Phono EQ: Origins and Design Elements

Phono EQ Origins and Design Elements:

Phono playback has come a long way from wind-up phonographs/gramophones. Before 1925, all shellac discs, whether 78, 80 RPM, faster or slower, the cylinders or discs were recorded acoustically. The musicians would gather around the bell of the horn, used as the only studio recording microphone, while the truncated throat of the horn housed a semi-flexible membrane with a centered, cutting stylus used to “cut” the foil, wax, or later lacquer discs in this "direct to disc" fashion. With the pre-1925 discs, the membrane/cutting needle was the “transducer” converting acoustical energy to mechanical energy, cutting a continuous groove onto the master disc. In acoustic record manufacturing, this wax disc was first coated, electroplated metallically, becoming the mother disc with a “negative” of the grooves, thus bulging, squiggly hills, processed to create one of the disc stampers. Two metal stampers were used to “press” a biscuit of shellac, and later vinylite, into the two sides of 78s, hydraulic pressure sandwiched together, for a “single” disc. As the record companies needed more copies to sell, their first or second generation master disc was used again to create fresh stampers, since the stampers do wear out. The above description is an over-simplification of quite the manufacturing wizardry.

During playback in the home, in essence the acoustic wind-up phonograph horn was an “acoustical equalizer” for the acoustically recorded discs, hopefully recreating all of the recorded sounds of all the instruments/performers surrounding the horn during the recording session. That was a bit too much to ask. During recording, there was no volume control, no equalizer, no audio electronics. Many early disc collectors treasure their acoustical reproducer, horn equipped phonographs. Some shellac collectors even shun the idea of playing these discs through more modern playback systems.

Let us explore some more mono disc chronology. Then, we can realize how we can reproduce these pre-1925 discs, electronically, yielding an even higher fidelity than using acoustic reproducer equipment, as well as increasing our enjoyment with later, electrically recorded discs.

Figure 1, below is a very informative frequency response graph of an early Victor acoustic phonograph, recorded with a 500 cps Turnover:


allegedly recorded at a state of the art sound studio, circa 1953, as seen in Audio Engineering, Vol. 37 #7, July 1953, pages 19-22 and 53-54 in author R.C. Moyer’s famous “Evolution of a Playback Curve” composition on behalf of RCA engineering, almost one year after RCA introduces the “New Orthophonic” EQ curve.

Note that there is very little “flat frequency response” indicated in this “musical snap shot” looking graph. In fact, this is a dismal, jagged response. The recording used had a wider response pressed onto the shellac disc, but the Fig. 1 graph represents the response of the wind-up, acoustical horn equipped phonograph “system.”

Before the recording studios converted to electrically recorded discs in 1925, disc response was presumed to have little or no extreme bass below 150Hz and rarely anything above 5000 Hz. Considering that later era Bell Telephones had a response of 300 to 3000 Hz, designed for intelligibility of the spoken words and recognition/identification of the speaker’s voice, acoustic phonographs could still present even higher fidelity, acoustically.
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When the sound studios began recording discs electronically in 1925, there was only one record cutter, at first, the Western Electric “Rubber Line” Disc Cutting Lathe system. The genius Bell Labs engineers devised a way to increase listening time per side, as well as soon enabling decreased noise for playback. By pre-emphasizing some of the high frequencies and de-emphasizing low frequencies during record cutting/mastering, the mission could be accomplished. However, this would require record playback systems to have the proper, exact inverse playback curve applied, in order to provide the most accurate representation of the original recording.

We can consider this recording mastering pre-emphasis/de-emphasis as “encoding,” while our playback systems provide the playback equalization, or EQ needed for “decoding.” Flattest frequency response, with the most natural sound reproduced, could be the result.

The part of the playback curve which now emphasizes the bass is known as the Turnover. Some folks call it a crossover point. This represents the transition point, +3db from the flat portion of the frequency response playback curve, toward the required bass boost for playback. Technically, this transition point is known as an asymptote. Likewise, for the pre-emphasized high frequencies during recording, the proper playback curve has another asymptotic transition point, now minus 3db from the flat portion of the curve, known as Rolloff.

Use of the proper rolloff will provide an essential reduction of the scratchy, pops and ticks inherent with the spinning disc medium, hopefully providing a more natural sounding reproduction for our listening pleasure. Many 1950s monaural, as well as some 1960s stereo preamplifiers can be recognized by their switchable phono stage’s many playback Turnover and Rolloff knobs/switches, plus their later gain stage’s bass and treble tone controls. One of my favorite classic, early stereo preamps, circa 1958-1960, the Lafayette KT-600 has both Turnover and Rolloff switches among it’s very versatile fascia knobs.

The above “acoustical snap shot graph” with very recessed low frequency response, below 1KHz, can certainly be better accentuated by increasing your bass control while the jagged looking mids and highs can be tempered with lowering the treble control. Even the 1929 RCA Victor 245 amp had a tone control to help tame the highs and scratchy noises heard during disc playback. Surely, the single tone control found on some early, radio/phono consoles was the beginning of phono playback equalization.

While RCA was modifying the WE cutter for their electrically recorded Victor discs, WE was also tweaking their own disc cutting system.
Originally, American 78s by Columbia, 1925 and up, chose a 300Hz Turnover, while RCA originally chose 500Hz as their crossover point. Also, originally in 1925, very little high frequency pre-emphasis was used during recording. By the mid to late 1930s, the NAB, National Association of Broadcasters, had introduced their NAB playback curve, synonymous with the NBC Orthacoustic playback curve. For the then popular 78s, Columbia had refined their record cutting equalization to mimic this NAB 78 curve, employing a 300Hz Turnover with a playback Rolloff specified as minus 16db at 10KHz. Comparatively, RCA originally chose 500Hz for their Turnover, their Rolloff specified as minus 8db at 10KHz, for a while. The beginning of recording studio and disc mastering engineering virtuosity, as well as better home playback was under way.

Here is Fig.2, a very cool graphical and chronological representation of record cutting curves from 1925 to 1952. Our playback curves would be the inverse of the second graph. Note the wider freq. response over time. These graphs are also seen in the same R.C. Moyer “Evolution of a Recording Curve” essay, as previously cited.
Unfortunately, the record industry catering to the home declined to standardize recording curves and playback curves, causing chaos for disc buying, music lovers. Different record companies would not adhere to the major curves, thus creating their own unique sounds, akin to a “house sound.”

With WWII ensnaring our country, pre-war “high fidelity” in the home was almost limited to the finest radio-phono consoles from EH Scott, McMurdo Silver and a few others. Sound studio and broadcasting equipment was not widely available to the public. In my opinion, the term high fidelity, coined by EH Scott, really began with the introduction of radio sets to the public in the 1920s. Astute radio listeners and disc spinners knew their sets might sound better, providing more musical reproduction with a simple swap of audio transformers or upgrading their speakers. In 1928, with Jensen providing competent speaker suspensions, allowing louder, lower distortion playback, HiFi was certainly developing. Unfortunately, the phono cartridge reproducer transducer was still the limitation.
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Post WWII, the American economy boomed pretty quickly, with suburbia popping up across the USA. The baby boomer generation was beginning. While families had gathered around their radios for the news during wartime, dads and moms, as well as the whole family now had good reason to kick back and begin relaxing while listening to music. 1946 was a banner year for the HiFi movement. Capehart introduced a beautiful, extravagant wooden console with clear Lucite knobs on their tuner-preamp, a push-pull parallel 2A3 triode amp made by Lincoln Walsh’s Brook company, including Capehart’s always amazing, record flipping changer, along with a full-range, very special 12 inch PM speaker by Western Electric. The key hi-fi element was their intro of a magnetic phono cartridge pickup in late 1946, the first public offering of the GE Variable Reluctance phono cartridge.

Quickly, in 1947, GE introduced a wider range, lighter tracking phono cartridge, complete with a user-replaceable stylus assembly, the RPX-040. This immediately gave serious competition to the pickups being used in the broadcasting world. Stateside, RCA, Western Electric, Radio-Music Corp., Fairchild, Audax and Clarkstan were familiar tonearms and cartridges in sound studios and TV/Radio stations. Before the intro of the RPX-040 by GE, the Clarkstan pickup was known as the highest fidelity magnetic phono cart, even though it used a needle type cantilever. It tracked at 10 or more grams, depending on the tonearm. The GE RPX could track 78s at 6 grams, in spite of GE originally publishing 8 grams as necessary. Plus, the GE RPX had wider frequency response. The Pickering magnetic cartridge was soon introduced. Yes, indeed, HiFi was now well established in 1947.

The top of the line Brook amps were still hard set on keeping their “Natural Sound” headline using sweet sounding, 2A3 triode output tubes, even after McIntosh entered the scene in 1948, with their 50W1 and soon after 50W2 fifty watt amps using pp 6L6G output tubes. Radio sets were now also mostly using these lower distortion (on paper), beam power and pentode output tubes. Brook introduced a preamp, with a number of switchable Phono Playback Turnover and Rolloff choices. McIntosh would soon jump on the bandwagon with variable phono playback curves, as the Columbia 33 1/3 RPM LP was introduced in the summer of 1948. Of course, the record disc “speed wars” would really happen in 1949, when RCA introduces their 45 RPM record players and 7 inch, safely stackable discs. If there wasn’t a previous reason for a quality home HiFi set in every home, now was the time.

When Columbia introduced the Microgroove LP in 1948, their “Col. LP” playback EQ curve was very similar to the NAB phono EQ, but with reduced bass boost, including the second version NAB phono EQ’s 500Hz Turnover, bass shelving at 100Hz and the -16db at 10KHz Rolloff.

RCA’s 1949 introduction of their 45 RPM discs originally specified another unique playback curve, or simply phono EQ. In the late 1930s, RCA’s 78 rpm curve changed from a 500Hz Turnover to an 800Hz turnover, with a Rolloff specified as -8db at 10KHz, like their original, early RCA "Orthophonic" playback curve. However, for their 45s, the treble Rolloff was increased to -12db at 10KHz and Turnover was originally specified at 800Hz. RCA’s EQ curves did not yet include any bass shelving that could help to reduce any rumble noises from turntables and record players.

While this multitude of phono playback EQ curves can easily sound confusing, just imagine what music lovers went through at the time. Without the proper phono playback EQ on hand, some records simply would not sound as good as others. Using bass and/or treble controls in tandem helps, but these tone networks are always after an additional gain stage, where adding more EQ could inevitably change the specific sound characteristics of musical instruments. Well, certainly some music was better than no music.

When the Audio Engineering Society began in the mid late 1940s, their magazine publication arm was “Audio Engineering,” later known as the consumer periodical “Audio” magazine. The AES really steps up to the plate in 1950, issuing a serious recommendation that everyone conform to their averaged mathematically, “AES” phono EQ to enable standardization within the audio industries. A noble intention indeed, their premise of standardized phono EQ would increase sales throughout the production, recording, mastering, reproduction playback, equipment manufacturing and broadcasting industries. AES had one caveat: Recording/mastering engineers could use any sound techniques they choose, but they should ensure that using the AES phono EQ playback curve would still sound excellent. The AES specified a Turnover of 400Hz and a Rolloff of -12db at 10KHz. Bass boost was at the engineer’s discretion to stay within the limits of the reproducing equipment then available.

Recalling that the earliest acoustic 78s had little or no bass boost, but many early electrically recorded, stateside 78s had Turnovers of 300Hz or 500Hz while European and Gt. Britain recordings were using a 250Hz Turnover, the 1950 "AES" playback curve with its' 400Hz Turnover was a fine compromise, mathematically. Considering that pre-vinyl 78s were rarely quiet during playback, the AES Rolloff of -12db at 10KHz was a welcome feature for many studio and home listeners alike, for many discs of all speeds in their record collections. If you wish to read more about the AES playback curve, the Audio Engineering, January 1951 issue, Vol.35 #1, pages 22 and 45 include their important “Standard Playback Curve” essay. Included is a frequency response plotted curve, an R-C passive EQ playback circuit, a chart with + and - db at specific frequencies, as well as passive Inductance-Resistance (L-R) EQ playback circuits. The AES recommended that “bass boost should stop rising at a freq. determined by the range of the reproducing equipment.” Here’s the link:

While the AES playback EQ was becoming somewhat of a standard for 78s, many high quality record manufacturers chose this EQ for their LPs, EPs and 45s, including Blue Note, some Capitol, Mercury, Prestige, Riverside, Westminster, plus others, making it a very versatile playback EQ curve. These discs, have a very revealing treble response using a precision AES phono EQ playback curve on our preamps.

So, from 1948, into the early 1950s, while the Columbia LP EQ curve was specifying a -16db at 10KHz Rolloff, RCA allegedly used their unique 45 RPM curve with their discs of all speeds from 1949 thru 1952 specifying -12db at 10KHz, what could possibly come next ? Why, of course, yet another averaged mathematically, playback EQ curve. Only this time, we return “back to the future” in a way, since the 1952 intro of RCA’s “New Orthophonic” playback curve, specifying a 500Hz Turnover, Rolloff of -13.7db at 10KHz, plus increasing bass boost resonance/shelving to a lowered 50Hz peak, this EQ was destined to become the next “standard” in 1953, then endorsed by the AES. The RIAA recommended this New Orthophonic curve be adopted by all record manufacturers as thee standard in 1954. As agreed upon in October 1954 among many record companies, the RCA “New Orthophonic” EQ, now simply called the RIAA EQ, became the standard phono EQ in the USA during 1955.

In order to convince the creative recording production studios, mastering and cutting engineers, broadcasting, playback and manufacturing equipment companies to all upgrade, modify, retool and manufacture new components, this did take some time, in spite of the contractual arrangements. Some companies adhered to the RIAA curve quickly. Others took months and some took years to comply. The notable for sound quality, Westminster Labs label continued to also use NAB, LP and AES curves with certain artists, specifying which playback curve to use on their LP rear covers, well into the late 1950s.

As confusing as this all seems, before the RIAA playback curve standard, there really was no standard. If you have some pre-’55 mono discs, many of these will still sound nice with our RIAA preamps. However, if you play a disc encoded with AES using the RIAA curve, you will be missing some upper midrange and definitely missing high freq. details, which you would only know about if you had the AES curve available on your phono stage. Conversely, if you played an RIAA encoded disc using the AES curve, you might sense more high freq. detail, but there might be added scratchy noise, as well. If you could quickly switch in the correct phono EQ, your ears will quickly know which sounds right. If the disc was mastered with a certain recording curve, having the corresponding playback EQ curve available will sort it out quickly, much quicker than fiddling with later gain stage tone controls, graphic or parametric equalizers. Our ears/brain will recognize which switchable phono EQ sounds more natural.
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Let’s think about playing an early Columbia LP or an NAB transcription disc with the RIAA EQ curve. First, the RIAA EQ will cause increased bass with these discs, sometimes a good thing. The turnover is the same 500Hz, but the Col. LP/NAB treble rolls off quicker than the RIAA EQ, so the LP disc might sound a bit more detailed in the treble or sound a bit shrill because the RIAA EQ has less severe rolloff. Using discs containing recognizable instruments, your ears can again recognize the most natural sounding, given the choice. Playing an LP/NAB encoded disc with the AES playback EQ will instantly alert your ears/brain that the treble is too shrill. Quickly switch to the RIAA EQ, and that NAB/LP encoded disc will sound much better.

What about that 800Hz Turnover EQ, you ask ? Well, besides using it for the 1949-1952 RCA discs encoded with it, there are some other discs which might benefit as well. Since the bass boost turnover begins higher in the midrange, playing an RIAA or Columbia LP/NAB encoded disc with this 800Hz EQ curve could sound interesting, with noticeably rich mids, but the highs can sound shrill. The best application for this obscure “800” EQ is actually for playing acoustical 78s or dull sounding, early electrical 78s. The fidelity obtained from these discs will easily outperform any wind-up, acoustical horn playback system.

The musical instrument defining midrange will sound much clearer and the “800” curve’s treble rolloff will reduce the usual, hissy and scratchy noises, since these acoustical discs have little upper treble musical content. We can run into a problem with increased low frequency, rumbly, pulsing noises, since the lower midrange and bass are enhanced using the “800” type curve. Playing some acoustically recorded discs can include necessary compromises. Perhaps, a rumble filter will be needed to be switched in, or a reduction in bass by turning the bass control down will help, if available, should you encounter low freq. noise while playing these often dull sounding, bass deficient, acoustically recorded, antique discs.

Shall we have a graphic look at the various curves, below in Fig. 3 ? This comes from a “The Fisher” variable EQ phono preamp brochure, circa 1952-1953, which I annotated for our knowledge base. You can easily recognize how much more midrange and mid-bass gets accentuated using the “800Hz” Turnover playback EQ. Then, check the High Frequency Equalization Rolloff curves between LP, RIAA and AES. Graphically, they look close, but sound wise, most listeners will easily discern which sounds correct, if you have the ability to quickly switch-in and choose the most natural sounding EQ.

For us “spinning mono discs in real time” fans, having a switchable set of playback curves in our preamplifier is the way to obtain the highest fidelity, with the right stylus tip on our phono cartridge transducer.
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Stylus shapes and sizes can also help or hurt our quest for the most natural sound with early mono discs. Use the wrong size and the noises could get louder. Use the right size and the music can snap into focus, overriding any noise. For early 78s, the 3 mil spherical tip is most recommended. Well worn early 78s might benefit from a 3.2, 3.5, even a 4 mil wide, specialist tip, which can help the stylus from not hitting bottom, where the noise is certain. There are truncated, rounded tips available, specifically for this reason. There are elliptical 78 tips and truncated ellipticals, at a cost.

The Transcription discs, those 16 inch discs, originally spec a 2.3 mil tip, served well by a 2.5 mil tip. Even a 2 mil tip will work, but it might swish side to side in some grooves. The usually available 2.7 mil or 3 mil tip will work with worn ones, sometimes.

The vinylite 78s were made for the 2.7 or 3 mil spherical tips, at first. Later vinyl 78s, circa 1954-55 are said to sound best with a 2.5 mil spherical. YMMV…The professionals who transfer these discs have an arsenal of at least one of each size stylus tip available for their reference cartridge, in order to transcribe each disc for the best sound, according to their needs.

Since I seek good sounding 78s from all eras, I usually spin with a 3 mil tip. If needed, I have a 3.5 mil, truncated elliptical diamond tip, which is a nude mounted, jewel-like beauty of a stylus tip, made by Expert Stylus, UK Ltd. decades ago. I also have a 2.7 mil, bonded elliptical on a cantilever waiting, but I rarely use it. When I sample average condition discs, I typically use a 3 mil or 2.5 mil sapphire conical/spherical. Sapphires wear quickly, but they run much cooler than diamonds, rarely “skipping” or getting stuck in the groove. Sound wise, with 78s, sapphires are my “go to” as they sound smoother than the much more durable diamonds, IMHO. The sapphire tips need replacement, long before their pre-published, rated fifty hours playing time. The AES, in 1950, recommended 15 hours playing time before replacement. Fortunately for me, I have been collecting GE RPX and Pickering V-15/Stanton 500/ Stanton Calibration Series 581 styli for decades.

With early mono LPs, EPs and 45s made before 1966-1967, a 1 mil, 25 microns wide, spherical stylus tip is usually my choice. You can use a “stereo” size .7 mil, 17-18 microns wide tip with these discs, but they can sometimes sound like they splash around in some of these microgrooves. Then again, there are some pro-transcribers who sometimes use a Line Contact or even Shibata type tip, in search of the best sound from each specific disc.

However, here comes the warning: Do not use a vintage, mono cartridge with a 1 mil tip on your stereo discs. Most early mono microgroove discs have only lateral, side to side groove info. If your mono cartridge has no vertical sensitivity (compliance) and no vertical output, it can ruin a stereo disc rather quickly. More modern mono carts, favoring more modern mono discs, can be limited as to which records they sound best with.

Stereo cartridges can be employed for all discs of all speeds, using the correct stylus, of course. Be aware that a stereo cart has lateral and vertical compliance sensitivity and output, since the stereo grooves were mastered/cut with a 45 degree by 45 degree alignment. So, one half of a stereo cart’s output is lateral and one half is vertical. This is why a stereo cart used to play early mono discs can produce too much noise during playback, even when we press our Mono switches to “sum” the L + R channels. Summing the channels at the headshell or using the mono switch reduces the scratchy noises, but never fully eliminates them. With scratchy early mono discs, a mono cartridge with only lateral output wins for the lowest noise floor award.

Stereo LPs begin in the summer of 1958, specifying a .7 or .5 mil, 15-17 microns wide, stylus tip. Stereo 45s did not really begin until the later ‘60s, for the public. There were Promo copies of the same song in Mono on one side with Stereo on the other for radio stations, plus some obscure early stereo 45rpm rarities, but most stereo 45s to the public were delayed until the later 1960s, unfortunately.

Growing up, baby boomers all remember hearing a new 45 over the radio on our way to school, work or play, only to stop at a record store on our way home to buy a copy, or even spring for the LP album. The single 45s either made or broke a band during that era. So, if you luck out and find some original, early to mid-sixties 45s you always liked, those discs were mastered/cut with a 1 mil wide tip, standard microgroove playback fare. Your stereo styli will work, but they can sound a bit scratchy/splashy. Many of these 45s even look like they have wider grooves, so please be aware. This is just another reason to build your own arsenal of carts and styli…

Researching vintage cartridges and replacement styli, especially from the stereo era, some phono cart and sty manufacturers offered what I call "wide ellipticals" specified as .9 or .8 mil wide, as opposed to the usual .7 mil wide stereo stylus tips. The first Shure V15 had an elliptical .9 mil wide tip. Pickering/Stanton offered ellipticals for numerous cartridges, listed as .9 mil wide, well into the 1970s. I have some .4 x .9 mil ellipticals which I cherish. Knowing which model #s to look for can net you some great sounds listening to your cleanest discs, in hopes of eliciting the best high frequency details from your early mono discs.

Keep on spinning the discs and Keep on Groovin’….
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So, how can you design your own phono equalization networks for your preamp, you wonder ? There are numerous sites which have calculators for some of the needed formulae. There are literally hundreds of pre-existing phono EQ circuits which you can emulate and build. Whether you use online calculators, pre-existing circuitry, or utilize known formulae for your own math and DIY exercises, you still will want to verify your end results with test gear. If you simply want to verify that your existing stereo RIAA phono stage adheres with the standards, some math analysis can verify compliance, but you might want to actually test the results. The same test gear is appropriate.

Before we get to the analytical or design math, let's understand what gear might be needed. Here is a tried and true, old school method. At the input of your phono stage, you would inject constant level test tones, from 20-20KHz, with a signal generator. Also at the input, connected from input to ground, would be a known, high quality, inverse RIAA curve, passive EQ circuit. Your results are only as good as this inverse network, so choose wisely. There are some available, rather inexpensively, which get the job done very well. At the output of your phono stage, impedance matching is critical, as you will be measuring each successive tone with a microvolt sensitive voltmeter, calibrated in smaller than or equal to 1/4 decibel increments. The signal generator output and AC millivolt meter must be calibrated. Charting the results is needed, for each test tone. Then, you can produce a frequency response plot.

The original publications for any and all of the phono playback curves specified a + or - 2db from the original curve's tolerance as acceptable. Well, that is just okay, not fine enough for design, laboratory or engineering folks. Fortunately, the original phono EQ publications often included a chart of frequency vs. level. This has enabled some phono stage designers and manufacturers to specify their phono EQ circuits as " + or - .1 db from 20-20KHz." However, the question will always remain: Is that + or - .1db of the accepted + or -2db tolerance for the RIAA, or any other published phono EQ curve you are testing ?

Reading the RIAA analysis treatise by genius sound engineer Stanley Lipshitz, as linked below, most of us will think it involves way too much trigonometry or very complicated algebraic math. Stan included analysis of all types of phono EQ circuitry, passive and feedback types. Stan knew that many of the readers would not be mathematicians, so he cleverly included this investigative formula method which works very well to verify the Turnover frequency: (R1 x R2) divided by (R1 + R2), then the resulting multiplier # is multiplied by (C1 + C2). The resulting Turnover # is in uSeconds, or uS or uSec. Then, the mathematical, uS constant for 1Hz, the # 159,155 is divided by the Turnover # in uS, to yield the actual frequency. This Turnover is the +3db point from the flat portion of the curve, the asymptote, which represents the beginning of the upward slope toward the bass boost. With my favorite, classic active feedback phono EQ circuitry, this formula is essential. If you prefer passive EQ phono preamps, the numerous online calculators can quickly and easily help your passive EQ endeavors.

Typical active feedback phono EQ equipped preamps have two, parallel connected resistor-capacitor networks, which we call poles, connected in series. In the above formula, let us simplify the bass "boost" pole network as R1 in parallel with C1, plus the treble Rolloff pole network as R2 in parallel with C2. Combined mathematically, they "derive" the actual Turnover.

Each network "pole" can be deciphered easily with R x C multiplication computation, yielding the + or - 3db point (Asymptote) from the flat portion of the frequency response. The flat portion of the RIAA curve is very small. It ranges from a point slightly above the 500Hz Turnover to a point slightly below the Rolloff asymptote, or transition point, 2122Hz for RIAA Rolloff. In all playback curves we would concern ourselves with, reference 0db is at 1KHz, 1000 cps.

Ready to analyze a typical, classic tube phono preamp type phono EQ circuit ? These numbers are similar to both an upgraded Dynaco PAS and an upgraded Eico HF-85 RIAA phono EQ circuit. These R-C networks in series work well for both, plus the Eico HF-81 integrated amp, as well as more than a few other tubed phono preamps using 12AX7 tubes. In the following, ( // ) means "in parallel with" and ( & ) means "in series with:" R x C = TC (Time Constant) in uS. R is in Ohms while C is in uf or microfarads. 159,155 / TC = Frequency in Hz.

2800pf // 2Meg Ohms & 830pf // 91K Ohms
2,000,000 x .0028 = 5600uS. 159,155 / 5600 = 28.4Hz. Bass "resonance" is at 28.4Hz.
91,000 x .000830 = 75.5uS 159,155 / 75.5 = 2108 Hz. Treble Rolloff is -3 db at 2108Hz, close enough to the RIAA spec of 75uS or -3db at 2122Hz, for this exercise. The two networks interact to derive the Turnover:

Using the Turnover formula as above: ( R1 x R2 / R1 + R2) x ( C1 + C2) = Turnover in uS. 159,155 / # in uS = Turnover in Hz.
2,000,000 x 91,000 / 2,000,00 + 91,000 = 87, 039.69 the "Multiplier." 87, 039.69 x .003630 = 315.95uS. 159,155 / 315.95 = 503.244Hz.

A 2800pf cap can be found from testing 2700 or 2750 pf caps on hand with a cap tester, or a trusted LCR meter. A 91,000 Ohm resistor can be found by testing 90,000 or 91,000 Ohm resistors, using a trusted Ohmmeter, as found in a trusted VOM meter, like my Fluke 77 which is still in calibration after 30 years. In order to maintain these values, very small copper or aluminum "alligator clips" are used as heat sinks, for the part and connected parts' leads during soldering. If you like polystyrene caps, "quick soldering" techniques must be used in addition to the heat sinks.

Since the Turnover is derived from the two network poles in series, changing any value changes the Turnover. Let's say you want precisely 75uS for the treble Rolloff Time Constant. Okay, let's change the 830pf cap to an 820pf standard cap value which actually measures 815pf.
92,000 x .000815 = 74.98uS. Now, let's see how that affects the Turnover. The Multiplier 87,039.69 x .003615 = 314.648uS = 506Hz.

Thus, a lower uS number yields a higher frequency. Conversely, a higher uS# = lower frequency in Hz.

One more caveat is in order. If you simply "juggle" network parts' numbers and arrive at the right time constants, there is a known ratio of the bass cap to the treble cap which must be maintained of at least 3:1. The bass cap is at least 3x the treble cap. Otherwise, the networks do not interact properly and could end up reacting in an additive way, increasing bass too much while changing the treble rolloff detrimentally. You will not see this mathematically. This is also why proper test gear must be used to verify your results.

Through the years, online "programs" have been devised for sand or tube gear designing. There are also programs and apps with a pink noise generator (equal energy per octave), combined with spectrum analyzer graphic displays, quick deciphering and impressive looking frequency response graphs, instantly obtained. The "looks good on paper" concept should still be verified by objective testing techniques, using calibrated test equipment. When it sounds right and measures right, you can then be sure what you are hearing is accurate.
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Here are some very informative links for our audio journeys with vintage and modern discs:

Playback equalization for 78 rpm shellacs and early 33⅓ LPs - Audacity Wiki a seriously admirable spinning disc p layback EQ compilation by a site specializing in digital transfers. You can DIY your own transcriptions online, without preamps, if you choose. They have some amazing programs for the pro-transcribers plus essential, informative reading for all fans of spinning audio discs.

CURVES OF OLD RECORDS Reference Grade Charts by Label, A-Z & Year ! + a Multitude of Reference Grade playback info for decades ! Continuous Thanks are due to Mr. Robinson for his midimagic.sgc-hosting site. His wealth of research and masterly competence with Phono Playback EQ is evident in his publishing. Very worthy of exploring his publishings !

Time constant and cutoff frequency calculator upper and lower corner frequency RIAA frequency response break RC pad tau and f RIAA NAB CCIR DIN FM conversion cut-off cut off EQ filter emphasis Pre-Emphasis De-Emphasis preemphasis deemphasis - sengpielaudio Sengpiel Berlin Truly, a SOUND ENGINEERING REFERENCE with all useful formulae, plus easy to use calculators. REFERENCE Information !

Stereo Lab - Historical Recording Characteristics Great info and history from a European/UK perspective. Europe and the United Kingdom used their own set of phono EQ curves until well after the RIAA curve appeared. Scroll down 2/3 of page 1 for the recording curve comparison. Remember, these curves are the inverse of their playback curves. Many Thanks for the perpetual treasure of useful knowledge shared and presented here. From this link, at the bottom of page 1, the Home Page is now their online StereoLab transcription suite site.

Reproduction of 78rpm records Essential info and playback charts. 78 EQ Chart.pdf Very informative. Nice charts !

Equalization of Older Records Extremely Informative ! - Sie sind im Bereich : Sound-Restoration-Teil 6 great site; super informative, features legendary British Archivist P. Copeland's phono playback treatise. Thank You for sharing this info worldwide.

https://jelabsblogspot/ Scholarly gentleman, professional musician/music listener, sound engineer and perpetual craftsman, using professional, tried and true, sound audio practices. Fantastic Site, info, with excellent visual sights and systems from worldwide DIY folks. Passive R-C Phono EQ Design calculator. Tremendous Thanks for sharing the info and providing phono playback goodies.

PHONO PHONOGRAPH PICKUP CARTRIDGES, 78 RPM needle, CUSTOM PHONO STYLI, NEEDLES Cool site...links to their cartridge & STYLI extensive info, suggestions, descriptions and catalog offerings. Transcription Specialist equipment supplier. Thanks for sharing the info and providing phono playback goodies. This is Thee Phono Playback EQ Design/Analysis treatise from the Jrnl. of the AES, June,1979. Very analytical + informative, plus, a mathematical reference.

The Ultimate EQ Cheat Sheet for Every Common Instrument Fun & informative reading. Very Helpful to develop your own ears and craft.

RADIO NEWS MAGAZINE - Hugo Gernsback's electronics magazine starting 1919 links to Radio & TV News library. Radio News, later called Radio & Television News was a gift of information in every issue.

In the beginnings of the home HiFi movement, post WWII, esteemed sound engineer Oliver Read began publishing his later textbook in almost every issue. His book, “The Recording and Reproduction of Sound” was published by Howard W. Sams & Co, Inc. in Indiannapolis, Indiana circa 1952 (2nd edition). It is essential reading for vintage sound studio info seekers and anyone interested in record disc cutting and playback. Whenever Oliver Read includes info about Phono EQ, he inevitably includes authored info from and refers to another expert sound engineer, Charles P. Boegli of the Cincinnati Research Company.

Passive Phono EQ devotees will delight in reading the Boegli writings, which can be read from the above link. These issues are very worthy, listed from the latest to his initial preamp phono EQ essay:

“New Developments in Phono Equalizers” in R&TV News, April 1953, V49 #4, pages 54, 55, 56, 102, 103.

“Phono Equalizer Chart” in R&TV News, November 1952, page 94.

“An Improved Equalizer-Preamp” in R&TV News, April 1951, pages 46, 47. (12AY7 preamp)

“A Preamp for Magnetic and Crystal Pickups” in R&TV News, July 1950, pages 36, 37, 38, 120, 121. (6SL7 preamp)

These vintage, passive phono EQ circuits that C.P. Boegli designed are still very worthwhile today. The R&TV News, April 1953, V49 #4 issue, with updated EQ circuit charts, includes refinements that are still used and will always be considered reference grade.

RADIO and BROADCAST HISTORY library with thousands of books and magazines now known as perhaps a more legible copy of the original AES EQ.

The Genesis Of Vinyl Stereo Record - Entstehungsgeschichte der Stereoschallplatte If you scroll down about 2/3 of the page to Appendix 1 EQ info and Table 1.1 EQ comparisons, you might find this and all the information very helpful.

Thanks to all of these sites for "sharing the knowledge, so that we can all share the wealth of enjoyment from listening to music."
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