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Old 7th July 2004, 05:09 AM   #1
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Default What should be the transformer rating in respect to wattage of a power amplifier,

What should be the transformer rating in respect to wattage of a power amplifier,

As we all know nobody will be feeding a sinewave signal to the amplifier,nor would anybody use it at maximum power , hence what should be the wattage of a transformer for a poweramp of say 250 watts for optimum performance ,keeping in mind the efficency of the amp and the value of filter capacitors.
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Old 7th July 2004, 10:53 AM   #2
djk is offline djk
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People building 'gain clone' type amplifiers tend to use very small filter caps and 'make up' for this by using larger than normal transformers.

Cheap commercial designs tend to use smaller transformers and try and 'make up' for this by using larger than normal capacitors.

I prefer a more balanced method.

For a capacitor input filter I find bigger 'sounds' better up to a point, the point being 2VA of transformer for every 1VA the ampifier is rated to produce. Beyond this point the improvement in sound vs cost and size seem to say 'not worth it'.

I have built with 1VA of transformer for 1VA of rated power, the amplifier worked well, but sounded a bit 'thin'.

During testing with the smaller transformer (driving hard into clipping on FM radio) the transformer burned up.

For a 250W amplifier I would use a 400VA~500VA transformer.

For 50hz and a single 4 ohm load I would use a pair of 8,000F filter caps with around 47F as a bypass cap.
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Old 7th July 2004, 11:55 AM   #3
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Wouldn't this depend on the amplifier design quite a bit. As an extreme example for a given output power a class A SE amp would need a much larger transformer than a class AB push pull design. Also differing amounts of small signal circuitry could be an issue if there were a lot of signal processing etc. One would need to know the quiescent current requirements first wouldn't they?

I guess your rule of thumb is based on a basic class AB push pull right?
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Old 7th July 2004, 12:03 PM   #4
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It can actually be calculated if you take into account all the Vbe's, drops across devices and resistors, quiescent current, etc. etc., but for a class-B amp, 2x wattage required is a very close approximation.
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Old 7th July 2004, 12:49 PM   #5
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Originally posted by djk
I have built with 1VA of transformer for 1VA of rated power, the amplifier worked well, but sounded a bit 'thin'.
I've always had satisfactory results with that figure, 1VA per watt RMS. I love lots of caps though.
Best-ever T/S parameter spreadsheet.
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Old 7th July 2004, 02:13 PM   #6
K-amps is offline K-amps  United States
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Regardless of class, as said earlier if you can calculate the total maximum dissipation of the amp, I would go conservative with 2X the VA per watt dissipated......

So for a 200 watt per channel amp (2 channels) I'd go preferably with a 800VA (minimum of 600VA ). This will give you good headroom if you went into 4 ohms. Also an occasional 2 ohm load will not kill the trafo instantly as it would if you went cheap. E.g. The ubiquitas Adcom 555 is a 200wpc amp and uses a 675VA toroid.

With larger Trafo's you'll see that the voltage drop under load will be less, meaning you can use lower DC rails in your amp thereby avoiding the typical glare high rail amps have and also make the OP stage more reliable.

For example, I have a Kenwood M2a power amp rated at 220 wpc @8e (247w at clip) that uses small trafos... dual 250VA E+I types (500va total) but high rails +/- 91.5 vdc. This set up is less desirable (in the purist sense) than maybe a Nakamichi PA-7II which is also rated at 220wpc (250w at clip) but uses only +/- 70 volt rails to achieve the same. But that uses a 700VA Toroid. It is more reliable and stable at lower loads. (Lets not get into the design of the two amps but speaking strictly from a PS perspective).

I then proceeded to increase the Trafo size on my Kenwood to experiment, I used a 800VA trafo that gave me rails of only +/- 86vdc (about 5.5 volts per rail lower than kenwood's own trafos).

The sound was fuller, less constricted, had less glare and was warmer.... oh and by the way, the output at clipping increased from 247watts per channel to 372 watts per channel...

The point I am making is that by using a larger Trafo, you can increase power and sometimes make the amp better sounding and more reliable.... that is if u can afford a larger trafo.

On the capacitance, I tend to go with a minimum of 10,000uF per 100 watt. So for a 100 watt (2 channels) amp I'd uses 10,000uF per rail for a total of 20,000uF. This is the absolute minimum I'd use...
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Old 7th July 2004, 05:01 PM   #7
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Default Transformer Spec

1VA per Watt is fine if you are reproducing sine waves. However, you are not. You are reproducing music. I find that a VA of about 2/3 of the amp's RMS power rating is fine for a Class AB amp. With Class D amplifiers, the VA of the transformer can be 55% of the RMS power rating.

Just be awaye that if you run the amp at full power with a sine wave, you will eventually trip out the thermal breaker. However, even the safety agencies do not do this test, At most, they use pink noise at 1/3 power to simulate music though most of their long term tests are pink noise at 1/10 power.

Myself, to simulate music, I run a tone burst with the peak 20db over the average level, 50mS wide repeated every 500mS. The peak is set so the amplifier is in clipping. I find this does best to simulate the power demand of rock music, simulating a 120 beat per minute song with a heavy beat. With this test, VA demand through the transformer never exceeds 1/3 of the RMS power rating.

Don't waste money sizing the transformer to play sine waves at full power for hours at a time. You're not building a power inverter. We don't listen to sine waves.
Dan Fraser
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Old 9th July 2004, 06:33 AM   #8
lucpes is offline lucpes  Romania
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Absolutely true for commercial amps, no need to oversize the transformer and for speakers that behave well, with nice impedance curves and good sensitivity.

When it comes to DIY I see no reason not to oversize the transformer (2x rating is ok).

I find this article good for DIY power supply issues: http://www.tnt-audio.com/clinica/ssps1_e.html

PS. We do not listen to sine waves but don't try weak amps on speakers like let's say an older Infinity Kappa 9 (86dB sensitivity and more like a short in the lower bass region, this speaker will happily do 29dB -2dB with a beefy amp, otherwise a lesser amp is likely to go presto kaboom or rush into protection mode/limiting if it has one)

Kappa 9 impedance curve: http://www.rageaudio.com.au/kappa9.jpg
We should no more let numbers define audio quality than we would let chemical analysis be the arbiter of fine wines. Measurements can provide a measure of insight, but are no substitute for human judgment. Nelson Pass
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Old 9th July 2004, 04:07 PM   #9
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Actually, commercial use is harder on the amplifier than home or DIY use. We play highly compressed music on our systems for hoursat a time with a low peak to average ratio. Our amps are consistently running at the clipping level or into heavy limiting. The amount of VA really needed in a home amp is seldom over 1/3 of the RMS rating of the amplifier.

In our commercial amplifiers we operate speakers that are like 100db 1W/1M efficient and routinely put out SPLs like 136db at 1 meter in order to get 105db at the audience. If you look at how UL test our products compared to a home amplifier, comercial amplifiers are tested at both 1/10 and 1/3 rated power while hi-fi amps are just tested at 1/10 power.

The safety agencies recognize that amplifiers reproduce music, not sine waves and they find transformers rated as less than 1/2 the RMS power rating of the amplifier to be acceptable.

That said, damping factor in an amplifier depends on their being a very low source impedance. This not only includes the speaker wiring and the amplifier itself but also includes the transformer, bridge reftifier, wiring and filter capacitors. We have determined that all of these affect the tightness of the bass but the biggest efect is from the filter capacitors and the wiring from the capacitors to the power amplifier.

This is because the capacitors are what supply the power to run the amplifier. The transformer and bridge merely recharge the capacitors. However, this recharge does happen faster if the transformer is over size. The effect of a smaller transformer is that the current pulse that recharges the capacitors is wider, meaning that at high volumes there is more ripple on the supply. On the other hand, less electromagnetic interference or buzz is created.

The point is, that an oversize transformer won't hurt anything and there may be the same sort of subtle effects that occur with the use of things like Monster Cable at work here. However, to make any effect, the use of an oversized transformer has to be accompanied by all the other audiophile tricks, including great speakers, to make the difference between a good amplifier and a true audiophile product.

Just realize that going past 1/2 the RMS rating of the amplifier for a transformer VA rating will do very little good in measurable results and is not required from a safety standpoint. You are entering on the difficult to quantify, often very subtle but very real differences you get in the audiophile realm.
Dan Fraser
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Old 9th July 2004, 05:38 PM   #10
Bricolo is offline Bricolo  France
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A 300W amp with a 150VA transformer?
Just remember: in theory there's no difference between theory and practice. But in practice it usually is quite a bit difference... Bob Pease
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