More dullard impedance questions

[john_the_bass]

Right, I've read the this - [url="http://wiki.basschat.co.uk/info:amps:impedance_and_wattage"]http://wiki.basschat.co.uk/info:amps:impedance_and_wattage[/url]

What I want to know is this. If I have an amp that is rated at say 300w at 4 ohms, and I connect it to a single 8 ohm load then I get, for the sake of argument half of that, = 150w out of the amp? Correct?!
So when I connect up a second 8 ohm cabinet either linked to the first one or direct to the amp (in parallel) to make a 4 ohm load in total, the amp goes "oh, 4 ohms - right then, I'll chuck out the full 300 watts). I have no switch or anything to choose 4 ohms load, the amp hypothetically knows it's a 4 ohm load.

So if I get the same amp, but this has 16, 8 and 4 ohm outputs (3 separate outputs) and I connect it to a single 8 ohm load, then I still get the same 150 (same argument applies as above). If I want to link a second 8 ohm cab, I have to link from the cab connected to the amp into the next cab and use the 4 ohm output on the back of the cab (ie I can only use one speaker out at a time) and this will give me the full 300w.

If I get a single 16 ohm load and connect it to the 16 ohm output jack on the back of the amp, I will get less than the hypothetical 150w at 8 ohms - I don't know what this is a proportion.
Am I on the right track here or am I completely confusing the situaion? I knew I should have listened in physics.

I think the simple answer is if I use a dedicated 8 ohm output into an 8 ohm load, do I get full power as if I was using the 4 ohm output into a 4 ohm load or does it still scale back the power because it's not running at minimum load?

[clauster]

John,

I take it you're talking about valve (power) amp?

They have the different outs because the output must be impedance matched to the cab. The same rules for power still apply as far as I am aware.

Tranny's will quite hapily work with anything down to their minimum load.

[BOD2]

+1 to what clauster says

Valves amps are different to solid state amps.

If you are using a valve amp you must connect the correct speaker load. This is either done with separate speaker outputs or an impedance selector. You can damage the amp if the wrong speaker impedance is used.

As I understand it (correct me somebody if I'm wrong) a valve amp gives the same power output regardless of the speakers used, provided these are correctly matched. So a 100W valve amp will give a maximum of 100W into 4, 8 or 16 ohms.


A solid state amp, on the other hand, will behave as you suggest adjusting its maximum power output automatically depending on the load connected. With a solid state amp you will NOT have speaker connectors marked with different impedances. You will only have a "minimum impedance" warning somewhere.

[john_the_bass]

Right - I'm with you, yes valve amps!
That makes sense and I was sorting of thing along the lines of what BOD said, but was wondering if it's the case of what Clasuter said re the rules of impedance and power.

[i]edit[/i]: I would be interested to know if the same rules apply

Edited June 12, 2008 by john_the_bass

[Hamster]

[quote name='clauster' post='217738' date='Jun 12 2008, 04:14 PM']Tranny's will quite hapily work with anything down to their minimum load.[/quote]

It depends on the tranny. My Auntie Bernard doesn't feel she's done a good days work unless she's shifted over 200 sacks of coal on her shift

>coat > go

[john_the_bass]

[alexclaber]

If you think of the amp as just a voltage amplifer then it makes more sense. Then treat the speaker as a simple resistor.

Power = Volts x Amps
Amps = Volts / Resistance

So:

Power = (Volts x Volts) / Resistance

So for a 300W into 4 ohm amp:

300 = (V x V) / 4

V = SQRT (300 x 4)

V = 35 Volts

Then into an 8 ohm load:

Power = (35 x 35) / 8 = 150 Watts

And into a 16 ohm load:

Power = (35 x 35) / 16 = 75 Watts

One more thing to bear in mind is that earlier on we stated that Amps = Volts / Resistance. So the lower the resistance, the more amps the amplifier has to supply for every volt it puts out. Unfortunately the power supply in most amplifiers cannot keep consistently increasing the amps as resistance decreases, so you end up getting less than double the power every time you halve the resistance.

That's the simplistic view. The real view takes into account that we're dealing with impedance which varies hugely with frequency instead of being a constant 4 or 8 ohms and also because the speaker is a reactive load the volts and amps are rarely in phase which mean the amplifier needs more current capability than you'd expect.

So to recap - your amp is simply a voltage amplifer and it does not 'know' what impedance your cab is but it will simply try to supply the current that is required to match the above equation. If it is asked for too much current then it will shut down or die - that's where the minimum impedance comes into it.

Valves amps are totally different because they use impedance matching output transformers.

Alex

[john_the_bass]

Cheers Alex,
Yes sorry - my simplistic view that the amp "knows" what impedance my cab is was based on V=IR, but I was getting myself confused by that point.
So with a valve amp and impedance matched transformers, you'll get the same power if you connect a 16 ohm load to the 16 ohm output as if you connected a 4 ohm load to the 4 ohm output?

[bass_ferret]

[quote name='john_the_bass' post='217784' date='Jun 12 2008, 05:17 PM']So with a valve amp and impedance matched transformers, you'll get the same power if you connect a 16 ohm load to the 16 ohm output as if you connected a 4 ohm load to the 4 ohm output?[/quote]
Yes.

[john_the_bass]

cool!

[clauster]

I was aware wrongly and stand corrected (in my defense it's now 21 years since I completed my electronics A level). Apologies for the confusion John.

[BOD2]

[quote name='clauster' post='217829' date='Jun 12 2008, 06:20 PM']I was aware wrongly and stand corrected (in my defense it's now 21 years since I completed my electronics A level). Apologies for the confusion John.[/quote]


I doubt very much you'd have looked at valves in any electronics course 21 years ago !

However, if you look at the power specs for a valve amp you'll see they only quote one power output e.g. [url="http://marshallamps.com/product.asp?productCode=JCM800&pageType=SPECS"]Marshall JCM800[/url] or the same power into different impedances e.g. [url="http://www.ampeg.com/products/classic/svtvr/index.html"]Ampeg SVT[/url]

[Musky]

[quote name='drthirkenstein' post='218679' date='Jun 14 2008, 02:19 AM']right. so i use an svt classic and on that you can select 4 or 2 ohms. so it'd be bad to use it with an 8ohm cab? i got a 4ohm 810 now so its not a big deal but i was hoping to use my old 8ohm 410 for practice.

thinking about it i cant wait for turn up at a gig and see someone else with another 810 so i can flick it over to 2ohms![/quote]

I had pretty much the same question a while back. The short answer is that you shouldn't use a valve into a higher load than the amp is set to - you'll risk destroying the valves. [url="http://basschat.co.uk/index.php?s=&showtopic=5427&view=findpost&p=57345"]http://basschat.co.uk/index.php?s=&sho...ost&p=57345[/url]