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Can Someone Tell Me if This EQ Circuit Looks Okay?

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hummusdude:
I'm working on adapting this 5 band active EQ circuit that I found online for stereo and I'm wondering about a node that seems wrong. It uses NE5532 op amps with a single supply and the author shows a connection from the output of the input buffer stage to virtual ground. The design uses multiple-feedback band-pass filters but I thought the multiple part referred to the inverting input.

Also, they seem to have gone out of their way to separate signal ground and power ground. I know this is important, at least for power amps, because of ground loop hum issues. Is that also true for low-gain circuits like this?

blackdog:
Hi hummusdude,

All opamps are set to half of the power supply, thats OK, but the capacitor in the inverting input off the last opamp to ground is wrong...  :palm: C24 of 0,1uF
The resistors for making the virtual ground are a bit on the high side with their value of 100K, I think more like 22K for bipolar opamps and decouple the node of this 2x22K with 220uF.
I don't know if all your opamps are properly set for your equalizing frequencies.

Kind regards,
Bram

hummusdude:
Thanks Bram.  I did the calculations on the cutoff freqs and they seem good. You mentioned C24 is wrong...is that because it will ground the ac output?

Also, Do you think it's okay if the output of the input buffer (and hence all of the filter inputs as well) is tied to virtual ground (circled in red on the schematic)? That seems odd to my inexperienced eyes.

thanks for the help!!

Benta:
Agree with blackdog, but I see more issues.
- R1 and R2 to are way too large, reduce to 10 kohms.
- The purpose of R3, R4, R6 evades me. They'll skew the DC level of the signal.
- C2 should have a helper cap for high frequencies in parallel, I suggest 100 nF .
- No reason for R8 and C3, all amps are at the same DC level, provided you redesign the R3, R4, R6 stage.
- C24 makes absolutely no sense.
- The connection you've circled is a schematic error, the dot shouldn't be there.


Yansi:
This kind of bandpass-slaptogether topology will have absolutely horrific response, that can't be even properly zeroed out.

Do not build.

Find a different topology, that uses gyrators.

//Such as this one - but verify correctness of the schematic first! I haven't checked, just providing an example of a correct approach:
http://www.hobby-circuits.com/circuits/audio/tone-and-volume-control/352/6-band-graphic-equalizer

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