Marshall 8008 impedance

benvall

I've just got a Marshall 8008 power amp and I'm not sure what is the best/safest way to connect it to my 2x12 cab.

Is it better/safer to run both channels and wire my 2x12 in stereo with both 8ohm speakers receiving an output from the amp. This will effectively be 40 watts at 8ohms per side.

Or should I rewire my cab for 4ohms and run the amp in mono? This way it will get 80 watts output, but is it ok to run this amp at 4ohms with only one output. Also, this way I can possible get another 4ohm cab and run it in stereo at 80 watts per side at a later date.

Being a solid state amp I am unsure of the safest impedance options when using it in mono.

Thanks

ecc83

Can't find a manual for the 8008 (well, not unless I download a "downloader" and I ain't doin' that!) but pikkies show it to be a two channel amplifier and so IN THE NORMAL COURSE OF EVENTS you should be able to connect a 4 Ohm speaker load to EACH output.

Do note however, IF you mean running the amp in mono "bridge" mode then the minimum impedance will be 8 Ohms I am sure. It should make absolutely no difference to a well designed SS amp if you run just "one lung" at max power but since you have the option why not share the load? I doubt that the total loudness will be much noticed?

Of course! The 8008s MIGHT be unreliable ***t! ICBM will no doubt advise!

Dave.

benvall

Thanks @ecc83

The hand book is here
http://www.drtube.com/schematics/marshall/8004-8008-hbk.pdf

It says that output b is "linked mono" whatever that means?

ICBM

The 8008 is reliable enough - it's the 8004 that's a piece of junk.

With all solid-state amps it's safer to stress the output section less than to the maximum power/minimum impedance if you can - even if they are actually OK at max power into min impedance - so it's definitely better to run it in stereo at 8 ohms per side. This should give you a combined power of slightly more than running one side in mono at 4 ohms too - the power doesn't drop quite to half when running into double the minimum impedance with an amp like this, due to the power supply not being perfectly regulated.

benvall

Many thanks @ICBM I don't know how I'd manage without your wisdom and this forum.

drwiddly

I've owned a couple of 8008's and they're very good, reliable power amps especially for the £100 or so you can pick them up for. They are a stereo amp and can't be bridged as far as I know so, unless you use it with two cabs or a stereo cab, you will not get full power from it. The linked mono thing is on the input so you can feed it a mono signal and still output to both channels.

ICBM

drwiddly said:

I've owned a couple of 8008's and they're very good, reliable power amps especially for the £100 or so you can pick them up for. They are a stereo amp and can't be bridged as far as I know so, unless you use it with two cabs or a stereo cab, you will not get full power from it. The linked mono thing is on the input so you can feed it a mono signal and still output to both channels.

Yes, the link is for the inputs - do not try linking the outputs! The result is blown power transistors.

benvall said:

Many thanks @ICBM I don't know how I'd manage without your wisdom and this forum. 

No problem, just trying to put back a lot of the stuff I've learned the hard way . (Or seen others do.)

ecc83

"Yes, the link is for the inputs - do not try linking the outputs! The result is blown power transistors."

Have you tried this IC? I ask because it is a generally well accepted way to get power amps, sstate and valve, to drive lower Zs. Amps should not object to being paralleled any more than being put in series for bridge operation.

Mind you, I DO understand that we are not talking "industrial" amplifiers here!

And if anyone wants to try it you can bridge any stereo amp. All you need is PI of some sort at the input . But don't blame me for the smoke!

Dave.

ICBM

ecc83 said:

I ask because it is a generally well accepted way to get power amps, sstate and valve, to drive lower Zs. Amps should not object to being paralleled any more than being put in series for bridge operation.

Yes… when the inputs are being driven with identical signals!

But if you connect the outputs together to try to drive the same cab when there aren't identical input signals then the output sections don't track each other - and the problem with the 8008 is that it has separate volume controls for each channel, so it's difficult to guarantee that the input signals to each power amp are the same.

I've definitely seen at least one 8008 blown like that.

Clarky

I've got an 8008. I run it into a pair of 4x12 cabs at 4ohms.. I've toured it for more than a decade.. it's only ever needed repair once [after my rack was handled particularly badly at an airport]..

I've found it to be really solid, great tone and plenty loud [at least for my needs]..