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.