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Old 9th December 2010, 05:32 AM   #1
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Default options for high amperage voltage regulation

The Pass F5 wants 24V with 6A continuous and 10A peak.

Yes, I should be an engineer but I am not. I have been burrowing through these threads for a few days and have found options to take my 24V AC transformer and regulate the 34V DC (or so) it will become but all the devices I find are 2A at most. I looked on Digikey and Mousers. I have not found a DIY thread for high amperage PS, but reading the PS for DAC and Pre-Amps has given me a pretty good lay of the land. I don't fancy using resistors to drop the voltage either. Do I have to buy 18V transformers or is there a regulator that can manage more current?
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Old 9th December 2010, 01:27 PM   #2
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No, you don't need to be an engineer. You just need to be able to learn. Asking for help means you are on your way. Good on you.

Take a look in the application notes for most 3 pin regulators and you'll find instructions for adding a larger transistor to increase the current capacity. It's really pretty simple.

A discrete regulator that I have built on breadboard several times can be found in here: http://www.passdiy.com/pdf/a75p2.pdf. Follow the same scheme as with a chip regulator to increase the current capacity using devices like MJL21193/21194 or MJL3281/1302 as boosters.

Practically, to pass that much current you'll need to parallel 3-4 high power devices. Use emitter/source resistors to help balance the current. You'll have ~32V after the rectifiers and maybe 30V after a CRC filter. Average dissipation will be around 36W per rail. Be sure to provide plenty of heat sinking.
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Old 9th December 2010, 01:34 PM   #3
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Quote:
Originally Posted by brucegseidner View Post
The Pass F5 wants 24V with 6A continuous and 10A peak.
A single channel F5 is biased to 1.3A.
A two channel F5 needs 2.6A continuous for it's output bias.
The peak requirement depends very much on the speakers and the music but could be up to 20Apk per channel.

Look up the PASS site for various regulated PSUs for his ClassA amplifiers.
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Old 9th December 2010, 02:46 PM   #4
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While exploring these helpful options I found a discrete regulator that might be appropriate. The LM338 is rated at 5Amps.

LM338 - 5 Amp Adjustable Regulator

The ratings I listed of 6Amps continuous and 10Amps peak were found in the F5 manual but I may have quoted those numbers out of context and without understanding their significance.

1.3 Amps is a less intimidating number. And, my interests are in very efficient full range drivers in large horn cabinets. No disco array type speaker builds are foreseen in my future.

Is there any downside to using a simple device like this to regulate the voltage before it goes to the conventional unregulated capacitor PS of the F5 amplifier? Other than reducing the voltage does it introduce problems that a native 18v transformer would not pose to the passive capacitor power supply filter of the F5?
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Old 9th December 2010, 06:03 PM   #5
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Would you be happy if your amp clipped anytime the total draw wanted to exceed 5A? That integrated regulator (all in once case, discrete is when you make it with a bunch of separate parts) has self protection current limiting.

Another engineering consideration is that the maximum current is limited by thermal considerations. From the data sheet, your maximum junction temperature is 125C. Drop it to 100C for reliability. Say your sinks are sized like Papa recommends, giving you 55C sinks. Now you are limited to a 45C rise from sink temperature to junction. What is the thermal resistance of the T package? 4C/W, so you are limited to 11W dissipation. But you need up to 60W. Time to boost it with a bigger pass transistor or 4.

High current regulators are tough beasts to tame.
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Old 9th December 2010, 06:06 PM   #6
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Quote:
Originally Posted by BobEllis View Post
Would you be happy if your amp clipped anytime the total draw wanted to exceed 5A? That integrated regulator (all in once case, discrete is when you make it with a bunch of separate parts) has self protection current limiting.

Another engineering consideration is that the maximum current is limited by thermal considerations. From the data sheet, your maximum junction temperature is 125C. Drop it to 100C for reliability. Say your sinks are sized like Papa recommends, giving you 55C sinks. Now you are limited to a 45C rise from sink temperature to junction. What is the thermal resistance of the T package? 4C/W, so you are limited to 11W dissipation. But you need up to 60W. Time to boost it with a bigger pass transistor or 4.

High current regulators are tough beasts to tame.
If you put say 20.000uF AFTER the reg, you can easily handle the peak requirements while the reg keeps the cap topped up.

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Old 9th December 2010, 06:19 PM   #7
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Ahh, why didn't I think of that? Guess that's why you're the man, Jan.
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Old 11th December 2010, 04:51 PM   #8
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Quote:
Originally Posted by AndrewT View Post
A single channel F5 is biased to 1.3A.
A two channel F5 needs 2.6A continuous for it's output bias.
The peak requirement depends very much on the speakers and the music but could be up to 20Apk per channel.

Look up the PASS site for various regulated PSUs for his ClassA amplifiers.
Now, that seems to be a contradiction: Classic definition of 'class A' states that it draws a (nearly) constant current. It draws continuous current which exceeds the maximum peak load (loudspeaker) current.
If it can draw up to 20A at peak output (some 18+A above its idle current), is it not then class AB?
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Old 11th December 2010, 05:27 PM   #9
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In the past I have built up kit amps, following directions, like a big Hafler. It was custom to put a farad or two after the rectifiers and before the amp board. This was early 80's when voltages where going down and down for digital equipment and the surplus market was awash in huge high voltage capacitors. So this issue of having juice available for peaks was a non issue. Electrocution, days after it was powered down, was however a hazard.

But, because I have not figured out the dynamics of these voltage regulators it appears that putting them before a large capacitor confuses the the regulator. This can only mean I don't understand how they work and need to be pointed to a schematic which has the transformer-bridge-regulator ahead of the capacitors and is set up to work correctly reducing the voltage to the capacitors from 35VDC to 24VDC.

I have looked at the Zen 4 regulated PS and the Linsley Hood regulated PS for his Class A amp. I would understand if this were something that were too much to ask but if I had the correct values and components for my particular need to scrub about 10V from my transformer before the capacitors, that would be very much appreciated. Otherwise, the best advice has been to suck it up and just buy an 18v-18v transformer which are not at all hard to find and can be had for about $50.
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Old 11th December 2010, 11:15 PM   #10
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Quote:
Originally Posted by MJL21193 View Post
Classic definition of 'class A' states that it draws a (nearly) constant current.
generally no.
A ClassA amplifier uses all it's devices to actively control the output current all the time.

A push pull ClassA output stage transfers to ClassAB if the load demands more current than the ClassA limit.

A single ended ClassA output stage transfers to severe distortion if the load demands more current than the ClassA limit.

Both single ended and push pull ClassA do not draw nearly constant current when the load demands current. There is a single ended topology that does draw nearly constant current but it is so rarely adopted that it can be virtually neglected.
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