panel. For the sake of symmetry I also allow 10mm on the right hand side. I plan to use 20mm diameter knobs for the level pots and 15mm diameter ones for the pan pots. The next step is to input these to front panel designer so I can get them made.

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