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FDP Forum / Amp Mods, Repairs, and Projects / Speaker impedance change



Jan 21st, 2017 08:59 PM   Edit   Profile  


Can I add a resistor somewhere between my 4 ohm output amp and an 8 ohm speaker so the amp sees 4 ohms?



Contributing Member

The spotlight

looks like a prison break
Jan 21st, 2017 10:02 PM   Edit   Profile  

No. Best bet would be to use a small attenuator that allows a 4-Ohm input and has an 8-Ohm output.

But--most tube amps can handle a 100% Ohms mismatch (what you have there) between the OT and the speaker. The thing to do is never run the amp wide open with a mismatch like this and things should hold together.

Te 52

Laws of Physics

strictly enforced
Jan 22nd, 2017 11:25 AM   Edit   Profile  

You could put a high-wattage 8 ohm resistor in parallel with the speaker, but you don't want to.

Reason is that speakers are reactive and resistors aren't, that is, speakers have different impedance at different frequencies, and resistors don't. Result would be a change in frequency response and hence tone quality. You'd lose highs and lows and be left with over-emphasized mids.

As Peegoo says, using a proper attenuator is the best approach.



Jan 22nd, 2017 02:05 PM   Edit   Profile  

What amp?
Might you overdrive its power amp?



Jan 22nd, 2017 02:57 PM   Edit   Profile  


The amp is a '67 DUAL (in block letters) Showman. I need to cast off some weight and over kill for the gigs I play lately. I have considered getting an EVM 15L but cannot find one in 4 ohms...I might check out Tedweber.com for a selection.





Jan 22nd, 2017 04:46 PM   Edit   Profile  

If you're already got a suitable speaker, you might consider an aftermarket OT with a tap to match it, eg see link.
But if you're not going to overdrive it (as I guess may be the case) you would probably be fine with the amp as it is and a single 8 ohm speaker.

External link

Contributing Member

So. Cal. USA

Jan 23rd, 2017 11:19 AM   Edit   Profile  

You might consider an Eminence Legend CA154 15. These are available in 4 ohms and can be had on Amazon. I don't believe EV made the EVM-15L in a 4 ohm variety. A recone may be possible.

I also have a '67 DUAL Showman. I loaded my 2-15 cab with Weber Neo 15s. Huge weight savings! Unfortunately the speakers have been discontinued by Weber but if you could find a pair?

The Neos sound more like JBLs however, less of the EVM smoothness and tolerance for distortion.

An OT swap is a good idea. I've been using multi tap OTs in my Fender clone builds and like the flexibility they offer. I typically wire the unused ground switch for the output load as they are not needed when a grounded power cord is used.

Contributing Member

USA / Virginia

Jan 25th, 2017 12:12 PM   Edit   Profile  

You could put an 8 ohm power resistor in parallel with your 8 ohm speaker and that would work.

Audio amplifiers are routinely tested into resistive loads on test benches.



Jan 25th, 2017 12:43 PM   Edit   Profile  

Just run it.

It's not worth adding an attenuator or swapping out the output transformer. You will barely hear a difference.

Steve Dallman
Contributing Member

Merrill, Wisconsin

Age is just a number...mine is big
Jan 26th, 2017 10:09 AM   Edit   Profile  

That amp will run just fine into 8 ohms. It can do 2-8 ohms. No resistor, attenuator or transformer necessary.

Blackface and Silverface Fenders will handle a 100% mismatch easily.



Feb 5th, 2017 12:10 PM   Edit   Profile  

Speaker impedance varies massively according to frequency, eg may be >10 x nominal at bass resonance perhaps around bottom E.
See linked Swamp Thang chart as an example.
Hence a fixed resistor of the same nominal impedance connected in parallel may take >90% share of the power and, due to the interaction of a tube amp's output impedance and its load, significantly alter the sound.

External link

FDP Forum / Amp Mods, Repairs, and Projects / Speaker impedance change

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