damping factor and speaker design
OK, up front here: I'm an SET enthusiast. There have been several comments in this thread about the importance of high damping factor, which SET amps don't have. So it seems a good place to say my piece. Which is basically that the damping factor should be matched to the speaker.
The total resistance - voice coil plus cable plus amplifier - is part of the cabinet design. The oldest JBL woofers (130A and its relatives) have an incredibly low Q, which provided great efficiency and worked well with amps of high output impedance, as was common before the Williamson amp made large amounts of negative feedback popular.
A modern Theile-Small optimum design for a 2220 with a high damping factor amplifier is 0.75 cubic feet, and extends down to 130dB - not much of a woofer! In a 5 cubic foot cabinet (such as JBL designed and recommended in the late forties/early fifties) you could get down to 60Hz, with a 3dB ripple and a strong, narrow hump at 70Hz. Pretty poor. But double the electrical Q by driving it with an amp whose resistance equals the DC resistance, and suddenly you get a beautiful maximally-flat Butterworth alignment down to 50Hz.
Now the 130A had a VC resistance of around 5-6 ohms, but was labeled 16 ohms in those days. An SET amp, or a triode push-pull such as was popular at the time, with no feedback, has a damping factor around 3 - i.e. an output impedance of about 5 ohms on the 16 ohm tap.
Put a 2235 in the same box, and you can get deeper bass, very flat to 35Hz with a high fedback, high damping amp. But drive it with the high impedance amp, and it acquires a giant booming peak of 6dB or more around 60Hz, unless you put it into a huge 25 cubic foot box.
So yes, an SET amp (or any amp with little or no feedback) will make a modern speaker design boom. But a high-feedback, high damping amp will generally make a well-designed vintage speaker boom too. Combine an SET (typical damping factor 2 to 3) with a speaker designed for that amount of amplifier damping, and it works just as well as a modern amp/speaker except the vintage setup is more efficient.
Some amps from the fifties, including my own pair of Heathkit UA-1's, had a complex feedback scheme so that a unity damping factor could be generated even with high amounts of feedback. Presumably this was so they could drive the "original JBL/Altec/etc" kind of speakers correctly.
And now, back to your regularly scheduled entertainment ...
Re: damping factor and speaker design
"Which is basically that the damping factor should be matched to the speaker."
How true! But pretty impractical for the person who is exhausted after programming a VCR.
Your points are one reason why I really enjoy the computer software packages of today because all these factors can be taken into account if desired. It sure beats all the myriad formulas I used to have to work through! :(
Re: damping factor and speaker design
Quote:
Originally posted by Paul Joppa
So yes, an SET amp (or any amp with little or no feedback) will make a modern speaker design boom. But a high-feedback, high damping amp will generally make a well-designed vintage speaker boom too. Combine an SET (typical damping factor 2 to 3) with a speaker designed for that amount of amplifier damping, and it works just as well as a modern amp/speaker except the vintage setup is more efficient.
I think you're absolutely right Paul. It is the question of finding an amplifier with the right amount of damping for a speaker with a given amount of damping. Or vice versa ...
As a lover of SET amps (I have a 300B Uchida amp which I don't use currently), I must admit that it was quite difficult to find a satisfying combination for my 4430s. I'm currently using a KT88 (triode) push-pull amp in my system. From time to time I swap with my Hafler 9505 (excellent MOSFET amp with almost no sonic contribution). I think the challenge is to design an amplifier (tube or SS or both) with SET sound, low output impedance and no feedback.
I'll face that ...
Best regards,
Norbert
