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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 19th Mar 2008, 10:54 am   #1
'LIVEWIRE?'
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Question Modifying Speakers 4 to 8 Ohm

I have been using a Pair of 3-way 4 Ohm Speakers for some time with a Pioneer Receiver which specifies 8 Ohms Min. for it's main Speakers. Before the output stages blow I am considering rebuilding the speakers with 8 Ohm drivers, and new crossovers. The exsisting, very good dome tweeters(Audax HD88D19M) are 8 Ohm, whilst the mid range and Bass units are 4 Ohm. My question is, should the tweeters now be changed for 16 Ohm types? I have seen 16 Ohm tweeters used in, e.g. Spendor 8 Ohm speakers, but wonder whether this is essential
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Old 19th Mar 2008, 11:06 am   #2
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Default Re: Modifying Speakers 4 to 8 Ohm

Unless you are playing it really loud, espeically on music that has high average power such as rock or organ, the amp isn't in any great danger.

Let's do a calculation. Say it's a 32W amp, designed for 8 ohm speakers. That means it's rated to deliver 16V and 2A at full power. I've chosen the numbers for easy calculation It's the current that can damage the amp so it will safely deliver 8V and 2A into 4 ohms. Hence 16W. Unless you really wind up the wick, most of the time the amp will be delivering a fraction of this so don't worry unless you're an organ or heavy rock addict. The heavy bass in this sort of music can push amps (and speakers) beyond their limits very easily.

PS: Many years ago I blew the bass drivers on my BC1s when doing a frequency sweep test. The very low bass sounded a bit quiet so I wound up the volume. 100W of 40Hz from a Quad 405 quickly burnt out the bass units.
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Old 19th Mar 2008, 11:39 am   #3
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Default Re: Modifying Speakers 4 to 8 Ohm

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Originally Posted by ppppenguin View Post
Unless you are playing it really loud, espeically on music that has high average power such as rock or organ, the amp isn't in any great danger.
Jeffrey is quite right. Using lower impedence speakers means that the amp can deliver more power, and this can overrun components, but it will only deliver that power if you turn it right up. In normal use it's perfectly safe to run with 4 ohm speakers.

Even if you like loud music, unless you are playing sine waves the amp will only be delivering peak output for a fraction of the time. As you're unlikely to want the peaks to clip the average output will be much lower than the amp is rated for, even with 4 ohm speakers.

Paul
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Old 19th Mar 2008, 1:17 pm   #4
geofy
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Post Re: Modifying Speakers 4 to 8 Ohm

The Ohmic value of 4 or 8 is for DC resistance of the speech coil, the AC impedance of the speaker varies with frequency and can be several hundred Ohms. The speaker should match the amplifier for power rating at the lower frequency when the impendence will be nearer the DC value allowing larger currents to pass. The 16 Ohm tweeter should be ok with the 8 Ohm bass unit as the increased impedance at high frequency will make negligible difference to the 16 Ohms DC value.

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Old 19th Mar 2008, 1:22 pm   #5
mastermanx2001
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Default Re: Modifying Speakers 4 to 8 Ohm

Just a couple of thoughts, what make and model are the speakers? Most Hi Fi speakers are 8 Ohm nomimal, but not all.

It would appear that the Pioneer Receiver has capability of driving two sets of speakers. Is this correct?

If this is so, assuming 8 Ohm speakers are connected to both outputs, the total load on the amp would be 4 Ohms.

Therefore, if you are using just one pair of speakers of 4 Ohm impedance, that should be OK (just so long as you do not connect a second pair of speakers)

I had a similar incident to Jeffrey, I bought a Quad 405 in 1976 and it was feeding speakers with KEF B200 bass units in, not on a test signal, but on music with quite a lot of bass. The speakers were not very sensitive, so quite a lot of input is needed to achieve a high volume. It was not long before one of the B200s voice coil fried. I had learnt my lesson. I had both B200 bass units rebuilt by KEF.
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Old 19th Mar 2008, 8:02 pm   #6
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Default Re: Modifying Speakers 4 to 8 Ohm

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Originally Posted by geofy View Post
The Ohmic value of 4 or 8 is for DC resistance of the speech coil, the AC impedance of the speaker varies with frequency and can be several hundred Ohms....
The impedance of a speaker will vary considerably with frequency. A single speaker will have a DC resistance which is constant and an inductive reactance which rises with frequency. This can be substantially modified by mechanical effects, notably the self resonance of the speaker where the impedance will usually rise to a peak. Capacitative effects can be largely ignored. A multi-unit speaker with crossovers will have an irregular looking impedance curve. A few speakers, notably Quad electrostatics present a very capacitative load to the amplifier. Not all amps are happy about that. Usually speakers are inductive and many amps have a Zobel network (R and C in series) across their output to fool the amp into thinking the total load is nearer to pure resistive.

Typically the rated impedance of a speaker is a pretty arbitrary figure, possibly an average across a frequency range which may or may not be specified. The DC resistance of the coil will always be lower than the quoted impedance.
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Old 19th Mar 2008, 9:08 pm   #7
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Default Re: Modifying Speakers 4 to 8 Ohm

Thanks, everyone for all your comments and suggestions. Having been a Radio/Audio Service engineer for over 36 years now, I should, and do know most of what has been said about impedance altering with frequency, etc. In fact, bearing all this in mind, and having just checked the specifications of my Pioneer VSX-505RDS receiver, it is, in fact rated 50wpc(DIN)into FOUR ohms(@1kHz) for each of it's 5 channels, so I don't know from where I got the idea that it was designed for 8 Ohms anyway. I only have two one speaker on each front channel in any case, plus the center speaker. My main speakers are a pair of Sharp CP-460E that came originally with their SG500E Music Centre, although the woofers have been replaced with a pair of Soundlab 4SP8W 60 watt Bass Drivers. None of these are exactly 'audiophool' I know, but they produce a good sound and work well, so I guess I will leave things as they are!
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