lowering noise floor on Aphex 104 Aural Exciter Type C2 w Big Bottom

[z11111]

Alright so check it out -- I got one of these units (dated 1992) for basically free and it totally rules. I've used it on bass guitar after a Retro 176 (opposite ends of the spectrum lol) on the past two mix projects I've worked on and love the sound. The one thing I don't love is that the noise floor is pretty nuts.

When I got it, I recapped the PSU and (I think) the rest of the electrolytic caps, but it sat on a shelf for about a year before I got the proper weird 24VAC wall wart to power it so my memory isn't clear. I couldn't find the schematic at the time but have attached one to this thread for reference.

I could go through and replace all the carbon film resistors in the signal path but I'm sure there's a better way to achieve less noise, I'm just not familiar with these Aphex Exciter circuits by any means and I know that some noise is inherently part of the design. When I saw Signetics 5532s on the input and output, I was like "oh that's good to see," but obviously doesn't make a difference. Has anyone in here ever done some mods on these?

 

[tommia]

Changing caps resistors and opamps is only marginal improving noise. Design is what changes noise. Aphex is well aware of that and are good designers. So I would asume it is inherent of the function, any mod would change the working.

 

[RoadrunnerOZ]

The Aphex works on adding overtones tuned to the original signal of a higher frequency as well as a low frequency overtone. I think it uses 4th and 5th order harmonics.
If you look at the schematic there are test points to view these program generated tones which are mixed with the original program tone. Check to make sure there is nothing other than expected with a test tone - they show a 1KHz test tone - look for noise on any of the added tones (and the pass through signal) - 100Hz and 5 KHz as per the diagram test points - those with a half darkened circle denote scope relative to mid-point “ground” at 1/2 VCC - you may need to set scope ground onto pin 1 of U301 for Ch1 and pin 7 for Ch2 (except at the input which is balanced input). Your scope needs to be ungrounded. Check for noise at the chip VCC points for each chip as well as the output of each at idle.
There needs to be caution regarding the high pass tune setting - too low a tune frequency results in harshness.
Too much drive causes overload distortion, insufficient drive results in noise. Make sure the +4 - -10 switch matches the level you’re driving with.
Too high a mix setting and it starts to sound artificial. Turn the mix up full to set the tune and drive, then drop to zero and slowly bring the mix up until it sounds right.
If you have noisy inputs to the exciter this noise will be amplified if it is HF above the tune set point. The noise floor in the 104 is -100dBv so check your input sources for noise.