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The Fisher Phono Preamp, and RIAA

Discussion in 'Fisher' started by dcgillespie, Jun 25, 2014.

  1. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    INTRODUCTION

    Over the years, I have generally defended the Fisher phono preamp design -- not to extol its absolute EQ accuracy, but simply to say that keeping it in perspective, it is not all that bad at all, and in fact, very much in keeping with the performance of that from many other manufacturers of the day. I've also defended the design against a rather (in)famous modern day review by a well known expert, as executed in a 500C receiver, wherein he rather summarily trashed their performance, based on testing conditions that were capable of anything but accuracy.

    In the early days of stereo where the RIAA curve's beginnings can be found, most manufacturers (including Fisher) didn't even provide specifications of EQ compliance, rather choosing to simply indicate that their unit offered a setting to produce the RIAA EQ curve. Where it was given (HH Scott for example), "good" compliance was indicated by a response curve that was specified as being within +/- 1 db of ideal. In the succeeding years of those heady days, the best manufacturers tightened this specification to produce within +/- .5 db of ideal as accuracy drew more attention, but measured accuracy within +/- 1 db was still quite common. Remember, the engineers of the day did not have CAD capability (let alone a computer), or even a pocket calculator for design work! All they had was a slide rule, chalk boards for group work, and pencil and paper for their notes and personal calculations.

    On the production end of things, the early stereo units (including Fisher) often employed discrete components in the EQ networks, which just as often, were often not high tolerance components, let alone matched between the channels. Some manufacturers (like Dynaco for example) did use close tolerance parts. But most (including Fisher) simply used standard 10% components, which then added their own flavor of error into the mix. Of course, time and use then further works its own brand of error into these components as well.

    In later years, Fisher and others used PECs for the EQ networks, which eased production issues, and also afforded more consistency between the channels and units of a given model. At the time that Fisher made this change, they also took the opportunity to tweak their (by then) almost universal RIAA network design (the X-202 however used a notable variation of it), to tighten the resulting curve produced for closer compliance. It wasn't a major change at all. In fact, it was small enough that so many decades ago when I first noticed the difference in component values, I assumed it more had to do with the manufacturing of all the EQ components into one small PEC package, rather than being the result of improving compliance accuracy. Today however, I realize that the move was indeed one of multiple goals, designed to ease production costs, improve consistency, and improve compliance.

    Overall however, when the front end errors from limited design aids were coupled with the back end errors produced from component tolerances (discrete or PEC), its no wonder then that a specification (in the rare cases where provided) -- and "good" measured performance in the lab -- only achieved compliance within a +/- 1 db bracket. Add in also a generally casual attitude towards compliance at the time, and you ended up getting what you got, relying on the better manufacturers to do a better job of achieving compliance, which they did, but which was still only typically within the aforementioned +/- 1 db. With a 2 db (total) spread however, the errors become audible. Today, this can be improved on.

    But improving the RIAA performance of yesterday's classics with today's technology can be fraught with its own set of problems. AK is filled with posts of those who have modeled various popular circuits, which they then proclaim as defective because the computer says so. Of course, these same people rarely if ever provide any proof of any such defect, instead relying on the computer to be judge, trial, and jury all at the same time, with their prowess to use it being king.

    Done properly, modeling can be a truly wonderful design AID, but should in fact be considered as just that, and hardly be counted on for the final word all by itself -- at least when used with vacuum tube circuit design. Accurate modeling in and of itself requires great knowledge of: (1) The modeling software capability, (2) The hard data it has been programed with, (3) A thorough understanding of electronic circuit design, which affects (4) The accuracy in which the soft data (the circuit to be modeled) has been input into the system.

    This is all on top of the realization that the results produced from modeling vacuum tube circuits should be considered as a ball park verification -- one that can be very close if the skill and effort are present to address all the myriad of variables involved -- but still, best used to verify actual results achieved, rather than taken as gospel all by itself. It's like a chain, which is always only as strong as its weakest link. Three of the four areas can be handled with incredible accuracy, but if one is off even by just a little, the results can then be way off from reality.

    These requirements then -- for me -- potentially bring into question 99.9% of the modeling results heralded by most diyers which is why I pay only so much attention to modeling results. Instead, I have always preferred to have measured results be the outcome of my efforts -- which is the safe bet, but surely is more time consuming without enlisting the aid of modern design tools. However, I am a creature who is lucky enough to know what I don't know, and since I don't know the intricacies of ACCURATE modeling, I simply don't do it -- but I do endorse it in the hands of those who truly know what they're doing with it.

    Against this backdrop, some of you may have followed my efforts recently with a Fisher 400C preamplifier ("A 400C Transformation"), and my goal to have it perform more in keeping with that of a Fisher 400CX/2. To date, those efforts have centered on improving the line stage performance, and eradicating the hum that so many of the stock units display. Along the way, I posted the some scope shots depicting the performance of the modified line stage, which perform admirably now with very little noise. I also posted some scope shots of the RIAA performance of the phono preamps in this unit, which still employed stock, but generally within tolerance EQ components for the RIAA network. They were less than admirable.

    To put a wrap on that project, I wanted to address the RIAA results produced. There was not only a discrepancy between the channels, but a notable discrepancy between both of them, and ideal. It just wouldn't be right to have the tail end of the audio chain then perform so well, but not the front end.

    As luck would have it, my efforts in working to improve the RIAA compliance in this unit were enhanced immensely by the efforts of George Ronnenkamp, who founded and operated Audio Regenesis. The quality of his work and the products of that company are without equal.

    George was indispensable in first accurately modeling the originally 400C phono preamp design (as judged against my own measurements), then comparing it to that produced by the updated design as used in the 400CX(2), and finally modeling the results of my work to improve them, by providing validation of the measured results I achieved.

    Modeling the basic Fisher preamp design presents problems from the get go, as apparently none of the modeling programs could properly deal with a grounded cathode (i.e. contact bias) 12AX7 circuit as employed in the second stage of the Fisher design. This was but one variable that had to be chased down, as were things like determining any differences between the reverse RIAA network he was using in his virtual models, versus the real one I was using in my lab. As I said earlier, modeling is fraught with potential problems, let alone those piled on by injecting vacuum tubes into the exercise.

    Through out the process then, and parallel with my own real world work, George was independently working on optimum values for the network based on what the model was indicating. Ultimately, the level of conformity we both achieved independent of each other, but verified both by his modeling work, and my real world measurements, was really quite amazing.

    Stay tuned!

    Dave
     
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  2. HiFiHarv

    HiFiHarv Active Member

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    CAINT WAIT, as we say down south.
     
  3. tube-a-lou

    tube-a-lou Super Member

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    This sounds great Dave, I always like the phono in my Fisher KX200 and X101C, But
    if it can be made better it will be great.

    Tube
     
  4. Moko

    Moko Member

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    Location:
    London,England
    I did all the mods recommended here...

    "The Fisher phono section will perform to world class standards with a few upgrades. Change the existing .01uF ceramic coupling capacitor out to .01uF Vitamin Q and change out the existing .022uF capacitor to a Russian K40Y-9 .022uF capacitor. The capacitors are located at the bottom of the 12AX7 socket. I have seen both film type and ceramic type .022uF capacitors on stock Fisher products. The Vitamin Q capacitor is most common in a 200 volt rating and the physical size is best vs 400 or 600 volt rated capacitors. I use 400 volt rated K40Y-9"

    And the phono stage sounds much better so any further improvements would be gratefully received, so keep up the good work.....
     
  5. sony6060

    sony6060 Super Member

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    The Vitamin Q & K40Y-9 capacitors are simple to install for better sound. But, more performance can be achieved. The RIAA upgrade. I thought about this, but realized the original design is not optimal using the original component values. I went no further, as not having enough tools to correctly redesign a RIAA.

    This is exciting as only three wires on the PEC is a simple upgrade for many to swap out. A kit in the future would be great.
     
  6. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    I replaced the PEC's on the EXECUTIVE's 800-C with identcal value parts, only change is tolerance of parts. All were 1%, and matched side to side for identical values. Compared to my standalone, there is not much improvement, but what there is is mainly in the mid range and female vocals. It's tighter, across the bandwidth, and orchestral LP's are putting me on the conductor's dias.

    I'd like to see Dave's improvements to install in my standalone and A/B with the Executive's 800-C.
     

     

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  7. sony6060

    sony6060 Super Member

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    An accurate RIAA should sound better balanced across the entire audio spectrum. This will require changes in component values. 10% factory tolerance is not good enough either and especially so if one has one RIAA channel tolerance say on the left and other RIAA channel on the right of center so to speak.
     
  8. notdigital

    notdigital AK Subscriber Subscriber

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    Always a pleasure watching Dave kick it up a notch :yes: :thmbsp:
     
  9. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Location:
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    Results

    The original Fisher RIAA network that started life in the phono preamps of their first piece of stereo equipment (the 400C) consisted of four discrete components:

    1. A sub-network consisting of a 2.2 meg resistor in parallel with an 1800 pF cap, with one end of this sub-network then connected to a 270K resistor,

    2. And a 390 pF cap then placed across free end of the 270K and the free end of the sub-network.

    The end of this overall network that consists of the 270K and the 390 pF is considered the input side (receiving the signal from the output of the preamp), while the other end of the overall network is considered the output side, supplying the equalized feedback signal to the cathode of the first stage.

    In my 400C, this produced the following results, relative to the ideal RIAA response curve:

    20 Hz = -.8 db

    30 Hz = -.6 db

    40 Hz = -.7 db

    50 Hz = -.8 db

    100 Hz = -1.2 db

    500 Hz = -.5 db

    1 kHz = +/- 0.0 db

    5 kHz = +.6 db

    10 kHz = +.6 db

    15 kHz = +.6 db

    20 kHz = +.5 db

    As mentioned in the first post, this performance would be considered as having good compliance back in the day. Visually, this performance can be seen in the phono preamp scope shots for channel B as supplied in the thread "A 400C Transformation", since the components in the network (and indeed the entire phono preamp section) for that channel were all good and of stock value, and within the tolerance given for the components specified.

    When Fisher went to using PECs for the RIAA network, they modified the original network slightly, keeping the same configuration, but changing the sub-network to consist of a 2 meg resistor (instead of 2.2 meg), and 1500 pF cap (instead of 1800 pF). The other two components in the overall network remained the same. This effort was a step in the right direction, but still produced rather loose conformity as compared to what can be produced. A spot check of the 400CX-2 I repaired for Ivan100000 still showed a maximum deviation of ~ 1.5 db total throughout the audio spectrum.

    From the extensive testing I did, and that which George did to properly set up his modeling software to verify my work, I modified the original network, keeping (again) the same configuration, but using these values:

    1. The 2.2 meg becomes 2.13 meg (a low value 2.2 meg resistor)

    2. The 1800 pF becomes 1350 pF (two appropriate 680 pF caps in parallel)

    3. The 390 pF becomes a 350 pF

    4. The 270K resistor remains the same.

    By comparison, this produced a very tight conformity over the whole of the audio bandwidth. For full disclosure, I present the measured AND modeled response characteristics of the new network in a measured/modeled response format:

    20 Hz = -.20/-.59 db

    30 Hz = +.05/-.17 db

    40 Hz = +/- 0.0/-.05 db

    50 Hz = +/- 0.0/-.04 db

    100 Hz = -.20/-.11 db

    200 Hz = -.20/-.15 db

    300 Hz = -.20/-.14 db

    400 Hz = -.15/-.11 db

    500 Hz = -.10/-.08 db

    1 kHz = +/-0.0/+/-0.0 (Ref)

    2 kHz = +/-0.0/+.08 db

    3 kHz = +/-0.0/+.12 db

    4 kHz = +/-0.0/+.14 db

    5 kHz = +.05/+.16 db

    6 kHz = +.05/+.16 db

    7 kHz = +.05/+.17 db

    8 kHz = +.05/+.17 db

    9 kHz = +.05/+.17 db

    10 kHz = +.05/+.18 db

    12 kHz = +.05/+.18 db

    15 kHz = +.05/+.18 db

    18 kHz = +.05/+.18 db

    20 kHz = +.05/+.18 db

    This represents very close conformity to the actual RIAA characteristic, as shown both by real world measured results, and by properly executed computer modeling backing it up. Frankly, the deviation is so small as to challenge my ability to visually resolve the change in db response from ideal on an analog meter. Within the response curve, the measured results show a worst case maximum deviation (from max over to max under) of just .25 db, while in no case deviating any more than .2 db from the ideal curve.

    Except at the very lowest frequencies (20 & 30 Hz), conformity between the measured and modeled characteristics throughout the bandwidth are well within .15 db, while at the lowest frequencies, some slight deviation does occur. Deviation can occur from modeled results in this region because the EQ NFB level becomes quite low at those frequencies, allowing discrepancies to more easily creep into the picture. In such cases, deference is given to the measured result, since it represents the real world characteristic produced, with the modeling compliance still providing strong support for the performance achieved in this region.

    Again I would like to thank George Ronnenkamp of Audio Regenesis for lending all of his expert modeling expertise to this effort. His involvement helped to cut my normal time consumption for such exercises to a fraction of what it would normally be, while helping to achieve and adding the highest degree of confirmation to the performance achieved.

    For those seeking greatly improved accuracy in reproduction from the phono inputs of their Fisher stereo equipment, this modification will produce it, being basically a drop in replacement for the original network installed.

    Pics include:

    1 & 2 Pics of both sides of the installation of the new RIAA networks in my 400C. Some components are hidden, while smaller caps to produce the 1350 pF cap in each network would have eased installation considerably. I simply used what I had on hand.

    3. A 200 Hz square wave first passing through a high accuracy Inverse RIAA Network, and then feeding the RIAA 1 inputs. The output is taken directly at the output of the phono preamps to eliminate any influence from succeeding stages. Channel A is on top, while channel B is on the bottom. Some delay characteristics of the circuit necessarily produce the slight degree of tilting shown at this frequency, as response down to near zero Hz is not possible nor desirable, which would be required to eliminate the tilting shown. But from the important response standpoint, it is the greatly improved flat wave tops that indicates the high degree of accuracy to the RIAA characteristic in the lower regions of the audio bandwidth, right down to 20 Hz.

    4. A 2 kHz square wave presented under the same conditions as described above. Without the drop off in response for the fundamental frequency presented, the waveform shows no tilting, with ruler flat wave tops indicating the high degree of RIAA conformance in the upper regions of the audio bandwidth, right up to 20 kHz. The presentations in both pics 3 & 4 also show the high degree of conformity between the two channels now.

    5. As a final indication of the significant input from George Ronnenkamp with this effort, he provided this graph for me which I am passing along, depicting both the original 400C RIAA EQ performance (in red), and that of my modified network presented here (in blue). The center line depicts ideal conformance to the RIAA characteristic. This easily allows then for not only a comparison between the two networks to be seen, but also a comparison of both of them to the ideal characteristic as well. The improvement achieved speaks for itself.

    I hope this new network will be of benefit to all those who love vinyl and Fisher equipment as much as I do!

    Dave
     

    Attached Files:

    Last edited: Mar 4, 2017
    bolero likes this.
  10. Sam Cogley

    Sam Cogley Last of the Time Lords Subscriber

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    Thanks for doing all of this so I can just sling solder and not risk spraining what's left of my brain... :D
     
  11. notdigital

    notdigital AK Subscriber Subscriber

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    Dave: In looking at the 500c schematic, are we talking about replacing PC1 and PC2? If so, am I understanding that the changes in your mod will work across all Fisher models??
     

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  12. JBL GUY

    JBL GUY Addicted Member

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    Dave, thanks for another very interesting thread.
     
  13. tube-a-lou

    tube-a-lou Super Member

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    Thanks again Dave, now to figure out how to do this to my KX200 and X101C this
    will be great!

    Tube
     
  14. buglegirl

    buglegirl In The Direction Of The Singularity Subscriber

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    Hiya,

    No offense to the Dutch Men or even Avery himself.

    But Fisher could have used you back in the day.

    Thanks for demystifying a very critical and overlooked part of these classic Fishers we so love.

    Send along thanks to Mr. Ronnenkamp as well.

    Frannie
     
  15. walyfd

    walyfd Well-Known Member

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    Location:
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    I'd love to hear the difference between the 2 setups! When I finally get the 500B worked on (Sony, I'll be dropping you a PM soon, I hope) I'd consider having this done!
     
  16. Moko

    Moko Member

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    89
    Location:
    London,England
    Thanks for doing such great work, will the same changes work on a Fisher 500c?

    Would it be possible to for someone with a bit more electrical knowledge to make a list/parts numbers for someone like Rapid or Farnells so we can then all get the correct parts and install them correctly.

    Thanks once again

    David
     

     

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  17. HiFiHarv

    HiFiHarv Active Member

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    You have managed to whittle the response deviation down to almost nothing. Not just flavor it with this cap or that cap. A monumental achievment, thanks for laying it all out so clearly for us.
     
  18. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    NotDigital, Tube, et al. If the PHONO PEC in your FISHER has a part # of 50187-3 the section in RED is the PHONO section. NO PROBLEM. From my SM research 99% or better of the FISHER gear from 1961 on use this same part # for PHONO/TAPE EQ.

    Just replace the parts in the RED section with 1% tolerance parts of the appropriate value, and go for it.

    [​IMG]

    You need to get only 4 parts per side. If the PEC's are side by side, you can either make 1 board with both sides on it, or 2 separate boards.

    I had to make 2 separate boards for my 800-C's as the Phono tubes are at different locations on the chassis. But at the time I did the whole PEC. So this should be easier to do.

    MOKO: Use 1% 1/4W metal film or Carbon Film resistors. Use 1% Silver Mica caps with at least a 250V rating. Mouser doesn't translate too well to FARNELL or RAPID so you'll need to do some legwork. The Silver Mica caps will be the most expensive parts, but total should come under $10.00 USD.
     

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  19. notdigital

    notdigital AK Subscriber Subscriber

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    Thanks Larry. When I posted this morning I had just returned from slapping the water for stripers all night. I've managed to get a couple of hours sleep :boring: and my head is clearer now. I've re-read Dave's post and I'm on board on what to do :D

    Before I left on vacation I built a jig for my LP12 to get that stupid bouncy thing right. Once that's done I'll be attacking the Fisher phono mods big time with Sony's cap change and, of course, Dave's latest refinement which is spectacularly simple. Thanks Dave :thmbsp: :thmbsp:

    Small favor if it's possible: If anyone has an 800c or 500c opened up can you post a pic of the PEC's in question. That is if it's already opened up (or will be soon) for surgery. I'll be away for for another 3 to 5 days and I want to begin visualizing what/where/how to attack. I know it's crazy, but hey, between hooking a slob and working on the Fisher I want my aging brain to be happily busy with anticipation :D
     
  20. sony6060

    sony6060 Super Member

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    That was a lot of work and the results are very flat. Thanks again Dave.
     

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