View Full Version : ground loop hum
04-17-2012, 07:17 PM
I'm new to the forum. I have a BC Rich acoustic with an older Barcus Berry passive pickup running through a Para DI. I'm getting what sounds like a ground loop. I've read on the forums that the Para doesn't have a ground lift. Any suggestions for getting rid of the hum? (The feed is perfectly quiet running my electric guitar through the box). Is my pickup just bad? Thanks for the help.
04-18-2012, 08:48 AM
Are you using both outputs from the Para DI?
04-18-2012, 02:24 PM
No, just the 1/4" output jack
04-18-2012, 03:27 PM
If you are only using one of the Para DI's two outputs, the hum you're experiencing is not likely due to a ground loop.
Ground loops can occur when you are sending the signal out both the 1/4" and XLR outputs to two separate destination devices, such as a mixer or PA head and an amplifier, for example. If those two devices do not share the same power supply or their power sources don't share the same resistive path to ground, interference blocked by the shielding cannot escape to ground at the same velocity. Some of the interference gets trapped in the ground circuit and therefor interferes with the signal.
With a single path like you're connecting through the Para DI, what's likely occurring is that some part of the circuit prior to the Para DI is exposed to interference and that is being boosted by the gain stage of the preamp. The source of the interference can be the same - other electrical devices in the environment or the wiring of the environment itself. Fluorescent lighting, CRT computer monitors and dimmer switches are three of the biggest culprits when it comes to 60Hz hum interfering with pickup signals.
I have a couple further questions:
Is the Barcus Berry pickup magnetic or piezo?
How long is the cable between the instrument and the input of the Para DI?
Does the pickup have any exposed shielding in the circuit that you can touch and observe a change in sound? If the shield is poor, but intact, touching the shield or metal in the ground circuit should diminish or eliminate the hum as you add yourself to a complete ground circuit. If the shielding circuit is broken, touching shield on the pickup side of a break should make hum way worse as you become part of an antenna picking up the interference.
Please reply with results if you can perform that test.
04-18-2012, 05:02 PM
OK, first off, forgive me for not being terribly tech savvy on pickups, but here goes. I believe I have a peizo pickup - based on the pictures of a similar model on the Barcus Berry website. It mounts inside the guitar under the saddle. (are the sound hole pickup variety then magnetic?) Anyway, there's nothing on the pickup itself to touch - it's enclosed in a wood block - but if I touch the ground (shield?) lead of the pickup at the end post jack, the hum diminishes. My cables are from Musician's Friend, about 8' long and say "high definition instrument" on the cable. Thanks for your help. I really appreciate this.
04-19-2012, 08:34 AM
Thanks for the info. That is likely most similar to what Barcus Berry calls the "Insider" pickup which is a very low-cost option. I would not expect many pickups in that price range to be very resistant to interference or produce a very natural tone.
It's possible that the pickup on your guitar is simply not shielded very well or has degraded with age. You might also get a little cleaner sound from using higher quality cables but I suspect the pickup is a greater factor when it comes to the hum.
04-19-2012, 10:07 AM
I've suspected the pickup from the beginning and have been looking at the M1 and the M1A. I bought the Para DI since I knew my old pickup was a passive one and now that I have it, my question is: Do I gain anything using the M1A vs. the M1? Thanks again for all your help.
04-19-2012, 12:03 PM
The M1 or M1A are very nice pickups. Both should be quite resistant to hum. I understand they are nearly but not quite as quiet as a metal-covered side-by-side humbucker like you'd find in most Les Pauls.
At this juncture, however, I'd like you to see the video in this link - http://www.lrbaggs.com/m80/ - because the M80 conveys more of the warm lower frequency resonances of the guitar body than the M1 and M1A do.
Some of the differences between those magnetic options and the Barcus Berry are that the M1, M1A or M80 can be easily moved from guitar to guitar, they should be less prone to hum in the wider variety of environments due to better shielding, they should have a snappier string response as well as being body sensitive and the magnetic pickups are all likely to be more feedback resistant than the contact sensor on the guitar's body.
04-19-2012, 06:50 PM
Thanks for the pickup suggestions. I listened to the M80 sample and also the M1A sample. If I hadn't already purchased the Para DI I'd probably go with either one of those. But since I do already have the PD, does that essentially turn the M1 into an "active" pickup? And with its built-in EQ, will that overcome what the video calls "the more electric" sound of pure magnetic pickup?
04-20-2012, 02:37 PM
When there is a preamp inside the pickup, as in the case of the M1A and M80, it means you don't NEED a preamp in between the guitar and the amp or PA but it also doesn't mean you can't benefit from one if it offers other useful features.
One major benefit to the active version is that the built-in preamp applies some gain to the signal which will reduce the gain required at the first active stage after the pickup. It translates into a higher signal-to-noise ratio.
In fact, it's very common for players to run active pickup signals through the Para DI or Venue DI devices. You may not need all the available gain but you can use the EQ, FX Loop and the balanced XLR output just as well as if the pickup were passive.
Also, I can say that the gain applied to the raw signal inside the M1A and M80 is not nearly all the signal can handle. There's plenty of room for optimizing gain before overdrive to ensure higher signal-to-noise.
So, having the Para DI doesn't necessarily mean you should get the passive M1. The passive M1 into the Para DI(after 10-12 feet of cable, gain set at half and no EQ applied) is a lot like the M1Active but with a slightly lower signal-to-noise ratio. The Para DI will not cause the M1 or M1A, however, sound like the M80. The M80's body response handles much lower frequencies and therefor is warmer, richer and fuller sounding.
04-21-2012, 07:39 AM
Once again, thanks for all the info. You've been a great help and given me a lot to think about. An active pickup seems the way to go and the M80 is clearly the best choice. Now all I have to do is be patient until my next paycheck or stoop to robbing my daughter's piggy bank !
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