Are High quality opamps important in active crossovers? - diyAudio
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Old 4th September 2005, 09:55 AM   #1
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Question Are High quality opamps important in active crossovers?

Hi everyone.

I'm in the process of upgrading my tri-amped system. I started with a pre amp and 3 way active crossover I built using boards I purchased from Rod Elliot (P09 & P97 respectively), and I was/am pleased with the results.

I recently rebuilt my 3 way crossover using plans I downloaded from Silicon Chip's site.

see http://www.siliconchip.com.au/cms/A_30278/article.html

The main reason I did this is everything is on one board, including input and output connectors and power supply. Also the filter stages and output buffers of the circuit are exactly the same as Rods' design. Rods' boards worked just fine,b ut it was a pain to wire all those screened cables and looked messy.

I'm in the middle of building another pre amp, using Rods' circuits for his "High quality Audio Preamp" (P88), and as he suggests I'm using the OPA2134 dual opamps.

It seems silly, though, that after going to the trouble of using good quality chips in the preamp to then use the el cheapo TL074's, from Jaycar, in the crossover. Or is device selection not so important in this role? If it is what would be a good (quad) opamp to use? OPA4134's (same as OPA2134 in a quad) don't seem to be readily available in Oz, not from Farnell or RS Components anyhow. I've spent hours searching thier web sites and I can't find a device with similar specs for less than $25 ea, and I need 6 of them!

Part of my problem, I'm sure, is I don't know enough about what to look for. I've a pretty good idea (I think) of what GBP, THD and slew rate are, but what actual figures are considered "good", and what else should I be looking at?

I read in another thread in this forum that AD8513's might be OK ($18.33 from RS). Data sheet here. However these seem to be surface mount only - can you buy little boards to solder them onto so they will fit the DIP14 sockets I used when building the board?

Thanks all.
Andy
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Old 4th September 2005, 01:54 PM   #2
jcx is offline jcx  United States
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try

http://www.ti-estore.com/

or

http://focus.ti.com/docs/general/dis...countryCode=AU

you might have to buy a tube/rail from a distributor but even 2 doz will be cheaper than the options you're looking at
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Old 4th September 2005, 02:19 PM   #3
hau is offline hau  Philippines
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hi

opa4134 is a soic type,
you can use opa4227/8 they are dip type.

i have some opa's left from my previos project , if you need them i can trade them to you , just send me an email.

best regards
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Old 4th September 2005, 03:33 PM   #4
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Audio General amplification requires GBP of 1MHz min, and slew rate of 0.5 v/uS min.


Active filters place exceptional demands on opamp slew rates, and the greater the Filter's Q (quality factor) the greater the Slew rate needs to be. Since slew rate is closely tied to bandwidth, a high slew rate opamp is likely to have high bandwidths. It's not uncommon in hi-fi active filters, to have to go to opamps with bandwidths well above the audio frequency range (20MHz+)

I would not use any thing with slew rates lower than 15-20 V/uS in an active filter.

For high fidelity, you'll want a noise level of 5nV/rtHz or less. NE5534 has been widely used in hi-fi and is a standard for comparing noise. It sits at 4 - nV/rtHz.

Some newer, exotic parts like INA103 feature noise levels as low as 1nV/rtHz, excellent slew rate and lo distortion. They are often used in pro mixing consoles and studio recording equipment.

The OPA4228 is good value, with a noise figure of 3nV, which is good for a car application, but its slew rate (10v/uS) is borderline for active filters.
NE5534, at 13V/uS, is not ideal either, but in a bind, can do for a low-Q active filter, and is widely available, and cheap.
So, this is what you look for, when deciding on op-amps for your active crossover: noise and slew rate.

Pro equipment generally use single (1-per-package) opamps, to minimize cross-talk, parasitic capacitances, and very importantly- to provide individual power supply decoupling capacitors for each individual op-amp. A dual or quad pack is a convenience, not a pro tool.
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Old 5th September 2005, 07:28 AM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
postscript to Funberry on slew rate:- In my opinion highest slew rate only required on treble Hi-pass and if fitted treble Lo-pass.
Mid and bass filters can easily manage with lower slew say 5 to 10 V/uS (3Mhz to 8Mhz bw).
Funberry, any comment?
How do you define Hi Q? above 1? or above 2? or some other figure?
Try an edge on piggy back PCB with one pair of dual opamps into a 14 dip.
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regards Andrew T.
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Old 5th September 2005, 11:52 AM   #6
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To me, Hi Q = elliptic, state variable filters.

Higher the Q, the higher the SR required. Higher the ferequency, higher the sr required.

If I were designing a filter now, I'd pull out my college physics book and calculate the required SR now. But I'm just speaking qualitatively, if there's one parameter I really need to pay attention to in an active filter, it's slew rate, more than offset oltage, input current, rail-to rail IO, temperature drift, or anything else.

Because it's easy to say " well, I'm working at audio frequencies, so 1MHz bandwidth is fine" and stick in a 741...

The rationale behind slew rate in an active filter, is a bit counter-intuitive to a beginner.
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Old 6th September 2005, 12:23 PM   #7
macka2 is offline macka2  United States
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Try the Moamps discrete opamp, its noticably superior the the TL072 and the 5532.

regards

Ian
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Old 6th September 2005, 12:47 PM   #8
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Quote:
Originally posted by AndrewT

Try an edge on piggy back PCB with one pair of dual opamps into a 14 dip.
Thanks Andrew, ggreat idea. I'm not sure if there is physicaly enough room, but definately worth investigating.


Quote:
Originally posted by funberry

Higher the Q, the higher the SR required. Higher the ferequency, higher the sr required.
Q is a concept I never realy quite understood. Something about a ratio of reactance...? I do recall that tuned circuits with a high Q hive a greater propensity to oscilate. Or is it the other way.
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Old 18th March 2006, 07:03 PM   #9
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Hi,

For an active crossover which are the best opamps that can be ordered as free samples?

Thanks!
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Old 18th March 2006, 11:45 PM   #10
AndrewT is offline AndrewT  Scotland
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none
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regards Andrew T.
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