Phono EQ Design + Retrofit to the Shure M65
Various Phono Playback Curves and annotated info.
The SHURE M-65 PHONO STAGE:
Two M65 Units, underside by underside. The first is lightly modified. The other is retro-resto upgraded with three different phono EQ choices: RIAA; AES; 700/800 Hz; plus beefy power supply capacitance "reservoirs" for each channel...
Introduced in 1960 and popularized quickly, Shure’s “Stereo Conversion Preamplifier” phono stage has come to be known as one cool looking and sounding phono preamp. It’s small, cute and sometimes the chrome finish can still be found in stunning condition. Sonically speaking, the factory installed phono stage has some unique playback equalization curves. With only one pair of input jacks and output jacks, the M65 is switchable for Phono, Special, Tape and Mic playback. Most spinning disc users will be using only the Phono position.
Shure uses a unique parts content in their Phono playback EQ curve with 44db of gain for the M65 Function Switch “Phono” position. The “Special” switch position is indeed very special. This is the “stereo conversion preamp” input, converting high impedance, ceramic cartridge inputs on an existing amp into magnetic cartridge inputs with 45db of gain. The Tape switch position is designed with NARTB playback EQ for tape deck heads, with 40db of gain. The Mic switch position is unequalized with 63db gain. 63db is a lot of gain for a 12AX7 phono stage to stay quiet. If you had your turntable plugged into the input jacks, then accidentally switched to the Mic input position, whoa now, that will be some loud sounds….Be careful when switching this input switch. Not only does the switch allow the blastingly loud Mic gain position, the switch itself is a sealed switch, which was noisy during the switching. Caution: Keep your Volume way down as you audition switchable, high gain stage noise and hum possibilities…
According to Shure’s one page, two sided 1960 brochure, the M65 phono playback EQ “incorporates a 3db rise at 10KHz, designed to complement the high frequency characteristic of most magnetic cartridges.” Deciphering the actual phono playback circuit, Shure digresses from the RIAA specified phono EQ. With the RIAA playback EQ curve specifying + or -2db from 30 to 15KHz for playback conformity, Shure can implement their unique playback curve and still publish a phono playback frequency response of (+ or – 1.5db) from 25 to 15KHz for this function’s “nominal characteristic.” Thus, like many manufacturers, their plus or minus 1.5db tolerance is in addition to the plus or minus 2db tolerance permissible within the actual, original RIAA playback curve’s “nominal characteristic” specification.
Let’s get right to the phono EQ circuitry implemented by Shure in their M65 preamp, shall we ? The type of equalization used here is the classic, active feedback topology. Two resistor-capacitor (R-C) parallel combinations of parts are connected in series. One R-C combo is the Bass Boost “pole” while the other is the Treble Rolloff “pole.” The overall feedback network connects to and “starts” from the output, then is fed back to an earlier gain stage. In this case, the overall feedback network connection “returns” to the first gain stage cathode. This active feedback topology utilizes two frequency discriminating network poles, wired in series, which mathematically combine to derive the actual midrange Turnover frequency. The M-65 also has a cathode to cathode, 100KOhm resistor, for some more lowering of distortion and output impedance. As I am initially "prototyping," I lifted this positive feedback R on one end, for possible implementation later...
The RIAA phono playback equalization curve, EQ for short, specifies three frequencies, which relate to the Turnover and Rolloff required. These three freqs. can also be referred to as time constants in microseconds (uSec. or uS):
50Hz (3180 uSec.) for bass, 500Hz (318 uSec.) for actual Turnover, plus 2122Hz (75 uSec.) for treble Rolloff. These RIAA specified frequencies and their associated time constants represent the transition points, or “asymptotes” of their frequency response plots. The 50Hz signifies the point of bass resonance or levelling off and shelving in gain. 500Hz signifies the +3db point above the flat portion of the response curve, with 0db referenced at 1KHz. 2122Hz denotes the -3db point, beginning the required treble Rolloff. So, the actual "flat" portion of the RIAA playback EQ response curve starts a bit above 500Hz and only extends to a bit below 2122Hz. The Turnover denotes the point where the midrange begins its’ upward slope toward bass boost. Rolloff signifies the beginning of the downward slope which “de-emphasizes” the pre-emphasis of the treble which was recorded and cut onto the master disc. Proper playback restores musical balance by reducing the inherent disc surface noise, while boosting the bass which was reduced during record cutting, originally intended to extend playing time on a disc. Hopefully, our preamp’s phono playback EQ curve is close to the exact inverse of the recording curve which was used to “cut” the master disc at the mastering/record cutting studios.
The Shure M65 has a bass pole of 1800pf (.0018uf) in parallel with (signified by //) 7.5Meg Ohms. The M65 treble rolloff pole is composed of 470pf(.00047uf) in parallel with 100K Ohms. Each pole has a time constant, in uSec. or uS, computed by simple R x C = Time Constant. With the two poles placed together in series, they mathematically interact to derive the actual Turnover frequency, also formulated in uSec. In order to convert uSec. to frequency, the mathematical constant of 159,155 (1Hz) is divided by the time constants to yield the exact frequencies, in Hertz.
Therefore, the M65 has its’ bass pole R x C measured as .0018 x 7,500,000 = 13,500 uS or almost 12Hz bass resonance, which can appear as close to turntable rumble territory in some playback systems. The Rolloff pole is .00047 x 100,000, which is 47uS or 3386Hz, not the RIAA curve spec of 75 uSec. or 2122Hz. Hence, this is the curve designed in by Shure, with the "+3db at 10KHz" indicated in their published M65 brochure. How will this sound ? Briefly, this little chrome cutie has impressive bass, sweet mids and lively highs.
But, it also has what I call juicy sounding, or rich mids. Why does it have juicy mids ? Let us look at the analyzing math to see why.
Introduced in the very famous Stanley Lipshitz treatise on analyzing all types of phono preamp EQ circuitry, as published in the Journal of the AES, Volume 27, Issue #6, June 1979, here is the not so complex formula to prove or disprove the specified playback curve’s Turnover frquency conformance:
(R1 x R2) divided by (R1 + R2) results in the resistive multiplier. Then, this multiplier # is multiplied by (C1 + C2), the capacitive multiplier, to arrive at the actual Turnover Frequency, in uSec. Finally, using the mathematical constant of 159,155(which represents the time constant of 1Hz), by dividing the derived uSec. # into 159155, we arrive at the actual frequency of Turnover, in Hz. To illustrate, here is the M65 math computation:
7,500,000 x 100,000 / 7,500,000 + 100,000 yields the resistive multiplier. That resistive multiplier #, multiplied by the (.0018 + .00047 cap combo), the capacitance multiplier, results in the Turnover, but in microseconds. R x C = uSeconds. 159,155 / Turnover uSec. # arrives at the genuine Turnover frequency, in Hz.
If my math was correct, the Shure M65 has a Turnover of nearly 710Hz, not the RIAA specified 500Hz. This is the reason the Shure M65 can be accused of possessing juicy mids…The M65 playback EQ curve begins its’ upward 6db per octave slope for bass boost at a higher than usual frequency, causing rich sounding mids, along with the impressive bass and lively treble. As a multitude of owners of M65s have realized, this sonic character, in combination with certain phono cartridges and sound systems, can sound endearing to many ears.
We can now think about how we can improve the M65 RIAA EQ adherence for better measurable and listenable, possibly reference grade results. Plus, how about we make the "Special" and "Tape" selector positions function as additional choices for differing phono playback EQ curves ? We can simply use the above analytical formulae and change R-C values according to our phono EQ curve preferences. Or, we can use the same math and parts’ values for all three M65 function positions, but use different capacitor and resistor type choices, like polystyrene vs. polyester (mylar) vs. mica vs. polypropylene caps and carbon vs. metal film resistors. It is your choice, once you can decipher the actual preamp EQ wiring layout with the function switch sequence. Tedious, yes, but it’s not rocket science…
As a confirmed audiophile, I can enjoy trying different combos of certain specific capacitors and resistors with the same circuit for a knowledge base of which caps I prefer. Yet, as a terminal music appreciator, the sweet sound of music via spinning grooved discs is more important to me, these days. It's all about playing the records....I do closely match stereo channel’s EQ Resistor-Capacitor parts values to 2% or lower, though. Tight tolerances for matching parts values give great results.
Some of the music before the RIAA curve was standardized circa 1955, was actually “cut” onto record discs with different playback curves. There is sufficient information available on which discs used which playback EQ curves. Moreover, when delving into the world of early mono 45s, EPs and 78s, the RIAA playback curve does not always work its’ magic.
Two of the playback curves I consider essential, besides the RIAA EQ curve, are the “AES” curve and the “800” playback EQ curves. In 1950, the Audio Engineering Society presented their recommended phono playback curve, mathematically averaged from the many playback curves record manufacturers used before the RIAA was recommended as the standard circa 1953-1954. The AES noble intention in 1950-1951 was simple. If the sound and broadcasting industries could actively standardize, then more equipment and records would inevitably sell better. Literally, commercial and consumer concerns could be met. We would have better phono playback and buy more discs, while the sound industries could produce more enjoyable, consumer compliant equipment, thus creating more demand for software and hardware.
The “AES” phono EQ curve specifies a 400Hz (398uS) Turnover and a 2500Hz (63.6uS) Rolloff for a “ -12db at 10KHz” spec. Bass boost was left optional for studio engineering and record cutting personnel to “mix and cut” at will, provided the end product sounds good via the “AES” playback EQ curve. Many of the earliest 78s and some of the early mono microgroove LP, EP and 45s sound really fine using this playback EQ, especially when the disc was encoded and cut with the complimentary, recording curve specifications. When you look at the top of the page playback curves chart, you would'nt think the 1.7 to 2 db difference in treble rolloff curves between RIAA and AES could sound so significant, but your ears/brain will recognize this and let you quickly know which phono EQ curve is correct for the spinning disc.
The original “800” curve had a Turnover at 800Hz with a -8 db at 10KHz Rolloff spec for one of a few RCA 78 rpm curves. Later, “800” EQ changed to the original 1949 RCA 45 rpm spec, with an 800Hz Turnover and -12db at 10KHz Rolloff, which RCA allegedly utilized from 1949 through 1952, with many of their discs. The “800” curve was known as the “Old RCA” playback curve, not to be confused with RCA's "Old Orthophonic" curve for 78s which had a 500Hz Turnover and -12db at 10KHz Rolloff. Yes, all these curves can be confusing...In late 1952, RCA introduced their all new, drum-roll please, shout the horns, “New Orthophonic” playback EQ curve for all speeds of their discs. Of course, the “New Orthophonic” phono EQ curve was soon destined to become the RIAA standard playback curve.
Have you ever heard the early generation 78s, acoustically recorded, up to 1925, played back with our more modern record players, using a magnetic cartridge through an RIAA equipped phono stage, only to comment how dull the old shellac sounded ? Those very dull sounding discs can often sound much better using the “800” playback curve. Looking at the chart at the top of the page, note the "extra mids" that a 700 to 800Hz Turnover can provide. Slightly dull sounding shellac discs, can often reveal more recorded details using the AES playback curve compared with the usual RIAA curve. This was some of the reasoning for my three EQ choices…
Various Phono Playback Curves and annotated info.
The SHURE M-65 PHONO STAGE:
Two M65 Units, underside by underside. The first is lightly modified. The other is retro-resto upgraded with three different phono EQ choices: RIAA; AES; 700/800 Hz; plus beefy power supply capacitance "reservoirs" for each channel...
Introduced in 1960 and popularized quickly, Shure’s “Stereo Conversion Preamplifier” phono stage has come to be known as one cool looking and sounding phono preamp. It’s small, cute and sometimes the chrome finish can still be found in stunning condition. Sonically speaking, the factory installed phono stage has some unique playback equalization curves. With only one pair of input jacks and output jacks, the M65 is switchable for Phono, Special, Tape and Mic playback. Most spinning disc users will be using only the Phono position.
Shure uses a unique parts content in their Phono playback EQ curve with 44db of gain for the M65 Function Switch “Phono” position. The “Special” switch position is indeed very special. This is the “stereo conversion preamp” input, converting high impedance, ceramic cartridge inputs on an existing amp into magnetic cartridge inputs with 45db of gain. The Tape switch position is designed with NARTB playback EQ for tape deck heads, with 40db of gain. The Mic switch position is unequalized with 63db gain. 63db is a lot of gain for a 12AX7 phono stage to stay quiet. If you had your turntable plugged into the input jacks, then accidentally switched to the Mic input position, whoa now, that will be some loud sounds….Be careful when switching this input switch. Not only does the switch allow the blastingly loud Mic gain position, the switch itself is a sealed switch, which was noisy during the switching. Caution: Keep your Volume way down as you audition switchable, high gain stage noise and hum possibilities…
According to Shure’s one page, two sided 1960 brochure, the M65 phono playback EQ “incorporates a 3db rise at 10KHz, designed to complement the high frequency characteristic of most magnetic cartridges.” Deciphering the actual phono playback circuit, Shure digresses from the RIAA specified phono EQ. With the RIAA playback EQ curve specifying + or -2db from 30 to 15KHz for playback conformity, Shure can implement their unique playback curve and still publish a phono playback frequency response of (+ or – 1.5db) from 25 to 15KHz for this function’s “nominal characteristic.” Thus, like many manufacturers, their plus or minus 1.5db tolerance is in addition to the plus or minus 2db tolerance permissible within the actual, original RIAA playback curve’s “nominal characteristic” specification.
Let’s get right to the phono EQ circuitry implemented by Shure in their M65 preamp, shall we ? The type of equalization used here is the classic, active feedback topology. Two resistor-capacitor (R-C) parallel combinations of parts are connected in series. One R-C combo is the Bass Boost “pole” while the other is the Treble Rolloff “pole.” The overall feedback network connects to and “starts” from the output, then is fed back to an earlier gain stage. In this case, the overall feedback network connection “returns” to the first gain stage cathode. This active feedback topology utilizes two frequency discriminating network poles, wired in series, which mathematically combine to derive the actual midrange Turnover frequency. The M-65 also has a cathode to cathode, 100KOhm resistor, for some more lowering of distortion and output impedance. As I am initially "prototyping," I lifted this positive feedback R on one end, for possible implementation later...
The RIAA phono playback equalization curve, EQ for short, specifies three frequencies, which relate to the Turnover and Rolloff required. These three freqs. can also be referred to as time constants in microseconds (uSec. or uS):
50Hz (3180 uSec.) for bass, 500Hz (318 uSec.) for actual Turnover, plus 2122Hz (75 uSec.) for treble Rolloff. These RIAA specified frequencies and their associated time constants represent the transition points, or “asymptotes” of their frequency response plots. The 50Hz signifies the point of bass resonance or levelling off and shelving in gain. 500Hz signifies the +3db point above the flat portion of the response curve, with 0db referenced at 1KHz. 2122Hz denotes the -3db point, beginning the required treble Rolloff. So, the actual "flat" portion of the RIAA playback EQ response curve starts a bit above 500Hz and only extends to a bit below 2122Hz. The Turnover denotes the point where the midrange begins its’ upward slope toward bass boost. Rolloff signifies the beginning of the downward slope which “de-emphasizes” the pre-emphasis of the treble which was recorded and cut onto the master disc. Proper playback restores musical balance by reducing the inherent disc surface noise, while boosting the bass which was reduced during record cutting, originally intended to extend playing time on a disc. Hopefully, our preamp’s phono playback EQ curve is close to the exact inverse of the recording curve which was used to “cut” the master disc at the mastering/record cutting studios.
The Shure M65 has a bass pole of 1800pf (.0018uf) in parallel with (signified by //) 7.5Meg Ohms. The M65 treble rolloff pole is composed of 470pf(.00047uf) in parallel with 100K Ohms. Each pole has a time constant, in uSec. or uS, computed by simple R x C = Time Constant. With the two poles placed together in series, they mathematically interact to derive the actual Turnover frequency, also formulated in uSec. In order to convert uSec. to frequency, the mathematical constant of 159,155 (1Hz) is divided by the time constants to yield the exact frequencies, in Hertz.
Therefore, the M65 has its’ bass pole R x C measured as .0018 x 7,500,000 = 13,500 uS or almost 12Hz bass resonance, which can appear as close to turntable rumble territory in some playback systems. The Rolloff pole is .00047 x 100,000, which is 47uS or 3386Hz, not the RIAA curve spec of 75 uSec. or 2122Hz. Hence, this is the curve designed in by Shure, with the "+3db at 10KHz" indicated in their published M65 brochure. How will this sound ? Briefly, this little chrome cutie has impressive bass, sweet mids and lively highs.
But, it also has what I call juicy sounding, or rich mids. Why does it have juicy mids ? Let us look at the analyzing math to see why.
Introduced in the very famous Stanley Lipshitz treatise on analyzing all types of phono preamp EQ circuitry, as published in the Journal of the AES, Volume 27, Issue #6, June 1979, here is the not so complex formula to prove or disprove the specified playback curve’s Turnover frquency conformance:
(R1 x R2) divided by (R1 + R2) results in the resistive multiplier. Then, this multiplier # is multiplied by (C1 + C2), the capacitive multiplier, to arrive at the actual Turnover Frequency, in uSec. Finally, using the mathematical constant of 159,155(which represents the time constant of 1Hz), by dividing the derived uSec. # into 159155, we arrive at the actual frequency of Turnover, in Hz. To illustrate, here is the M65 math computation:
7,500,000 x 100,000 / 7,500,000 + 100,000 yields the resistive multiplier. That resistive multiplier #, multiplied by the (.0018 + .00047 cap combo), the capacitance multiplier, results in the Turnover, but in microseconds. R x C = uSeconds. 159,155 / Turnover uSec. # arrives at the genuine Turnover frequency, in Hz.
If my math was correct, the Shure M65 has a Turnover of nearly 710Hz, not the RIAA specified 500Hz. This is the reason the Shure M65 can be accused of possessing juicy mids…The M65 playback EQ curve begins its’ upward 6db per octave slope for bass boost at a higher than usual frequency, causing rich sounding mids, along with the impressive bass and lively treble. As a multitude of owners of M65s have realized, this sonic character, in combination with certain phono cartridges and sound systems, can sound endearing to many ears.
We can now think about how we can improve the M65 RIAA EQ adherence for better measurable and listenable, possibly reference grade results. Plus, how about we make the "Special" and "Tape" selector positions function as additional choices for differing phono playback EQ curves ? We can simply use the above analytical formulae and change R-C values according to our phono EQ curve preferences. Or, we can use the same math and parts’ values for all three M65 function positions, but use different capacitor and resistor type choices, like polystyrene vs. polyester (mylar) vs. mica vs. polypropylene caps and carbon vs. metal film resistors. It is your choice, once you can decipher the actual preamp EQ wiring layout with the function switch sequence. Tedious, yes, but it’s not rocket science…
As a confirmed audiophile, I can enjoy trying different combos of certain specific capacitors and resistors with the same circuit for a knowledge base of which caps I prefer. Yet, as a terminal music appreciator, the sweet sound of music via spinning grooved discs is more important to me, these days. It's all about playing the records....I do closely match stereo channel’s EQ Resistor-Capacitor parts values to 2% or lower, though. Tight tolerances for matching parts values give great results.
Some of the music before the RIAA curve was standardized circa 1955, was actually “cut” onto record discs with different playback curves. There is sufficient information available on which discs used which playback EQ curves. Moreover, when delving into the world of early mono 45s, EPs and 78s, the RIAA playback curve does not always work its’ magic.
Two of the playback curves I consider essential, besides the RIAA EQ curve, are the “AES” curve and the “800” playback EQ curves. In 1950, the Audio Engineering Society presented their recommended phono playback curve, mathematically averaged from the many playback curves record manufacturers used before the RIAA was recommended as the standard circa 1953-1954. The AES noble intention in 1950-1951 was simple. If the sound and broadcasting industries could actively standardize, then more equipment and records would inevitably sell better. Literally, commercial and consumer concerns could be met. We would have better phono playback and buy more discs, while the sound industries could produce more enjoyable, consumer compliant equipment, thus creating more demand for software and hardware.
The “AES” phono EQ curve specifies a 400Hz (398uS) Turnover and a 2500Hz (63.6uS) Rolloff for a “ -12db at 10KHz” spec. Bass boost was left optional for studio engineering and record cutting personnel to “mix and cut” at will, provided the end product sounds good via the “AES” playback EQ curve. Many of the earliest 78s and some of the early mono microgroove LP, EP and 45s sound really fine using this playback EQ, especially when the disc was encoded and cut with the complimentary, recording curve specifications. When you look at the top of the page playback curves chart, you would'nt think the 1.7 to 2 db difference in treble rolloff curves between RIAA and AES could sound so significant, but your ears/brain will recognize this and let you quickly know which phono EQ curve is correct for the spinning disc.
The original “800” curve had a Turnover at 800Hz with a -8 db at 10KHz Rolloff spec for one of a few RCA 78 rpm curves. Later, “800” EQ changed to the original 1949 RCA 45 rpm spec, with an 800Hz Turnover and -12db at 10KHz Rolloff, which RCA allegedly utilized from 1949 through 1952, with many of their discs. The “800” curve was known as the “Old RCA” playback curve, not to be confused with RCA's "Old Orthophonic" curve for 78s which had a 500Hz Turnover and -12db at 10KHz Rolloff. Yes, all these curves can be confusing...In late 1952, RCA introduced their all new, drum-roll please, shout the horns, “New Orthophonic” playback EQ curve for all speeds of their discs. Of course, the “New Orthophonic” phono EQ curve was soon destined to become the RIAA standard playback curve.
Have you ever heard the early generation 78s, acoustically recorded, up to 1925, played back with our more modern record players, using a magnetic cartridge through an RIAA equipped phono stage, only to comment how dull the old shellac sounded ? Those very dull sounding discs can often sound much better using the “800” playback curve. Looking at the chart at the top of the page, note the "extra mids" that a 700 to 800Hz Turnover can provide. Slightly dull sounding shellac discs, can often reveal more recorded details using the AES playback curve compared with the usual RIAA curve. This was some of the reasoning for my three EQ choices…
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