1. ## adding line out to a speaker output

I want to add a line out to a 100 watt SS modeling amp with an 8Ω speaker. The following is from another forum, Enzo in fact.

"All in all, the impedance shouldnlt much matter to a line out. A line out from a speaker line is just a voltage divider. The line out will not draw any current, so higher resistances will be invisible to the amp. The ratio of resistors is what determines the voltage division, not the resistors themselves. IN other words, If I put two 10k resistors in series then across the speaker output, the junction of resistors would be at half the signal level of the speaker. If I did the same thing with 22k resistors, the junction point would STILL be half the speaker voltage. SO if you see something like a 10k resistor from the speaker hot joining a 1k resistor to ground, at their junction there would be about 1/10 the speaker signal at the junction. Feed that junction out as a line signal.

Let's make up an example: 50 watt solid state amp into an 8 ohm speaker, full output. SOlving for voltage, we find 20v of signal across 8 ohms is 50 watts. SO we have 20 volots at the speaker, and you want to drop that down to about a volt - line level. SO you;d need a 20/1 voltage divider. COmes to mind easily a 22k and a 1k resistor. Put them in seriess, then 22k free end to speaker, and 1k free end to ground. 1v signal then is across the 1k resistor."

The old fella who plays the amp will never get near 100 watts, so why can't I use the above values for a 50 watt amp to make sure I get a good signal level at lower volumes? If someday someone does drive it harder, they could just trim down the input on whatever board or whatever they are feeding, right? The owner said he doesn't care about a level control, and would rather not pay extra for it.

My other question is about polarity. Is the junction between the two resistors in phase with the speaker, and why?

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2. Originally Posted by Randall
The old fella who plays the amp will never get near 100 watts, so why can't I use the above values for a 50 watt amp to make sure I get a good signal level at lower volumes? If someday someone does drive it harder, they could just trim down the input on whatever board or whatever they are feeding, right? The owner said he doesn't care about a level control, and would rather not pay extra for it.

My other question is about polarity. Is the junction between the two resistors in phase with the speaker, and why?
27k, 1k would give a 1V line out from a 100W amp driving 8 ohms but I don't see why you can't use 22k, 1k for a little more output.

Yes, the junction of the two resistors is in phase with the speaker. It's a resistive divider so there's no capacitance or inductance to cause phase shift.

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3. Thanks Dave, now one more question, my math tells me 1 watt resistors would be plenty, but can someone confirm?

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4. Originally Posted by Randall
Thanks Dave, now one more question, my math tells me 1 watt resistors would be plenty, but can someone confirm?
Consider that the load from the line out voltage divider will be at 23,000 ohms. If you divide that by 8 ohms you get 2875. So 1:2875th of the wattage will be across those resistors. If the guy is cranking the amp and there are peaks to 150W (hypothetical) then 150/2875 = .052 watts. You could use 1/8W resistors for this circuit. I generally just use the 1/2W resistors I have on hand. And, FWIW... The impedance of this circuit DOES matter because the user will be running cables to whatever it's fed into. Since there is truly a lot of leeway here in the impedance margin I would suggest going to a 470 ohm dropping resistor and a 15k series resistor for the divider. The lower impedance will lose less top end when longer cables are used. Not that a 1k would be that bad, but why not give the circuit the extra edge if you can? It's not a big concession and I'll bet you have 470 ohm and 15k resistors on hand.

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5. Chuck, I am down with lowering the impedance, but wouldn't 15K/470 be taking it from 22:1 to 32:1? Wouldn't that drop signal as well? What about 10K/470, that would be closer, no?

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6. Originally Posted by Randall
Chuck, I am down with lowering the impedance, but wouldn't 15K/470 be taking it from 22:1 to 32:1? Wouldn't that drop signal as well? What about 10K/470, that would be closer, no?
You guys! Splittin' hares again I see.

Jeepers creepers it's just a line out. As Enzo reminds us, in a guitar amp! Pick a combination & go. In the time you take typing you could wire up 50 of 'em.

Whilst we're splitting, with these values you can use half watt resistors. Split that!

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7. I know, but it is to get my head around the theory to make sure I get it. So maybe splitting hairs, but is my question correct?

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9. Originally Posted by Randall
I know, but it is to get my head around the theory to make sure I get it. So maybe splitting hairs, but is my question correct?
Yes. Some of us like to try a few hypotheticals in order to make sure we know WHY it works and can we work it out outselves in the future with different parts, and not just want to know IF it will work. So to everyone else, forgive us for the "check my math!" posts!

Thanky Thanky!

Jusrin

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10. +100!

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11. re: the 15k or 10k series resistor value with a 470 ohm dropper,..

With the 22k/1k values being discussed there was already a concession being made toward the low ratio side. For a 100W amp the correct ratio would be about 28/1. 10k/470, at 21/1 would be another tad lower in ratio where the 15k/470 at 33/1 is actually closer to the ideal ratio. YMMV because as you said, your guy is not likely to ever push the amp to it's full output. I think 10k would be fine and make for a better noise floor in use when the amp isn't making many watts.

EDIT: For that matter, consider that the EQ of the line out is not affected by the frequency limitations of a typical guitar speaker. Since a guitar speaker will roll off considerably at either end of the spectrum you can make things more manageable for whatever is being used as an interface with the line out by running the circuit through a 470n capacitor and paralleling a 33n cap across the dropper. This would approximate the sound from the speaker better for re-amp at a board or processing in a recording unit. The typical peaks and dips in a guitar speaker response can only be achieved with inductors and that's more complicated than anyone wants to get. I'm sure.

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12. Well, I wired it up today, and the cold slap of reality hit me in the face. It doesn't work. It seems in this amp the speaker neg is not grounded, so the sound goes really low if the line out jack is touching the chassis, even with nothing plugged into it. Also, with the jack isolated the sound goes very weak if a cable is connected to a mixer. Clearly this is a problem. I thought this would be pretty straight ahead, but I'm stumped.

http://www.fmicassets.com/Damroot/Or..._schematic.pdf

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13. The Mustang III phones output is designed so it can be used as a stereo line out. If you don't want the speaker muted, use the stereo FX send jacks instead.

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14. "If you don't want the speaker muted, use the stereo FX send jacks instead. "

I have come to the same conclusion. Man, that was a long way to get there, a hole added to the back, and no charge. I'm going fishing.

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15. https://drtube.com/schematics/marshall/5213psu.gif

It works convincingly the best.
Insulated jack (JS3) and R27 (100 ohm) to avoid ground loops (ground loop buzzing)
Instead of VR1, it can have a trimmer pot or a fixed resistor.

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16. Originally Posted by vintagekiki
https://drtube.com/schematics/marshall/5213psu.gif

It works convincingly the best.
Insulated jack (JS3) and R27 (100 ohm) to avoid ground loops (ground loop buzzing)
Instead of VR1, it can have a trimmer pot or a fixed resistor.
And as you can see, the line out is wired with the standard series resistance and a dropper to 0V. The circuits already suggested will work like this. If the negative speaker terminal isn't at 0V then it can't be used as the 0V reference for a line out. That's why it didn't work.

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17. Originally Posted by Chuck H
If the negative speaker terminal isn't at 0V then it can't be used as the 0V reference for a line out. That's why it didn't work.
Yes. The problem with the Mustang amps is they used a bridged configuration. They also have a ground wire to the speaker frame so you run into big trouble if you ever replace the speaker with a type that has the negative lug connected to the frame.

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18. Originally Posted by Randall
Well, I wired it up today, and the cold slap of reality hit me in the face. It doesn't work. It seems in this amp the speaker neg is not grounded, so the sound goes really low if the line out jack is touching the chassis, even with nothing plugged into it. Also, with the jack isolated the sound goes very weak if a cable is connected to a mixer. Clearly this is a problem. I thought this would be pretty straight ahead, but I'm stumped.
Transformer. Mouser has fingertip size transformers that will sort this out, Xicon brand. Get a 600:600, use the primary instead of your smaller resistor. Secondary is "floating" iow doesn't care whether one lead is grounded or not. So you send the output to another unbalanced input, or send balanced to the input of a mixer or mic preamp.

https://www.mouser.com/ProductDetail...%2FZ8KUebnk%3D

2 bucks, ya can't go wrong! Don't worry much about the 300-3K bandwidth, that's at the power extreme. With a 10K-30K padding resistor, there's plenty of bandwidth. If that's still a worry, there are better transformers that cost only @ \$15.

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19. With a SS power amp I wouldn't expect any benefit from taking the line out signal at the speaker out.

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20. Originally Posted by Helmholtz
With a SS power amp I wouldn't expect any benefit from taking the line out signal at the speaker out.
Yep, that too.

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21. If you can get to it, may have to be a member, take a look at Jensen AS045 or AS061. It is Jensen's design for guitar line out. Weber use to sell a xformer to do this. Don't know if they still do. Also you may want to do a search this forum for Slave line out, started by mac dillard. I have found that the transformer is critical.

Doesn't appear that Weber sells this little xformer(WBALUN) any more...The spec. that I have says frequency responce +/- 2db 80hz to 20 khz,,,DC resistance 150 ohms Pri and Sec, Max Idc 80 ma., typical AC 1V RMS.

I don't know how to post a schematic but they come off of the hot on the speaker jack to a 5K to 100K (select value for best range of output signal level ie: your voltage divider) to a 1KA pot. Wiper of pot is connected to the primary of the xformer...Secondary of the transformer to out put jack/XLR connector. You can put a low value cap in parallel with the 5K/100k resistor to peak the high end.

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22. "With a SS power amp I wouldn't expect any benefit from taking the line out signal at the speaker out."

Why not?

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23. Originally Posted by Randall
"With a SS power amp I wouldn't expect any benefit from taking the line out signal at the speaker out."

Why not?
There's not much interaction between the power amp & speaker the way tube amps do. SS amps more or less simply make the input larger and that's that. What you put into the input is what you get at the output, only bigger.

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24. Originally Posted by Helmholtz
With a SS power amp I wouldn't expect any benefit from taking the line out signal at the speaker out.
Line out signal at the speaker out, uses instead microphone to connect amp to the sound system (PA system)
For the sound system, it doesn't matter, whether it's an SS or a tube amp.

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25. Originally Posted by Randall
"With a SS power amp I wouldn't expect any benefit from taking the line out signal at the speaker out."

Why not?
SS power amps (especially switched mode) typically don't have any (desirable) sound shaping properties.
If the power amp doesn't add anything but power, it makes no sense to use the power output for line out.

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26. "SS power amps (especially switched mode) typically don't have any (desirable) sound shaping properties.
If the power amp doesn't add anything but power, it makes no sense to use the power output for line out."

I would have to respectfully disagree here that it does in fact make plenty of sense to pick the signal off the power output if only as purely a matter of convenience dealing with a SMT board. It's a modeling amp, all the "sound shaping properties" are done on a computer. Who plays a shitbox amp like this and worries about if their line out to the mixer for sound reinforcement at a dinner club is having slight plus or minuses to their tone due to the signal being taken before or after their SS power amp? Certainly not me.

The amp is out the door, with the customer advised the simple solution to use the FX send as his line out, which works pre MAster Vol. I should have suggested this sooner, if I hadn't been too focused on what I was asked to do, add a line out.

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27. Indeed! Short of hacking into a crowded and complicated build topology it seems that the output is at least a convenient place to tap in. Plus there's the convenience of the outputs low impedance making the design of the line out a simple matter.

Randall, if the customer decides they REALLY want a line out after the master volume, or if it ever comes up again in the future, just take the dropping resistor of the circuit already discussed to 0V rather than the negative speaker terminal and it'll work fine.

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28. DOH!!!

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29. I would have to respectfully disagree here that it does in fact make plenty of sense to pick the signal off the power output if only as purely a matter of convenience dealing with a SMT board. It's a modeling amp, all the "sound shaping properties" are done on a computer. Who plays a shitbox amp like this and worries about if their line out to the mixer for sound reinforcement at a dinner club is having slight plus or minuses to their tone due to the signal being taken before or after their SS power amp? Certainly not me.
Well you asked why and I gave you my reasoning.
Didn't say it's wrong or bad, just don't see any benefit over using the FX send or phones output.
The output impedance of op-amps is generally very low as they use heavy NFB, maybe lower than the source impedance of a voltage divider at the speaker out.

You could have avoided some trouble and discussion by providing a schematic from the start.

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30. Originally Posted by Randall
DOH!!!
- - - is what I said after spending time offering solutions...

Oh well, on to better things! Gotta sort out my socks. They are in disarray, more than usual.

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31. Originally Posted by Leo_Gnardo
Oh well, on to better things! Gotta sort out my socks. They are in disarray, more than usual.
If it helps, I put one on each foot. works every time.

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32. Originally Posted by Chuck H
If it helps, I put one on each foot. works every time.
I notice, amongst the hipsters, they don't put on sox. I s'pose because urban legend has it, Einstein often forgot to put his on. Therefore "look at me, I'm such a genius!"

To counter this trend I put two on each foot. But only in cold weather. Now I'm such a geezer who cares if it's stripes on one foot & polka dots on the other. Gotta amuse people somehow...

OK we're into extra innings now. Game over. Send the teams to the bar, it's time for a cool refreshing adult beverage. Socks optional!

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33. "You could have avoided some trouble and discussion by providing a schematic from the start."

Hindsight being what it is, yes.

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34. Originally Posted by Leo_Gnardo
Oh well, on to better things! Gotta sort out my socks. They are in disarray, more than usual.
I settled on a simple solution. I threw away all of my old socks and replaced them with a big pile of identical new ones. No more sorting. When they start to wear out, I replace the entire pile with new.

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35. ## Nice and Easy !

Never realized how easy this was ! I always thought you had to go with a much more elaborate circuit, perhaps including a few caps, resistors, and some type of impedence matching transformer.

Good to see it's just a simple voltage divider !

I will pay heed to the appropriate ohmages and add some caps to simulate speaker response as Chuck H suggested.

Good Stuff !

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