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Old 27th May 2004, 08:02 PM   #1
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Default how to calculate/approx rails

ok i have ran into this problem before
i bought an avel toroidal transformer that is 330VA and +-35 secondaries
now i hooked up one 4700uF in the positive and negative rails
i get a whopping +-55 volts from this psu
i was wondering how you calc/approx what rails are going to be after you put caps on them
this caused problems because my amp was doing 180watts into 8 ohms and my speakers can only handle 125 watts
any comments quesions? please guide me with some useful info
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Old 27th May 2004, 08:51 PM   #2
K-amps is offline K-amps  United States
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The usual formula is you multiply the AC secondaries by 1.4 to get an approximate DC value. (Actually you take the peak values of the AC so you can also divide it by 0.70707) and then subtract 1 volt or so (or whatever your bridge diode needs to bias it on, an approximate value of 1 volt turns on most diodes) so the subtracting of 1 takes into account the voltage drop across the diode.

So 35 / 0.70707 = 49.5 -1 = 48.5 vdc.

Now your Avel (if its a Y23 model) is rated for 35 volts at 115Vac.

The usual AC you get at home is anywhere from 117 to 124 vac in most US homes. So the Actual AC on your secondaries can be anywhere from 35.6 to 37.74vac.

meaning your DC rails will be anywhere from:

( 35.6 / 0.707) -1 = 49.36 to
( 37.74 / 0.707 ) -1 = 52.37.

If you are getting, 55 volts, I am guessing your Home AC voltages are close to : 129vac or so. Using schottkeys will alter the calculation a bit but not much.

Under load, your DC will drop to approx +/- 43volts or so. More power is not a bad thing, its how you use it.

Good luck,

K-
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Old 27th May 2004, 09:28 PM   #3
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You haven't loaded the transformer yet, so it will drop.
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Old 27th May 2004, 10:06 PM   #4
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i keep frying tip142 and 147
i only have one of each in the output stage class AB
i am wondering why they keep starting on fire
its not my speaker because it wokrs fine
do i need to have one more each of the tip142 and tip147 so there is not so much current
keep in mind i am using the above rails +-55 reality -+ 50
am i exceeding the rated amperage?
i am using 8 and 4 ohm loads
the tip142 and tip147 are starting on fire with either speaker after about 30 seconds of play
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Old 27th May 2004, 11:12 PM   #5
K-amps is offline K-amps  United States
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YOU ARE EXCEEDING THE tip142/47'S PRIMARY vce RATING TO BEGIN WITH. (Sorry for caps)

They are rated at 100vce, so any rails above +/- 50 vdc will be beyond spec.

Also using one pair of those to get 180 watts into 8 ohms, you are definitely exceeding the SOAR specs too.

6-8 pairs will be more in order...
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Old 28th May 2004, 05:07 AM   #6
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what would be more suitable transistors then because i cant find any for on semiconductor
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Old 28th May 2004, 05:35 AM   #7
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how about your idle current, what is your quiscient dissipation on the output trannies? how about heatsinking are they sifficient?

does your amp's 'sinks get hot even at no signal?
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Old 28th May 2004, 05:51 AM   #8
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my transistors dont even get that hot
heat sink is more than adequate
my q current is really low
i think the problem is that i am using rails that are +-55
any one know of some substitute trans wth a higher voltage rating than the tip 142/147
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Old 28th May 2004, 09:50 AM   #9
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Try any cheap alternative like the 2sd1047/ 2sb817 pair. They can be used with +/- 70 volt rails. Other characteristics are similar to the TIP142/47. 2-3 pairs will work.

Any higher then you cannot go wrong with genuine 2sa1302/2sc3281 variants or Toshiba's own 2sc5200/ 2sa1943's. Those you can use with up to +/- 100 volts if sufficiently applied i.e multiple pairs.

For your application 2 pairs will be more than adequate, heck even one pair will work but its better to be conservative.
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Old 7th June 2004, 05:28 AM   #10
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how does this work when reading the SOAR
so reading the tip142 i see that vce at like 1 volt can hadle about 10 amps
so that would be more the fine for 150 watts at 8 ohms
with 55 volt rails correct?
the only problem is that it cant handle more the 100vce which would happen becuase the total swing is 55*2 =110 right
so vce is to high? correct?
i am trying to figure this out becuase i keep frying tip142
i have great heatsinking so that is not the problem
i am wondering if i am reading my specs sheet wrong

heres my scenario
i play music ts fine i turn the amp up higher and its loud and playing fine then all of a sudden the tip142 and 147 fry
i am using class AB
so when i turn it up the signal is swinging bigger and exceeds my vce for the transistors right? then fries?

so if i use -+45 or 50 rails i should be fine
because max amps would be 5.625 or 6.25 into 8 ohms
and the tips can handle 10 amps

tell me if my thinking is correct and what is not correct
thanks
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