Amp Impendance Trouble

kiefer182

Didn't know where to post this, sorry if this is the wrong forum,

Here's my problem;

i have a LANEY GS412LA cab and have recently bought a Mesa Boogie single rectifer 50w, the cab is 16 ohms but the head only has 2x 4 ohms outputs and 1x 8 ohms output.

Will i still be able to use my cab with my new head or will the impendance mismatch harm my amp? ive read up somewhere about impendance mis-matching but im unsure whether it is safe and if it is what would be the safest option for me considering my set up?

cheers for any help

smokiebeverage

Having a higher impedance speaker probably wont hurt the amp as it wont be over drawing current, but I may sound off and the combo would be wasted. You'd be better getting a impedance matching transformer. The following web page will explain it better than I can!

http://www.horrorseek.com/home/halloween/wolfstone/Audio/spkimp_SpeakerImpedanceMatching.html#Why

darrenw5094

Mesa Boogie gear will be well fit to take the mismatch. On the official Mesa site somewhere, they even advise to try a mismatch, as you might like the sound of it.

Other brands like Marshall etc., maybe not a good idea.:rolleyes:

Paolo_M

I'm pretty sure the Mesa Boogie Rectifier series actually promotes impedance mis-matching, for the tonal variation it gives, in their manuals.

There are many myths regarding impedance mis-matching.

Firstly mis-matching either way is equally as dangerous (not very dangerous assuming it's a quality amp and not a big mis-match like 4 ohm - 16 ohm).
One causes excessive current draw from the power amp, the other causes excessive fly-back voltages.
Either scenario can harm an output transformer and/or output valves.
Excessive current is actually slightly less dangerous as a valve is self regulating to a degree, plus 99% of the time the HT is fused), and so it could be argued that mis-match downwards is the lesser of two evils.

Secondly, a decent output transformer and decent sets of output valves are very forgiving, and can easily tolerate mis-matches of 100%. As mentioned, Mesa Boogie actually promote doing this.

Marshall historically used under-speced their output transformers and so mis-matches caused problems, especially when the amp was played at 10 on every dial which Marshall players tended to do back in the day.
Marshall continue with this tradition, the DSL/TSL series are some of the most pathetic excuses for transfomers I've seen in any amp.

In short; check the Mesa manual, but I'm fairly certain that mis-matches of 100% are fine, and even suggested in there.

An impedance matching transformer is certainly not the way to go here BTW.

eoin5

Its probably possible to wire the cab for 4 ohms but as the lads said you probably dont need to with your Mesa.

zafo

eoin5 wrote: »

Its probably possible to wire the cab for 4 ohms but as the lads said you probably dont need to with your Mesa.

This is where I'd be looking to solve the problem, I'd also echo that the "impedance matching transformer is certainly not the way to go". (in fact I'll just quote Paolo...).

smokiebeverage

Like I said the you won't damage the amp, but you will be seriously underdriving the voice coil in the speaker, now that may produce a sound you like and if so thats what you want, but you have to be careful because in a stage setting you need to crank the power up on the amp to get the volume you require which may produce a clipping at the top which can damage the voice coil as they are not made to handle square waves. The amp will handle it, the messa's are really tollerent of under and over loading but as you try to drive the cab hard over time you may cause the return on the coils to change or stick. Its funny people always worry about the amp but ususlly think the cab is indestructable! Eitherway if you are only home playing or recording it wont be an issue and even live it may not be, but don't expect the combo to be super loud. The sound change my be great, I had a little small hand built amp at 4Ohms driving and 8Ohm speaker for ages and it sounded great but never really had a kick but it was lovely to practice at home with. Apart from the power transfer issues with impedance you also have to remember that by not matching them you are essentally putting a filter in the system and producing reflections (mostly at the high end and at the frequencys the guitar hits they are relatively minor) thats what the sound change is. Either way you're not going to kill the amp or the cab so have a go and see what your ears think! thats the most important thing.

Paolo_M

smokiebeverage wrote: »

Like I said the you won't damage the amp, but you will be seriously underdriving the voice coil in the speaker, now that may produce a sound you like and if so thats what you want, but you have to be careful because in a stage setting you need to crank the power up on the amp to get the volume you require which may produce a clipping at the top which can damage the voice coil as they are not made to handle square waves. The amp will handle it, the messa's are really tollerent of under and over loading but as you try to drive the cab hard over time you may cause the return on the coils to change or stick. Its funny people always worry about the amp but ususlly think the cab is indestructable! Eitherway if you are only home playing or recording it wont be an issue and even live it may not be, but don't expect the combo to be super loud. The sound change my be great, I had a little small hand built amp at 4Ohms driving and 8Ohm speaker for ages and it sounded great but never really had a kick but it was lovely to practice at home with. Apart from the power transfer issues with impedance you also have to remember that by not matching them you are essentally putting a filter in the system and producing reflections (mostly at the high end and at the frequencys the guitar hits they are relatively minor) thats what the sound change is. Either way you're not going to kill the amp or the cab so have a go and see what your ears think! thats the most important thing.

It's a valve amp, not SS. The standard electronic power transfer theorems do not apply here. The amp will attempt to deliver exactly the same power to the load (speaker) either way.
Almost no detectable change in volume will be noticed.
The guys with good ears will hear a tonal shift as the entire frequency response of the power amp gets skewed, but it's not major TBH.

smokiebeverage

Paolo_M wrote: »

It's a valve amp, not SS. The standard electronic power transfer theorems do not apply here. The amp will attempt to deliver exactly the same power to the load (speaker) either way.
Almost no detectable change in volume will be noticed.
The guys with good ears will hear a tonal shift as the entire frequency response of the power amp gets skewed, but it's not major TBH.

Solid State or Valve you still lose power, you are correct that the Valve amp will push more power to compensate but you still lose power. The two (or three really) effects are power loss and bandwidth reduction (and damping response). The difference between SS and valve is best explained at the extremes, no load on SS no problem, no load on valve big problems, massive load on SS problem, massive load on valve no problem. Thats why you should never switch on a valve amp with no load the OT pumps voltage to compensate, so if the load is bigger than the match impedance the voltage goes up, now on most amps at a factor of two there is no problem, just a shorter valve life. As for the formulas applied for impedance matching they are the same for all matching whether for amps or not, what is different is how the amps react. But back to the original question, fire away I think everyone is in agreement on that!