Post by KeithL on Dec 28, 2014 2:41:25 GMT -5
Right you are; although I'm inclined to classify autoformers as simply a specific type of transformer- McIntosh used autoformers on most of their solid state models (otherwise sometimes called auto-transformers). Other than not having isolation between the primary and secondary windings, an autoformer works about the same way as a transformer when it comes to impedance matching. The old Mac tube amps used very cool transformers; they were bifilar wound, and had separate windings that were coupled into both the cathode and plate of each output tube; their marketing term was "unity coupled". This enabled McIntosh to deliver very good performance from their tube amps, better than most of their competitors (and better than some early solid state equipment), but the transformers were very expensive to make, which was one of several reasons why their amps were so expensive.
The other amp you mentioned was the original Sunfire Load Invariant Power Amp - back when Bob Carver ran Sunfire (I had one; it was a very nice amp - although I can't say I ever heard any benefit from putting the resistor in line and screwing up the damping factor.) However, saying it was operating as "a current source amplifier" was a bit of a humbug. A true Voltage Source amplifier has a very low output impedance, and modern solid state amps come pretty close to that; but a true Current Source amplifier would have a very high (theoretically infinite) output impedance, and neither tube amps nor the Sunfire are close enough to be considered as such. They're simply voltage source amplifiers that are very far from optimum operation as such.
(Speakers designed to be used with tube amps, especially older speakers, tend to be mechanically damped on their own. If you pair such a speaker with a modern amplifier which also exerts good electrical damping, the speaker may end up sounding "dry" or lacking in bass. Likewise, if you pair a modern speaker, which "expects" to be electrically tightly controlled by the amplifier, with an amplifier with a low damping factor, it will often sound "flabby" and "blubbery". I can't honestly say I've heard very many speakers that sounded better with less damping, but I'm sure there are a few. It's easy enough to try for yourself - a 1 ohm 20 watt power resistor costs only a few dollars - just make sure to set it someplace safe while you're trying it... power resistors can get very hot.)
As it turns out, it's also actually pretty simple to design a real Current Source amplifier, and there are some theoretical advantages to be had if you use one with certain drivers, and over certain frequency ranges. Unfortunately, drivers and crossovers designed to be used with normal amplifiers interact rather badly with a true current source, and not a lot of work has been done towards designing ones that would work well.
One of the biggest differences between TRUE bi-amping (with the crossover between the preamp and power amps) is damping. As long as the passive crossover components remain in-circuit between the amps and the speaker drivers, as they do with "vertical biamping", they raise the impedance of the circuit, and so drastically reduce the actual damping which the amplifier can use to control the speaker. In a true bi-amplified speaker, each amplifier is connected directly to the speaker driver it runs, with no crossover components between the amp and the speaker drivers; this gives the amplifier much tighter control over the movement of the driver. Many midranges, and even tweeters, can benefit from the damping they get from being connected directly to an amplifier.
2 DPOLMAN:
I run a PAIR of Parasound amps with my Maggies. They are setup for easy biwiring or biamping. However, I don't consider it biamping when you still have the speaker level crossover involved. Follow this: When you remove the speaker level crossover AND add a Line Level crossover BETWEEN the amp and preamp, you can gain as much as 3db in amp power. That's like DOUBLE the amp power. Your amps are currently called upon, as are mine, to deal with 'out of band' signals which take power for NO audible benefit.
My goal is to DELETE the speaker level crossover and connect EACH driver to its OWN amp. Between the amp(s) and preamp will be a crossover. And in doing my homework, I find I have a choice of 2 major types. The 'Passive' requires knowing the input and output impedance of the connecting gear and SHORT cable runs. The 'Active' crossover is generally opamp driven doesn't care too much about the impedance of anything AND can drive a longer run of cable. BUT, you need a very good (quiet) power supply and more engineering. Kits are available. Personally? I'm going passive. I can EASILY duplicate my speakers transfer function and locate the crossover right AT the amp. The run FROM the pre is no problem at <8 feet a side. Also, most active crossovers can be as great as 24 db/octave which I've never heard anything GOOD about, except maybe as a sub crossover. I'm not about to Re-Engineer and second guess the guys that designed and built my Maggies.
And Keith, Mc uses AUTOFORMERS on many SS amps. NOT transformers, as you correctly noted. If anyone cares, I'll do a little writeup on the Differences between 'em…….
If I remember correctly, Carver had an amp with both Current Source (similar to tube amps) and Voltage Source (the ideal SS amp) which basically inserted some additional resistance into the line. Some speakers Prefer one or the other.
The other amp you mentioned was the original Sunfire Load Invariant Power Amp - back when Bob Carver ran Sunfire (I had one; it was a very nice amp - although I can't say I ever heard any benefit from putting the resistor in line and screwing up the damping factor.) However, saying it was operating as "a current source amplifier" was a bit of a humbug. A true Voltage Source amplifier has a very low output impedance, and modern solid state amps come pretty close to that; but a true Current Source amplifier would have a very high (theoretically infinite) output impedance, and neither tube amps nor the Sunfire are close enough to be considered as such. They're simply voltage source amplifiers that are very far from optimum operation as such.
(Speakers designed to be used with tube amps, especially older speakers, tend to be mechanically damped on their own. If you pair such a speaker with a modern amplifier which also exerts good electrical damping, the speaker may end up sounding "dry" or lacking in bass. Likewise, if you pair a modern speaker, which "expects" to be electrically tightly controlled by the amplifier, with an amplifier with a low damping factor, it will often sound "flabby" and "blubbery". I can't honestly say I've heard very many speakers that sounded better with less damping, but I'm sure there are a few. It's easy enough to try for yourself - a 1 ohm 20 watt power resistor costs only a few dollars - just make sure to set it someplace safe while you're trying it... power resistors can get very hot.)
As it turns out, it's also actually pretty simple to design a real Current Source amplifier, and there are some theoretical advantages to be had if you use one with certain drivers, and over certain frequency ranges. Unfortunately, drivers and crossovers designed to be used with normal amplifiers interact rather badly with a true current source, and not a lot of work has been done towards designing ones that would work well.
One of the biggest differences between TRUE bi-amping (with the crossover between the preamp and power amps) is damping. As long as the passive crossover components remain in-circuit between the amps and the speaker drivers, as they do with "vertical biamping", they raise the impedance of the circuit, and so drastically reduce the actual damping which the amplifier can use to control the speaker. In a true bi-amplified speaker, each amplifier is connected directly to the speaker driver it runs, with no crossover components between the amp and the speaker drivers; this gives the amplifier much tighter control over the movement of the driver. Many midranges, and even tweeters, can benefit from the damping they get from being connected directly to an amplifier.
I run a PAIR of Parasound amps with my Maggies. They are setup for easy biwiring or biamping. However, I don't consider it biamping when you still have the speaker level crossover involved. Follow this: When you remove the speaker level crossover AND add a Line Level crossover BETWEEN the amp and preamp, you can gain as much as 3db in amp power. That's like DOUBLE the amp power. Your amps are currently called upon, as are mine, to deal with 'out of band' signals which take power for NO audible benefit.
My goal is to DELETE the speaker level crossover and connect EACH driver to its OWN amp. Between the amp(s) and preamp will be a crossover. And in doing my homework, I find I have a choice of 2 major types. The 'Passive' requires knowing the input and output impedance of the connecting gear and SHORT cable runs. The 'Active' crossover is generally opamp driven doesn't care too much about the impedance of anything AND can drive a longer run of cable. BUT, you need a very good (quiet) power supply and more engineering. Kits are available. Personally? I'm going passive. I can EASILY duplicate my speakers transfer function and locate the crossover right AT the amp. The run FROM the pre is no problem at <8 feet a side. Also, most active crossovers can be as great as 24 db/octave which I've never heard anything GOOD about, except maybe as a sub crossover. I'm not about to Re-Engineer and second guess the guys that designed and built my Maggies.
And Keith, Mc uses AUTOFORMERS on many SS amps. NOT transformers, as you correctly noted. If anyone cares, I'll do a little writeup on the Differences between 'em…….
If I remember correctly, Carver had an amp with both Current Source (similar to tube amps) and Voltage Source (the ideal SS amp) which basically inserted some additional resistance into the line. Some speakers Prefer one or the other.
