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Old 30th September 2012, 12:00 PM   #1341
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I would concur with Andrew.
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Old 30th September 2012, 11:54 PM   #1342
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my quick calculation for power:

28-0-28vac gives rails of +39-0-39VDC or 78volts total
Vac = 78/3 = 26vac or 85watts into 8 ohms...

OTOH, 35-0-35vac provides rails of 49-0-49 or 98 volts total
Vac = 98/3 = 32vac or 133watts into 8ohms....

notice that real world traffos have finite dc resistances in their windings and that the rails at full power drops some so that actual power figures are lower than calculated...
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Old 1st October 2012, 01:00 AM   #1343
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The circuit will work, but the transformer won't be happy, it will steadily cook itself while you're testing what's going on at full power. I believe there are many commercial units out there that have undersized transformers, that get away with it because people in general don't try and do silly things too much of the time ...

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Old 1st October 2012, 01:05 AM   #1344
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the relation between amplifier output power and traffo VA capacity is a strange one....i have asked about this from the audio gurus here and the answer i got was, "get the biggest traffo that will fit in your box"......

if you were just playing music, i'd say a 100watt power amp can use a 100va power traffo...but if you are going to test your amp on sine waves, then the 100va power traffo definitely will not do...
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Old 1st October 2012, 03:09 AM   #1345
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Quote:
Originally Posted by Tony View Post
my quick calculation for power:

28-0-28vac gives rails of +39-0-39VDC or 78volts total
Vac = 78/3 = 26vac or 85watts into 8 ohms...

OTOH, 35-0-35vac provides rails of 49-0-49 or 98 volts total
Vac = 98/3 = 32vac or 133watts into 8ohms....

notice that real world traffos have finite dc resistances in their windings and that the rails at full power drops some so that actual power figures are lower than calculated...
Andrew noted that I had corrected my erroneous 150-Watt calculation result in the next post, 1338.

Your 85 Watts for a 28 VAC RMS transformer secondary output is too high because there has to be some minimum voltage across the amplifier that is between the power supply output and the high side of the load.

If the bottom of the ripple voltage waveform and the top of the output voltage waveform get closer together than that minimum voltage needed across the amplifier, then the signal voltage is imprinted with gross distortion in the form of ripple-voltage-shaped chunks being carved out of its upper extremeties, whenever the ripple voltage and signal voltage come too close to each other.

The minimum voltage required across that part of the amplifier, i.e. the minimum voltage required between the bottom of the ripple voltage and the top of the output signal voltage, will depend on the amplifier configuration. In the simulation I am using, for example, there are a transistor and a 0.22 Ohm resistor, with Vce of the transistor in series with the resistor. In that case, for the transistor used, the minimum voltage across them must be at least approximately 3 Volts (neglecting the resistor's voltage drop, in that case, and I think that Vce min = Vbeo agrees with the sim results) but could need to be slightly higher.

If you subtract that 3 Volts from the theoretical peak output voltage before calculating the RMS value, the maximum RMS value comes out to roughly 24.5 Volts, giving a max sinusoidal output power of about 75 Watts.

Last edited by gootee; 1st October 2012 at 03:16 AM.
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Old 1st October 2012, 03:50 AM   #1346
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we are of course discussing "theoretical transformers" (because we do not know how big it is and what are its dc resistances) and theoretical amplifiers..........computations are at best estimates....

if we want to be accurate, the proper thing to do is wire up the actual amp with the actual transformer, then using a dummy load, a signal generator and a scope we can verify actual output power.........until then, our talk is merely mental excercises....
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Old 1st October 2012, 04:26 AM   #1347
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Quote:
Originally Posted by Tony View Post
we are of course discussing "theoretical transformers" (because we do not know how big it is and what are its dc resistances) and theoretical amplifiers..........computations are at best estimates....

if we want to be accurate, the proper thing to do is wire up the actual amp with the actual transformer, then using a dummy load, a signal generator and a scope we can verify actual output power.........until then, our talk is merely mental excercises....
There's a lot we don't know but we should use the significant things we do know to improve our estimate.

I didn't do a proper engineering estimate. But the right engineer would be able to get pretty close (or as close as we wanted to pay for), without building the thing.

That is, after all, one of the main reasons there are engineers, i.e. so we can be certain-enough that something will work the way we think it will, before building one.

Measurement is no panacea, since each system measured is made of components with values and characteristics that are random variables (i.e. probabilistic) and it is unlikely that any two "identical" circuits would give the same measured values. So in general we would need to measure a large-enough sample of the circuits and calculate the mean and variance, etc. (Or, the engineer could calculate an estimate of the mean and variance of any performance parameter, given the mean and variance of each component's value and characteristics.)
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Old 1st October 2012, 04:38 AM   #1348
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Quote:
Originally Posted by danielwritesbac View Post
Tom? Howabout 28+28vac 144va transformer, 160va transformer, 200va transformer as comparison points? That would be really fascinating for comparing what happens with the needed capacitance.
Daniel,

I have done only one, so far, which used a 35-Watt-RMS-rated output power into 8 Ohms, with square waves (making it 35 Watts per rail), with 1470 uF of smoothing capacitance, a 28-0-28 V transformer, and VA ratings (for each secondary) that were swept from 20 to 220 VA in 20VA steps.

Attached are the data points and a plot of the square wave distorion vs VA Rating.

Tom
Attached Images
File Type: jpg VA_Sweeps_35W_1479uF_.jpg (125.6 KB, 92 views)

Last edited by gootee; 1st October 2012 at 04:45 AM.
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Old 1st October 2012, 05:12 AM   #1349
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what makes the discussions even more interesting is that power traffos with a certain volt-ampere rating varies from manufacturer to manufacturer.....one more variable to consider....
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Old 1st October 2012, 07:58 AM   #1350
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Quote:
Originally Posted by gootee View Post
Daniel,
I have done only one, so far, which used a 35-Watt-RMS-rated output power into 8 Ohms, with square waves (making it 35 Watts per rail), with 1470 uF of smoothing capacitance, a 28-0-28 V transformer, and VA ratings (for each secondary) that were swept from 20 to 220 VA in 20VA steps.
Attached are the data points and a plot of the square wave distortion vs VA Rating.
Tom
Thanks Tom! There's some really disturbing news when it comes time to pay for the right size transformer. The chart is totally plausible though. I've seen a few real life situations where there's just not enough bass until buying the much higher current transformer. So, it would have been less expensive to get the transformer big enough on the first try.
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