Hissing noise when using wireless system with LM386 amp - thoughts on solving with a capacitor?

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Piny

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Joined 2022
#1
Hi - I'm back with more travails in the LM386 guitar amplifiers.

Here's the issue. In order to try to be able to play from across my living room without tripping my family, I purchased a cheap wireless Tx/Rx system from Amazon. When I plug it into my home-made, 9v battery powered LM386-based amplifier (basically a Little Gem amp below, but without the C2 capacitor), there's a horrible hiss. The hiss does change when the gain is changed, so I believe the problem to be some noise coming in on the input.

I also tested dropping a .01uf capacitor between pin 2 (guitar line in) and ground, and the hiss did go away. But I have never seen an amplifier schematic that includes a low-pass filter like that at the input, making me think it's a bad idea (e.g., killing treble before it even hits the amplifier). But maybe I'm just being paranoid.

Any thoughts on using a capacitor in this way? Any suggestions for capacitance values? Alternatively, any thoughts on alternative solutions that won't suck tone from my jamming?

Thanks in advance.

Edit to add link to video with sound:

1683777368331.png
 

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#2
Does this amplifier have to work with an electric guitar plugged straight into the input as well? The 50 kohm input resistance of an LM386 is rather low for a guitar amplifier.

I would guess that your receiver is some digital thing with a sigma-delta DAC built in with insufficient filtering of its ultrasonic quantization noise. The LM386 then produces audible intermodulation products. Or at least that's what I hope, because if the noise is already in the audio band at the receiver output, you can't do anything about it with a filter without killing treble.

The difficulty is to come up with a filter that doesn't reduce the input impedance much when you connect a guitar instead of the receiver, unless you can arrange it such that the filter is only in the path when you use the receiver, for example by making it switchable or mounting it inside the receiver.
 
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#4
MarcelvdG said:
Does this amplifier have to work with an electric guitar plugged straight into the input as well? The 50 kohm input resistance of an LM386 is rather low for a guitar amplifier.
Click to expand...
Yes, ideally it would work with both. If necessary, I could put the filter on a switch inside my amplifier so I could just cut it out completely when needed.

MarcelvdG said:
The difficulty is to come up with a filter that doesn't reduce the input impedance much when you connect a guitar instead of the receiver, unless you can arrange it such that the filter is only in the path when you use the receiver, for example by making it switchable or mounting it inside the receiver.
Click to expand...
Agreed. And I was testing out some RC and LC low-pass filter values (assuming the only R/L in the filter was the guitar pickups, which may or may not be appropriate - I'm not an audio/electrical engineer), and it appears basically impossible to select a single capacitor value that would provide the right filtering over all pickup ranges. Maybe I'm thinking about this wrong, though.
MarcelvdG said:
Does a 10 kohm resistor from the input connector to the LM386 input and 100 pF from the LM386 input to ground help much (much meaning much more than 2 dB)?
Click to expand...
I'll have to check that. Are you basically suggesting this?
1683815317076.png

If so, I can try that. Would you expect that this would work with both direct-input guitar and the wireless receiver?

On that point, am I correct in thinking that the resistance/inductance of the pickups would need to be considered in this circuit? So the "equivalent" schematic would be more like this?
1683815620771.png


Thanks for your help on this! I'm learning a lot.
 
#5
Yes, that's what I meant, and yes, the guitar's impedance (pick-up, volume control, tone control if applicable) will have a big influence, as will the 50 kohm input impedance of the LM386.

I hope the RC filter is of sufficiently high impedance to not worsen the sound too much when you connect a guitar to it, yet effective enough against ultrasonics to help against the hiss issue when you connect the receiver.
 
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#6
Here's a weird bit of data - there's no hissing from these when I plug into my Vibro Champ (tube amplifier). Does that just mean that the Vibro Champ is more tolerant of and/or does not amplify these high-frequency signals? Or that something about the circuit of the Vibro Champ filters/attenuates these types of frequencies better than the LM386 amp does?
 
#7
Probably. Chances are that it can better handle ultrasonic noise because the characteristics of tubes are less curved than those of the bipolar transistors in the LM386, but it could also have some filtering built in. (I assume now that both amplifiers have about the same gain, otherwise it could just be that noise in the audio band gets amplified more by one amplifier than by the other.)
 
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