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Old 5th January 2005, 08:38 PM   #1
ainami is offline ainami  United States
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Default Need opamp recommendations

Hi,

I'm looking at upgrading parts in an active subwoofer crossover that I currently have. I would like some input on it's current opamps and what I should swap them with.

The power for this unit is supplied with a 24VAC transformer. There are only 2 1N400X diodes in the power supply section, so I assume it's a single 24VDC power supply. I don't think there is a voltage regulator there, but worst-case scenario would indicate that the opamps would need to be able to handle a +24VDC supply.

There are four MC33078 DIP8 dual-opamp chips, made by ST, on the main board and it looks like they drive the high-pass / low-pass outputs. The spec sheet says it has a large output voltage swing (+14.3V/-14.6V), but I'm not sure if this is critical. It looks like they are all bypassed pretty good because there a little of .1uF caps surrounding these opamps.

There is also an LM324 DIP14 quad-opamp chip on the main board, also made by ST. It's not apparent what this is used for, but it provides a large voltage gain (100db) and a very low supply current (376uA). Thinking about it, it's probably for the gain stage.

Finally, there are two MC33179 quad-opamp chips (DIP14) on a sub-board along with 4 pots. The pots are used to adjust hi-pass frequency, low-pass frequency, phase and gain. The only unique quality that I can really see on this one is its ability to drive a 600ohm load. I would think this is to handle the different impedances from the pots. It's also a bipolar device (but the others could be bipolar, too).

I don't intend to use the high-pass outputs, so I'm not really worried about frequancies above 200Hz or so.

Recommendations are welcome.
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Old 6th January 2005, 12:49 AM   #2
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Default Re: Need opamp recommendations

Quote:
Originally posted by ainami

I don't intend to use the high-pass outputs, so I'm not really worried about frequancies above 200Hz or so.

Recommendations are welcome.

so why do you want to improve anything? noise issues?
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Old 6th January 2005, 04:11 AM   #3
ainami is offline ainami  United States
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In a lot of applications, you can improve the sound by replacing opamps. If these opamps are fine, then let me know.
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Old 6th January 2005, 12:08 PM   #4
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What he's saying is, that for a unit that outputs only below 200Hz you won't notice the difference. It's the mid and high where the differences become apparant.
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Old 7th January 2005, 12:34 AM   #5
ainami is offline ainami  United States
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Well, from what I've read, there is a lot of difference between opamps at that low of a frequency. The Burr-browns usually are described as softer or more "tuby" at the bottom end, as opposed to the Analog Devices opamps.

What I ended up deciding on is to replace the four MC33078's with AD823's. They seemed to be regarded as having a tighter bottom end than the bb's. The crossover is for a home theater application, so I don't want a loose bottom end. The circuit also requires something that will run off a single supply at 24V. I did measure the supply pin of the opamps at 24V, so the power supply is not regulated at all.

The LM324 is getting replaced with a OP484 (Analog Devices), which seems be a better solution for a quad opamp using a single supply. There a lot of negative feedback on the LM324.

I left the MC33179's alone because they seem to be regarded as pretty good. Also, I couldn't find a supplier for them, so I can't switch back if the new ones don't work. The OP484's above seemed like a good solution for this, but I don't know how they react at 600 ohms (which the MC33179 is designed for)
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