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Hengy 25th January 2013 03:58 PM

Parametric EQ based bass boost circuit
So I needed a circuit to act as a bass boost. I ended up adapting this circuit (by ESP Sound) by excluding everything but the 35-150hz part.

This simulates perfectly in LTspice.

However, when I try and convert it so that the op amps use a single supply - GND to 12.6V - something goes wrong, and I don't know what.

I added R1 to bias the input. R4/R5 and R7/R8, as well as R10, represent pots. They adjust the volume, amount of bass boost, and the frequency of the boost, resp. The output is 0V - no signal. The transient analysis shows the output is attenuated by about 100dB.

Can anyone help me fix the circuit? Using a dual PS is not an option.



Frank Berry 25th January 2013 04:47 PM

Re-check your schematic C2 is shorted. U2 is not biased to 1/2 of the VCC supply.

Hengy 25th January 2013 08:58 PM

If you take a look at the schematic at ESP sound, C2 is shorted there as well.. Is this just to enable shelving? I don't want peaking, then I can just take C2 right out, correct?

To bias U2, I need to have a resistor to VCC. Would I put this "above" R9?

Thanks, Hengy

Hengy 26th January 2013 12:08 AM

So I fixed what you suggested, but it didn't fix it.


stratus46 26th January 2013 04:28 AM

What others told you about a shorted capacitor and missing bias is true. What isn't true is that being a parametric equalizer. Those are tunable gyrators but you can't easily alter the 'Q' in this scheme and have a narrow tuning range. Look up 'state variable' filters.

The Technics SH-9010 form the late '70s used state variable filters and allowed 9:1 tuning range and 10:1 'Q' (7 to 0.7) on each of 5 bands. They called it a 'universal frequency equalizer'.


Jay 26th January 2013 05:19 AM

U3 inverted

rsavas 26th January 2013 02:55 PM

Years ago I made 2-channel/2-band parametric eq's using a ckt that was published in Popular electronics mag, sept 1979, still have a few spare parts and a working unit. I can sell you some of these parts, Alps pots, switches. I guess posting the article, would be okay, if I am not in violation of a copyright?
Hello copyright police, is this okay?
It used TL074CN, as they were new at that time. Let me know if I can help.
Low ban was set at 40-960Hz,+/-20dB@Q=0.16 octaves, +/-12dB@Q=2 octaves
It worked really nice!! I sold a few of them.
There is also Walt Jung's SV EQ ckt's as he published in "Audio IC OPA App's" handbook, p161,fig5-25, it was a 3-band design and had a larger Q range,0.74-4. Maybe Walts' web site has this information, so I do not have to provide it, or violate a copyright.
This stuff brings back memories. I do not use tone control much anymore, as I have very good speakers that do not need tone control, but there are times they can be used, as I guess that you have a reason for needing them.


Hengy 26th January 2013 05:34 PM

I did find that mistake, and all my simulations are with it correctly oriented. It is also correct in my third post. Thanks

The project I referenced, by ESP Sound, called it a parametric EQ!
Are you saying that this won't work? That I should try a design based on a state variable filter?
The goal of this circuit is to provide both a volume control and bass gain stage before a headphone amplifier.

Thanks for the offer, but I am a student in a CS/Physics program, and would very much rather try building one myself (and learn from my mistakes!).
I will look into those articles you mentioned.
And thank you for understanding. I do not consider myself an audiophile, but I can recognize, and do appreciate, good sound quality and systems. I tend to turn up the bass in almost all of my music, as a personal preference.

Thanks, Hengy

rsavas 26th January 2013 05:53 PM

SVF, parametric eq = the same thing
Suggest +/- v rails = more dynamic range
rather than a single supply, not having to bias OPA's at 1/2 Vcc = more components.
SVF, Sure an excellent bass control ckt, to tune a speakers low end to compensate for response roll off.
As an effect, can sweep the freq, with low Q setting, to get the ole flanger effect. Need a sweeper (triangle osc) & a var res for Freq control, say a LDR/LED combo ckt, lots to play with.
Built my SVF (ParaEq) as a college project (tone control section) , part of a pre-amp design, was impresses with the sound, used NE5533 (obsolete = 2x5534=16pin dip ) as gain cell set at a gain of ~6 (1V/150mV), plus a MM phono pre-amp, old LM381. I am amazed that this design would still be valid today as a low cost pre-amp. Of course could use newer OPA's for increased performance and change the MM phono of course.
Like to know if a young lad, would be able to hear the difference between a 5534 and a newer LME49xxx part. Let me know how it goes.
Good Luck

Hengy 26th January 2013 07:06 PM

Can anyone suggest a solution other than a dual supply? The headphone amp I am putting this into just has a single supply.


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