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Topic: Correct Speaker Impedance |
Buck Dilly
From:
Branchville, NJ, USA * R.I.P.
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Posted 26 Jan 2003 4:47 pm |
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I reviewed the previous threads on the topic of impedance, but did not get my question answered. I have a solid state L-5 Lab Series amp designed for an 8 ohn load. I lunched the speakers years ago and I have a beautiful vintage Altec Lansing that is probably an alnico speaker. Unfortunately it is 16 ohms. I know this will work, but will it put a strain on the amp?
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Steels and Guitars. Emmons PP, Nationals, Dan-O's, ES 340, Tube Amps only! "Blue Sparks From Hell", "Kings in Disguise".
[This message was edited by Buck Dilly on 26 January 2003 at 04:57 PM.] |
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Ken Fox
From:
Nashville GA USA
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Posted 26 Jan 2003 5:27 pm
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| The Lab series is a transistor amp. It will just put out less power at 16 ohms and will be a happy camper! It's output transistors will run cooler into a 16 ohm load. The problem with not matching is more critical to tube power amps, as the reflected signal goes back thru the transformer as a high voltage to the tubes plates. Transistor amps should not be ran into a lower than rated impedance, it attempt to produce more power than the output transistors are rated for and will occur in their early demise. |
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Bruce Bouton
From:
Nash. Tn USA
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Posted 26 Jan 2003 8:31 pm
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Ken
I've enjoyed your posts as you obviously know your stuff. Could you please expand , in laymens terms about the rules for impedence matching in tube amps.
Thanks
Bruce |
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Ken Fox
From:
Nashville GA USA
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Posted 26 Jan 2003 9:55 pm
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There's a lot of views about impedance matching in tubes amps on the various forums. The general agreement among most is it will not hurt to mismatch up or down one, that is to go to 4 or 16, if rated at 8 ohms. For a Twin, rated at 4 ohms, up to 8 or down to 2 is considered my many to be OK.
For the electronic "purist" it is not OK! There is a flyback voltage created by the speaker voice coil on an impedance mismatch that is refelcted to the secondary and coupled back to the primary of the output transformer. This results in a much higher voltage (transformer step up action). This voltage is a bit rough on the plates of tubes. I must say, I am no expert in this area, but I have seen a similar thing with relay coils. When a relay coils voltage is removed from the holding coil of a simple 24 VDC relay, the relay coil gets a voltage flyback of 300 to 600 volts induced back into the supply wiring! I have seen this over and over again in relays used for building controls in fire alarm and security industries. A simple diode is placed acroos the coil, in reverse polarity, to absorb this type of spike voltage. Without it, the voltage can get back into a systems microprocessor and do severe damage. Unfortunatly, there is no way to buffer the flyback voltage of the resulting from an impedance mismatch in a tube amp.
A transistor amp is typically direct coupled or capacitor coupled and such a problem does not exist. Transistors will not do well into less than their rated load, they develop more power than the transistor is designed to handle and overheat. Typically this results in thermal runaway and transistor meltdown. At higher load impedances they devlop less power and therefore run even cooler.
Again, I am not an enginner in this area! If someone can add to or correct me here, please do so by all means! |
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Kevin Mincke
From:
Farmington, MN (Twin Cities-South Metro) USA
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Posted 27 Jan 2003 6:53 am
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I too have enjoyed reading Ken's posts as he explains in lay terms and it makes sense. Another one is Donny Hinson Thanks!  |
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Ken Fox
From:
Nashville GA USA
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Posted 27 Jan 2003 8:18 pm
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| I was hoping Donny would check me out on that answer. I think I got it right, but the memory unit ain't what it used to be!!! |
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Stephen Gambrell
From:
Over there
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Posted 27 Jan 2003 9:39 pm
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Well, Ken, I ain't Donny, but you sound close. Old tube TV's had those flyback diodes, to keep high voltage from the yoke out of the transformer, right? But it's been my experience that tube amps will hold up to impedance abnormalities better than transistors. A big ol' transformer is gonna handle higher current better than a little bitty transistor!
But seriously Primary(of the transformer) impedance=a(squared)xload impedance, and a=#turns(primary)/#turns(secondary), and, by Ohm's law, secondary current=voltage/total resistance, voltage, of course, being the incoming signal. Since this signal's magnitude is constantly varying(see "pedal, volume") the output voltage is, of course, also varying. Impedance, then, is constantly varying, so a variance of a few ohms probably won't hurt anything.
Joe Walsh said it best, I think. In "Guitar Player," Joe listed his hobbies as going to get some more beer, and"hooking things up to see if they'll work." |
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Ken Fox
From:
Nashville GA USA
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Posted 27 Jan 2003 9:50 pm
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| I have to agree with Joe's philosophy! I find a lot of things that make sense on paper are not always the best! Of course transistors are a lot less forgiving than tubes when it comes to a lower than rated impedance or a dead short! I have seen several speakers catch on fire from shorted output transistors. |
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Buck Dilly
From:
Branchville, NJ, USA * R.I.P.
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Posted 28 Jan 2003 8:23 am
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| If, as you say, a transistor amp operates cooler at a higher impedance, I assume that this reduces wattage. Is this by half or less than half? If the amp runs at 130 watts into an 8 ohm speaker, what will the wattage be into 16 ohms? -Also: thank you, the previous information was very helpful to me. |
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Michael Brebes
From:
Northridge CA
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Posted 28 Jan 2003 9:38 am
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| Usually the resistance change doesn't quite cut the power in half. If it's 130W into 8 ohms, then chances are it will be 70-80W into 15 ohms. |
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Michael Frede
From:
Sonoita, AZ, USA
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Posted 28 Jan 2003 11:03 am
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| At any rate,less output power also means reduced headroom before the power amp clips. |
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Ken Fox
From:
Nashville GA USA
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Posted 28 Jan 2003 4:59 pm
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| As power equals voltage squared divided by resisitance, then a 130 watt amp at 8 ohms will be 32.25 volts RMS. For 16 ohms this would yield about 65 watts RMS. That is assuming the voltage stays the same, and I believe it should be based on the power supply rail voltages. |
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Terry Downs
From:
Wylie, TX US
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Posted 28 Jan 2003 10:07 pm
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The biggest difference in tube and solid state amps with regard to load impedance is damping factor. A solid state amp may have an output impedance of 100-500 times less than the speaker impedance. Damping factor is the ratio of the load impedance to the source impedance.
Since tube amps require a output transformer, the output impedance is higher that a solid state amp. Solid state amps have direct connection from speakers to the emitters/collectors of bipolar transistors. This dampens the counter electromotive force (EMF) from the speaker's inductance.
To lower the output impedance of a tube amp output, a significant amount of negative feedback is applied. The speaker output signal is fed back to the phase splitter input so the amp will "over achieve" the drive required, so the output will more closely match the signal present at the preamp.
The PRESENCE control on a Fender amp reduces the amount of negative feedback applied at high frequencies. This increases gain at high frequency, but also increases output impedance at high frequencies as well. The speaker at high frequencies is controlled in a more sloppy fashion.
So, Ken is correct about lower power at 16 ohms. And to answer Bruce's question, it is simply a matter of preference. Going up or down (moderately) on tube amp loads is not stressful on the amp. It is just a matter of taste.
Changing speaker impedance on a solid state amp basically changes power delivered with barely noticeable change in tone. Changing speaker impedance on a tube amp changes the tone more significantly. This all goes back to damping factor.
Regards,
Terry |
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