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  1. #1
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    Possibly weird question about bypassing RIAA EQ

    I've been doing a lot of record digitization lately with a heavy focus on 78s. Because they mostly pre-date RIAA equalization, getting the best sound usually requires running the raw captures through software that un-does RIAA and applies the correct curve for the disc/era in question. I've been using Equalizer, a free program by Brian Davies, the same guy who gave us ClickRepair.

    That got me to wondering more generally about bypassing RIAA or any other pre-amplification at all and doing everything in software. A bit of Googling suggests that this is actually sort of a trendy thing at the moment and there are even a few pre-amps out there made for exactly that purpose. But my fantasy involves plugging the phono cartridge directly into the recorder (presumably the mic input) and having nothing in between to potentially color the sound.

    However, cartridges generally prefer to be loaded with specific impedance and capacitance, which of course a dedicated pre-amp would be expected to provide. I'm wondering if that's something that can also be emulated in software and if anyone has ever tried it.

    I wrote to Mr. Davies to see if he had any insight into whether Equalizer could be used for this, but he simply replied that (paraphrasing) it's a hardware issue. Knowing NOTHING about the ramifications of impedance and capacitance, I didn't argue with him because for all I know the math I'm asking about is way more complex than what an RIAA-to-whatever equalizer is designed to do.

    Has anyone else tried this? If so, did you have any luck or do you have any advice? "You're trying to fix something that isn't broken" is fine, too. I don't personally hear that I have a problem I need to correct, I'm just intrigued by the concept for the sake of general tinkering nerddom.

    Thanks!

  2. #2
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    Quote Originally Posted by atrocity View Post
    But my fantasy involves plugging the phono cartridge directly into the recorder (presumably the mic input) and having nothing in between to potentially color the sound.

    However, cartridges generally prefer to be loaded with specific impedance and capacitance, which of course a dedicated pre-amp would be expected to provide. I'm wondering if that's something that can also be emulated in software and if anyone has ever tried it.
    I have no practical experience to offer.

    I think you may need to rethink what you mean by "colouring the sound". The behaviour of the cartridge is determined by its mechanical and electrical construction, and must be influenced by the electrical input impedance of whatever it's connected to. "Coloration" is, therefore, unavoidable, at least at a theoretical level, because what you record will, of necessity, reflect this composite state of affairs.

    Perhaps the behaviour of the cartridge can be adequately characterized by an "output impedance", which will be both mechanical and electrical in composition and, presumably, known to the manufacturer. Perhaps this characterization can be deduced from the manufacturer's specification, or advice as regards the expected "input impedance" of the pre-amp. No doubt the type of cartridge would have a large role in determining it.

    Perhaps, also, you have have an adequate characterization of the actual input impedance of your chosen input device. Then you might be able to construct a digital filter that modifies your recorded input to reflect what it would have been had your input device had the same impedance that the manufacturer designed his cartridge to drive. Or any other input impedance of your choosing.

    Where does that get you ?

    I suspect that there's much to discover about what "real world" devices actually do, and the practical effects of that. Have fun !

  3. #3
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    Quote Originally Posted by mrw View Post
    I think you may need to rethink what you mean by "colouring the sound". The behaviour of the cartridge is determined by its mechanical and electrical construction, and must be influenced by the electrical input impedance of whatever it's connected to. "Coloration" is, therefore, unavoidable, at least at a theoretical level, because what you record will, of necessity, reflect this composite state of affairs.
    This is an excellent point. Perhaps it would be more accurate to say that I'm trying to avoid any additional colo[u]ration. Until recently I used a hardware "re-equalizer" when digitizing 78s. It has a switch that allows the signal to simply pass through and the manufacturer swears that it's safe to leave inline when not being used because it does nothing. Unfortunately, despite that claim, it adds audible hiss. Not really a problem when the source material is a 78, but definitely not something I'd want to inject into a modern recording.

    That's part of what got me wondering if my phono pre-amp (which is definitely nothing fancy) is also potentially doing something undesireable, which in turn made me think about the possibility of simply not using one at all and "fixing it in post."

    Quote Originally Posted by mrw View Post
    Perhaps, also, you have have an adequate characterization of the actual input impedance of your chosen input device. Then you might be able to construct a digital filter that modifies your recorded input to reflect what it would have been had your input device had the same impedance that the manufacturer designed his cartridge to drive. Or any other input impedance of your choosing.
    I have found a claim out there that the mic input impedance of my device (a Roland R-05) is 7,000 ohms. Phono cartridges generally want 47,000 ohms. It's nice to know that, but what I don't know is what the real-world ramifications of that mismatch are. Too many highs? Not enough highs? Something wildly non-linear that would be extremely difficult to fix via digital EQ? Unfortunately, while the EQ curves for RIAA and its predecessors are reasonably well-documented and easy to adjust, I have no clue where to begin to emulate additional impedance.

    I have no idea what the capacitance of that input is, though I deduce that higher capacitance = attenuated high frequencies. Back in the quadraphonic days it was important to have low-capacitance phono cables because CD-4 demodulation required absurdly high frequencies. In fact, it's also a desire to preserve some CD-4 records for potential maybe-some-day software decoding that makes me think about this project. I doubt my cheap phono pre-amp is passing 45kHz, though I suppose that I don't know that it isn't. Certainly my cartridge (an AT440MLa) is not rated for anything that crazy, but it actually will perfectly activate a CD-4 demodulator.

    Making it even more interesting, I've read (but have no idea if it's true) that impedance/capacitance can actually have a mechanical effect on the cartridge/stylus. If that's the case, I wonder if it's simply a bad idea to attempt to fix via software something that's traditionally done in hardware for a good reason.

    If I sound like I'm completely lost and am blindly theorizing about something I simply don't understand at all...well, I am!

  4. #4
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    Quote Originally Posted by atrocity View Post
    I've been doing a lot of record digitization lately with a heavy focus on 78s. Because they mostly pre-date RIAA equalization, getting the best sound usually requires running the raw captures through software that un-does RIAA and applies the correct curve for the disc/era in question. I've been using Equalizer, a free program by Brian Davies, the same guy who gave us ClickRepair.

    That got me to wondering more generally about bypassing RIAA or any other pre-amplification at all and doing everything in software. A bit of Googling suggests that this is actually sort of a trendy thing at the moment and there are even a few pre-amps out there made for exactly that purpose. But my fantasy involves plugging the phono cartridge directly into the recorder (presumably the mic input) and having nothing in between to potentially color the sound.

    However, cartridges generally prefer to be loaded with specific impedance and capacitance, which of course a dedicated pre-amp would be expected to provide. I'm wondering if that's something that can also be emulated in software and if anyone has ever tried it.

    I wrote to Mr. Davies to see if he had any insight into whether Equalizer could be used for this, but he simply replied that (paraphrasing) it's a hardware issue. Knowing NOTHING about the ramifications of impedance and capacitance, I didn't argue with him because for all I know the math I'm asking about is way more complex than what an RIAA-to-whatever equalizer is designed to do.

    Has anyone else tried this? If so, did you have any luck or do you have any advice? "You're trying to fix something that isn't broken" is fine, too. I don't personally hear that I have a problem I need to correct, I'm just intrigued by the concept for the sake of general tinkering nerddom.

    Thanks!

    The loading of the cartridge controls the mechanical damping and therefore resonance (Mainly R) and the overall frequency response (Combination of R and C) of the stylus and cantelever.

    BUT theres no reason you cannot apply the R and C loading to a flat gain stage (Use the recommended loading for your cartridge) and apply the RIAA (or other) EQ in software

    Jeff

    Edit: You'd need to check your overload margins as (as you probably know) theres 40db of difference between the eq at 20 and 20KHz so the HF end might saturate the IP amp (This is still true of RIAA preamps)

    Edit 2: Found this with a very quick search https://www.mysonic.space/phono-cart...edance-loading
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  5. #5
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    Quote Originally Posted by atrocity View Post
    I have found a claim out there that the mic input impedance of my device (a Roland R-05) is 7,000 ohms. Phono cartridges generally want 47,000 ohms. It's nice to know that, but what I don't know is what the real-world ramifications of that mismatch are. Too many highs? Not enough highs? Something wildly non-linear that would be extremely difficult to fix via digital EQ?
    I think you can expect a roll off that is shifted, possibly significantly, towards the lows, because of the additional damping of 7,000 ohms versus 47,000 ohms. That is a guess based on the calculations below.

    I looked up your cartridge. It is, apparently, of the moving magnet type. https://www.audio-technica.com/cms/c...c54/index.html

    Amongst other things is quoted:
    Coil impedance : 3,200 ohms at 1 kHz
    Coil inductance (mH, 1 kHz) : 490
    This implies a coil resistance of 872 ohms, by my calculations.

    If we make the thoroughly unreasonable assumption that the mechanical parts plus the magnet and coil act as a simple voltage generator (they won't), and that the cable plus mic input present no significant capacitative load, we can model the arrangement as a voltage generator (Vo) driving the coil (complex impedance Zc, with resistance Rc and inductance Lc) in series with the resistive input load (Ri).

    The load will therefore see a voltage of Vo x Ri / (|Ri + Zc|).

    At DC, that will be Vo X Ri / (Ri + Rc).

    The 3dB (half power) point of the roll off happens when Frequency = (Ri + Rc) / (2 x pi x Lc).

    Numbers:
    Ri = 47,000 ohms, Rc = 872 ohms, Lc = 0.49H.
    Frequency when 3dB down = (47,000 + 872) / (2 x 3.14 x 0.49) = 15,557 Hz.
    DC voltage = Vo x 47,000 / (47,000 + 872) = Vo x 0.98

    Ri = 7,000 ohms, Rc = 872 ohms, Lc = 0.49H.
    Frequency when 3dB down = (7,000 + 872) / (2 x 3.14 x 0.49) = 2,558 Hz.
    DC voltage = Vo x 7,000 / (7,000 + 872) = Vo x 0.89
    That's 0.8dB below the 47k case.
    I hope I got all of that right...

    In practice, I would expect that mechanicals will also play a significant part, although I have no idea what the impact would be, as I have no experience of the matter.

    It is possible to create a model in which mass, springiness, and friction are combined with the electrical components to give additional elements to the "electrical" network, with inductance, capacitance, and resistance replacing the mass, springs, and friction of the mechanicals. The above numbers would change, perhaps substantially.

    An experiment:

    (A) Record something directly into your mic.
    (B) Record it again, through a 40k resistor in series with the input (thereby creating a 47k load).

    Get the levels sorted, and see if you can form any conclusion about the respective frequency responses. I wonder if, in practice, they would bear any resemblance to my unreasonable calculations.


    Quote Originally Posted by atrocity View Post
    Making it even more interesting, I've read (but have no idea if it's true) that impedance/capacitance can actually have a mechanical effect on the cartridge/stylus.
    Yes. It is a composite system. The cartridge/stylus is generating electricity which is then passed through the load. If you think of the load as imposing friction (through its resistance) and springiness (through its capacitance), and even having mass (through inductance), you might expect this to impact the cartridge/stylus just as if there were mechanical friction, springs, and masses in play. It gets quite complicated to model these things, though. I believe that motor car manufacturers used to use analogue computers operating under these principles to model car suspensions and the like.

  6. #6
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    Quote Originally Posted by atrocity View Post
    But my fantasy involves plugging the phono cartridge directly into the recorder (presumably the mic input) and having nothing in between to potentially color the sound.
    Have a look at a Behringer UFO202 - it may colour the sound - its effectively an inline pre-amp that will take the tuntable output and send it via USB to your PC.
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  7. #7
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    Thank you everyone for your thoughtful (and detailed!) responses. It's been a long, long time since I've soldered anything, but the idea of getting a couple 40k resistors is somewhat tempting just for fun.

    But it sounds like in general I may be worrying about something that isn't really a problem. Or is only a problem for CD-4, which is admittedly a very obscure issue.

  8. #8
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    Quote Originally Posted by atrocity View Post
    Thank you everyone for your thoughtful (and detailed!) responses.
    Let me add a little more detail ! Although perhaps too much time has passed by now.

    It was a bit wimpy of me not to have a go at estimating the input capacitance of your device of choice, so here we go: 3 feet of input cable at, say, 30pF per foot = 90pF, plus, say, another 30pF for the device itself = 120pF. I don't know how good an estimate that actually is, but I don't suppose it would be miles out.

    The result surprised me. With a 47k input resistance that level of capacitance actually extends the HF response in a pretty flat manner. Much more and we get a characteristic overshoot at HF - which, I guess, would contribute 'brightness'. Increasing the input resistance (reducing the damping) also results in overshoot.

    So choice of cable is clearly quite significant. And a judicious reduction of input resistance could tame unwanted highs. But 7k would be overkill !

    I've attached a (pdf) plot, capturing my earlier comments and this one. It may be of interest. I've also attached the 'gnuplot' script which I used to generate it, should you be minded to tinker for yourself.
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  9. #9
    Senior Member Jeff07971's Avatar
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    Quote Originally Posted by mrw View Post
    Let me add a little more detail ! Although perhaps too much time has passed by now.

    It was a bit wimpy of me not to have a go at estimating the input capacitance of your device of choice, so here we go: 3 feet of input cable at, say, 30pF per foot = 90pF, plus, say, another 30pF for the device itself = 120pF. I don't know how good an estimate that actually is, but I don't suppose it would be miles out.

    The result surprised me. With a 47k input resistance that level of capacitance actually extends the HF response in a pretty flat manner. Much more and we get a characteristic overshoot at HF - which, I guess, would contribute 'brightness'. Increasing the input resistance (reducing the damping) also results in overshoot.

    So choice of cable is clearly quite significant. And a judicious reduction of input resistance could tame unwanted highs. But 7k would be overkill !

    I've attached a (pdf) plot, capturing my earlier comments and this one. It may be of interest. I've also attached the 'gnuplot' script which I used to generate it, should you be minded to tinker for yourself.

    How have you modelled the equivalent circuit of the cartridge and what values did you use for them ?
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  10. #10
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    Quote Originally Posted by Jeff07971 View Post
    How have you modelled the equivalent circuit of the cartridge and what values did you use for them ?
    I haven't, other than the electrical characteristic of the pick up coil itself (Lo and Ro), which is what I am trying to say by my comment (in the title) "Assumes negligible electrical loading on hardware".

    I have no experience of these things, nor any feel of what the magnitude would be.

    I guess the equivalent circuit might be something like adding inductance (mass), capacitance (spring), and resistance (friction), all in series with the coil. The gnuplot script as drafted does not cater for the capacitative element, although it would be easy to add. The inductive and resistive elements could be done by simply increasing the existing values for the coil.

    But, as I say, I am clueless on what those values should be. Perhaps a full technical specification of the cartridge might give us the real model to be used. This model is really only a 'toy', perhaps helpful in determining the direction of things but certainly not definitive as regards values.

    The manufacturers seem to have had a 47k input resistance in mind, and must have had an idea about typical cable/device capacitance, and designed accordingly. Possibly this model isn't too far from reality, but I don't have the knowledge to be able make a judgement.

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