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Old 24th March 2011, 05:16 AM   #1
ddietz is offline ddietz  United States
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Default Transformer alternatives

Hi all,

I recently discovered this site and I'm very pleased. I just finished my first project, a cmoy based on the tangent tutorial, and I might be hooked

For my next project I'm considering a dual mono amp to power small 8ohm monitor speakers (something in the ~20 watt range).

I wonder if there are any powersupply alternatives than the bulky toroids? I'd like a smaller and more portable case. Are AC -> AC wallworts a possible alternative? Some sacrifice in sound would be ok.

As form better toroidal (or suitable square ones), are there any recommendations for places to salvage suitable transformers? TVs, old radios, etc?


Thanks and looking forward to continuing to learn from this site!
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Old 24th March 2011, 01:24 PM   #2
AndrewT is offline AndrewT  Scotland
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The usual recommendation for a transformer for a power amp is that VA should be one to two times the total maximum output power.

If you have chosen your transformer voltage to give 20W+20W output into your 8+8ohm speakers, then the smallest transformer I could recommend is a 40VA.
This will impact on the sound quality of the amplifier, but it will work.
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Old 24th March 2011, 02:24 PM   #3
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People have sucessfully used Swithmode Power Supplies with good results. Perhaps a couple of old laptop power supplies could be what you are looking for? As Andrew rightly points out, ideally they would be able to provide at least the maximum continues output of the amp.
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Old 24th March 2011, 02:28 PM   #4
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by ianpengelly View Post
Switchmode Power Supplies ............ ideally they would be able to provide at least the maximum continues output of the amp.
Be careful the continuous output and transient output capabilities of the SMPS may not suit the very variable nature of the current demand of an audio amplifier.
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Old 24th March 2011, 05:17 PM   #5
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40 VA is suposedly the minimum, to be safe.
laptop charger i might not recomend, usualy SMPS has a turnon voltage spike , You may need to use a dummy load and a delayd relay to make things work in order.

If You have a wall pulign adaptor You may give it a go, if You do not need to drive the amp to the max even less than 40 VA can do the trick.
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Old 24th March 2011, 06:02 PM   #6
ddietz is offline ddietz  United States
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Thanks for the feedback everyone!

My situation is that occasionally I need to use two small 8ohm monitor speakers. Playback quality does not have to be audiophile level (though higher quality is always nice) as it is used to play back unmixed/unmastered track captures during recording sessions. A friend has used a cheap berhinger headphone amp to drive them and the sound was acceptable. For portability sake, I'd like to fit a stereo amp (maybe BrianGT's LM1875) into a small as possible (reasonably) package and power it with a wallwart.

Obviously, I'm pretty new to all of this, though I'm learning.

Id also be fine with a normal powersupply using toroids or center tap transformers but the $50 price tag is more than I'd like. If I could find a source for one good transformer (free from a broken device) I'd live with the larger size.

Any thoughts of the types of older devices I might be able to pull either a small transformer for lower volume listening or a larger transformer for higher output assuming I'm powering a LM1875 based amp like BrianGT or others?

Thanks again. I love learning new things!
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Old 24th March 2011, 06:11 PM   #7
ddietz is offline ddietz  United States
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Quote:
Originally Posted by Arty View Post
If You have a wall pulign adaptor You may give it a go, if You do not need to drive the amp to the max even less than 40 VA can do the trick.
Arty, what do you mean by wall plugin adapter? Something like these:
14 - 24 VAC Adapters

Thanks again!
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