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Topic: Speaker ohms ? for electronics guys |
Tim Whitlock
From:
Colorado, USA
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Posted 21 Aug 2012 7:01 am |
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I have an amp that wants to see 4 ohms. It's a combo with two 12" 8 ohm speakers wired in parallel. I want to put the amp in a separate head cabinet and build my own speaker cabinet using a single 8 ohm 15" speaker.
My questions: Can I wire the speaker in parallel with an 8 ohm resistor to bring the load to 4 ohms? Wouldn't this be the same as wiring two 8 ohm speakers in parallel? |
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Earnest Bovine
From:
Los Angeles CA USA
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Posted 21 Aug 2012 7:21 am Re: Speaker ohms ? for electronics guys
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| Tim Whitlock wrote: |
| Can I wire the speaker in parallel with an 8 ohm resistor to bring the load to 4 ohms? Wouldn't this be the same as wiring two 8 ohm speakers in parallel? |
A resistor would not sound as loud as a speaker. |
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William Lake
From:
Ontario, Canada
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Posted 21 Aug 2012 7:41 am
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Tim, the ohms on speakers are NOT resistance, but impedance.
You would have to wire an 8 ohm coil in parallel. Don't think it's a good idea.
Your 8 ohm will work but have a slighly less output if you are loading a solid state amp. If tube, you best not fool around.
_________________
Bill |
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Tim Whitlock
From:
Colorado, USA
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Posted 21 Aug 2012 8:16 am
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William - Thank you for your kind response to my dumb question. Forgot that ohms is a measure of impedance. 
Earnest -  |
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Pete Nicholls
From:
Macon, Georgia, USA
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Posted 21 Aug 2012 9:18 am
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Tim, I believe you could add a four ohm resistor, and it would work if you found a resistor that would handle the same power as the speaker. The biggest difference between impedance and resistance is that impedance will change with frequency but the resistor would not.
_________________
Justice The Judge SD-10, 2007
Justice Pro Lite SD-10, 2011
Quilter Steelaire
Quilter Labs Tone Block 202 Head
Roland Cube 80-XL
American Stratocaster - Yamaha Bass Guitar
1 Fender Telecaster Nashville Edition
Ham Call: N4BHB |
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Jerome Hawkes
From:
Fayetteville, North Carolina, USA
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Posted 21 Aug 2012 9:45 am
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i am in this same situation with wanting to use a 8ohm 15" Altec 418 with my '63 Bassman head.
most of these old speakers dont measure out to true 8ohm - often they come in the mid 5's.
_________________
'65 Sho-Bud D-10 Permanent • '54 Fender Dual-8 • Clinesmith T-8 • '38 Ric Bakelite • '92 Emmons D-10 Legrande II |
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William Lake
From:
Ontario, Canada
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Posted 21 Aug 2012 11:17 am
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Pete just gave me an idea that might be considered.
A wire wound resister would have some impedance I would think.
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Bill |
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Stephen Cowell
From:
Round Rock, Texas, USA
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Posted 21 Aug 2012 2:13 pm
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| Jerome Hawkes wrote: |
i am in this same situation with wanting to use a 8ohm 15" Altec 418 with my '63 Bassman head.
most of these old speakers dont measure out to true 8ohm - often they come in the mid 5's. |
DC resistance is always less than 'nominal'... trust the manufacturer's rating unless you're dealing with a recone. Rule of thumb: DC is always about 2/3 of nominal. 4 ohms out at about 3, 8 at 6, 16 at 12ohms.
I'd hate to hack a blond Bassman up... but you can put a Twin transformer in it and that will convert it to 8 ohms... same as if you cut one speaker and pull two tubes (one from each side!) on a Twin.
Adding a dummy load is going to mess with the tone... even a wirewound resistor is not going to 'react' (literally) like a speaker would. Somebody (Weber?) is making impedance-matching transformers for just this application... let me go look... oh yeah!
https://taweber.powweb.com/store/zmatch.htm
"The Impedance Matcher allows you to match the output impedance of your amp to impedances of speakers and cabinets that are not the same as your amp output impedance. Matches 2, 2.6, 4, 5.3, 8, and 16 ohms to 2, 2.6, 4, 5.3, 8, and 16 ohms. Includes an extra output jack and a balanced and unbalanced line out with a level adjustment. "
For the price difference I can't see buying the 50watt one when the 100watt one is only $20 more. I'd forgotten about this box... it even includes a bal or unbal DI line out with attenuator control. Super Reverb into a Marshall 4x12? No problem!
_________________
Too much junk to list... always getting more. |
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Jonathan Cullifer
From:
Gallatin, TN
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Posted 21 Aug 2012 8:01 pm
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| William Lake wrote: |
Pete just gave me an idea that might be considered.
A wire wound resister would have some impedance I would think. |
Do you mean reactance?
http://www.aikenamps.com/DummyLoads.htm
There are also some good articles about resistive vs reactive loads as they relate to attenuators...all very similar.
I'm surprised that the amp won't handle an 8 ohm load...for most amps, tube or solid state, that level of mismatch is not an issue. |
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Cartwright Thompson
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Posted 22 Aug 2012 6:08 am
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Using an 8ohm speaker in a Fender tube amp that has a 4ohm output transformer works just fine. The amount of power you loose is negligable and it won't hurt the amp. I've done it for years and never had a problem. I actually like the sound better with the missmatch.
Now I shall repair to my closet and don my flame-proof suit.... |
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Dave Mudgett
From:
Central Pennsylvania and Gallatin, Tennessee
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Posted 22 Aug 2012 6:40 am
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To answer Tim's original question -
If the amp is a direct-coupled, solid-state amp, let's say like a 4-Ohm-rated Peavey steel amp, there is no reason you shouldn't run the amp with an 8 Ohm load. It makes absolutely no sense to artificially increase the load (reduce the impedance) to 4 Ohms by soaking the power - you'll generally get less output (the power transfer increase is more than offset by dropping half the power over the resistor), and you're working the amp harder - a bad combination. Running a solid-state amp like this at a higher-than-rated impedance is actually easier on the amp. There is some loss of power transfer, but it's not usually down to 50% of the matched-impedance load.
If the amp is a transformer-coupled tube amp, it is generally best to run the rated load, but I would never match it by adding a power resistor in parallel to the speaker. That Weber impedance matcher is a great idea, or if you're trying to reduce the delivered power, use an inductive power soak like one of Weber's speaker-motor attenuators or a THD Hotplate.
I agree that some tube amps like old Fenders have pretty robust output transformers and can handle up to a 100% mismatch. But it's generally better for that mismatch to be in the higher-load (i.e., lower impedance) direction. When you add an extension speaker using the extension speaker output - let's say an 8 Ohm speaker with a Twin or Deluxe Reverb - that's what you're doing, lowering the impedance from 8 Ohms to 4 Ohms. More here - http://bb.steelguitarforum.com/viewtopic.php?t=228139
I would never mismatch the speaker load unless I was pretty confident that the output transformer was pretty robust. Never with something like an old Marshall tube amp - part of the classic rock and roll Marshall sound was pushed output transformers. They want to see the correct load. |
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Earnest Bovine
From:
Los Angeles CA USA
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Posted 22 Aug 2012 7:27 am
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| Dave Mudgett wrote: |
| It makes absolutely no sense to artificially increase the load (reduce the impedance) to 4 Ohms by soaking the power - you'll generally get less output (the power transfer increase is more than offset by dropping half the power over the resistor), |
That was my point in saying that the resistor is not as loud as a speaker. Why would you want to waste half of the amp's power in a resistor? If your amp is too powerful, there are better/lighter/cheaper ways to get a less powerful amp. |
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Tim Whitlock
From:
Colorado, USA
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Posted 22 Aug 2012 7:59 am
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That was my point in saying that the resistor is not as loud as a speaker. Why would you want to waste half of the amp's power in a resistor? If your amp is too powerful, there are better/lighter/cheaper ways to get a less powerful amp.
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My original question was not about how to reduce the power, rather it was how to best deal with the mismatch in impedance when going from a 4 ohm load to an 8 ohm load.
Just as an FYI - the amp in question is a 1969 Standel Artist with four main transistors - no output transformer. It came in a huge particle board cabinet that is as unattractive as it is heavy. I am generally reluctant to butcher a vintage amp, but in this case I am building a head cabinet for the very lightweight amplifier section and a 1x15 speaker cabinet for an 8 ohm JBL D130F. |
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Dave Mudgett
From:
Central Pennsylvania and Gallatin, Tennessee
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Posted 22 Aug 2012 10:33 am
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Yup - we didn't know what your application was, hence my long-winded answer trying to cover different possibilities.
There should be no inherent problem running your solid-state amp, rated down to 4 Ohms, into an 8 Ohm load. You'll lose some power, as compared to running into the 4 Ohm load. But there should be less current through the output transistors, which I generally consider to be a good thing, especially with some of those old transistors - some early transistors were subject to thermal runaway when pushed. Not saying that's the case with these, but in general, I think it pays to be conservative with older transistor amps.
You're not butchering the amp if you just take the chassis out and put it in a different cabinet. In principle, you can put it back to stock if you ever want to. Makes total sense to me. |
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Donny Hinson
From:
Glen Burnie, Md. U.S.A.
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Posted 22 Aug 2012 4:32 pm
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Using a higher-than-normal impedance speaker may...or may not...affect the audio power output (which is different than the electrical power output of the amp). Speaker efficiency plays a far larger importance when it comes to actual audio output.
In general, it's quite safe to go higher in impedance, as long as you're not exceeding the rating by more than a factor of 2. In other words, it's okay to run a 4-ohm amp at 8-ohms, or an 8-ohm amp at 16-ohms. In normal applications, with all other things being equal, the output differences will barely be noticed.  |
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Tim Whitlock
From:
Colorado, USA
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Posted 23 Aug 2012 6:12 am
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| Thanks Dave and Donny. Exactly what I was looking for. |
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Jerome Hawkes
From:
Fayetteville, North Carolina, USA
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Posted 23 Aug 2012 6:32 am
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| Dave Mudgett wrote: |
I would never mismatch the speaker load unless I was pretty confident that the output transformer was pretty robust. Never with something like an old Marshall tube amp - part of the classic rock and roll Marshall sound was pushed output transformers. They want to see the correct load. |
this has puzzled me for some time - but on my OLD plexi Marshall 50watt head there is a selector on the back for the intended ohms - its a little pin type knob you pull out and set to 2 - 4 - 8 whatever. which i have never understood as i havent seen this on other (mostly Fender) amps. oh, and that is a HUGE output transformer on those Marshall 50w - serious iron in that amp.
_________________
'65 Sho-Bud D-10 Permanent • '54 Fender Dual-8 • Clinesmith T-8 • '38 Ric Bakelite • '92 Emmons D-10 Legrande II |
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Donny Hinson
From:
Glen Burnie, Md. U.S.A.
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Posted 25 Aug 2012 10:08 am
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Some amps have tapped output transformers, so they have switches or jumpers to access those various taps. Also, whenever you're talking speaker impedance mismatches, you really have to consider the musical load the amp is seeing. Pedal steelers are, almost without exception, very conservative players, so the amount of stress on their amps is quite small. Most rock players, on the other hand, with their penchant for distortion and sustained overloaded chords, are an entirely different story. An amp that Lloyd Green could use for 20 years might not last a single gig for a windmilling/distorted rock player! Everything must be taken in context, and I feel it's important to realize that when giving advice on such matters.  |
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