FDP Forum / Speaker impedance change/ 11 messages in thread.

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macguy



USA

Jan 21st, 2017 08:59 PM        

Greetings,<br /> <br /> Can I add a resistor somewhere between my 4 ohm output amp and an 8 ohm speaker so the amp sees 4 ohms?<br /> <br /> Thanks,<br /> <br /> MG



Peegoo

Contributing Member
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The spotlight

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

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

You could put a high-wattage 8 ohm resistor in parallel with the speaker, but you don't want to. <br /> <br /> 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. <br /> <br /> As Peegoo says, using a proper attenuator is the best approach.



pdf64



UK

Jan 22nd, 2017 02:05 PM        

What amp?<br /> Might you overdrive its power amp?



macguy



USA

Jan 22nd, 2017 02:57 PM        

PDF64,<br /> <br /> 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.<br /> <br /> <br /> Thanks,<br /> <br /> MG



pdf64



UK

Jan 22nd, 2017 04:46 PM        

If you're already got a suitable speaker, you might consider an aftermarket OT with a tap to match it, eg see link.<br /> 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.



Hammond101

Contributing Member
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So. Cal. USA

Jan 23rd, 2017 11:19 AM        

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.<br /> <br /> 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?<br /> <br /> The Neos sound more like JBLs however, less of the EVM smoothness and tolerance for distortion.<br /> <br /> 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. <br /> <br />



ECS-3

Contributing Member
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USA / Virginia

Jan 25th, 2017 12:12 PM        

You could put an 8 ohm power resistor in parallel with your 8 ohm speaker and that would work.<br /> <br /> Audio amplifiers are routinely tested into resistive loads on test benches.



guitarcapo



U.S.A.

Jan 25th, 2017 12:43 PM        

Just run it. <br /> <br /> It's not worth adding an attenuator or swapping out the output transformer. You will barely hear a difference.



Steve Dallman

Contributing Member
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Merrill, Wisconsin

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

That amp will run just fine into 8 ohms. It can do 2-8 ohms. No resistor, attenuator or transformer necessary.<br /> <br /> Blackface and Silverface Fenders will handle a 100% mismatch easily.



pdf64



UK

Feb 5th, 2017 12:10 PM        

Speaker impedance varies massively according to frequency, eg may be >10 x nominal at bass resonance perhaps around bottom E.<br /> See linked Swamp Thang chart as an example.<br /> 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.



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