Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: shooter on October 23, 2015, 07:08:20 pm

Title: mixing ?
Post by: shooter on October 23, 2015, 07:08:20 pm

I need some help with mixing, I drew in a pot, but I’m guessing something is “wrong/missing”.  I can *cheat* if I only use the 1/4” by putting a shorting rca plug, but if I want to use the AUX side, it seems like the pot would be floating and have no effect.

Thanks

Title: Re: mixing ?
Post by: sluckey on October 23, 2015, 07:17:50 pm

This should help...

http://el34world.com/charts/Schematics/files/bogen/bogen_cha-20.pdf (http://el34world.com/charts/Schematics/files/bogen/bogen_cha-20.pdf)

Title: Re: mixing ?
Post by: shooter on October 23, 2015, 08:26:40 pm

 :think1: :think1: :think1:
I have that schematic...BUT....I only looked at the  TS!!!!!
thanks steve

Title: Re: mixing ?
Post by: shooter on October 23, 2015, 08:48:59 pm

do the 2 470k Rs form a 50% voltage divider for the in-coming signal?  I can see the 470k to ground as a ref. for the grid?

thanks

Title: Re: mixing ?
Post by: HotBluePlates on October 24, 2015, 07:10:43 am

Quote from: shooter on October 23, 2015, 08:48:59 pm

do the 2 470k Rs form a 50% voltage divider for the in-coming signal?

Yes, but the 470kΩ from the phono path and the 220kΩ from the mic path work against the 470kΩ to ground to mix the 2 channels. So the mic path signal is reduced less than the phono path signal.

There are other ways (a feedback/virtual-earth mixer) to mix signals with control over the relative strength of the mixed signals and without throwing away the existing signal strength. However, you'd have to add an active device (tube/BJT/FET) to the circuit to be used just for the mixer.

Title: Re: mixing ?
Post by: shooter on October 24, 2015, 09:47:32 am

thanks HBP, I should have some *throw away* signal since it will be driven by mostly line-level, off-amp volume controlled type signal. 

Title: Re: mixing ?
Post by: PRR on October 25, 2015, 01:20:22 pm

What you drew is a cross-fader. It mixes, but best used between two *similar* sources, mostly one or the other but with smooth transition.

And if the Line-In L&R mix resistors are near as big as the pot, the pot won't cut well.

There's no gain control for the guitar for several stages. It is sure to overload in stage 2, 3, and 4, before you get to the only gain control.

Your line inputs are likely to be much higher level than guitar, and also less range of strength.

Bogen knew their stuff. Mixing background music with announcements WAS(is) their paying gig. Steal their plan.

As guitar is similar but a bit hotter than MIC, you may not need the 470K+470K divider, use 220K resistor.

Mixing L&R on all modern sources may be two 10K resistors.

Title: Re: mixing ?
Post by: shooter on October 25, 2015, 04:00:32 pm

Quote

Steal their plan

Thanks PRR, it's re-drawn, just not re-posted, and I did leave off 1 470k. 

Quote

Mixing background music with announcements

Sorta my *concept* here, to be able to jack in music while playing the guitar if you wanted to.