transformer capacity

krish2487 · 15th January 2009, 06:55 PM

hi guys i am a newbie here and wanted to ask a question abt transformer capacity.

I am building a 2 channel chipamp on 1875
i wanted to parallel 2 1875s per channel

Now what is the transformer capacity i should go for?

assuming 40 w per channel
it is 80 watts total
and a efficiency of 80% will a 100VA t/f with 18 0 18 do for my chipamp??

i asked because when going through the posts i noticed that for all chipamps the minimum t/f rating everyone was using was a 160VA

is it because they are easily available or any other specific reason??

Nuuk · 15th January 2009, 06:59 PM

80VA per channel is pretty much the minimum most would recommend.

krish2487 · 15th January 2009, 07:01 PM

is that 80VA per channel??

rjb · 15th January 2009, 07:16 PM

Actually choosing a transformer rating is not quite as straight forward as you might think. If you look at the transformers from commercial amps they are often less than a calculation of load at maximum power output would suggest.

Take the quad 405 the Input power is stated as 80-320 Watts, (VA), depending on volume. As most of the time the volume listeners use is much less than maximum, the transformer is able to take brief overloads.

Transformers mostly fail by overheating. Excess current (usually protected by fuses), can cause catastrophic failure, but long term milder overheating also shortens their life. So brief overloads can be absorbed by the thermal mass, but at a cost in terms of life-span. Of course there is a limit to that, but for commercial designs this results in cost savings.

When experimenting, running close to the limits may be acceptable, but for long term reliability I recommend playing safe and running transformers cool. The small additional cost is worth the peace of mind, and who knows, you may want to build a more powerful amp next time.

krish2487 · 15th January 2009, 07:20 PM

@rjb
thank you

I just wanted to know how the t/f was chosen.
cost is a factor
but i ll take your advice and play safe by going for a bigger t/f
thank you again.

krish2487 · 15th January 2009, 07:35 PM

@everyone

i have so far read only nationals app notes on bridging and paralleling amps
they are good
can anyone give some more references ??
for knowledge and understanding sake..

15th January 2009, 06:55 PM	#1
krish2487 diyAudio Member Join Date: Dec 2008	transformer capacity hi guys i am a newbie here and wanted to ask a question abt transformer capacity. I am building a 2 channel chipamp on 1875 i wanted to parallel 2 1875s per channel Now what is the transformer capacity i should go for? assuming 40 w per channel it is 80 watts total and a efficiency of 80% will a 100VA t/f with 18 0 18 do for my chipamp?? i asked because when going through the posts i noticed that for all chipamps the minimum t/f rating everyone was using was a 160VA is it because they are easily available or any other specific reason??

15th January 2009, 06:59 PM	#2
Nuuk diyAudio Member Join Date: Feb 2003 Location: Somerset, SW England	80VA per channel is pretty much the minimum most would recommend. __________________ The truth need not be veiled, for it veils itself from the eyes of the ignorant.

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15th January 2009, 07:01 PM	#3
krish2487 diyAudio Member Join Date: Dec 2008	is that 80VA per channel??

15th January 2009, 07:16 PM	#4
rjb diyAudio Member Join Date: Jun 2004 Location: Piha	Actually choosing a transformer rating is not quite as straight forward as you might think. If you look at the transformers from commercial amps they are often less than a calculation of load at maximum power output would suggest. Take the quad 405 the Input power is stated as 80-320 Watts, (VA), depending on volume. As most of the time the volume listeners use is much less than maximum, the transformer is able to take brief overloads. Transformers mostly fail by overheating. Excess current (usually protected by fuses), can cause catastrophic failure, but long term milder overheating also shortens their life. So brief overloads can be absorbed by the thermal mass, but at a cost in terms of life-span. Of course there is a limit to that, but for commercial designs this results in cost savings. When experimenting, running close to the limits may be acceptable, but for long term reliability I recommend playing safe and running transformers cool. The small additional cost is worth the peace of mind, and who knows, you may want to build a more powerful amp next time.

15th January 2009, 07:20 PM	#5
krish2487 diyAudio Member Join Date: Dec 2008	@rjb thank you I just wanted to know how the t/f was chosen. cost is a factor but i ll take your advice and play safe by going for a bigger t/f thank you again.

15th January 2009, 07:35 PM	#6
krish2487 diyAudio Member Join Date: Dec 2008	@everyone i have so far read only nationals app notes on bridging and paralleling amps they are good can anyone give some more references ?? for knowledge and understanding sake..

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