Thd, Wpc, & Lies!!!!!!!!!!!!!!

Discussion in 'Archived Threads 2001-2004' started by Chris_A_J, Jun 15, 2002.

  1. Chris_A_J

    Chris_A_J Extra

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    Can someone explain this to me:
    I just put a down payment at my local hi-fi shop on a Marantz SR8000. The salesman told me that it was 120x5. I came home to double check the specs and found out it was only able to push 105x5. I called the hi-fi shop and told the salesman about my discovery and he said that the receiver did push 105x5 at 0.05% THD. He continued saying that this receiver pushed out 120x5 at 0.1% THD.
    Is he lying? I thought a receiver's WPC only increased if you went to 6 or 4 ohms....I cancelled my order since I felt he mislead me, but I would still like to know the truth. I guess I'll have to go ahead and bite the bullet and fork over the extra $$$ for the SR8200 [​IMG]
    Thanks
     
  2. Joseph_W

    Joseph_W Stunt Coordinator

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    Maybe not lying, but definitely missleading. Noise (Total harhomic distortion - THD) definitely gets higher as the amp is pushed harder. But the ability to drive more watts at a lower ohms depends on the current capability of the amp. For an amp to deliver clean power, it must not clip voltage or current. Since lower impedence (ohms) requires less voltage and more current for the same watts, an amp which can easily carry more amps delivers more watts to a 4 or 6 ohm speaker.

    That being said, I'd be upset if a saleman set his own limits to the specs he quoted me. He should be quoting manufactures specs, in my opinion.

    Joe
     
  3. Saurav

    Saurav Cinematographer

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  4. Chris_A_J

    Chris_A_J Extra

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    Thanks for the info....
     
  5. matthew_rm

    matthew_rm Second Unit

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    How does THD matter? I know it's not hiss, and I have read that it has nothing to do with sound unless you play some kind of "1KZ sine wave" or something like that.[​IMG]
     
  6. Saurav

    Saurav Cinematographer

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  7. Wayne_T

    Wayne_T Stunt Coordinator

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    Based on the many threads on this topic, and published test results, it seems that it is common practice to over rate receiver power or to mislead, for example quote power with one channel driven vs all channels driven. On the other hand, I can't remember ever seeing an instance where a power amp was overrated. I guess if the only thing you do is amplify, then there's no where to hide, whereas receivers can be hyped for all kinds of things - DACs, supported formats, bass management, tuners, # and type of inputs, etc, so who's gonna notice a little lie about power output.
     
  8. Adil M

    Adil M Supporting Actor

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    HK, no lies, just clean sound.
     
  9. Paul Clarke

    Paul Clarke Supporting Actor

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    Amen, Adil.
     
  10. StevenK

    StevenK Second Unit

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    but questionable build quality and lacking features....(about the H/K that is)
     
  11. Chu Gai

    Chu Gai Lead Actor

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    I don't know if i'd have cancelled the order...105, 120...not a big thing. If I'd really have been satisfied with the receiver, I'd have bought it although I'd have asked him for something hard copy to substantiate it. Goes back to the joke...
    Q: what's the difference between a used car salesman and a person selling audio?
    A: the used car salesman knows when he's lying.
    no offence to anyone [​IMG]
     
  12. Phil Mays

    Phil Mays Second Unit

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    Chris,
    If power is what (watt)[​IMG] you are looking for then why not purchase an amp.
    After much debate in my mind and help on this forum from others I have decided to upgrade my Onkyo 797 at 100WPC (or whatever) with an exteranal amplifier. I was going to purchase a new A/V receiver for $2,900. I have found I can get a great used 5 channel amp for about $1,000. I have even seen them down to around $400.
    I do not know the cost difference between the units you mentioned but certainly if the lesser expensive unit has the features you want (sounds like it did as you had initiated the purchase process) then perhaps the saving could go towards a amp.
    Of course you have the issue of the sales person...I hate being mislead.
    Phil
     
  13. Blake R

    Blake R Stunt Coordinator

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    I don't believe a .05% differential in THD over an ouput range of 15 watts is hardly worth getting agitated about. It's like someone else has said unless your listening to pure tones (1 kHZ sine wave for ex.) it makes little difference. If you were listening to a 1 kHz tone and your amplifier introduced a second harmonic at .1% of the input signal amplitude, do you think you'd hear that 2 kHz harmonic in there? Probably not.

    If you really like the receiver why not get it? If you don't trust the salesman, get it somewhere else.
     
  14. Chris_A_J

    Chris_A_J Extra

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    Well that's a good point....

    Back to the possibility of having an external amp, what is the cheapest receiver(with good soung quality) that has at least 5.1 inputs(DVD-A) and 5.1 preouts? PLII and all the ES modes are not important to me. Is something like a Parasound 855 going to be on the same output level as the SR8200? I'm just looking to get the cleanest and loudest sound I can get for less than $1,500.

    BTW, this will be pushing Paradigm Monitor 9's, CC-370, and ADP-370's. 80% HT and 20% music.....
     
  15. Rich Malloy

    Rich Malloy Producer

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  16. Adil M

    Adil M Supporting Actor

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    The Sony ES line has great Bass management in the triple xover. This paired w/a solid amp would be a hell of a start. If you really like the Marantz, just get it elsewhere. Is sounds like the salesman deliberately misguided you in convincing you to get it. Only you know the context of the situation.
     
  17. Mark Tranchant

    Mark Tranchant Stunt Coordinator

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    Chris,

    Reading your first post it seems to me that you are uncertain of exactly what power output is - it's your reference to "a receiver's WPC" that unsettles me. If I'm barking up the wrong tree here then please forgive me.

    We are only really talking about the power amp section of the receiver here, as that is what "pushes out the watts". A power amp takes a signal voltage in (from the pre-amp) and gives out what should be an exact copy, but multiplied by its gain factor. The key feature of this output voltage is a very low output impedance, meaning that it can drive low impedance loads like speakers.

    Now the number of watts the amp actually puts out depends on the input voltage from the preamp, the (fixed) gain factor and the impedance of the speaker. What that power output then means in acoustic terms is down to the efficiency of your speakers. Let's take an example (using rms values throughout):

    With a gain factor of 10, and an input voltage of 2V, the output of the power amp will be 20V. Ohm's law says V=RI, and we know that power is voltage times current (P=VI), so P=V²/R. An 8-ohm speaker will therefore be getting 50W from the amp (20²/8 = 400/8 = 50). This is 20V at 2.5A. Into 4 ohms, the amp will be putting out 100W (400/4), 20V at 5A.

    If you turn the volume down, the voltage from the preamp decreases - at 0.5Vrms, the amp will be putting out just over 3W into 8 ohms (try it yourself). In fact, most of the time you'll be using surprisingly low output power, due to the square in the equation above. This is how the world's most expensive valve hi-fi amps get away with outputs of 10W and less.

    Every power amp has a current output limit and a voltage output limit, which is usually dependent on the power supply design. As you can see in the example above, lower impedance speakers require a higher current output to hold a given voltage, so you're more likely to hit the current limit with lower impedance speakers. An amp with a voltage limit of 30V and a current limit of 5A can drive up to 112.5W into 8 ohms (30²/8, current is 3.75A) limited by voltage and 100W into 4 ohms (5²x4, voltage is 25V) limited by current.

    With multi-channel amps, the voltage limit is the same across all the amps, but the current limit is distributed between them (very simplistically). This is why the single-channel output is higher than the all-channels-per-channel output. With a huge (heavy, expensive, overkill) power supply, you could design a receiver with no multi-channel downrating.

    Prior to hitting one of these limits, distortion increases with output (and thus input voltage). So it is perfectly fair to quote an amp at 105W at 0.05% THD and 120W at 0.1% THD, so long as the statement is correct. What he's saying is that if you give it an input voltage that drives 105W into 8 ohms, you'll measure 0.05% THD; if you then turn it up a bit, 120W will give you 0.1% THD. You could probably turn it up further at the expense of more serious distortion - many of the popular power amp ICs (midi system applications) rate their power at 10% THD!

    As usual, don't worry too much about the numbers, listen to the sound.

    Joseph - noise is not THD. THD is a function of the input signal, noise is independent of the input signal.
     
  18. Tung Enkhych

    Tung Enkhych Extra

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    Q: what's the difference between a used car salesman and a person selling audio?
    A: the used car salesman knows when he's lying.

    ----------------------------------
    I don't neccesarily think that it's the salespersons fault here, as too often the manufacturer publishes differant specs for the same unit.....
    Ask yourself when looking at wattage ratings these questions:
    Are they testing all frequencies or just testing at 1khz?
    Are they measuring with less than 0.05% THD or are they allowing for distortion at the measured output?
    Are they rating 1 channel driven at a time , 2 at a time , or all channels driven?
    Are they rating current output sustainable or peak ?

    Understand that they are marketing to fsr less sophisticated a consumer than we might find here....
    Understand that when bb,cc, gg haS no one who can explain the differances then most mfgrs will publish the same kind of misleading spec's that Aiwa uses for their boomboxes!!!!
    We are Americans BABY, the bigger the wattage # ,the better it must be right?
     

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