Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: shooter on September 04, 2016, 02:41:02 pm

Title: Mixing help
Post by: shooter on September 04, 2016, 02:41:02 pm: have 2 ch, CF outs that I want to mix, without pots if possible. Can I simply tie the 2 470ks together, then attach to the following stage? the .02 before the reverb dropping R can be eliminated?
thanks, a brain fart day, so you can slap me now :laugh:
Title: Re: Mixing help
Post by: kagliostro on September 04, 2016, 03:05:28 pm: That is usual for Anode Followers, I think it will be the same also for CF

220-470k are "standard" values for this purpose

Franco
Title: Re: Mixing help
Post by: sluckey on September 04, 2016, 03:48:06 pm: You can also just tie the two cathodes together and eliminate two caps and two resistors.
Title: Re: Mixing help
Post by: kagliostro on September 04, 2016, 04:16:48 pm: Quote
You can also just tie the two cathodes together

:w2: :w2: :w2:

those are two separated channels Steve, is possible to do that ?

Thanks

Franco
Title: Re: Mixing help
Post by: sluckey on September 04, 2016, 05:14:42 pm: I think I've seen that somewhere. Maybe I just dreamed it?

http://www.diyaudio.com/forums/instruments-amps/292730-dc-cathode-follower-mixer.html (http://www.diyaudio.com/forums/instruments-amps/292730-dc-cathode-follower-mixer.html)
Title: Re: Mixing help
Post by: kagliostro on September 05, 2016, 02:16:57 am: OK, Merlin ....... didn't remember at all that (in truth I do not remember a lot of other things besides this :sad2: )

Thanks Steve

Franco
Title: Re: Mixing help
Post by: shooter on September 05, 2016, 10:57:51 am: Quote
Maybe I just dreamed it?
I gotta believe you have at least as many varied circuits, taking up gray matter, that it's hard to tell dreams from reality! :laugh:

I'll puzzle over the link and figure 20.

thanks for the nightmares!
Title: Re: Mixing help
Post by: PRR on September 05, 2016, 10:59:32 am: The cathode mixer has very low maximum output.

Say one side is pulling high and the other side pulls low. The low side can not fight the large conductance of the pull-up side.

Two resistors WORKS. After decades of silly stunts using active devices to mix, essentially all recent (last 40 years) mixers do it in resistors (often with active devices for make-up gain and to keep gain constant as resistors are switched in/out). (A variant with some advantage in large systems is active Current Sources instead of resistors. A pentode plate mixer is not the worst thing in the world, but ends up more active devices than a resistor mixer and one pentode make-up stage. ICs allow much more complexity but may be outside our interest.)

One specific exception for exactly two inputs: the Long Tail Pair has two inputs. Sometimes the other input is used for NFB. If not, it can be a second input.
Title: Re: Mixing help
Post by: PRR on September 05, 2016, 11:29:43 am: My gut said the cathode mixer is in trouble when Vpeak is near B+/Mu, say 3V for our 12AX7 and 300V supply.

Simulator says this is not far off.

As a "worst" case I mixed two identical 3Vpk signals out of phase. The "right" answer is zero (out of phase nulls out). The two 220k resistor mixer (pink trace) does just that. The cathode mixer (yellow) actually half-wave rectifies to give the higher of the two inputs.

This may be of interest in an "octave up" sound effect. (A similar thing with two BJTs has long commercial and clone success.) But do you want a whole amp with octave-up and no defeat switch?
Title: Re: Mixing help
Post by: PRR on September 05, 2016, 11:42:21 am: A second test case with one 3Vpk input and the other at zero also shows gross bentness.

The average of one 3V signal and one zero V signal should be 1.5V.

The resistor mixer does this correctly. The cathode mixer runs flat.
Title: Re: Mixing help
Post by: PRR on September 05, 2016, 11:58:13 am: The octave-up flaw simulates remarkably well (if you want octave+ with nearly no fundamental). I am sure it won't do this well in real life due to non-perfect matching.
Code: [Select]
HARMONIC FREQUENCY FOURIER NORMALIZED NO (HZ) COMPONENT COMPONENT 1 1.000E+03 4.543E-07 1.000E+00 1V (ref) 2 2.000E+03 1.195E+00 2.631E+06 2,600,000V 3 3.000E+03 3.480E-07 7.660E-01 0.7V 4 4.000E+03 1.979E-01 4.356E+05 435,600V 5 5.000E+03 3.474E-07 7.647E-01 0.7V 6 6.000E+03 6.000E-02 1.321E+05 132,100V 7 7.000E+03 2.259E-07 4.972E-01 0.5V TOTAL HARMONIC DISTORTION = 266,964,400 Percent
Code: [Select]
FOURIER COMPONENTS OF TRANSIENT RESPONSE V(T-mix) HARMONIC FREQUENCY FOURIER NORMALIZED NO (HZ) COMPONENT COMPONENT (1KHz nulled fund) -128dB(!!) 1 2E+03(2KHz) 1.195E+00 1.000E+00 (reference 0dB @ 2KHz) 2 4.000E+03 1.979E-01 1.656E-01 -15dB 3 6.000E+03 6.000E-02 5.020E-02 -26dB 4 8.000E+03 1.843E-02 1.542E-02 -36dB 5 1.000E+04 3.249E-03 2.719E-03 -51dB 6 1.200E+04 2.114E-03 1.769E-03 -54dB 7 1.400E+04 3.411E-03 2.854E-03 -51dB TOTAL HARMONIC DISTORTION = 17 PERCENT (on 2KHz main component)
Title: Re: Mixing help
Post by: shooter on September 05, 2016, 03:01:26 pm: Quote
The resistor mixer does this correctly. The cathode mixer runs flat.
Nice, thank you, I get the pic's n comparison, the numbers chart lost me! But.....to be sure, leave the CF in my drawing as is, mix after with the 2 R's shown? also dump the redundant .02 after the mix?

thanks for the time
dave
Title: Re: Mixing help
Post by: shooter on September 08, 2016, 11:26:02 am: Quote
The cathode mixer (yellow) actually half-wave rectifies to give the higher of the two inputs.
Ok, I'm loosing sleep here :laugh:
when you say *higher*, that's signal amplitude? I re-read and seen results hold true over frequency. Guessing if you ran the same sim on *anode out* mix, it would be same/close to resistor mix with 0 180 cancelling?
thanks for the insomnia :laugh:
Title: Re: Mixing help
Post by: PRR on September 08, 2016, 11:28:05 am: The cathode "mixer" only mixes for very small signals.

Larger signals are distorted.
Title: Re: Mixing help
Post by: shooter on September 08, 2016, 08:41:51 pm: Thanks