please help a noob with impendance

chrishill901

Hi there, I wondered if I could have a little science lesson and advice on what are the various safe ways to operate my marshall 8008 poweramp.

First of all, here is my cab:

It has 2x 16ohm WGS speakers in it but I kept the wiring the same.

In a separate thread, ICBM very kindly contributed this:

In mono into 8 ohms, one side will deliver about 45-55W I think. (I did check once, can't remember the exact figure but that's close enough.)

In stereo into 16 ohms per side, it should deliver around 25-30W per side, or 50-60W total… basically the same, but you can use stereo processing if you want and it's slightly more reliable since the amp is running even more conservatively.





How does stereo work? Would it be possible/safe to use the channels of the 8008 for different purposes, for example have an 11r in channel a, output to the left side of the cab set to stereo, and a zoom g3 in channel b in the right side? Do the channels directly relate to the speaker, so in stereo the 11r would only come out of the left speaker and g3 only out of the right? (I'm thinking when a mate comes to jam to save him lugging an amp.

If the above is possible, is it possible to just have one output of the 8008 plugged in to the cab but it still set to stereo? (I have no reason why, just curious) does the cab need a load into both jacks if set to stereo?

Also, why is it slightly more reliable to run the 8008 in stereo rather than mono as it is running more conservatively? Surely the best way to run it would be at full 80w into 4ohm?

Another one, I read that into a 16ohm speaker the 8008 would give me between 25 and 30w, therefore into 2 16 ohms in stereo it would be 50-60w? How come? If the channels and outputs are independent then surely it would be no different to using 2 separate 30w SS amps... Surely using 2 amps wouldnt double your power?

Finally, the 8008 manual says that input B can accept a signal from mono equipment via linked mono mode that could be output via both speaker outs. That will give me the same power to the speakers as having stereo inputs right?

Thanks

ICBM

I'll try to take these one at a time!

Stereo means that the two speakers in the cab are operating independently, yes. You can use the two speakers for completely different purposes if you want, exactly as you describe. Since the cab has a switch, make absolutely sure you don't switch to Mono when you're doing this - it could damage the amp.

If you have the cab set to stereo and only one input, the sound will only come out of one speaker. Neither the cab or the amp will mind - the amp might if it was a valve amp though, so if you ever get one, be careful. Solid-state amps are almost always safe with no load. Remember that the power handling of the cab is only that of the one speaker you're using.

It's marginally more reliable to run the 8008 in stereo into 16 ohms per channel because each power amp is putting out less power and hence is running more conservatively - to be honest it probably makes little to no difference when you're comparing 8 to 16 ohms though - the amp is not at maximum stress into 8 ohms anyway. The real difference is with running at 4 ohms, which is the minimum impedance the amp can take. Unlike valve amps, which are safest at the correct matching impedance, solid-state amps are under most stress at the minimum impedance and less if you increase it. (Don't ever run a solid-state one into less than the minimum. Valve amps are usually quite happy though.)

The power of each channel into 4 ohms is 80W, into 8 ohms about 45-55W (I forget, I did measure it once - it's almost always a little more than half for a solid-state amp though), and into 16 ohms a little more than half as much again, ie probably just under 30W. The reason it's not a straight half into double the impedance is exactly because the amp is under less stress - the power supply voltages rise slightly because there's less load, so the power output 'bounces back' a little.

Then, if you combine both channels you do get double the power. (Actually possibly slightly less, because the power transformer is common to both channels and so the voltages will *fall* again slightly - still with me? ) So with both channels at 8 ohms the amp would give a combined power of probably around 90W, and with both channels at 16 ohms probably around 60W. You do indeed get exactly twice the power if you use two fully independent identical amps, which means separate transformers as well.

Finally, yes - it's exactly the same as both channels each getting identical mono signals. It's just a little bit more conservative and efficient than running one side in mono - essentially because you're spreading the load between both power amps - which is why Marshall give you the option.

Hope that all makes some kind of sense! I know it's a bit complicated to begin with .

octatonic

Just to add to ICBM's post:

http://www.mnbigbirds.com/images/SFM/Jan 13 NL/Ohm's Law.jpg

chrishill901

That's absolutely brilliant @ICBM - makes perfect sense! Thank you for taking the time to write that, I really appreciate it

Lol @octatonic

Out of curiosity, on a valve amp that has 1x8ohm out or 2x16ohm outs, with I cab like mine that can run stereo 16ohm, what would be the better way to run it? With one cable in mono or 2 in stereo?

ICBM

chrishill901 said:

Out of curiosity, on a valve amp that has 1x8ohm out or 2x16ohm outs, with I cab like mine that can run stereo 16ohm, what would be the better way to run it? With one cable in mono or 2 in stereo?

You're unlikely to find it that way round - it would normally be one 16 and two 8-ohm outputs, in which case you need to run it in mono at 8 ohms. You still have to use the 8 ohm outputs even if you're using two cables - two 16-ohm speakers in parallel make 8 ohms.

It makes no real difference except a tiny bit of reliability if you use two cables - if one fails or falls out you still have the other. (The resulting mismatch of 8 ohm amp into one 16-ohm speaker is safe.) You'd need to be really paranoid to bother though!

chrishill901

Thanks again

My ht50 has 2 sockets labeled "1x8ohm or 2x16ohm"

octatonic

Chris, 

Bit of a random questions- do you happen to live in Ealing and play rockabilly?

I know a Chris Hill of that description.

chrishill901

octatonic;126217" said:

Chris, 

Bit of a random questions- do you happen to live in Ealing and play rockabilly?I know a Chris Hill of that description.

Haha no, afraid not! Its a remarkably common name. I had a very good friend at uni called chris hill too, and no, he wasn't from Ealing either!!

ICBM

chrishill901 said:

My ht50 has 2 sockets labeled "1x8ohm or 2x16ohm"

Ah, right - that's just two jacks with a single impedance of 8 ohms, so you can either use one 8-ohm cab or two 16-ohm ones together.

With your cab it then doesn't matter whether you use one cable or two, but it's really overkill to use two!

You'll find a lot of amps with one 16 *and* two 8-ohm jacks (ie three jacks total) to give you the option of 8 or 16 ohms.

Jalapeno

It's Impedance BTW

ICBM

Jalapeno said:

It's Impedance BTW

Nah, it's ohmage. Same as wattage but ohms. Everyone knows that.

Jalapeno

Well ok, but it's not impeNdance ! :P

chrishill901

told you I needed educating!! :P

ICBM

Jalapeno said:

Well ok, but it's not impeNdance ! :P

No, impendance is the state of impending .

(Jon & Vangelis, covered by Donna Summer)