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Old 27th December 2008, 08:04 AM   #171
zikinho is offline zikinho  Croatia
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I have another tranny 150VA, 2x20VAC. Thanks for the answers.
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Old 27th December 2008, 08:08 AM   #172
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Quote:
Originally posted by zikinho
I have another tranny 150VA, 2x20VAC. Thanks for the answers.
20vac x 1.43 = 28.6vdc rails. This is nice for 16 ohm speakers.
Edit: But this voltage is so high that its not documented for use with LM1875. Please choose lower voltage.
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Old 27th December 2008, 08:54 AM   #173
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You can use up to 30 V with 8 Ohm speakers and up to 25 V with 4 Ohm speakers. The latter could run into heatsinking limits.

If you have the 200 VA 2x18 V transformer lying around, use it. The voltage is okay and the power,...

well, you will have lots of headroom even with a two-channel amplifier.
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Old 27th December 2008, 09:02 AM   #174
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Quote:
Originally posted by pacificblue
You can use up to 30 V with 8 Ohm speakers and up to 25 V with 4 Ohm speakers. The latter could run into heatsinking limits.

If you have the 200 VA 2x18 V transformer lying around, use it. The voltage is okay and the power,...

well, you will have lots of headroom even with a two-channel amplifier.

There's a problem with the thermal junction, along with a few things I didn't want to mention because there could be electroshock or full-rail DC offset from a damaged chip because of what happens when it has expired.

Okay, so I gave conservative figures and you caught me.

Any moment, I expect AndrewT to come along and tell me that my conservative figures are too high.

What can be done?
Maybe this:
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Old 27th December 2008, 09:22 AM   #175
zikinho is offline zikinho  Croatia
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I allready made this one using LM3886, and also one small amp using LM3875. Wanted to try LM1875 just to compare to the other two.
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Old 27th December 2008, 09:41 AM   #176
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@PacificBlue Look at that! There's an exposed heatsink and a ground reference within 1 cm of each other. My voltage figures were correct. I gave the voltage figures assuming an insulator pad will be used with LM1875 for safety.

@zikinho Its not difficult to create a parallel LM1875 amplifier. That can use your 18vac+18vac transformer along with either 4 ohm or 8 ohm speakers.
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Old 27th December 2008, 09:45 AM   #177
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Quote:
Originally posted by zikinho
I have another tranny 150VA, 2x20VAC. Thanks for the answers.
Is this a center tap (3 wire output) or is it a dual secondaries (4 wire output)? I ask, because it is possible to create a very inefficient rectifier for a dual secondaries transformer. Some purposeful inefficiency at the rectifier will drop the rail voltage. This is just slightly complex to do, and there's no need to discuss all that bother if that is a center tap transformer for which the technique wouldn't make a sufficient voltage drop.
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Old 27th December 2008, 09:46 AM   #178
zikinho is offline zikinho  Croatia
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Inside...
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Old 27th December 2008, 10:04 AM   #179
zikinho is offline zikinho  Croatia
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Quote:
Originally posted by danielwritesbac


Is this a center tap (3 wire output) or is it a dual secondaries (4 wire output)? I ask, because it is possible to create a very inefficient rectifier for a dual secondaries transformer. Some purposeful inefficiency at the rectifier will drop the rail voltage. This is just slightly complex to do, and there's no need to discuss all that bother if that is a center tap transformer for which the technique wouldn't make a sufficient voltage drop.

It is a center tap. Both of them are.
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Old 27th December 2008, 12:02 PM   #180
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Well, the 18+18vac transformer can be used with a parallel LM1875 amplifier on either 4 ohm or 8 ohm speakers. Its a pretty good option because it solves a difficult problem with LM1875's current handling capacity.
When used in parallel, it spots half the load (it sees your 4 ohm speakers as 8 ohm speakers).
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