LT1028 in an active filter - diyAudio
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Old 1st April 2004, 01:09 PM   #1
Vigier is offline Vigier  Netherlands
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Default LT1028 in an active filter

Hi,

I planned to build a 6 channel filter (that means 6 channels, each 2 way, 4th order) for a 7.1 system.
The idea was to use OPA627 but they are pretty expensive (I've had quite a lot of them, but I sold them all because I thought I didn't need them anymore).
A few minutes ago I got a pretty good offer for LT1028's (CN8).
Since I need 42 single opamps, the price/quality is pretty important.

1) I read that the LT1028 is unity gain stable but it performs much better with a higher voltage gain??
2) Is it better to use OPA604 than LT1028?

I've read the datasheet from LT, and the specs of the LT1028 are VERY good in my opinion: max noise of 1,1nV/Sqrt(Hz), 11V/µS slewrate (GBW 75 MHz at 20kHz fo), CMRR of 122dB, PSRR of 130 dB)

Thank you for any information! So.... should I go for the LT1028CN8?

Grtz, Joris

P.S. Datasheet: www.linear.com/pdf/1028fa.pdf
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Old 1st April 2004, 05:54 PM   #2
jcx is offline jcx  United States
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the 1028 isn't unity gain stable in general, some specific source/load combo's may be, there is a compensation pin so it can be slowed down for general purpose unity gain apps

the main objection i would have for audio active filter use is that it is optimized for low noise with very low source impedance, you will need 50 - 200 Ohm resistive source and active network R to use the low noise potential, this makes for unusually large C values for typical crossover freqs - with higher R the current noise of the 1028 increases the total circuit noise over that of "higher noise" op amps used at more reasonable KOhm impedancve levels

assuming line level inputs, exceptionally low voltage noise op amps are not required

a simple formula for reduced distortion with op amps is to use inverting circuits such as the multiple-feedback biquad and to buffer inside the loop to eliminate output loading/thermal feedback effects - the buffer can be as simple as a resistively biased ef
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Old 2nd April 2004, 02:26 AM   #3
Vigier is offline Vigier  Netherlands
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Hi jcx,

thank you for your fast reply!
I don't think I'll do it. As you say: it seems there are too many "workarounds" necessary to get any advantage of the LT1028.

Grtz, Joris
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Old 2nd April 2004, 07:08 AM   #4
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The low noise in LT1028 isn't needed in an active filter, I suspect. Get the noise of your signal source first and then subtract 10-20 dB. 115-125 dB is what you need at the most.

I vote for OPA134/2134/4134 for budget and AD8610/20 for deluxe but note that AD8610 doesn't come in DIL08 AND max supply voltage is +-13 volts. OPA627 is also good but not worth the price if you pay for them.
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