Quad Eight EQ question(s)

[Gwaggin390]

Hi all,
i have a couple Quad Eight Line/EQ modules and was thinking of altering the EQ points.
I've talked to Dave Geren @Cinemag, who has the original info for the inductors, but i cannot figure out a part of the low end.

The "low Frequency" section has the lowest 2 frequencies and the higher
2 frequencies on separate inductors.
The 2 lowest frequencies are 50hz and 140 hz. From Dave the inductor values are 3.25H and 1.15H.(marked L1 on the schematic)
The question i have, is that there are no capacitors on these, and doing the math in the simple cap/freq/ind calculator i have, i cannot get it to make sense. It seems to get 50hz there would need to be a cap of 2mf with 3.25H or it would need to be a much larger inductor.

I'll add the schematic to the bottom of the post, but there is no there caps in there for those 2 frequencies, or even physically close to be part of the circuit.
All the other cap value/inductor/frequency combinations pan out in the calculator.

That one has me confused and any help or clarification would be great.

thanks

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[peterc]

The first 2 positions look like a shelf type EQ, then the others with the cap are bell type EQ's.

Peter

 

[syn]

What op amp is A2?

 

[peterc]

If I remember correctly, it is the Q8 AM4 in that position.
Peter

 

[syn]

I was hoping that it might be the AM10. I like the sound of it very much
but i've never seen any schematic that is using am10 (apart from the mic
preamp)...Was it used anywhere else at all?

 

[Gwaggin390]

Yes in this case the amp is an AM4.
There were some factory original disctrete based 310 modules, but i've only ever seen them once.
The AM10, I believe, was used all over the Pacifica and Ventura and mixers of that era.
I don't think they were fully modular, so you don't see the modules for sale.

If the lower 2 frequencies are shelving type, does anyone have an online "calculator" or just the straight up equation
to figure out what Inductor Henries will give me X frequency?

 

[peterc]

I simulated these 2 values, see below:

The peak is at 34Hz & -3dB point is 114Hz

Peak is at 60Hz, -3dB is 300Hz

Peter

 

[CJ]

that thing was blowin up my monitor, so i shrunk it.
here is the orig, cropped a tad:

http://vacuumbrain.com/The_Lab/Quad_8/Q8_310eqa_full.jpg

 

[Gwaggin390]

Hi peter, thanks for doing that.

Curious, if it matters, but voltage on the amp would be +/-28v
also assuming the resistances of 7.2K and 1.3K are the 1.3k of full boost and the sum of the resistors in the opposite direction?

So is this really just a "cheater" shelf?
Frequencies so low and Q so wide as to not matter?

 

[peterc]

The +-15v was just for the sim, normal opamp model used.

The resistor balance is the max boost position on the schem.

I think the HP shown in the sim is caused by the input caps? The Q is very wide.... Definitely "vibe" EQ's

Peter

 

[syn]

thanks for the info on Pacifica and Ventura... any one has any schemos of those?

 

[Gwaggin390]

I've never seen them floating around, but Ken at Orphan Audio owns the name(i think) and sells schematics(when/if he returns emails.....)

 

[ioaudio]

[quote author="Gwaggin390"]...sells schematics...[/quote]

http://www.avensonaudio.com/tech/Quadeight/71%20schematicsm1.jpg

 

[Sleeper]

I've got a pair of factory discrete 502 modules and that has a discrete op amp. round plastic can with the op amps all potted up.
I think it's called an am100
Kelly
here's something similar ( 302 I think... the 502's have a mic pre built in.)

 

[Gwaggin390]

that actually is a real Electrodyne
from before Q8 stole all their drawings.

Quad 8 were similar, but i've never seen that potted thing in a Q8

 

[Sleeper]

oh your right electrodyne is what I have.
I also have the schemo for the op amp and board layouts...
After New York Dave's recent dilemma with people posting schematics I'm reluctant to put it up. I'd promised Ken years ago that I would keep this off the web. I'm not sure why, but a promise is a promise.

It's an interesting design though, 2 small round boards, 1 top and one bottom that makes a little cage with the 2 transistors stacked one on top of the other (one to the top board, one to the bottom board to heat couple them.)

They do They sound bitchen though. Almost always my first choice for recording and those eq's... man they are so sweet.
Good luck
Kelly