best opamp for equaliser

I've built a 4 stage parametric bass equaliser based around Rod Elliotts design (http://sound.westhost.com/project28.htm) - it works well. It replaces a Behringer DSP8024 digital eq that worked well, but in my system levels ran too few bits so hashed the sound up a bit I felt. The analogue eq seems pretty transparent, though it is in the main signal path (well tape loop actually). Anyway, being completely obsessive I'd like opinions of what op amps might be better than the 5532's I've used so far. The OPA627 is well recommended in the op amp discussion on this board, but I just discovered what they cost! Any more sensibly priced recommendations please?

My circuit is effectively the 35-150Hz stage (trimmed here and there) repeated 4 times, though I only link in stages as required 1 on one channel, two on the other for now. The object is to judiciously cut room resonances only - works wonderfully when WAF prevents other solutions! The circuit is well laid out with good decoupling.

Any suggestions appreciated.

rgds

MurrayP
 
Don't knock the 5532!

People carp about the "lowly" 5532. But even some 20 years after its introduction, there aren't many op amps available that are significantly better... and those that are cost a LOT more money!

Having said that, I have had good results with the TI (Burr-Brown) OPA2134 as a general purpose device in musical instrument gear.

In an equalizer application, you can pick and choose your devices for each frequency band. It would be a waste to use an AD797 for the low frequency filters, but it might be appropriate on the high end.

The input buffer amp should be capable of driving low impedance loads with low distortion, and for this the 5532 is hard to beat. The main summing amp needs to have very low noise voltage at low source impedances as well as good drive capabilities, and because the loop gain is >> 1 you can often use a decompensated device like the Analog Devices OP-37 or Linear Technology LT1115 or LT1126. Again, the 5532 is not a terrible choice either.
 
On second thought...

Just had a look at the schematic, it wasn't quite the topology I expected but is not too different. I was thinking of the one that has active filters in the feedback loop, instead of the series-resonant "LC" shunt to ground in this version.

Gee, I might have to build this one for myself!

The input buffer can use almost any device as long as the slew rate and noise aren't abysmal. The author suggests a TL072; I might suggest an LM833 (dual) or LM837 (quad), a bipolar input device which will be quieter than the '072 with most line-level sources, and is about as cheap. It's also not as power hungry as the 5532.

The output stage appears to need unity gain stability, so forget what I said about decompensated chips.
 
oops

Sorry should have said 5534 (single version I think) - anyway - same thing. Anyway just thinking maybe there could be better. Should also have said rest of system is a bit flash - good valve preamp, Plinius SA250/4 and N801's so the eq is in pretty strong company, but I really don't think I'm loosing much going thru it, and gaining heaps by eliminating a really nasty room mode that masks other detail like crazy,

thanks for the ideas,

Murray
 
MurrayP said:
Sorry should have said 5534 (single version I think) - anyway - same thing. Anyway just thinking maybe there could be better.

At the risk of repeating myself, I think the 5532 or 5534 is hard to beat in this application, and better than many people want to believe it is.

But don't let me stop you from experimenting with more exotic devices!
 

djk

R.I.P
2001-02-04 4:23 am
USA
You only need one op-amp here, the rest are just buffers.Get a McIntosh MQ104 service manual, it gives the formulas for calculating frequency and Q for this circuit.The early MQ104s use a single transistor in class A for each buffer.Later ones use cheezy op-amps.Two guesses as to which version sounds better.
 
The output/summing opamp can be an OP27 instead of NE5534 and I can almost bet that there will be a significant improvement to the overall sound quality. The same principle of gyrators with frequency sweep is used by Rod Elliot in his Mixer Project. I recently used the tone control section in conjunction with a different input section. I faced problems with the value of the feedback resistor (2k7) in the output/summing opamp, in that a large negative dc voltage appeared at the output with hardly any signal at all; I had to increase the value above 18k (actually I used 22k in the final circuit board). Comparisons between NE5543, LM301, OPA604 and OP27 in this position, showed that OP27 performed significantly better. I had a TLE2141 at hand but forgot to use it; I assume this would have been a better choice.
 
yeah man,

I´m shure you have the 741 in your amp too, to drive the 2N3055s.

Bernhard
 

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I recently designed an active tone control board for a friend of mine that builds custom electric bass's and used gyrators in a very similar fashion to Rod's design. Low power was a primary requirement since it runs off a 9V battery and I ended up selecting the MC33178 (dual)/MC33179(quad). These are the best sounding, low noise and low power (200 uA/channel) op-amps I could find. The MC33178 actually gave better noise performance when corrected to a high impedance source (like a magnetic pickup) than other parts that on paper had lower noise specs. Another bonus is that the MC33178 is able to directly drive a 600 ohm load so you can plug this unit directly into a PA mixer input without having to use an impedance matching transformer.

I also included a 'Q' control on each band (a 1K trim pot) in series between the low frequency limit cap and the node that connects to the high frequency limit cap and the feedback resistor to the opamp output. This control is not for the musician (they can barely handle setting the filter frequency), the builder sets this by ear to suit the particular bass he is building.

Needless to say the design used all plastic film caps and metal film resistors. Sound quality is very good. Has a much lower noise floor than commercially available products (EMG) and the builder likes the flexibility of design, I designed a version for him that lets the player switch the center frequency of the mid-range control between 300 Hz and 1KHz.


: To Boost/Cut Pot
|
_
_ Low Frequency band limit Cap (larger value)
|
\
/ 'Q' Adjust Trim Pot (1K)
\
|
------/\/\/------ : To opamp output
|
_
_ High Frequency band limit Cap (smaller value)
|
 
The LT1115 is really a special purpose device, optimized for low-Z sources and high loop gains. I'm not clear how you would use it in an equalizer; the summing amp is about the only application that might make sense.

Oh, and it gets hot too; LT says about 9 mA quiescent current. That's the price tag for the low voltage noise.

The LT1028 seems like a better choice, because it's stable at loop gains of -1 and has lower current noise. I still wouldn't call it general purpose though.
 
haldor said:
I recently designed an active tone control board for a friend of mine that builds custom electric bass's and used gyrators in a very similar fashion to Rod's design. Low power was a primary requirement since it runs off a 9V battery and I ended up selecting the MC33178 (dual)/MC33179(quad). These are the best sounding, low noise and low power (200 uA/channel) op-amps I could find. The MC33178 actually gave better noise performance when corrected to a high impedance source (like a magnetic pickup) than other parts that on paper had lower noise specs.

Yeah, that MC33178/9 is a surprisingly good part for instrument on-board preamp applications. I've substituted an OP-284 for the MC33178 that was OEM in my own bass, but I'm not entirely sure it's an improvement.

I'd be interested to see the schematic for your tone control, or at least get contact info for your friend.