Tech: Guitar amp/speaker impedance

Subject: Tech: Guitar amp/speaker impedance
From: michaelr
Date: 21 Feb 11 - 03:56 PM

Punkfolkrocker recently had a similar query. My 50-watt guitar amp has switchable 4, 8, and 16-ohm speaker output. Thr problem is that the amp is so loud that I can't turn it up to its full tonal potential. I run two 8-ohm speakers in my cabinet, with the amp switch at 4 ohms.

My question is: Can I reduce the effective volume by switching the amp to 8 or 16 ohms? Would this risk damage to the amp or speakers?

Apologies if this has been covered before.

Subject: RE: Tech: Guitar amp/speaker impedance
From: pdq
Date: 21 Feb 11 - 03:58 PM

Use less efficient speakers.

Subject: RE: Tech: Guitar amp/speaker impedance
From: michaelr
Date: 21 Feb 11 - 04:42 PM

Thanks, that doesn't address my question at all.

Subject: RE: Tech: Guitar amp/speaker impedance
From: GUEST,Jon
Date: 21 Feb 11 - 04:45 PM

Your speaker impedance would still be 4ohms and the amp would be expecting 8ohms. Sounds a bad idea to me.

Subject: RE: Tech: Guitar amp/speaker impedance
From: GUEST,Auldtimer
Date: 21 Feb 11 - 05:32 PM

Wire them in series.

Subject: RE: Tech: Guitar amp/speaker impedance
From: Bernard
Date: 21 Feb 11 - 05:42 PM

What he just said!

According to Ohms Law, an amplifier giving 50 watts into 4 ohms will only deliver around 16 watts into 16 ohms... but it's not quite that simple, so you may find you get as much as 25 watts. Half power doesn't equate to half volume...

Okay, you may still not get the reduction in volume you want, but you may be able to get your local, friendly amplifier repair man to put a volume control pot between the pre-amp and output amp, so you'll get the full distortion of the pre-amp, but at a much lower volume.

I take it by 'full tonal potential' you meant 'distortion'!

Subject: RE: Tech: Guitar amp/speaker impedance
From: Bernard
Date: 21 Feb 11 - 05:44 PM

Erm... that should have read '12 watts into 16 ohms'...!

Subject: RE: Tech: Guitar amp/speaker impedance
From: s&r
Date: 21 Feb 11 - 05:51 PM

Auldtimer's right; assuming that the voltage out of the amp is constant, then wiring the speakers in series would increase the speaker impedance by 4x, giving 1/4 the current through the speakers. The results might be poor in other ways but I can't see any potential damage. The current from the amp is also reduced by 4x.

If you're worried start with low volume.

Stu

Subject: RE: Tech: Guitar amp/speaker impedance
From: JohnInKansas
Date: 21 Feb 11 - 06:26 PM

Assuming that your two 8 ohm speakers are wired in parallel, and the speaker pair input impedance is nominally 4 ohms, running on the amp's 8 ohm (or 16 ohm) output impedance probably will introduce so much distortion due to the impedance mismatch that it won't make much difference what gain (volume) setting you crank the amp to.

Whether using the wrong amp output impedance will damage the speakers or the amp depends somewhat on the details of the components, but damage to either or both is fairly likely. (The cheaper component will be protected by the failure of whichever parts are most expensive to replace.)

This is especially true if you're attempting to crank the amp to higher power levels based on the mostly mythical notion that the amp has to run at high power to reduce distortion.

The lack of "tonal purity" you hear most likely is because you've been sitting in front of (or too close behind) too big an amp with too powerful speakers, for too many years, you're getting old, and the best cure for the "bad sound" would be a trip to a good audiologist. In this context, "old" means two or three years - or less - in the presence of high volume sounds, and some estimates are that more than half of people have "old ears" before they get out of high school in the US (about age 18). (That's why screaming has replaced singing in much of the "entertainment" business(?).)

On the possibility that there is a real problem with your sound kit, the only realistic fix is to get a smaller kit (especially the amp) for the times when you don't really need 50 watts. You could consider a pair of "low efficiency speakers" but speakers of the kind you'd need would likely cost as much or more than a decent 10W or 20W amp.

Note that the power rating of the amp does have some relationship to the level at which it's intended to operate. The "power rating" on the box for speakers usually is only the maximum power that you can put in without destroying them, and generally has little to do with what amount of power you need to put in for them to sound good.

You haven't indicated whether your problem is in a performance situation, large or small venue, or home practice etc., and the situation might affect recommendations; but a 50W amp is generally more than needed for a small indoor venue, and in a place big enough to justify it you'd often be better off with a preamp to plug into the house system where the "sound engineer" can take care of the amount of your noise that competes with the crowd.

John

Subject: RE: Tech: Guitar amp/speaker impedance
From: michaelr
Date: 21 Feb 11 - 06:50 PM

Thanks all for your responses.

I should have clarified that this is about playing electric guitar, so "preamp into the house system" does not apply. The way my amp is built (it's a clone of the Dumble Overdrive Special) means that the tonal "sweet spot" comes with overdriving the output tubes, not just the preamp tubes. But I'm finding that with 50 watts this just can't be done except at ear-shattering levels (I don't play stadiums).

It's also not financially feasible for me to buy a different amp or speakers. I'll look into wiring the speakers in series instead of parallel.

I welcome more input!

Subject: RE: Tech: Guitar amp/speaker impedance
From: Richard Bridge
Date: 21 Feb 11 - 06:52 PM

Transistor output stage will not object to raising impedance. Valve output stage will.

If you have a valve output stage you need a thing like the Marshall "Powerbrake" which is a variable dummy load that you put between the output of the amp and the speaker to reduce the perceived volume once youhave got the sound you want.

Alternatively turn everything else up too.

Subject: RE: Tech: Guitar amp/speaker impedance
From: michaelr
Date: 21 Feb 11 - 10:37 PM

Ha! I'd face a mutiny! And I don't want to get any deafer than I am...

Subject: RE: Tech: Guitar amp/speaker impedance
From: GUEST,punkfolkrocker
Date: 22 Feb 11 - 12:21 AM

If it's not already mentioned here, the 'best' practicable solution is to use an attenuator/power soak
between the amp and the speakers.

I don't have the knowledge or skills to make my own.
but the simplest DIY devices use minimal cheap components.

After a fair bit of market research I settled on a 50w Weber Mini-Mass
as probably the most cost effective [despite delivery cost to UK and extortionate import tax & handling charges]
'pro' quality device to help quiet my valve amps;
ranging from a measly but still too loud 1watt Blackheart Killer Ant,
5 to 20watt VHTs and Laneys,
to a brutal 30watt Hayden Mofo.


"Some Weber Attenuators use power resistors to effect a purely resistive load on the amp.
These attenuators are much like the other attenuators on the market,
and provide a less natural sounding tone when heavily attenuated.
Our resistive models are the High Power Load Dump, Low Power Load Dump, Mega Dump, and Headphone Tap.

Other Weber Attenuators utilize an actual speaker motor to apply a reactive load on the amp.
The attenuators are unlike any other on the market,
and provide a much more interactive, natural sound when in use. They affect the tone less than the resistor-based models. The speaker motor units are the MASS, MiniMass, MicroMass, Stereo MASS, MASS 150, MASS Lite, and Power Tap."

Weber recomend a 50 watt amp would require an attenuator rated for 100 watts to be safe.

http://www.tedweber.com/atten.htm

It's a compromise, the tone wont be perfect, but definitely pleasing enough hitting the power valve sweet spot
at more civilised volume levels..

Good overdrive/boost pedals can also help balance out any desired tone whilst still pummelling the power tubes.

Even with this setup, I still feel self concious about playing at home later than 7.30pm
as I live in a town centre terraced house,
but now consider my home practise volume to be fairly reasonable during the afternoon.

I've also just spent a few quid more than I can afford
on new reproduction old fashioned inefficient 20watt 96db greenback speakers
which are making even my solid state amps sound significantly better.

Subject: RE: Tech: Guitar amp/speaker impedance
From: GUEST,punkfolkrocker
Date: 22 Feb 11 - 12:28 AM

forgot to mention, the weber mini mass, provides a 'line-out'
and an option to switch the speakers off completely & safely
while still connected..
This enables me to run the signal post power tubes to a mixer,
speaker simulator device or software, recording equipment,
and small monitoring speakers or headphones.

Really is a flexible system for home or gigging.