Portable two-band Parametric EQ (ESP/Urei 545)

[abbey road d enfer]

Hi @abbey road d enfer , sorry for insisting on this but what is the reason the SVF oscillates when forced to cover a full range spectrum (ie. 40Hz-15KHz). I thought it would be due to particular opamp limitations they 'run out of juice', but I tried with other opamps with very high output specs and bandwidth and same happens. Could you please share with me what the limiting factor is?

The probable reason is that the two halves of the tuning pot don't track closely enough and at some point the loop gain is too high.
Does the circuit oscillate whatever the position of the tuning pot?
40-15k is a ratio of 300+. SVF's filters have a typical range of 1:30, which often uses switching for covering the audio spectrum in two ranges.

 

[domingo]

The probable reason is that the two halves of the tuning pot don't track closely enough and at some point the loop gain is too high.

That's an interesting hipothesis, didn't think of that.

Oscillation occurs only (and always) at low frequency and high gain/Q. Roughly when fq is below 80Hz, Q 6.9 and gain above 7.

If I increase the resistor between the frequency pots and ground oscillation doesn't occur, for the pot ends then at 80Hz. But then I loose range which is what I'm trying to increase and understand where the obstacle is.

Another symptom I have is that, when Q is low and gain high (roughly Q 2 and gain 7), the filter simply collapses and stops filtering, letting the source signal passthrough after a few intermittences. But this is not oscillation, seems like the opamp runs out of power, but other opamps fail at the same point.

 

[abbey road d enfer]

That's an interesting hipothesis, didn't think of that.

Oscillation occurs only (and always) at low frequency and high gain/Q. Roughly when fq is below 80Hz, Q 6.9 and gain above 7.

If I increase the resistor between the frequency pots and ground oscillation doesn't occur, for the pot ends then at 80Hz. But then I loose range which is what I'm trying to increase and understand where the obstacle is.

You are pushing the limits of the design. I guess (would need to make a sim to be more affirmative) the 100uF caps at the bottom of the pots introduce significant phase-shift that impairs stability. Can you try replacing them with larger values (470 or even 1000uF)?
Why don't you use a range switch, that would solve your problem?

 

[domingo]

You hit right on the spot @abbey road d enfer ! I'm very glad that I asked and that you were there with an answer... After increasing the caps to some 220uF (solid) elcos oscillation immediately stopped )) So the problem at low fq and high Q/gain is over. I've been dealing with this problem for quite a while so I am really deeply happy and thankful with your suggestion. And my ears too!

You are pushing the limits of the design.

I'm attached to an equalising practice that I inherited from digital filters (spoiled by!), of which I can hardly get rid of. It normally starts with a band without knowing if lows or highs will be filtered, then as I scan the whole range I create a curve and then move to the next one. In other words I usually don't know if I'll filter highs or lows until I start experimenting and find something. If ranges are separated I easily get trapped. Also, another habit from digital filtering, very often I need to create peak curves that are close to each other, so the shared range between bands should be wide as possible. Switching ranges/caps unfortunately doesn't help me for the intended use/workflow, which involves recording while eq'ing.

Line apart.
If not too much already. I'm still having issues on the opposite corner: when Q and gain are to a wide minimum low, while frequency is to a maximum high (ie., Q 0.4, A -11, 15KHz). Right before reaching the end of any three pots, being the other two already at the end, the signal starts suffering from very fast interruptions; if pushed even further, the filter simply shuts off and stops working. With certain extreme ratios (+180) I sensed that, instead of shutting down, the filter's gain gets sort of inverted, sudently amplifying the whole high to ultra-high range rather than attenuating it. But in less extreme ratios that I'm intending to use, around 140, the filter just just off and an attenuated source signal goes through instead. Would you have any idea on the cause of this? Ground caps don't seem to influence, neither the type of opamp. Maybe the integrators' resistors are too low (R200), but not fully sure that's the cause of it. Maybe unfiltered DC, not sure.

 

[abbey road d enfer]

If not too much already. I'm still having issues on the opposite corner: when Q and gain are to a wide minimum low, while frequency is to a maximum high (ie., Q 0.4, A -11, 15KHz). Right before reaching the end of any three pots, being the other two already at the end, the signal starts suffering from very fast interruptions; if pushed even further, the filter simply shuts off and stops working. With certain extreme ratios (+180) I sensed that, instead of shutting down, the filter's gain gets sort of inverted, sudently amplifying the whole high to ultra-high range rather than attenuating it. But in less extreme ratios that I'm intending to use, around 140, the filter just just off and an attenuated source signal goes through instead.

I don't understand what you mean by "ratio". Ratio is a term taht pertain to compressor/gates, not to EQ's.

 

[domingo]

I don't understand what you mean by "ratio".

I mean the ratio between the highest and lowest frequency of the range covered by a band (ie. 107Hz-15KHz = ratio 140).

 

[sahib]

I have read all of the posts now. I am wondering if increasing the gain (reducing R146 to 2k) range is giving you this headache as Abbey mentions of curves moving and touching unintended frequencies.

 

[abbey road d enfer]

It's very hard to tell, but as I said earlier it's really pushing the limits. At max frequency, the opamps see a very low load impedance (100 ohms) which is not quite right. The amplitude of boost/cut is about 27dB, it can even reach 36dB in some positions, when it's typically about 15.
Is your schemo in post #42 accurate? The 100uF caps should go to ground, not virtual ground.
Many possible causes for the problem, here the issue is probably due to parasitics, like power supply sag or bad ground circulation, or not enough capacitance on virtual ground.

 

[sahib]

Maths side is extremely hairy. I wish I had the time to work out what the maximum allowed gain would be before the stability is compromised. It would be good for learning.

But Rod Elliot seems to have kept R146 , R135 and R160 to the same and for a very good reason.

I have checked the original Urei schematic and R135, R160, R125 and R214 are all kept to the same value and R146 is pretty much half of this value to limit the gain range at max, all for again a very good reason.

It may come to nothing, but I would increase R146 back to 22k again and see how the circuit behaves. If it behaves good then incrementally reduce it to work out the maximum gain allowed before things become unstable.

At some point Domingo mentions of circuit ceasing to work at certain settings and pass the signal unaffected to output. That means the transfer function is hitting a zero (again a hairy math). What I am trying to say is that there s a lot of interaction among R135, R160, R125 and R214, and now R125 and R214 being 100R you plug one hole but let the water in from the other.

As you said the problem of course may turn out to be a simple wiring error and I have talked a lot of nonsense.

 

[domingo]

Hi @sahib! I appreciate that you went through the whole thread and join the conversation.

After testing with other 'gain resistors', as you suggest, I figured that the problem starts happening only when it is about half 22K Rod Eliott's value. Also anything exactly below 11K starts failing I would say. Of course at 2K it becomes much more evident.

I've made a video to show the failure, hope it goes through. The signal is coming into the spectrum of a software parametric equalizer via a USB interface (only for visualisation), it is a real time recording of the breadboarded circuit over pink noise. An updated version of the diagram at test goes attached as well.

Is your schemo in post #42 accurate? The 100uF caps should go to ground, not virtual ground.

Yes @abbey road d enfer, I've testing with the caps to ground, as suggested by you before and shown in the new diagram. I've also simplified the bandwidth resistors to two (there were 3 before) and the caps are now 220u solid elcos.
View attachment parametric_equalizer-test1.mp4
In the video I start with max Q, moving from one extreme to the other of the fq range. Thanks to the 220u caps it doesn't oscillate in the lowest fq point anymore Then I move to min gain and min Q (widest bandwidth), where the problem is. Distortion starts, then the filter collapses and sort of inverts itself all of a sudden. So apparently the symptom is not that the unfiltered signal passes through really, but that when gain is too low (max cut) it flips to max gain (max boost) alone. I provoke the failure twice at the end.

I've read in an extensive discussion in another forum that gain can also be achieved placing a resistor to ground close to the boost/cutoff opamp. It suggests that some gain can be provided by the filter and the extra needed by the boost/cutoff differential amp (ie. see post #24). In my diagram this would be done placing a resistor between R351 and R359 to ground, I think. A value of 3.9K to GND is recommended when general resistors are 5.1K. I haven't tried this, don't understand the logic neither the value I should try. But seems like some light after the tunnel.

 

[sahib]

Hi @sahib! I appreciate that you went through the whole thread and join the conversation.

After testing with other 'gain resistors', as you suggest, I figured that the problem starts happening only when it is about half 22K Rod Eliott's value. Also anything exactly below 11K starts failing I would say. Of course at 2K it becomes much more evident.

Pleasure. It is a good learning for me too.
However, as I have mentioned before in the original Urei schematic it is about half of the other four resistors. So, the designer knew his limit.

I've read in an extensive discussion in another forum that gain can also be achieved placing a resistor to ground close to the boost/cutoff opamp. It suggests that some gain can be provided by the filter and the extra needed by the boost/cutoff differential amp (ie. see post #24). In my diagram this would be done placing a resistor between R351 and R359 to ground, I think. A value of 3.9K to GND is recommended when general resistors are 5.1K. I haven't tried this, don't understand the logic neither the value I should try. But seems like some light after the tunnel.

I have checked that link. I do not understand the logic in Increase the gain in one hand and attenuating on the other but it seems to be working.

However, the Guitar DooDaa guy has all the resistors mentioned above in equal value. You still have R354 and R366 different than the rest, not only that, extremely low as Abbey pointed out. All the examples that I know have R364, R366, R363 and R368 kept to the same value to make the maths manageable. I have looked up some references today and found only one transfer function for a text book non-inverting SVF, and it is already as hairy as a bear without the cut/boost stage thrown in.

Abbey is an expert on this and I would take his word that the design is already pushed beyond its limits. So, I think you may have to cut your losses and stick with 11k. It seems this is the best we can get.