power amp question...

sorry, this is a rather basic question, but perhaps someone can help enlighten me...?

I've got a Musical Fidelity x-as100 power amp, which has a mono/stereo switch to allow bi-amping. The manual shows an arrangement (with mono switch in) where a an As100 is used to power each speaker, with one set of outputs going to the HF connections, the other to the LF connections (obviously, used with a biwirable speaker).

Can I do this with a non-biwirable speaker (ie + on speaker connected to both + outputs on amp, - on speaker connected to both - amp outputs)? The description on the MF website seems to suggest this is fine:

"By 'mono-ing' the X-AS100 and joining the speaker connections, the X-AS100 can be turned into a 100 watt, 60 amp monobloc. This allows a second unit to be added, for a stereo pair. This simple upgrade path gives even greater dynamic control. Alternatively, two X-AS100s can be 'mono-ed' easily for passive bi-amping."

It's just there's no reference to this in the manual so I want to be sure I'm not going to blow anything up....

The 2nd part of my question is...
Used as a stereo amp, it's rated I think as 100W, 35 amps per channel.
As a monoblock, it's 100W, 60 amps. What does this actually mean? Surely the same amount of power (100w) is going to the speaker, with higher current but (presumably) lower voltage? Is this going to provide any benefit? I would have thought the idea would be to provide a higher wattage?

 

Q 1: yes, you can do this on speakers with a single terminal.
Q 2: What they try to tell you is that the voltage swing stays the same, but the amp can put out more current. Depending on the impedance of your speakers, this may or may not result in a small gain in loudness capability. It also may lead to better sound, particularly bass control. But forget the specs, just try it and see if you like it enough to justify the extra money.

 

Thanks for that, I'll give it a go and see what happens.

Of course, you're right, it doesn't really matter about the specs, it's whether it makes any difference to the sound that counts.

Just that I'm curious to know how things work... from my (not very extensive) understanding of physics, Power = voltage x current, so if the voltage swing stays the same, but the current increases, then surely the power goes up?

 

Hi.

The peak voltage for 100W is 40 volts, to deliver 60A this will require a speaker (and cable) impedence dropping to well below one ohm. if you actually need this much current i would suggest you would get better quality by changing your speakers

Chris

 

So can someone explain to me the relationship between power, current and voltage, as far as amps / speakers are concerned?

I mean, if an amp is rated at, say, 200W rather than 100W, does that mean it's providing a higher voltage at a similar current, or vice versa? Or something else altogether?

 

Hi.

Amplifier power is normally quoted as watts delivered to an 8 ohm load (sometimes 4 ohms). This power is measured by applying an 8 ohm resistor and turning up the output voltage of the amplifier until a given level of distortion is reached (often 1%). The power output is calculated by using the formular voltage squared devided by resistance eg. 40V RMS gives 40*40/8 = 200 Watts RMS. The current will be given as the voltage devided by the resistance, in this example 40/8 = 5 Amps RMS. It should be noted that for a sine wave the peak current (and voltage) will be 1.414 * bigger, eg 7 Amps and 56 Volts.

Unfortunately speaker loads are not as kind as our measurement 8 ohm resistor, their impedence (resistance with the current not always in phase with the voltage) varies wildly with applied frequency. Many modern speakers have impedence dips that go as low as 2.5 ohms, to achive the 56 Volt peak of the previous example would require a current delivery of 56/2.5 = 22.4A, and to make matters more dificult for the amplifier this peak current may not need to be produced at the peak of the voltage.

For a 100W amplifier the figures would be:

28.3 Volts RMS - 28.3*28.3/8 = 100 Watts
28.3 Volts RMS - 28*1.414 = 40V peak
RMS current for 8 ohms - 28.3/8 = 3.5 Amps
Peak current for 8 ohms - 3.5*1.414 =5 Amps

For the 2.5 ohms speaker the peak current requirement would be 40/2.5 = 16 Amps.

HTH

Chris

 

Thanks very much for that clarification, it all makes sense.

But I still don't quite get what happens when I switch my MF X-AS-100 (described as 100wpc stereo) into mono mode.

It doesn't necessarily become the equivalent of a 200W monoblock, right?

If I were to switch it to mono mode and connect it up to an 8ohm load, I'm assuming one could turn up the output voltage higher (until reaching the given level of distortion) than would be possible doing the same with one of the channels in stereo mode? So therefore, the power rating will be greater (as well as the peak or RMS current)?

So why do MF refer to it as a "100W, 60amp" monoblock in mono mode?

 

Hi.

In mono mode, the two halves of the amp are connected in parallel, with this connection scheme the voltage output will be the same. The current is shared between the two channels so the output can deliver more current, but only if the load demands it.

Chris

 

thanks,

I think I get it now.

by the way, yesterday I tried connecting the 2 +terminals together, and the 2 -terminals together, switched it into mono and tried connecting it to one of my speakers...

as soon as i turned the amp on I got a rather alarming squeaking kind of sound (this is with the pre still switched off, so no input to the power). So I switched it off again rather quickly and decided not to meddle further...

 

Owl

You need to be very careful if you are bridging your amp (s) to monoblock status.

I have a pair of Nad 218 power amps which are conventionally bi-amped into a pair of Triangle Zerius 2002 speakers. Quite content and happy with the sound so ergo it's time to mess about with it (again)

Sussed out the monoblocking bit which in the Nad's case TRIPLES the power output from 200wpc to over 600wpc into 8 ohms, and 1.2kw into 4 ohms, 2.4kw into 2 ohm. Scary stuff. Houses burn down with that.

The furthest I got with it was contacting Nad for their advice......I can still hear the guy laughing and falling off his stool...........

Anyway, didn't have the bottle for it and (sorta) lost interest in it.

Will