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Impedance Switch on Amplifiers

Discussion in 'AV Receivers & Amplifiers' started by General Skanky, Jan 25, 2004.

  1. General Skanky

    General Skanky
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    Just a bit confused.

    Why do amps have an impedance switch? I know what they are supposed to do, ie speaker load tolerance etc.

    But..................

    SMR reviews I've read in the past have not been too keen on seeing them on the back of amps implying a poor 'quality' power supply in the amp. Switches like those are found typically on the back of Yamaha amps and the new Arcam AVR300. IMHO, I wouldn't have said they are bad amps at all, in fact, the total opposite. I've owned a couple of Yamahas and found them to be pretty good. Their new AZ9 looks to be a total monster in that dept.

    So the crux. If the switch is frowned upon, why include them on good quality amps. I'm sure I've seen John Dawson comment here on the forums about the new AVR300 being a ground up design, with a lot of time invested by him in it. Surely contradictory if the switch is really a bad thing.

    :confused: :confused: :confused:
     
  2. John Dawson

    John Dawson
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    Hi Guy,

    The speaker impedance switch is, in my opinion, a very useful device in products such as AV receivers in order to extract the best possible performance from products where both price and space are limited.

    I can't comment on other manufacturers' implementations but in the Arcam receivers the switch activates an internal heavy duty relay which alters the voltage of the main power rails supplying the power amplifiers. This is not done with anything so crude as resistors but by having taps on the secondaries of the main power transformer.

    It then allows the unit to meet its specification into both 4 and 8 ohm loads without risk of overstressing the output stages or overheating. So, in the case of the AVR300, we can get 100 WPC of continuous output power into either 4 or 8 ohm (nominal) speaker loads without having heavily to overengineer or alternatively excessively current limit the power amplifiers. Incidentally these power ratings are continuous, with all 7 channels driven simultaneously, whereas many receivers with huge quoted power specs simply and audibly run out of grunt under such conditions. There's quite a bit of misinformation out there!!

    I hope this helps clarify matters somewhat.

    John Dawson (Arcam)
     
  3. General Skanky

    General Skanky
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    Thanks very much. That has cleared that right up. Conflicting write ups in the past have always made me wonder.
     
  4. Gatto

    Gatto
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    General Skanky,

    There are receivers, which do not need a switch at the Back to set the impedance for the speakers being connected too. Speaker rated at 4 ohms, can go down to 2 ohms at certain instances. A good power supply will be able to drive and adapt itself to low impedance speakers without the need of any switch at the back.
     
  5. General Skanky

    General Skanky
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    That I know and understand. But aren't you going against what John has just said?
     
  6. fraggle

    fraggle
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    What John said was the amps output at both 4 and 8 ohms was 100W x 7.

    Since, given a particular output voltage at the speaker terminals, the current flowing (and hence power) will depend on the speakers impedance. Halve the speaker impedance and (theoretically) the power should double.

    So the only way to maintain 100W at 4 AND 8 ohms is to reduce the voltage powering the output stages, which is what the Arcam switch does. (I've no doubt there are other ways of doing it that involve electronic wizardry, but more complex designs muck around with the output signal more IMHO)

    Without the switch in the power supply, the power supply, output transistors and everything else in the output line would need to be massively overrated to be able to supple the huge output current needed at 4 ohms and below.

    Of course if the PSU has sufficient reserve on tap to power high transients (at 8 ohms) and you stick lower impedance speakers on it the PSU might be reasonably happy but if the output transistors were rated at 100W (or say 150W to allow for a bit of headroom) they'd overheat and blow at 200W.

    Swings and roundabouts! :) And counting the pennies, satisfying everyone, etc, etc...

    In a nutshell if you've got a cheap amp the PSU won't have enough "oomph" to need any form of limiting, it'll just distort.
    Move up a bit and the PSU will be beefier to cope with high transients, if its beefier enough it might blow the output trannies UNLESS its got electronic protection OR a switch.
    Move a lot higher up and you get the amps that can say deliver 2000W at 1 ohm, 1000W at 2, 500W at 4, 250W at 8 ohm, all RMS. But it'll need 3 people to lift it and a second mortgage to buy it :rotfl: :rotfl:
     
  7. General Skanky

    General Skanky
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    So................. it is a method of achieving a desired performance from the power supply within a specific budget.

    ie, the AVR300 is an AV receiver and does so much more than a stereo amp let's say, so budget is spread about a bit more. The switch allows a level of performance without sacrificing elsewhere.

    Whereas on a similairly priced hifi amp a greater proportion of the budget allows for a 'different' spec power supply to be used.

    How it sounds is then a matter of opinion. But the AVR300 should then come close to a hifi amp of the same price - ish.
     
  8. General Skanky

    General Skanky
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    Easy to see why a statement about impedance switches being 'undesirable' is not actually totally correct.
     

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