Thiel 2.4 and Quad 405-2

Discussion in 'Home Cinema Speakers' started by barkersoldbean, Aug 17, 2012.

  1. barkersoldbean

    barkersoldbean
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    Does anyone have actual experience of successfully driving a set of Thiel 2.4 speakers (impedance 4ohm - 3ohm) with a Quad 405-2 power amp?

    I note that the 405-2 typically doesn't like low impedance speakers, but electrostatics are 4ohm and they seem to work ok?
     
  2. Mark.Yudkin

    Mark.Yudkin
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    The ESL-63 is nominally 8 Ohms, the actual impedance varies from 3.5 Ohms at 10KHz, to over 20 Ohms below 100Hz (this is level-dependent). [Service Manual, page 8].

    The highest current draw is at lower frequencies, where the Quad ESL's have the highest impedance. The Thiels are likely rather different in this respect. OTOH, 4 Ohms isn't all that low, my current ESLs go down to 0.8 Ohms and I'm happily driving them from my 7 channel Arcam P1000.

    The Thiels have a sensitivity of 87dB @ 2.83V / 1m, so I'd guess that the Quad 405 would be OK at sensible levels, but perhaps not at party volumes. Can't you arrange a trial? Ands while you're at it, also a trial of some ESLs, to get an idea of real sound quality:)?
     
  3. barkersoldbean

    barkersoldbean
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    Thanks for the reply - Despite being a lay person in the electronics area, I think that I am getting it now - the issue is not the impedance per se, but more the fact that as you try and drive them hard, the amplifier runs out of puff at higher volume levels and then distorts?
    = at lowish (non-party) volume levels = no problems.
    T
     
  4. Mark.Yudkin

    Mark.Yudkin
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    The problem isn't so much distortion as clipping - trying to draw more power than the 405 can supply.

    When I write "party levels" I mean the sort of volumes that drive your neighbours crazy and get the police around. The 405's should be quite fine at producing and maintaining levels that at a work place would legally require ear protection.

    The impedance plays a role in the sense that the power draw is: W = V**2/Z, (W: power, Z: impedance, V: voltage - the thing an amplifier amplifies, as per its volume control). So speakers with a low impedance will draw more power for the same voltage, although it's the voltage that determines the level.

    As mentioned, the Thiels have a senstivity of 87dB @ 2.83V / 1m. That means that a 2.83V signal (that's 1W into 8 ohms: 2.83**2/8 = 1) gives a sound pressure level of 87dB measured at 1 meter from the speaker in an anechoic room. Using two speakers in a real room raises the actual SPL for 1W at 1m to over 90dB. Hearing damage due to prolonged exposure commences at 85dB.

    The reality is that you don't need much power when staying with sensible listening levels - sensible being defined as not harmful.
     
    Last edited: Aug 19, 2012

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