Sunday, 9 February 2014

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 :

"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.

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