Modular Parametric Eq

I have been working on the prototype of a 500 series form factor parametric equalizer. The goal is to have a discrete signal chain, at least for the main signal path. Packaging in this form is a real problem, it appears, since making a single board solution to match the 500 spec creates a problem with the layout of a usable front panel.

The unit has four bands with a peak/shelf switch for the HF and LF bands and Q controls on the LF, ML, and MH bands.

Here is a picture of the DIY main board unstuffed and with most of the major parts in place.

You might take a look at the Scheme, if you can find one, for the Aphex EQF 1 or EQF 2. Might point you in a direction for some space saving options. Its got 2 boards but it might be helpful. Looks great!

Thanks for the suggestion. The Aphex units used a switch to select the Q. I wanted to try to keep the Q variable.

I have found that being able to vary the filter width is helpful for difficult situations like hum removal where you don't want to take out too many of the surrounding frequencies. But I also like having broad peaks for subtly tweaking an instrument in a mix.

There are a couple of module eqs out there like the ones from S&M Audio and Speck Electronics that move the left hand board right up against the left edge of the module and then use a daughter board that has the card edge contacts to mate with the connector in the chassis. This give a lot more freedom for the front panel layout.

Hey John can you check your pm?
Soryy to hijack your thread!!

Thanls

Hey John can you check your pm?
Soryy to hijack your thread!!

Thanls

Some views with the metalwork added. Clearly, this is only going to work as a prototype unless you don't mind making adjustments with a pair of long nose pliers.

Nice work!

What opamps belong in all those DIP sockets? And is the 2520 just providing some output oomph driving that Cinemag CMOQ?

The opamp sockets are set up for dual 8pin dips like an OPA2604. I have a couple of hunderd LF353s which have about one fourth the GBW of the OPA2604 which I will be using for testing. Both of these amps are FET input devices.

The first DOA is used as a differential input amp for the balanced line. The second DOA is the summing amp. This type of equalizer, based on the Sontec/McNeal/Massenburg/Forsell/(anyone else I may have missed) topology requires two signal inversions to produce the boost and cut bus signals. And I am growing fond of the 2520/CM02S combination for the sound.

I guess the original Sontec eq used discrete opamps everywhere but I don't think that doing it all with discretes will fit in this format unless I am able to get the debigulator ray going again.

Nice work!!

[quote author="burdij"]Some views with the metalwork added. Clearly, this is only going to work as a prototype unless you don't mind making adjustments with a pair of long nose pliers. [/quote]

Why not just build a extender card for tweaking
and then fully inserted for work aday?

Testing, testing . . .





The two fully parametric midband channels are working. The lower ranges from about 70Hz to 8KHz. The upper ranges from about 400Hz up to 20KHz. The main audio path, consisting of the two discrete op amps and the transformer, is passing 60KHz, 3 db down with very good waveshape. I haven't done any real distortion measurements yet. I am testing with LF353 op amps in the filters.

Me want...

Finally got to try the eq in a rack. Here it is with a couple of mike pres:



Very clean sounding. The low and high band need some gain adjustments to match the middle channels. Noise seems very low. I have just tried it on speech so far.

:shock:
NICE JOB MAN!

Looking real good guy. Fine project

What are those pots and where did you get them?

Thanks for your comments. I am doing a redesign of the boards to better fit the panel although the solution will need three seperate boards not including the two DOAs.

I am getting my prototype parts from a RAMSA mixer I am de-constructing. I am trying to locate suppliers for the pots, though. One solution is to use three seperate pots, one being the dual 100K anti-log frequency control (cheaper approach) but I really like the dual concentric frequency and gain control (less panel space).

[quote author="burdij"] am trying to locate suppliers for the pots, though. One solution is to use three seperate pots, one being the dual 100K anti-log frequency control (cheaper approach) but I really like the dual concentric frequency and gain control (less panel space).[/quote]
Colin (forum member Slenderchap) of Audio Maintenance Ltd. has a few dual concentric pots in his webshop. Of particular interest for parametric EQs are the 47k rev log / 47k rev log / 22k lin with center indent models by OMEG. They're not cheap, but they're perfect for tight designs. Colin may also be able to supply matching knobs.

JDB.
[I've some of Colin's pots sitting in a box here, waiting for me to get around to finishing my own design]

Thanks for the link. I had forgotten about those. They are pricey but not nearly as bad as a custom run of pots, with minimum orders, etc. Need to tweek the cap values somewhat for the frequency range.

I just ordered some.
Nice useful webshop.

Very nice job Burdij!!!!
Especially the dual concentric knob design and the "home made" (no??) 500 racks!
:thumb: :thumb: :thumb: