Ashly SC-40 Input Noisy

bk201

Member
2010-08-29 5:11 pm
I have an Ashly SC-40 guitar preamp thats extremely noisy, and I've narrowed down the source of the noise to the input stage. I tried swapping out all the components and replaced the 4558 with a spare lm4562 but to no avail. How can I change this circuit to improve its performance?
 

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Mooly

Administrator
Paid Member
2007-09-15 8:14 am
I take it you mean hiss... and it's silent with the 680 ohm isolated ? Is it silent with no input connected ? Does the hiss (noise) alter as the pot is turned ?

The input impedances are pretty high depending on what you hang on the inputs... a FET opamp may be better... but it's not going to be night and day difference.

Perhaps the way the gain stages are set out (the whole chain) are sub optimal with the noise of each stage adding together... or is the source intrinsically noisy perhaps and you are just ampliyfing and making it worse.
 
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bk201

Member
2010-08-29 5:11 pm
I would call it white noise, it almost has a tearing sound to it, and its volume is completely controlled by the input pot. I've attached the full circuit, if I remove R18 (the 680 ohm resistor) and leave the connection open, the noise disappears. Theres still a small amount of hiss without the input circuit connected but its within the expected noise floor. Connecting a guitar to the input has no effect on the noise.

The noise floor with respect to the guitar signal makes the unit pretty much unusable for live performances because the noise gets too loud with the volume turned up.
 

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Mooly

Administrator
Paid Member
2007-09-15 8:14 am
Can't really suggest much tbh as far as the circuit goes... it's all text book stuff and conventional.

The pot R1 has the ability to bring the gain up to very high levels. The gain at max is (10000+100)/100 which is 101 or 40db, so if you are having to run at max gain then it's not surprising it's noisy, also I wonder if the sound is still "too low" whether there is another issue such as the signal from the guitar is too low or a mismatch somewhere (sorry I know next to nothing about what's used in practice on such an instrument... what the specs would be etc).

It must be worth trying a different opamp for the noise, and the gain bandwidth product would also be far better... the NE5532 is about as quiet as you will get for a bjt device.
 

Bonsai

Member
Paid Member
2003-07-25 10:44 pm
Europe
www.hifisonix.com
You may be picking up some RF here. Lets take a look.

1. Short the input right at the high level input and set the volume to minimum. The noise should be all but gone, maybe a faint hiss from the input
2. Turn the volume up to max. with a gain of c. 100, there will be some hissing.
3. Now plug your guitar in. You should be getting similar results to steps 1 and 3 - maybe slightly more hiss. If not and its a lot more noisey, you are probably picking something up like CFL lamp noise, or a laptop adaptor of PC power supply - so try turning all of those things OFF to see if it makes a difference.

BTW, I notice that the 68p cap is across the 2.2M resistor and before the 1k input resistor on non-inverting input. Are you sure you have drawn this correctly? I would expect the 68p cap to be after the 1k series resistor.

Note, because of th e fairly high impedances around this circuit, it is going to be quite noisey, but definitely not 'unusable'.

good luck