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4 ohm outputs?????????

ferninando

ferninando

Lunatic Member
dont even have my amp yet and find out it only has 4 ohm outputs. So is it not going to drive my 8 ohm speakers? Did I just waste alot of $$??
Can it be changed over to 8 ohm outputs with different transformers?
Very depressed!!:tears:
 
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Essentially, output trafos don't have a specific impedance of their own. They reflect the impedance from one "side" to the other based on the turns ratio between primary and secondary windings.

So if you use an 8 Ohm speaker on a 4 Ohm tap, the end result is that the impedance of the primary will be roughly twice what it would be if you had a 4 Ohm load on the 4 Ohm tap.

Again, generally speaking - the higher primary impedance seen by your power tubes will mean they produce less power and (this is the good part) less distortion. It might be a very nice setup, as long as it has adequate power to drive the speakers to the level you want.
 
Even though higher primary impedance seen by your power tubes means less distortion, it also means reduced frequency response (bandwidth) at both low and high ends due to limited primary inductance and leakage capacitance.
 
ultralc said:
Even though higher primary impedance seen by your power tubes means less distortion, it also means reduced frequency response (bandwidth) at both low and high ends due to limited primary inductance and leakage capacitance.
Click to expand...

Exactly. That being said, it would be odd if the OT's only have 4 ohm secondaries. Even if there are only one set of binding posts per/ch on the outside, 8 ohm (and higherr) secondary taps may be bundled inside the amp. If so, it will be easy to fix. :yes:
 
the OTs are stock, 6.6k Z in, 4 ohms out. So primary Z-in is not high. I got the data from the photofact for the amp, magnavox 8802. If there are other taps inside I'll have to wait till I get the amp to find out.
So are you saying that using 8 ohm spkrs on a 4 ohm output will reduce freq resp?
What if I get OTs with pri Z=6.6k and 8 ohm out? That shouldn't be a prob. Right?
 
Whether any change in frequency response is audible remains to be seen. You likely have negative feedback at work in your amp which tends to extend frequency response at both ends of the audio spectrum. The loss of response may be outside the range of audibility, especially if the OPTs are good quality. Your speakers are not likely to be 8 Ohm impedance at all frequencies, dips in impedance are common. That impacts the tone too.

There is only one way to find out, and that's to try it. :yes:

FYI, deliberate mismatching is not uncommon. Some guys do it because it sounds better to them in their systems.
 
Although the 4 ohm secondary to 8 ohm load is not ideal, you may or may not hear any reduction of frequency extremes. Part of that is going to depend on the speakers in question. Their impedance plot may fall notiecable below that of 8 ohms. Speaker impedances vary with frequency. I'd listen awhile first before doing anything. You may find they sound great together as is.
 
oldhifiguy said:
dont even have my amp yet and find out it only has 4 ohm outputs. So is it not going to drive my 8 ohm speakers? Did I just waste alot of $$??
Can it be changed over to 8 ohm outputs with different transformers?
Very depressed!!:tears:
Click to expand...

No you didn't waste your money. A tube amp will work fine driving an 8 ohm load from the 4 ohm tap. It won't burn up or go up in a cloud of sparks or smoke or anything. It won't be as efficient, but it will work without damaging the amp or tubes. But, I would bet you that if you open the amp up, you may well find extra wires emanating form the secondary of the output transformer. Since most people don't care about the 16 ohm output of my amps, I generally only have connectors on the back for 4 and 8 ohms. But any amp I build has a transformer that has taps for 4, 8, and 16 ohms. all one has to do is open the amp up and swap the taped off wire with one of the ones going to the output terminals on the back to get a 16 ohm output. (although I now just put all three connectors (4, 8, and 16 ohm) on the back).
I'm betting your amp may well have the leads there for 8 and 16 ohms. They are just not connected. Never hurts to ask the builder.
Bill
 
Well seta1/Bill, the amp's a Magnavox 8802 so can't ask the builder. Only have the sam's photofact that has specs for stuff and it says OT pri Zin is 6.6kOhms and 4 ohm output. When I get the amp I'll definately check for more wires. I can also get aftermkt OTs of same Z and 8Ohm outs if I decide to go that far. And I may just because I can.
Thanks
cheers.
 
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