Slave line out

[mac dillard]

Would someone please explain the theory behind the resistor selection of the slave line out circuit. I know that it is a simple voltage divider circuit but what sets the value of R1 (first resistor) and R2 (second resistor). Looking at old schematics I see values all over the place. From R1=2K2, R2=270 of the Fender Deluxe reverb to R1=220K and R2=1K. Have seen some recommendations of 54K for R1 and 1K for R2. The math says that you can get the 10:1 voltage ratio with almost any combination..What am I missing. Thanks

[MWJB]

It's not the voltage ratio that is critical, it's the voltage out of the voltage divider.

Line level voltage is 0.775VAC into 600 ohms...there are 2 ways to look at this...

1) use a dropping resistor between 1/50 & 1/40 of the following volume control (assume 100K), hence we see the 2.2K (R1) dropper in the Fender circuits. Load resistor (R2) is then selected to give the desired ratio).

2) Stick with 600ohms (560 or 680 in reality) as the load (R2) and select the appropriate value of R1 to give the correct ratio.

0.775VAC might be seen as the ideal, but in reality could be anywhere from 0.5VAC to 1.5VAC

So for a few examples...

5W amp * 4ohm speaker load = 20.
square root of 20 is 4.47VAC at the OT secondary
4.47/0.775 = 5.76:1
R2/(R1+R2) = 470/2670
or a drop ratio of 5.7:1, or 0.18 of the voltage fed into the voltage divider
Therefore 4.47 * 0.18 = 0.8046VAC...close enough to 0.775VAC
If the smallest resistor you had handy was 4.7K, then you could use that for R2 & 22K for R1, you might just want to be careful about plugging into a slave with a low-ish input load in case it drags down the signal level. Conversely if you had a 100ohm resistor you could use that for R2 and use a 5.6K for R1.

Want to use 600ohms as your load?
5.76*600= 3456
Nearest common values might be 3300 for R1, 680ohms for R2.

50W amp * 16ohm speaker load = 800
Square root of 800 = 28.28
28.28/0.775 = 36.5:1
R2/(R1+R2) = 60ohms/(2200+60)
Drop ratio of 37.66:1 (near enough) so you could use either a 68ohm or a 56ohm for R2 giving 0.848VAC or 0.702VAC respectively.

Want to use 600ohms for your load?
600ohms * 36.5 = 21900
Nearest common values for R1 are 22K or 27K, with a 680ohm at R2 giving 0.848VAC or 0.7VAC respectively.

Make up a little XL formula then plug in your values for quick results. Common resistor values will usually decide the precise ratio, rather than the other way around (e.g. you're not going to bother stringing resistors together to exactly 60ohms in the above example).

A lot of mixers & PAs have lo-Z 1/4 jacks, rather than hi-Z, these days so a DI box is also handy.

[mac dillard]

Thanks for the reply MWJB. I'm going to try to digest all of that. I have been playing with a little circuit that I built that has R1=68K and R2 a 10KA pot. It works using two 5 watt amps that I have. (one for master and one for slave). Am getting ready to use a 40 watt amp as the slave..All of a sudden it occurred to me that I really did not understand exactly what I was doing. Know it works but did not quiet understand the math behind it.( Actually it is a little more complicated than that. I am also using a 1:1 matching transformer ala Jensen to break any ground loops.) Works pretty good. Just having a hard time selecting the line matching transformer. Seems it is either a $40 really good transformer that will cover 20hz to 20Khz, 600 ohm or a cheap $2 telephone system transformer that covers 300hz to 4Khz, 600 ohm.

[MWJB]

I'd be inclined to put the pot on R1, say a 25KL pot (wired as a variable resistor) in series with a 2.2K fixed resistor. Leave R2 as 470ohms to 680ohms and this should cover most eventualities. You will have ratios from 5.7:1 (2.2K & 470) to 59:1 (27.2K & 470) ...if you have too much series resistance at R1 it'll take the edge off fidelity, so just back off until you get the clarity you want.

[Enzo]

Just remember that power to your speaker is a function of speaker impedance and the signal voltage coming out of the amp. So 40 watts will represent a larger voltage than 5 watts across the same impedance speaker.

In making the line out, we are working with that voltage, the speaker impedance doesn't much matter to the line out.

The line out voltage should remain in the same range, regardless of the amp output power. SO moving from a 5w amp to a 40 watt amp means we have a higher signal voltage to knock down, but the same output signal level requirement. MWJB said that in different words.

So considering the output signal from the speaker, you will need to determine how much voltage division will be required to provide line level. That explains why you see different ratios.

Since we are slapping a relatively high resistance across the very low resistance of the speaker, the actual value is not all that critical. In other words, a voltage divider comprising a 10k and a 2k won't be any different from a 20k and a 4k, as far as the circuit is concerned.

[Jazz P Bass]

Please keep in mind you still need the speaker hooked up.
On a tube amp anyway.

[mac dillard]

Thanks for the input guys..Another thing that I might add is that the output level in this set up is also dependent on the vol. level of the master amp. I originally started out to do a pre amp line out mod and talked myself into doing the slave line out just to be able to use the power section of the amp. Most of the amps that I fool with are either 5 watt, 15 watt or 40 watt. looks like the set up that I have will cover all of these. The 5 watt'ers I generally us the FireZog line out with a load resistor and take the speaker out of the system. This mod leaves the speaker in and allows me to just clip on to the speaker terminals of the amp that is being used as the master. I also have a 0.1uF cap inline and a .022uF cap around the 10K pot. This is a take off on the "Slave output Project" by Joe Varela that was published in Guitar Player, November 1983.Should be able to do some recording with it later this week. Anxious to see how it is going to sound. I suspect that I will have to up grade the 1:1 matching transformer.

[d95err]

There are two things that are affected by the total value (i.e R1+R2 ohms) of the voltage divider: output impedance and heat. The lower the combined value of the resistors, the lower output impedance you'll get. Lower output impedance means the signal can drive longer cables without treble loss and is less sensitive to outside interference (noise).

The signal current through the voltage divider will dissipate heat, so the lower you go, the more heat will be generated. The lower the values, the higher power ("Wattage") the resistors need to be.

[MWJB]

Simplified formula for finding R2 (load) value when using 2.2K as R1 (dropper)...

Square root of (Watts RMS * Speaker impedance) gives you VAC at the OT secondary.
(VAC @ OT secondary -0.775VAC)/0.775VAC gives you the voltage drop ratio
So R1/drop ratio gives you the value for R2.

E.g. ...

Sq. root of (50W*8ohms) = 20VAC
20VAC - 0.775VAC = 19.225VAC
19.225VAC/0.775VAC = 24.8
2200ohms/24.8 = 88.7ohms
Therefore R2 = 88.7ohms ...nearest common value is 91ohms.