DX7 noise and opamps (was: Re: Midi to CV -> ZN4

[Haible Juergen]

> Something completely different while I'm at it.....
> I attacked my DX7 and installed sockets at all op amp locations.
> I then replaced all 4558 (and even TLO72's) with 5532's just to see if
> I could hear a difference/improvement.  All I heard was a reduction
> in standing noise level (post volume control).  The noise level of the
> actual signal remained the same and the character did not change.
> I put the old parts back in - I want to use the 5532's where they
> will be appreciated!  The LM386 headphone output amp could be
> replaced for much better S/N if one were up to it.  I was depressed
> that all my work was for naught so I just closed it up and said the
> heck with it.

Yes, the FM process used in the DX-7 is very noisy by itself.
No problem for bright sounds, but very annoying if you want to
program dark and mellow sounds. Wurlitzer Piano - what a
wonderful Wurlie I had on my DX-7, even with the typical
distortion on every individual note - but it was drowned in noise.
You can make things a *little* better by tweaking the trimpot
in the DAC section, to optimize linearity. But even then ...

I remember the headphones output being even more noisy than
the FM process. This was the only chip I replaced when I used
to own a DX7. I *think* I used a 5534 there, which can drive 600
Ohm Headphones very well. But I don't remember the details.

BTW: My greatest disappointment with FM synths was that
the later models like TX-802 or SY-77 were almost as noisy
as the old DX-7.
I still haven't got a satisfactory Wurlie sound on *any* synth!

JH.