Parametric with only 2 opamps

[cuelist]

This is an interesting alternative to the common "state variable" implementation of a parametric mid filter. I like it for its simplicity.
Q is variable between approx 0.6 and 4 in this case.

The only drawback I have been able to find is a slight interaction between Q and gain. It can be minimized by making the Q-pot a dual gang and using the second gang to modify RL19.

Could be a nice basis for a "fully discrete" parametric if someone wanted to pursue that idea.

 

[abbey road d enfer]

Typical Wien-bridge parametric. This interaction between boost/cut and Q is the reason why it is generally implemented as a semi-parametric (sweepable). I believe Amek used it a lot.

 

[JohnRoberts]

I've used it as a fixed Q  swept mid EQ.  It is actually a very nice topology for that, low noise gain so very clean.  I never considered it for variable Q.

JR

 

[cuelist]

Typical Wien-bridge parametric.

Typical?  Perhaps if you consider a fixed Q sweep type a "typical wien bridge parametric".

I've only seen this on a couple of Crest consoles (V12 and GTX) and on David Deardens DDA CS3 and the Audient ASP8024.

There may very well be others but it's not exactly a textbook circuit...

 

[skipwave]

Thanks for sharing. 

Are the standard (if you can say that) markings used on the potentiometers to denote taper?

e.g., VR3 is "10KB..." so linear taper

 

[JohnRoberts]

IIRC yes 10kB is linear.

JR

 

[jp-apb]

Thanks for sharing.  

Are the standard (if you can say that) markings used on the potentiometers to denote taper?

e.g., VR3 is "10KB..." so linear taper

As JR already indicated, "B" is Linear, and the "CD" suffix means Center-Detent.
Some other vendors use "CC", Center-Click
This is the Boost/Cut control of the EQ and typically has a detent at the center (0dB) point.
Some other "standard" tapers you may encounter:
A= 15% forward-log (volume controls)
C= 15% reverse-log (for EQ and filter tuning)
D= 10% forward-log
RD= 10% reverse-log

JP

 

[jp-apb]

Typical Wien-bridge parametric.

Typical?  Perhaps if you consider a fixed Q sweep type a "typical wien bridge parametric".

I've only seen this on a couple of Crest consoles (V12 and GTX) ....

There may very well be others but it's not exactly a textbook circuit...

At Crest, I "borrowed" it from a BSS circuit used for their plug-in crossover cards. Works well, but as noted, you can't adjust the Q too far without affecting the B/C.

JP

 

[cuelist]

At Crest, I "borrowed" it from a BSS circuit used for their plug-in crossover cards. Works well, but as noted, you can't adjust the Q too far without affecting the B/C.
JP

So you designed those Crest consoles? 

 

[jp-apb]

Cuelist,
Yes, I was chief eng at Crest from when it started (1992), through some of the Peavey years, until 2004.
At that point, the console team left and we started APB-DynaSonics.
I lurk on some of these forums....

JP

 

[danjpiscina]

Like an API  550, 550a, 550b, right?

 

[JohnRoberts]

For any who haven't figured it out JP is the P in APB.

JR

 

[cuelist]

APB-DynaSonics...  very nice to see a smaller dedicated company continue the fine art of analog engineering!

 

[cuelist]

Yes, I was chief eng at Crest from when it started (1992), through some of the Peavey years, until 2004.

Aha... I got to use Century GTX a bit in the later 1990's - very very nice. I would rate it higher than a Midas XL3 actually. But the "Crest" name never had the pull that Midas had.

 

[PRR]

>> I lurk on some of these forums....
> ...the P in APB

"APB-DynaSonics ... Each company owner plays an active role in the audio community and is intimately involved with user response to its products on a first-hand basis."

Watch out; we can get pretty "intimate" here with gear we love/hate.

Totowa?? If I'd been more wide-awake, I should have thrown a job-app under your door. I once drove a skid-jack in the Tandy (Radio Shack) warehouse. My lover soldered crunchy-caps for Gavin when they were still making TV amplifiers in Bridgewater. Gofering for you guys might have been more fun than what I been doing the last 5 years. However I just bought a house in Maine, have to sell the Jersey house to cover my debt, I'm leaving.

 

[jp-apb]

Well, thanks everyone for the greetings; I'll try to contribute when appropriate.
Yeah, we're a small company making (so far) analog gear for live-audio.
We plan to expand into more markets/technology as time/money permits.

JR and I share a little bit of history, we both worked under the PV banner for awhile 
We've kept in touch and JR contributed to our latest product by designing the meter assembly.
Displays both Ave and Peak simultaneously (a reimplementation of one of his old patents  ;D)
I've seen threads here about meter arrays; JR should chime in there.

JP