Sorry, just another boring extension cab impedance query...but better safe than sorry!!

Voxman

My Marshall JCM2000 DSL401 1x12 combo has a 16 Ohm internal speaker (upgraded to a Celestion G12 Classic Lead 80):

• With just the 16 Ohm internal speaker - the impedance selector is set to 16 Ohm
• When connecting to a 16 Ohm cab PLUS the internal 16 Ohm speaker - impedance selector is set to 8 Ohm
• An 8 Ohm cab on its own, with NO internal speaker -  impedance selector is set to 8 Ohm

But I'd like to connect it to a separate 1x12 Laney Cab that has an 8 Ohm Celestion Seventy-Eighty and accordingly only an 8 Ohm out.  Is it safe to connect this in addition to the internal 16 Ohm speaker and if so, what's the correct impedance selector setting?  

ICBM

Set the amp to 8 ohms. The total load will be a bit of a low mismatch - 5.33 ohms - but although not ideal, this is safe with any valve amp I know of, which will normally be fine with a 2:1 mismatch in either direction. It only really matters if you're cranking it into power-stage overdrive anyway. Most valve amps are actually less at risk from a low mismatch than a high one, contrary to popular belief.

(Although the popular belief is correct for solid-state amps, which shouldn't be run below their minimum impedance - that's why it's described as a *minimum* rather than a *matching* impedance.)

Voxman

ICBM said:

Set the amp to 8 ohms. The total load will be a bit of a low mismatch - 5.33 ohms - but although not ideal, this is safe with any valve amp I know of, which will normally be fine with a 2:1 mismatch in either direction.

It only really matters if you're cranking it into power-stage overdrive anyway.

Most valve amps are actually less at risk from a low mismatch than a high one, contrary to popular belief.

(Although the popular belief is correct for solid-state amps, which shouldn't be run below their minimum impedance - that's why it's described as a *minimum* rather than a *matching* impedance.)

Thanks @ICBM.  If I did fully crank the amp up (on any channel) will I still be OK?  

ICBM

Voxman said:

If I did fully crank the amp up (on any channel) will I still be OK?  

I would normally say yes, but this is a DSL401 so I would have to say be careful… they're not known for their reliability really . Although they're not as bad as the DSL201.

Voxman

ICBM said:

Voxman said:

If I did fully crank the amp up (on any channel) will I still be OK?  

I would normally say yes, but this is a DSL401 so I would have to say be careful… they're not known for their reliability really . Although they're not as bad as the DSL201.

OK, thanks @ICBM - I wouldn't be cranking everything to the max with the Laney cab anyway as this is more for home playing.  If I did want to crank the DSL401 for gigging with an extn cab I have a 2x12 Vox AD212 extn cab with 2 x Celestion Neodymium 'Neodogs' (80w versions of the 60w Celestion G12 Century 'Nyodmium' Vintage) that I could connect at 16 Ohms so it would then be a true 8 Ohm overall impedance with the internal speaker.  I've tried that at home and its bone-crushingly loud even at 50%!!  

I must say that adding the 1x12 extn cab to both my Laney Cub 12R and DSL401 (both have 100dB internal speakers) makes a noticeable difference re perceived volume/fullness - more noticeable with the Cub actually but probably because it has a smaller cab than the DSL401. 

ICBM

That should be OK.

To be fair, I haven't yet seen a DSL401 with a blown output transformer, which would be the most likely failure other than a valve. The 201 will blow transformers even when run at the *right* impedance though… so I'm just inherently cautious with them!

ecc83

"Although the popular belief is correct for solid-state amps, which shouldn't be run below their minimum impedance - that's why it's described as a *minimum* rather than a *matching* impedance.) "

Yes ^ The term 'matching' when applied to amps and speaker impedance (aka 'Z') IMO needs dragging out to the pub car par, soaking in petrol and burning and the remains stamped on and hosed away!

The internal, source Z of an amplifier, valve OR transistorized is never* anywhere near the value of the speaker. Valve OP stages are generally much higher than load and transistors much lower, near buggerall most times.

True signal source/load matching only happens at RF and with digital and video signals.

*A big, ~100W valve amp might have an internal Z of around 10 Ohms but that is just serendipity.

Dave.

Voxman

ecc83 said:

"Although the popular belief is correct for solid-state amps, which shouldn't be run below their minimum impedance - that's why it's described as a *minimum* rather than a *matching* impedance.) "

Yes ^ The term 'matching' when applied to amps and speaker impedance (aka 'Z') IMO needs dragging out to the pub car par, soaking in petrol and burning and the remains stamped on and hosed away!

The internal, source Z of an amplifier, valve OR transistorized is never* anywhere near the value of the speaker. Valve OP stages are generally much higher than load and transistors much lower, near buggerall most times.

True signal source/load matching only happens at RF and with digital and video signals.

*A big, ~100W valve amp might have an internal Z of around 10 Ohms but that is just serendipity.

Dave.

 

I'm sure that was a brilliant post Dave - if only I had the brains to understand it!  

ICBM

Matching in this case means the impedance which produces the best power transfer - which is not only the most efficient but also the least stressful on the valves and transformer - rather than the same impedance for both.

In fact, that's how you can identify the correct impedance on an unmarked amp, or the taps on an unknown transformer - measure the power output at 4, 8 and 16 ohms and see which produces the most. Start with 8, then 4, then 16 - if you get more power at 4 than 8 then don't try 16 since it's more risky to run a 4-ohm amp at 16 than a 16-ohm amp at 4.

sm55onl

Voxman, Hughes &Kettner seem to have answered your question on their website ?...(as well as ICBM having done so) 
http://blog.hughes-and-kettner.com/ohm-cooking-101-understanding-amps-speakers-and-impedance/

Am not quite sure what bee has got into ecc83's bonnet regards matching ;-))

ICBM

sm55onl said:

http://blog.hughes-and-kettner.com/ohm-cooking-101-understanding-amps-speakers-and-impedance/

I hadn't seen that before, but finally - finally - a single article that gets pretty much everything there is about speaker mismatching right.

The only other thing I can think of that's worth mentioning is that if you're ever presented with a cab of unknown impedance which you have to use and you have no way of measuring it, just assume it's 8 ohms - because running an amp set to 8 ohms into either a 4-ohm or a 16-ohm cab is less risky than running either of them into the opposite one...

Voxman

Really good article and its reassuring to read that the mixing of 8 ohm and 16 ohm is a really common query that causes confusion, so I don't feel so stupid in seeking advice. Doubly reassuring that all of the 'ticked' combinations are safe and no amp has blown up - of course it all completely echos @ICBM's post, who I always have complete confidence in anyway!

ecc83

"

Am not quite sure what bee has got into ecc83's bonnet regards matching "

The 'bee' sm chap is because the term 'matching' in electronics refers to a signal transmission condition that is never now used for audio signals (and never was for valves or transistors) .

Yes, the transformer reflects a load to the valves  but this is ideally the 'Optimum Load' and this is chosen for best power output at minimum distortion. It is NOT a 'match' to the anode resistance of the valve.

I agree these matters are a bit esoteric in the rough-ass world of guitar amps but these questions DO come up in other forums and 'matching' e.g. S/PDIF signals, Word Clock are also discussed and I feel it important to keep the differences in mind?

Dave.[i

sm55onl

Ecc83, thanks for your reply.
A bit off-topic from the basis of the thread, but am i assuming correctly that you are are referring to 'impedance matching' for audio connector issues regards the s/pdif and word clock ?

ICBM

Ha, yes it's a bit like 'RMS power' - technically nonsense, but very commonly used to the point it's become almost the accepted term. It needs correcting every now and then .

sm55onl

ICBM, Sorry not quite sure if the 'RMS power' was a reply to myself or ecc83 but, yes, there's rms voltage and rms current....but rms power ? Power is power.

ICBM

That was a reply to Dave .

'RMS power' is actually very commonly used, despite being nonsense. It seems to be defined as the continuous maximum clean power of an amp - I assume it's derived from the RMS voltage output into the load impedance. Peavey do actually state that, but most other companies use 'W RMS'.

sm55onl

It's all the Federal Trade Commission's fault apparently
http://www.n4lcd.com/RMS.pdf

Another good explanation ?
http://www.n4lcd.com/RMS.pdf

and to get mathematical about it
http://www.skm-eleksys.com/2011/06/instantaneous-average-and-total-power.html

clarkefan

Sorry to hijack but I have to ask, @sm55onl, what's the story with the Tele in your avatar? Looks amazing, worn or what?! Very cool   

ICBM

sm55onl said:

It's all the Federal Trade Commission's fault apparently
http://www.n4lcd.com/RMS.pdf

Nicely typical American dig at anything 'government' in that… "the FTC saw it as an opportunity to regulate (control)." Well, that's because the free market can't be trusted to be honest. (And still can't, judging by some of the modern amps which claim greater output power than their power draw.)

That does sound like the explanation, anyway.

sm55onl

Clarkefan,
It's a back shot of a '63 Fender Tele which i bought in the noughties from an ex-player/collector. It's original and very light in weight but with the worst paint/tippex 'job' around the side to make it look like a 'Custom' whilst on stage (carried out by an earlier previous owner)....and, yes, it's been played ;-))
The original dowel marks should be visible if you look closely enough.