Portable two-band Parametric EQ (ESP/Urei 545)
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The problem with TLO7X is noise voltage. I deemed them not suitable for my 5 and 9-band parametrics. With typical 5k resistors at the input, the noise du to noise urrent is less than that due to noise voltage.
However, today there are FET opamps with noise voltage comparable to 5532.
OPA1652 and even better OPA1656, both only in SOIC package.
I typically used >5k Ohm resistors around SVF to keep capacitor values small.
Do what works for you.... FWIW I haven't designed any parametric EQs this century.
JR
I had to use relatively large caps (about 150nF) for the low range
Actually I did a DC-controlled one; turned out the VCA's noise was dominant to a point I gave up.
I don't recall all but I used 8200pF for the low frequency range in my 1979 kit parametric EQ
Have you ever considered DPOTs?
There are multiple ways that the SVF parametric filter topology can negatively impact noise floors. I have seen approaches that incorporated an elevated BP section gain to generate a narrow bandwidth (I used a variant on that in my old kit design). Likewise there are variable Q/bandwidth adjustment approaches that result in elevated noise gain in the HP and BP filter outputs. I have seen too many different parametric approaches to list (or even remember).
A possibility with DPOTs or VCAs is to vary Q/bandwidth by shifting the integrator poles further apart to get a broader bandwidth (unsure how narrow you can get by moving them closer together). Sliding integrator poles apart or together should have no obvious negative noise gain consequences.
When considering modern technology pure digital domain filters should be excellent.
JR
That would have implied rather high value resistors, hence the need for FET opamps.
Yes I have. Actually the project was a dynamic notch filter and I was not comfortable with the idea of having the logic needed to drive DPOT's co-existing with the analog path.
I agree, but the customer wanted a full analog signal path (vinyl mastering).
Back in those days I was using lots of 100K resistors
A couple decades ago, I actually designed a digitally controlled analog (using DPOTs) automatic mixer for friend's "analog only" mixer company. While I got respectable sound quality (to my ears) the company fell on hard times because "analog only" customer base got seriously eroded by digital consoles with tons of features at impossible to ignore prices. That project got abandoned before reaching production prototype stage.
My breadboard layout had both DPOTs and VCAs. I populated the DPOTs first and they worked so well, I forgot about the VCAs.
That seems like a pretty small market but the customer is always right as long as they have the money to support such requirements.
JR
How relevant is the opamp's 'gain bandwidth' for this application? Some opamps are very fast like above 20MHz, I wonder how beneficial the extra bandwidth in real terms compared to a humble ie. 5.5MHz. I understand that faster opamps normally have higher slew rate and output current, but I wonder for directs benefits for this application.
Here I share some short noise tests with 5 opamps: OPA1656, OPA2145, AD8620 and OPA2132. The results seem similar and noise at the lowest fq produced by the 68K quite dramatic.
Here I share some short noise tests with 5 opamps: OPA1656, OPA2145, AD8620 and OPA2132. The results seem similar and noise at the lowest fq produced by the 68K quite dramatic.
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Not necessarily regarding output capability; output current is unrelated with BW. OTOH many newer opamps have improved output capability.
It allows reducing the resistor values for optimizing noise.
In terms of audio perception I don't think there's any advantage using high speed opamps. The integrators noise is dominated by the time- constant, only the summer may benefit from extended BW but since it operates at moderate gain (about 30dB at min BW), even with 5.5MHz it means performance will be kept up to ca. 180kHz.
It must be said that older opamps, such as 748's and LM358 were actually challenged, but not the TLO's and 5532's.
A good enough reason to keep resistor values low, which is where opamp's output capability becomes a concern.
My concern was with the dynamic part of the design.
Setting frequency and BW was not an issue, but the dynamic part required converting the detector voltage, with possible interference, and the possible zipper noise. Being a strictly analog designer, it was too challenging for me.
Vinyl mastering engineers are ready to invest in 100k+ custom monitor speakers. They would spend a significant fraction for a dedicated all analog dynamic notch.
Not enough to make a business out of it, but filling a niche.
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I see! Is there an easy way know or estimation to predict how low input resistors can be for a certain opamp's gain bandwidth? Or is it something to try and see if it runs out of steam? I like the idea of having a minimum Q of around 17, equivalent to a chromatic semi-tone. But this requires around 160R at the summing inverting input (Q pot) when using 5.1K general bandwidth resistors. If you can see what I mean.
The discussion on DPOTs is fascinating. What was your experience with steps using DPOTs @JohnRoberts? Do you remember how many steps did you use and for what range more or less? I wonder if steps are really noticeable because I have an ARM MU in my system already, so very tempting to explore this in the future, once I manage to set the analog filter properly with carbon pots on PCB.
Resistor value is not limited by GBW. Limitations come from the drive capability of opamps.
At max boost the level at the outputs of the mixer and the integrators is a few dB below the output, you could dimension the resistors for the lowest load recommended by the manufacturer. For 5532 it's 300R IIRC.. For OPA1656 it's about 150R.
I don't think there's any issue with that. The limitation comes from the GBW product. High "Q" * means high gain in the mixer.
BTW I hate using "Q" for this parameter. Q is valid only for transfer functions that go asymptotically to minus infinity, which is not the case for EQ's. Actually Q varies with the amount of boost/cut.
BW is a much more "organic" description of perception. It has been debated a lot, and never came to a common standard. Still, I prefer using BW over Q.
The computer I did the PCB layout on lost its hard drive years ago, but my recollection is that I used 12 bit DPOTs. I included hooks in the software architecture for later adding zero crossing synchronization and the MCU had enough A/Ds that I could use them for synchronization, but during debugging I didn't perceive a pressing need for it. FWIW the target application was spoken word. I used TV talking heads audio for source material.
My speculation is that I was updating the gain changes so frequently that the gain steps were tiny and not very audible. I expected more critical listening tests later as this SKU got closer to production, but that never happened.
JR
