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Power supply voltage keeps changing from transformer output, [TOS #6: moved from General Audio]
john11
post Jan 9 2013, 04:34
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Hi And thanks for reading this post.

The ac output on my quad 405-2 amplifier's transformer is 71 volts but keeps going up and down by approximately 0.1- 0.3 volts everytime i measure it. Is this normal and is this the correct voltage. It is quite and old transformer from the 70's and i want to connect some new quad amp boards to it but am not sure if the output voltage is correct or if anything can be gained by exchanging for a new toroid/audio transformer.

Many thanks in advance. John.
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DVDdoug
post Jan 10 2013, 19:10
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With a constant load, the output (secondary) voltage of a transformer is proportional to the input (primary) voltage... If the AC line voltage changes by 10%, the ouput voltage will change by 10%.

If you crank-up the volume (with speakers connected), you'l pull more current out of the transformer and the voltage will probably drop a bit more. (But, it's not worth blowing-out your speakers doing experiments like that. wink.gif )

QUOTE
...or if anything can be gained by exchanging for a new toroid/audio transformer.
No. Assuming your amp is not a lousy design, a "better" transformer isn't going to change anything.

QUOTE
It is quite and old transformer from the 70's and i want to connect some new quad amp boards to it
Why? Are your boards burned-out? What are you trying to do? Is there something wrong with the sound? (Noise, distortion, frequency response?)

Good quality solid state amps from the 1970s are about equal in performance to modern amps. With integrated circuits and the widespread use MOSFETs, it's now cheaper & easier to build a good amp, and high-power is a LOT cheaper, but modern amps with similar power are not necessarily better than older amps. (Most older tube amps weren't that good.... Tube amps require audio transformers in the signal path which makes it very difficult and costly to make a good high fidelity amp. But with solid state electronics, the transformers could be eliminated and it became simpler. ICs have have made it super-easy!)

If you just want to "upgrade" your sound, and you want a truly audible difference, upgrade your speakers.

This post has been edited by DVDdoug: Jan 10 2013, 19:21
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