Phase Inverter Balancing

[pompeiisneaks]

If anyone has an amp with a phase inverter balance pot, here's a demo on how you can use the scope to set it.  I did a video on this once before but it was part of my tweedle dee deluxe series, or so I recall, so I figured I do another on just that.

~Phil

[proaudioguy]

Just an FYI, John Baptiste Joseph Fourier is pronounced like the French water Perrier.  4 EE AY.


Now for my real question.  Don’t I want an imbalance for a sweeter sounding distortion?  Seems like for hifi, I would want it perfect but for guitar, I probably want some added harmonics from a mismatch.  What do you think?

[pompeiisneaks]

Sorry I didn't see this until now,  I don't come to this forum as much as I used to, due to being the forum admin/mod of ampgarage now. 

I speak fluent french, and was americanizing the name per how most americans would know/be familiar with on purpose. 

The imbalance IS actually very useful for guitar but you almost have to dial it to your preference at that point.  BUT it's good to balance it perfectly first, know that, and then shoot for the imbalance that gives you the best second order harmonics and tone... this is entirely personal preference at that point.

~Phil

[pdf64]

I wouldn’t adjust the balance using that method.
Firstly, many amps have a NFB loop around the power amp. Signals within a NFB loop are a combo of the desired signal and the error correction signal generated by the NFB. So trying to analyse them is akin to chasing ghosts.
Secondly, what’s the purpose? The balanced signal handling stages are more than just the phase splitter. ie there’s the output valves and the OT. A balance trimmer affects the balance of that whole system.
Hence I monitor the amp’s output signal at the onset of clipping, and tweak the balance trimmer such that the upper and lower sections of the wave are evenly clipped.
If the output valves are perfectly matched and the OT primary is perfectly balanced, then the result will be the same as balancing just the phase splitter outputs (assuming no NFB loop in operation).
But that’s probably an exception, rather than the norm.

[sds1]

I wouldn’t adjust the balance using that method.
Firstly, many amps have a NFB loop around the power amp. Signals within a NFB loop are a combo of the desired signal and the error correction signal generated by the NFB. So trying to analyse them is akin to chasing ghosts.
Secondly, what’s the purpose? The balanced signal handling stages are more than just the phase splitter. ie there’s the output valves and the OT. A balance trimmer affects the balance of that whole system.
Hence I monitor the amp’s output signal at the onset of clipping, and tweak the balance trimmer such that the upper and lower sections of the wave are evenly clipped.
If the output valves are perfectly matched and the OT primary is perfectly balanced, then the result will be the same as balancing just the phase splitter outputs (assuming no NFB loop in operation).
But that’s probably an exception, rather than the norm.

Correct. Dumble's procedure for setting this trimmer involves scoping at the speaker load. The calibration is to balance the entire push/pull amplifier. For more info we can reference the calibration procedure printed on Ampeg SVT schematic.

The information in this video as well as the information that is sticky at Amp Garage is not accurate and should be corrected IMO.