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Old 18th June 2010, 11:59 PM   #1
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Default Tranformer size how much is too much?

I amp building a pair of ESP P101 Mosfet amps. The planned outout power will be ~150W per channel. The design note sugest the supply transformer should be 200VA per amp. So 400VA for the pair.

I have an opertunity to purchace a toroidial unit with two secondaries that are 38V @ 15Amps = 570VA x 2 = 1140VA. This ia ~ 3x what I need. The unit has an internal electrostatic shield and the price is very good ($50)

Any issues? I read somwhere that excess transformer capacity increases ripple current. I don't understand how. Is this a concern in my situation?

PS: Later I am planning to build another amp pair for biamp setup. So in that case perhaps this big transformer would be ideal?
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Old 19th June 2010, 12:22 AM   #2
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I am no expert, but a larger capacity transformer should not cause extra ripple. Excess ripple is generally caused by not enough microfarads in the power supply. Modern power amplifiers have lots of power supply rejection and moderate ripple should not be a problem. All power supplies except for regulated ones have ripple. Regulated ones will have a lot less but it still will be there to some extent. 76Vct is cutting it close if you want to get 150 RMS @ 8 ohms. You need around 35 volts rms to get 150 watts rms @ 8 ohms ie (35*35)/8=153. (35vrms * 1.41)=49 peak . Add a few volts for VCE losses Re losses fuse loss etc ripple, power supply sag and you will end up needing around +/- 65-70 volts DC. your tranny will give you a ball park figure of around 53-55 volts depending on line voltage, diode losses etc. If your target is 150 watts rms @ 4 ohms, no problemo. If you have an unshielded transformer keep the signal lines away from the tranny and all should be fine. There are bigger things to worry about. Remember most class AB amps are around 50 -60% efficient so multiply your desired out put by 2 per channel to get your ideal tranny rating. Hammond a transformer manufacturer used to rate their tranniess at 200% for a given period of time and 100% for almost ever. $50 is a great price.
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Old 19th June 2010, 12:31 AM   #3
AndrewT is offline AndrewT  Scotland
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using a 230:40+40Vac transformer run on 240Vac, I got >170W into 8r0
150W should be possible from 38+38Vac, but it will depend on the available mains voltage.
regards Andrew T.
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Old 19th June 2010, 12:41 AM   #4
jcx is online now jcx  United States
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you may want some sort of inrush current control at that power level, even more so with the oversize xfmr
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Old 22nd June 2010, 11:04 PM   #5
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Default Toroid Power Tranformer for Audio from Noratel UK


For those interested. There five more availble on ebay at

I got specs from Noratel UK- I list the basics below and i've attached the datasheet.

Four Primary windings
2 x 100V
2 x 15V

Four Secondary’s (all non inverting)
2 x 38V @ 15 Amps (39.1V Unloaded)
2 x 36V @ 2 Amps (37.5V Unloaded)

One electrostatics shield (Yellow Wire)

- soul-wind
Attached Files
File Type: pdf 063499_sc.pdf (49.3 KB, 21 views)
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Old 23rd June 2010, 09:33 AM   #6
space is offline space  Norway
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I read somwhere that excess transformer capacity increases ripple current.
It will cause lager input ripple current from your socket to amp. This should not affect your amp performance, but It might trip your "house hold" fuses at startup so jcx's advice is a good one.

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