Why are different 500watt heads rated differently at 8ohms?

[dave_bass5]

In my quest for a new head I've been looking at 500watt class D heads to replace my Matkbass F1 head.
I've just got a Barfaced BB2 which is 8ohms, and MB quote 350watts at 8 ohms. I've seen a few others also stating this, or close to this, but some only 300-250watts.

I'm not asking about real loudness, or specific heads, just trying to understand the big difference between some manufacturers and if it's just good marketing or not all output stages are the same when plugged in to an 8 ohms cab.

Edited November 9, 2015 by dave_bass5

[paul_5]

I wouldn't worry. By BF Compact is more than capable of competing with 2 Hundred Watt valve heads and an 'enthusiastic' drummer. My head delivers about 400W into 8 ohms (bridged) and it rarely goes above 30%. BF stuff is really efficient, so more of the power from your amp is converted into lovely low end grunt, and less of it becomes undesirable heat etc...

[Chienmortbb]

A power amplifier has two main circuits, the power amplifier and the power supply. Either can be we working close to their limit on either current or voltage and that will determine the power output.

In the case of the F1 et al, the user manual quotes 300 watts into 8 Ohms should give 600 watts into 4 ohms i[b]f there is enough in the power supply and the output devices in the power amp. [/b]Clearly the F! has limitations in one or both areas. Of course if the F1 and the other amps really output the powers they say, the only real difference will be the sound the preamps impart.

[xgsjx]

I could be wrong on this, but every time I've looked at watts & ohms, 8 ohm usually works out about 2/3 of the 4 ohm rating.

[MOSCOWBASS]

You would be better off with an 800 ish watt head, so you would get 500+ watts into the BB2. It's not for volume as much as headroom.

[dave_bass5]

Thanks for the replies.
I dont think my post was worded right. I was just asking why the differences in the 8 ohms output from different 500watt heads.
I think i have enough volume at the momnet (yes, 300watts and not 350watts as i first thought), and yes, i was origionaly looking for a 800watt head for the extra headroom. It was just that during my research i noticed slghitly different 8ohms figures and was wondering why this is.

I might still get an 800watt head if i find im pushing my F1 too hard, but i know what gigs ive got coming up and i have done them all with an old Ashdown MAG300 C-115 so i know ive got enough power for those.

[Phil Starr]

Hi Dave, you have the answer already from Chienmortbb. I'll see if I can explain differently.

An amp can only produce a certain maximum voltage into a speaker and that determines it's power output.

However if you reduce the ohms the speaker will take more current and more power with the same voltage, so if you have an amp that produces 100W into 16 ohms it will produce 200W into 8ohms. Theoretically this means the amp could go on doing this so 400W into 4 ohms and 800W into 2 ohms.

The problem is that this needs more and more power from the amp and the current demand keeps rising. Eventually either the amp gives up and goes up in smoke, automatic protection kicks in, or the power supply inside the stops making extra current. Usually the power supply is the limiting factor. Power supplies are often expensive and if they are transformer based really heavy. This is why most amps don't like 2ohms and why they can't produce twice the 8ohm power into 4ohms. In practice I doubt you'd notice the difference between say 350W and 400W so it doesn't matter. That's another story though.

So the answer to your question is 'because they have differing power supplies.'

On a practical level I don't think you need to choose an amp on the basis of 25% more power, you just won't notice it.

*Valve amps work slightly differently as they have a transformer to adjust to different speakers so they will produce the same power if you have an output for that ohmage speaker.

Edited November 9, 2015 by Phil Starr

[MOSCOWBASS]

Sorry Dave, that is an interesting question as to why some amps reputedly deliver more or less into 8 ohms. I’ve read that the Glok Blue Sky delivers less than expected, yet the GK mb800 is pretty healthy at 8 ohms.

[dave_bass5]

Thanks Phil. I did think that was the answer but wasn’t too sure, it did make sense though (so thanks Chienmortb), but your explanation adds to the clarity of the info.

Originally the idea was to get an 800watt head so i would have 500watts at 8ohms, i dont think my F1 likes being pushed over half way, and this would take me back to what i had before getting my BB2. By the time i realised i didn’t really need it i had GAS for a new head, so started to look at the 500watt class D heads, specifically what they give out at 8ohms. I just wondered why some were less powerful than others.

As xgsjx said above, i too always just thought 8 ohms was 2/3 of the full output, but obviously thats not the case.

Edited November 9, 2015 by dave_bass5

[Phil Starr]

[quote name='dave_bass5' timestamp='1447081469' post='2904656']

As xgsjx said above, i too always just thought 8 ohms was 2/3 of the full output, but obviously thats not the case.
[/quote]

It was a good question.

obviously the power can be 2/3 or 1/2 depending upon the power supply. I think my old MAG600 was about 2/3 and it isn't a bad rule of thumb but that's why it isn't a fixed proportion. theoretically double but a bit less in practice.

[alexclaber]

A better way to look at it is to ask why amps don't put out twice as much power into 4 ohms at they do into 8 ohms. An amp is primarily a voltage amplifier, so halve the impedance whilst keeping the voltage the same and twice as much current will flow, hence double the power.

So what we're seeing happen with most bass amps is that they don't double their output power when going into 4 vs 8 ohms, because they don't have enough current capacity. We've been looking at some amplifier modules and by changing the power supply they're running on we can change the maximum power into different loads, so with a higher voltage supply lower current they produce more power into 8 and 4 ohms but less into 2 ohms whilst with lower voltage but higher current we can keep the 8 and 4 ohm power output a bit lower and the 2 ohm output higher (for when you're running lots of cabs).

[Chienmortbb]

I have to be honest, having once re-designed a protection circuit for a class A/B power amp, I would not recommend you rely on one at 2 ohms. it is too close to a short and at some frequencies the speaker will go below 2 ohms.

In fact we found that if you don't use jack plugs, amps rarely short. The, and the current limit of most jacks, is why I prefer speakons.

[Count Bassy]

[quote name='Phil Starr' timestamp='1447080059' post='2904643']
So the answer to your question is 'because they have differing power supplies.'
[/quote]

And output devices/Heat extraction.

Edited November 11, 2015 by Count Bassy

[Balcro]

Hi dave_bass5,

Going off at a tangent - your earlier question about outputs into 8 or 4Ohms has
already been answered.

Rather, picking up on your post #9.

Why do you say the MarkBass doesn't like going beyond half way? What speakers are you using
with it. As per your equipment profile or something else? Just 1 speaker or 2?


Balcro.

[dave_bass5]

Over the years I've used a MB15P traveler, Epifani UL115 and UL212, Schroeder 1210R and 1212L, Ashdown 1x15. The UL212 and Schroeders were 4 ohms cabs, the others 8ohms, and only one cab at a time.
Whenever I've gone over noon on the master I've just found the tone to start sounding compressed and they dynamics seem to get lost.
Could be I was pushing the cabs too much, although the Schroeders are rated quite high.
I''ve never had much luck getting louder than whatever is at noon, maybe 1 o'clock, and I haven't been using any effects of active basses (on the whole).

There is of course technical reasons for all this, and on the whole I've never been under powered for nany gigs, but the fact is ive found I can runout of headroom when pushing the head too hard.