SOLVED: 60 Hz hum when re-amping bass (but not guitar)

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mhschmieder
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SOLVED: 60 Hz hum when re-amping bass (but not guitar)

Post by mhschmieder »

I encountered a bizarre problem tonight, but it's also the first time I've ever tried to re-amp a bass track vs. a guitar track.

Using the exact same setup as for guitar re-amping, and the same routing within Digital Performer, I get a 60 Hz hum in the bass head from the re-amper output, with a barely audible bass signal.

If I simply plug that same 1/4" cable into the guitar head, there are no issues -- nor are there any problems when re-amping the Guitar DI track.

Could this be an impedance issue? Mesa doesn't document the impedance of their amp inputs, but I wouldn't think it would be that different between the Bass Prodigy head and the Royal Atlantic head.

I am using the Radial Engineering passive JCR re-amper box. As I couldn't find TRS cables or a 1/4" to XLR of the right gender, I am connecting the third output channel of the RME Fireface UFX to the front of the re-amper using 1/4" TS -- the Radial box itself is balanced and appears to have the same functionality and specs when using the rear XLR input or the front 1/4" input.

From there, a 1/4" goes from the re-amper's output to the amp's input.

There is a ground lift on the re-amper, but it is currently inaccessible to me as it is not a button and I can't reach into my rack to change its setting without first removing the Radial boxes from the rack. Yet I can't see how it would make THAT big of a difference to engage the ground lift.

If I turn up the output level of the re-amper box, the 60 Hz hum gets louder, but the barely audible Bass DI signal does not increase at all (or at least it seems that way). This is another mystery.

Plugging a bass guitar into the amp head does not exhibit this problem; nor does the guitar head gets any mains hum even if the re-amper's output is maxed out.

I feel like I've isolated every possible variable, to no avail. Even switching the re-amper's low-cut filter to off (I use low-cut on guitar tor educe boom) has no impact. The hum doesn't go away if I flip the ground on the Mesa bass head either. And it's there even if DP isn't playing the track that is routed to the re-amper box.

My active monitors are turned off, and all but the necessary output is muted in RME TotalMix. I have verified that the sends in DP are all as expected, with no trim, and a single mono route to the third output channel of the RME Fireface UFX. Identical settings for Guitar DI and Bass DI in DP.
Last edited by mhschmieder on Tue May 23, 2017 12:13 pm, edited 1 time in total.
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Re: Unable to re-amp bass, w/ same settings as guitar re-amp

Post by mhschmieder »

I'm giving up on re-amping for now, as I can't figure out what's causing the problems -- I just spent several hours trying to narrow down the issues, which are separate for guitar and bass.

For guitar, even if I push up the gain on the Mesa head, and the treble and mids and lower the bass by just a bit, it sounds way different.

Going to an insane volume level with the master level knob on the Mesa head doesn’t change the timbre for the better — the original tracks were much warmer and articulate.

I am wondering if the guitarist had a boost pedal during the sessions, and I forgot to notate that, and it was between the DI and the amp vs. pre-DI?

It makes no sense. Even if I boost the DI track output to account for my giving a lot of headroom, and max the re-amp output, it sounds dark and muffled -- although less boomy due to the amp settings changing a tiny bit.

The bass guitar amp hum remains, and even if I boost the bass DI track by +24dB, the hum greatly overpowers the bass. Ground lift does nothing to alleviate this!

Even stranger, it MUST be at the amp end, because I can cut/paste the bass track to the guitar track to eliminate DAW internal issues/mistakes. And of course sending it to the guitar amp works fine in terms of signal level being high and hum barely existing.

So why does plugging the bass in directly to the amp, or from the regular DI box, not cause this problem? Can a mismatched impedance between re-amp box and amp head make this big a difference? I don't know either impedance as neither is documented.

I am even now using a balanced signal from the audio interface to the re-amp box, and this had only minimal impact on the bass and guitar results.

Perhaps the last time I re-amped, I wasn’t comparing to previously amped tracks and was only dealing with DI-only, so didn’t notice the difference?

Yet one is supposed to be able to get the same results via re-amping as through original amping, correct? My mic changes are NOT the reason -- I only changed the room mic, and yet the close dynamic mic and the mid ribbon mic sound WAY different from before, on the guitar tracks!
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Re: Unable to re-amp bass, w/ same settings as guitar re-amp

Post by mhschmieder »

Obviously, I can isolate the guitar timbre difference by recording a fresh guitar DI track on my own, along with miked tracks, and then compare to the re-amped tracks. That will eliminate any mystery about whether the guitarist had intermediate pedals in the signal path that I forgot about.

The bass amp hum is of bigger concern, as it blocks me from re-amping bass entirely, and is a complete mystery that I can't find an explanation for, as everything has already been isolated as a cause other than an impedance mismatch, and/or maybe a problem with the ground lift feature on the Mesa bass amp head.
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Re: Unable to re-amp bass, w/ same settings as guitar re-amp

Post by mhschmieder »

Obviously, I can isolate the guitar timbre difference by recording a fresh guitar DI track on my own, along with miked tracks, and then compare to the re-amped tracks. That will eliminate any mystery about whether the guitarist had intermediate pedals in the signal path that I forgot about.

The bass amp hum is of bigger concern, as it blocks me from re-amping bass entirely, and is a complete mystery that I can't find an explanation for, as everything has already been isolated as a cause other than an impedance mismatch, and/or maybe a problem with the ground lift feature on the Mesa bass amp head.

I'll try my EBTech Hum Eliminator between the power cable and the wall, after dinner, as that will probably tell us whether the Mesa Prodigy amp head's ground lift feature is at issue. Sometimes new gear can have problems you don't know about, in features you don't use at all, or rarely use.
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Re: Unable to re-amp bass, w/ same settings as guitar re-amp

Post by mhschmieder »

Using the EBTech Hum Eliminator between the bass head and the wall circuit greatly diminishes the hum, but neither eliminates it nor results in a usable S/N ratio for the main audio signal to come through and not still be dominated a bit by the hum.
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Re: Unable to re-amp bass, w/ same settings as guitar re-amp

Post by mhschmieder »

I was able to get near-identical results before and after re-amping, on a fresh track I had full control over, with no pedals but using the high gain channel of the Mesa Royal Atlantic head (at 50w vs. 100w) and removing the 15 dB pad from the Radial PZ-DI direct box.

I used my new Eastman T486RB thinline semi-hollow guitar, as it overdrives the easiest of any guitar in my collection, and in a very musical way right from the outset. Lollar low-wound pickups.

The guitarist from the earlier sessions probably had a boost or overdrive/distortion pedal that I forgot to notate in my usually fully-detailed project notes.

At any rate, I don’t feel the re-amped tracks sound as organic, even though timbrally they are now almost identical. The reason is that there are subtle differences in the harmonic structure of the distortion from the overdriven pre-amp, and the rhythmic pulse of these harmonics is different with just the slightest change in levels.

So it seems like too much “fix it in the mix” work is required for really getting inspired and musical results from re-amping vs. original amping. Perhaps re-amping is best used just for doubling with a different amp, than for serving as the main (and/or only) amp tracks.

Perhaps if I had recorded the DI track at near to zero dB peak vs. my usual -12 dB down, the subtle "organic differences" between original miked amp tracks and miked re-amp tracks would diminish, but now it's 9pm and I need to move on to an hour of working on bass parts for one of my bands.

I remain disturbed and perturbed by the hum issue with the bass re-amp attempts, however. I have run out of ideas as to the cause and a potential solution. I only need to re-amp the bassist because he insisted on having his headphones up full blast, and also a few songs had drum bleed in the room mic (but not the ribbon mic 22" from the grille or the MD421 3" from the grille).

For live all-at-once band recordings, I am using a cardioid pattern on the room mic, but prefer the sound and frequency balance of figure-8 overall. Omni gets too boomy, in the room I record in.
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Re: Unable to re-amp bass, w/ same settings as guitar re-amp

Post by daniel.sneed »

I've made many re-amped guitar tracks with *in-the-box* amps.
Got satisfying results with stock DP amps and cabinets.
BTW, I used both bass and guitar cabinets.
Sometimes I doubled tracks and fed a *guitar channel strip* and *bass channel strip*, starting with hard high / low pass.
In these projects, I ended with none of the original miked amp tracks at all in the mix.
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Re: Unable to re-amp bass, w/ same settings as guitar re-amp

Post by mhschmieder »

The guitarist has confirmed that he did not have a boost, overdrive, or distortion pedal at the sessions, but his super-hot EMG pickups may have produced a signal that doesn't re-amp so well?
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Re: Unable to re-amp bass, w/ same settings as guitar re-amp

Post by mhschmieder »

I reviewed the signal flow within RME TotalMix a bit more carefully today, and noticed that Analog 3-4 were stereo-bound both for the software outputs and the hardware outputs, so I split those to mono instead, pulling down channel 4 to minus infinity in the software outputs, and also bringing channel 3 back up to zero in the hardware outputs, to eliminate unintentional mono-summing, gain-followed-by-trim, or "open circuit" situations.

Eliminating the unintentional stereo output from the DI track, and the mistake in trimming that output by -28 dB, did not get rid of the hum problem when trying to re-amp to the bass head, but it did significantly improve the S/N from before, so every little bit is helping (along with using the hum eliminator at the power plug end), but it still isn’t enough to result in a usable signal level sans hum for recording purposes (even if I filter for the hum later, given that it is too close to the range of the bass guitar).

This remains an unsolved mystery overall, except that the remaining issue may indicate a flaw in Mesa’s design of the Prodigy bass head (or a flaw in my specific unit), since it doesn’t happen in the Mesa Royal Atlantic guitar head.
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Re: SOLVED: 60 Hz hum when re-amping bass (but not guitar)

Post by mhschmieder »

If I swap which of the two vertical panels-of-two-outlets that the bass and guitar heads are plugged into, in the four-outlet wall panel, the hum problem inverts to the other amp.

I can’t have everything plugged into the same wall circuit due to the maximum allowed current. The audio interface and pre-amps are on a third outlet, on another wall.

I only have one of the two instruments amps on at a time, but clearly the 60 Hz hum is an indicator of which of those two vertical strips is on the same circuit as the other wall.

I am also going to use the active selector switch for bass re-amping though vs. passive (all my basses are now passive, as are those of people I record), and an LPF for highs.

The bottom line is that all issues with re-amping are resolved. I thought about what remaining isolation I hadn’t done yet yesterday, and realized the outlet choice was critical.
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Re: SOLVED: 60 Hz hum when re-amping bass (but not guitar)

Post by mhschmieder »

It is likely that ground isn’t present on one of the vertical sections of the quad outlet.

Old houses are typically retrofitted with three-pronged outlets for plug compatibility only, without performing an electrical re-wiring to add ground.

I'll be buying some stuff at the hardware store tomorrow to better diagnose such situations.
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Re: SOLVED: 60 Hz hum when re-amping bass (but not guitar)

Post by wylie1 »

These are handy to have for a few bucks.
Most the hardware places have them.
http://www.frontiernet.net/~kf2b/tesla/ ... check.html
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Re: SOLVED: 60 Hz hum when re-amping bass (but not guitar)

Post by HCMarkus »

Ground loops can certainly be a b****.
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Re: SOLVED: 60 Hz hum when re-amping bass (but not guitar)

Post by mhschmieder »

Yeah, I'll look for that device, even if standard probes can be used as well. It's simple and straightforward, and unambiguous in its readings. :-)

Oh, I just checked email from my co-worker who's our main electro-mechanical engineer, and he pointed me to an even better one:

http://www.osh.com/Osh-Categories/Elect ... /p/2899607

We have an OSH right next door so I'll check it out this afternoon and see if it's in stock.
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