Grayhill 71 Series Switch Stops

Grayhill 71 Series Switch Stops

The demo mixer uses a lot of PCB mounted 71 series Grayhill switches. These are all basically 12 position switches but they can be adjusted for fewer positions if required. This is done by inserting one or more pins into small holes in the front of the switch to act as end stops. To stop them falling out, Grayhill provides a self-adhesive cover. With no pins fitted the switch just goes round and round which can be useful in some circumstances.

Fortunately, every time I have used the Grayhill 71 series switches, I always laid out the PCBs starting from position 1 so that if any positions were left over they always started at 12 and worked downwards. This means there is one standard way of adding the stops to the Grayhills that only varies with the number of positions required.

Even if all 12 positions are used, you still need to add one pin if you want to stop the switch continuing from position 12 back to position 1. This is exactly the case with the mic pre gain control where the maximum gain position is 12 and the minimum gain position is 1. The last thing you want is to accidentally switch from minimum to maximum gain. So for 12 positions you need a single pin and for fewer positions you need two of them. The diagram below shows how to locate the right holes into which to fit the pins and lists the various switches used in the mixer and their corresponding pin positions.

I Hate Mechanics

I willingly admit I am not good at mechanics. I can't saw a straight line or reliably drill a hole on the right place, but sometimes mechanics is unavoidable. Below the modules of the mixer is a 2U panel where the channel rotary faders are to be. It really needs to be made of aluminium so it can be engraved but even 3mm thick aluminium is not strong enough such that it it can reliably span 19 inches and survive being leant on - I know I accidentally did it.

A long time ago, when I was building another mixer that was intended to use the same RackZ 19 inch console housing as this one, I had some special wide aluminium extrusions made that were designed to fit right across the RackZ console. I never used them and still have them so I thought I might be able to use them to add strength to the aluminium panel. Unfortunately, when I 'designed' the extrusion, I forgot to account for the 19 inch rack panel mounting built in to the RackZ console so they don't fit!

However, I have a lot of other bits of extrusion accumulated over the years so I had a rummage through them and found a couple that seemed ideal. Each is just shorter than the panel width and is about 6mm thick and 12mm wide. More importantly, they have several 3mm tapped holes along their length, ideal for attaching to a panel. So I stuck them to the panel with Sellotape and used a small drill to make pilot holes in the front panel where the tapped holes were. I removed the extrusions, drilled out the holes in the panel to 4mm and then bolted the extrusion to the panel. I am pleased to say the result is a very strong assembly that, when fitted to the RackZ console, can be leant on without fear of it bending.

I don't like doing mechanics but occasionally you win.

Mix Amps

The mix amps are nearing completion. These amps are responsible for raising the mix bus signals back up to line level. The Twin Line Amp (TLA) was designed specifically with this in mind and two such boards are used to provide the four required bus amps - two for the L&R master and two more for the two AUX buses. The PCBs are shown below:

The TLA board is very similar to the mic preamps. The same three tubes are used in the same topology. The main differences are that in the TLA the mic/line/pad/phase/48V switches have been removed and the stepped gain control has been replaced by a trimmer. The space made available by these changes has allowed an extra input transformer to be fitted. The TLA therefore provides two uncommitted line amplifiers with gain variable from 6dB to 40dB and a pair of uncommitted input transformers. The transformer footprint allows 10K:10K bridging or 1:10 mic input transformers by Cinemag, Jensen or Sowter to be fitted.

For operation as a mix amp no transformer is needed. The AUX send mix amp is the PCB at the bottom of the picture. The bus inputs from the edge connector are simply connected direct to the AUX send master pots (each 10K log) and thence to an amplifier which provides the necessary gain make up to compensate for the passive mix bus loss and any gain 'in hand' required for the master fader plus any loss in the output transformer. With four channels feeding the AUX buses via 47K resistors and with a 10K master AUX pot, the total bus loss is close to 19dB. So with 0dB in hand on the AUX send master control, the mix amp gain would need to be set to 19dB. However, since a balanced output is required, a 2K4:600 transformer is fitted externally which drops the level by 6dB. A total gain of 19 +6 = 25dB is therefore required.

The upper board is the main L&R bus mix amp. On the front panel of this board are the AUX return controls consisting of level and pan controls for each of the two AUX returns. The reason they are on the main L&R bus mix amp is because the AUX returns need to be fed to the L&R mix buses so this is a convenient place to put them. Again a TLA board is used but this time a pair of 10K:10K input transformers is fitted to provide balanced input to the AUX returns. The transformer outputs feed the level and pan controls, the output of which feeds the L&R mix buses via 47K resistors. The two amplifiers on this board act as the master L&R mix amps. As before the mix bus signals end up at the master level control but in this case the master L&R fader is external to the module. As before there are four channels feeding the master L&R buses but in this case the AUX returns also feed it. With 47K bus feed resistors and a 10K master fader, the the total bus loss is just under 21dB and if we need a balanced output then the make up gain will need to be set to about 21 + 6 = 27dB

The AUX send board is complete and ready for testing. The master L&R board is waiting for the pan pots to arrive.

AUX/MON panels and Smart Pan PCBs

The panels for the AUX send and return modules and the monitor panel arrived the other day.

The AUX send is very simple; it just has the two AUX master send controls. The AUX return is a little more complex; as well as a level control for each of the two returns, there is a pan pot for each return. This allows two mono returns or a single stereo return to be used. You can compare the actual panel layouts with my original sketches posted at the beginning of August.

The monitor panel is shown at the bottom of the above picture. The monitor is entirely passive and balanced throughout. The lower hole in the centre is where a 4 pole 3 way switch is fitted that acts as the main monitor select switch, selecting between the stereo bus (centre), a 2 track play back device (right) or an individual signal source (left). The left position connects to an additional 2 pole 6 way switch that can be switched to any one of the four direct outputs or the AUX send outputs. As these are mono signals they are fed to both left and right channels of the monitor. Following the main selection switch is the monitor volume control, the output of which goes to a pair of XLRs at the rear of the mixer for connection to the studio monitor amplifier. In parallel with the monitor volume control is one for the headphones. This feeds the headphones amplifier housed in the meter bridge which in turn feeds a TRS socket at the rear of the mixer.

Smart Pan

I have also been given some consideration to the implementation of the Smart Pan controls. This is an idea I came up with when I was at Neve back in the 70s. The idea is that a pan pot has two switches associated with it. When both switches are up the channel is muted. If the left switch is down, the channel is routed directly to the left bus. The pan control is not connected so there is no crosstalk via the pan control. Similarly, if just the right switch is down, the channel is routed directly to the right channel. However, if both the left and right switches are down, the pan pot is engaged and works as you would expect. The advantage of this circuit is you can route channels directly to buses with no crosstalk or you can pan as normal and you get a mute for free. The problem is it means you have to wire up two double pole double throw switches and a stereo pot, that's a total of 18 connection points, and you have to get it right each time for it to work. Doing this by hand is going to be prone to errors as well as not looking very neat so I thought a PCB solution was called for. I could then include connectors for the cables to the buses and from the channel amp to make assembly easier too. However, the pins on the switches are at right angles to those on the pan pot so a single PCB was not going to be possible. However, the PCB area required was so small it seemed wasteful and expensive to use two of them. I then realised that I could lay out a single PCB as if all the pins were in the same plane, but leave a gap between the pan pot and its switches, and then cut the PCB into two pieces; one to fit on the pan pot and the other to the switches. And that's what I did.

The picture above shows the completed PCB on the left. It is 50mm long and 30mm wide. I got 10-off made at Iteadstudio for less than £10.  As well as all the interconnect between the switches and the pan pot, it also has space for the pan pot slugging resistors that determine the pan pot law and has provision for an input connector and two bus output connectors (Molex). The PCB in the middle is after I have crudely cut it in two with a hack saw and on the right you can see them fitted to the two switches and the pan pot. There are just four connections between the two PCBs for which I intend to use a short ribbon. Although the PCB attached to the switches extends upwards over where the AUX send pots are, it is high enough to clear those pots.

Rear Panel Connections

On the back of most mixers you will find a panel filled with an array of connectors, mostly XLR and TRS jack types along with an occasional D-type, power socket or IEC mains inlet. Many of these require quite large holes which are not easy for the DIY constructor to make simply due to their size. Often it is convenient to make the rear panels compatible with 19 inch racking. As 19 inch panels are only supported at the edges they are generally made of mild steel to stop them bending in the middle. For making large holes in aluminium I have used Q-Max punches. These work very well and make a nice clean hole. Unfortunately they do not work so well with mild steel and it is quite easy to break a Q-Max punch on a steel panel. Making lots of holes is anyway a royal PITA and something I therefore prefer to avoid if at all possible. What you really need is a simple way of building panels with a variety of sizes and types of holes.

Fortunately, manufacturers of rack equipment have also realised this and produced some nice modular systems. The one I prefer, because it is flexible, low cost and British, is made by a company called Monacor:

Monacor International

They do a number of pre-punched 19 inch rack mounting panels which are fine if you happen to want the number and type of holes they have available. However, if you want something more flexible they have a 2U frame onto which you can mount a number of different panels:

prosound-rackpanels

The basic frame has space for 10 segments which can be  filled with pre-punched panels one or two segments wide. Here's a frame with a selection of panels laid on it:

From right to left we have a 2 segment panel punched for Neutrik XLRs into which I have fitted four plugs, then we have the same panel unpopulated, next a 2 segment slotted ventilation panel (very handy for tube mixer designs), then a one segment blank, a one segment panel punched for two TRS jacks and lastly a 2 segment blank which I will use to mount the the IEC mains inlet connector. Monacor do have an IEC pre-punched panel but the mains inlet I am using on this mixer is rather bigger as it includes a fuse, an on/off switch and a mains filter. At least I only have one big hole to make myself. All the other connections needed by the mixer are taken care of by the Monacor pre-punched panels.

The only downside is that there is no means of easily labelling the connectors. Now you can get a different pre-punched panel system from Canford Audio that does include the ability to label the connectors but it is five times more expensive so for the moment I am happy to think of an alternative means of achieving this.

And before anybody asks I have no association with Monacor at all.

Power Supply Build

The custom toroid mains transformer has arrived so I can complete the build of the power supply. This toroid had dual primaries so it can be configured for either 115V or 230V mains and it has secondaries of 250VAC at 150mA for the HT and 50VAC at 100mA for the phantom power supply. It has a GOSS band (Grain Oriented Silicon Steel ) for reducing the radiated magnetic field and an electrostatic screen between the primary and secondary windings for minimum noise. This transformer is made by Terry at Canterbury Windings Ltd. They carry a wide range of standard toroids and will also wind any toroid to your specifications at very reasonable prices.

I also received the remaining components for the two power supply boards. The estimated total HT draw is 90 mA (6 Eurocards at 10mA each plus 30mA for the headphones amp) so to obtain 10V drops across each of the three dropper resistors in the HT supply PCB, each resistor needs to be 110 ohms. See the HT Power Supply Design document for details. The first of the three resistors is a 5 watt type.

I also made some changes to the heater elevation circuit. Normally I use a potential divider consisting 220K and 75K resistors to produce a nominal 75V heater elevation voltage from a 300V HT supply. However, many tube manufacturers specify a maximum resistance value between heater and cathode of rather less than 75K. To date this has never been a problem but I thought this was a good opportunity to update the design. One problem with using smaller value resistors s that the current they draw increases and so they dissipate more heat. As many tube data sheets mention a heater cathode resistance of around 20K, I decided to change the 75K resistor to 22K. With 75 volts across it, this will dissipate 255mW so a 0.5W type is required. The 220K I decided to change to a pair of 33K resistors in series. Each one of these will dissipate nearly 400mW of heat so 1W types were used. The heater elevation voltage is now exactly one quarter of the HT voltage. An unplanned side benefit of the smaller value resistors is that the HT supply does discharge more quickly when turned off.

Lastly I needed to set the SOT resistor in the phantom power supply. There is a nominal 1.25V dropped across the 120 ohm resistor between the output and the ADJ pin of the TL783 regulator which means for a 48V output voltage, the SOT resistor from the ADJ pin to ground needs to be about 4.6K. I made this from a 2.2K and a 2.4K resistor connected in series. The measured output is 49.8V which is due to the tolerances in the resistors and the chip's on board reference. Fortunately the phantom power spec is 44V to 52V so we are comfortably within that. In future I might update the PCB to replace the SOT resistor with a 10 turn pot. However, one of the reasons for using a resistor is that it dissipates 500mW of heat so a 1W type is needed. I used two 0.5W types in series.

Here is a picture of the completed power supply. The white cable is the mains input which will come from a filtered, switched and fused IEC connector mounted at the rear of the mixer.

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.