8pre noise from turning gain knob

[gfurgiuele]

I recently purchased an 8pre, and I liked it except I found out during a recording session that when the gain knobs are turned up past 60-70%, then touching the gain knobs again at all (and turning them up or down) results in a lot of sweeping electrical noise polluting the input.

I contacted support and they told me to get the unit replaced, so I did, but the next unit I received does the exact same thing.

What should I do? Is this the expected behavior of the 8pre? Did I just get REALLY unlucky with 2 bad units?

[Terrym]

Hi, The 8 pre have a lot of gain and for instruments i seem to not need too much gain. what do you have plugged in when moving the knobs.  are you using correct gain stage methods ?

[gfurgiuele]

Hi, The 8 pre have a lot of gain and for instruments i seem to not need too much gain. what do you have plugged in when moving the knobs.  are you using correct gain stage methods ?

Thanks for the reply, but your comment is implying that I shouldn't use the gain that the 8pre provides because.. it's too much? Then why design the interface to provide that much gain if can't handle it?

I have microphones plugged directly into the inputs, not instruments. The noise is not the result of issues with gain staging, or clipping, or the specific microphone plugged into the unit, or even what outlet it's plugged into. It happens when the gain is turned up, and seems to build-up and get worse the longer that the gain is turned up. Again, the knob is not turned up all the way, just 60-70% is all it takes, which is what I would expect to put it at for gain-hungry dynamic mics.

This is clearly a hardware defect, but what I'm trying to figure out is if it's a defect that exists on ALL 8pre's, or if it just exists on some and I got unlucky.

EDIT: I want to be clear, because I realize I might not have explained it well in the original post--I'm not talking about the noise that exists after you turn the gain up. I realize that there will be underlying noise when you turn the gain up high, that's not the issue. The issue is that extra noise gets added specifically while turning or touching the gain knob. It's like jiggling a loose cable, when you get loud static.

[Terrym]

Hi
I tried a few dynamic mics today and had a listen for noise at around the 60/70  region and besides the analogue pot noise i had near full volume when i stopped turning
i was only left with noise from the amps magnifying input noise.
There was no added noise from moving the gain after this so I'm not sure what is causing your issue.

[gfurgiuele]

Hi
I tried a few dynamic mics today and had a listen for noise at around the 60/70  region and besides the analogue pot noise i had near full volume when i stopped turning
i was only left with noise from the amps magnifying input noise.
There was no added noise from moving the gain after this so I'm not sure what is causing your issue.

Hey Terry, when you say "besides the analogue pot noise", are you saying that you hear noise due to physically turning the gain pot? Because that's exactly what I'm talking about.

[c1m]

I have the same pot noise issue, it seems to trigger if I set gain somewhere above 70%. Doesn´t matter if I turn it or touch it gently it still makes noise. Is this is considered a defect? It certainly is annoying for a new unit. My headphone output also has the same issue.

[gfurgiuele]

I have the same pot noise issue, it seems to trigger if I set gain somewhere above 70%. Doesn´t matter if I turn it or touch it gently it still makes noise. Is this is considered a defect? It certainly is annoying for a new unit. My headphone output also has the same issue.

I'm curious to hear back from Terry, if that's what he's talking about, but I asked if any other owners on Reddit experienced this, and I did get a confirmation from someone. So it appears to clearly be normal behavior for the unit, desirable or not. Here was the post from /u/calvinistgrindcore

Ok, confirmed, mine does this too. Only happens when the mic preamp is activated, not line or instrument. The steady-state background hiss level is much lower if the dummy plug has a 150-ohm loading resistor across it (this much more accurately simulates having a mic hooked up, and happens with all mic preamps). But the noise of wiggling the pot is the same regardless of loading, about 40dB down from full scale with gain maxed. So the noise is not originating in the input stage, but in a subsequent gain stage.

To be honest, this seems pretty normal to me. I have to crank my headphones and the preamp itself to hear it. These preamps do +72dB of gain (~3900x). If 0dBFS represents +18dBu (6.5V), and you have gain maxed and a noise is hitting -40dBFS, that means that noise is 16 *microvolts* in amplitude. That's down in the range of broadband input noise for a high-quality opamp chip, about 100 times lower than the nominal output voltage of a quiet ribbon mic at 1 Pa pressure. And that noise only happens when you physically wiggle the pot, which of course we don't typically do while recording. The steady-state noise floor is way way lower with any normal microphone source impedance strapped across the input.

What's probably happening here is that the slight movement of the pot wiper on the carbon track is creating small random changes in the DC current flowing through the pot, which are showing up as noise. This is totally normal for DC-coupled, high-gain, high-current/low-impedance amplifier stages. A lot of preamps insert a high-value electrolytic capacitor in series with the pot to reduce that noise, but then you have the trade-offs that come with capacitor-coupling.

I don't entirely agree with /u/calvinistgrindcore regarding the impression they're giving about how quiet the noise from touching the gain pot is, numbers be damned... but regardless, it at least makes me feel a bit more comfortable knowing that there's a reason this could be considered normal behavior, and that it doesn't seem to be indicative of a larger problem. Seems like this might just be one of those tradeoffs you get by having an analog gain pot rather than digital controls, along with whatever special sauce it is, exactly, that Arturia is doing with these preamps. And, it's nice knowing that I shouldn't do another return-and-replace, since every unit does this.

I'll note, based on your post, that I haven't experienced any issues with the headphone output, on either unit I've tried.

[Terrym]

Hi Guys,
I think the comment above is what is happening on the mic gain as the pots are analogue so any movement is amplified. a quick test of line and inst shows the noise is not there.
I could ask arturia for a detailed description. but i think we have it above.

[gfurgiuele]

Hi Terry,

I think it'd be great if we could hear from Arturia, just to get some acknowledgement that this is expected behavior, and that it is indeed doing what we think. I feel pretty confident at this point, but it still would be great to hear from them.

[glorybearer]

I’m experiencing the same issue, except one pot seems to be louder than the rest. And creates static noise throughout the whole rotation. This shouldn’t be normal behavior. I’ll be exchanging this unit for another. Hopefully the next unit doesn’t exhibit this inconsistency.

[gfurgiuele]

I’m experiencing the same issue, except one pot seems to be louder than the rest. And creates static noise throughout the whole rotation. This shouldn’t be normal behavior. I’ll be exchanging this unit for another. Hopefully the next unit doesn’t exhibit this inconsistency.

Good luck, although so far, this does this for literally everyone I've talked to who owns one. The noise does vary between the different pots, and seems to vary depending on the mic plugged in as well. The noise is particularly bad on a couple of mics that I own, I have to turn down my headphones before adjusting the gain, it's very loud and disruptive. As long as I don't touch the gain knobs after setting levels, though, the unit does work as advertised.