Want to build a SS PA

[J M Fahey]

Quote Originally Posted by Steve Conner View Post
Yeah, think about it dude. There's no winding on a Champ PT that would supply a SS power amp.

am i allowed to use an output transformer?

Of course you are!!
You only need to build the approppriate topology SS power amp and you can use both Champ Power and Output transformers.

[kg]

Of course you are!!
You only need to build the approppriate topology SS power amp and you can use both Champ Power and Output transformers.

all right!

well, i'd start with a couple of these http://www.st.com/internet/com/TECHN...CD00149569.pdf
or any HV mosfet with reasonable gate capacitance.

have to rig up a small voltage divider to generate a positive bias of a couple of volts for the gate, and heatsink the mosfet, but it should work pretty well.

[R.G.]

Chia Yu Co Ltd at Apex Jr is a dual secondary. Are you suggesting that I connect the 2 non-center-tapped secondaries as 1 center-tapped secondary? So connect the lower end of the top primary winding with the top of the lower winding, thus making a center-tap? I'm a bit unclear here.

Yep, that's right. There is a bit of technical detail to be observed - you have to connect the two windings so that they are phased properly.

To do this, you hook the "non-dot" end of one to the "dot" end of the other. Manufacturers use this dot convention to indicate the end of a winding that goes positive (or negative) at the same time. IF the info sent with the transformer does not tell you which is which, hook up the secondaries in a test connection in series. If the voltage on the non-connected ends is twice the voltage across one secondary, they're correct. If they are wrong, the voltage across the non-connected ends will be nearly zero. Do this test with secondaries not connecting to anything else, of course.

[Steve Conner]

Excellent find KG! I see they're in stock at Farnell. I must get one and substitute it for an EL84 to see what happens.

[lowell]

Thanks RG! I was aware of the possible phasing issues, but glad you chimed in as I would not know how to test for that.

[lowell]

Where can I get hardware for the apex Jr toroid? You know, the large metal disc washer and rubber washer?

secondly, how do I go about knowing if my chassis is a suitable heatsink for the LM3886? It's a hammond aluminum one.

[lowell]

Where can I get hardware for the apex Jr toroid? You know, the large metal disc washer and rubber washer?

secondly, how do I go about knowing if my chassis is a suitable heatsink for the LM3886? It's a hammond aluminum one.

[R.G.]

Where can I get hardware for the apex Jr toroid? You know, the large metal disc washer and rubber washer?

Well, for mine, I bought a surplus mouse pad and cut out neoprene foam washers from it. Then I took an old piece of 1/8" thick aluminum and bashed a dimple in the middle with a ball peen hammer on a sandbag (gently, many light taps), then drilled a hole in the center of the dimple for the screw, and then hacksawed a concentric circle around the dimple/hole. I have several of those transformers. Great value.

Another way might be to coil up corrugated cardboard to stuff the center hole and keep the toroid centered over the bolt. That would let you use a flat top plate. Do NOT let anything connect from a bolt+top plate to the chassis. Many sparks and smoke result.

secondly, how do I go about knowing if my chassis is a suitable heatsink for the LM3886? It's a hammond aluminum one.

The simple way is to clamp a power resistor +heat goo inside the chassis, and feed it current from a power supply. This makes it easy to determine the power in. You feed power in, and measure the box temperature right under the power resistor. This tells you degrees of temp rise per watt of dissipation. Convert to degrees C/W and you can then make educated guesses about it from the LM3886 datasheet specs. Another way is to look at the LM3886 datasheets for the power supply voltages and loads you'll use, and get watts. Then run the power resistor at that power, measuring the temperature. If the temp is more than about 55C, it will be too painfully hot to touch, although the LM3886 *may* be OK.

Another way is to mount it, run it up to full power, and see if the LM3886 turns itself off because it's too hot. If it does not, try (carefully!) to put your fingertip on the case right over the LM3886. If you can do this without burning yourself, the temp is under 130F/55C and is OK.

[lowell]

Another way might be to coil up corrugated cardboard to stuff the center hole and keep the toroid centered over the bolt. That would let you use a flat top plate. Do NOT let anything connect from a bolt+top plate to the chassis. Many sparks and smoke result.


The simple way is to clamp a power resistor +heat goo inside the chassis, and feed it current from a power supply. This makes it easy to determine the power in. You feed power in, and measure the box temperature right under the power resistor. This tells you degrees of temp rise per watt of dissipation. Convert to degrees C/W and you can then make educated guesses about it from the LM3886 datasheet specs. Another way is to look at the LM3886 datasheets for the power supply voltages and loads you'll use, and get watts. Then run the power resistor at that power, measuring the temperature. If the temp is more than about 55C, it will be too painfully hot to touch, although the LM3886 *may* be OK.

Another way is to mount it, run it up to full power, and see if the LM3886 turns itself off because it's too hot. If it does not, try (carefully!) to put your fingertip on the case right over the LM3886. If you can do this without burning yourself, the temp is under 130F/55C and is OK.

Rg can you please explain the part about the not connecting the bolt or "top plate" to the chassis? I don't understand why this would be a problem?

thanks for all the other great tips!

[Steve Conner]

Corrugated cardboard, is that UL compliant?! ;-O A cork from a Jack Daniels bottle might have more rock mojo anyway.

When mounting a toroid transformer you must avoid making a loop of metal that passes through the centre hole. It will be a shorted turn capable of generating hundreds of amps and mucho magic smoke.

Fun experiment: If you mount a toroid in the usual way, you can take your DVM set to AC voltage and measure between the chassis and the big metal washer on top of the transformer. You will see a few hundred millivolts AC. This is the voltage corresponding to one turn, that you mustn't short out.

[J M Fahey]

What's the size and sheet metal thickness of your "Hammond aluminum chassis"?
How many LM3886 do you want to mount there?

[lowell]

Corrugated cardboard, is that UL compliant?! ;-O A cork from a Jack Daniels bottle might have more rock mojo anyway.

When mounting a toroid transformer you must avoid making a loop of metal that passes through the centre hole. It will be a shorted turn capable of generating hundreds of amps and mucho magic smoke.

Fun experiment: If you mount a toroid in the usual way, you can take your DVM set to AC voltage and measure between the chassis and the big metal washer on top of the transformer. You will see a few hundred millivolts AC. This is the voltage corresponding to one turn, that you mustn't short out.

A loop? I'm totally confused by this. I swear I've worked on many an amp with a toroid that had a metal disc and a bolt running straight through the center of the transformer to the chassis. Am I taking crazy pills?

Like the hardware here:
http://fivefishstudios.com/index.php...d=47&Itemid=64

[lowell]

Okay okay I get it now. You guys are saying that the bolt and plate cannot touch the top and bottom of the CHASSIS. Right? That I get. Please confirm.

[kg]

Okay okay I get it now. You guys are saying that the bolt and plate cannot touch the top and bottom of the CHASSIS. Right? That I get. Please confirm.

yes. no electrically continuous path through the center and around the outside of the donut.

[R.G.]

Corrugated cardboard, is that UL compliant?! ;-O A cork from a Jack Daniels bottle might have more rock mojo anyway.

Actually, that was my first choice. 8-)

It's distinctly NOT UL compliant to let the toroid slide sideways and skin it's insulation on the bolt. 8-)

I kind of like the toroids that have the center poured full of epoxy or some such with a hole in the middle. I've never liked the metal dish stuff much.