Parametric Eq Theory

[Animatic]

John, thank you that's EXACTLY the reason I joined.

Post something I CAN find,
and get some info even MORE enlightening.
PRR was right this is da place.

I have read a few of your posts and they are very informative.

I will have to wake up and hit the coffee to follow you,
since midnight ain't my best circuit analysis time....

I am just trying to transistion for tube design/copy to SS,
and then combine the two, and EQ is my biggest desires
for this hybrid plus gain control of tubes remotely,
and level monitoring remotely.

If I could pop EQ this between 2 12AX7's I would be a happy camper.
:grin:

 

[opacheco]

[quote author="JohnRoberts"]Thats a good start but I can't fully agree with his statement "To make gain and Q individually adjustable requires the use of two-opamp summing stage (the first opamp)."

You can vary the Q independently of the BP gain with minor tweaks to that basic 3 opamp SVF. Instead of using R1 for signal input, apply the audio input to R2 (ground old R1 input). Now the BP gain is roughly unity inverting (I'd make R2 and R3 the same for exactly unity gain.)

Adding a resistance to ground from the junction of R2 and R3 will shift the Q to narrower bandwidth, without changing the BP gain. Alternately applying the resistance to ground at junction of R1 and R4 will shift the Q broader, also without changing BP gain.

To vary the center frequency without changing the gain or Q, vary R6 and R7 with a dual pot. Any pot mistracking can cause a slight Q change if the poles shift relative to each other. Note: intentionally offsetting the two poles is another way to vary the Q of that simple circuit without changing the gain.

JR

[/quote]

JohnRoberts thats is great explenation!!!....and I would like to know what happend with the Group Delay for each band in this kind of parametric filter????....What kind of pole/ceros location funtion it is?????.....Bessel, Butherworth, LR, Elliptical......?????

Thanks a lot,
Opacheco

 

[Animatic]

I looked this with coffee and it seems more logical.
Gotta bread board one.

Can you add another BP in the middle without issues?
Always like top and bottom mids control.
There just seems to be more clutter cumulatively in those bands
in a mix , and also live on stage for that matter, IMHO.

John, I just looked at your Resotune, pretty cool box.

As a studio engineer I found myself becoming a drummer
after WAY too many drummers couldn't improve their drum tone
after listening to 1st test take and groaning big time.
I heard them setting up and knew in advance
that I would likely be behind the kit for a bit...

They think it's good until they hear it when NOT playing.. OOPS!
Eventually I picked up quite a bit of drum licks.
But also years of time tuning heads....

Your box looks like a great studio tool.
It's now on my list.

 

[JohnRoberts]

The poles are simply R6xC1 and R7xC2 however the multiple feedback paths alter the way they combine.

Logically each integrator section forces 90' of phase shift, so for signals present in all three outputs at the same time The HPF output will lead the bandpass 90', while the LPF lags the BP 90'.

These quadrature outputs can be used for sin and cos outputs if SVF is used as sine wave generator.

Yes, you can stack additional integrator sections. Back in the '80s I designed an adjustable loudspeaker crossover that was switchable between 3 pole and 4 pole response. As I recall I had to play with gains of the sundry feedback paths to keep it from oscillating, and it was some work to get acceptable tracking from a 4 gang pot (alps).

JR

PS: Life is too short for poorly tuned drums...

 

[Animatic]

Yeah no bad drums, the bane of any recording engineer.

I would like to build one of these cicuits to stick between two 12AX7 stages,
and also a simple single parametric filter for same type placement.

Any suggestions on actual parts #s are more than welcomed,
Since you DID build one similar.

I have a problem; I fix the console, I am now building tube amps,
for guitarists and harp with some success recently.
About to start tube mic pres.
But designing Opamp circuits is still greek black arts.
Following a schematic and reading levels is fine.

I am trying to upgrade my functions, at an advanced age it seems...
since NO ONE down these parts can really help me. So DIY.

 

[JohnRoberts]

The TL074 quad bifets I used 30 years ago are probably still adequate for the SVF, while if you have too much money there are slightly better modern opamps.

I have no advice to offer about 12AX7s or any tubes... My total tube experience is limited to fixing radios and TV sets as a kid by replacing with new, or sometimes just swapping around the same part numbers into different sockets.

I've yet to find a compelling need to learn more about them. I'm busy enough now trying to learn how to program DSP, since I'm more interested in performing a FFT than ..... whatever it is that tubes alone can do.

JR

 

[Animatic]

TL074 quad bifets Cool that's a place to start.
Newer, slightly better, lots more expensive... hmmm
Old tried and true reasonably clean
and so cheap I can blow them up and not care; works for me.

I do have one decent electronics fix it and parts shop on the island,
My tube exploits seem to amuse him. He makes a good living fixing
a wide variety of shear crap, so has lots of parts in stock.

My 110v '61 Champ blew up this winter, after 10 years
in 220v France, no problems, but Thailand is 230v and a
mish mash of plugs standards. One gig could have USA, Europe
and Austrailian plugs all on stage and 230v so loaded down it's
pulling 175v and wavering. So my transistor amps were not happy campers.

Tubes seemed a good way to NOT have amps going south during songs...
And has proved true. And they sound killer if tuned in well.

Someone plugged my old Champ into 230v... I had to fix it.
I ended up on Hoffman tube amp board, where I met PRR.
(the grand echoing voice of experience).

I figured out how to parts out the Champ, fixed it, mod'd it
into high gear and then built another SE amp from scratch
and turned it into a screamer. I also got 2 modding and restoration
gigs from others recently, both successful and lucrative.

So I am building amps that ignore the flakey island voltage,
and give my soon enough studio visiting pickers several amp choices.

PRR mentioned Prodigy, one look and I signed up.
Wanting to build some studio gear too, and having almost NO
studio techs around for 500km, and those not up to snuff...
a re-education seemed in order.

So I am trying to interface the two worlds I am interested in
tubes, which are cool, but have limits, and SS which can bridge
those limits too if interfaced well.

I am looking at both worlds as a systems integrator,
more parts of the larger studio entity, but also from a
builders point of view. As a builder I am playing catch up.

Thanks for your assistance.

Here's a link to one of my tunes from the french studio days, if your interested.
River's Edge

 

[opacheco]

Well,

I am searching for advise about the GROUP DELAY for the State Variable Filters.......Does is it a really linear phase filter (in order to reduce to min. the delays problems)????


Thanks a lot
Opacheco.

 

[PRR]

> whatever it is that tubes alone can do.

With an output transformer: pure sweet audio amplification.

Anything tricky, like filters, "can" be done with tubes, but it gets ugly and hot real fast. That's TL074 turf.

> GROUP DELAY for the State Variable Filters.......Does is it a really linear phase filter

Yes. It obeys classic R-L-C filter rules. The phase and delay can be predicted from the amplitude response, or vice versa. It IS an R-L-C tuned circuit, except we make a fake L by flipping a capacitor twice, and we can use the amplifiers to multiply small parts to "unrealistic" values, and also buffer-out in/out impedance issues.

Don Lancaster has a book about opamp filters. Active Filter Cookbook It has been in and out of print for 30 years. "by far the best-selling active filter book of all time". Get it. It won't take you all the way to Audio EQ Guru, but gives you the basic background and plenty of easy builds to try. Here is the latest printing as sold on Amazon, but you can find old copies of the same thing. (One of the few electronics books which has NOT needed re-writing!)
http://www.amazon.com/Active-Filter-Cookbook-Second-LANCASTER/dp/075062986X

 

[Animatic]

Great thanks again PRR
I just ordered
Active Filter Cookbook Don Lancaster
and
"Op Amp Applications Handbook (Analog Devices Series)" Walt Jung

Another couple of notches more informed.

 

[opacheco]

[quote author="PRR"]> whatever it is that tubes alone can do.

With an output transformer: pure sweet audio amplification.

Anything tricky, like filters, "can" be done with tubes, but it gets ugly and hot real fast. That's TL074 turf.

> GROUP DELAY for the State Variable Filters.......Does is it a really linear phase filter

Yes. It obeys classic R-L-C filter rules. The phase and delay can be predicted from the amplitude response, or vice versa. It IS an R-L-C tuned circuit, except we make a fake L by flipping a capacitor twice, and we can use the amplifiers to multiply small parts to "unrealistic" values, and also buffer-out in/out impedance issues.

Don Lancaster has a book about opamp filters. Active Filter Cookbook It has been in and out of print for 30 years. "by far the best-selling active filter book of all time". Get it. It won't take you all the way to Audio EQ Guru, but gives you the basic background and plenty of easy builds to try. Here is the latest printing as sold on Amazon, but you can find old copies of the same thing. (One of the few electronics books which has NOT needed re-writing!)
http://www.amazon.com/Active-Filter-Cookbook-Second-LANCASTER/dp/075062986X[/quote]

Thanks a lot for your comments PRR!!!!.
Opacheco.

 

[opacheco]

[quote author="PRR"]> whatever it is that tubes alone can do.

With an output transformer: pure sweet audio amplification.

Anything tricky, like filters, "can" be done with tubes, but it gets ugly and hot real fast. That's TL074 turf.

> GROUP DELAY for the State Variable Filters.......Does is it a really linear phase filter

Yes. It obeys classic R-L-C filter rules. The phase and delay can be predicted from the amplitude response, or vice versa. It IS an R-L-C tuned circuit, except we make a fake L by flipping a capacitor twice, and we can use the amplifiers to multiply small parts to "unrealistic" values, and also buffer-out in/out impedance issues.

Don Lancaster has a book about opamp filters. Active Filter Cookbook It has been in and out of print for 30 years. "by far the best-selling active filter book of all time". Get it. It won't take you all the way to Audio EQ Guru, but gives you the basic background and plenty of easy builds to try. Here is the latest printing as sold on Amazon, but you can find old copies of the same thing. (One of the few electronics books which has NOT needed re-writing!)
http://www.amazon.com/Active-Filter-Cookbook-Second-LANCASTER/dp/075062986X[/quote]


PRR,
Which ones consoles(SSL, Neve, Yamaha....) do use the state-variable filter in they eq section????....

Thanks
Opacheco.

 

[JohnRoberts]

I don't know what the other designers did, but I put full parametric (using SVF) in a LOFT console back in the early '80s. At Peavey I put parametric EQ in only one console (due to cost) and in that case I only put parametric on a couple of channels.

The vast majority of consoles I did at Peavey used fixed or at most a swept frequency (with fixed Q). This can be done with a single opamp.

In consoles it's almost more an issue of panel space to fit the controls, but number of opamps take PCB real estate and PS current (and money).

JR

 

[Animatic]

John I used a couple of LOFTS back in those days and like the EQ.

I also did use one Peavey that had semi parametric for a few live gigs
and it did the job. The LOFT was better. I compared them to my Trident
in the studio and they were OK, but I loved that Trident.

Can you post a schematic of the LOFT eq you did?

 

[JohnRoberts]

I would if I had one but that was decades ago...

I recall in the old LOFT console parametric the top and bottom bands were switchable between shelving and peaking, which was unusual.


JR

 

[opacheco]

[quote author="JohnRoberts"]I would if I had one but that was decades ago...

I recall in the old LOFT console parametric the top and bottom bands were switchable between shelving and peaking, which was unusual.


JR[/quote]

Animatic, Right, This is the way I would like to design for a proyect!!!!!!......

JR, But do you have a schematic for this swept frequency (with fixed Q) filter using a single opamp???

Thanks a lot,
Opacheco.

 

[JohnRoberts]

[quote author="opacheco"][quote author="JohnRoberts"]I would if I had one but that was decades ago...

I recall in the old LOFT console parametric the top and bottom bands were switchable between shelving and peaking, which was unusual.


JR[/quote]

Animatic, Right, This is the way I would like to design for a proyect!!!!!!......

JR, But do you have a schematic for this swept frequency (with fixed Q) filter using a single opamp???

Thanks a lot,
Opacheco.[/quote]

The computer ate my long answer but short version is no.., but EQ was used widely in Peavey consoles/mixers so schematics may be available from service dept.

JR

 

[bcarso]

I found one of the Rane articles which is really good on EQ in general. It is their RaneNote 122, written by the indefatigable Dennis Bohn. Highly recommended.

See:

http://www.rane.com/note122.html

 

[JohnRoberts]

Yup, I actully linked to that myself in my first attempted answer but the website timed out and I lost my response.

I couldn't find an obvious example of a variable peaking style wein bridge EQ but the Rane page came up on google.

I'd advise caution about following 20C literally.. :roll:

JR

 

[opacheco]

JR and Bcarso, Thanks a lot for your comments.

please let me to know anothers books, notes, tutorials in order to learn a really goods issues about the State Variable Filters and Parametric Eq.

Opacheco.