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Re: early 70s v late 70s

Posted by Bold Eagle (A) on March 17, 2004 at 15:07:01

In Reply to: early 70s v late 70s posted by Tadlo on March 17, 2004 at 12:04:27:

I give you my take on this. I've noticed the same thing.

First, you have to understand that speakers and amplifiers interact by virtue of the source impedance of the amplifier and the impedance curve of the speaker. Source impedance is usually expressed as Damping Factor (DF), which is calculated from measurements of source impedance. Low DF equates to high source impedance.

Tube amps of the 50's and 60's had low DF, so the source impedance was fairly high and the interaction moderate. Probably on the order of 0.5 to 1.5 dB difference in frequency response over the whole audible range. Speaker designers would "voice" the speakers to have the desired response with DF in the range of 6 to 17 (typical 60's power amp sections).

Along comes solid state. The DF can be a lot higher, like 30-60. Now the interaction with the speakers is on the order of 0.5 dB or less, nearly inaudible change. But all the speakers available are voiced for much lower DF. So the amplifier people tricked up the circuits to sound more like a tube amp with those speakers. Capacitive coupling helps this, but it's not the only thing needed. It's not why they went to capacitive coupling, by the way. If you didn't modify the amp circuits, the bass would be too thin, and the mids recessive with the highs too bright in the upper mids and too soft in the extreme highs. If you drive a 60's speaker with a late 70's amp, that's what you get: "transistor sound".

But when solid state amps were clearly going to be in the majority, speaker manufacturers now started to voice the speakers for the higher DF of the solid state amps. So now the amp manufacturers didn't need to build in "backward compatibility", and could design for a high DF and a pretty normal circuit. This began in the middle and 70's and by the late 70's both speakers and amplifiers were fully worked out for direct coupled SS amps.

Because of this I feel it is always best to match amps and speakers of the same vintage. Makes sense, does it not? If you were in business to sell amps or speakers you'd design to sound right with the contemporary models of speakers or amps in the same price range, wouldn't you?

I think this is why McIntosh designed their first generations of SS amps with output transformers, so they'd have similar source impedance and interactions to their tube line. Most people think they sound very much alike. Later, Mac changed to direct coupled, and revoiced the speakers as well.

Jerry

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