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Old 12th October 2012, 05:46 AM   #1
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Default Discrete opamp vs regular op amps

I have a Yamaha preamp channel that I liked so i pulled out a channel And rack mounted it. However it is a little noisy on the EQ stages cause it still has jrc4458d op amps in that stage. Would it be beneficial to switch them discrete op amps? How big a difference would something like a seventhcircleaudio discrete opamps vs burr brown opamp have?
It is running at +-15v currently and I am installing sockets to play around with components in the EQ section, but I can't help but think that discrete op amps play a role in the magic that big name companies like API use.
The preamp sounds great, the EQ just gets too much hiss when boosting high midrange and highs, but I am working on it for fun.
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Old 12th October 2012, 06:30 AM   #2
jcx is online now jcx  United States
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you need to know the circuit impedance to choose better noise spec op amps - many heavy bias BJT "low noise" parts wll be worse than what you have in kOhm Z EQ/feedback impedance circuits because of their higher input current noise
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Old 12th October 2012, 07:35 PM   #3
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I replaced JRC4558 (not 4458) Op amps with ST33078 op amps. There was a considerable reduction in hiss. I had to put .1 ceramic bypass caps on the power supplies within 2" of the socket, and a 22 pf cap around the feedback resistor, to eliminate the resulting oscillation. I don't know your circuit, but it is a cheap experiment. The Amp dip sockets were $.56 and the op amps were $.38 each. The shipping was $6. The capacitors were junk from a dead PCAT switcher power supply. I had to change the rail voltage dropping resistors (330 ohm) the designers were using to suppress hum, since the 33078 uses more power. They were running it off +-3.5 v with no filter caps after the resistors; I'm using +-8v off 1n5344 zener diodes as regulators, and electrolytic caps before and after the zener diodes.
If I want to do further experiments with $6 apiece op amps or something, the sockets are in there. Matter of taking the cover off. I have other equipment much more broken, however.
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Last edited by indianajo; 12th October 2012 at 07:39 PM.
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Old 12th October 2012, 11:57 PM   #4
sreten is offline sreten  United Kingdom
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Hi,

5332's are the obvious cheap* substitute for 4558's, a lot better,
and well good enough for any sensibly designed tone control.

rgds, sreten.

*Cheap, yes, but near untouchable performance versus price.
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Old 13th October 2012, 12:12 AM   #5
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Despite of all beliefs and fashions 4558 when biased by 6k8-7k5 resistors to the + rail sound obviously better than 5532. I experimented with different channel strips in TOA console when wanted to upgrade it, so could check and compare immediately. As the result, left all 4558 there, biased them, but replaced in summing amps only, by OPA1612.
One more "obsolete" opamp that I never replace is 5534. It is still better than the majority of modern opamps.

If you hear "hiss" boosting highs that does not mean that the EQ itself hisses. It may boost hiss of what is before.
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Old 13th October 2012, 12:01 PM   #6
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Wavebourn View Post
Despite of all beliefs and fashions 4558 when biased by 6k8-7k5 resistors to the + rail
Is this a conversion of the output stage to single ended ClassA?
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Old 14th October 2012, 03:00 AM   #7
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Quote:
Originally Posted by AndrewT View Post
Is this a conversion of the output stage to single ended ClassA?
Yes. Also it increases a bit base current of output emitter follower and collector current of the driver stage on some linear value, making it a bit more linear and faster, except class A benefits.
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Old 14th October 2012, 03:28 AM   #8
sreten is offline sreten  United Kingdom
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Hi,

One persons opinion can't argue with real measurements.

rgds, sreten.
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Old 14th October 2012, 05:33 AM   #9
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Quote:
Originally Posted by sreten View Post
Hi,

One persons opinion can't argue with real measurements.

rgds, sreten.
Some persons that design best in the world equipment know better what to measure than the majority that does not have a clue what and how to measure.
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Old 14th October 2012, 05:52 AM   #10
Jay is offline Jay  Indonesia
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Some persons that design best in the world equipment know better what to measure than the majority that does not have a clue what and how to measure.
Cool. I should put that in my footer signature
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