Are High quality opamps important in active crossovers?

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#24
Hi,
it depends a bit on your output level and gain/sensitivity of the poweramp/speakers.

For conventional gain/sensitivity I would guess that 10nV/rootHz is quiet enough you might even get away with 15nV/rootHz particularly if you optimise towards low resistances throughout the filter system.

High sensitivity speakers and/or high gain power amps will require much lower input signals and these noisy opamps even when optimised may be a bit hissy and you may then require <6nV/rootHz.

As a guide 30db amp gain and 86db speakers giving 116dbA/2.8Vac input is my interpretation of conventional.

Someone using 26db+82db=108dbA is therefore low and 34db+90db=124dbA is high. There is a very high category but generally these are valve(tube)/horn based.

I hope this helps towards your decision.
 
#25
dfdye said:
How is the cross channel interference in those? Do you have a frame of reerence as to those vs the 2134's? I've never used the 4134's but they do sound really convenient.

Thanks!
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I could not find crosstalk specification, but I guess anything above 40 dB - most certainly easily met by these devices - should be of no consequence. I do not have direct experience in this respect, I use the 2134 for input stages in my amplifiers.

Rodolfo
 
#26
jacco vermeulen said:


So, what is the minimum noise level of opamps for use in a crossover ?
IIRC, noise level of the OPA2134 is worse than that of the NE5534.
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In my case I have 37 dB overall gain, 89 dB woofer and 91 dB tweeter. 4th order L-R crossover with 4134 (0 dB passband gain) and power amp input 2134.

In this conditions, noise is noticeable only putting your ear directly against the drivers. (and some residual hum I will supress anyway). Half meter away and they are completely quiet.

Rodolfo
 
#28
These are Sallen Key filters mostly, they are not very demanding to op-amp. Moreover you may find it nicier to use some other buffer circuits, like source follower of Sziklai pair instead of opamp follower. Some like avoiding class B high feedback opamps wherever possible.

regards
 
#31
peranders said:

Try to disconnect the filters and short the input of the power amp. How much difference in noise?
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Well, tried it and noted no difference. In fact most of what I could hear was mains related ant then only placing the ear against the drivers. A hint of hiss could be perceived in the tweeter.

Yet just stepping back about half meter all noise vanishes. I took care to close doors and turn off computers, radio of course, and observe only when there was no street traffic (about 100 m away and sparse).

As said, system gain is 37 dB (for the woofer channel, the tweeter has 2 dB more conversion gain and being 4 ohms against 8 for the woofer, the corresponding amplifier channel was adjusted for 29 dB voltage gain).

4134 crossover is up front and 0 dB gain. I may probably change this anyway, for it does not make much sense to process crossover at a nominal - 10 dBV level, may be I lower the amplifier gains say to 27 dB (for the woofer) and put 10 dB in front of the crossover.

Rodolfo
 
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