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an mm/mc phono stage
an mm/mc phono stage
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Old 6th October 2017, 11:26 PM   #1
rjm is offline rjm  Japan
Richard Murdey
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an mm/mc phono stage
Default an mm/mc phono stage

A new circuit from me, I'm posting it give it a public timestamp.

Two stage active/passive phono pre-amplifier. The main point is the simple jumper switch between MM and MC configuration. It's also a well-balanced circuit from a noise/drive aspect at both gain settings and has very accurate RIAA response invariant of switch position. mm is 38 dB, 47k input, mc is 58 dB, 100 ohms input. S-Reg voltage regulation.
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File Type: jpg pcb-emerald-10a-sch.jpg (511.8 KB, 1013 views)
File Type: png emerald response.png (26.3 KB, 969 views)
File Type: png pcb-emerald-10a-brd.png (40.7 KB, 904 views)
File Type: png pcb-emerald-10a.png (47.1 KB, 889 views)
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Last edited by rjm; 6th October 2017 at 11:48 PM.
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Old 8th October 2017, 01:04 AM   #2
rjm is offline rjm  Japan
Richard Murdey
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an mm/mc phono stage
update to 10c, BOM uploaded

(Jumper positions switched, capacitor parts numbers fixed)
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File Type: png emerald10c.png (176.8 KB, 823 views)
File Type: png pcb-emerald-10c-brd.png (40.8 KB, 126 views)
File Type: jpg pcb-emerald-10c-sch.jpg (510.8 KB, 223 views)
File Type: png pcb-emerald-10c.png (47.3 KB, 217 views)
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File Type: zip pcb-emerald-10c-bom.zip (348.9 KB, 31 views)
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Last edited by rjm; 8th October 2017 at 01:06 AM.
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Old 8th October 2017, 01:49 AM   #3
anatech is offline anatech  Canada
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an mm/mc phono stage
Hi Richard,
Looks like it should perform well. What makes it different from all the ones that have come before with the same basic idea?

-Chris
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Old 8th October 2017, 06:20 AM   #4
Calvin is offline Calvin  Germany
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Hi,

sorry, but I think it has certain issues.
- the OPA134, beeing a JFET-input OPA has typically highish voltage noise and very low current noise. The former plays a dominant role in MC application and in MM-application below ~2kHz. The figures of >>5nV/sqrHz are not noise-transparent any more (not yet taken into account the rather high 1/f intercept point). For MC useage where one rather aims for sub-1nV figures it'll be even more something like a noise generator. IIrc a LME49710 would be the better choice for the first stage (not really MC capable too)
- also regarding noise ... its common to design a great deal of the amplification into the first stage. This design though rather functions like buffer than an amplifier (A=2.5, adding ~20dB in MC mode). This has probabely been done so to keep the gain in MC mode at a reasonably low value as to preserve sufficient bandwidth .... It'd been advantageous to omit with the MC mode alltogether and to give the first gain stage more gain and add a dedicated MC prepre stage with a LT1115 for example.

jauu
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Old 8th October 2017, 06:50 AM   #5
rjm is offline rjm  Japan
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an mm/mc phono stage
@anatech

Of the two-stage op amp phono stage circuits, there are what I'll call "the LM833 derivatives" (active RIAA in first stage, passive RC interstage filter, flat output buffer) from the LM833 datasheet and the more common "passive interstage RIAA" types which feature in the OPA606 datasheet. Both are presented as MM stages only.

My approach is more along the lines of putting variable gain head amp in front of a VSPS. Compared to a regular VSPS though the high frequency RIAA cut is moved out of the VSPS feedback loop and into the interstage region, the low impedance drive of the input op amp being wasted otherwise.

It ends up close to the LM833 circuit, I admit, just with the bass boost part of the eq. moved from the input to the output, and the coupling cap from the interstage to the output. As you suggest, there are only so many different approaches to solving this. The LM833 circuit does not, however, have provision for MC gain out of the box, and the all passive circuits, due to their greater inefficiency, are not MC compatible without a separate head amp / step up transformer.

@Calvin

It's true I hadn't thought too much about what op amps to use. The OPA134 just kinda ended up there as a holdover, I use OP27 for the input stage of MC amp usually. Though with a dual purpose design some compromise here is inevitable. Re. your second comment, in MC mode the midband gain is split evenly 30/30 dB between the two stages. For MM it is less, but again, the output is just a VSPS which normally does double duty covering both amplification and output drive. Here, some of the gain is farmed out to the input, so compared to the normal VSPS it has better performance.
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Last edited by rjm; 8th October 2017 at 06:59 AM.
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Old 8th October 2017, 10:45 AM   #6
MarcelvdG is offline MarcelvdG  Netherlands
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Quote:
Originally Posted by Calvin View Post
IIrc a LME49710 would be the better choice for the first stage (not really MC capable too)
An OPA134 is indeed not a good choice for a MC amp, but your alternative is not that good either. The current noise of an LME49710 is a bit high for MM and the voltage noise is a bit high for MC. (See LME49710 Phono Stage for details.)
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Old 8th October 2017, 03:12 PM   #7
anatech is offline anatech  Canada
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an mm/mc phono stage
Hi Marcel,
Thanks for the link!

-Chris
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Old 8th October 2017, 06:39 PM   #8
vulejov is offline vulejov  Serbia
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Quote:
Originally Posted by MarcelvdG View Post
An OPA134 is indeed not a good choice for a MC amp
That opamp isn't good choice for anything..
I use VSPS as MC phono, 65dB gain with AD797, work excellent..
Simple R1-R2 value change and one can use it as MM/MC..
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Old 8th October 2017, 09:03 PM   #9
MarcelvdG is offline MarcelvdG  Netherlands
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AD797 is great for MC, but not for MM: 2 pA/sqrt(Hz) of input noise current.

Noisewise the very worst MM amplifier ever is probably the moving-magnet version of the Elektor Supra 2.0: four LT1028s in parallel, which reduces the op-amp noise voltage from 0.9 nV/sqrt(Hz) to 0.45 nV/sqrt(Hz), but increases the op-amp noise current from 3.25 pA/sqrt(Hz) to 6.5 pA/sqrt(Hz).

(The LT1028 datasheet says 1 pA/sqrt(Hz), but that only applies when the impedances driving the positive and negative inputs are equal, which they never are in a real-life application; a graph at the back of the datasheet shows that with unmatched inpedances it is 3.25 pA/sqrt(Hz).)
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Old 9th October 2017, 04:50 PM   #10
vulejov is offline vulejov  Serbia
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Quote:
Originally Posted by MarcelvdG View Post
AD797 is great for MC, but not for MM
I'm thinking about sound.. noise isn't problem for MM phono, as for MC..
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