Some Discussion on Amp EQs

[Passinwind]

Here is a new one for the thread. This is a VB-2 EQ, which is a negative feedback type that is unique to Peavey. This is with one setting of the mid shift switch. The frequency seems a little off from the actual amp, but I'm going to blame the tube models in this case. This is also just the EQ, not the actual speaker output curves.

Thanks Bobby. The dashed lines are phase, right?

 

[BbbyBld]

I'd like your take on B-strings question quoted here when you have time...

Not sure if that was directed at me...

I have an 800RB schematic and maybe a 1001RB schemo. The GK mid topology is a peaking EQ. Many companies including Peavey have used nearly identical circuits for years in various bass amps, active bass preamps, and various live sound products such as mixers. The only appreciable difference between a GK EQ and others is of course that the ranges are limited so that the GK tone shaping stays intact.

Nice to see an amp designer from a big company get in on the fun. Thx for sharing, Bobby!

They should have never let me start uploading pictures!

The dashed lines are phase, right?

Yeah. Couldn't remember how to turn off the phase plot. But, kind of a whole other can of worms and is something that I am always trying to learn more about. A waveform can also be called a harmonic series. The phase AND amplitude of various harmonics determine the shape of a signal wave. For example, start with a fundamental frequency and add integer harmonics (2,3.4,5,6,7, etc) halving the amplitude each time. Set all the even harmonics 180 degrees out of phase and all odd harmonics in phase. You'd end up with a sawtooth wave. Harmonics from the bass fall in different areas as you play across the neck, which means that the phase of various harmonics changes depending on what gear you use and how you set the controls, which changes the shape of signal waves. Obviously, hi-fi means controlled phase response across the spectrum but maybe that's not what you want for a musical instrument and maybe it is.

 

[BbbyBld]

Since it was brought up, gain and phase response of a GK 800RB EQ set flat...

Sweeping the low mid control...

Sweeping the high mid control...

 

[Passinwind]

Yeah. Couldn't remember how to turn off the phase plot. But, kind of a whole other can of worms and is something that I am always trying to learn more about.

Assuming you're in LTspice: left-click on the right vertical axis an untick the phase plot box. I can never remember that either. As you say though, the phase plots are often pretty danged important when working at the design level. My first real electronics gig was building synths and doing unnatural things to harmonic series chains was pretty much the game.

 

[hennessybass]

Just revisiting this thread after a discussion about a Bassman 135 eq. Great thread!

 

[markjazzbassist]

links and images aren't appearing, any chance someone can update them? i have a bassman and would love to learn more.

 

[cfsporn]

@dunwichamps

Thank you for the thread! Some of the images you linked earlier are broken. Can you reupload them please?