Active Passive Input

[J Luth]

I need to add a switch or change one of the input jacks for using an active bass on the amp that has been fixed in this thread, http://music-electronics-forum.com/t45926/. Would a switch be the correct way or is there a way to wire one of the inputs for active and one for passive? The inputs are currently wired the way they are on a Fender amp. I suppose it would be better if one input was active and one was passive but they don’t have to be.

One way I found is to use a DPDT switch and resistors but not sure of the value of resistors that should be used, see attached image. Probably need to drop the input 15 to 20dB, just a guess.

If the switch would work, would anyone have an idea of what value resistors to start with?

[The Dude]

I would run a resistor to ground through a SPST switch to lower the level in active mode. Value will depend on taste, input impedance, and what's already there. You can experiment with resistor value. Maybe even use a pot first to test. Dial it in, then measure it to add a fixed resistor. Here's a picture of a Trace Elliot input circuit for example. You could even use a pot and make it variable.




If you just want 2 different inputs, one for active and one for passive, just add a resistor to ground on the active input to lower the level.

[J Luth]

Thanks. I'll give the switch that way a try.

[Enzo]

If you have two input jacks and they are wired like a zillion Fenders, then you ALREADY have active and passive jacks. They are 6db different. The Fenders used 68k to make a 2/1 voltage divider, that Trace schematic above uses 100k to make a 2/1 divider. Same same. Back when they didn't call them active and passive, but that is all it means, The active is the one padded down for the hotter signal.

[J Luth]

Your correct about the input wiring, I mentioned that in the post, I probably should have mentioned that it's not enough of a drop with the active bass I have. Figuring probably -15 to 20 dB might do it.

[Enzo]

The two resistors in the Fender arrangement are either in parallel for high gain or in series, forming a voltage divider for low gain. When both are the same, like 68k, then the voltage division is half. 2/1. if you want an even larger pad, make the resistors different. 47k and 100k would make 3/1 say. But the two jacks and their switching functions work the same, you just change the numbers to suit.

[pdf64]

-20dB seems like overkill; are you sure that input stage overload is the issue?

[J Luth]

Thanks Enzo

[J Luth]

-20dB seems like overkill; are you sure that input stage overload is the issue?

Agree with the -20dB being a bit much for myself but just thinking about it due to occasionally I have to be part of the house band at a couple of Blue's jam's, OH BOY, and there are a few knucklehead's that use an active bass that they like to turn it up toooo loud. Don't particularly care for doing the jam thing but sometimes I get roped into it.
I usually bring in 2 amps and only let them use the one they can't turn up loud before it clips hard, but bringing two amps in at some places is a bit of a pain.

Maybe I'll put in 2, one on one of the input jack's for myself and an additional on a switch to knock it down more and label it KH.