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Old 11th May 2004, 02:38 PM   #1
xcortes is offline xcortes  Mexico
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Default Transformer sizes and their effect on sound

Hi everyone:

There seems to be some consensus that you need at least 100VA per channel for a gainclone (more specifically an LM3875 one but I guess this applies to others as well).

Why is that?
What do you loose when you go to a lower rating transformer?
How about for low volume listening?

I'm thinking about building a portable system: a GC amplifier, a couple of minimonitors based on the Jordan JX92S drivers (3.5 to 4 liters each) and an ipod with lossless compressed music (the new Apple format), all tightly fitted within a solid pelican case.

For this application the size and weight are as important as the sound quality (well maybe just below!).

Any thoughts?
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Old 11th May 2004, 02:50 PM   #2
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A larger xfmr gives a more ' effortless ' sound. A reasonable rule of thumb is at least 1.5 x max power (RMS) I.E. for a 20 watt amp you would want (at least) a 30 VA xfmr..............mike
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Old 11th May 2004, 03:22 PM   #3
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Default Re: Transformer sizes and their effect on sound

Quote:
Originally posted by xcortes
Hi everyone:

There seems to be some consensus that you need at least 100VA per channel for a gainclone (more specifically an LM3875 one but I guess this applies to others as well).

Why is that?
The issue is that a transformer cannot maintain it's specified output voltage at all current levels. It is rated at some moderate current level. It also has a "regulation" specification that is a measure of how well it can keep the voltage up as the current increases.

As the amp puts more load (draws more current) on the transformer, the transformer output voltage drops some.

If the VA rating is very low, the current draw as a % of capacity is high and the voltage drop is large. It is easy to imagine that this would affect the sound as the amp output would be not be proportional to all input sounds.

If the VA rating of the transformer is high enough this effect is minimal since the current as a percentage of capacity is smaller.

Larger power supply capacitors can also help overcome the load since they have some stored energy they can release but the effect is only for a very short time (on the order of milliseconds?). So they can help for transient loads but not for a sustained high volume load.

There are some different rules of thumb. An example using the 2x rule of thumb for a solid state discrete amp would be:
An amp with 2 channels of 60w each would need a minimum of 120x2=240 VA transformer. Of course if the regulation in the transformer is especially poor the transformer may need to be bigger etc.

From a “maintaining voltage” point of view a higher than 2x VA is even better but the benefit becomes less and less.
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Old 11th May 2004, 06:48 PM   #4
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Default Re: Re: Transformer sizes and their effect on sound

Quote:
Originally posted by moving_electron


An example using the 2x rule of thumb for a solid state discrete amp would be:
This rule of thumb would be for a class AB amp. A class A amp would draw more current in general due to the design and would require the transformer to be a higher VA than 2x.
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Old 14th May 2004, 04:14 AM   #5
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I have read that the Creative Nomad players have the best sound quality, and are also much cheaper than an ipod.

However if you have a mac, then go with that.

Iriver make some nice players as well, not sure on the quality. A bit cheaper than the ipod.


If you are listening at low noise levels you should be fine. Just be careful when turning it up, as the transformer will heat up and possibly short if its not good enough.
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Old 14th May 2004, 04:14 PM   #6
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Default Re: Transformer sizes and their effect on sound

Quote:
Originally posted by xcortes
Hi everyone:

ipod with lossless compressed music (the new Apple format), all tightly fitted within a solid pelican case.
One of the advantages of the iPOD over Windows Media format and MP3 players is the compression scheme. I do not think it is lossless though. The filesize seems to small for it to be lossless. It does sound good though compared to the other compression formats.

If we discuss this futher I think we should move it to the digital forum.
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