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A Line Out jack on an amplifier provides a versatile way to send the amp's output to a recording console, a PA system, or another amplifier. Some amps put the Line Out between the pre-amp stage and the power section, but the magic tone of some amps come from the phase inverter, power tubes, or output transformer, and such a Line Out bypasses those. Plus, adding a Line Out in the pre-amp often requires a dozen or more components, which may possibly include another triode.
The simple way to do add a Line Out is with two resistors and a output jack:
R1 and R2 need to be high enough values to not change the overall load on the speaker. R1 and R2 in serial connection form a voltage divider at their junction (the line-out jack), thus attenuating the output signal down to a voltage that is acceptable to use as an input to another amp, a recording console, or a PA.
Choose the values of R1 and R2 such that the sum total is 2K or higher, and that the voltage divider attenuates the signal down to a voltage that is acceptable to send to a mic input, or another amp. A 5W amp's speaker voltage is much lower than what you'd have with a 100W amp. If the desired Line Out voltage is 1V for either 5W or 100W amps, you'll need different voltage dividers for either one. If you aren't sure, to can always make it variable (see below).
The output impedance of the Line Out will more or less be set by the value of R2. 100ohm is a good versatile lower bounds of impedance, but up to 300ohm should be okay for recording devices or PA's.
A Variable output is an simple as making R2 be a Potentiometer, with the center terminal being the Line Out signal (below). R1 + P1 values should still be 2K or greater. You might decide what your lowest attenuation might be, and put a lower bounds resistor under the pot, such that at the lowest setting, the voltage divider ratio isn't 100:0.
If this looks like voodoo, or somehow would tamper with the speaker, consider that any Negative Feedback (NFB) loop or presence control of an amplifier is this exact same circuit, except, instead of an output jack, it is connected to the cathode of a pre-amp stage. Below is a common power section of an amplifier with a NFB circuit. To help illustrate, all but the NFB and speaker circuits are greyed-out. The resistor R11 serves two purposes: it is the cathode resistor for the triode, and it is the "R2" resistor in the R1/R2 voltage divider pair of the NFB circuit.
Below, I've moved the voltage divider resistor pair to the other side of the schematic to show how it is exactly the same as the Line Out circuit at the top of the page.
It is most certainly the case that adding a Line Out, or an NFB circuit has some effect on the load of the output transformer, but if the overall resistance of R1+R2 is more than 2K, the effect is so tiny, its not really worth doing all the math. "load impact = insignificant".
Still not satisfied?
The Line Out R1+R2 pair is a impedance device parallel to the speaker. for resistors, DC resistance = AC impedance. So the impedance of the Line Out pair is R1+R2. Simple parallel impedance math is the same as parallel resistance:
Z= (Z1 * Z2) / (Z1 + Z2 )
Give it some real world values, like the ones in the example amplifier NFB circuit above:
Z2=22K + 5.6K
Z=( 16 * 22000 ) / 22016
So adding the NFB changed the impedance that the transformer sees from 16 to 15.98. The same calculation for a Line Out R1+R2 pair of 2K changes that impedance to 15.87.
The resistors at left are way over-sized, from a power handling perspective.
If your speaker is rated at 25 watts, the Line Out resistors can be selected with power rating to match that. Use Ohm's law to calculate a suitable wattage rating:
I = current (amps)
P = power (watts)
R = resistance
V = voltage
I barely remember enough high school algebra to take V=I×R and P=V×I and make this leap:
So if you plug some numbers into that forumla, like 25W (P) and 16ohm (R) for the speaker, you'll come up with 20 volts (V).
Plug that 20v and the Line Out 2K resistance into P=V×V÷R and you've got 0.2watts. This shows that ¼watt resistors are minimum. Plug in your numbers, but for a 100W amp, Line Out resistors rated at ½watt should be fine.
The following images show a small pedal size universal Line Out device, that allows you to tack on the Line Out resistors to any speaker connection that uses a 1/4" mono plug. Although physical device is placed in between the amp and the speaker, we know that the circuit "hangs off the side" and places the speaker and the the Line Out in parallel, not in serial. The pot provides a variable attenuation, with 2.2K:150ohm being the minimum voltage divider, and 27K:150ohm being the maximum voltage divider.
Vintage 47 has a nice example of this type of box that you can purchase.
If you have questions, or you've found an error contact me