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Old 4th October 2012, 07:44 PM   #1371
Krisfr is offline Krisfr  United States
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Daniel

I agree with you. I would like to know also what is going to be recommended in the future.

How many transformers for a stereo amp. 1, 2, 4?
How many capacitors per rail to get to the needed farads? 1 big one SOME smaller ones or many little ones.
There are so many questions, and so many answers...

How about 16 or 24 mid-ranges used to make a woofer, each about 4 or 5 inches in diameter? Each in its own box, like an old PE article on the sweet sixteen speakers system.

Why limit the 750 to just sub woofing? You might blow a tweeter or two or three, hehehe.

I would like to see a no holds barred 400 stereo group design build on DIYAudio to be based on the 5U chassis coming in soon. I think that would generate a great deal of interest. But that is MHO. I think a choice of front end topology would make it go a long way too. A OStriper modulo type would work.
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Old 5th October 2012, 02:43 AM   #1372
gootee is offline gootee  United States
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Quote:
Originally Posted by DF96 View Post
You should add the transformer effective secondary resistance to your table, as this is the crucial parameter. VA rating by itself only tells you whether the transformer will get too hot. Two transformers with the same VA could have different resistances.
DF96,

I was finally able to set it up so that the transformer subcircuit parameters' measurements can print automatically, into the LT-Spice log file, making it much easier to acquire them.

I have attached some tables that show the primary and secondary inductances and resistances, and the magnetizing inductance, for the originally-measured input and output conditions, plus two different input and output voltages and line frequencies.

If someone who knows about scaling transformer model parameters would care to comment on how far they might reasonably be able to be scaled, that would probably be quite useful.

Cheers,

Tom

Edit: Added the 250VA case to the "as-measured" table.
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Last edited by gootee; 5th October 2012 at 02:55 AM.
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Old 5th October 2012, 05:26 AM   #1373
gootee is offline gootee  United States
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Nico,

After filling in the spreadsheet I posted earlier, with lots of combinations of transformer voltage and maximum output power, and 8 Ohms for the load, I pulled out some data for a few values of reservoir capacitance and made it all into a table, just to illustrate the trends and to have as a handy reference.

Of course, some assumptions are embodied in the data, such as the 1.7-Volt rectifier drop, and the 3-Volt minimum between the ripple voltage and the output signal voltage. So your mileage may vary. Still, it's an interesting type of table.

Cheers,

Tom
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Last edited by gootee; 5th October 2012 at 05:32 AM.
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Old 5th October 2012, 06:11 AM   #1374
gootee is offline gootee  United States
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Here are two examples of the latest version of my CMIN-calculating spreadsheet. They each contain lots of combinations of values of the transformer output voltage and the maximum output power. One is for 8 Ohms and the other is for 4 Ohms (They are the same spreadsheet, except for the values entered.). Both .XLS files are in the zip file.

Cheers,

Tom
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Old 5th October 2012, 06:50 AM   #1375
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I forgot to mention that the table in post 1373 is for a 60 Hz line voltage. For 50 Hz, the minimum capacitance values would be larger.
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Old 5th October 2012, 09:39 AM   #1376
DF96 is offline DF96  England
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Quote:
Originally Posted by gootee
After filling in the spreadsheet I posted earlier, with lots of combinations of transformer voltage and maximum output power, and 8 Ohms for the load, I pulled out some data for a few values of reservoir capacitance and made it all into a table, just to illustrate the trends and to have as a handy reference.
Interesting. Seems to show that even for high power there is little point in going much above 10000uF, and for lower power diminishing returns set in from 5000uF.

Turning it around, if you design a 100W amp then find that it can't quite do continuous 100W low bass then you can do one of the folowing:
1. massively increase the capacitance size
2. somewhat increase the transformer size/voltage
3. just call it an 80W amp, with excellent transient capability!
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Old 5th October 2012, 10:23 AM   #1377
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Quote:
Originally Posted by DF96 View Post
Interesting. Seems to show that even for high power there is little point in going much above 10000uF, and for lower power diminishing returns set in from 5000uF.

Turning it around, if you design a 100W amp then find that it can't quite do continuous 100W low bass then you can do one of the folowing:
1. massively increase the capacitance size
2. somewhat increase the transformer size/voltage
3. just call it an 80W amp, with excellent transient capability!
I did an experiment. First an 70w amp with a 96va (2a) transformer and 10,000u. The low bass just wasn't very good. But I had two of these transformers and they're dual secondaries models. So, I paralleled the secondaries of both, to create a 192va (4a) transformer. The bass was much better. Lastly, I added a second amp for stereo, but the bass did not change. Well, that is somewhat mysterious.
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Old 5th October 2012, 03:19 PM   #1378
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Quote:
Originally Posted by danielwritesbac View Post
I did an experiment. First an 70w amp with a 96va (2a) transformer and 10,000u. The low bass just wasn't very good. But I had two of these transformers and they're dual secondaries models. So, I paralleled the secondaries of both, to create a 192va (4a) transformer. The bass was much better. Lastly, I added a second amp for stereo, but the bass did not change. Well, that is somewhat mysterious.
Connect one of them to a dummy load
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Old 6th October 2012, 06:27 AM   #1379
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The attached might be interesting.
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Old 6th October 2012, 10:27 PM   #1380
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How to calculate transformer RMS AC Output Voltage and change it into common specs used with transformer purchases, such as their AC, Current, and VA figures?

Same question more simply: Can the chart show real transformers?
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