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Old 22nd October 2007, 11:47 AM   #11
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You stick to your "understanding" and I will stick to mine.
Since I do this for a living I do take the numbers seriously. The load is no different than what it would be if the amplifier was driving a 70 volt transformer and a run of 70 volt speakers.

If for example the amplifier was driving a 70 volt transformer and the amplifier was rated to drive a 50 watt load and the load on the amplifier was say 75 watts the amplifier would be overloaded.Therefore the 4 ohm load does in effect present an overload to the amplifier when the amplifier is rated for a 6 ohm load.

The amplifier was rated for a specific minimum load and saying that I'm not pushing it hard therefore I can run a load that I choose to run just plain isn't smart. It certainly isn't good advice to give someone when they deserve an honest answer to a question.
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Old 22nd October 2007, 12:42 PM   #12
sreten is offline sreten  United Kingdom
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Hmmm..........

Someone seems to think rating an amplifier at 8 or 6 ohms is a get-out
clause for the manafacturer such that if too low impedance speakers
are connected its not their fault if the amplifier blows up.

This not generally true, most commercial mainstream amplifiers are
protected against any load, the rating is recommended loading.

70V/ 100V line systems are a classic case where you cannot have your
cake and eat it, power limiting is effected by the step-up transformer
rather than overloading the amplifier.

The simple fact is an easy non-reactive 4 ohm load is easier than a
highly reactive 6 ohm load, the truth is not told by the numbers. One
should always regard an 8 ohm (resistively) rated amplifier with deep
suspicion as to it load driving capability and one should also think
about what 8 / 6 / 4 ohm nominal impedance actually means.

sreten.
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Old 22nd October 2007, 10:56 PM   #13
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What would be the result of connecting 4-ohm DIY speakers to my Onkyo amplifier which specifies a 6-ohm minimum speaker impedance?
The only way I can see this is that the speakers are 4 ohm nominal impedance which I see as a load that could very well dip to 2 ohms or so on the low end.

Still want to gamble? Its your amp and I'm sure it can be rebuilt again.

However its an Onkyo and that isn't the most robust unit around.
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Old 23rd October 2007, 12:16 AM   #14
tman204 is offline tman204  United States
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I suppose you could also buy some el cheapo speakers (4ohm or higher), and run them in series with your DIYs. This should fix your problem.

You could also modify your crossover via resistors to compensate for the 4ohm load you expect.

my $0.02
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Old 23rd October 2007, 12:33 AM   #15
troystg is offline troystg  United States
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Or just throw a 2 Ohm 10-20 watt resistor (NON inductive!) in series with each speaker.

Not the BEST solution, but it will let you use the speakers on the amp.

I doubt you will be able to "hear" it through an amp that can't drive a 4 Ohm load.
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Old 23rd October 2007, 01:00 AM   #16
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What's the arithmetic mean of "probably OK" and "absolutely not?"



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Old 23rd October 2007, 02:05 AM   #17
Ron E is offline Ron E  United States
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Quote:
Originally posted by diesel_88
What's the arithmetic mean of "probably OK" and "absolutely not?"
The arithmetic mean is "do it if you want and don't blame me if it breaks "

There are too many "if's" in the question. Which 4 ohm speakers, which amplifier, how low a level?

I ran an older Onkyo with an 8 and 6 ohm rating (it was rated for "dynamic power" into 4 and 2 ohms) with a pair of 4 ohm speakers. I listened at very low levels (<80-85dB most of the time) and had 97dB sensitive speakers. The speakers would get reasonably loud driven by a walkman. Looking at the board, there are some small value resistors (~1 ohm/1watt IIRC) that at some point got hot enough to darken the board. They were obviously intended to get hot as they were mounted above a hole in the board.

The only times I played loudly was for ~5 minutes at a time to shut up my noisy neighbors. One time an upstairs neighbor played the Eagles Hotel California loudly about 6 times in a row, so I played it at ~12 o'clock on my receiver and I never heard loud music from that neighbor again. About 116dB in room on that one

I had a speaker protection relay failure at 4 years, not caused by overloading, AFAIK. The amp still works, but one of the channels cuts out every now and again after 20 years of use. Turn it up a bit and the channel comes back because the protection relay contacts arc. I'd replace the relay, but it is a pain to get at. Maybe I'll pull off the relay cover and see if I can burnish/clean the contacts.
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Old 23rd October 2007, 02:49 AM   #18
Ron E is offline Ron E  United States
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Quote:
Originally posted by Cal Weldon
Ron, would you venture a guess as to what you might be able to run? I know we shouldn't at all but could you pull an honest 5 watts from a 50 watt amp without future concerns?

EDIT: Mostly I am thinking of an 8 ohm channel with a 4 ohm speaker.
Playing around with ohms law, say you have a 50W / 8ohm amp.

50 Watts Rms at 8 ohms is 20Volts Rms.
20/8 (or 50/20) is 2.5 amps Rms

2.5 amps Rms into 4 ohms is 4*2.5^2=25 Watts Rms

Resistively, you are in the same operating window a long as you are under 25Watts at 4 ohms. To give yourself some kind of safety factor you might then derate the amp by a factor of between 2-4 (consider your 50W 8ohm amp to be a 12.5-25 Watt / 4ohm amp) and my opinion is that you will probably be safe in doing this. The trouble is knowing where 25 watts is for whatever music you are listening to. To be really sure you need to monitor operating temps, etc. In the end, buying an amp that is rated to handle the load is probably simpler

BTW, full output from a typical receiver from a CD source is frequently reached with the volume control at the ~10 o'clock position, and actually lower for much music recorded recently. This is quite hard to gauge with newer digital readout volume controls. In other words, half way up is not half power. Half way up is most likely "nasty clipping" level. Volume control position has little or no consistently predictable relation to power output, and it is most definitely not a linear relation.

I used a test CD and a calibrated VU meter to test the output of my old Onkyo amp for various level control settings. Rated power was reached when the amp volume control was set to 14 out of 40 (3.5 of the once common 10) divisions, with a full scale CD signal at 1kHz. Much of the music I had at the time was recorded at around -20dB, but over the full range of frequencies, for about the same average power demand. Modern pop music is often recorded 6-10dB louder and has a demand much (4-10 times, in this example) higher. For example, what would have been within power limits with old recordings at a setting of 14 on my volume control now needs a setting of 6 or 8 out of 40 with new music to prevent clipping. So you may not think you are listening "loudly", based on volume control position, when in fact you really are.
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Old 23rd October 2007, 03:24 AM   #19
tman204 is offline tman204  United States
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Half way up is most likely "nasty clipping" level. Volume control position has little or no consistently predictable relation to power output, and it is most definitely not a linear relation.
I can definitely vouch for what Ron E is saying. I used to own a set of decent HiVi Aluminum 8" woofers in my home studio. This studio is driven by a large QSC PLX2402 amplifier capable of delivering up to 2400 watts in bridged mono mode. As anyone who has used Pro-Audio gear before knows, typically, amps of this nature have two gain controls that vary the INPUT sensitivity, not the OUTPUT gain. I had it in stereo mode with a couple nice DIY studio monitors rated at 230 watts each @ 6ohms, at about 1/2 max input gain - no sweat for this amp, and speakers should have been safe.

One evening, while listening to some music in the basement at moderate levels (wife was asleep upstairs), I decided my beer was getting low, so, I decided to venture upstairs for a refill. Before I made it back to the basement, the randomizer on my PC (which was the DJ for the evening) decided to pick some Jurassic 5, which was recorded at significantly higher gain than the early Floyd before it.

Needless to say, the amp, without modification of any volume adjustments, managed to literally hammer the voice coil through the aluminum cones - leaving a perfect cylinder imprint in the cone. Pretty neat in hindsight.

Not all music is recorded the same, and even if your speakers claim to be 4ohm, they'll probably be more like 2-3ohms at certain frequencies, and a setting of "low" on the amp volume control doesn't always mean "low." I learned this the hard way...
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Old 23rd October 2007, 03:57 AM   #20
GDJ is offline GDJ  Canada
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I'd have to concur with Burntfingers on this one. Running speakers with lower impedance than the amplifier is rated for is just asking for trouble. Not worth it.

Find a nice set of 8 ohm speakers. Safer all around.
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