Re: Reducing hiss by changing op-amps ?

From: Pooh Bear (rabbitsfriendsandrelations_at_hotmail.com)
Date: 03/19/05 

Date: Sat, 19 Mar 2005 11:53:30 +0000

pcmangler@ntlworld.com wrote:

> Hi,
>
> I have an audio mixing desk from the 80's that is a bit 'hissy' (i.e.
> background noise with no inputs). Would I be able to reduce this by
> replacing the op-amp ICs with lower noise versions ? ...or is this
> sort of noise nothing to do with the op-amps ?
>
> I have obtained and scanned the schematics, in case anyone fancies a
> quick look. It's only a 1Mb PDF file, located here:-
>
> http://homepage.ntlworld.com/anengineer/1410_Schematic.pdf
>
> The vast majority of the op-amps are physically labelled:-
> '4558 DD'
> ' JRC '
> ' 1266B '
>
> I'm presuming these were made by the Japan Radio Company. (The
> schematic refers to them as 'TA4558NB')
>
> The desk incidentally, is an 'INKEL MX-1410' made in Korea.
>
> Any help would be very much appreciated.

Ok - this is my call since I design for pro-audio.

The 4558 is pretty rubbish, in fact I don't even recall a noise figure
being specified.

Simply replacing the op-amps won't overcome *thermal noise* due to higher
than required resistance values however. Given a mixing desk of this
vintage - thermal noise is unlikely to have been optimised.

It's likely that you can improve matters though. Certainly the 4558 is no
'audiophile' op-amp !

You need to watch current consumption though. Replacing with some devices
could easily double the current required from the power supply and it may
not be able to do this..

Simplest substitution option is the very much better 4560 also from JRC.

Graham

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