Fisher 50-PR/ 50-PRC = Variable EQ Phono Preamps, circa 1953....Design + Retrofit

[tubeactive]

Definitely a touchy tube of sorts...the shared cathodes limited preamp design choices. When grid leak biasing was popular, simply grounding cathodes and using very high Ohmic value grid resistors, plus needing an input coupling cap, the phono EQ Bass boost/Turnover network designs usually used very high Ohm value or no resistors across the bass EQ cap in order to provide flattest response. Note the Fisher factory's 22 Meg across the bass/turnover caps. Of course, the Columbia/NAB "LP" EQ always required a much lower value resistor across the bass/turnover EQ caps.

With no resistor, or a very high value R across that network's cap, the bass boost slope continues to boost into the subsonic region, which can be problematic for rumble/tt bearing and mismatched cartridge/tonearm setups. With mono era amps and preamps, rumble was not yet an issue. With our "thrilling dual mono" setups using our stereo (or dual mono) amps and stereo speakers, more precise phono EQ networks and designs were needed.

In time, I might eventually convert the 50-PRC to 12AX7s, 6SL7s or build an "inspired" version on different chassis...

 

[tcdriver]

...In time, I might eventually convert the 50-PRC to 12AX7s, 6SL7s or build an "inspired" version on different chassis...

The Fisher 50-C or Heathkit WA-P2 would make excellent starting points for 12AX7 based designs.

 

[tubeactive]

tcdriver, There are lotsa cool old preamps. Do you have a 50C ? I tried a very nice one. The split passive EQ among two gain stages doesn't seem to do it for me. I always prefer the feedback EQ. That's why I traded away a pair of the 50-PRs long ago. I should have kept them and rewired the 50-PR passive Rolloff EQ into the EQ arrangement I prefer, as in the super-rare 50PRCs. For pre-RIAA mono discs, especially many 78s, passive EQ can indeed sound very nice...

The 50 PRCs, circa 1953, are among the first preamps with "classic" active feedback EQ topology for both Turnover and Rolloff using the same tube, available to the public. Certainly, the 50-PR and 50-PRC are also among the first Variable EQ, switchable with levers, phono stages. So, they must be one of Avery's Fisher's genuine Firsts in High Fidelity......


Fisher 50Cs are early '50s, also pre-RIAA. I wonder what the last year was that Fisher still offered the 50C ? Probably, 1953 or early 1954...I prefer the 80C and had a sweet pair in smooth cases; Lovely sounding phono. 80Cs are like genuine mixers, too. The 80C definitely gives and gave the Marantz One and Mac C8 sound competition...For my stereo pre, it's usually a retro-resto upgraded HF-85 Eico, Paragon E or Lafayette KT-600. The Lafayette has intense phono EQ versatility. My current, reference HF-85 has three phono EQs available: RIAA for 2 sets of stereo input jax and the Tape jax input for "AES" or Col/NAB "LP"

The dual mono preamps have no stereo "dimension" or Blend control or Mono/Stereo and Stereo Reverse switched easily....

 

[tcdriver]

tcdriver, There are lotsa cool old preamps. Do you have a 50C ?

Yes, I have three Fisher 50-C preamps with three different sets of phono equalization choices. They are in the
queue, waiting to be rebuilt.

...Fisher 50Cs are early '50s, also pre-RIAA. I wonder what the last year was that Fisher still offered the 50C ? Probably, 1953 or early 1954..

There was an Fisher advertisement for the 50-C, 50-A and 70-A in the January 1955 issue of Audio magazine (page 43).

 

[tubeactive]

Cool...Wow ! You have three differing versions ? Did they list the RIAA EQ on the fascia as "New Orthophonic" on the few versions you have ? Or, did they print "RIAA" in any of the versions ? What a long, sometimes expensive journey to find truly matched pairs of this early gear....

 

[tcdriver]

Cool...Wow ! You have three differing versions ? Did they list the RIAA EQ on the fascia as "New Orthophonic" on the few versions you have ? Or, did they print "RIAA" in any of the versions ? What a long, sometimes expensive journey to find truly matched pairs of this early gear....

On the latest version, there are five EQ settings each for LF and HF. They are marked as follows:

LF Turnover: AES, LP, New Ortho Standard, NAB, 800
HF Roll Off: 0 Flat Response, -8, -12, -13.7 New Ortho Standard, -16 LP NAB​

It seems that RIAA is named New Ortho Standard on this late 50-C version.

 

[tubeactive]

Awesome...Historical stuff....When you open that one up, it will be interesting to almost know the date of manufacture...I guess 1954...As you know, even though the audio manufacturers and record companies "agreed" on the RCA New Orthophonic EQ to become the standard, there were indeed holdouts or delayed compliance among the industry.

Some of my mono, pre-RIAA (new Ortho) "demo" discs, known to be recorded with the AES (1951) EQ, sound absolutely fantastic. With EQ able to be quickly changed, recognizing which is right is almost immediate. One of my usual demo discs actually makes me wonder why the AES was not the everlasting standard. That jazz disc is so alive and fast sounding, with piano fingering and drum strikes sounding like "real time" with incredible dynamic range.

tcdriver, I look forward to your 50C threads down the road...

 

[tcdriver]

Awesome...Historical stuff....When you open that one up, it will be interesting to almost know the date of manufacture...I guess 1954...As you know, even though the audio manufacturers and record companies "agreed" on the RCA New Orthophonic EQ to become the standard, there were indeed holdouts or delayed compliance among the industry...

I l dragged my Fisher 50-C and looked for date codes. Unfortunately, the original yellow tag was missing. Inside the unit, I found the code 646 413 on the high voltage electrolytic capacitor and 137 417 on the input level pots. This would suggest, my late version Fisher 50-C was assembled some time after the first week in June, 1954. According to the Audio Cyclopedia the RIAA standard was originally adopted by the disc recording industry in June, 1953. I do not know if there was a later Fisher 50-C with RIAA markings.

The Heathkit WA-P2, considered by some as a circuit copy of the Fisher 50-C, was originally released in 1954 and it has RIAA marked positions on both the TURNOVER and ROLLOFF equalization switches.

 

[tubeactive]

Interesting....With the "Audio Engineering" periodical, later evolved into "Audio" magazine, publishing industry events and news within a month of them occurring in the AES, RCA engineer C. Moyer published "Evolution of a Recording Curve" in the June 1953 Audio Engineering issue.

www.lencoheaven.net/forum/index.php?topic=25066.0

I am under the impression the AES, Audio Engineering Society actually quickly recommended this "New Orthophonic" playback curve and adopted it to become the new standard. However, the EIA (Electronics Industry Association) and the RMA (Radio Manufacturers Association) as well as the RIAA had a bit of convincing to do in order to persuade all of the industry professionals among the manufacturers, production and recording studios plus the powerful record companies. Apparently, sometime between this 1953 publication and 1954 was the "year" the RIAA approved and adopted the New Orthophonic playback curve, but was still undergoing industry compliance. As the AES EQ was somewhat of an averaged curve among the many available 78 rpm curves circa 1951, the commercial successes of certain record companies, like RCA, must have brought some politics into the mix, so to speak. Only two to three years after the 1951 AES curve was agreed upon, the record industry was changing, again. The RCA New Orthophonic curve could also be considered an "averaged" EQ curve, as the industry was forced to press records equalized to sound natural somewhere between the accepted AES and NAB/LP recording and playback curves. And, this was only stateside....

While 1954 is considered the "acceptance" year, it has been written in many publications that 1955 was the actual industry "compliance" year. However, some industry pros, especially producers and engineers within the recording studio and record mastering worlds, still retained their "signature sound" and were still hesitant to actually fully comply with the specific New Orthophonic, now known as the RIAA playback EQ curve.

Looking at the 50-PR and 50-PRC fascia, their 1953 labeling of the "New Orthophonic" EQ as the -12 db Rolloff position, (New Ortho/RIAA is -13.7 db@10KHz, not -12 db), was still indicative of an industry's state of flux.

 

[fred soop]

And we are worried about getting to ±0.1 dB response in the phono preamp.

 

[tubeactive]

Who is the "we..." ? Original publications of the curves in Audio Engineering actually indicate adhering to + or - 2 db over the response 30-15KHz is within spec...

However, as the AES publications of the AES and RIAA (New Ortho) curves actually specify the db boost or cut at many freqs, the industry continues striving to lower the tolerances. So, would + or - .1 db of the original curve's spec (+ or - 2 db from 30-15Khz specified by the RIAA) mean one tenth of a db more than the acceptable plus or minus 2 db ? LOL....

 

[fred soop]

Another problem is finding a graph with enough resolution to find the correct compensation for a given frequency. The lines on the graphs are often several dB wide and response between the frequency lines must often be estimated.

Someone prepared a spreadsheet (can't find the link at the moment) doing calculations for response individually for each pole and zero, which is accurate to 4 significant digits. Then the individual responses were algebraically summed for an exact table. There is a second calculation that includes the IEC amendment.

 

[tubeactive]

Calculations and then testing/measuring the circuit are the only way to ensure low tolerance response. Looking good on paper and sounding great is one thing. Using a reference grade, inverse RIAA (or other inverse EQ) circuit, then inputting calibrated, constant level tones, while monitoring the preamp circuit's output with a calibrated, AC Microvolt meter, is the engineering method of choice.

Fancy simulation and spectrum analyzers look great in photos and on paper. Expensive test gear can work, no doubt. When you can read uV increments and plot the flattest curve as + or - 1/4 db, via old-school technology, that is plenty precise, IMHO.

Unless you are manufacturing, rather than strive for tighter response, how about spinning some more discs ?.....

 

[tubeactive]

Summer Greetings ! It sure is hot in the east...I am wondering if anyone has seen any 50-PR preamps with the "later" 50-PRC type topology, using active feedback for both bass boost/Turnover as well as the treble Rolloff circuitry. In 1954 articles and ads, Fisher claims the 50-PRC is..."identical to the 50-PR, but with the addition of a volume control..."

 

[Plato65]

Summer Greetings ! It sure is hot in the east...I am wondering if anyone has seen any 50-PR preamps with the "later" 50-PRC type topology, using active feedback for both bass boost/Turnover as well as the treble Rolloff circuitry. In 1954 articles and ads, Fisher claims the 50-PRC is..."identical to the 50-PR, but with the addition of a volume control..."

I have two that I think are of the later model.

 

[tubeactive]

You and I are quite lucky. Pairing these pre-stereo and pre-RIAA, early '50s phono stages was not easy. Have you modernized the phono EQ R-C parts
values ? I think we both posted at a thread or two on lencoheaven in recent past, yes ? Best Regards !

 

[Plato65]

I did modernize and wanted to change the adjustments to be more relevant for my record collection, but I had the wrong schematic so the values went wrong.

 

[tubeactive]

I certainly hope this thread can help you retrofit the right parts' values...

 

[Plato65]

Definitely, but a bigger picture of the schematic would be very valuable in that regard.

 

[tubeactive]

OK...Happy to oblige you...Here is my latest scheme...











I sure hope these pix are better for you all. I use very matched parts per channel, matched with my calibrated VOM and LCR meters. With matched 105,800 Ohm resistors for the HF pole and 2.2 Meg. for the LF pole, these values can only be retained utilizing careful soldering with heat sinks applied liberally to all leads involved while soldering. The capacitors, especially polystyrene types, are very fragile with regards to heat. For the best, lowest tolerance results, measure the parts' values after the soldered parts have cooled down. In order to adhere precisely with mathematical calculations, heat sinking is necessary.

My EQ parts' choices for my mostly stateside pre-RIAA discs, will probably differ from European collectors, like Plato65. I chose to keep the "0" HF Rolloff position "open" for no capacitance across the 105.8K resistor, but this is optional. For the "-8" switched position, I chose 471 pf for a 50 uS HF time constant, close to the -10.5 db@10KHz as requested. If I wanted a -12 db@10KHz for the actual "AES" phono EQ adherence, I would choose 600 pf. If I wanted -11 db @10KHz, I could use 534 pf. These would be switched in, parallel with the 105.8K resistor.

In my units, my "-12" HF position is actually set for -13.7 db@10KHz, adhering closely with the genuine RIAA/New Orthophonic phono EQ. This requires 707 pf, switched in across/parallel to the 105.8K resistor. That provides the needed 75 uS (74.9) uS time constant.

My "-16" position, adhering with "NAB/Columbia LP" phono EQ, requires a 100 uS time constant for -16 db@10KHz. Thus, I chose 945 pf to be switched in across the 105.8K resistor.

For my bass boost/turnover poles, I use a 2.2Meg resistor across the switched in cap:
3200 pf for the "AES" EQ adherence.
For the "New Ortho/RIAA," I use 2450 pf.
For the "NAB/LP" that same 2450 pf and a 750K resistor get switched in across the 2.2Meg.
For the "800" EQ, 1200 pf was chosen. 1150-1200 pf will work splendidly.

Lastly, for this post, how about a quick math refresher, regarding analysis for phono EQ circuits according to the Lipschitz/Jung writings ? I know, it can be boring, but needs to be remembered for these exercises and applications. Let us call the bass boost/turnover resistor 2.2Meg = R1, the rolloff resistor 105.8K = R2. The bass boost/turnover cap is C1 while the HiFreq. rolloff cap we will call C2. Thus:

(R1x R2 divided by R1 + R2) multiplied by (C1 + C2) = Time Constant in uS (microseconds). In order to convert to frequency in Hz, the math constant 159,155 is used, divided by the obtained uS, yields the actual Turnover frequency in Hz.

Noting that the look of simple "product over sum" math methodology for "resistors in parallel" is used, while caps in parallel get added together, the formula for RIAA/New Orthophonic is: 2,200,000 x 105,800 divided by 2,200,000 + 105,800 yields the "multiplier 100,945.44. Then,
100,945.44 x .003157 (which is C1 + C2 or .002450 + .000707 =.003157) = 318.68 uS Turnover time constant, close to the 318 uS (500 Hz) specified by RIAA.

159,155 divided by 318.68 results with the actual +3 db point of Turnover = 499.42 Hz, very close to the 500 Hz turnover as needed for precision RIAA phono EQ. This actual point of turnover, is often referred to as an "asymptote" or transition.

For the "AES" EQ values, the same multiplier 100,945.44 x .003800 = 383.59 uS
159,155 / 383.59 = 415 Hz, adhering nicely with the AES phono EQ needs.

I hope this enlightens all concerned....