Adding a slave out..

[jussi]

Hey,

I built an amp a few years ago so these things are not in very fresh memory. I want to add a slave out from the output of the amp (like in Orange AD200) but I don't know how to decide the resistor values for the slave out circuit. I know R1 is limiting how much it's "slaving" and R2 limits the output. Any help?

[Chuck H]

For a slave out you probably want a signal capable of both "line" and "low" level signals. Rather than use a switch I prefer a pot because the amps output varies with it's setting and you may need the flexibility.

This slave output is a voltage dependent circuit running from a current source, basically. Technically the circuit is a "voltage divider". So the circuit isn't a division of power, it's a division of the voltage that happens to be present in the creation of power. There won't be any significant power consumption from the circuit because of the relatively high values of the circuit resistances and the relatively high input impedance of any device plugged into it.

Assuming the amp is making about fifteen watts at full tilt, the signal from the 16 ohm tap will have about 15.5V on it (NOTE: if you used a different impedance tap the calculations below would also be different). Line level is about a volt. So we need a voltage division of about 15:1 to operate our slave amp. Much less if the slave amp requires a low level signal, but we'll get to that.

R1 is the series resistance and R2 is the load. The division represented by the resistors will result in an equal division of voltage.

You'll want about 600 ohms or less for the load. Why? Because it's a fairly common output impedance and most inputs designed to work with it. I mentioned using a pot rather than a resistor (you'll see why). So R2 will be a 500 ohm pot. Now we can calculate our voltage division. 15 x 500 is 7500, so 7500 ohms could be the resistance for R1. But you should go lower than that. Why? Because you may not always have the amp at full output when you need to get 1V at the output of this circuit. So let's just make it 4700 ohms.

Since you're not passing any significant power through the circuit you won't need high watt resistors. So R1 could be a 4.7k 1/2W resistor and R2 could be a 500 ohm pot. You can wire the pot as a variable resistor or a second voltage divider, like a volume control is wired. It's fine either way.

With this arrangement you can get line level from the amp at most useful volumes. You can also get too much output when the amp is cranked up, but not a dangerous amount and you'll know if it's too much because it'll sound bad. Because the pot is fully variable you can also adjust down to very low signals for inputs and devices designed for "low level" input signals. Easy peezy.

Probably more information than you wanted but I always think it's better to understand a problem rather than just know the answer rote. Now you can design this circuit for different amps and for use from different impedance taps.

[Chuck H]

Here's a useful chart that shows voltages for common amplifier power and impedances: