Audio graphic EQ architecture.

[schmitt trigger]

It has been over 30 years when I last tore down a graphic EQ, so called by the shape suggested by its sliders.

Back then, the board would be filled with dozens of opamps, hundreds of precision resistors, and many film capacitors.
The norm back then were 10 band/ channel units, as the 30 band/channel units were quite expensive.

How are these units built nowadays? Browsing thru Amazon, I found this one.....
https://www.amazon.com/gp/product/B01LZL8LGT?pf_rd_p=c2945051-950f-485c-b4df-15aac5223b10&pf_rd_r=FSB9B3AZT191Q1EAYYR5

What sort of circuitry is used to be able to sell a stereo 31-band EQ, with subwoofer crossover, all for less than US$100?
Is it still analog? DSP?
Has anyone tear down a recent model?

[Yansi]

Analog ones (cheap ones) still use gyrators and sound crap (cheap opamps, high noise due to bad circuit design, etc).

Digital ones may be implemented as a biquad cascade (a cascade of individual PEQ filters) or by summing individual bands, like this: https://www.nxp.com/docs/en/application-note/AN2110.pdf

Not sure which of the two is better and why, there will be numerous factors in play.

Also there is the "FFT way" of implementing things, but this may (probably is?) way more computational intensive to achieve the same result. Also I have a feeling, that the FFT/iFFT solution will provide unacceptable signal delay for real time audio use.

I should have a few photos somewhere, where I have torn apart a Behringer 31band GEQ.. Yuck! Those LM324s there....

[schmitt trigger]

Indeed, yucky LM324s.

Nevertheless, the sheer amount of precision caps and (yucki) opamps would still amount to a significant cost if employing a analog topology. After all, there are 62 independent bands.

My feeling is that this must somehow be done via a DSP or FPGA, otherwise how can you sell it retail for $100 bucks? That is my curiosity.


Hopefully someone reading this post has a similar unit and has already looked inside.

[Yansi]

No, it is not done with DSP. That would cost way more and cheaping out on the digital solution would sound even worse.

There is nothing that expensive on a bunch of LM324 and TL074s, with a shitload of SMD ceramics, resistors, a bunch of electrolytics and maybe few foil caps. The set og faders probably costs more than the rest of it.

[schmitt trigger]

I think you nailed it. A boatload of ceramic chip capacitors would be the way to go for the lowest possible cost.

Wonder about the temperature accuracy of the center frequencies though, as some capacitance values roughly above 1 nF would only be available in X7R ceramics.

[Yansi]

Not really.  I've seen more than 20nF in C0G available, for not that high price.  However sure, ceramics are now expensive.

Can't remember what caps were used in the behringer, but guessing there was a ton of those green mylar caps, which are dirt cheap and quite adequate.