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Old 29th December 2001, 08:46 PM   #21
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yes, well, thank you all, but,

could somebody please answer the shielding question? Like I mentioned at the beginning of the thread itīs somewhat crowded inside my amp.......

william
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Old 29th December 2001, 09:27 PM   #22
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Hello again...

Originally posted by GRollins
> For those who don't remember a certain thread
> from last summer, Keith likes to argue more than > anything else in the world.

Heh!

> As for intuitive...PI filters seem quite
> reasonable to me; make perfect sense.

I never said they didn't. What I guess I'm trying to say is that a properly designed PI filter may be helpful in reducing hum and noise, while a poorly designed one may be less than useless. I'm trying to be helpful by providing a rederence for information.

Keith
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Old 29th December 2001, 09:57 PM   #23
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Hi Wuffwaff,
In my Ampzilla amplifier it is very crowded too.
I managed to find a place on the heatsink as far away as possible from the transformer. As the inductors are ferrite cored the field is concentrated in the coil and they don't seem to pick up hum.
As you can see the inductance of the coils I use it much higher than yours with only slightly higher DC resistance.
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Old 29th December 2001, 10:21 PM   #24
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As for shielding, it'll need to be something ferromagnetic. Iron, nickel, cobalt, or alloys of same (i.e. steel). Then ground it. If that doesn't do the trick, then there's nothing like distance to do the trick--i.e. a remote power supply. Yes, they're a pain in the rump sometimes, but they work.
I don't remember if it's been mentioned yet or not, but try reorienting the transformer this way and that. Transformers do not have uniform radiation patterns and sometimes you can fix things that way. Of course, the underlying assumption is that you've got room to move things around.
Someone--grataku(?)--had a similar problem last spring and ended up moving the offending transformer outboard to cure the problem. Bummer if you hadn't planned on doing it ahead of time, but I believe it got things going for him.

Grey
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Old 29th December 2001, 10:50 PM   #25
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OK, useful information regarding the coils used in PI filters.....

Make an assumption. The coils are largely carrying DC, with an AC component that is due to two factors.
1. The ripple voltage
2. Current varying as the output level of the amplifier varies.

These will cause a varying magnetic field around the coils. If the inductance is low or a non-saturating iron based core is used, the size of this field is likely to also be low. Thus, if the coils are mounted as far as possible away from the input circuitry, then all should be hunky dory.

Due to the small size of the inductances, the cross coupling between the coils should be minimal, but to minimise this, mount the coils at right angles to one another.

Of far greater import are the values of the inductors and capacitors. Take the standard PI filter. The two capacitors and the inductance form a parallel resonant circuit. Under certain dynamic conditions, this circuit may add a ringing voltage to the D.C. output which will affect the amplifier. The requirement in solid state amplifiers for low resistances will exacerbate the problem as the Q or quality factor of the circuit will be high. The higher the Q, the higher the ringing voltage superimposed on the power supply voltage. The cure is to ensure that the inductance is sufficiently large to keep this resonance well below the amplifier's passband. Use the standard formula fo=1/(2 x pi x sqrt(LC)) where C is the value of the two capacitors in series (in farads) and L is the value of the inductance (in Henries). A rule of thumb is to have the resonant frequency down around 2Hz for a 20Hz to 20kHz passband.

This phenomenon may explain why some people perceive a change in the sound characteristics of amplifiers when they add inductances into the power supplies.

Hope this help.

regards, Keith
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Old 29th December 2001, 11:07 PM   #26
jam is offline jam  United States
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Elvis is on the loose again!
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Old 30th December 2001, 12:34 AM   #27
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Originally posted by jam
Elvis is on the loose again!
I thought he was working in a Burger King in Des Moines....
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Old 30th December 2001, 01:08 PM   #28
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Hello Keith,

according to your formula this would mean between 0,3 and 3farad for an inductance of 2 to 20mH. This 0,3 to 3 farad would be the series value of the two capacitors!
Since I would need this 4 times I think the extra case I have to build for the power supply will be to large to fit my living room.

How critical and under wich circumstances will this ringing occur?

william
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Old 30th December 2001, 09:00 PM   #29
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Quote:
Originally posted by wuffwaff

How critical and under wich circumstances will this ringing occur?

william [/B]
That's the problem. Much will depend on the physical characteristics of the components involved. In solid state amplifiers, we go for large wires, low resistors and low ESR values for the capacitors. As stated, these characteristics can cause a high ringing voltage.
When does the ringing occur? Well there are several theoretical causes. Sudden large changes in current requirements can cause a step in the current going through the inductance. This step may excite the ringing. Another possible cause is if the current is varying at the resonant frequency of the PI filter. Another thing to look at is the length of time the ringing lasts once it's started. It may only last for one or two cycles. It may last last for a considerable period . All this will depend on the damping factors, and they're affected by the resistances around the circuit.

When will ringing occur in reality? Who knows. It may be there and you can't observe it.

I remember looking at this some years ago and, in the end put it in the too hard basket. I decided that the physical construction of the power supply had a far greater effect on the system than the addition of a couple of inductors. If you're determined to use inductors, the L filter doesn't suffer from the same problem.

For those of you thinking about valve amplifiers, they too may suffer from the same problem but to a lesser extent. The filtering chokes used usually have high resistances (of the order of several 10s of ohms). There are also often resistors in series with the diodes and sometimes bleeder resistors across the capacitors. There are also sometimes load resistors across the power supplies in order to maintain a minimum load as some filters work best with a minimum DC load.

Then there is the standard RC filter used in many situations. In combination with lead inductance of wires and PC board tracks, these can also ring.

The design and implementation of power supplies is in no way simple. If you modify an already designed PSU, you really need to know what you're doing.

Enough of my blathering. Have a great New Year.
regards, Keith
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Old 30th December 2001, 09:38 PM   #30
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Actually an oscilloscope and a square wave at the input will do a very nice job of detecting any problems with ringing.
In my experience, it's absolutely a non-issue. In theory, it can happen, but in practice I've never built a circuit that had a problem with a ringing power supply. The only thing I've ever had come up was motorboating if too many gain stages were run off the same rail, but that's a no-brainer to detect and cure. Oh, and oscillations with chips, but that's easy to cure also (...don't use chips, duh...just kidding).
When I first started keeping bees, I read over twenty books on the subject. Every one of those books had at least one chapter on diseases and pests that can effect your bees. Dozens of nasty possibilities: American foul brood, European foul brood, tracheal mites, varroa mites, chalk brood...the list goes on and on. It's intimidating. You know how many of those problems have ever cropped up here? Two. Varroa mites and the small African hive beetle, which is such a new problem that it wasn't even in the books. The moral of the story? Too much theory about problems that <i>might</i> happen will drag you down. The only cure for too much theory is to leaven it with hands-on experience. That way you learn what really is important.
The only ringing you'll get is if someone calls on the phone while you're soldering.

Grey
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