Aengus graphic EQ (thumbwheel) information sought--any leads?

[soapfoot]

The old Aengus graphic EQ is one of my favorites on bass and kick drum.

I can't find schematics or documentation of any kind anywhere--it doesn't seem like this project has been undertaken yet.

Has anyone reverse-engineered one of these, or does anyone have access to schematics?

 

[pinebox]

I dont think they are on enough peoples radar to be remade really. From what I understand they are very rare and kinda a narrow use. Not many people are aware of them, I myself just heard of these recently, and had them confused with Sphere 900 EQs. Thumbwheel switches are also pretty expensive, so to have a similar setup you could be close to $500 in just switches. Too much for modern 500 series market imo. Dont think anyone would bother cloning them, very small pool of potential buyers.

I do have a client searching for four of these, so if some pop up for him I will get some good pictures of the inside and post them, but I dont expect us to find him some for a little while.

 

[soapfoot]

I’m more interested in making them for myself.

I’d really just like a schematic if anyone has one

 

[NOON]

I'm also curious about the circuit and what makes them so interesting. Those who know them seem to love them.

 

[soapfoot]

I'm pretty sure I remember learning that it's gyrator-based... no inductors or anything like that.

I just know they sound great on kick and bass, are fairly rare, and have gotten way too expensive. I'd love to see if I could make a copy.

 

[dmp]

These look cool, there's video on youtube and it looks like it has steps of -15,-6,-4,-2,0,2,4,6,15
For DIY you could use stepped switches, but probably have to move to a 1U
Similar to other graphic EQs (API 560), using a gyrator to simulate an inductor EQ - but the exact details of the circuit would be needed to know the Q and the important mojo I think
Maybe someone who owns one will reverse engineer it

 

[dmp]

Best picture I could find. The JE-123 is easy to get.
Those metal can opamps are probably long obsolete

 

[soapfoot]

Thanks for that! The part numbers on those metal can ICs seem *just* too small to read from that pic. But now we at least have a transformer part number…

 

[Walrus]

The metal cans are 741's.

 

[Walrus]

 

[dmp]

Wow, nice. Two caps and two resistors for each band around the 741 opamps, like the API 560. Then the switch daughter boards replace the fader with a ladder of 11 resistors(?).
Resistors look like 330R and 62k. Can you read any values off those caps?

 

[Walrus]

Unfortunately I can't read the value of the caps between the switch boards. Going home shortly, but I may disassemble it further tomorrow.

Hidden resistor behind switch is another 180 Ohms.

 

[soapfoot]

This is fantastic!

Can’t thank you enough

 

[dmp]

Gyrator calculator

L = C2*R1*R2
F = 1/(2*pi*sqrt(C1C2R1R2)
Not sure how to calculate Q, but the ratio of the two caps is critical. In the API 560, ratio is 100

 

[dmp]

Unfortunately I can't read the value of the caps between the switch boards. Going home shortly, but I may disassemble it further tomorrow.

Cool - if we can figure out a schematic I'd like to build one too. I have a pair of API 560s and I really love graphic EQs.
The 12 resistors on the switch board look like 3100R-1100R-680R-300R-180R-180R-300R-680R-1100R-3100R with a pair of 1M?
Possibly log scaled moving away from the center? The 560s use a S taper fader, so that would make sense.
If you post a picture of the back of one of those switch boards with all the resistors I'll draw it out

 

[Walrus]

Ok, here is what I managed to do today....

 

[dmp]

Incredible! Extremely generous of you Walrus

 

[dmp]

Component values on the bands all look right for frequencies. Could add a ninth and tenth band at high frequencies quite easily in a DIY version

 

[soapfoot]

This is fantastic—and so generous with your time @Walrus

I’m looking into small rotaries (of the size where eight may fit on a 500 series front panel) and there are a few promising leads.

Looks like we need 12 positions, yes?

Unless somebody makes, would make, or has surplus for the old thumbwheel switches this seems like the best way.

These are unbalanced in, I believe—so maybe adding an input Tx is advisable? Would love some thoughts.

 

[dmp]

Output amp card schematic. Someone check my work, seems strange (edit, fixed a mistake)

edit: deleted, see Slendershap's schematic:

http://www.audiomaintenance.com/downloads/ez404_Schematic.pdf