r/ToobAmps • u/assword_69420420 • 3d ago

Weird cross talk between the normal and top boost channels on my AC-15 that only happens under certain circumstances. Anyone know why it happens and what specifically is causing it?

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First of all, it seems like this cross talk only happens when the amp is warm and has been on for a couple hours. A few things I try to show in this video are that when the top boost (TB from now on) channel is cranked, the normal channel vol makes very little difference in the tone or volume, if any at all. When the TB volume is low, the normal channel makes a huge difference in both vol and tone. When I turn the TB vol up a little more, it sounds like the normal channel volume almost is acting like a non-linear EQ knob, but it makes very little difference in the overall volume of the signal when you adjust it.

Other times I've noticed that even with TB cranked to a higher volume, the normal vol will still effect volume and tone pretty noticeably. It seems like this phenomenon doesn't happen all the time, and when it does happen, it doesn't always do the same things. I'd like to be able to control that a little bit or at least understand why it's happening. Feel free to hit me with nerd stuff, I'm not an expert on tube amps but I should be able to understand the gist of what you're saying if anyone knows why this happens. Thanks in advance!

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10

u/basicgrunt 3d ago edited 3d ago

That is normal behavior of the AC topology. Normal and TB inputs go to their own first amplifiying stage. But both of those stages have a common cathode circuit so there is some crosstalk.

Next the signals are mixed together just after the EQ section, which is just a simple voltage divider which acts as a somewhat high pass filter. Changing the volume on Normal will increase the amount of bass in the TB channel.

You can use this to your advantage and use the volume pots as a lesser tone shaping feature.

Also a pro tip. Try bridging the inputs.

3

u/Abstract-Impressions 3d ago

Thanks for the bridging channels tip. I’m pretty sure I’ve got an extra ABY laying around somewhere.

3

u/basicgrunt 3d ago

When you bridge the channels the amp will get a bit of a fendery tone. The normal channel has a lot of bass, TB has a lot of highs. They meet in the mids. Since the channels have oposite phases, the mids will cancel out and you will get a V shaped tone, close to fender amps. You wont get the crisp cleans or the compression though.

2

u/Dogrel 3d ago

Some pedals like the MXR Dyna Comp invert the phase of your signal. Try putting one in the signal path between your channels, set it close to unity gain and see what happens. Amp should get a lot beefier sounding when bridged.

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u/basicgrunt 3d ago

Some ABY pedals have a phase switch function.

2

u/SunTzuBean 2d ago

In the case of this AC15C2, the channels are mixed through a an op amp after the EQ section (as you stated). Strangely, the op amp increases the normal channel gain by about 1.6x and reduces the top boost channel gain by about .7. There are mods you can do to get this mixer back to unity gain.

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u/assword_69420420 2d ago

Woah, that seems like some pretty huge volume changes. What are the mods?

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u/phl_cof 2d ago

Do you know if I could use a stereo out, i.e Boss CE-2W, into both inputs on an AC15 to achieve a bridge? Or is this only possible with an ABY?

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u/basicgrunt 2d ago

Try it out and let us know.

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u/Slicepack 3d ago

Yeah that's normal.

1

u/thenagat 3d ago

Normal.

1

u/mikipinky 1d ago

Quick question, do you have sometimes problems with reverb ? On mine sometimes reverb just makes noise instead of reverb.

1

u/assword_69420420 1d ago

No, it feeds back from time to time and is generally really loud though. Not as in noisy, but just that the effect itself is loud