Which basic amps can drive 1 ohm loads ?

There was a guy here on Audiogon that was running Perreaux PMF-2150's in bridged mode to drive Apogee's. I can't remember who it was, but that is one helluva testimony to the ruggedness / reliability of the Perreaux. In bridged mode, the amps were seeing somewhere around half an ohm (.5 ohms) !!!

As a side note, my Perreaux 3150's have no problem driving very low impedances. Much of this has to do with the heavy gauge output inductor that Perreaux uses. Not only does the amp maintain a low output impedance as power is increased*, it can pass more current on demand as needed without any type of restriction. That's why these amps, when running properly, have great bass impact. Most amps have relatively scrawny output inductors, so they lack the capacity to pass enough current as needed. The end result is a lack of control as impedances are reduced and a reduction in "slam" at any impedance.

Other than that, the two channel Sunfire Signature will drive Apogee's with no problem. Sunfire even has a specific set of "factory mods" that they do for Apogee's. I've had this done to my Signature and it noticeably increased the bass impact. One of the reasons for better bass response ? They install larger output inductors to pass more current : ) Sean
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* damping factor varies with the dynamic load and output levels that the amps sees and responds to. Many "techs" & "engineers" will say i'm off my rocker, so believe whom you want.

Just to clarify things, the "Apogee mod" is the slang term that Sunfire uses for the mods that they do to their amps to better deal with very tough to drive, low impedance, low sensitivity speakers. Since Apogee's are the "ultimate" in terms of "low impedance, low output, high reactance" speakers, they chose to name the mods for the amps after those speakers. As such, Sunfire does not perform modifications to Apogee speakers.

Other than that, i'm sure that there are mods and tweaks that one can do to the various Apogee models. Sean
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Ghostrider: Thanks for the clarification. For some reason, i was thinking that these were quite reactive loads.

As far as 1 ohm speakers go, some of the Acoustats ( and possibly other E-stat's ) hit 1 ohm at very high frequencies. I know that my Ohm's drop well below 2 ohms but do so at low frequencies. Nothing like asking an amp to deliver all it can at frequencies that already need gobs of current...

As far as the Scintilla's dropping down to .6 ohms, good luck : ) If we put 2.83 volts into an 8 ohm speaker, we have about .353 amps flowing through the output stage of the amp. This is equivalent to one watt of power. If we put that same 2.83 volts into that .6 ohm load, we've got almost FIVE amps ( 4.717 ) flowing throw the output stage. This is literally 13+ times the amount of current. Needless to say, MASSIVE heatsinks and / or forced cooling would be pretty much mandatory. This is especially true if you were "standing on the throttle" for any amount of time. Sean
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PS... I don't think that the guy that was using the bridged Perreaux 2150's was using them to drive Scintilla's : )

PPS... People with Scintilla's can charge folks money to break in their power cords. If they rig up adapters, they can use power cords as speaker cables and draw as much or more current through them as a refrigerator would : )

I wanted to make note that the use of an output inductor in an amp will LOWER damping factor at higher frequencies. By using a heavier gauge conductor, the effects of this are minimized AND more current can be passed as discussed above. Sean
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I've got what would be a great deal for ya on a modified Sony receiver. I know it will work like gang-busters on those speakers.... : ) Sean
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Here's something else to think about folks. That is, the lower the impedance, the less time that the amp stays in Class A and the sooner it switches over to Class B. The more switching of operating Class that takes place, the "grungier" and less consistent the amp will sound ( in most cases ).

With that in mind, a "typical" amp that is of a "rich AB" design is typically 6 - 10 wpc at 8 ohms in Class A and then switches over to Class B at power levels above that point. If one has a 4 ohm load, the same amp will only run in Class A up to about 3 - 5 wpc. At 2 ohms, you get 1.5 - 2.5 wpc in Class A and at 1 ohm, you end up with .75 - 1.25 wpc in Class A. As such, one would have to have an amp that is VERY richly biased if you wanted to stay in Class A mode for the majority of listening on a very low impedance, low sensitivity speaker. In order to do this, you need an amp that has GOBS of heatsinking and current capacity to say the least. Otherwise the amp will both starve itself and cook itself to death at the same time.

With that in mind, a rather unique approach regarding bias levels were chosen when John Curl designed the Parasound JC-1's. The JC-1's are switchable between "rich biasing" and "very rich biasing" for these purposes. In the lower bias mode, it runs in Class A up to 10 wpc and then switches over to Class B mode. In the high bias mode, it runs in Class A up to about 25 - 30 wpc* and then switches over to Class B above that point. Using this approach, one can let the amps idle in "low bias" mode when not in use, which will keep them as warm or warmer than most other amps, and then switch over into high bias mode when listening. This approach allows one to have the benefits of "ultra high bias" in terms of sonics yet retain the benefits of "medium to high bias" by cutting down on power consumption and heat build-up. I'm not suggesting that this amp is the right amp for Apogee's or any other specific speaker, but that it is a very unique and "user friendly" approach to designing real world products.

Doing the math with the JC-1 as a reference and starting off with the conservative rating of 25 wpc of Class A operation available at 8 ohms, this would give us something like 12.5 wpc @ 4 ohms, 6.25 wpc @ 2 ohms and slightly over 3 wpc @ 1 ohm. All of these figures are based on the amp operating in Class A and then switching to Class B operation above that point. Given that most listening occurs with less than a watt or two being used ( except for rock music ), the JC's should retain their sonic characteristics relatively consistently for most uses with just about any load. This is due to the consistent electrical characteristics that the amp would display while remaining in Class A operation and the lack of switching distortion that all amplifiers will demonstrate when pushed harder.** Sean
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* Stereophile measured 27 watts in Class A on their review samples.

** This is all based on theory and not first hand familiarity with this amp. I chose it as i was familiar with the electrical characteristics of the amp as advertised and as measured by Stereophile. Other amps, like the Bear Labs Symphony, Pass Labs X series, larger Krell's, etc..., may perform quite similarly due to the very high levels of bias applied to the output stage.