Does this parametric EQ circuit look right? Not textbook looking...

[mr coffee]

Hope one of you analog designers can look this schematic over and explain to me how it works, or if it's just plain got errors in it. I've been studying it a while and I just don't quite get all the signal going 'round and 'round. It doesn't look like any textbook State Variable Filter I've seen, or a Bi-Quad design either. I think it is supposed to be a bandpass filter of some kind.

It's supposedly a lower noise design for use with electric guitar signal level and can be used with well over the common 9v. battery - up to 30 volts.

The setup with the first two op amps doesn't make sense to me with where the output is taken, especially because it is labelled "GAIN".

 

[mskeete]

Looks like an SVF to me. Just drawn a bit differently

 

[JohnRoberts]

the way it is drawn gives me a headache... Yes SVF

JR

 

[mr coffee]

Thanks for looking it over, guys.

@ John,
Gives me a headache, too!

What do you make of the feedback from the first integrator to an op amp OUTSIDE the 3 op amp SVF - the one labelled IC2_B?

I've never seen that before and really don't understand what is going on there.

And what does the pot labelled "GAIN" do?

The circuit sort of looks like the UREI 545/546 sections that do boost and cut - at first look. But it's NOT.

It's different in a way I don't get. Any reason it should have the claimed better noise performance?

Thanks so much for any understanding you can share of this topology.

 

[JohnRoberts]

Thanks for looking it over, guys.

@ John,
Gives me a headache, too!

What do you make of the feedback from the first integrator to an op amp OUTSIDE the 3 op amp SVF - the one labelled IC2_B?

I've never seen that before and really don't understand what is going on there.

And what does the pot labelled "GAIN" do?

The circuit sort of looks like the UREI 545/546 sections that do boost and cut - at first look. But it's NOT.

It's different in a way I don't get. Any reason it should have the claimed better noise performance?

Thanks so much for any understanding you can share of this topology.

sorry I've already got enough headaches already working today.

A typical SVF parametric section used a 2 pole SVF which involves 2 simple op amp integrator sections, One providing the BP output and the other the LP output. The HP output is generally another op amp without an integrator involved. The parametric boost/cut section involves a 4th op amp section.

Perhaps try to redraw that puppy in a more conventional order. It is difficult to follow, especially if you say there may be mistakes.

JR

 

[abbey road d enfer]

What do you make of the feedback from the first integrator to an op amp OUTSIDE the 3 op amp SVF - the one labelled IC2_B?

IC2_B is the inverter with the pot that swings from input to output to control Boost/Cut.

I've never seen that before and really don't understand what is going on there.

It is part of almost any existing EQ, including Baxendall, SVF or Wien.

And what does the pot labelled "GAIN" do?

It's actually the Boost/Cut control.
It is a pedal. Musos are known to use inappropriate tech language, like their guitars have an "input" jack.

The circuit sort of looks like the UREI 545/546 sections that do boost and cut - at first look. But it's NOT.

I don't see a fundamental difference. Blocks are identical.
IC2_A &C are the integrators, IC2_D is the adder.

Any reason it should have the claimed better noise performance?

Compared to what?

 

[mr coffee]

Thanks for taking a look, too.

@John Roberts
Here's a redraw that may or may not make this clearer.

@ abbey

The circuit sort of looks like the UREI 545/546 sections that do boost and cut - at first look. But it's NOT.

you said: I don't see a fundamental difference. Blocks are identical.

Look more closely, abbey. It looked like the UREI circuit to me at first glance, but it's different. I know the schematic is drawn awkwardly/weirdly, but even laid out more conventionally, it's not the same. Check out the redraw I attached below per JR's request.

I've included the excerpted part of the UREI 545/546, which is at least somewhat different. The UREI circuit doesn't have the same feedback setup for the SVF loop or for the Q control. The second integrator feedback loop goes to a different op amp outside the SVF loop in the SOK, which is different from the UREI.

If the two circuits function the same, please explain what I'm not getting here. That's why I posted this.

 

[JohnRoberts]

The Urie looks like a more conventional SVF parametric drawing.

JR

 

[abbey road d enfer]

I've included the excerpted part of the UREI 545/546, which is at least somewhat different. The UREI circuit doesn't have the same feedback setup for the SVF loop or for the Q control. The second integrator feedback loop goes to a different op amp outside the SVF loop in the SOK, which is different from the UREI.

I don't see a significant difference. In all cases the output of the 2nd integrator goes to the inverting input of the adder.

If the two circuits function the same, please explain what I'm not getting here. That's why I posted this.

To me the basic principles are the same. The only significant difference is the way the BW control is implemented, but I don't doubt there are possibilities to adjust values in order to get identical response.
The only majot difference I've seen in SVF EQ's is when the filter is driven from its inverting input, which makes the Boost/Cut pot act reversed.

 

[mr coffee]

@ abbey
Thanks for your explanation. When I looked back at my redraw and your explanation, I realized I was misreading the original schematic when I said,
"The SECOND integrator feedback loop goes to a different op amp outside the SVF loop in the SOK, which is different from the UREI."
It was really the FIRST integrator feedback loop that was going to an op amp outside the loop, but it was drawn so strangely that I kept confusing which integrator was which. THAT iwas why it wasn't getting through my thick skull.

NOW I get it!

@John
Thanks for suggesting a redraw in a more conventional manner. I just should have looked at it longer myself!

Couple more questions if I may

1) Can you imagine any justification for the claim this topology has better noise performance (with the difference being how the adder and BW adjustment is set up)? I suppose, in comparison to the more conventional UREI circuit (and assuming current-day better noise performance op amps in both).

2) Would noise performance suffer if the ganged frequency pot were allowed to go closer to ground, i.e., if the 1K resistors to ground were decreased in value, rather than using the integrator capacitor range-switching arrangement? Could this range switching be the justification for the "better noise performance" claim?

Thanks much for the help guys

 

[abbey road d enfer]

1) Can you imagine any justification for the claim this topology has better noise performance (with the difference being how the adder and BW adjustment is set up)? I suppose, in comparison to the more conventional UREI circuit (and assuming current-day better noise performance op amps in both).

Considering the TL06x series has a noise voltage density of 30nV/sqrtHz and the LF35x series only 12, this affirmation is false.
Now, using true low-noise opamps in both circuits, the noise performance would not be significantly different.
When I made parametric EQ's, last century, I used 5532's and 2068's everywhere, because every stage contributes noise.
Today, I would probably use OPA1612's (1.1nV/sqrtHz).

2) Would noise performance suffer if the ganged frequency pot were allowed to go closer to ground, i.e., if the 1K resistors to ground were decreased in value,

Yes. Simply because the wiper goes closer to the ground, the voltage fed to the opamp is lower and noise is more perceptible.

Could this range switching be the justification for the "better noise performance" claim?

Indeed, range switching helps optimizing noise, but it's not fair comparing the noise of this single-band EQ with that of a 3 or 4 band one.

 

[JohnRoberts]

Couple more questions if I may

1) Can you imagine any justification for the claim this topology has better noise performance (with the difference being how the adder and BW adjustment is set up)? I suppose, in comparison to the more conventional UREI circuit (and assuming current-day better noise performance op amps in both).

The dominant noise in SVF would be from the op amps but keep in mind we are mainly listening to the band pass output, the other bandpasses can contribute some. Another consideration is how the Q/bandwidth gets varied. I have seen this done a number of different ways with different pros/cons. One obvious con is bandwidth adjustment circuitry that dramatically increases the noise gain of what Abbey call the "adder" stage and I call the HP output. You can't avoid the high noise gain when commanding a narrow bandwidth but the approach used in the Urie doesn't suffer the elevated noise gain all the time.

Note: there are different ways to tweak the Q/bandwidth, we can detune the tuning frequency of the two integrators further apart for wide bandwidth, this would not have any noise penalty cost. Another way I have seen the Q/bandwidth varied is with the gain of the bandpass stage. As I recall higher gain there narrows the bandwidth. The additional gain needs to be scrubbed off and this arguably eats into headroom. Clipping any stage of a SVF in use does not immediately clip the bandpass output but it can generate odd artifacts in the BP output

In a consumer parametric I did back in the 70s (Phoenix system P-94 kit) I intentionally allowed the boost/cut to interact with the Q/bandwidth. For general Hifi EQ this worked satisfactorily to compensate for the apparent loudness change from adjusting bandwidth wide/narrow. I was able to generate something like 20 dB of boost/cut but only at the narrowest bandwidth.

2) Would noise performance suffer if the ganged frequency pot were allowed to go closer to ground, i.e., if the 1K resistors to ground were decreased in value, rather than using the integrator capacitor range-switching arrangement? Could this range switching be the justification for the "better noise performance" claim?

The frequency adjustment strategy has several popular variants. One problem is that the pots are not very precise making accurate frequency setting difficult. If they mistrack between bands that can cause q/bandwidth to shift. Of course the Johnson noise of the resistance matters (a little). I just did a search for a past discussion of this and didn't find it. My preferred approach is to ground the bottom of the frequency pots and use them how they are most accurate (as voltage dividers). To establish the LF extreme a high value resistor feeds around the pot to the integrator section. This way both the HF and LF accuracy are defined by the external resistors... Back in the day ALPs specified decent section to section tracking but not perfect.

Of course my description is somewhat simplified.

Thanks much for the help guys

I half considered making a SVF parametric using DPOTs to vary the sundry adjustments, but why bother if one could just do the whole thing in DSP (No not me )

JR

 

[mr coffee]

Thanks guys!