Impedance Hell

[MichelG]

Hi guys,

I wonder if I can plug both my AFD100 and Super Champ xd in my 1936 cab.

Thing is that I didn't succeed in understanding anything on the subject of impedance.... (Ok, I may be lazy  :orczz:...)

Could anybody tell me if it's possible or not? Maybe it's possible but with some custom circuit thingy...

Technical details as follows :

SuperChamp xd output

• 8 Ohms

AFD 100 outputs

• 4 Ohms for a single 4 ohm guitar cabinet or two 8 ohm guitar cabinets.
• 8 Ohms for a single 8 ohm guitar cabinet or two 16 ohm guitar cabinets.
• 16 Ohms for any 16 ohm guitar cabinet

1936 cab inputs

• 8 Ohms mono
• 16 Ohms per side in stereo

Thanks a lot !

[Dmoney]

Doesn't sound possible.

You'd need your AFD going into one speaker with the head set to 1x 16ohm load only.
The Champ however would also need a 16ohm output, but it only has an 8.
I dunno what speakers you've got but you probably wouldn't want to crank the AFD at full power through one speaker either!

[MichelG]

Yep, it's 70W max per speaker.

I've never cranked up the AFD. It's too loud (or maybe I'm too old). Pushed it to 50W max. It may sound even better fully cranked but it's already very good half way up.

So for you there is no may to make any kind of LRC circuit to transform the 8Ohm output of the SuperChamp into a 16Ohms... too bad but thanks for the answer dude!

[Copperhead]

DANGER DANGER DANGER!!!

NEVER connect two amps to one cab!
Only way this is possible is through a device like a Radial Tone Bone Switcher.
Safe mismatch with impedance is cab value higher than the output value of the OT.

[Dmoney]

DANGER DANGER DANGER!!!

NEVER connect two amps to one cab!
Only way this is possible is through a device like a Radial Tone Bone Switcher.
Safe mismatch with impedance is cab value higher than the output value of the OT.

I think you are wrong with all your points here.

The OP was referring to a mono/stereo cab. 8ohm mono, 16ohm stereo. When in stereo the speakers will be working independently but housed in the same cab and electrically disconnected from each other. Connecting a head to each 16ohm input will be exactly like connecting each head to a 1x12. It's absolutely possible and perfectly safe unless the speaker can't handle the power of the amp or the two amps you use are out of phase.  

The radial switcher you mention is the Headbone used to switch 2 heads into the same input on one cab... like an advanced A/B switcher. A normal A/B switching box won't work in this position (between head and cab) and introduces a high likelihood of damaging the amp. The Headbone is specifically designed for this application, like you said.

The safe impedance mismatch you mention here is wrong. Think about the worst thing you can do with your valve amp... turn it on with no load!
'No Load' is not zero ohms... having nothing attached to the output jack is in effect an infinitely high impedance, an open circuit. Solid State amps (in general terms) are ok with this. The problem comes when they don't have enough impedance... ie a short from tip to sleeve on the output jack.
Valve amps are different. Using the amp set to 4ohm's into a 16ohm cab is not the safe method. You're setting the amp to drive a low load, and then plugging it into a higher load. With a valve amp, the opposite way is the safer way. Running at 16ohms into 8ohm or 4ohm cab.  

  

[Copperhead]

No, I'm correct on all points.
Did not know he was referring to a stereo cab.
Connecting two amps to the parallel inputs on a "normal" cab is a no-no.

Thank you for correcting me on the exact name of the Radial product I was referring to.
"Radial Headbone ToneBone Amp Head Switcher"

On the mismatch, it is safer to go one step higher on the load than the other way around.
I would not go 4:1 but one step is generally accepted as a safe mismatch.
Too low impedance for the OT is like a direct short.
Neither SS or tube amps will survive this.

[Copperhead]

... In stereo mode, the Champ, 8ohm output will be fine into a 16 ohm load.

[Dmoney]

Oh right,

I read...

1936 cab inputs

• 8 Ohms mono
• 16 Ohms per side in stereo

Thanks a lot !

...and assumed he had a stereo cab.
I agree what you mentioned is a bad idea, I've never seen a cab with 'parallel inputs' though... one is usually labelled as an extension speaker connection rather than an input, but it would be the kind of wiring you describe.

Correct. Shorted outputs will damaged both types of amplifier, but running a valve amp into too high an impedance is a good way to damage your amp, even if running 8ohm out into 16ohm might work.

EDIT: And having no speaker at all connected is not a short.

[jpfamps]

On the mismatch, it is safer to go one step higher on the load than the other way around.
I would not go 4:1 but one step is generally accepted as a safe mismatch.
Too low impedance for the OT is like a direct short.
Neither SS or tube amps will survive this.

Valve amps will generally survive a dead short (that is of course NOT to say this is a good idea); this is why Fender used a shorting jack in their amps.

What can and does damage valve amps is too high a loading, which causes excessive screen grid current, large voltage spikes, and even oscillation at idle.

I can honestly say I have never seen a valve amp that has been damaged by too low a loading, or even a dead short, but regularly see amps damaged by too high loading.

A DC coupled SS amps will only survive a dead short if it has effective protection circuitry. My personal experiences would suggest that of the amps that have "short circuit" protection, and a surprising number don't, few have effective protection circuitry.

[MichelG]

Hi guys!

Thanks a lot for all the information devilvered.

To wrap up things, I understand that pluging the 16ohms AFD100 output on on side of the cab (16 ohms) and pluging the 8 ohms Champ output on the other side of the cab (16 Ohms) is at least a safe try.

What I also understood is that the Champ sound may lack a bit of bottom end due to the impedance mismatch.

Is that correct?

If yes, I'll give it a try on thursday afternoon :)