Sorry to bother - 1800W for $400???

[BlueWizard]

Actually, I'm looking for an old thread to settle an argument in another discussion group.

This is the thrust of the thread I'm looking for and I'm sure it was at this website that I read it. -

- - - - - - - - - - - - - - - - - -

The original poster wrote in reporting that he has built a really nice Power Amp, and one of his friends said he had wasted his time since he could by a 1800 watt amp for only $400. As it turns out, that was for a used commercial power amp.

I questioned whether 1800W amps really existed, and he provided a link to the manufacturer's website, and low and behold, there it was a $10,000 1800w amp.

- - - - - - - - - - - - - - - - - -

Now my details may be wrong, maybe it was a 1,000w amp, but overal, above is how the conversation went.

The original poster eventually posted picture of the amp he built.

Does this sound familiar to anyone? Could anyone point me to this thread?

I have tried searching for it, but I'm a little foggy on the details so I haven't been able to find it in any forum here.


The reason I'm asking, is that someone in another forum is claiming that 4 ohm speakers are EASIER to drive than 8 ohms speakers. Several of us have pointed out that it takes more current to drive 4 ohms and that means the power amp and the power supply have to work harder. I've also pointed out that more power consumption equals more work, but he is absolutely convinced low impedance is easier. Of course, I sense he is using a relatively obscure definition of 'easier'.

He keeps bring up the issue that current demand is only a problem for crappy £1000 (US$2,000) amps. That REAL amps, meaning outrageously expensive commercial amps, don't have this problem.

In the thread above, the original poster, tested the power output on a expensive commercial amp, and posted the voltage and current draws for various power outputs up to the maximum capability of his amp. I need those voltage and current draws to demonstrate that even expensive commercial amplifiers still have current limitations.

The other discussion is going on here. You can read but you can't post without joining.

"Ohms help"
http://www.avforums.com/forums/showt...50#post5319950

Steve/BlueWizard

[pinkmouse]

Steve, you can't argue with people like that, whatever evidence you produce. Just ignore him and he'll go away.

[Magura]

Well, PassLabs comes to mind. At least I am fairly sure the data is reasonably correct and not masked. There you can see big dollar amps with current limitations as well

www.passlabs.com

Magura

[KSTR]

Originally posted by BlueWizard
The reason I'm asking, is that someone in another forum is claiming that 4 ohm speakers are EASIER to drive than 8 ohms speakers. Several of us have pointed out that it takes more current to drive 4 ohms and that means the power amp and the power supply have to work harder. I've also pointed out that more power consumption equals more work, but he is absolutely convinced low impedance is easier. Of course, I sense he is using a relatively obscure definition of 'easier'.

Well, "easier" in this case might mean the fact that one needs less supply voltage to produce the same output power into 4 ohms than into 8 ohms, at the cost of increased current. And if you go bridged it is even less, again. Available power in terms of current times voltage from the supply is one factor, power dissipated in the amp another. For a given power to be delivered into specific load impedance there is one optimum set of trade-offs between supply voltages and currents, internal power dissipation, drive voltage etc. Under this conditions amplifier/supply "works equally hard" for the same output power, it's the power that counts.

Very low impedance can compromise amplifier performance, but not primarly from increased current demand per se.

Of course it's simple math that a 4 ohms spkr with the same sensivity will play louder ("easier") with a given volume setting (== output voltage) than a 16 ohms specimen.

Regards, Klaus

[richie00boy]

Alan Mac post #67 is correct and you are wrong, but the co55ie bloke saying 32 ohms is not easier to drive than 6 ohms is not understanding of basic electronics and practical implementation.

Must point out I only looked at the last page so no idea what the original thread was about!

[Brett]

Originally posted by BlueWizard
He keeps bring up the issue that current demand is only a problem for crappy £1000 (US$2,000) amps. That REAL amps, meaning outrageously expensive commercial amps, don't have this problem.

Most commercial 'Pro' amps are actually very cheap for what you get for the money. Some of them sound quite good too on low efficiency speakers.

[BlueWizard]

Well, as long as I have some people interested in discussing this, what really drives a speaker? Is it Power or is it Voltage?

Certainly Power is consumed by the speaker, but if you give speakers of similar sensitivity the same input signal won't you reasonably get the same acoustical power?

So, I say, speakers are driven by Voltage, and in the process consume power.

So, I question which is the more fair test of Sensitivity, to drive speaker with an identical signal source (voltage; 2.83v) or to drive them with equal power(1 watt) which implies different voltages.

Let me illustrate -

Applied signal for fixed power and variable impedance-

1W, 4 ohms, = 2.000 volt
1W, 6 ohms, = 2.449 volts
1W, 8 ohms, = 2.828 volts
1W, 16 ohms = 4.00 volts

This would measure sensitivity to a power standard. But note that the signal to the 16 ohm speaker is very high.

Fixed input signal, variable power, variable impedance-

2.83V, 4 ohms, = 2.000 W
2.83V, 6 ohms, = 1.335 W
2.83V, 8 ohms, = 1.000 W
2.83V, 16ohms, = 0.501W

This would measure sensitivity to a voltage standard, but conversely, notice the difference in real power.

This is how the discussion started-

Someone wanted to know which was EASIER to drive -

8 Ohm (88db sensitivity)

6 Ohm (86db sensitivity)

But it hinges on how you define easier.

88db is louder than 86db, so in that sense, 88db is easier to drive. Depending on on what you mean by easier.

Then someone claimed that the 6 ohm speaker was easier to drive because it had a lower impedance. Which in turn lead to an argument that LOWER Impedance is HARDER to drive because for a given input signal, it consumes more power, more power means more work, more work mean harder, not easier. So, more work, but not necessarily more resulting acoustical output.

And the argument has gone on for 71 post arguing the point back and forth.

So, do we drive a speaker with an input voltage, and achieve an acoustical output, and the power burned in between is incidental. Or, conversely, do we drive our speaker with power, and voltage means nothing relative to acoustical output.

I'm saying that for a given input voltage, and assuming similar speakers, a 4 ohms speaker consumes twice the electrical power, but doesn't deliver twice the acoustical power relative to an similar 8 ohm speaker driven with the same voltage.

Essentially, you consume twice the power at twice the rate and get nothing extra for it.

As to Allen's comment, he misunderstood what I said. I do agree with the point he made in post#67, though it is unrelated to the point I was trying to illustrate.

Frustrated-
Steve/BlueWizard

[MJL21193]

Originally posted by BlueWizard

Someone wanted to know which was EASIER to drive -

8 Ohm (88db sensitivity)

6 Ohm (86db sensitivity)

But it hinges on how you define easier.

It's simple math:
8 ohm, 88 db will take less power to get the same spl.
Doesn't that sound easy?

[djk]

Tapco J2500, USD$399 delivered.

Most pro power amps are only rated at 1Khz for power. The Tapco is rated per channel, both channels driven,20Hz-20kHz:
* 2 ohms: 1400W
* 4 ohms: 750W
* 8 ohms: 575W

If that won't make your 88dB audiophile speaker sit up and bark, nothing will.

Same basic design as Behringer EP2500 and QSC RMX2450, but more transformer.

[SpittinLLama]

Power is what counts, not voltage in testing and in determiniting 'easier'.

It is very easy to get a nice 200V driver (National's LM4702, for example) to drive a ton of voltage but there is no power because there is no current capability. Even an op amp can drive tons of voltage but not much current. So voltage is not the right parameter to use. Power is a combination of voltage and current and covers both bases when talking about different load impedances. High impedance means more voltage but less current, low impedance is more current but less voltage. Comes out 6's either way you figure it. This is why speakers are often rated at 1W, not 2.83V (only 1W for 8 ohms). But in practical applications, lower impedance loads take slightly more work (power) to drive. Not output power but total power. This is due to IR loss increasing because of higher current. There are also other issues like cost for higher part counts for higher output current, stability differences between loads, etc. Anyway, easier could have many definitions so that has to be locked down before any real discussion can take place because if defined in terms of voltage or power with a given supply voltage, then 4 ohms are easier to get more power to.

-SL