MP-1 EQ: mid-band is not centered at 1.6 kHz.

[Kazinator]

Back in January, was simulating parts of the MP-1 EQ circuit. It confirmed what my ears were telling me for years: the midrange control is not at 1.6 kHz.

It doesn't sound anything like moving the 1.6 kHz slider on my equalizer.

I always pegged it at around 2.5 to 3 kHz.

Well ... the sim matched that hypothesis perfectly according to its frequency sweep:



The circuit fragment I actually simulated:



Now here is the thing. There are reasons to suspect that there was intent to have this be 1.6 kHz. When you run some naive calculations on the resistor-capacitor network in isolation, it does look like a band-pass filter on 1.6 kHz.

And for the other bands in the EQ, I found that the naive calculations do jive with the behavior.

I also feel the EQ would have been better with the 1.6 kHz band. Because it so happens that I actually cut around that area with my 31 band EQ, while the mid control of the MP-1 is at zero. THat's where my ears led me. With a 1.6 kHz mid control, I would probably be able to leave more of the external EQ flat.

[rnolan]

Hey Kazinator, interesting. According to the v2 manual the Treble control is supposed be centred at 1.6kHz:
Manual version 2
BASS Narrow band EQ adjustable in 2 dB steps ±18 dB centered @ 180 Hz.
MID Wide band EQ adjustable in 2 dB steps ±12 dB centered @ 600 Hz.
TREBLE Narrow band EQ adjustable in 2 dB steps ±18 dB centered @ 1.6 kHz.
PRESENCE Wide band EQ adjustable in 2 dB steps ±12 dB centered @ 3 kHz.

The v1 manual doesn't list the centre frequencies. It does list the Factory patch settings, while the v2 manual doesn't.  Just looking through the patch settings, they are all either 0 or boost, i.e. no cut.  I find that interesting as, when I used a MP-1, I typically cut the mid to -4 or -6.

Another difference is the v1 manual says Bass and Treb boost/cut is +/- 16dB where v2 has them at +/-18dB
Manual version 1
BASS
Narrow band low frequency EO control adjustable in 2 dB steps ± 16 dB.
MID
Wide band mid frequency EO control adjustable in 2 dB steps ± 12 dB.
TREBLE
Narrow band high frequency EO control adjustable in 2 dB steps ± 16 dB.
PRESENCE
Wide band high frequency EO control adjustable in 2 dB steps ± 12 dB.

There is a mod some do to lower the Bass centre freq.  Not that I ever had an issue with my MP-1 bass but at 180Hz it's a bit high

[Kazinator]

Hey Kazinator, interesting. According to the v2 manual the Treble control is supposed be centred at 1.6kHz:
Manual version 2
BASS Narrow band EQ adjustable in 2 dB steps ±18 dB centered @ 180 Hz.
MID Wide band EQ adjustable in 2 dB steps ±12 dB centered @ 600 Hz.
TREBLE Narrow band EQ adjustable in 2 dB steps ±18 dB centered @ 1.6 kHz.
PRESENCE Wide band EQ adjustable in 2 dB steps ±12 dB centered @ 3 kHz.

Sorry, right. I wanted to say treble, not mid! Yikes! The treble is claimed to be centered on 1.6, but the actual behavior is 2.5.

The presence control is definitely not centered on 3 kHz. I somehow didn't catch this claim in the V2 manual. It is mainly active from 3 kHz to around 11 kHz or so. Based on simulation, I found the middle frequency to be around 5.7 - 5.8 kHz. That more or less checks out in a side-by-side listening comparison compared to a graphic EQ.

The September 1987 dated schematic of the EQ also claims similar numbers to the V2 manual. There are inscriptions of "160 Hz", "1.6 k", "3 kHz" and "600 Hz".