Transformerless Amp Safety

[atmars]

I know this topic has been talked about before here (I found some info already), but I'm looking for clarification. I guess it is a good topic to cover again because of it's potential for fatal consequences.

So my question is: I just got an amplifier (filmosound) with no power transformer. I didn't know such a thing even existed. Can this amplifier be used for guitar safely with modifications, or should i just take it out to the driveway and beat it into dust with a hammer before it kills someone?

I've read so far about these topics but cannot seem to find a definitive answer.

Isolation transformer: If this is a permanent part of the setup will it protect the user? If so, how exactly does it work? Is there more than one kind? I've heard of a version that has a ground the passes across the windings and one that doesn't. Any input on which is best?

GFCI: If a consumer GFCI was wired into the enclosure would this protect the user?

Polarity and grounding: Old radio sites say to put a polarized plug with the hot side interrupted by the switch for a margin of safety. Some say a three prong grounded plug can be added others say it cannot. What is correct and how can it be accomplished?

Can all or any of these methods be used together?

Please help me avoid being the guy who is "just smart enough to be dangerous" .

Thanks

[Enzo]

Transformerless amps simply rectify the AC mains as it comes in. Now there is DC, but it is directly connected to the mains. If you ground the negative side of that DC to chassis like most amps, then the chassis is DIRECTLY connected to the wall outlet. Depending on which way the plug is in the outlet, the chasis could be hot - like 120VAC - or 240.

Now one side of the mains is neutral - at "ground" voltage - in a wall outlet. The longer of the two slots. SO one COULD wire a three wire power cord onto the amp so the neutral side is always on the chassis. Except that relies on the wall outlets being wired correctly. Believe me they are wrong more often than you'd think. if the outlet is wrong, your amp becomes a dead short across the mains circuit.

Normal transformer supplies are safe - at least in regards to hot chassis - because there is no direct connection from the mains to the circuits. The mains go into the transformer winding, and other wiindings supply the circuit.

Do not confuse ISOLATION transformers with STEP-UP/STEP-DOWN transformers. Some steppers also isolate, but some are single winding things which do not. SIngle windings will NOT protect you at all. They are called AUTOTRANSFORMERS or sometimes AUTOFORMERS.

Isolation means the input and output of the transformer are separate windings of wire around the core. No direct connection, hence isolation. REgular power transformers do provide isolation by their nature. An isolation transformer is specially made for the purpose, and usually has a 1:1 voltage ratio. SO 120v in, 120v out. Connect the circuit to the iso, and it still gets the 120VAC it wants, but now that 120V is no longer directly connected to the mains.

SO there are a couple choices. An isolation transformer could be put in front of the amp, and the amp plugged into it instead of the wall. Then use the amp as always. Alternately, add a power transformer to the thing. Iso will make the amp safe from power line exposure.

SInce the circuit is now isolated from the mains, we can ground the chassis. SO a true iso can ALSO have a ground from the wall outlet. ONly the mains voltages themselves are isolated. Usually isos with earth ground are like a finished product with actual wall-type outlets on it and a power cord. You can also buy just plain old iso transformers - like a electronic component. That would be a plain old transformer you wire into the citrcuit and solder. YOu can ground the chassis then, but the transformer itself in that case would not have a ground of its own.

I wouldn't rely on a GFI to do anything useful here.

ONCE IT IS ISOLATED I highly recommend a three wire power cord be installed. POlarized two wire plugs are dependent on the outlet being right. if it is backwards, your amp becomes deadly.

[cbarrow7625]

100% correct advice Enzo! I second your recommendation.

Use a 1:1 isolation transformer that passes earth ground and add a 3-prong power cord to permanently gound the chassis.

Don't use this amp without a chassis ground. If the wall plug is backwards you will have 120 V (or 240 V, depending on where you are) directly on the chassis. Certain death for you (or anyone else) who touches the amp (or grounded-connected guitar strings) and, say, a microphone that is properly grounded. Live AC into your arms, through your heart & out your lips. Not pretty....

[kg]

unless it's a battery-powered amp, there is no such thing as transformerless amp safety.

[Dave Curtis, dB AudioTech]

I wouldn't rely on a GFI to do anything useful here.

Actually, it would hurt like the dickens if it were ever called upon to save your life. And your amp would shut off.

[hasserl]

Certain death for you (or anyone else) who touches the amp (or grounded-connected guitar strings) and, say, a microphone that is properly grounded. Live AC into your arms, through your heart & out your lips. Not pretty....

Not certain death. Is death possible, yes (but then death is always possible, no?); but not probable. In fact, can anyone point to a single instance of death by electrocution from a transformerless amp? I don't mean to downplay the importance of safety, but when we overexagerate things it also has a way of degrading the importance. Let's keep things in perspective.

BTW, someone playing through one of these tranformerless amps would not likely be using a microphone. You don't really need to amplify your voice to be heard above one of these amps, we're talking about 1 watt of power or so.

[Wakculloch]

I personally know someone killed at a gig from the live chassis on guitar amp, touch microphone, death scenario.
It aint pretty, and totally avoidable with a trip switch feeding on stage power.
Transformerless amps are insanely dangerous.

[trevorus]

I would only use a transformerless amp for home audio stuff... Nothing where I might become part of the circuit. Transformers=good.

[atmars]

I would only use a transformerless amp for home audio stuff... Nothing where I might become part of the circuit. Transformers=good.

If I put a 1:1 isolation transformer in between the amp and the power supply, doesn't that mean it's no longer a transformerless amp?

[atmars]

100% correct advice Enzo! I second your recommendation.

Use a 1:1 isolation transformer that passes earth ground and add a 3-prong power cord to permanently ground the chassis.

Don't use this amp without a chassis ground.

I realize you haven't seen the circuit (I could post it if anyone cares), but I believe you are saying after the iso is in place I can attach the ground wire of an added three prong plug to the chassis and the amp will be grounded and the circuit will function normally?

Sorry to be such a dunce - my only previous amp mod experience has been with the Valve Jr. and there is an enormous amount of info available so its not so easy to get lost


thanks

[Enzo]

Yes, it would no longer be a transformerless amp. The lack of a transformer is what makes it dangerous. There is no inherent advantage for not having a transformer other than cost to the people who originally made the thing.

I realize you haven't seen the circuit (I could post it if anyone cares), but I believe you are saying after the iso is in place I can attach the ground wire of an added three prong plug to the chassis and the amp will be grounded and the circuit will function normally?

Exactly.

[kg]

There is no inherent advantage for not having a transformer other than cost to the people who originally made the thing.

well, they ARE a lot lighter without all those pesky cores weighing it down...

[Dave Curtis, dB AudioTech]

I personally know someone killed at a gig from the live chassis on guitar amp, touch microphone, death scenario.
It aint pretty, and totally avoidable with a trip switch feeding on stage power.
Transformerless amps are insanely dangerous.

I'm guessing by "trip switch", you mean a GFCI device.

Even though I don't have a "hot chassis" amp, I keep a non-contact voltage tester in my gig-kit, similar to this:

http://www.harborfreight.com/cpi/cta...emnumber=97218

Probe the chassis after you plug it in; if it glows, flip the plug in the receptacle.

Even the ultra cheap neon tester will tell you if your chassis is hot. It's way cheaper than a properly-sized isolation transformer, and much less painful than a GFCI. Here's one:

http://www.electricsupplyonline.com/...14_w000849.php

And some neon tech-stuff related to this thread:
http://www.tinaja.com/glib/muse136.pdf

The iso-transformer is the safest way, without using a sledgehammer.

[Enzo]

well, they ARE a lot lighter without all those pesky cores weighing it down...

All of a sudden I have visions of an SVT portable record player with little Disney characters painted on it.

[Arthur Dent]

What tubes are in it?

I've added transformers to a couple of hot chassis amps. Much less importantly than the iso transformer, adding a filament transformer and switching to tubes with conventional heater voltages will help to lessen the loud hum which I'm sure this amp has.

If this is like the first amp I modded with a 12ax7 preamp, 50L6 power, and 35Z5 rectifier, then you just need a 6.3 VAC 1A+ transformer, a 6V6 and a SS diode. Somewhat "brown" tone, kind of nice. I've seen a couple amps like this.

If it's got a 12au6, 50C5, and some rectifier tube like my Kent amp did, then you have an amp that can potentially sound absolutely awesome. I think these are somewhat common. Replace the 50C5 with a 12C5 (or 6CU5 and the 12au6 with a 6au6), and find a proper rectifier tube. Mine probably has the best tone of any amp I've yet encountered, VERY smooth distortion, a warm, sweet, chocolaty tone that normally doesn't get muddy.

The setup in mine is a 250Vct transformer with a 6.3vac winding, EZ90 full-wave rectifier, 6AU6 preamp tube and 6CU5 power tube. A bit noisy, but the tone is so worth it!