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no input noises from amp to speaker, pot variates

sonder

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Mar 6, 2023
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Wondering if somebody can explain what's going on, after experimenting a bit.

If I remove all inputs from my amplifier and set it to SE, I can amplify a small hiss using it's internal pot, get it a bit louder, hear it about a meter.
Repeat with balanced selected, essentially nothing, if i squeeze my ear to the tweeter i can hear a very slight something.

Introduce to the mix, the NS-05P passive preamp with switchable RCA/XLR in/out, and a 20k alps 09 pot - note it's passive and no mains wire.
Sbef1859c16b94b7aa102373124630f88S.jpg


With the amp at full gain/it's pot wide open, the NS-05P connected via RCA to the amp (but nothing connected to the RCA/XLR inputs):
- the input set to RCA or XLR: I can amplify the small hiss using the ns-05p's pot/volume, get it a little bit louder than just bare amp.

However, with the amp at full gain/it's pot wide open, the NS-05P connected via XLR to the amp (but nothing connected to the RCA/XLR inputs):
- the NS-05P input set to RCA: I can amplify the hiss to a greater extent using the ns-05p's pot/volume, it's also lower in frequency. Hear it about 2m+ away.
- the NS-05P input set to XLR: nothing at any level.

Second point:
Additionally, under all scenarios there's a slight static noise as I twist the volume on the NS-05P. If I touch the NS-05P with my thumb or other hand whilst I spin the volume, no static noises introduced. Is this just my own electrical signal/static being ungrounded and amplified vs grounded?

Third point:
If I connect my DAC via RCA to the NS-05P and play something at -80db, such that when both pots are wide open I can faintly hear the music, retaining the RCA connection flow to amp, I can hear me "changing" the tone of the hiss between 9-3 o'clock, is this the varying ohm's of the pot changing it's tone?

So I guess the specific questions are:
- what is the hiss when RCA is in the signal path and not connected to anything.
- why does the 20k pot on the NS-05P appear to amplify it a little bit (maybe it does't, arguable)
- why does changing to RCA in and XLR out at the NC-05P out amplify it much further and make it a lower tone? (double signal via xlr?)
- why does removing any RCA sockets from the signal path completely wipe it? (this is perhaps a stupid question, but I'll ask it anyway, in case it indicates that there's no bleed through between signal paths on the NC-05P or amp for example).
 
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Differential amplifier input amplification principles AND an understanding of how the audio signal is transmitted via XLR vs. RCA are the 2 (main) ingredients here. Also, you need to understand what happens when a differential transistor pair (the first gain stage inside an amp) is fed single-ended input (RCA), instead of a differential (XLR) input.

You could research the above....

In a nutshell, when a differential signal is fed to the differential input, the shield is not required... so both inputs (of that differential input transistor pair) see the exact same voltage and there's no hiss/buzz/hum.

When an RCA signal is fed to the same differential input transistor pair, one side (ONLY) of that differential pair is fed the ground potential as the result of the fact that some current always runs through the shield, due to slightly different potentials that the amp and the source (their chassis) sit at. This current, when presented with the input impedance of the input transistor, turns into voltage. Now, this voltage is directly fed to (ONLY) one side of that differential input pair (voltage amplifier) - hence, it gets amplified, i.e. the noise/hiss/buzz gets amplified (search for "differential input OPAmps"... and in particular the common mode rejection ratio).

An audio signal transmitted via RCA from the source towards the amp uses interconnects' shield as a return path (back to the source).
An audio signal transmitted via XLR from the source towards the amp uses only hot and cold (pins 2 and 3... also called a twisted pair) - the shield is not used to transmit the audio signal (the shield is used only to protect the hot and cold from RF noise and magnetic radiation, where all that crap is sunk into the chassis potential)

You could tell that... RCAs should never be used in audio... but are used because it is cheap... the truly balanced chain requires everything doubled (inside the source, as well as inside amps... and that is expensive)... but once you go XLR you'll never go back to RCA.

I hope the above gives a good starting point... you can take it from here... with a bit of research.
 
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