[sdiy] MN3007 Chorus Acting Weird

[Richie Burnett]

A sinewave mixed with a time-delayed (phase-shifted) sinewave just results 
in another sinewave with different amplitude and phase.  So you mostly just 
perceive a variation in the intensity (volume) as you get 
constructive/destructive interference between the dry and delayed signal at 
that one frequency.

If you fire a saw or square into the Chorus effect there's a rich spectrum 
of harmonics and you hear that lush "swooshing" chorus effect as you get 
constructive & destructive interference at all of the harmonic frequencies 
at different times during the periodic sweeping delay.  The movement in the 
resulting spectrum is much more complex and interesting to the ear.

The delay time in cheap BBD chorus effects is also typically only 5ms or 
thereabouts, so you need a reasonably high pitch input signal before you get 
much "beating" at the actual fundamental frequency of the note.

-Richie,



-----Original Message----- 
From: Jacob Watters
Sent: Saturday, February 15, 2025 10:16 PM
To: Richie Burnett
Cc: SDIY
Subject: Re: [sdiy] MN3007 Chorus Acting Weird


You were correct! I used a square wave, and I could immediately hear a 
normal chorus effect. I checked the scope, and I could see left and right 
movement.

I am curious to learn more - is this because the BBD circuit works on a 
limited frequency range, and a low frequency sine is outside of that range, 
but the harmonics in a square wave are within the range? This would explain 
some of the unique character that people say a BBD has.

Thank you so much!

I am going to swap the rate trimpot for a panel mounted pot, and then this 
thing is ready to use :)

Best regards,

Jacob Watters
JacobWatters.com




On Sat, Feb 15, 2025 at 4:50 PM Richie Burnett 
<rburnett at richieburnett.co.uk> wrote:
Try testing it with a sawtooth or square waveform.  With just a sinewave you
can only really get amplitude modulation, especially if the test tone is
quite a low pitch.  And that could sound like panning if you alternately get
constructive interference on one side and destructive on the other side of
the stereo image.  With a bright saw or squarewave at a decent pitch you
should be able to hear some periodic detuning or at least some "phasing"
between the dry and the chorused waveform edges.

-Richie,



-----Original Message----- 
From: Jacob Watters via Synth-diy
Sent: Saturday, February 15, 2025 9:35 PM
To: Richie Burnett
Cc: SDIY
Subject: Re: [sdiy] MN3007 Chorus Acting Weird


I am testing it with a sine wave from a signal generator. When I look at a
scope, I only see vertical movement. When I listen with my ears, I only hear
panning.

I found this photo of the service manual. It's difficult to read the exact
values, but maybe someone familiar with the IC will be able to tell how it
should be functioning. The BBD part is in the rectangle at the bottom with
the "DC 1/2" label.

https://rvb-img.reverb.com/image/upload/s--wzg-Dhdj--/f_auto,t_supersize/v1669410662/zuo0v17w68dkc6c0n99s.jpg

Jacob Watters
JacobWatters.com



On Sat, Feb 15, 2025 at 4:28 PM Richie Burnett
<rburnett at richieburnett.co.uk> wrote:
What input signal are you testing it with?

-Richie,


-----Original Message----- 
From: Jacob Watters via Synth-diy
Sent: Saturday, February 15, 2025 9:17 PM
To: SDIY
Subject: [sdiy] MN3007 Chorus Acting Weird


I am repairing a Yamaha KM602 mixer, which has a builtin MN3007 circuit. The
chorus circuit behaves a bit odd. Instead of being a modulated delay that
causes a pitch shift, it seems to behave like an auto-pan circuit. The sound
level increases on one side, while decreasing on the other, and alternates
back and forth. It is labelled as CHORUS on the front panel, so that is not
the behavior that I expected.

Is this normal, or is something wrong with the circuit?

I tried finding a pdf of the service manual online, but I was unsuccessful.
Does anyone have the service manual and can provide some input as to how the
circuit should work?


Thanks for your help!

Best regards,

Jacob Watters
JacobWatters.com





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