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

I wouldn't do it, the adcom would be seeing a 2ohm load when bridged, plus if they haven't been serviced your definitely asking for trouble.

Thanks.  You really think they'd see a 2 ohm load with just the bass panels?

The amps should still see a 4 ohm load but if they’re used bridged, they’ll lose power into low impedances. Even unbridged, these amps have trouble with low-z (they do not double power as impedance is halved). Further, distortion rises in a bridged amp. The OP likes to tinker, so I suggest he do just that. But I wouldn’t be optimistic about the outcome.

Thanks again. There are some posts in various forums that it's worked, but just about as many opinion that it won't.  

I guess bridged mode isn't all that useful, given that most of the really power hungry speakers are 4 ohms.  I 'm sure there are exceptions. 

@jji666 jji666 Since the Magnepans are 4 ohm rated, however it’s only the bass panels I’d be powering, what are your thoughts on stability?

Bridged.., unstable. Not that desirable. Sharing an experience if it helps, and firstly:

Why? What are you trying to accomplish? More bass...? Quality bass?

Quality-stable amplification is more important than driving amps bridged into 2ohms or 1-ohm for that matter. Speaker PLACEMENT and the ROOM itself what I found challenged or benefited me most on the 3.6s I demoed for a few weeks. In my case, it turned out the speakers were just way too big for my room. Smaller Maggies did much better in my situation. A friend brought a pair over. Way better. We tried moving them all over the place, wider, more forward, straight forward, toed in/out, try everything. And, why does a single GFA-555 not do it for you, sounds like a room issue or some kind of a synergy mismatch. A 555 is no slouch in stereo mode, 331.9W into 4 ohms. If that’s not enough power, something else going on there, Best of luck.

I’m sure you will notice an improvement. I would try it, you have nothing to lose and you won’t damage the amplifier.

And, why does a single GFA-555 not do it for you, sounds like a room issue or some kind of a synergy mismatch. A 555 is no slouch in stereo mode, 331.9W into 4 ohms. If that’s not enough power, something else going on there

There's no problem to solve.  I had a single Classe CA-200 powering the bass panels and it sounded wonderful.  I moveed the Classe to power some newly acquired KEF R11s and temporarily put in a Nakamichi Pa-7 powering the bass panels, and it still sounds pretty good.

But since I had the open question of what is best in place of the Classe amp, and I have two GFA-555s, I thought why not try and see if they can well power the magenpan bass panels as monoblocks. Intuitively it seems two monoblocks of that power should do better than a single Nakamichi, but not if it's going to damage anything. 

The 555 will power Magies just fine but I would not bridge them. 

Thanks.  Seems to be the consensus that it will "work" but not sound great, with some chance of actual trouble. I may stick with the Nak there for a while - it's not bridged, and it's beefy.

My 16 year old came upstairs and asked me to turn down the volume!

Back in the late 80's I was evaluating a pair of Apogee ribbon panels, stunningly beautiful and wonderful sound. The dealer modified the 555 to drive 2 ohm loads, if I remember the costs was 1/2 the power but I could be wrong. Still using the Adcom today but I returned the speakers.

I did much like you I had a pair of sim audio w3 moons and a bryston  10b active crossover. My room was 16x28x8. There was absolutely  no lack of power there. I really  doubt you are going to need more than you have running them as stereo  amps. 3.3 are power piggies but you should have lots. 

 the bass panels will still be 4ohm but the amp will see a 2ohm load when bridged.

I had Adcom separates in the 90's.  They are not a high quality product.  I actually GAVE them away.

I think the poInt is bridging disadvantages the amp’s ability to drive low Z loads. 
You have one. So why would you think it a good use of your gear?  The 555 was heralded as “high current” in its day (relative to its class peers, not Krell!), but bridging compromises that quality. I would try vertical bi-amping, however. 

Vertical biamping is an interesting idea - as I've always used different type amps it's always been horizontal.  But I have better amps than the Adcom as a stereo amp to use for the biamping (except the vertical as an experiment).  

Right now I do have a Krell amp as part of the biamp but it's on top.  I figure the better spot for the Nakamichi is on the bass. 

What sound levels are you hoping to achieve? At low to moderate levels you’ll be just fine, but if you’re going for loud, you may be looking for trouble.

All the best.

JD

Have you tried the Krell on the Bottom?

What sound levels are you hoping to achieve? 

Loud, but I can get there with my current setup and it sounds good. This was just a thought that maybe 800WPC monoblocks would be good to try on the bottom.  But not if they're going to struggle because they're bridged. 

Have you tried the Krell on the Bottom?

Yes, had it there for years and sounded good.  I thought the Classe CA-200 was actually a little better on the bottom with the Krell on top which is what I had for a long time.  I just moved the Classe so thought I'd see what else might sound good there. 

I think the basic point is there's a stability issue into 4 ohms with bridged amps, and if my goal is to be able to pour a lot of current into the bass panels, this isn't the most stable way to do it.

That's fine.  The Nakamichi actually is pretty muscular there.  I may try the vertical biamping idea at some point.  I wonder if the uneven load on the channels in that configuration would be a long term issue?

Personally, I would not do it.  My experience (repairing) with Bridging amplifiers is they become very unstable when powering low impedance speakers and driving them hard.  If a 16 y.o. tells you to "turn it down", I suspect you are driving them hard.  States nominal 4 ohms.  Some speakers dip below the 4 ohms.  Especially on the bottom end.  You will be lucky to just blow fuses.

But hey!  Sacrifices must be made in order to Rock and Roll.   It's only electronic gear.  Not like it is the last pair of amplifiers in existence. With trepidation I would say go for it.  Rock On!   LOL  I just remembered, the last bridge set I blew, I was playing Mahler.  I don't know why I assumed Rock.

No, there's nothing precious about the Adcoms.  I have them mostly as backups or to run stuff I'm testing.  

But yes, mostly rock.  I've been spending more time listening to my (new to me) KEF R11s.

I think I've decided not to bridge the Adcoms. 

I wonder why so many amps are bridgeable when most of the inefficient speakers are 4 ohms anyway. 

Run  some fans.

I had a Krell Ksa250  running B&w 801#3's

Got a Pair of Adcom 565 monos on loan from a steroshop.

Adcoms did very well, Sounding a Bit steely on highs but the lows were neck in neck.

The adcom will pry need to be checked for DC offset and or caps replaced.

Fine amps for the money

Which do y'all think will sound better with KEF R11s:  a single Classe CA-200 or 2 Adcom GFA-555's in vertical passive 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).

@oldrooney 

Thank you very, very, much for taking the time to respond so in-depth to a complete stranger.  I really appreciate it.  I am going to take time to read and reread what you have written.  Indeed, Ohm's Law is at the top of the "Electrical Side" of this hobby that totally confounds me.  I'm 58 and retired, so I have all the time in the world -- I'm glad to see someone else later in life pursued this interest through Community College.  After digesting your info, be prepared for more questions!  Thank you again.

Allen

To Tackle your simpler question:

Take a look at the Stereophile measurements on any speaker.  Regardless of what the manufacturer specifies as its nominal impedance, the solid line in the 1st graph will show the impedance (up and down) varies significantly by frequency (bass to treble is left to right).  The discontinuities normally correspond to port tuning, crossover frequencies, cabinet resonances, etc.  While the nominal impedance (along with sensitivity) should give you some idea of their power requirements, it's not unusual for the speaker impedance to vary from half (or less) to a few times that value.  It is quite common for the lowest impedance to be reached in the bass, where power requirements are highest.

You won't find that graph for SS amps, as their output impedance is usually a fraction of an ohm and much less variable across the audio band.  However, any distortion or clipping graphs will normally climb slowly on the right side of the graph, then suddenly and dramatically turn north.  You don't want to push your amp into this region!

Depending on the nature of the mismatch, the result could be an amp or speaker fuse blown, or a speaker driver or an amp output section.  Your house & neighborhood are probably safe, though!

@petaluman 

"Depending on the nature of the mismatch, the result could be an amp or speaker fuse blown, or a speaker driver or an amp output section.  Your house & neighborhood are probably safe, though!"

That's good to know!  Usually when I do some tinkering and something unwanted happens, it's never small.  I go big!

@allenf1963 If I may comment on @petaluman ’s answer, in terms of Ohm’s Law, let us say you have an old-time filament-type flash light. If you Ohm out the bulb, it shows continuity, just as a speaker coil does, but when you turn your old-time flash light (or ‘torch’ as the Brit’s are wont to call it) on, the filament lights up and gets warm, the warmth is energy (power) being dissipated. The bulb (or more correctly lamp’) is rated in Watts because of the energy (power) it is releasing, mostly in the form of heat, and only secondarily as light. [Light Emitting Diodes (LEDs) emit more light than heat, but that’s another story.]

Now, suppose that this is a magical flash light and capable of illuminating the space in any color of the spectrum and beyond, shining different colors like a loud speaker reproduces different frequencies of the audio spectrum. What @petaluman is saying is that this ability to ‘play different frequencies’ is not without cost. As the wavelengths get longer (that is, frequencies get lower), it takes more power to generate them. For light waves, this means that red light is harder to generate than blue or violet; or, more to the point, that bass is more difficult to generate than treble. In fact, when really low bass notes are generated, unless the amp and speaker are designed for it, the speaker appears as a near short to the amp. Remember, to an Ohmmeter, it IS a short, it is only because the wire is wrapped in the form of a coil, and has AC current running through it at a particular frequency that it exhibits a resistance, or, more properly ‘reactance’ (XsubL). So the speaker coil gets warm and expands, possibly shorting, the fuse, if installed blows, the output transformer (if part of the amp’s design), heats, shorts, and blows, any integrated DSP chip starts complaining and sending messages to the screen before it blows —and the music dies. In our flash light analogy, running the red light all the time burns the batteries up more quickly than light of higher frequencies.

A disclaimer, the flash analogy is used for illustrative purposes only, I have no idea how much energy it takes to produce light of a specific color, although, since red LEDs were produced well before the relatively recent white LEDs I may have it backwards. I was trying to emphasize the longer wavelengths of bass compared to treble.

Hope this helps.

@allenf1963 I forgot the math. Since P = I * V, and since V = I * R, then P = l * (I * R). When you turn the flash light on, and the bulb heats up, it’s resistance increases, and you find the source voltage across the lamp (don’t try this with an LED). If it takes more power to produce red light than blue or violet, then either the resistance has increased and more voltage is required, or the resistance has decreased allowing more current to flow for the same amount of voltage ‘driving’ the circuit. In the case of a speaker’s impedance (XsubL) drop producing the lower bass frequencies, because the speaker offers less resistance at the lower frequencies (XsubL = 2(pi)fL), more current passes through the circuit for the same amount of voltage driving the circuit. (With semiconductors it is easy to enter a ‘death spiral’ in which more current generates more heat which draws more current, which generates more heat, etc. the semiconductor offers less and less resistance until it finally burns itself up). Unless there is a microprocessor monitoring the circuit’s power usage, or appropriate and well-placed fuses in the circuit, part destruction is a real possibility where impedance ‘droops’ occur in the bass region, or if you hook up 4 Ohm (or 2 Ohm) speakers to an 8 Ohm tap (at least in theory, many folk recommend trying your speakers on different taps, just to see if they sound better). In practical terms, it isn’t going to make much difference unless or until you turn the volume up and start to drive the amplifier into clipping or at least start to use significant power (unless there is a short in your speaker or connections).

@oldrooney 

Again, solid info and I'm starting to gain a grasp.  Another simpleton question, in its most basic use, is the purpose of the speaker's crossover then to alter the voltage or signal that is going to the bass, mid, and treble drivers -- to produce the sound signal you want without burning out those individual drivers?  Again, I know this is Kindergarten stuff to most of this Forum, but even though I've been a music fan and around bands my whole life, this was the area that just puts me in a "deer in the headlights" mode.  I know I'm not the only "lurker" in this Forum this puzzles over these questions...I'm just not afraid to admit my ignorance!  😂

Thanks again.

Allen

I miss my GFA-585 LE, what a beast!  

@allenf1963 You’re asking questions about a topic on which I’m a bit fuzzy. I studied the design of filters for ‘high pass,’ and ‘low pass,’ performed the basic calculations, and became conversant in ‘first order,’ ‘second order’ and the like; ‘notch filters,’ etc. but I don’t have much practical experience with building, or tuning them. I would have go to my textbooks to get a refresher to go into detail. I’m much more comfortable discussing the sort of filtering that takes place in the power supply, trying to convert AC to DC and eliminating ripple.

That said, I can describe the basic function and components. For a passive crossover, the high pass filter takes advantage of the properties of a capacitor to ‘pass AC.’ The amount of capacitance determines the frequency at which the the ‘highs’ pass to the tweeter. Stages of filtration can be built up and at each stage (or ‘order’) the ‘pass frequency’ becomes more and more selective. That is, the slope of the frequency response curve gets steeper and steeper. I think a first order slopes off at 6 bB per octave, and a second order filter slopes off at 12 dB per octave, but I’m not sure, it may be 3 dB and 6 dB. The inductor is used for a low pass filter, and to be honest, I’m not sure how to describe its operation, but again, first and second order filters can be made quite simply. Resistors are also employed in the design of cross-overs, if you find a ‘vintage’ speaker, there is often a knob or two offering to attenuate either the highs or lows: the potentiometer is serving to ‘trim’ the frequencies at which the filter (cross-over) is effective.

So, yes, cross-overs serve to direct frequencies to the appropriate driver within the speaker (bass frequencies to the woofer, high frequencies to the tweeter, those frequencies that are nether high nor low, to the midrange driver.

The issues with passive cross-overs are at least twofold, (1) the fact that they are passive means that some of the signal’s energy will be absorbed by the cross-over components: the capacitors will want to hold onto their voltage; the inductors will want to hold onto their current, the resistors will restrain current flow and will maintain a voltage across them with a polarity opposing the source as long as they are in the circuit (it’s what they do). An alternative is ‘active’ cross-overs, which must have their own power supply, which ‘massage’ the frequencies without using the signal’s energy, but which may also add to the sound a coloration of their own. I’m not sure what components are used in an active cross-over, but I’m assuming transistors and ICs like op-amps are involved.

If you want to pursue this topic further, you probably need to pm me, as I fear we’ve hijacked this thread. We’re not all on the thread’s stated topic.

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

that's the ok way if your going to do it.

While the Adcom is at its best controlling woofers like some of the infinity speakers

In my experience i cant thing of a worse combination.

These amps are way over damped for any of the Magnepans.

Try any vintage Bryston and you will quickly see what works.

 listen to The Planets, Gustav Holst 4th movement with the opening Acoustic Basses you will appreciate the proper dampening once you experience the difference.

 JohnnyR Magnepan Dealer

Don't forget when you take part of the crossover out  the passive box and replace that part with the active cross over the speaker impedance  is likely  different  than what the manufacturer  has published.  When I had my 3.3 actively  blamed I had the horizontal  that way I was able to ture the base separately  form the mid and top end. By using different  powercords and interconnect s. 

I find it perplexing  what you really are wanting. On paper your amps are bigger than my w3 amps the moons were known to be able to swing large current.  But that being said Maggie's need that and not only was there never a need at any volume  the need of more power there was the sense of ease  that comes with a hi-fi  with big power. You don't need a power number to accomplish  great sound in fact the numbers game is a mid fi low fi game. In horizontal  configuration  you have separation before  the amps so that each amp only receives  the signal it is going to drive.. I should say that the room I was I n at the time wasn't  huge but not small either. Around 3200 cubic feet. 

Adcom GFA 555 and a Nak PA-7...that's a blast from the past ! We were selling both of those brands w-a-y back in the early 1990's when I was running the Harvey Electronics chain of audio stores in the NYC/NJ area ! We sold a ton of the Adcom, fewer of the Nak as it was pretty expensive and though built well some people felt it was a little lean sounding...I think Nelson Pass may have had a hand in the design of that but not certain...

Adcom GFA 555 and a Nak PA-7...that's a blast from the past !

They are fun amps.  I do have a number of more modern, beefier amps for the Maggies but I've been doing some swapping as I also added some speakers I wanted to hear with the better/newer amps.  

I think especially the Nak is a good value - bought it for like $250.  Basically a buck a watt for the current the thing will put out.  Also helps to heat the room in the winter!

I am using two Bryston 3B amps to biamp similar to as you propose but horizontal biamp. I use a Bryston 10B crossover. Per the directions in the Magnepan manual (I have IIIa Maggies), i do not use the crossover boxes and changed the jumpers on the back of the speakers. I can provide you with my crossover setting if you like. I use one amp for the bass and one for the midrange/high.

i do not use the crossover boxes and changed the jumpers on the back of the speakers. 

This is what I do as well. Magnepans are the best for biamping because of the external crossovers. I have a Marchand crossover - or to be precise a custom built one using a Marchand board.