Post by Owen Y on Jan 15, 2020 12:14:58 GMT 12
In view of recent mention of 'current-mode' phono preamps in harvey's Works for me system thread, it's worth highlighting STEREOPHILE Editor Jim Austin's brief auditioning of 2 such 'current-mode' Phono preamps: Ideally suited to low impedance, Moving Coil cartridges. - SUTHERLAND Loco - US$8200 - CHANNEL D Lino C 2.0 - US$2400 
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Post by michaelw on Jan 15, 2020 12:38:49 GMT 12
dear admin, your STEREOPHILE Editor Jim Austin link is broken, it refreshes this page instead of taking the reader to the stereophile page.  another current mode phono stage is the CH Precision P1 used by m fremer et al.
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Post by Citroen on Jan 15, 2020 12:48:13 GMT 12
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Post by Owen Y on Jan 15, 2020 13:51:39 GMT 12
Thanks, folks (I was wondering if Fremer preferred the CH in current mode or voltage mode.)
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Pundit
Post by garym on Jan 15, 2020 18:07:00 GMT 12
The Dynavector is also one of these, in Dr T mode. Its pretty hard to design something that does current mode without using an opamp, in fact I'd say almost impossible. It depends on the cartridge connecting to a virtual earth node and this is generated by negative feedback. There is a resistor between the output of the amp and its inverting input. The cartridge connects in between ground and the inverting input. In effect the cartridge current flows to the inverting input, and due to negative feedback is met by the same current inverted. Cancellation takes place keeping the virtual earth at essentially zero volts under all signal conditions. The gain is given by the feedback resistor and Ohms law... I x R = output voltage. Note that this requires very high open loop gain to keep the virtual earth ideal, hence why it is hard to achieve with any sane discrete circuit. Whew...
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Pundit
Post by harvey on Jan 15, 2020 18:55:50 GMT 12
I'm glad you said almost impossible.
Just as an FYI Gary cause I'm way over my head here but the Aqvox website states 'no integrated opamps in signal path'.
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Post by andrewp on Jan 15, 2020 19:34:31 GMT 12
Crikey Im glad Garys cleared that up...lol!!
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Post by colinf on Jan 16, 2020 0:26:04 GMT 12
Well I understand what he’s saying, what’s the problem?😉🤔 You don’t necessarily need an opamp to generate the low impedance node, bipolar transistors can be made to do it with the signal injected into the emitter, and discrete circuitry to generate enough NFB to lower and linearise the input impedance.
AMR-iFi R&D
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Pundit
Post by garym on Jan 16, 2020 9:18:29 GMT 12
Well I understand what he’s saying, what’s the problem?😉🤔 You don’t necessarily need an opamp to generate the low impedance node, bipolar transistors can be made to do it with the signal injected into the emitter, and discrete circuitry to generate enough NFB to lower and linearise the input impedance. Well getting that emitter input at 0V to allow a DC connection to the cartridge would be a challenge. Don't want to bugger the cartridge with DC current..
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Post by colinf on Jan 16, 2020 9:31:16 GMT 12
No worse than using a bipolar input opamp. You’d use an opamp for dc servo duty I expect. Come to think of it, I recall making a circuit like that around mid-1990s that used 3 parallel jfets operated at 0v gate voltage biased with a constant current source running at the right current to make Vgs 0v, and the cartridge connected to the source of the jfet. The output of the jfet formed the input stage of a valve NFB circuit that lowered input impedance. I think I didn’t like it in the end as although it worked fine, I couldn’t adjust the cartridge loading, which made more difference to the sound. So I went back to connecting the jfets as voltage input and normal cartridge loading resistors. The current input circuit was a modification of a valve I-V converter I designed at the time for the TDA1541 dac chip.
AMR-iFi R&D
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Pundit
Post by garym on Jan 17, 2020 9:59:34 GMT 12
Bipolar input opamps bias current is in micro amps worst case. The current at the emitter or source of a common base / gate amp is in milli amps. You could servo it, but if anything fails your cartridge is toast. It would make me very nervous.
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Post by colinf on Jan 17, 2020 10:11:33 GMT 12
That’s true!
AMR-iFi R&D
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Post by colinf on Jan 17, 2020 10:25:41 GMT 12
You could always construct a discrete opamp! In most current mode inputs using opamps, if the opamp fails you’ll also have lots of mA going through your cartridge from the NFB resistors. You could use a protection circuit that disconnects the cartridge if it detects more than a few mV across it....or a snubber circuit that allows a few mV across it at most. Perhaps RF schottky detector diodes back to back across the cartridge...0.2V at 1mA, or pre-bias them to get lower threshold voltage. That raises a question, how much power can a typical MC cartridge dissipate in its coils before it expires, under circuit failure conditions? That’s possibly the thing that needs defining before proceeding.
AMR-iFi R&D
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Post by colinf on Jan 17, 2020 11:14:22 GMT 12
Thinking about it, my Dynavector can take the 1mA from the diode check function of my multimeter. At 12 ohms DC resistance that’s 12mV across it. The protection circuit would have to trip or limit at that or less than that.
AMR-iFi R&D
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Post by colinf on Jan 17, 2020 11:32:49 GMT 12
To construct a protection circuit for the cartridge I’d possibly use a jfet input opamp, dc connected, with gain set to 300 or so. At 12mV the output will be 3.6v, enough to operate a 5v relay and disconnect the cartridge. Different opamp gain would change the mV threshold. Using a latching relay to short out the cartridge instead of the sound going through relay contacts might have better sound quality.
AMR-iFi R&D
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Post by foveaux on Jan 17, 2020 11:41:02 GMT 12
Go for it garym and colinf,  "...RF schottky detector diodes back to back across the cartridge...0.2V at 1mA, or pre-bias them to get lower threshold voltage..."  so foreign to me, it could be a new language... I will keep reading these 'details' threads, google at the ready. But alas, to date, I'm soundly beaten by the 'Old Dog, New Tricks' conundrum. p.s. reassuring and grateful thanks to have your, Owen and others, expertise and experience available to us on DL.
"I see music as a lifetime affair." [Rory Gallagher]
"Free - I miss that band, but when I look back, we were very young" [Paul Rodgers]
848 posts
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Post by Citroen on Jan 17, 2020 16:30:39 GMT 12
foveaux Exactly. Its just like when you hear people talking to each other in a foreign language. They obviously understand each other but I ain't got one helluva idea what they are on about. All I know is that I preferred most of my carts in Dr T mode with my Dynavector phono preamp.
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Post by michaelw on Jan 17, 2020 17:17:46 GMT 12
i found dr t mode a lucky dip even with dv cartridges,
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Pundit
Post by garym on Jan 17, 2020 18:08:48 GMT 12
I like the KISS principle. In design.
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Post by Graham on Jan 17, 2020 18:17:13 GMT 12
I like the KISS principle. In design. Hear hear ! You're talking my language.  I was involved with race car design many moons ago, and that was out guiding principle. Always aim for the simplest way to achieve your goal.
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Post by Owen Y on Jan 17, 2020 19:35:49 GMT 12
Go for it garym and colinf, "...RF schottky detector diodes back to back across the cartridge...0.2V at 1mA, or pre-bias them to get lower threshold voltage..." so foreign to me, it could be a new language... I will keep reading these 'details' threads, google at the ready. But alas, to date, I'm soundly beaten by the 'Old Dog, New Tricks' conundrum. p.s. reassuring and grateful thanks to have your, Owen and others, expertise and experience available to us on DL. Thanks foveaux, but it's beginning to sound a bit Spanish to me too
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Post by colinf on Jan 17, 2020 20:44:16 GMT 12
More elaborate and complicated circuits evolved from the flaws of simple circuits.😀
AMR-iFi R&D
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Pundit
Post by belbo on Jan 17, 2020 21:45:54 GMT 12
May I ask what is the advantage of Current-mode amplifiers in general?
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Post by colinf on Jan 18, 2020 8:29:24 GMT 12
I believe the main advantage is in noise performance and coil generator damping, for MC cartridges only. In normal cartridge loading, to have the cartridge mechanism dampened electronically you’d need a very low value loading resistor. That means the tiny power from the MC cartridge is partly wasted into the loading resistor and thus signal to noise ratio is degraded. Current input has extremely low input impedance, like a short circuit to the cartridge, but all of its current instead of part of it is used to be converted into voltage output with a special circuit, called a transimpedance amplifier, or I-V amp (current to voltage amp). Noise can be very low with such a circuit. But there is no cartridge loading adjustment, it’s already set at next to zero! Apparently some cartridges work well with current input and some don’t, you just need to try it, or rely on reviews of current input phono stages who have tried and liked certain cartridges. Another technical advantage is increased immunity to RF pickup through the turntable leads. As the gain of the transimpedance amplifier relies on the impedance of the cartridge, and since the cartridge is like a small inductor, the amplification factor naturally rolls off at RF frequencies. That can also improve high frequency noise performance.
AMR-iFi R&D
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Post by Owen Y on Jan 18, 2020 9:43:58 GMT 12
I believe the main advantage is in noise performance and coil generator damping, for MC cartridges only. In normal cartridge loading, to have the cartridge mechanism dampened electronically you’d need a very low value loading resistor. That means the tiny power from the MC cartridge is partly wasted into the loading resistor and thus signal to noise ratio is degraded. OTOH, for SUT-loaded MC cartridges, the gain is increased with decresed cartridge loading, because of the higher gain ratio (squared) of the SUT or tapped SUT ( colinf, I think I'm correct here?)
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Post by colinf on Jan 18, 2020 10:28:39 GMT 12
Assuming the load at the output of the SUT is the same in each case, usually 47k, there’ll be a point at which the power output of the cartridge is balanced with the step up ratio and efficiency of the transformer and gain will increase no further, when you lower the loading impedance by increasing the turns ratio. The cartridge has finite resistance, around 10 ohms for most. My Dynavector has 12 ohms. SUTs are also naturally RF-resistant and have the advantage of being able to accept either a single ended or a balanced input without using a balanced input preamp circuit.
AMR-iFi R&D
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Post by colinf on Jan 19, 2020 8:59:25 GMT 12
So Dr T mode on the Dynavector P75 must be current input mode.... I assume Dr T is Dr Tominari who designed the DV cartridges? michaelw, which carts did you like in Dr T mode? Which weren’t so good?
AMR-iFi R&D
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Post by michaelw on Jan 19, 2020 22:30:06 GMT 12
hi colinf,
with the original p75 i used a shelter 501 mk2, the p75 made the shelter sound like a dv, and that wasn't good.
p75 mk2 - shelter, ortofon tmc200, dv20x2 low.
2nd implementation was better but i preferred using non-dr t voltage mode.
p75 mk3 - ortofon tmc200, dv20x2 low, denon dl-a100
this version hit most bases, dr t was so-so with the ortofon and denon but made the dv sing !
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