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  • john11
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Power supply voltage keeps changing from transformer output

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.

Power supply voltage keeps changing from transformer output
Reply #1
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.


Probably. You are choking on a grain of rice!

Measure the AC voltage on the primary, its probably wandering around a tiny bit, too.

If the transformer were bad the variations would huge and mostly there would be no voltage at all.



  • pdq
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Power supply voltage keeps changing from transformer output
Reply #2
You are choking on a grain of rice!

  Love that expression!

  • db1989
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  • Global Moderator
Power supply voltage keeps changing from transformer output
Reply #3
This is at least the third time you’ve posted a thread in General Audio that obviously doesn’t belong there. Please stop doing that.

  • john11
  • [*]
Power supply voltage keeps changing from transformer output
Reply #4
Many apologies. I saw a few other people had posted amp questions here so assumed it would be ok.

Power supply voltage keeps changing from transformer output
Reply #5
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.


Do you think the line Voltage is a constant? It wobbles all over the place. If you try your test with a brand new first time plugged in unit you'll find the same thing. If the line Voltage WAS constant, why would we need regulators? - And nearly every solid state gadget you own has regulators - LOTS of them. Power supply regulators are an art and science of their own.


  • DVDdoug
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Power supply voltage keeps changing from transformer output
Reply #6
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.  )

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.
  • Last Edit: 10 January, 2013, 01:21:19 PM by DVDdoug