Welcome to Roger Russell's
McIntosh Equalizer History Part 1
The ML-1C Speaker plus the MQ101 and MQ102 Equalizers

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What's the McIntosh Equalizer All About?

The McIntosh Environmental Equalizers all began with my first patent number 3,715,501 "Loudspeaker System". The McIntosh ML-1C loudspeaker system consists of the ML-1C speaker (above) and the MQ101 equalizer (below). Together they form a complete system that can maintain flat response down to 20Hz within 1dB. No subwoofer was needed! The system works just as well today as it did in 1970.

There are equalizers and equalizers. They aren't all the same. There are speaker equalizers that work with a speaker system to achieve a specific acoustic output. There are equalizers that compensate for different speaker locations in a room. There are equalizers that compensate for response peaks and dips due to room resonances. There are also program equalizers (tone controls) that adjust the frequency balance of program material. Each kind of equalizer is tailored to accomplish a specific task. Each covers a specific frequency range with a certain shape and amplitude. A bass control, for example, does not make a good speaker equalizer. It’s too broad and while it may correct one frequency range, it can also exaggerate another, resulting in different bass that still doesn't sound good.

The McIntosh MQ101 and MQ102 equalizers were never completely understood by most sales people or customers. To further complicate matters, the equalizers were sold separately from the speakers instead of being included in the price of the system. It was treated as an optional enhancement instead of an essential part. There were no owner manuals for the ML systems and the equalizer manuals never explained how to use the equalizer to achieve specific acoustic results or even why it was needed with the ML speaker systems at all. As a result, only one person in three bought an equalizer.

In the design of normal speaker systems there is a trade off between efficiency, box size and low frequency cutoff. This is all in accordance with the Thiele-Small parameters and Hoffman's Iron Law (See Note 1). Low frequency response is normally desired to be as flat and extended as possible. This can be done with either a very large box or a very inefficient speaker. A large box is expensive and usually not compatible with room decor. An inefficient speaker requires a large power amplifier, which is expensive and places excessive demands on the speaker power handling ability. Therefore, design of a speaker with flat response to 20Hz would seem to be impractical. The final choice is invariably a compromise. Typically, a sealed system is designed to extend response down to system resonance, which is often around 50Hz. It is typically an underdamped system having a Qts ranging from 0.7 (green curve below) to 1.0 (blue curve below).

Going Outside of the System

If we accept these rules as a limit, then it's as far as we can ever go with speaker systems, and golden opportunities are being missed. What if we go outside the system and cheat? Like in the movie Star Trek "The Wrath of Kahn" we learn that early in the film Captain Kirk is able to change the scenario by going outside the accepted limits and changing the conditions from what was otherwise a no win situation.

In the ML system design, that's what I did. I designed an overdamped woofer that had a very high BL product and used it in a small sealed enclosure. The amount of acoustic material in the enclosure was chosen to provide a smooth continuous response through the system resonance, which was about 50Hz. The Qts was about 0.4 (red curve). Efficiency was much higher in the upper bass, but response began rolling off as high as 200Hz.

The ultimate slope of the low frequency rolloff for all sealed systems is 12dB/octave. As a system becomes more underdamped, there is a greater tendency for it to ring. It means the cone motion does not stop when the signal stops. It keeps on moving and gradually decays. Critical or overdamped systems have no tendency to ring. The overdamped approach is not normally a good design because it has relatively few low frequencies compared to the upper bass. This was where the equalizer came into being. I createed an electrical equalization circuit that was just the opposite of the speaker rolloff. Wherever the response was down, the equalizer was up by an equal amount, effectively maintaining a flat response for the speaker and equalizer combination. What was nice about this concept was that the equalization continued to correct for response right through the system resonance and right on down to 20Hz and below. Electronic equalization was made prior to the power amplifier so there were no losses or load impedance problems introduced between the power amplifier and the speaker. That was what my patent was all about.

I chose 20Hz as the limit and began to roll off frequencies below there. There were several reasons for this. At the time there was turntable rumble, record rumble and tonearm resonance. There was also the cone excursion limit to be considered. Each octave lower means four times the cone excursion. Even though the equalizer was rolled off below 20Hz, it didn’t just suddenly die. There was still some response even at 10Hz. The system just wasn't flat any more. Today, where CD's and other means of recording are no longer plagued by rumble, arm resonances etc., flat response could be extended down even further. Of course, the woofer must still not be overdriven.

In the 12" woofer design I used a 2" high temperature four-layer voice coil wound on a black anodized aluminum voice coil form that helped to dissipate heat. The magnet structure had two 6" X 6" X 1/2" steel plates and a 54 oz barium ferrite magnet. To prevent the voice coil from bottoming, the back plate was machined out 1/4" deep in a circle where the voice coil could hit. The woofer then easily made a 3/4" peak-to-peak excursions without hitting. This proved to be adequate.

The tinsel leads that connected to the voice coil were brought out on opposite sides of the basket for good dynamic balance and were a unique feature of the McIntosh woofers. Another reason for doing this was that I found if both tinsel leads were brought out on the same side to a commonly available dual terminal strip, the leads could touch at high power and burn out.

I used a urethane surround. I found that rubber surrounds were unsatisfactory for several reasons. The added mass to the moving parts lowered the output by several dB. The rubber surround was only available with a small roll. This limited the excursion that I needed. The rubber roll introduced resonances of it's own. I also tried a cloth roll surround. This collapsed and went inward at long outward cone excursions and made a cracking noise like a whip. I thought it was the voice coil bottoming until I saw it with a strobe light. See my McIntosh Drivers History page for more information about this and other McIntosh woofers.

When the system was combined with a crossover coil, Hookup wire and amplifier, the combined resistance in the circuit brought the system Q to about 0.5, or critical damping. Another distinct advantage with a more efficient woofer was that a more efficient mid-range and tweeter could be matched with the upper bass output of the woofer.

Two Equalizers in One Package!

When a speaker is placed in a room, the low frequency response is altered depending on where it is placed with respect to the room surfaces. This effect is called room gain. It is not related to standing waves. When a speaker is placed in a corner on the floor, you may have noticed there is more bass response than when it is out of the corner and off the floor. This effect occurs below 200 Hz. Each time the solid angle of radiation is halved, the response below 200 Hz is increased by 3dB. It became obvious to me that since the speaker equalization is also accomplished below 200 Hz, that the effect of room gain can be combined with the speaker equalization. Reducing the amount of low frequency equalization in 3dB steps can do it. This way flat response can be maintained for various speaker locations in the room.

The equalizer range is divided into five low frequency switch positions. Four of them corresponded to the solid angle that the speaker is radiating into. This is 360°(2p), 180°(p), 90°(p/2), and 45°(p/4) The low frequency switch is used as follows: Position 5: When the speaker is away from all surfaces. Position 4: When the speaker is against the wall and away from the floor. Position 3: When the speaker is on the floor against the wall. Position 2: When the speaker is in the corner on the floor. Position 1: is electrically flat. Dual concentric knobs are used for independent equalization on each channel. Separate adjustments can be made, for instance, when one speaker might be located in the corner and the other is not.

The price that's paid for the outstanding low frequency performance of the ML-1C and equalizer combination is that more power is needed to maintain flat response as the frequency decreases below 200Hz--apparently as much as 20 dB at 20Hz. It really isn't as bad as it looks, however. Compared to a system with a Qts of 1 (blue curve), that's flat down to resonance, the McIntosh woofer has as much as 6dB more output. This means that above approximately 75Hz the power requirements are less for the McIntosh woofer for the same sound level. It's only below 75Hz that efficiency gradually decreases, and is down 14dB at 20Hz.

That's still a little over 11 times the power. In practice, however, the power requirements are demonstrated to have increased very little compared to a system that is flat to 50Hz. Why? Records and music in general don't have that much deep bass output compared to the upper bass and higher frequencies. To verify this I used the Bruel & Kjaer 3347 real time analyzer with the peak hold feature to find some records that have deep bass. Some of the records current at that time are:

 

Some of these records have substantial output to 32Hz and a few to 25 Hz. Even these selected records have higher output in the upper bass, but of course not always at the same time. Obviously CD's and DVD's did not exist in 1970. However, today's recordings still exhibit the same characteristics.

The ML-2C, ML-2M, ML-4C and ML-4M systems are floor models and are normally used against a wall, but can still be placed in or out of a corner. The equalizer settings apply in the same way as for the ML-1. The ML-2's have two 12" woofers and the ML-4's have four woofers. Low frequency distortion is reduced even more with these systems.

When the first Power Guard amplifier (the MC2205 at 200 watts/channel) came out in 1973, it provided added insurance that the power amplifier cannot be overdriven with the bass boost equalizer. In addition, the ML-1C can actually handle program peaks much higher than the conservative 100-watt rating. In 1997, I tested an original woofer and equalizer and found them still able to meet my original specs. I have played the ML-1C's at very high levels using an MC7300 amplifier (300 watts/channel) and CD's as a sound source. The bass is still impressive with no sign of woofer bottoming.

The following speaker systems are designed for use with the MQ101 and MQ102 equalizers.

(Note 1) "Hoffman's Iron law, described by Henry Kloss in the mid 1950's and later turned into exact mathematical form by the engineers Thiele and Small, governs the behaviors of woofers. Essentially, it says that a woofer's efficiency is proportional to the volume of its cabinet and to the cube of its cutoff frequency. For example, suppose you have a woofer whose response is flat down to 40 Hz in a 2.0 cubic foot box. To make its response flat down to 20 Hz, you must either increase the cabinet volume by eight times (to 16 cubic feet) or use 8 times as much amplifier power to achieve the same listening volume. This, in a nutshell, is why flat loudspeaker response down to 20 Hz is rare." Stereo Review July 1992 "All about Subwoofers" by Peter W. Mitchell

ML-1C Specifications

ML-1C Service Information

The ML-1C is listed in the New Products section of Stereo Review April 1971. It is described in detail including the ML-4C and ML-4M. The MQ101 and MQ102 Environmental Equalizers and their function are also described. The original selling price for the ML-1C at that time was $312.00 each. The ML-4C and ML-4M were $1012.00 each.

Update for the ML-1C

After more than 30 years, two different updates may be needed. The most common one is the deterioration of the 12" woofer and some 8" lower-mid surrounds. Only the surrounds need to be replaced. New drivers or new cone assemblies are not needed or recommended. Surrounds can be replaced with new polyether ones that will not deteriorate like the original polyurethane surrounds. Some early 8" mids had coated cloth surrounds that don't need to be replaced.

Be certain the new woofer surrounds are made of polyether and that they have a 3/4" diameter roll. Many speaker repair centers do not supply the larger 3/4" roll. A surround that has only a 1/2" roll will restrict the woofer excursion and cause distortion for deep bass notes. If you have new surrounds put on the woofer for you, be certain they are NOT the rubber types. These heavy surrounds reduce the efficiency of the woofer and it will no longer match the 8" lower mid-range output. The heavy surround will also alter the woofer response and it will no longer match the MQ101 or MQ102 equalizer compensation required for the system to provide flat response to 20 Hz.

The new 8" surround must also be polyether and the roll must be 1/2" in diameter. The 12" and 8" surrounds can be replaced with do-it-yourself kits or you can have them replaced for you by a McIntosh dealer such as Newfoam.

The second update is for the upper-mid domes or tweeters. This is only needed if they are not working properly. If the ML-1C has been overdriven by an amplifier that did not have Power Guard, the upper-mid domes or tweeters may have been damaged. They may look all right, but the voice coils may be rubbing or burned out, causing distorted sound or having no sound at all.

Direct replacements are no longer available. I have redesigned the system using new upper-mids and tweeters that will handle much higher power with even lower distortion. Since these are not direct replacements, new crossover networks must be used. I have designed new networks to match the original ML-1C sound. I have included an option in these crossovers for you to choose the original ML-1C sound, or a slightly brighter, but still very smooth sound. I can supply these new supplementary crossovers completely wired and ready to install, including 6 pages of illustrated instructions. The drivers are available from Madisound. For further information email to rogerr4@earthlink.net

The MQ101 Environmental Equalizer

The MQ101 Environmental Equalizer has an illuminated glass panel that matches the appearance and width of other McIntosh electronics. It also has Panloc fasteners that allow it to be locked in place in the cabinet or it can be unlocked to slide out. It is shipped complete with a wood cabinet. It can also be custom mounted using the panloc brackets from the cabinet. There is no power switch in the MQ101. It can be plugged to a switched outlet in the preamplifier or receiver.

The MQ101 also has mid and high frequency controls. Some speaker systems have controls for the mid and tweeter that are part of the crossover network. These controls or switches are located at the rear of the cabinet that require reaching behind each speaker to make an adjustment. In addition, the potentiometers or switches that are used often become intermittent. The speaker system impedance also changes with various settings, sometimes going too low. In addition, the frequency range covered by each control is limited by the frequency range and efficiency of the mid or tweeter in the system.

In comparison, the MQ101 mid frequency and high frequency controls can be remotely adjusted, do not affect the impedance, and avoid high currents. The frequency band for the mid and high frequencies is not limited to only the frequency range or efficiency of the mid or tweeter. It covers a range that relates better to music.

 

The input to the MQ101 normally comes from the preamplifier main output. The output from the MQ101 then goes to the power amplifier input. However, when used with a receiver, access to the preamp output and power amp input is not always available. A way around this is to use the tape monitor section of the receiver. Tape Out from the receiver goes to the MQ101 input and the MQ101 output goes to Tape In on the receiver. The tape monitor switch in the receiver must then be left at the IN position. The Tape monitor function in the receiver is, of course, lost. A tape monitor switch on the front panel of the MQ101 is provided to restore this feature. Input and output jacks are located at the rear of the MQ101.

When The MQ101 is used with the tape monitor circuit, this would be equivalent to driving the MQ101 with the volume of the preamplifier up full. The MQ101 can then be overdriven. A three position input attenuator slide switch is at the left side in the rear. It is normally used in the 0dB position but when the MQ101 is driven by the tape monitor output, the switch must be placed in the –12dB position.

MQ 101 Specifications

Response: 20Hz to 20,000Hz (+0.5 -0.5dB)
Distortion: 0.1%
Hum and noise: -80dB
Input sensitivity: 2.5V
Input impedance: 60k ohms
Input attenuator switch: 0dB, -6dB and -12dB
Output: 2.5V into 47k ohms
Tape monitor: inputs and outputs with switch
Equalization low frequency: 0 to +17dB at 20Hz in 5 steps (0, +6, +9, +13 and +17dB)
Equalization mid frequency: -5 to +5dB at 4000Hz in 5 equal steps
Equalization high frequency: -4 to +4dB at 20kHz in 5 equal steps
Front panel: illuminated gold/teal with Panloc mounting
Size: 2-15/16" high, 16" wide and 10" deep (1-1/2" knob clearance)
Cabinet size: 3-1/2" high, 16-5/8" wide and 9-3/8" deep
Weight: 15 lb.
Sold from: 1970-1978
Last retail price: $250.00 (cabinet included)

An MQ101 Owner’s Manual can be found on a separate page.

The MQ102 Equalizer

Shortly after the ML-1C and MQ101 went into production, the MQ102 was introduced. This lower cost equalizer provides the same low frequency compensation as the MQ101 but has a none of the extra features except the tape monitor function. The box is black and chrome with black silk-screened lettering on the chrome. The left and right channel low frequency equalization controls are near the top of the picture. Each is selectable for positions 1 through 5. There is no power switch in the MQ102. It can be plugged to a switched outlet in the preamplifier or receiver.

The input to the MQ102 normally comes from the preamplifier main output. The output from the MQ102 then goes to the power amplifier input. However, when used with a receiver, access to the preamp output and power amp input is not always available. A way around this is to use the tape monitor section of the receiver. Tape Out from the receiver goes to the MQ102 input and the MQ102 output goes to Tape In on the receiver. The tape monitor switch in the receiver must then be left at the IN position. The Tape monitor function in the receiver is, of course, lost. A tape monitor switch on the side of the MQ102 is provided to restore this feature. Input and output jacks are located on the side of the MQ102.

When The MQ102 is used with the tape monitor circuit, this would be equivalent to driving the MQ102 with the volume of the preamplifier up full. The MQ102 can then be overdriven. A three position input attenuator slide switch is at the top of the box at the center. It is normally used in the 0dB position but when the MQ102 is driven with the tape monitor output, the switch must be placed in the –12dB position.

MQ 102 Specifications

Response: 20Hz to 20,000Hz +/- 0.5dB
Distortion: Will not exceed 0.1% at rated output from 20Hz to 20kHz,
Hum and noise: -80dB below rated output
Input sensitivity: 2.5V
Input impedance: 60k ohms
Input attenuator switch: 0dB, -6dB and -12dB
Output: 2.5V into 47k ohms or more
Tape monitor: inputs and outputs with switch
Equalization: 0 to +17dB at 20Hz in 5 steps (0, +6, +9, +13 and +17dB) in positions 1, 2, 3, 4, and 5 respectively.
Finish: Black and chrome
Size: 2-1/2" high, 6-3/4" wide and 4-1/8" deep. Knob clearance 1-1/2"
Weight: 2-1/2 lb.
Sold from: 1970-1978
Last retail price: $74.50

Related Patents

Number 3,721,861 assigned to Sidney A. Corderman at McIntosh Laboratory. This patent contains mostly background material of my speaker-equalizer design including a diagram of the ML-4 crossover, MQ101 circuitry and response curves. The actual patent is for a protective circuit used in the MQ101. It consists of a reed relay and RC timing circuit that shorts the equalizer output for about 1 second and prevents turn-on transients from being sent to a power amplifier.

Number 3,803,359 assigned to Sidney A. Corderman at McIntosh Laboratory. This patent contains more background material of my speaker-equalizer design including details of the ML-1C operation, a diagram of the ML-4 crossover, MQ101 circuitry and response curves. The actual patent is for the cascading of the three low, mid and high frequency contours in the equalizer.

Interesting Sidelights

Although my 1973 patent is for equalization of an overdamped woofer, the equalizer includes compensation for different radiation angles but is not part of the patent. In 1975, Acoustic Research came out with the AR-10pi speaker system that has a similar stepped compensation. The equalization is accomplished not with a separate active equalizer like the MQ101 but with passive crossover network elements and a transformer inside the speaker enclosure. A flip down panel on the front of the AR-10pi covers three switches for adjusting the response. I had the opportunity to test the 10pi at that time. Unfortunately the transformer goes into saturation at frequencies approaching 20 Hz, causing significant distortion beyond that generated by the woofer. The system efficiency is a little below 85 dB compared to 89 dB for the ML-1C (above 200Hz). The 10pi was reviewed in High Fidelity magazine in July 1977. It sold for $425.00 and the ML-1C sold for $399.00.

My patent was cited as a reference in the AR patent for the 10pi. The patent, number 4,021,614 Woofer Equalizer, was awarded to John Bubbers on May 3, 1977 and assigned to Teledyne, Inc. AR was owned by Teledyne at that time. A patent search shows this to be his only patent. I remember that in a conversation, when visiting with John Bubbers in October, 1973, I had told John about my equalizer and features. Of course, it was no secret then because it was in production. Perhaps he decided to patent the part of the equalizer that wasn't covered by my patent.

Further, I have found mention of this from an article in the October 1957 issue of Audio magazine. This is by Ed Villchur at Acoustic Research titled Loudspeaker Damping. He mentions the effect of solid angle seen by the speaker: "It would seem to be a good idea for someone to design an equalizer network to provide variable bass boost to compensate for this effect on performance due to change in solid angle." No patent had been filed. The behavior of speaker radiation into various solid angles is common knowledge. It is explained in detail in books such as Acoustics by Leo Beranek published in 1954 and Hi-Fi Loudspeakers and Enclosures by Abraham Cohen published in 1956. While at the Sonotone Corporation, I spent the day once in the early 1960's with Abe and ran response curves for him on his Bi-phonic Coupler speaker system.

AR further pursued the idea of electronic bass equalization. The patent number 4,113,983 Input Filtering Apparatus For Loudspeakers was issued to Paul Steel on September 12, 1978 and assigned to Teledyne Acoustic Research. It describes a bass boost system but with a feedback circuit to limit the low frequency power delivered to woofer. This prevents bottoming of the woofer cone, spider or voice coil. The greater the power to the woofer at the lowest frequencies, the higher the cutoff frequency for the low end of the woofer. The net effect is to limit deep bass from being reproduced at loud levels.

In my opinion neither of these ideas patented by Acoustic Research will reproduce the lowest frequencies as accurately as the ML-1C and MQ101 combination. When the MQ101 is used, there is no significant distortion introduced by its use and there is no need for gimmicks to restrict the bass response right where it is needed and wanted for accurate reproduction.

In 1972 the Concept EQ-5 was introduced by University Sound Division of LTV Altec. According to advertising, this system has a 12-inch overdamped woofer. The equalizer reportedly restores flat bass response and permits contouring of other portions of the audio spectrum to suit the acoustics of individual listening rooms. Three contouring controls are provided: low frequency (-6 dB to flat response in three steps); mid bass (-6 to +6 dB) in five steps); high frequency (-6 to flat response in three steps). The system also has a 5-inch mid-range and a small dome tweeter. The two speakers and equalizer sold for $399.00.

Although this appears to be a copy of my patents, Gordon Gow felt this was not worth pursuing and the system was apparently discontinued soon after it was introduced. The idea was undoubtedly brought to the attention of Altec by Dale, who worked for me for a short time and learned about the theory. After leaving McIntosh, he went to work for Altec. No patents for this system were applied for. Later, Dale was awarded two design patents at Altec for speaker appearance.

In 1979, KLH announced the KLH-3. This system uses an active analog bass “computer” (equalizer) in a separate cabinet. It essentially extends the bass down to 40 Hz but limits the woofer excursion if the amplitude of the low frequency program material becomes too great. The novelty is that the extended bass is accomplished in a speaker cabinet only 8” X 12” X 6”. The pair of speakers and equalizer sold for $450.00. Again, this is a much compromised attempt to extend low frequency response. The system was discontinued soon after.