The Fisher "Series 50" components included some cool items. The Fisher 50 phono cartridges were rebranded, plated Fairchild pickups. Besides the legendary 50A and 50AZ amps, which this series undoubtedly revolved around, Fisher offered some novel, cute, plasticized, genuine imitation bakelite cased preamps with switchable phono EQ, same cased mixer/faders, as well as same cased Hi/Lo Filter units. The 50-PR and 50-PRC preamps used the 6SC7 tube, while the mixer/fader unit and filtering unit used the 12AX7.
This pair took me over 12 years to match up. Along the way, I also had a pair of 50-PR preamps, looking quite similar, with a pilot light jeweled bezel where the volume control resides on the 50-PRC units. While I enjoyed the 50-PR preamps briefly, the 50-PR uses active feedback EQ for the bass Turnover EQ, but then utilizes passive EQ for the treble Rolloff, after the gain stages.
In spite of the 50-PRC advertised as "identical with the 50-PR but includes a volume control...," the later 50-PRC actually uses active feedback EQ for both Turnover and Rolloff. Now, that's the way I like it....
Since these units were manufactured before the RIAA phono playback EQ became the industry standard, the switchable EQ resistor-capacitor values do need some tweaking and upgrading to include the actual RIAA or "New Orthophonic" playback curve EQ.
Note that the fascia panel has Rolloff positions for 0, -8, -12 and -16 (db@10KHz), but the "New Orthophonic" soon to be known as the RIAA EQ, requires adherence to a playback curve with -13.7 db@10KHz. The AES EQ and (old)Orthophonic EQ both used -12 db@10KHz curves.
I believe the only record manufacturer using the -8 (db@10KHz) Rolloff position would be RCA for 78s and their new 45 rpm phono EQ, used from 1949 until 1952, when RCA then introduced the "New Orthophonic" EQ. However, these preamps seem to indicate the industry was still in a state of flux concerning standardization. RCA also used the rare "800 Hz" Turnover with some of their 78s and 45s from 1949-1952. Are you confused yet ? I was, for a while, until I seriously researched pre-RIAA Phono EQ circuitry. With only four Turnover and four Rolloff positions, some design compromises had to be made, especially considering the 1951 "AES" phono EQ was expected to remain/become the real industry standard.
With LPs arriving in 1948, courtesy of Columbia (CBS), the older "NAB" Phono EQ was modified into the "LP" playback curve EQ, with a bit less bass boost than the NAB (later known as NARTB) EQ. The 45 discs, courtesy of RCA, arrive in 1949, bringing the industry's record speed and format competition, plus unique phono EQ, as mentioned above. Before the LPs, 78 rpm phono EQ used so many EQ possibilities, the AES, Audio Engineering Society, implored the record industry to standardize, by offering their unique, "averaged" phono playback curve EQ while politely suggesting that recording studios and disc mastering facilities can have all the freedom they desire in "creating" their unique, house sounds, provided the playback of their records sounded good utilizing the "AES" playback curve. This was indeed a noble effort to stimulate industry-wide sales. If discs kept selling, more equipment would sell, as the home HiFi movement was well under way. Standards were needed but they were slow to evolve.
Fisher "fit in" as best as they thought they could, with a degree of anticipation, yet real uncertainty concerning which phono curve EQs would reign. Just looking back at the era, with the TOTL "all triode" Brook amps and preamps appearing in 1946-47 and the much higher power McIntosh amp (50W1 and soon after 50W2) appearing in 1948, Fisher's early 1950s 50A amp chose a mix, with all triode tubes, including triode connected 1614/6L6 output tubes operated in Class AB2 for 50 watts. Fisher knew how to fit in...
The versatility of the 50PR and 50PRC are easily evident, as soon as you realize that the four Turnover and four Rolloff positions can provide sixteen different EQ curves ! Nevertheless, with bass equalization and treble equalization networks wired, and switched-in, in series, the two EQ networks "interact" to yield the actual Turnover frequency. So, with the faceplate labelled as it is, each labelled "Turnover" frequency is accurate with only one or two complimentary "Rolloff" positions.
I hope to be explaining and illustrating this variable EQ connundrum within this thread, while describing how I retro-resto upgraded the EQ circuits to accommodate more discs more accurately. For now, here are some more pics:
This pair took me over 12 years to match up. Along the way, I also had a pair of 50-PR preamps, looking quite similar, with a pilot light jeweled bezel where the volume control resides on the 50-PRC units. While I enjoyed the 50-PR preamps briefly, the 50-PR uses active feedback EQ for the bass Turnover EQ, but then utilizes passive EQ for the treble Rolloff, after the gain stages.
In spite of the 50-PRC advertised as "identical with the 50-PR but includes a volume control...," the later 50-PRC actually uses active feedback EQ for both Turnover and Rolloff. Now, that's the way I like it....
Since these units were manufactured before the RIAA phono playback EQ became the industry standard, the switchable EQ resistor-capacitor values do need some tweaking and upgrading to include the actual RIAA or "New Orthophonic" playback curve EQ.
Note that the fascia panel has Rolloff positions for 0, -8, -12 and -16 (db@10KHz), but the "New Orthophonic" soon to be known as the RIAA EQ, requires adherence to a playback curve with -13.7 db@10KHz. The AES EQ and (old)Orthophonic EQ both used -12 db@10KHz curves.
I believe the only record manufacturer using the -8 (db@10KHz) Rolloff position would be RCA for 78s and their new 45 rpm phono EQ, used from 1949 until 1952, when RCA then introduced the "New Orthophonic" EQ. However, these preamps seem to indicate the industry was still in a state of flux concerning standardization. RCA also used the rare "800 Hz" Turnover with some of their 78s and 45s from 1949-1952. Are you confused yet ? I was, for a while, until I seriously researched pre-RIAA Phono EQ circuitry. With only four Turnover and four Rolloff positions, some design compromises had to be made, especially considering the 1951 "AES" phono EQ was expected to remain/become the real industry standard.
With LPs arriving in 1948, courtesy of Columbia (CBS), the older "NAB" Phono EQ was modified into the "LP" playback curve EQ, with a bit less bass boost than the NAB (later known as NARTB) EQ. The 45 discs, courtesy of RCA, arrive in 1949, bringing the industry's record speed and format competition, plus unique phono EQ, as mentioned above. Before the LPs, 78 rpm phono EQ used so many EQ possibilities, the AES, Audio Engineering Society, implored the record industry to standardize, by offering their unique, "averaged" phono playback curve EQ while politely suggesting that recording studios and disc mastering facilities can have all the freedom they desire in "creating" their unique, house sounds, provided the playback of their records sounded good utilizing the "AES" playback curve. This was indeed a noble effort to stimulate industry-wide sales. If discs kept selling, more equipment would sell, as the home HiFi movement was well under way. Standards were needed but they were slow to evolve.
Fisher "fit in" as best as they thought they could, with a degree of anticipation, yet real uncertainty concerning which phono curve EQs would reign. Just looking back at the era, with the TOTL "all triode" Brook amps and preamps appearing in 1946-47 and the much higher power McIntosh amp (50W1 and soon after 50W2) appearing in 1948, Fisher's early 1950s 50A amp chose a mix, with all triode tubes, including triode connected 1614/6L6 output tubes operated in Class AB2 for 50 watts. Fisher knew how to fit in...
The versatility of the 50PR and 50PRC are easily evident, as soon as you realize that the four Turnover and four Rolloff positions can provide sixteen different EQ curves ! Nevertheless, with bass equalization and treble equalization networks wired, and switched-in, in series, the two EQ networks "interact" to yield the actual Turnover frequency. So, with the faceplate labelled as it is, each labelled "Turnover" frequency is accurate with only one or two complimentary "Rolloff" positions.
I hope to be explaining and illustrating this variable EQ connundrum within this thread, while describing how I retro-resto upgraded the EQ circuits to accommodate more discs more accurately. For now, here are some more pics:
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