Inductor Saturation in ‘Swinging Input’ EQ

[freshtapescent]

Dearest enlightened persons of groupdiy-
I’ve got a set of Opamp Labs EQs, which are ‘swinging input’ a la ‘EQ AMP’ and everything Connected between pins 1&8 here: https://groupdiy.com/attachments/opamp-labs-1204-2a-jpeg.51868/

My questions-
1. What happens when an inductor saturates in this circuit? From theory I know…the value of inductance value plummets when the core saturates. So…in practice…you’re boosting 40Hz on the drum bus…+2, +4, +8…+12dB…sounds linear until saturation, then peaks of waveform saturate the inductor and for those moments it sounds like you’re not boosting 40Hz any more? Like the loudest kick drum on the downbeat of the chorus actually gets less (or no) low end cause its so loud it saturated the inductor? Or…peaks that saturate the core max out at +12dB of boost and then there’s just no more boost as you turn up the input. Like…it’s cranked up to +12, and pushing more level into the EQ basically doesn’t give you any more boost?
2. Do HF band inductors have to be large enough to handle the low end signals? For example, regarding question #1…will I lose a high frequency boost because a kick drum just saturated the high frequency inductor? Or is it just audio level within the band the inductor is passing?
3. Will they saturate the same whether they’re boosting or not? Same with cuts? If you saturate the inductor, does the answer to #1 still ring true? Like, cutting cutting cutting…loud kick comes thru, inductor saturates, you’re back to 0dB of cut all the sudden…it’s like the EQ is bypassed?
4. Do we hear increased harmonic distortion as inductors saturate…or are any distortion products somehow nulled because they’re injected into the opamp differentially?

Thanks!!

 

[gyraf]

- Saturating inductors in boost mode acts slightly like dynamic EQ - if you boost a lot AND there's a lot of signal in that region, boost amount reduces. This imo the origin of the pultec's high-freq boost popularity. In cut mode I dunno, haven't investigated. Sounds neutral'ish though, perhaps a bit of expansion mode that way..

-hf (and mf) inductors are in series with their frequency-determining caps, forming resonant ccts, thus not subjected to whole-range power level

- when centered, +phase and -phase from each end of the pot cancels

- yes, we hear increased partials when inductors saturate - but mainly around the frequency tuned (!) which masks the effect significantly

All these side effects are probably the reason why we like inductor/capacitor and passive eq's so much

/Jakob E.

 

[Tubetec]

I had a similar question in mind for Jakob
more relating to core material ,frequency and distortion .

 

[ruffrecords]

I think it is highly unlikely any of these inductors in this EQ will get anywhere near saturation. Boost is achieved by reducing the impedance from the -ve input of the op amp to ground. WIth maximum boost the gain is close to 11K/1.5K which is at most 17dB but in practice will be close to the 12dB specified. Even with full boost a 0dBu input signal will be +12dBU at the output but still only 0dBu across the inductor.

Similarly in cut, the RLC circuit forms the bottom arm of an attenuator. So once again, with maximum cut, an input signal at +12dBu will only be 0dBu across the RLC

Cheers

Ian

 

[freshtapescent]

@gyraf Thanks, Top marks! Thank you @ruffrecords!

I get it. Explains a few things…

 

[Somkereki]

Dearest enlightened persons of groupdiy-
I’ve got a set of Opamp Labs EQs, which are ‘swinging input’ a la ‘EQ AMP’ and everything Connected between pins 1&8 here: https://groupdiy.com/attachments/opamp-labs-1204-2a-jpeg.51868/

My questions-
1. What happens when an inductor saturates in this circuit? From theory I know…the value of inductance value plummets when the core saturates. So…in practice…you’re boosting 40Hz on the drum bus…+2, +4, +8…+12dB…sounds linear until saturation, then peaks of waveform saturate the inductor and for those moments it sounds like you’re not boosting 40Hz any more? Like the loudest kick drum on the downbeat of the chorus actually gets less (or no) low end cause its so loud it saturated the inductor? Or…peaks that saturate the core max out at +12dB of boost and then there’s just no more boost as you turn up the input. Like…it’s cranked up to +12, and pushing more level into the EQ basically doesn’t give you any more boost?
2. Do HF band inductors have to be large enough to handle the low end signals? For example, regarding question #1…will I lose a high frequency boost because a kick drum just saturated the high frequency inductor? Or is it just audio level within the band the inductor is passing?
3. Will they saturate the same whether they’re boosting or not? Same with cuts? If you saturate the inductor, does the answer to #1 still ring true? Like, cutting cutting cutting…loud kick comes thru, inductor saturates, you’re back to 0dB of cut all the sudden…it’s like the EQ is bypassed?
4. Do we hear increased harmonic distortion as inductors saturate…or are any distortion products somehow nulled because they’re injected into the opamp differentially?

Thanks!!

I wouldn't worry about saturation, since they are in the feedback line of the operational amplifier, so the amplitude is very small. Rather, it avoids the use of inductances from a production technology point of view. A capacitor is much cheaper and easier to manufacture. It is also not sensitive to the magnetic field of transformers. Pot iron cores with a closed magnetic circuit are much less sensitive to it. For example, I made the input microphone transformer of the old mixing desk with a Siemens 25 mm diameter, Al 2700 pot iron core, and the sound was perfect even on the kick drum channel. Our drummer used two kck drums with AKG D12 microphones if I remember correctly. The sound was perfect. At that time we had Crown DC300A power amplifiers. I used National Semiconductor LM301M (Military) integrated circuits in the mixing console, I was surprised that the cheaper LM301A with a plastic case from the same manufacturer sounded much worse. I soldered gold-plated sockets in place of all ICs and capacitors in a channel of the mixing console and replaced the components. The signal source was a UHER 4400 Report tape recorder at tape speed 19. The usual sine 20 Hz - 20 kHz measurement showed no difference, but the mentioned tests showed big differences! Hope I helped. Good luck!

 

[abbey road d enfer]

Actually, in swinging input EQ's the LC tank sees the full input voltage, except at resonance, where it sees a fraction. But the subject here is the voltage across the inductor, which can be much higher than the input signal.
Counter intuitive it may be, but at max attenuation, the voltage across the inductor can be about 10-12dB higher than the input signal.
Since attenuation is usually used to deal with high levels, it can result in significant distortion, particularly when useing narrow BW settings.