Fender Tube Amp and Mismatched Speakers

Leslie Hammond · 24th July 2011, 04:33 PM

Can I power 8 ohm AND 16 ohm speaker cabs at the same time with a Fender PA-135? The amp has two output jacks labeled 8 OHM MINIMUM. Will the mismatch hurt the amp? Can I put a volume control on the 8 ohm cab to bring the volume down?

payitforwardeddie · 24th July 2011, 06:56 PM

If you google the amp Schematic, you'll find the absolute minimum output load is 4 ohms. You plug one speaker (8ohms) into one o/p socket and the other speaker (16ohms) into the other socket and all will be fine. The 16 ohm speaker will only use half the power that the 8ohm speaker will take but if its more sensitive (SPL@1m), it'll probably sound fine.

mario13 · 24th July 2011, 07:02 PM

Using both speakers on the amp will not hurt the it. The total impedance will be around 5 ohms since the two speakers will be connected in parallel. The total minimum load for the amp is 4 ohms or two 8 ohm speakers parallel connected hense the 8 ohm minimum on each speaker jack. Dont put a volume controll on the speaker!
Mario.

Leslie Hammond · 24th July 2011, 11:59 PM

Won't the

Won't the 8 ohm speaker be much louder? It would be nice if I could balance the two cabs.

ChrisA · 28th July 2011, 12:59 AM

Quote:

Originally Posted by Leslie Hammond

Won't the Won't the 8 ohm speaker be much louder? It would be nice if I could balance the two cabs.

Place a 2 to 8 ohm power resistor in series with the 8 ohm speaker. Just make sure the resistor is rated for at least twice as many watts your amp and it will run cool enough. Pick the exact value by ear.

They make things called an "L-pad" for speaker volume control but they preserve the speaker's impedence. In this case you don't want to do that, you want to force more of the power into the 16 ohm speaker.

Enzo · 28th July 2011, 11:44 PM

Don't confuse power and loudness.

If the two speakers were absolutely identical except for the impedance, the 8 ihm speaker would draw twice the power compared to the 16 ohm speaker, but the result would only be 3db louder. In most ears that is not "much" louder. 3db is a detectable difference, but not substantial.

But the speakers are not the same anyway. The speakers themselves are probably very different, and the cabinets they are in also make a difference as to the sound making ability.

Twice the power is a 3db increase from a speaker. But if one speaker is 3db more efficient, then it would be 3db louder. So 50 watts into a 100db efficient speaker willbe the same loudness as 100 watts into a 97db efficient speaker. All other things being equal, of course.

SO the two may be mismatched, but just like a singing group where one voice is a trifle louder than another, they still can sound great together.

24th July 2011, 04:33 PM	#1
Leslie Hammond diyAudio Member Join Date: Jul 2011	Fender Tube Amp and Mismatched Speakers Can I power 8 ohm AND 16 ohm speaker cabs at the same time with a Fender PA-135? The amp has two output jacks labeled 8 OHM MINIMUM. Will the mismatch hurt the amp? Can I put a volume control on the 8 ohm cab to bring the volume down?

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24th July 2011, 06:56 PM	#2
payitforwardeddie diyAudio Member Join Date: Feb 2011 Location: South Wales, UK	If you google the amp Schematic, you'll find the absolute minimum output load is 4 ohms. You plug one speaker (8ohms) into one o/p socket and the other speaker (16ohms) into the other socket and all will be fine. The 16 ohm speaker will only use half the power that the 8ohm speaker will take but if its more sensitive (SPL@1m), it'll probably sound fine.

24th July 2011, 07:02 PM	#3
mario13 diyAudio Member Join Date: Jul 2010	Using both speakers on the amp will not hurt the it. The total impedance will be around 5 ohms since the two speakers will be connected in parallel. The total minimum load for the amp is 4 ohms or two 8 ohm speakers parallel connected hense the 8 ohm minimum on each speaker jack. Dont put a volume controll on the speaker! Mario.

24th July 2011, 11:59 PM	#4
Leslie Hammond diyAudio Member Join Date: Jul 2011	Won't the Won't the 8 ohm speaker be much louder? It would be nice if I could balance the two cabs.

28th July 2011, 11:44 PM	#6
Enzo diyAudio Member Join Date: Sep 2004 Location: Lansing, Michigan	Don't confuse power and loudness. If the two speakers were absolutely identical except for the impedance, the 8 ihm speaker would draw twice the power compared to the 16 ohm speaker, but the result would only be 3db louder. In most ears that is not "much" louder. 3db is a detectable difference, but not substantial. But the speakers are not the same anyway. The speakers themselves are probably very different, and the cabinets they are in also make a difference as to the sound making ability. Twice the power is a 3db increase from a speaker. But if one speaker is 3db more efficient, then it would be 3db louder. So 50 watts into a 100db efficient speaker willbe the same loudness as 100 watts into a 97db efficient speaker. All other things being equal, of course. SO the two may be mismatched, but just like a singing group where one voice is a trifle louder than another, they still can sound great together.

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