Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[Bob Leonard]

I've been doing a certain amount of tinkering lately, time permitting with the holidays and all. I'm not against applying a fix to the line in side of the amp, and a standard GFI will work to that point. What I have found that concerns me is that there is still enough residual charge left in the PS capacitors to kill you, even after tripping the GFI. I tested this against a few amplifiers and as suspected the charge will last up to 5 seconds, maybe longer.

[John Roberts {JR}]

I've been doing a certain amount of tinkering lately, time permitting with the holidays and all. I'm not against applying a fix to the line in side of the amp, and a standard GFI will work to that point. What I have found that concerns me is that there is still enough residual charge left in the PS capacitors to kill you, even after tripping the GFI. I tested this against a few amplifiers and as suspected the charge will last up to 5 seconds, maybe longer.

That argues in favor of the capacitor coupled ground inside the guitar itself. There is no perfect solution.

My friend who designs guitar pedals and amps says he has heard of people using transformer isolated guitar inputs. I am uncertain about the high impedance guitar pickup tone being unaffected. Guitar pickups can output decent voltage and are sensitive to capacitance. The only good thing in favor of transformer coupling is that the lead guitar LF response does not go very low, but nether side is low impedance so not an obvious audio transformer application. 

They do make transformer direct boxes but these are usually used with bass guitars that have lower source impedance, for lead guitar active direct boxes are preferred.  Another downside to just hanging a DB transformer in front of a guitar amp is that they also scrub off several dB of voltage gain (intended for mic inputs).

My suggested modifications were to protect against a hot external safety ground. To protect against the amp itself you want to bond the chassis to safety ground, which is back were we started from and how modern guitar amps are manufactured today (but that still can kill people in the case of a power wiring fault.).

Do your own calculus... are you more likely to encounter a live mic, or an amp meltdown?

JR

PS: a relay that breaks the guitar cable circuit (both leads) when ground current is detected seems promising while not very practical. These things would all be easier to build into a new guitar amp design, while it might be difficult to get musicians to pay the cost for such improvements. 

[Mike Sokol]

These things would all be easier to build into a new guitar amp design, while it might be difficult to get musicians to pay the cost for such improvements.

The real problem is a culture of performers accepting being shocked. If they demanded a shock-proof guitar and were willing to pay for it, then this thread wouldn't be necessary. So in addition to coming up with an affordable and practical anti-shock solution for new backline amplifiers, we also need to educate performers that NO shock is normal for a modern sound system and backline gear. Of course, antique music gear is highly collected and played (plus it sounds great  ) so an affordable retrofit is needed. But again, without education that any shock can be dangerous and should be avoided, there won't be any push from musicians for a shock-safe solution. Still, a practical design is needed that won't mess with the sound of the amplifiers. I think we're on the right track here...       

[Stephen Swaffer]

.A capacitor discharge is different than a line voltage shock in that it is a DC current vs AC.  The danger thresh hold is different, and while I am sure a cap charged to 150 volts carries a painful "bite", I am not sure how lethal it is in and of itself (the reaction to the "bite" might be more hazardous) .

[Bob Leonard]

The B+ side of these amplifiers is usually in a range above 400V @ 1.5 to 3 amps. There's going to be more than a bite.

JR/Mike,

Looking back, and I mean way back, the path to ground through a PA system was generally more direct. The PA's of the day from Bogan, Shure, Marshall, etc. usually had the mic jacks tied to chassis ground. Combine that with AC plugs that had no ground lug and your chances of being shocked was 50/50. In those days one hand on the strings and a wet finger or tongue to the mic was the test, regardless of the acts level.

Today I don't feel the need to tongue test every mic I lean into as electronics and isolated microphone circuits are the norm. I also understand that new equipment designs don't guarantee you won't be shocked, and I have been. My most recent event was just over a year ago at a club in Somerville, MA. I was standing on a Persian rug, leaned over to check the mic level and was knocked on my ass. I spent the next half hour chasing the ground path until finally I removed the rug, and that is what cured the problem.

My idea would be a GFCI for the AC side and for the guitar side of the amp. I'll keep it simple and suggest an off the shelf GFI in the guitar cable side after all of the effects and just before the input jack. If I get time I'll jury rig a box this week and run some tests.

[John Roberts {JR}]

The B+ side of these amplifiers is usually in a range above 400V @ 1.5 to 3 amps. There's going to be more than a bite.

JR/Mike,

Looking back, and I mean way back, the path to ground through a PA system was generally more direct. The PA's of the day from Bogan, Shure, Marshall, etc. usually had the mic jacks tied to chassis ground. Combine that with AC plugs that had no ground lug and your chances of being shocked was 50/50. In those days one hand on the strings and a wet finger or tongue to the mic was the test, regardless of the acts level.

Today I don't feel the need to tongue test every mic I lean into as electronics and isolated microphone circuits are the norm. I also understand that new equipment designs don't guarantee you won't be shocked, and I have been. My most recent event was just over a year ago at a club in Somerville, MA. I was standing on a Persian rug, leaned over to check the mic level and was knocked on my ass. I spent the next half hour chasing the ground path until finally I removed the rug, and that is what cured the problem.

Not sure I understand.. Static build up from walking on the rug? Damp rug grounded? After living in the south for a few decades I kind of forget all about static, but there was one milk cooler at Walmart that would give a decent static shock when grabbing the metal handle. Probably an open safety ground and an unintentional van de graf generator going on. 

My idea would be a GFCI for the AC side and for the guitar side of the amp. I'll keep it simple and suggest an off the shelf GFI in the guitar cable side after all of the effects and just before the input jack. If I get time I'll jury rig a box this week and run some tests.

I'm not sure I follow but let us know what you come up with and be careful testing it (don't hurt yourself or any amps in the process. )

JR

PS: I still advocate using stock GFCI for back line, but lean toward shunting FOH safety ground to back line safety ground through a few diodes. Now I need to figure out how to avoid shocks while plugging into the safety gadget. 

[Scott Holtzman]

Not sure I understand.. Static build up from walking on the rug? Damp rug grounded? After living in the south for a few decades I kind of forget all about static, but there was one milk cooler at Walmart that would give a decent static shock when grabbing the metal handle. Probably an open safety ground and an unintentional van de graf generator going on.  I'm not sure I follow but let us know what you come up with and be careful testing it (don't hurt yourself or any amps in the process. )

JR

PS: I still advocate using stock GFCI for back line, but lean toward shunting FOH safety ground to back line safety ground through a few diodes. Now I need to figure out how to avoid shocks while plugging into the safety gadget. 

I am going to toss another one at you.  Cheap chinese charge plugged in convenience outlet on power bar at FOH.  Ground connect on DI to iPod.  Connect USB charger, plug in watch phone and wire catch on fire.  So many permutations.

I had almost forgotten about that.

[John Roberts {JR}]

I am going to toss another one at you.  Cheap chinese charge plugged in convenience outlet on power bar at FOH.  Ground connect on DI to iPod.  Connect USB charger, plug in watch phone and wire catch on fire.  So many permutations.

I had almost forgotten about that.

Generally using modern UL approved gear should prevent such hazards. If you buy non-UL products that plug into mains power you put yourself and those around you in harm's way.

JR

[ProSoundWeb Community]