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Step 11Installation
Ok, now to hook everything up...
Fix the module in place
Yep. That means attaching the module somewhere inside the cabinet. I used wood screws; whatever is adequate will work. Mounting it some distance from the chassis is fine, and might be advantageous in some circumstances.
Attaching the earth ground (from the three-prong plug & cord)
An important safety feature in any amp is a valid external earth ground. This helps protect the amp (and the player) in a very simple way:
Should any parts fail, or any connections loosen and cause a short circuit, the ground wire provides a "safe" current path, while ensuring that the current flow from a short will also blow the fuse. If the fuse blows, you know there's a problem to fix. And you won't be using potentially dangerous equipment.
The center prong wire from the three-prong cord is the earth ground. In the US, this should be the green wire. Test it anyway, to be sure.
Connect it directly to the chassis. It does not go through the isolation transformer.
Connect the Power switch
Route a two-conductor wire from the switch on the front panel, down to the incoming AC line. Line cord, like the type used in lamps or extension cords works fine. Buy it by the foot at hardware and home improvement stores (Home Depot, Lowes, etc.)
Drill a hole through the chassis if necessary (I did.) Install a rubber grommet in hole, to prevent the wire from rubbing across the chassis, a creating a short circuit.
Route the wire away from the signal path, if possible.
Connect the transformer secondary to the amp
As discussed in the "half-wave" step, there are several way to do so.
But in any case, a double-conductor wire should be connected to the RED secondary wires on the isolation transformer. The wire can then be fed through the chassis using the original power cord entry hole.
Add the solid-state bridge rectifier
This is discussed in depth in Step 8, and schematics are included. Check the photo below for a wiring example.
A bolt-on type of rectifier was used. A new hole was drilled in chassis to accept the mounting bolt.
Once soldered in place, heat-shrink tubing was added.
Fix the module in place
Yep. That means attaching the module somewhere inside the cabinet. I used wood screws; whatever is adequate will work. Mounting it some distance from the chassis is fine, and might be advantageous in some circumstances.
Attaching the earth ground (from the three-prong plug & cord)
An important safety feature in any amp is a valid external earth ground. This helps protect the amp (and the player) in a very simple way:
Should any parts fail, or any connections loosen and cause a short circuit, the ground wire provides a "safe" current path, while ensuring that the current flow from a short will also blow the fuse. If the fuse blows, you know there's a problem to fix. And you won't be using potentially dangerous equipment.
The center prong wire from the three-prong cord is the earth ground. In the US, this should be the green wire. Test it anyway, to be sure.
Connect it directly to the chassis. It does not go through the isolation transformer.
Connect the Power switch
Route a two-conductor wire from the switch on the front panel, down to the incoming AC line. Line cord, like the type used in lamps or extension cords works fine. Buy it by the foot at hardware and home improvement stores (Home Depot, Lowes, etc.)
Drill a hole through the chassis if necessary (I did.) Install a rubber grommet in hole, to prevent the wire from rubbing across the chassis, a creating a short circuit.
Route the wire away from the signal path, if possible.
Connect the transformer secondary to the amp
As discussed in the "half-wave" step, there are several way to do so.
But in any case, a double-conductor wire should be connected to the RED secondary wires on the isolation transformer. The wire can then be fed through the chassis using the original power cord entry hole.
Add the solid-state bridge rectifier
This is discussed in depth in Step 8, and schematics are included. Check the photo below for a wiring example.
A bolt-on type of rectifier was used. A new hole was drilled in chassis to accept the mounting bolt.
Once soldered in place, heat-shrink tubing was added.
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To ballpark: amperage = watts / volts. A 40 watt amp (including 5% added for the iso transformer) is 40/120 = .333 amps. Might have to use a 0.5 amp fuse.
If the setup works witha .35A or a .25A fuse, then that's even better...
If you were to run the power through the rectifier first, and feed the filaments DC with a series resistor to drop the extra voltage, do you need the extra bulk and expense of the isolation transformer?
-- Ohms law-- current = watts / voltage.
-- Rectified voltage is about 170 V (120 * 1.4)
-- This isolation transformer limits at 50 watts (50 VA, but close enough).
-- a 1n4007 diode bridge has a 1A limit (but it may take time to fail).
Iso transformer draw: 50 watts / 170V = 0.294 Amp max
1N4007 draw: 1 Amp max
Other rectifying diodes may draw 3 amps or more...
The issue is, that as we then earth (or ground) the chassis anyway, and the circuit ground is generally tied to that, we have just re-referenced our de-referenced ground anyway, which makes me wonder why the heck I'm using an isolation transformer.
According to the math above, if I were to use a 200VA transformer, the amp is suddenly dangerous again? There's got to be something we're missing here?
Don't get me wrong--enough current can pass through a 50VA iso transformer to do considerable damage. It's hardly a harmless amount of current. But there's a LOT more potential current in the mains...with or without a rectifier.
When people are often partly insulated by clothing, shoes, etc., it can make a huge difference.
Earth and neutral are NOT interchangeable. They are usually close in relative potential, but they are not the same. In fact, I bet that's one of the 'ground faults' that ELCBs, GFIs and RCDs trigger on...
Where are you planning on getting the earth reference for the chassis if an earth / neutral connection is by definition a "fault?" (neutral being connected to ground through a full-wave bridge rectifier, or directly with a half-wave rectifier.)
Also those devices also introduce a level of complexity, with their own set of faults-- they can fail themselves. Simply wiring one up incorrectly can render it useless.
Personally, I would never depend on one alone--especially not when I'm holding the signal ground in my hand. I might use a GFI together with a transformer, though.
Not to mention that you're assuming that all wall sockets are wired correctly... And that other people's equipment is safely wired and referenced.
Nothing is set in stone...witness newer switching power supplies which forgo isolation (although all have sophisticated current sensing circuitry). But how many of those power supplies are used in applications where the the user is literally grabbing on to one pole of high voltage?
Transformers limit current by their nature. If you try to pull too much current, the core saturates and that's the limit.
Diodes have a current / voltage limit, but they can exceed that for a time before they fail. Too long a time to protect from electrocution. Diodes can also fail "closed," which would limit NO current. Could still result in a "hot" chassis in the worst case, if only one or two diodes in a rectifier fail closed (and other safety changes weren't done).
Of course, both can fail "closed"...transformers can short internally. And that's why we change the other stuff... Like adding the fuse (and the earth ground). But a transformer short would prevent any current passing beyond that short.
A fuse can't react quick enough to prevent a serious (fatal) shock. It's there to prevent damage to the amp and act as a fault indicator.
Maybe I should drop in an ELCB prior to the fuse?
Most of the amps that are missing power transformers are pretty small, so a small iso transformer isn't very expensive... This 50VA model was only about $12 USD. I think that's pretty reasonable.