Lineout/Load schematic help bitte!

[mth5044]

I worked up this schematic massed from a few different schematics around the web. What do ya think?



My main concern is the ground connection. IIRC, speaker jacks (and therefor speakers) aren't connected to the common ground, rather, a + and - connection? Would tying all the grounds together in the circuit cause some bad business?

Should there be another resistor going to ground off of lug 1 of the volume pot?

What ratings should I be aware of? Wattage for the switches, pots, etc?

I'd really like make it for 100w, but with the cost of a 50w resistor being 25% of the price of the 100w, this seems logical.

Thanks guys.

[CynicalMan]

The ground would just be the common connection for that circuit, it shouldn't be connected to power ground or any other ground. So it can just connect to the speaker negative output from the amp.

I'd leave off position A4 in SW1A. It's too easy to set the switches so that there's no load, which could damage your OT. Do you want a no load setting at all? I can't think of any situations off the top of my head where it would be useful.

If you want the minimum output at line out to pass some signal, put a resistor on pin 1 of POT1. If you want the minimum output to be silent, leave it as it is.

[mth5044]

Thanks! Good point about the common ground.. I totally forgot there was no power going to this 'effect' 

There will only be three choices in the rotary switch, that's just the eagle schematic symbol I had in my library. However, I figured if there was going to be some settings on the dial that weren't going to be used, I could just tie them onto one of the loads for safety.

Thank you for your help

[PRR]

You show a rotary switch carrying speaker power. The $2 switches will wilt. There are power switches but they are expensive.

Is this supposed to be idiot-proof (no such thing), or can the user use his brain?

Do you have a BIG tube-amp, or small-tube and transistor?

Two toggle switches can select 16@50W, 8@100W, and 4@200W using only four 16r 50W resistors. If you check transistor-amp specs, they usually give much less power in 16 than in 4.



Your line out-- no pot value specified. Since Speaker is 4V-30V and Line is near 1V (and someday the line will get pluged while the pot is full-up) you need considerable attenuation. If the pot is 3.3K, that's good, but an odd value. 5K is not bad.

The maximum safe voltage on the dummy-loads is like 100W 8 ohms or 28 volts. We could allow for the 10K:5K division in the Line Out except some fool might short the jack. Then full 28V is applied to 10K resistor. 0.08 Watts; I'd use a 1/2W part for cheap mechanical robustness so heat is not a problem. Un-shorted the pot could get nearly 10V, in 5K is 0.02 Watts, 1/4 or 1/8W are standard ratings so heat is not a problem.

100 feet 30 meters of cable is 1,000pFd. Worst-case line-out resistance is 10K||5K or 3K. 3K driving 1,000pFd or 47KHz roll-off. So cable up near 1,000 feet is no real treble droop for guitar.

[mth5044]

Thanks PRR, cool trick with the resistors. User has brain capability and, unlike most circuits built here, this will most likely not be tweeked. No real reason the change ohm settings on the fly, more necessary when a different amp is going to used.

Tube amps used on it currently would be no more than 50W tube. Good at the toggle settings you posted.

I was going to go with a 10k pot, but I'll try 5k like you say.

Thanks!

[mth5044]

Does anyone have any recommendations for a heat sink for these resistors?