mixing resistors

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Rockwell666
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mixing resistors

Post by Rockwell666 »

a friend wants me to build a 2 channel amp with no relays. a marshall crunch channel and a Garnet Stinger channel. both channels would have their own inputs and EQ, I have the schematics drawn up but im not sure having the two channels verge thru mixing resistors and into the master volume will work properly with no mute circuitry. That or I just install a DPDT switch to choose channels.
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xtian
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Re: mixing resistors

Post by xtian »

Mixing resistors will work fine, assuming the signals from each channel are in phase. Let's see your schematic!
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sluckey
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Re: mixing resistors

Post by sluckey »

The phase of the signals is not important unless you plan to run a single guitar into both channels (jump the channel inputs).
tubeswell
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Re: mixing resistors

Post by tubeswell »

What the other two already said. Lots of amp designs use 2 channels with mixing resistors. They work fine, are adaptable, and the low-tech solution for blending channels is virtually bullet-proof. If you want to run both channels in parallel, then be aware that the number of inverting signal stages in each channel will affect how the signal blends when the channels are jumpered. Out-of-phase signal blending can be used to create all sorts of interesting tones and sounds as the tone and volume controls are varied. The range of tones from something as simple as jumping both channels in a Fender Super Reverb is a case in point.
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Rockwell666
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Re: mixing resistors

Post by Rockwell666 »

okay so I added the schematics. My apologies for how messy they are written. two parallel channels. I think instead of "mixing" them before master volume I think a channel volume and then a global master would be after the mixer resistors.
He has a Garnet Pro PA we are going to do the conversion on.
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tubeswell
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Re: mixing resistors

Post by tubeswell »

Forgot to mention before that you can also 'mix' separate pre-amp channels by running each one into opposite grids of the LTP (and I see you are contemplating a LTP)
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Rockwell666
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Re: mixing resistors

Post by Rockwell666 »

tubeswell wrote: Tue Sep 26, 2017 8:56 pm Forgot to mention before that you can also 'mix' separate pre-amp channels by running each one into opposite grids of the LTP (and I see you are contemplating a LTP)
Now that is an interesting idea, never heard of that before. it would seem I need to remove the global feedback for this to work.

yes we are keeping the stock LTP.
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jjman
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Re: mixing resistors

Post by jjman »

I recently converted an amp from a pre-PI mixer (pot) setup to injecting into both PI grids. The “channels” (actually a parallel loop situation,) are out of phase. I had a phase flipping tiny transformer on the loop side with the mixer-pot setup, but I wanted more potential post-return loop signal. I also added a “dry”-side volume control on that Dry PI grid. Now both “channels” have more drive to the PI and I can still bring down the dry for more (relative) wetness if needed.

Mixers create a voltage divider for each signal with the power supply at the end of each divider. If the amp is designed for this, the signal strength is considered and gain structures are set for the goals. My goal was pretty rare since parallel loops are rare. Most amps would be fine with normal mixing resistors unless relative phase is important and is 180degrees out.
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tubeswell
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Re: mixing resistors

Post by tubeswell »

Rockwell666 wrote: Tue Sep 26, 2017 9:12 pm
tubeswell wrote: Tue Sep 26, 2017 8:56 pm Forgot to mention before that you can also 'mix' separate pre-amp channels by running each one into opposite grids of the LTP (and I see you are contemplating a LTP)
Now that is an interesting idea, never heard of that before. it would seem I need to remove the global feedback for this to work.
...
The classic Vox AC15s and 30s (from 1960) do this.
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Rockwell666
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Re: mixing resistors

Post by Rockwell666 »

well ill be damned. and the global NFB is intact on the AC30 1960.

Thanks guys! I will keep ya posted as I start digging into this fun project.
tubeswell
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Re: mixing resistors

Post by tubeswell »

Rockwell666 wrote: Wed Sep 27, 2017 3:21 pm ...and the global NFB is intact on the AC30 1960...
There isn't any global NFB on the 1960. That line coming from the OT secondary on the 1960 schematic is the common (ground return) path.
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sluckey
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Re: mixing resistors

Post by sluckey »

and the global NFB is intact on the AC30 1960
There is no NFB on that amp.
Rockwell666
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Re: mixing resistors

Post by Rockwell666 »

DERP!
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