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Old 31st January 2005, 10:03 AM   #1
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Default replacing 4558s in SM Pro Audio PR8

Hi all,

I'm not very flash with electronics but into recording in a big way. I'm about to receive an SM Pro Audio PR8 microphone preamp (8 channel preamp) and it uses 4558s. I have heard these are somewhat noisy, and would like to replace these. I have been looking around and the OPA2134 seems to be a favourite. Can I just use this straight in the place of the 4558s without any other modification rather than soldering them in?

And is this a good choice? I'm basically after a low noise, very neutral sound, and reliability.

Thanks!
Jonathan
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Old 31st January 2005, 11:06 AM   #2
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If it were me I would put 5532's in it. Should work fine without any problems. Just my opinion.
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Old 31st January 2005, 01:26 PM   #3
macboy is offline macboy  Canada
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The OPA2134 is pin-for-pin compatible with the 4558's, and they are a huge step up in terms of audio performance. However, since it is a faster op-amp, it needs a more careful design to ensure stability. The exisiting circuit may work fine, but it also may not.

There are a few things that you can do to prevent/cure oscillation. Update the power supply decoupling to make the new opamps happier. Install a good quality 10 uF electrolytic from each power supply pin to ground, and a smaller (say 0.1 uF) ceramic between the two supply pins. Other people will have other opinions about specific power supply decoupling requirements...

Another thing you may need to do is to add a small ceramic capacitor (I mean really small, like 10 to 47 pF) in parallel with the feedback resistor. This creates a low pass filter at a very high frequency (100's of kHz), helping to prevent high frequency oscillation. If you still have stability problems, you may need to place a ceramic cap across the two input pins (usually in the range of 100's of pF).
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Old 31st January 2005, 11:40 PM   #4
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Thanks for the help guys!

The 2134 sounds like a good idea... but I'm not sure about all the other mods to prevent oscillation.

Can anyone suggest another opamp (my main concern is that I want very low noise) that I could basically just use in place with no further modifications?

Is the 4580 a better idea for a replacement?
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Old 31st January 2005, 11:40 PM   #5
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Would the OPA2604 likely work without decoupling anything, just as a straight replacement?
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Old 1st February 2005, 01:06 PM   #6
macboy is offline macboy  Canada
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Sorry to have scared you with my threats of oscillations. Honestly, any of the opamps you mentioned will probably work straight away without any other mods. But you do have to exercise some caution... if they do oscillate and you don't know it, you are likely to cook the tweeters of any speaker you connect to the system. If you have an oscilloscope handy then you can check for oscillations before connecting anything valuable to the unit.
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Old 1st February 2005, 10:26 PM   #7
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I've decided to not buy the PR8 after all, I'll probably just get something better...

But thanks for your help anyway! And I've found these forums in search for an answer. I'd have probably never heard of them otherwise!!
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Old 1st February 2005, 10:41 PM   #8
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The 5532 should work without any problems. Its a drop in part/same pin out.
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Old 1st February 2005, 11:49 PM   #9
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Quote:
The 5532 should work without any problems. Its a drop in part/same pin out.
I agree with burnfingers. Shouldnt give any oscillations problem because it is not sensitive to layout design. Many people reject this opamp disregard that it is widely used in the recording studios.
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Old 2nd February 2005, 02:11 AM   #10
digi01 is offline digi01  China
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Quote:
Originally posted by burnedfingers
The 5532 should work without any problems. Its a drop in part/same pin out.
I agree with burnfingers too. I guess it will work more reliability without any modification in this case.

please noticed:the opa2134 is a fet input opamp with high input impedance.
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