MG1.6 Mid-Boost Mod

Ed Grochowski

Introduction

This article describes a modification to the Magnepan MG1.6 that boosts the speaker's output by 2.5dB at the crossover frequency of 600Hz. This voicing gives the MG1.6 a more prominent midrange compared to the stock speaker. If you're looking for ways to improve the MG1.6, and have already tried the tweeter attenuator resistors, you may want to try this as well.

Stock Crossover

The electrical response of the stock crossover is illustrated below (green=woofer, red=tweeter, blue=sum amplitude, yellow=sum phase). The crossover has been designed with an intentional gap between the woofer and tweeter sections. At the crossover frequency, the electrical response is almost 4dB down.

The gap exists for two reasons: its compensates for a rising tweeter acoustic response at lower frequencies due to resonance, and it gives the MG1.6 the impression of stronger bass and more extended treble than it otherwise would have had.

At the time I developed the mid-boost mod, I did not have the equipment to measure the acoustic output of the MG1.6. Hence, I relied on listening. To my ears, the stock crossover sounds a little soft in the midrange. So I devised the following network to increase the midrange output by partially closing the gap.

Schematic

The network consists of two 2.2 ohm resistors connected between the amp and speaker as shown in the schematic below. The resistors are inexpensive 5%, 10W wirewound types available from Digi-Key (part number 2.2W-10-ND). 10W resistors are adequate up to moderate listening levels; 25W resistors should be used if you listen at high volumes.

A photo is shown below.

I built the resistor network into a pair of DIY speaker cables. Unlike most mods, this one requires no internal changes and does not void the warranty.

An astute reader noticed that in the photo, the (-) speaker wire is connected to the (-) input terminal with the jumper installed, whereas in the schematic, the (-) speaker wire is connected to both biwire (-) terminals without the jumper. The two methods are electrically the same.

The resulting electrical response is as follows (green=woofer, red=tweeter, dark blue=complex sum magnitude, yellow=complex sum phase, light blue=sum of magnitudes).

Effects

The effects are:

The speaker's output level is reduced by 3.5dB.

The woofer section's -3dB point is raised from 200Hz to 330Hz.

The tweeter section's -3dB point is lowered from 1.6KHz to 1.1KHz.

The crossover frequency remains unchanged.

The bass/treble balance remains unchanged.

The midrange dip is reduced to 1dB.

Sound

The sound is smoother, as though it were produced by a single full-range driver! The timbre of vocals, pianos, and acoustic guitars is noticeably improved. The slight graininess of the QR tweeter is gone, as is the tendency to sound hyper-detailed. I find the modded MG1.6 much easier to listen to. It also sounds more like the conventional speakers I've A/B'ed it against (while retaining the Maggie boxlessness of course!)

Because the bass/treble balance is not changed, the modded MG1.6 does not sound dull, as with the tweeter attenuating resistors installed.

The downsides are the mid-boost mod brings back some of the traditional Maggie weaknesses: the bass can sometimes seem weak, and there's a bit of the Maggie opaqueness.

Acoustic Response

In January 2004, I measured the acoustic response of the MG1.6 using a white-noise CD, my stereo, an inexpensive condenser microphone, microphone preamplifier, computer, and FFT software. The green line shows the stock speaker while the blue line shows the modded speaker. The levels were matched at the frequency extremes.

The graph shows a rising low-frequency response due to the bass panel resonance. As with the Stereophile measurements, the tweeter level is shelved down relative to the woofer. The output of the modded speaker is 2.5dB higher than the stock speaker at the crossover point of 600Hz. The peaks at 60Hz and 120Hz are due to hum.

Amplifier

The above resistor values assume a solid-state amp with negligible output impedance.

A tube amp can have significant output impedance that is already having a similar effect on the MG1.6 crossover. For a tube amp, subtract the amplifier's output impedance from 2.2 ohms to determine the proper resistor value. Output impedance is sometimes stated in terms of a damping factor. For example, a damping factor of 20 at 8 ohms is equivalent to 8/20 = 0.4 ohms.

Other Models

The mid-boost mod may be applied to the Magnepan models listed below. Since I haven't tested the modification with these models, please let me know how well it works!

MG1.5: Use a 3.3 ohm resistor in the bass section and 2.7 ohms in the treble. Reverse the polarity of the tweeter.

MG12: Only 1.5dB of midrange boost is required. Open the back plate to access the woofer and tweeter sections. Use a 0.75 ohm resistor in the bass section and 0.68 ohms in the treble. Replace the 50uF woofer shunt cap with 25uF.

MMG (without woofer shunt cap): Open the back plate to access the woofer and tweeter sections. Use 2.2 ohm resistors in both bass and treble sections.

Happy listening!