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Old 23rd February 2004, 08:58 PM   #31
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Default Additional clarification

I am interested in this thread as I hope to begin the PS on an Aleph project soon as well.

When modeling the PS in PSUD2, trying both a CRC and a CLC arrangement, the CRC seems to have less ripple. If this is correct, then is the only reason a CLC is more often recommended (it seems) because of the lower power loss? If I am doing the simulation correctly, then it seems that the difference between a 2.2mH and a 4.4mH inductor would be around 1/2 a volt. I was always under the impression that it was more.

Are the above observations consistent with the group's experiences? My real concern is that I too am hoping to use the 68000uf 50V caps, but was planning on an Aleph 2. It seems that Iím on the edge if I use a standard Avel 40-0-40, which modeled 44.5VRMS at the rails with a 7.5A (est 300W/40V) constant current draw. However, the peak on C1 is 49.8V, which seems a bit too close for comfort with my knowledge and the fact that they are 50V surplus caps. Should I consider getting some 63V caps for the first set of caps or just play it conservative and go for a 35 or 37V torroid? Finally, with low/no load conditions, the caps are way over their limits (around 57V). Is this an issue, since it is my understanding that a class A amp always draws constant current due to the constant dissipation across the transistors (the bias current) regardless of signal? It seems that the only time I could get in trouble would be during the trimming of the amp during the initial testing.

The above is how I understand these items from all the reading here about amp design. Please point out anything I have misunderstood, as I donít want to build on a false foundation.

Sandy.
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Old 20th March 2004, 08:58 PM   #32
SteveA is offline SteveA  United States
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Default Puzzled about psu

I assembled the PSU (for Aleph 5). I used a 30-0-30 toroid with a 2 mH inductor "sandwiched" between two 68000 50 v caps (for each rail). Duncan's software suggested I should see +/- 32v. I measure +/- 43. I used the rectifier board (and diodes) from the Aleph 2 posted on Passdiy by Wayne Sankey. Why the difference between the predicted and observed?
Sure wish this was chemistry.
Steve
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Old 21st March 2004, 02:26 AM   #33
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Default Re: Puzzled about psu

Quote:
Originally posted by Sandy H.
I am interested in this thread as I hope to begin the PS on an Aleph project soon as well.

When modeling the PS in PSUD2, trying both a CRC and a CLC arrangement, the CRC seems to have less ripple. If this is correct, then is the only reason a CLC is more often recommended (it seems) because of the lower power loss? If I am doing the simulation correctly, then it seems that the difference between a 2.2mH and a 4.4mH inductor would be around 1/2 a volt. I was always under the impression that it was more.

Are the above observations consistent with the group's experiences? My real concern is that I too am hoping to use the 68000uf 50V caps, but was planning on an Aleph 2. It seems that Iím on the edge if I use a standard Avel 40-0-40, which modeled 44.5VRMS at the rails with a 7.5A (est 300W/40V) constant current draw. However, the peak on C1 is 49.8V, which seems a bit too close for comfort with my knowledge and the fact that they are 50V surplus caps. Should I consider getting some 63V caps for the first set of caps or just play it conservative and go for a 35 or 37V torroid? Finally, with low/no load conditions, the caps are way over their limits (around 57V). Is this an issue, since it is my understanding that a class A amp always draws constant current due to the constant dissipation across the transistors (the bias current) regardless of signal? It seems that the only time I could get in trouble would be during the trimming of the amp during the initial testing.

The above is how I understand these items from all the reading here about amp design. Please point out anything I have misunderstood, as I donít want to build on a false foundation.

Sandy.

Quote:
Originally posted by SteveA
I assembled the PSU (for Aleph 5). I used a 30-0-30 toroid with a 2 mH inductor "sandwiched" between two 68000 50 v caps (for each rail). Duncan's software suggested I should see +/- 32v. I measure +/- 43. I used the rectifier board (and diodes) from the Aleph 2 posted on Passdiy by Wayne Sankey. Why the difference between the predicted and observed?
Sure wish this was chemistry.
Steve
Sandy, Steve,

Duncans is good for a quick look to get you in the ballpark. You have to have the DCR of the inductors and the ESR of the caps set as real as possible. Same holds for the other components. Then you need to very the load as it will vary in real life. In the case of the Aleph amps where there's a large current draw, you can get away with marginal cap voltage values. However, if you use tricks like limiting the bias current during non-use, or if you use a safety disconnect on the DC rails where the load current is allowed to go to a low value or zero, the cap voltage will go to the maximum no load voltage. This value can be found by setting the load to 0A in PSU2. (Sandy, applying voltage with no-load during testing and setup is also a place where you can over voltage the caps.)

Steve,

You should see voltages in the 40+ Volt range if you're measuring the static supply. The voltage drops come in to play when you have a load current flowing. You need to attach a load resistor on the supply output to get a true picture of the "in use" Voltages. (Be sure the load resistor is of a high enough wattage.)
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Old 22nd March 2004, 12:59 AM   #34
SteveA is offline SteveA  United States
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Hi Rodd-
Thanks, I inserted a 10R 22W resistor between the + and - outputs and measured ~33.5 volts. However, within seconds the resistor was starting to smoke. Is 22W not adequate?! Is it too much? I thought I would simulate an 8 ohm load but all I had available was a 10 ohm 22 Watt resistor. What should I have used?

Even though the output side of the supply seems to be providing the correct voltage I am concerned about frying the circuit boards were they to be connected at this stage.

Thanks again
Steve
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Old 22nd March 2004, 02:48 AM   #35
Blues is offline Blues  United States
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SteveA, you were dissipating 115W on a 22W resistor...
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Old 22nd March 2004, 03:11 AM   #36
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Steve,

The Aleph amp output sees a different load than its PSU. If you're testing the actual PSU alone you need to consider the load it sees. The total power consumption of the Aleph 5 is about 300W. Some of that will be consumed by the supply itself, but for testing the PSU, it's a good place to start.

300W delivered by 2 X 35V = 70V mean you need a total load of a little over 4 amps. That would equate to a resistor of 70V/4A ~ 18ohms @ 300W. For this you can use 2 - 10ohm resistors @ 150W, with the resistors tied to the + rail to ground and ground to the - rail.

First, you should be using a variac so you can bring the voltage up slowly. You can increase the resistance to simulate a lighter load (~2A at idle at the PSU output).
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