How would you wire an extension speaker output in stereo?

[gbuzz]

I aquired a fender M80chorus 2 X 12. Being it has a stereo loop, I figure it outputs in stereo to the 2 12's...do I sound on par or am I off course already?

If I'm on course, I'd like to run 2 speakers out...1 for mono and use the other if I choose to hook up instereo. Should I just use the wires going to the speaker from in the chasis and wire them to a jack...also, it's not printed on the fender designed speakers..anybody happen to know the ohms?

 

Thansk for any help!

[Druxar]

 

I aquired a fender M80chorus 2 X 12. Being it has a stereo loop, I figure it outputs in stereo to the 2 12's...do I sound on par or am I off course already? If I'm on course, I'd like to run 2 speakers out...1 for mono and use the other if I choose to hook up instereo. Should I just use the wires going to the speaker from in the chasis and wire them to a jack...also, it's not printed on the fender designed speakers..anybody happen to know the ohms?

 

 

I can't find any specs so I don't know the speaker impedance - you should figure that out before hooking up anything, even ifyou have to spend $10 to get a very cheap digital meter at some hardware store. It will let you measure across the terminals (unplug it to make sure nothing else interferes).

 

So I'm assuming since I can't find specs, this is a 2 speaker combo and each speaker is a separate amplified (stereo) unit and you want to install 2 jacks, one for each speaker so you can plug in external speakers, choosing only one arbitrary one if you want mono, or both into separate other cabs or a stereo cab if you want both? In my thinking then it's the same as if you just had a single speaker mono amp and wanted to put an ext. jack except you're doing this project x2 for 2 speakers.

 

The point of clarifying that is whatever gets plugged in, it won't be shorting the 2 jacks together hopefully (you'd be shorting 2 separate amplifier outputs to each other and destroy something if your external cab ends up putting 2 jacks in parallel).

 

So in that sense I think you should be getting one of those cheap meters anyway - let's say you do have a stereo single cab with 2 speakers in it and however many jacks on the back and you want to use this with the modified amp jacks. I'd plug in a cable on each speaker input on the cab and then do some probing between the 2 cables to make sure they are totally separated, tip to tip and sleeve to sleeve, make sure nothing is shorted when plugged into the cab. Then you know you can take those 2 new outputs you will make and plug them into a single cab without blowing up the amp.

 

So with all of that out of the way assuming you won't destroy anything and my understanding is so far good, that you just want to put a jack for each of the combo amp's speakers and enable hooking up an external speaker, what you next want to do is get a jack that has a switched circuit path on it so that it disconnects the amp's internal speaker when you do plug in an external speaker, otherwise you will end up with the external and internal speakers in parallel and the impedance will be reduced, which is not good.

 

This is where I'm at. I have a little single speaker combo amp and I'm adding an external jack because in my temporary hook up so far I disconnected the internal speaker and used alligator clips to hook up a 1/4" jack to plug into a cab and it sounded much better. So I went and bought a MONO 1/4" phone jack with THREE wiring terminals on it, not 2. There are many types of jacks even with the same number of pins, the switching can be different. So what this does, one terminal is the sleeve of the plug, another terminal is the tip of the plug, and the 3rd terminal will have a movable arm so that when there is nothing plugged in, terminals 2 and 3 are connected together. When you plug something into the jack the lever moves away and breaks contact between 2 and 3, so 3 is left disconnected to anything but 2 is now connected to the plug's tip.

 

In that example, the internal speaker's - terminal goes to jack pin 1 sleeve which also stays connected up to the amplifier's existing circuit board connection (just adding an extra new wire to the new jack), the internal speaker's + wire gets cut from between the amp and speaker so then the wire hanging from the actual speaker + terminal goes to jack pin 3 switched lever, and the jack's pin 2 goes to the actual amp output for the original speaker + terminal (so the jack's switch between pins 2 and 3 is going to be placed in-line with the cut speaker + wire and when nothing is plugged in, it's like you never changed anything but when you plug in, the internal speaker is disconnected from the amp and the amp output goes to the jack output).

 

Assuming I didn't make any obvious errors you should get the idea of what type of jack is needed and how it works and that it must cut the internal speaker off so the impedances aren't messed up. I already have a jack like this and will be drilling the hole in the back of the amp this weekend.

 

So a side note from my experience so far, I had to get a jack with a plastic casing rather than one of those metal ones where the ground sleeve jack terminal is connected to the jack's body and mount hardware because my amp's outputs are not at all part of the chassis ground circuit. Some amps have the speaker - terminal just on ground which would be fine but mine can't touch ground so when I'm drilling into the metal chassis to add a jack I have to make sure the jack is plastic and I don't get shorts.

 

So whatever the exact situation is, those are some tips to get going with the type of jack to get and how to figure out the speaker impedance with a cheap meter, and how to make sure whatever you plug the 2 jacks into, it won't short them together.

[gbuzz]

Thanks sooooo much! Much valuable info! Thanks again!

[WRGKMC]

I have an M80 Head I eventually built into a cab so I'm quite familure with its circuitry. Its a a mono head, not stereo. The output is normally 8 ohms, when you plug an extension in it changes from 8 to 4 ohms. Theres a send and return in it that allows an effects unit to work between the preamp and power amp, In your case it probibly allows a mono send and stereo return. I highly doubt its a stereo poweramp.

 

Theres quite a few posts on harmony centrals equipment review including mine. Some are about the chorus like yours, and others are about various versions including the one you have.

 

The heads are quite good really for transistor. Puts out a loud 100w. The chorus may be a littel lower wattage I'm not sure but there are some bad reviews on the chorus version. Maybe it has something to do with the 12" speakers they use. I'm running 4X10" on mine and it sounds pretty good.

 

To get a schematic, its one of those versions that you have to buy a copy. Maybe this is a testimate as to their durability, or it was a limited release where they didnt sell many.

[Druxar]

 

I have an M80 Head I eventually built into a cab so I'm quite familure with its circuitry. Its a a mono head, not stereo. The output is normally 8 ohms, when you plug an extension in it changes from 8 to 4 ohms.

 

 

So when you plug in an external I assume the internal still works (connected in parallel to get 4 ohms?) Do you need to switch anything to change the amp impedance? If not either it's really a 4 ohm amp running an 8 ohm internal speaker and finally getting 4 ohms when going external/internal, or maybe it's got one of those jacks with switches built in to select the amp's impedance automatically based on what's plugged in or not...do you remember how many wires are going to the ext speaker jack?

[gbuzz]

It doesn't have an ext speaker jack...just wires out of the chasis to the speaker. I wasn't sure if it was mono or stereo..

 

If I bypass the speakers could I run an 8 ohm ext jack? Or could I set it up to use both the stock speakers and add an 8 ohm 4 X 12?

 

Kind of confused!

 

Thanks everybody for the help!

[WRGKMC]

"So when you plug in an external I assume the internal still works (connected in parallel to get 4 ohms?) Do you need to switch anything to change the amp impedance? If not either it's really a 4 ohm amp running an 8 ohm internal speaker and finally getting 4 ohms when going external/internal, or maybe it's got one of those jacks with switches built in to select the amp's impedance automatically based on what's plugged in or not...do you remember how many wires are going to the ext speaker jack?"

 

Its actually a switched jack that uses a high wattage ceramic resistor on the board to change the impediance. I discoverd this on my EBay special because someone had disconnected the second jack for some reason before I got it. I originally thought it may have just been a parallel jack and the head was designed to run at 4 ohm minimum, but this wasnt the case. Using the jacks to split the ohms maintains maximum fidelity and negates issues with additional heat developed pushing a high wattage head at a lower impediance.

The head is likely 4 ohms. When you plug in a single jack marked 8 ohms, it uses a 8 ohm ceramic resistor to make the total 4 ohms. when the second jack is used the second cab gets substituted for the resistor giving a total or 4 ohms again.

 

The disconnection may have been made to match a 2X12 cab running at 4 ohms only, i'm not sure, I had no need to map it out but its and educated guess this is what the person who disconnected it was attempting to do.

 

 

"It doesn't have an ext speaker jack...just wires out of the chasis to the speaker. I wasn't sure if it was mono or stereo..

If I bypass the speakers could I run an 8 ohm ext jack? Or could I set it up to use both the stock speakers and add an 8 ohm 4 X 12?"

 

If theres only 2 wires coming out of the chassis, its definately mono.

You can disconnect the speakers and measure they're resistance to see what impediance they're running at (If the speakers dont already say what ohms they are) I'm guessing you have two 8 ohm speakers connected in parallel to give a total of 4 ohms. IF the two speakers are original and are 16 ohms in parallel, this would give you an 8 ohm total.

 

The combo probibly doent have the impediance split like mine does so you'll need to run any extension cab to match what the internal speakers are with the internal disconnected. You can install a switching jack that disconnects the internal speakers and routes the signal to your extension cab but the cab needs to be the same impediance as what the internal speakers total.

 

In some cases you can run transistor amps at a higher ohmage like 8 ohms if the head normally runs 4. You definately wouldnt want to go lower to two ohms because heat buildup can blow the power section. I only mention this because some transistor amps will run with a higher impediance and it wont damage them. Other amps are narrowly targeted for a specific impediance for maximum fidelity.

 

I mention this because it would be your decision and your risk to try running the amp at a different impediance from what it was designed. Knowing that my head does switch impediance with internal circuitry, my advice would be to Only run this particular head at its designed impediance if you do add an extension jack. If you only have the option of an 8 ohm extension cab, you could probibly add a large ceramic resistor to do the job of lowering the impediance to 4 ohms. Just be sure to have someone competent wiring it in. I think the resistor is about 3" making it 10W. Its pc mount so it should float inside the chassis. Do not heat sink it, The chassis is alluminum and heating it with the resistor can also heat the output transistors up.

 

Thats about it, Good luck.

[gbuzz]

Thanks for all the help! It has 2 speaker wires out of the chasis for each speaker. The speakers are fender designed but doesn't label the impediance...or ohms. But definately 2 different sets of wires to the speakers.

 

So, if I'm getting it right, assuming the 2 speakers having separate wires going to each, would that mean if I bypass the internals, I could run a 4X12 8 ohm cab?

 

I'll definately have someone else do the work, but I have the need to understand things..ya know what I mean? For future reference when I grow a set to do the work myself.

 

Thanks again...you're a godsend!

[WRGKMC]

So, if I'm getting it right, assuming the 2 speakers having separate wires going to each, would that mean if I bypass the internals, I could run a 4X12 8 ohm cab?

 

No having 2 sets just means you' have to open the chassis and trace the circuit to see what you got. The speaker ohms will also need to be checked with an ohm meter. fender made 4 8 and 16 ohm speakers so using a meter on them is the only way to get a ballpark idea of their impediance.

 

I see nothing about the head being stereo in these reviews, so I'm still thinking the amps mono, and there are two 8 ohm speakers wires in parallel for a total of 4 ohms. I'm suspecting they ran 2 sets of wires from the same source, vs running a single set and paralleling the speakers together from eachother. This is common but should be looked at just to be sure. Chances are the wires come from the same place on the cuircuit board.

 

http://reviews.harmony-central.com/reviews/Guitar+Amp/product/Fender/M-80+Chorus/10/1

[Druxar]

 

So, if I'm getting it right, assuming the 2 speakers having separate wires going to each, would that mean if I bypass the internals, I could run a 4X12 8 ohm cab?

 

 

Definitely need the ohm meter to measure the individual speakers to see what they are, and then you can also use the meter as a shortcut to determine if the two sets of wires are meant for speakers in parallel - suppose each pair has one black and one white wire. Take the ohm meter and measure the resistance between the two white. If it's zero ohms or close enough, they are common wires. Double check if the two black are also short circuits with each other. If so, the two pairs of wires are for the same path and the two speakers are ending up in parallel and the resistance you are looking for externally is half of what the speaker impedance is (if you have two 8 ohm speakers and they are in parallel with the two pairs of wires, it's a 4 ohm speaker load you really have).

 

If you don't see a short circuit with the pairs of wires, they really might be stereo and then it's a whole other story like we were originally talking about.

 

So once you figure out what those wires are really meant for and you figure out what each speaker measures and determine what the actual speaker load for the amp is, you can determine what external cabinet will work. It doesn't matter how many speakers are in that ext cab, just its overall impedance when you are plugging into it the way you are (that can be tricky too, you might have one jack for one impedance or another jack as a stereo cab option that gives a different impedance because only half the speakers are used on that jack...) So again all you'd do there is plug in a cable to the cab and on the free end measure the resistance across the two parts of the plug and see what the cabinet impedance reads on the meter.

 

Without that meter we won't know any more than we already came up with. Even if you visually trace the two pairs of wires and find out if they're common or not, you still need to measure the speakers and the target cabinet to be used.

[gbuzz]

Thanks a WHOLE lot! I'm gonna dig in this weekend!

[Druxar]

When using the ohm meter to check for a short circuit (common wire path), assuming there is no continuity beeper mode on the meter if it's going to be a resistance measurement, consider 2 ohms or less to be a short. "Less than 1 ohm" is considered a good short, but with a cheap meter with some error plus resistance in the probes, it might be 1.x

 

If you get something higher, report the findings and judgment calls can be made.

[gbuzz]

Thanx bro!