Using Adcom GFA-555s as monoblocks to power magnepan 3.3s in active biamp

In a somewhat analogous situation, I ran a pair of Hafler DH-220s bridged into a pair of Acoustat 1+1 and RHLabs sub. I thought  it sounded fine until someone smarter than me pointed out that the amps were likely quite unstable in that configuration. So as an experiment, I switched them out of bridged mode and used only one channel of each amp to drive the speakers.

I could not believe my ears at the improvement! Even with "less" power the sound was much better, more cohesive top to the now much better bottom. Others here have explained it better than I could but the bottom line is, I have practical experience to say that bridged is NOT the way to go in a low ohm situation.

Happy Listening!

Wow, thanks.  So, another question: is there any harm in running 1 channel of each Adcom?

Bridging an amplifier effectively doubles its output impedance, which halves the damping factor.  A 4 ohm speaker remains 4 ohms, but the amp's ability to stably drive the load is compromised significantly.  So, you can safely bridge into 4 ohm speakers if the manufacturer has designed the amp to be able to handle 2 ohm speakers when used normally.

There's no correlation between a speakers output impedance & its sensitivity.  However, most SS amps produce a lot more watts into 4 ohms than 8, so bridging tends to be more attractive to people with 8 ohm or higher speakers.  The amps are more stable bridged into these loads.  One thing to note, though - reducing the damping lowers the amp's control of the speaker.  This could result in looser sounding bass.

Pardon my limited knowledge on this subject and a question that may be very basic to many of you.  My biggest hurdle with audio equipment has always been the inability to grasp the "electrical side" of the hobby.  I've given great consideration to taking Electronic courses at my local Community College, just so I don't feel like a foreigner in a new country when I read discussions similar to this one.  If any members could suggest a very easy to understand book on this subject, I'd appreciate any and all book titles.  Again, I apologize, but here goes:

So what is the problem if a speaker or amp rated at 4 Ohms is operated at 2 Ohms?  What are the worst case outcomes -- blowing all the fuses, burning the amp up, blowing the speakers or frying the crossovers, catching the house on fire, or Three Mile Island Part 2?  This is why I am so eager to take some classes or read books that aren't written for someone with an Electrical Engineering degree.  I look forward to learning something.  Thanks.

@allenf1963 I tried responding to you earlier today, but lost my carefully typed reply disappeared when I tried to exit “preview.” I shall attempt a recreation. Most of the discussion on the ‘Electrical Side,’ as you put it, revolves around Ohms Law and it’s many implications for circuits in general, and AC circuits in particular (music that has been converted to an electrical signal constantly varies in voltage and current similarity to how standard AC voltage varies in phase and current). The AC power which feeds your amplifier is converted to DC fairly quickly, and is then modulated by the tubes or transistors which serve to in an ‘amplify’ at the output the signal which has been input. This amplification process can take place in more than one stage, each stage taking its input from the previous stage’s output. Great care is taken in the early stages to remove noise from the (AC) signal and the (DC) power so that the music coming out is at least as good (if not a bit better) as the signal going in.

The devices used to clean the signal and the power, filter out noise and what might be termed ‘resonances,’ also introduce phase changes themselves; capacitors ‘pass AC,’ and ‘store (or block) DC,’ for current will lead voltage through a capacitor; in like manner, voltage will lead current through inductors (or chokes). There are many factors to consider in the design of optimal circuits, and many parts from which to choose. There is no one clear path to success, but many options, each with trade-offs of their own. Yet each may impart a bit of color to the sound and/or tonality of the whole, or be better at some jobs than others, like powering speakers with low impedances.

Ohms Law states that Voltage (in Volts) = Current (in Amps) * Resistance (in Ohms). A corollary of Ohm’s Law is that Power (in Watts) = Current (in Amps) * Voltage (in Volts). Many permutations of this formula are used to solve for one or the other of these quantities.

In AC circuits, “resistance” can also be defined in terms of frequency as ‘impedance’ generally, and “reactance” specifically, as in ‘capacitive reactance’ (Xc) and ‘inductive reactance’ (Xl) [the symbols for reactance are written italic capital X sub small caps C and italic capital X sub small cap L]. Xc = 1 / 2(pi)fC [where (pi) = 3.146, f = frequency, and C = capacitance in Farads]; and Xl = 2(pi)fL [where (pi) = 3.146, f = frequency, and L = inductance in Henries or Henrys]. You will note that capacitive reactance (Xc) decreases with frequency and inductive reactance increases with frequency.

I could go on, but I fear either losing you or boring you (or both). I was where you are twenty years ago when, at age 49, I went to Community College and earned an Associates of Science degree in Electrical Engineering Technology. I have been, and still am, active on the faculty as an adjunct instructor. I encourage you to seek out training in the following two topics ‘DC/AC Circuit Analysis’ and Electronic Devices.’ Textbooks on both topics by one Thomas L. Floyd informed my study (and still does). There are other avenues these days, including interactive online seminars and courses. A YouTube channel, ‘Mr. Carlson’s Lab,’ offers to train you in the field by working your way through repairs of various radios, amps, and test equipment while teaching fundamentals and techniques. YouTuber ‘XRayTonyB’ walks you through extensive repair and restoration of vintage HiFi gear, and another fellow in Arizona likes to repair vintage, tuned guitar amps. His name escapes me at present, but he too walks you through the schematic, explains how the circuit works, and also teaches you some rather crafty techniques to restoring the cabinets as well. Both he and Mr. Carlson don’t hesitate to ‘improve’ on circuits they find with known problems, which I also find very interesting.

To reiterate, I found formal education necessary to answer the questions I was coming up on in my work; plus, I was just plain curious. There is math involved, up to and including Algebra II and trigonometry. While it takes forms of calculus to truly solve some of the equations on the AC side, you can get by with some instructions for using special functions (imaginary numbers comes to mind). I found the exploration of the topic well worth my time; I’ve been involved in electrical/electronic pursuits ever since.

Good Luck

(By-the-way, this write-up only faintly resembles what I wrote this morning at 5:30a, I guess I get a bit loquacious in the afternoon (and it’s not even 5:00p yet).