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Old 18th October 2012, 08:00 AM   #1
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Default psu transformer impedance??

Scenario: A stereo amplifier powered by one 100VA torroid. Add a second 100VA transforer so each channel has a seperate psu. Or one 1000VA transformer feeding two channels. I did a quick mesurment with my DVM . The primary mesures 0.8 Ohm's on the 100 VA And 0.2 Ohm's on the 1000Va transformers. Please be aware my DVM will not be very acurate at these resistances. Not sure wich is the best way to go.. im guessing this will attract a few diffrent camps . how important is the primary impedance of a transformer in a power amplifier.. Is infinate stereo seperation a good thing?

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Old 18th October 2012, 08:48 AM   #2
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well, you seem to have measured the DC resistance of the secondary. The impedance presented during operation is something quite different. Generally speaking, a separate supply for each channel is better, if and only if, properly designed. There is too much to further elaborate on.
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Old 18th October 2012, 10:25 AM   #3
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Hi demeterart. i stand corrected yes it was the secondarys i measured.. any links to some information regarding this.. seperate 100 va psu for each channel virtually no interaction between the channels but higer psu impedance.. surley a transformer with a lower impedance on the secondarys will sag less under transients.. but give less isolation between channels ...
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Old 18th October 2012, 10:28 AM   #4
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As for there being to much to eleberate on .. go on eleborate after all this is diy audio and we like to elaberate and investigate . I agree it might lead nowhere but atleast it would have been an intresting journey..
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Old 18th October 2012, 10:29 AM   #5
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Your example is 100VA per channel versus 500VA per channel. It's not really a contest.
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Old 18th October 2012, 10:31 AM   #6
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The usual impedance for larger transformers is 5%, this means that the voltage will drop by 5% at rated load. Easy enough to measure, Just apply a known resistive load and measure the voltage drop. The impedance is split between leakage inductance and DC resistance.
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Old 18th October 2012, 10:34 AM   #7
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Quote:
Originally Posted by metalsculptor View Post
The usual impedance for larger transformers is 5%, this means that the voltage will drop by 5% at rated load. Easy enough to measure, Just apply a known resistive load and measure the voltage drop. The impedance is split between leakage inductance and DC resistance.
just like i said a larger transformer will have less voltage drop for a given load opposed to a smaller transformer.. Could this be a reason lots of people like to use an oversized transformer
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Old 18th October 2012, 10:41 AM   #8
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do not overlook the filtering/regulation after the transformer. in a linear PSU the capacitors handle half of the time the load and are responsible for transient behaviour of the system. still, the use of a large enough transformer for both channels (accounting for peak current demand) with proper filtering/regulation and of course thoughtful layout/wiring serves adequately with out the need for separate supplies. done properly and the difference might not even be measurable.
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Old 18th October 2012, 11:02 AM   #9
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Hi demeterat.
I havent overlooked the filtering and regulation.. Im talking about a transformer 10 times the required size 1000va compared to a 100va.. the only testing instruments i have is my fluke DVM and my ears.. so i am at the most intresetd in the effect on the sound . by changing smoothing capacitors voltage and value the same but different manufacturer has a noticable effect on the tonal footprint of an amplifier. i have seen undersized transformers used with lots of capacitance to overcome the problem sort of.. So just because a said transformer might be hiding behind regulators and capacitors im sure it must still have an effect on the sound .. .. Also surley if a transformer has better regulation the need for regulation further up stream is reduced. ( less work for the regulators).... Why make it faulty then throw components at it to make it right . why not start out right then less components to fix it later and to a point i belive the less components to do the job required is the way to go .. This is early days and i was hoping maybe this post would start a discusion with other members giving there opinions and expiriences as to the diffrence it may make to the sound ..
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Old 18th October 2012, 11:14 AM   #10
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can your fluke measure true r.m.s ? can it measure AC beyond 400Hz ?
if yes then you can do a lot more towards building a robust supply. on another note, if the 100VA transformers were to be used to the max. i would go for the single 1000VA case and separate capacitance multipliers.
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