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 selec '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 EMI '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??

Cheers

Ian

Hey Ian,

I only just saw this. Well done. I did similar a few years back after plugging the original REDD eq schematics into spice and tweaking things to match using a similar topology to yours.

The original is a very well designed circuit but it isn't really practicable or necessary to use the original topology of two 400 ohm constant impedance stacked circuits for a 200 ohm with 10dB total loss - rather than the usual (at the time) 10dB loss per band (+20dB total).

For the 'Pop' presence and the brilliance controls, using a Pultec/Helios type circuit, I found it necessary to adjust the Q slightly for each step to match the original topology but I doubt it made much difference to what was heard/ perceived.

Regarding Dick Swettenham and EMI, I'm not sure there is any connection with the circuits. The approach is very different, even if similar results can be obtained by manipulating things. The REDD eqs were based on an earlier EMI desk eq that had 2OdB insertion loss which, in turn, was pretty much based on original research by Alan Blumlein and others.

The Helios EQ was simply a lesser expensive to build passive LCR EQ when constant impedance wasn't needed. It was quite commonly used before everyone adopted Peter Baxendall's feedback type EQ. The Sound Techniques original A Range desks at Trident et al. used a similar EQ to the Helios before switching to Baxendall type for the System 12 etc.

That's all I can think of at the moment.

Cheers.