Safety considering output of amplifier - Page 1
Forum section: electronics / beginners

author: TNb
author_rank: Regular Contributor
timestamp: July 07, 2014, 04:42:31 pm
content: Hi! Few months ago I built a attenuator for my tube guitar amp(100W).It works well, everything is fine, but when I was designing it I omitted few things because I don't really have knowledge about it.1) So, at first, since I have a stuff that possibly can have 125 volts on input it is pretty scare to think of touching the wires inside.The problem is that I don't know how to do a beautiful metal case for this without having this case touching wires and kill myself.Other attenuators on the market have metal cases, so there is a way to have it safe.How to do that? I obviously can't get access to ground through mono jack... have no ideas.For now I just made a wooden case, eh...here's example of commercial attenuator - as you see it's metal.2) The second problem is that I want to protect the output transformer of amplifier from burning in case of high current, so I put the fuse on the output.... but what output should I fuse? I mean it's two wires coming out, which do I need to put the fuse on? I suppose it makes not much difference, but there must be an explanation how to do better. For now I have both wires with fuses, which is probably stupid, but anyway...Here's some photos

author: xwarp
author_rank: Frequent Contributor
timestamp: July 07, 2014, 04:51:32 pm
content: Just curious, wouldn't an L-pad work for you?

author: katzohki
author_rank: Frequent Contributor
timestamp: July 07, 2014, 04:59:08 pm
content: If you have 125 Volts coming in I assume that's mains voltage. You would connect GROUND to your metal case, that way if any exposed wires touch the metal case they are shorted to ground. It will cause a short and probably blow a fuse / throw a breaker, but it will fail SAFE.Also, don't let any wires touch the metal case!

author: TNb
author_rank: Regular Contributor
timestamp: July 07, 2014, 05:38:59 pm
content: Quote from: katzohki on July 07, 2014, 04:59:08 pmNooo, I'm in Europe, mains are 220V here125 V is the output of amplifier on the full volume and attenuation turned to maximum(I have a potentiometer there). Sorry, didn't make it clear...Quote from: xwarp on July 07, 2014, 04:51:32 pmEh... It was quite time ago, I don't remember exactly why I decided not use just simple L-pad, I think it was because in this case I needed a super-powerful resistor(pot) and it will heat like hell on the full power going in. Now I have a L-pad resistor and one big-ass high power resistor like that on the image(not that one, just for pic) and the thing remains relatively cold.

author: homebrew
author_rank: Frequent Contributor
timestamp: July 07, 2014, 05:39:50 pm
content: Some questions and suggestions:1) Where does the 125V input come from? If I understood you correctly your device is a box that goes between the output of the amp and the speaker. You talked about an output transformer at the output of your tube amp. If this thing is correctly built (one can never be sure ...) the output of the amp might not be ground referenced at all...As with all other devices having a metal case there are basically two options: Either earth the whole case (including every touchable part), or apply double insulation. The first one might not be option for you as you don't have ground (PE) available. Putting on double insulation seems to be a good idea. Hence extra pvc flexible tubes goes over the wires and extra heat shrinking tube goes over every contact. In essence every component should be insulated twice so that if one layer of insulation fails, the other one is still there. Also pay attention to fixate the wires that they can not touch the case if a solder joint breaks.2) When in unattenuated mode (R1=4R and R2=8R) you are dropping serious amounts of power on those resistors. So you should pay attention to cooling. Mounting them in a wooden case might not be an ideal solution!3) Why are you using Mono-Jacks at all? I know that some vendors do so but this is a very bad connector for that purpose i my opinion. Use Speakon-Connectors instead. Insulated, Locked, rated for high currents, can't be confused with microphone cables and very robust on stage.

author: TNb
author_rank: Regular Contributor
timestamp: July 07, 2014, 05:49:36 pm
content: Quote from: homebrew on July 07, 2014, 05:39:50 pm125V is from the output of amplifier on the maximum volume and on maximum attenuation(adjusted by pot). So on the input of the pox I have signal to the speakers that is approximately 125V.Quote from: homebrew on July 07, 2014, 05:39:50 pmyeah, I calculated the power and I use high-power resistors that are enough to handle that. Also since it's 2 resistors and not one L-pad thing it is distributed and doesn't heat so much.Quote from: homebrew on July 07, 2014, 05:39:50 pmMono-jack is standard for all guitar amplifiers.Quote from: homebrew on July 07, 2014, 05:39:50 pmeh.... this is the good idea, though I was hoping there is some other solution. Double-insulating everything is pain in the assanyway, thanks for a great answer

author: IanB
author_rank: Super Contributor
timestamp: July 07, 2014, 05:55:29 pm
content: 1. Voltage will only kill you if it makes a circuit through your body.2. The output from an amplifier, especially if it comes from an output transformer, is likely floating.3. If the output is floating, it won't hurt you unless you touch both speaker terminals at the same time while powered. (Don't do that.)4. If you put something in a metal case, make sure no wires touch the inside of the case. It's not hard. An internal case short could easily damage some circuit components.5. 100 watts into 4 ohms is about 20 volts RMS. Where does this 125 V come from?

author: TNb
author_rank: Regular Contributor
timestamp: July 07, 2014, 06:08:32 pm
content: IanB, thanks for clearing outQuote from: IanB on July 07, 2014, 05:55:29 pmOn full volume and the R2 pot on max the impedance of R1(4ohm) goes to 155 ohms and overall impedance is equal to 157 ohms. Hence V=sqrt(100*157)=125 volts. Isn't it like this in AC circuit? Or do I say something dumb?

author: IanB
author_rank: Super Contributor
timestamp: July 07, 2014, 06:18:14 pm
content: For correct analysis you need to ignore the attenuator for the moment and look at the amplifier specs. What is the specified output of the amplifier into what loads?The amplifier cannot do more than it is designed to do.If the amplifier is designed to deliver 100 W into a 4 ohm load, it cannot deliver 100 W into a 150 ohm load. The output voltage of the amplifier is more or less limited to what it needs to be for the design.For instance, if you gave it a 150 ohm load, the maximum output power to a first approximation would be about 3 W (= 4/150 x 100).

author: TNb
author_rank: Regular Contributor
timestamp: July 07, 2014, 06:24:02 pm
content: oh... damn, you are right, it's not 100W in this case... don't know how I missed thatthanks! So, it's pretty much ~20-25 volts on output... maybe I need to check it with oscilloscopeaaa, yeah - what about the fuses? or maybe I can forget about them if it's not 125V as turns out...Quote from: TNb on July 07, 2014, 04:42:31 pm

author: IanB
author_rank: Super Contributor
timestamp: July 07, 2014, 06:31:25 pm
content: The need for a fuse doesn't much depend on the voltage, it depends on how easy it is to damage the amp if you accidentally short the output. Some amplifiers have overload protection to protect themselves if this happens. If yours doesn't, you could try putting a 5 A slow blow fuse in one of the output wires.

author: TNb
author_rank: Regular Contributor
timestamp: July 07, 2014, 06:40:27 pm
content: Quote from: IanB on July 07, 2014, 06:31:25 pmThanks

author: IvoS
author_rank: Frequent Contributor
timestamp: July 07, 2014, 08:36:06 pm
content: Can someone explain to me what kind of advantage this load has compared to potentiometer attenuation on amplifers input?

author: homebrew
author_rank: Frequent Contributor
timestamp: July 07, 2014, 08:52:07 pm
content: Quote from: IvoS on July 07, 2014, 08:36:06 pmBecause you want the amp to be working at its maximum to get the distortion sound typical for guitars...

author: TNb
author_rank: Regular Contributor
timestamp: July 07, 2014, 09:38:29 pm
content: Quote from: IvoS on July 07, 2014, 08:36:06 pmIt is mostly for tube amps.Tube amps tend to sound better on max power because tubes get hotter and somehow the sound gets better(debatable).But on max volume amp is freaking loud, that's why you need attenuator to play quite and with good sound.Though attenuator doesn't solve all problem concerning "ideal" guitar sound since on low volume the 12" speaker behaves differently than on loud volume.well... that's debatable, some people tend to say it doesn't make any difference, but for me sound is actually slightly better

author: IvoS
author_rank: Frequent Contributor
timestamp: July 07, 2014, 10:23:46 pm
content: Quote from: homebrew on July 07, 2014, 08:52:07 pmThat kind of doesn't make a sense to me, because you can easily overdrive the input stage and achieve the desirable distortion that way.

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 07, 2014, 11:07:26 pm
content: Quote from: IvoS on July 07, 2014, 10:23:46 pmYeah, on one level, deliberately causing distortion doesn't make sense to many of us.OTOH, overloading the front end gives a different effect than over-driving the output stage.Over-driving various stages has become an important "performance effect" for electric instruments.You can't argue with "art"!125V into 8 ohm implies a 2000 watt amplifier. WOW! Even those guys with the "wall of sound" probably werent' using 2KW guitar amps!

author: TNb
author_rank: Regular Contributor
timestamp: July 08, 2014, 08:59:15 am
content: Quote from: Richard Crowley on July 07, 2014, 11:07:26 pmyaeh, we discussed above that I miscalculated this 125V, it's 25V, sorry