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Moving-Magnet Head Amp - without RIAA
Moving-Magnet Head Amp - without RIAA
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Old 7th January 2019, 09:50 AM   #1
rsdio is offline rsdio  United States
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Default Moving-Magnet Head Amp - without RIAA

Has anyone published a schematic for a Moving-magnet head amplifier that merely loads the cartridge correctly and boosts the level, but without applying any RIAA equalization?

I understand that this sort of thing is common for Moving-coil cartridges, where a step-up transformer is either too expensive or undesired for other reasons. Such a Moving-coil head amplifier is designed to be connected to the input of a MM stage with RIAA.

I listen to all music through an excellent DAC, but there is still some music in my collection that's only available on vinyl. I have written my own RIAA equalization plugin in the digital domain, and my DAC also implements optional RIAA equalization in its DSP, and I hope this explains why I want to avoid the analog RIAA circuitry.

I've gone through an evolution of several stages so far:

1) Since I needed an adapter cable from RCA to 1/4" TRS or XLR inputs on my computer audio interface, I started by building in the 47 kΩ load in the adaptor cable. I started with the specifications for the input impedance of my audio interface, and calculated the resistor value that I needed to add in parallel to end up with 47 kΩ. This setup worked well, but the ground lug for my tonearm kept coming loose, as evidenced by obvious hum until I wedged it in somewhere.

2) After the ground lug came detached too many times for my patience, I decided to buy a cheap phono preamp with an actual screw terminal on the case. I then gutted the original RIAA circuit, measured the mounting points for its PCB, and fabricated my own passive adaptor. This time, I also added a capacitor to provide the 200 pF to 600 pF load specified by my cartridge manufacturer. The PCB is available at OSHPark. This worked well, but when I moved the cable to a different audio interface and again when I upgraded the electronics in the audio interface, the passive components are no longer loading the cartridge correctly.

3) My goal now is to build an active phono preamp, but without any of the RIAA curve. This circuit would be tailored for my specific cartridge, and the output would be at standard -10 dBV unbalanced and +4 dBU balanced levels so that any input on my various audio interfaces would work (without being fragile due to parallel resistor and capacitor combinations). I'm trying to fit everything in the same phono preamp case that I have, since the grounding screw is going to be a necessary component.

4) In the future, I hope to upgrade from MM to MC, and might just build Douglas Self's Moving-Coil Head Amplifier. I assume this could be connected directly to my audio interface instead of a MM preamp, and I could then continue to realize RIAA EQ in DSP.

By the way, I recently obtained Self's "Small Signal Audio Design" book, and this probably has everything I need. However, there are a few questions I have about my slightly atypical application that aren't directly anticipated in his text.

Despite the fact that I've not seen anyone talk about this, it seems like modern listening with DSP available might benefit from such a flat-frequency-response phono preamp. On a related note, I'd much rather archive my rare vinyl via such a flat preamp setup and then apply the RIAA curve during playback, rather than having to invest thousands into an analog RIAA EQ that doesn't suffer from the usual noise and component tolerances.

Brian

p.s. I'm planning to design around a +34V single-ended supply, since I'm a bit too cramped for space to pull off a +/-17V bipolar supply. To keep things simple, I'm using an AC wall-wart, but those have only two conductors - no center tap. Obviously, this power supply requires a virtual ground and capacitive coupling. Douglas Self recommends always using capacitive coupling on the cartridge connection to protect the cartridge from DC current. That seems like sound advice. I'm not too worried about the AC coupling on the output that my unipolar supply requires, since I'll probably use a subsonic filter that would defeat any potential advantage of DC coupling on the output anyway.
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Old 7th January 2019, 10:01 AM   #2
billshurv is offline billshurv  United Kingdom
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Moving-Magnet Head Amp - without RIAA
Scott Wurcer published a few schematics for that and there is a kit Wayne Kirkwood designed. There is also a thread I can locate where phantom powered MM/MC is discussed if that is of interest. Focus specifically on interfacing into the sort of mic inputs that people have on home studio ADCs



You can take pretty much any phono stage and remove RIAA of course, adjusting gain to suit.



Many choices
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Old 7th January 2019, 10:18 AM   #3
billshurv is offline billshurv  United Kingdom
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Moving-Magnet Head Amp - without RIAA
Found the thread Designing a universal diff-in/diff-out Head Amp
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Old 7th January 2019, 10:53 AM   #4
billshurv is offline billshurv  United Kingdom
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Moving-Magnet Head Amp - without RIAA
Missed edit window but realised I needed to correct my first post due to muddy memory. Hans Polak has also contributed to the head amp discussions. I've got a focusrite Scarlett coming soon so have on my list to do some testing.
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Old 7th January 2019, 06:25 PM   #5
rsdio is offline rsdio  United States
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Originally Posted by billshurv View Post
There is also a thread I can locate where phantom powered MM/MC is discussed if that is of interest.
I got excited about phantom power when I started this project, but then realized that I have the audio interface model with line level inputs and therefore no phantom power!

I would still be interested in looking over that thread, since I plan to power from +34V and that should be quite close to powering from a nominal +48V.
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Old 7th January 2019, 07:29 PM   #6
BrianL is offline BrianL  United States
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Moving-Magnet Head Amp - without RIAA
There is no reason why you can't generate +/- supplies from your wall-wart. I believe Doug Self shows how in his book. It's pretty easy and works well as long as the current draw isn't too high as each 'half' is being charged with half-wave rectifiers.
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Old 7th January 2019, 07:46 PM   #7
rayfutrell is offline rayfutrell  United States
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You may be interested in this Revox B291 turntable schematic on page 6-14. Revox called it an impedance converter but it's just an RIAA preamp without the RIAA.
STUDER REVOX B291 SM Service Manual download, schematics, eeprom, repair info for electronics experts
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Old 7th January 2019, 11:28 PM   #8
PRR is offline PRR  United States
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Originally Posted by rsdio View Post
....the output would be at standard -10 dBV unbalanced....
At what frequency??

If we define the RIAA-corrected response to be "flat", then a non-corrected response will be 20dB down at 50Hz and 20dB up at 20KHz.

i.e. the bass will be down in the mud.

It looks like the top octave will be STRONG, but in fact most music (and anything you can get from a needle) has around 20dB fall-off above 1KHz.

That's before we get to the wide range of recorded levels on LP, which has no real "limit" like digital, or FM, or even tape; only a trade-off between loudness and time per side.)

Taking a rough guess: if RIAA gain of 100 at 1KHz gives a good nominal line level, take the caps and bass R out of the RIAA and it will probably give a tolerable level. For two common topologies:
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File Type: gif no-RIAA.gif (33.1 KB, 435 views)
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Old 8th January 2019, 12:48 AM   #9
sgrossklass is offline sgrossklass  Germany
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A flat amp is going to require substantially more GBW and slew rate than one with active RIAA. I would use at least two opamp stages at 20 dB each. Maybe start off with NE5534As (910/100 ohm) and continue with a good FET input dual of highish slew rate (AD8620 w/ 4k3/470 ohm? I know they'll only do +/-13 V, but in this application a +/-9..12 V supply would be plenty).

Software / DSP RIAA is doable (and has been done before, even commercially) but you do want a decent ADC, and it goes without saying that you do not want to be fighting a ground loop issue in your setup, as the amp-DAC connection is going to be ~20 dB more critical in this regard.

As a compromise, I might try implementing the second gain stage as a good approximation of an integrator across the entire audio bandwidth. (Maybe inverting, 2k2 + 6.8n poly || 1M?) This characteristic would be easily reversed in software, and any parts tolerances would only result in some channel imbalance (easily enough brought in line either in software or by making one channel's gain slightly variable with a trimpot, applying a signal split to both channels to adjust) or frequency response deviations in the subsonic range. It would make ADC high-frequency noise a bit more critical, of course, but in the days of 100+ dB ADCs I don't see that as a major problem.
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Old 8th January 2019, 01:01 AM   #10
billshurv is offline billshurv  United Kingdom
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Moving-Magnet Head Amp - without RIAA
How much more slew rate? People who have done this have not reported any major issues. It's seen as a boogieman but does it really exist?
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