troporobo wrote:Synergy gurus: can somebody help me understand this please? I have been reading on the 'net from time to time that some people deliberately connect their 8 ohm speakers to the 4 ohm taps, or vice versa. I have not been paying close attention, but maybe this is more prevalent with low power tube amps. What's the theory behind doing it one way or another?
I was thinking about this last night after hooking up a pair of speakers I have on trial (many thanks again to a kind pinoydiophile!). The impedance wasn't labelled and I had forgotten to check, so left my cables on the 4 ohm taps (on an 8 wpc EL34 SET amp). The speakers sounded terrible, really flabby bass, flat mids, recessed highs. Turns out the speakers are 8 ohm and only 86 or 88 dB sensitivity. So I changed to the 8 ohm taps . . . success! Bass tightened up dramatically, highs and mids were brought forward and balanced in proportion, all was right with the world.
What I want to know is, why is this? What is the speaker / amp interaction that's happening? And why would some people deliberately use a seemingly mis-matched connection?
Thanks in advance
Sirs,
For tube and solid state amplifiers, it is necessary that the output impedance must match speaker impedance for maximum power transfer. Since tube amplifiers have transformer coupled output stage it is easier to match impedance. As such, if output transformer impedance is 4ohm, speaker impedance must also be 4ohms. This is the same scenario for 8ohms.
In this configuration, if the amplifier is capable of delivering 10watts output power, 5watts each will be consumed by the amplifier and speaker corresponding to a 50% and 50% power ratio.
As an example, if there is impedance mismatch as follows:
output transformer = 4ohm, speaker = 8ohm
the power ratio will be 1/3 and 2/3. This is not good for the amplifier or speaker as they will be consuming less/more than necessary. In example above, 1/3 or 3.33watts for amp and 2/3 or 6.66watts for the speaker. This is bad for the speaker as it is consuming more power than required. For the opposite scenario (output xformer = 8ohms, spkr = 4ohms), 6.66watts for the amplifier would mean driving the power device eg vacuum tube 33.33% more than required. There is more power stress in the tubes.
In the impedance mismatch scenario, there is more current to the power device - vacuum tube, output transformer and speaker. This can't be good to either of them regardless if the design had extra margin.
As such, connect 8ohm speaker to 8ohm output terminal and 4ohm speaker to 4ohm output terminal. Amplifier or speaker life must always be considered before pleasure.
Hope this information helps.
Best regards,
Rascal101