Transformer Panel Wiring Complete

After fitting the large output transformers to the panel, the next step was to wire them up. First you need to wire the links on both the primary and secondary windings to set the correct ratio (2:1 in this case). Two links are required on the primary as the two windings are wired in parallel. They are the red and black wires shown on the picture below. The secondary needs only a single link to wire the two windings in series. This was done with yellow wire. The primary sides are connected to unbalanced amplifier outputs so these need a single core screened cable terminated in a two pole Molex KK connector to plug into the backplane PCB. These are the grey cables you can see.The secondaries are balanced and use a twin core screened cable (black cables). Only the hot and cold are connected to the transformer. The screen is not connected at the transformer end but only at the XLR end. The XLRs are not connected yet. They will be connected after the panel has been mounted in the mixer.

The four cable pairs on the right are the the direct outs of the four channel amps. The four on the left are the master bus L/R and AUX1/2 send bus outputs which come from the two Twin Line Amps fitted next to the four channel modules.

I have a simple convention I use to identify the hot and cold wires in the Van Damme twin screened cable I use. The two wires have white and blue insulation so I make the white hot  and the blue cold for obvious reasons.

If Only..

I am not a fan of mechanics but there is the odd occasion when things go just right. Tonight I needed to drill the 16 holes for the VTB2291 transformers mentioned in the last post. I discovered I could set them two inches apart and start 3 inches from one end of the panel. Fortunately the ears of the transformer have slots rather than holes so you don't need to be dead on with the positioning of the holes - got to be a good thing with my mechanical skills. I marked it all out with a ruler and a Sharpie and whacked each intersection with my trusty hole punch. I then placed a transformer at each position to make sure all the punch marks lined up with the slots in the transformer mountings and then drilled sixteen 4mm holes. I de-burred them on both sides with my old three eighths inch drill and attached the transformers using a locking nut on each to make sure they held firm. Job done in less than an hour. If only all mechanics were that straightforward. Here's the result, ready to be wired up:

Quart Into A Pint Pot

The other day I completed wring up the mic and line inputs and the pre-fader unbalanced insert points including the wiring to the four channel faders themselves. The next job was to wire up the direct outs. These are fed from OUT2 of the channel amp, via a VTB2291 transformer to the direct output XLR connectors at the rear. As there are four channels there are four transformers. There are also the two AUX send outputs and the two main bus outputs that need output transformers for a total of eight. Initially, I intended to mount these eight transformers on the bottom of the Rackz enclosure towards the back. The main sub-rack is plenty high enough to fit the transformers in and there are some convenient ventilation slots in the bottom of the enclosure that could double up as fixing holes for the transformers. However, I had not allowed for the mass of wiring I had just put in. Here is a picture of the bottom of the enclosure looking through from the back:

You can clearly see there is no room to fit the transformers on the right hand side because of all the wiring and when the AUX send and returns and the main outs and the 2 track playback all get wired in there will be no room on the left either. Clearly the transformers can no longer fit there. So where to fit them?

Above is a view of the partially completed rear panel. The mic, line and insert connectors are annotated. Above the connector panels is a good sized black panel marked 'X'. This panel can be removed and behind it you can see the top of the rear of the sub-frame:

The bottom of the panel is 80mm from the sub-rack and the top of the panel is 130mm from the sub-rack. This should leave plenty of room to fit the transformers to the rear of this panel. The only downside is that the channel output transformers are now quite near the channel input transformers.Fortunately they are at 90 degrees to each other so hopefully there will be no magnetic feedback! Here is how the transformers will fit onto the panel.

Now all I have to do is drill 16 holes on the steel panel, bolt on the transformers and wire them up. Did I mention I hate mechanics?

Looks a Bit Like a Mixer

It's funny how sometimes you end up doing some things twice. Recently I wired up the PSU to the sub-rack and fitted them both into the 19 inch Rakz box. For the heater wiring I used heavy gauge mains cable. Nice and thick, low voltage drop but not exactly flexible. Thing is, you do need some flexibility in the power wiring so you can move things about for wiring up and testing other parts of the mixer. So you make the power cables somewhat longer than they need to be to provide this flexibility which is just what I did. The trouble is they do not sit very neatly in the bottom of the mixer because the heater wiring is so stiff. What you really need is some high current but flexible wire for the heaters.

This evening I was starting to wire up the XLRs at the back of the mixer and I was looking for the cable to do this. In this search I came a cross some Van Damme loud speaker cable that I had forgotten I had purchased. Examining it showed it had really thick cores, ideal for heater wiring, but the cable as a whole was really flexible. It seems that in a moment of lucidity some time ago I had ordered a few metres of this cable for just this purpose. So that is how I ended up re-wiring the heaters this evening. Here is a picture showing the sub-rack removed from the main chassis and laid on its back on top of the VU meter bridge:

The black and red twisted pair is the HT supply, the blue and white is the phantom power and the thick blue cable is the new heater cable. Next I replaced the sub-rack into the chassis in order to check that the cable curled up neatly and it did.

Lastly I connected to the mains to make sure the supply worked and lastly I fitted all the finished modules and ran a burn in test to make sure the power supply does not get too hot. In fact it just gets luke warm. At last it is beginning to look like a real mixer.

The Last Two Panels

The last two panels for the demo mixer arrived the other day from Frank Röllen. The first one is the 19 inch wide panel that sits at the bottom of the mixer and houses the channel and master rotary faders. I made the scales nice and big so I can use big fader knobs. Here's a picture with the knobs laid in position on top of it:

The second panel is the new design for the REDD EQ channel amplifier. Originally this mixer was going to have two Helios and two Pultec style EQs but the more recently designed REDD EQ has proven very popular, and it sounds really good too, so I decided to have just one Pultec channel and use the REDD EQ instead of the other one. Here is a picture of the front panel attached to the channel amplifier and REDD EQ PCBs:

The REDD EQ is designed so it can be fitted into a 3U high module if desired. This means the 3 band controls are closer together than on the Helios and Pultec EQs. This makes the legend a little cramped and forces you to use quite small diameter knobs. Maybe I am just getting old and blind but I prefer big knobs and large lettering. The control above these three, the one with the blue shaft, is the frequency control for the mid boost/cut. As this EQ uses 100% stepped controls, the response is flat when the controls are zeroed so strictly speaking you do not need an EQ in/out switch. However, this is very useful for comparing the effect of an EQ so I have included one. You will also notice  I have fitted the Smart Pan controls. All I need to do now is to fit the AUX send pots and then I can wire up the channel.

The REDD EQ is based on two EQs used in the famous EMI REDD 47 consoles as used to record many of the Beatles tracks.  The design came about completely by accident.  Here is a quote from a thread I started at groupdiy.com :

"I was recently asked if I could design an EQ that worked like the 'pop' and 'classic' EQ plug ins that used to be used in the REDD47 consoles and predecessors. Using the curves published in 'Recording The Beatles' I came up with a circuit based on a stripped down and modified Helios 69 EQ with a switch to select 'pop' or 'classic'. I was then asked if I could not make it so both the the 'pop' and 'classic EQ curves were available at the same time. In doing this, a very strange thought occurred to me. The Helios treble EQ is virtually identical to the 'classic' EQ curves (the frequency, step size and gain range are identical) as is the bass cut except for the frequency it works at. The bass boost is very nearly the same but tweaked from a shelf to a bell curve. The clincher is that the 'pop' 4.7KHz peaking EQ is a stepped version of one of the Helios 69 mid boost frequencies.

I then realised that Dick Swettenham, who designed the Helios 69 EQ, had previously worked at Abbey Road studios in the service and design departments so he must surely have had a deep understanding of the innards of the REDD EQ.

I am sure you can now see where I am going with this. Is the Helios 69 EQ simply a modified and expanded version of the REDD EQ?

I simulated a cut down version of the Helios 69 EQ (pic attached)  and it is surprisingly easy to get curves very close to those of the REDD EQ.

What do you think/know??"

From this realisation it was a relatively simple step to incorporate the frequencies of the EMI RS127 'brilliance box' into the EQ as additional mid boost/cut frequencies. This was made much easier by a groupdiy colleague Dylan who measured the curves of the Abbey Road plug-in both for boost and cut. The result was a new hardware EQ that incorparates both types of classic Beatles EQ in one package. If you are interested in how the design developed from there you can read it here.

For anyone interested in how the REDD EQ sounds you can listen here. The right channel is a track being played via the REDD EQ and the right channel is me saying what the EQ settings are.

Bus Testing

Using the channel testing technique described in the previous post, I soon identified a problem with the first Helios channel I tested. It was OK when the EQ was bypassed but when it was switched in, the level dropped by about 30dB. Clearly something was not right. A quick inspection of the PCB near where the signal is input to the EQ revealed a simple wiring error. I corrected this and the EQ worked as expected.

So, with the channel testing set up and working and with a working channel to play with, it was time to start testing the buses. To do this, all that was necessary was to wire up the buses to carry the mixed signal to the mix amplifiers, and to connect the Left and Right mix bus to the mix amplifiers via the master volume control. (There was no need to do this for the AUX sends as their master controls are already wired up in the AUX send module). The picture above shows the new test set up. The input, output and channel fader are as before. The added bus wiring and the master (stereo) level control are indicated on the picture above.

Before adding the stereo master level, I checked the AUX send buses. It was a good job I did because I soon discovered I had wired the AUX send module to the L&R master buses instead of the AUX send buses. Changing the connections of two wires in the AUX send module fixed this. I then connected the master level control and tested the stereo bus. The Smart Pan worked exactly as expected and I measured the Pan mid drop as 4.3dB, very close to the calculated value. In fact it all went very smoothly.

As part  of building the test set up, I had to do quite a lot of crimping and I have to say that, with practice, it becomes quite easy. You need to use the right sort of wire and I found that lightly tinning stranded wire was a great help but other than that it is becoming quite straightforward. I might at some point in the future revise the motherboard slightly to change the bus connectors from Molex to IDC for two reasons. First, it is easier to make a cable of an accurate length using IDC and secondly, it would allow me to include a ground wire between each pair of bus wires, thus reducing bus to bus capacitance and hence reducing high frequency cross talk. As yet I have no reason to suspect there is a high frequency cross talk problem but so far I have only been making tests at 1KHz.

Channel Testing

Now we have some channels fully built, it is time to test and calibrate them. The picture above shows the set up used to do this. It shows the main sub-rack from the rear. You can see the three motherboards into which the modules plug, each motherboard taking two modules. Towards the bottom of each motherboard you can see the power bus wiring that links the heater and HT voltages between motherboards which are made using heavy duty terminal blocks. All the signal connections to and from each module are made to Molex connectors soldered to the rear of the motherboards. Connections from these to other parts of the mixer are made using crimp connection Molex plugs (see earlier entry on crimping).

For module testing we first need an input connection. This comes from the XLR connector labelled 'Input' which connects to a 3 way Molex crimp connector that can be plugged into the mic or line input Molex on the motherboard. The output comes from another Molex crimp connector to a VTB2291 output transformer and then to the XLR labelled 'Output'. These two are all you need to test the first amplifier stage in a channel. You plug the input into either the mic or line input and take the output from OUT1 of the module.

In normal operation the OUT1 signal goes to the channel fader and the return from the fader is connected to the Fader input on the motherboard. This input is then connected internally to the input of the EQ section of the channel and the EQ output goes to the input of the second amplifier (IN2) and the channel output comes from OUT2. So for testing the whole module you need the additional connections shown to and from a fader and the VTB2291 is connected to OUT2 instead of OUT1. Thanks to Tony Reeves of  MTR for the bargain 100mm throw faders at just £2 each

I started testing the Helios channel module and the first stage worked fine. However, with the fader connected, the signal drops a lot when the EQ is switched in. Clearly there is a fault on the EQ board that needs to be fixed.