Ultra high spec. opamp MC/MM phono, warp "elliptic" filter, line, headphone amps.

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wyn palmer

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This is a DIY project that has been on Audio Karma for about a year and has had a number of revisions, additions and upgrades. The original thread is c.110 pages long so it's time to do a re-boot.
High gain opamp based preamp for low output MC cartridges. | Audiokarma Home Audio Stereo Discussion Forums
The original thread has many test results and design philosophy/trade off discussions plus some construction experience and a lot of irrelevant chatter.
There are about 230 of the units in various versions out in the field. The design has been added to and revised several times and is now at v3.1.
As the title suggests, it is opamp based, and is hardly unique, but it has some unusual features, has stellar specs, and is pretty easy to build.
Some features:
1. Optional parallel input amps-
2x LT1115/AD797/OPA1611 or OPA1612 bipolar opamps for MC- extremely high S/N ratio for low output moving coil cartridges.
2x OPA1641 or similar or OPA1656/OPA1642 FET input opamps for MM- also providing extremely high S/N with high cartridge resistance/inductance.
Cartridge load components, both R and C, can be altered without soldering, allowing for loads from a few 10s of PF to uF.
2. 75us TC in input stage to optimize overload characteristic- approx. 28dB at rated -9dBv output at all frequencies up to greater than 50kHz with +/-15v supplies- an important feature matching high performance, commercial designs.
3. Additional standard RIAA TCs in output stage- again maintaining maximal overload margin.
4. Gain partitioning/opamp choices made to keep negative feedback factor high over the entire audio range to minimize high order harmonics and many tone IM distortion.
No visible degradation in noise floor at levels much larger than rated output due to IM even with many tone inputs, high order harmonic distortion (e.g 9th) below noise floor minimizing degradation of sound quality.
5. Inter stage TC added to compensate for the transition to unity gain of the input gain stage to maintain RIAA characteristic compliance. No unity gain transition in output stage due to choice of architecture. This contributes to the compliance with the standard RIAA characteristic.
6. Phase adjusted DC offset correction in the phono amplifier - so no coupling caps in series with the signal path and no LF peaking due to the correction loop.
7. Ultra Low distortion, high stability, high accuracy Polypropylene caps used in all of the signal paths.
8. Off the shelf SMPS used with custom designed input LC filter. >100dB of supply noise rejection at SMPS switching frequencies.
7. Extensive on board power supply decoupling.
8. MC and MM versions can be built with some component changes, with essentially qualitatively identical performance.
9. 62dB gain for standard MC stage, 42dB for MM. Nominal sensitivity- 250uv @1kHz, 5cm/sec for the MC, 2.5mv for the MM.
10. RIAA simulated compliance c. 20mdB p-p, 20Hz-20kHz. Measured with non selected off the shelf components to be <50mdB p-p. Simulated worst case <100mdB p-p.
11. Unmeasurably low (limited by measurement system) distortion at rated output, including harmonic and many tone. Also applies to line and warp filter sections.
12. Phono Warp first order "elliptic" filter with vertical (L-R) rejection of c. 3dB at 140Hz and c. 36dB at 2Hz. Nominally 0dB rejection of horizontal (L+R) rejection. Great for rejecting noise from warped records or for some rumble components without disturbing the actual music frequency response.
12. Additional 10dB gain can be selected in the warp filter section to increase overall gain to 72dB/52dB.
13. Ultra low distortion, low noise high dynamic range Buffers/amplifiers for volume control and line driving added, 47 ohm output impedance.
14. Mono switch and balance control functions available.
15. Optional balanced output drive, 47 ohms output impedance/side (excludes "14")
16. Optional (separate board) OPA1656/LMH6321 headphone amp. On board heatsinking. Can drive loads as low as 15 ohms. Unmeasurably low distortion for 30 ohms and higher loads. Output impedance c.0.1ohms. 6dB gain.
Can be powered using same SMPS as the preamp.

Measurement system. RME ADI-2 PRO FS R with REW.
High quality boards available. Schematics, measurement results, BOM and build guides available.
installed.jpg

Mouser carts for the v2.0 boards.

https://www.dropbox.com/sh/3ax8p175lu2izoc/AADfl-SVvf84CR37Ra2lGMjla?dl=0

Pin strips for adapters

SMPS filter

RIAA MM single specific parts

RIAA MM parallel specific parts

RIAA MC single specific parts

RIAA MC parallel specific parts

RIAA Common parts

I/O parts

Line-buffer parts

Warp Main board parts

Balance control

Note: For the balance control, the 1.5k and 3.6K Rs replace R77/78 (9.1k) and R83/84 (5.11k) in the original line stage.
The balance board is assembled and wired between the Left mono and Right mono pads- left to left and right to right. Mono will still work, (i.e. a switch can short left mono to right mono) but it will disable the balance function if activated.

Headphone amp:


boards.jpg

Headphone schematic.

HP-schematic-19.jpg

Headphone wiring

https://www.dropbox.com/s/fu6nambryiqcots/HP-input-wiring.jpg?dl=0

The full schematic is too detailed to be visible except as a pdf.

v1.3 build notes.
https://www.dropbox.com/sh/r2rzhgtnb6b0m2t/AAAbOAk_lmSC9HwhT7G8KLiKa?dl=0

v2.0 build notes addendum
https://www.dropbox.com/sh/473t9jsaiv3jo1d/AAC6j1aOZU_10xlFV64ATMaPa?dl=0

V3.0, 3.1 full info, addendum.
https://www.dropbox.com/sh/92gig3oftxjqi9w/AACbKqpwnxBSPys9XDdVbqqSa?dl=0

https://www.dropbox.com/s/m62gn2j6ntb81hc/LOMC-v3-Addendum.pdf?dl=0
 

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BOM instructions below.
 

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Nice, Maybe add the link to the original thread in your OP?

EDIT: Here is a link to a thread for discussing the BUILD of the preamp chassis and its relative
options and features and how we implement them. This is to keep this thread about the actual electronics or the boards themselves and the theory of operation, specs etc.

Link to the Build thread

Athanasios
 
Last edited:
wyn palmer said:
Bumping up as the original thread is getting accessed.
Click to expand...


Ok, now maybe put a link to this thread in the other threads first post describing the purpose of this thread?

This way both threads can be navigated from each threads first post. :D

Make sense? Or would that be too confusing?

I still need to start building my two phono boards, not even sure which version they are. :dunno:

Athanasios
 
So, lots of links to BOMs and schematics, but I don't see any link to the v2.0 Build Guide. Where can I find that? As someone contemplating a DIY project that's always the first thing I look for, to determine if this is a project I want to tackle. To be clear, I can read schematics and BOMs and could even build one of your high gain phono stages without one, but a Build Guide is a real time saver when assessing the time and resources needed to complete the project.
 
MCM_Fan said:
So, lots of links to BOMs and schematics, but I don't see any link to the v2.0 Build Guide. Where can I find that? As someone contemplating a DIY project that's always the first thing I look for, to determine if this is a project I want to tackle. To be clear, I can read schematics and BOMs and could even build one of your high gain phono stages without one, but a Build Guide is a real time saver when assessing the time and resources needed to complete the project.
Click to expand...
The v1.3 build guide exists. Alas, my partner in crime @HalfApt is still polishing the v2 one up, and he has been rather busy of late.
I've tried to attach his latest document which can be combined with the v1 document on the original thread, but it's too large to download to AK.
I've asked him if I can link his latest v2 document.
To put it in context. I was able to assemble the v2.0 boards in a couple of hours using the mouser delivery. Most of the power supply wiring is either trivial or twisted/screwed, and the remainder is connections to optional switches/RCA sockets. There is no adjustment, no tweaking. All the components are identified on the board, including outlines, and in most cases, values. All of the IOs are marked, although perfect clarity is very hard to achieve.
I've built half a dozen boards and they've all worked perfectly first time. The hardest part is configuring the case, and I have no control over that.
 
Last edited:
wyn palmer said:
The v1.3 build guide exists. Alas, my partner in crime @HalfApt is still polishing the v2 one up, and he has been rather busy of late.
I've tried to attach his latest document which can be combined with the v1 document on the original thread, but it's too large to download to AK.
I've asked him if I can link his latest v2 document.
To put it in context. I was able to assemble the v2.0 boards in a couple of hours using the mouser delivery. Most of the power supply wiring is either trivial or twisted/screwed, and the remainder is connections to optional switches/RCA sockets. There is no adjustment, no tweaking. All the components are identified on the board, including outlines, and in most cases, values. All of the IOs are marked, although perfect clarity is very hard to achieve.
I've built half a dozen boards and they've all worked perfectly first time. The hardest part is configuring the case, and I have no control over that.
Click to expand...

To me, that's the part I am most interested in. Assembling and soldering the PCBs is the easy part. When I built the CNC phono stage, it took less than 45 minutes too solder both the phono stage and power supply boards. Building the enclosure was much more time consuming. That was one reason I was interested in a build guide, to see how much work is involved in the mechanical assembly, what size enclosure is needed, how many holes I'd need to drill, etc.

When I built the CNC, I made a custom wood case with cherry sides and a birdseye maple top. I lined the inside with adhesive backed copper foil for shielding. The v1.3 build guide says the enclosure must be made from metal. Is that really true, or can it be a copper foil lined wooden enclosure. Is the metal case requirement just for shielding, or is the metal case used as a heat sink for some devices?

In your photo above, I see what looks like eight RCA jacks. Is that two sets of independent inputs and outputs, one set each for MC and one for MM. My current phono stage has two sets of inputs, one for MC and one for MM but only one set of outputs. So, I can't run two tables off it at the same time and switch between them. It's either one LOMC or one MM, but not both at the same time. My question is, can your design be used with two turntables simultaneously, one with a LOMC cartridge and the other with an MM/MI cartridge. If so, how do you switch between the two tables?

The v1.3 document says it can be used as a stand alone preamp with a volume control. If so, does it support additional inputs for an SACD player, DAC, etc.? I didn't see a switch or knob for selecting different inputs.

If all I want is a standalone LOMC phono stage, is it possible to simply not populate the MM section and the volume control pot? I already have other MM capable phono stages (including a CNC) and am looking for a second LOMC phono stage and unless your preamp supports multiple inputs, I would run both phono stages (and probably my SACD player) into the line level inputs of a Nikko Beta III preamp that I would use for input switching and volume control. In which case, I don't need MM support or a volume control.
 
wyn palmer said:
I do have the v2 add ons,
I joined Dropbox, and the build guides should be visible.
V1.3
https://www.dropbox.com/sh/r2rzhgtnb6b0m2t/AAAbOAk_lmSC9HwhT7G8KLiKa?dl=0
V2.0
https://www.dropbox.com/s/15mupxdbqdt6nj2/LOMC v2 build options.pdf?dl=0

If someone can tell me if it works I'd appreciate it.
Click to expand...

OK, thanks. I had found the Drop box location for the v1.3 build guide and downloaded it a couple days ago. I wasn't aware of the v2.0 options document. I'll check it out. It may answer some of my questions above.
 
MCM_Fan said:
To me, that's the part I am most interested in. Assembling and soldering the PCBs is the easy part. When I built the CNC phono stage, it took less than 45 minutes too solder both the phono stage and power supply boards. Building the enclosure was much more time consuming. That was one reason I was interested in a build guide, to see how much work is involved in the mechanical assembly, what size enclosure is needed, how many holes I'd need to drill, etc.

When I built the CNC, I made a custom wood case with cherry sides and a birdseye maple top. I lined the inside with adhesive backed copper foil for shielding. The v1.3 build guide says the enclosure must be made from metal. Is that really true, or can it be a copper foil lined wooden enclosure. Is the metal case requirement just for shielding, or is the metal case used as a heat sink for some devices?

In your photo above, I see what looks like eight RCA jacks. Is that two sets of independent inputs and outputs, one set each for MC and one for MM. My current phono stage has two sets of inputs, one for MC and one for MM but only one set of outputs. So, I can't run two tables off it at the same time and switch between them. It's either one LOMC or one MM, but not both at the same time. My question is, can your design be used with two turntables simultaneously, one with a LOMC cartridge and the other with an MM/MI cartridge. If so, how do you switch between the two tables?

The v1.3 document says it can be used as a stand alone preamp with a volume control. If so, does it support additional inputs for an SACD player, DAC, etc.? I didn't see a switch or knob for selecting different inputs.

If all I want is a standalone LOMC phono stage, is it possible to simply not populate the MM section and the volume control pot? I already have other MM capable phono stages (including a CNC) and am looking for a second LOMC phono stage and unless your preamp supports multiple inputs, I would run both phono stages (and probably my SACD player) into the line level inputs of a Nikko Beta III preamp that I would use for input switching and volume control. In which case, I don't need MM support or a volume control.
Click to expand...
To try and answer.
Two boards can be used to construct MC/MM configurations. They can be used with a single warp filter on one board, with a switch in between, or with two warp filters with a switch afterwards.
The line amps are optional, and can be placed after an input selector switch, or not, and with a volume control, or not, in the single ended mode. They can also be configured as a balanced output, with all options except the volume control.
The configurations are described in the documentation, and the options are all independent so they need not be populated if not wanted.
My configuration is unusual- the joys of DIY. I have the stereo phono inputs- I'm MC only- with stereo outputs which leave to go to an external passive, remote controlled, relay selected, input selector and volume control with a LED display of input and volume. The stereo output from this comes back in and goes to the line amp and from there to two parallel sets of stereo outputs as I use multiple amps/drivers. The high driving capability of the line stage is very handy for this.
The metal box does provide valuable shielding. Without a cartridge connected and the external phono cables in place there are no measurable hum components, even with the IEC input and the SMPS in action.
The power consumption is extremely low. The total, excluding headphone amp, is in the order of 5W with two phono boards (MM and MC) and warp filters with the roughly 60% efficient SMPS that is recommended. Only the headphone amp requires heatsinking, and that is entirely contained on the headphone board.
The MM stage has the same level of excellence as the MC, and includes FET input opamps- which is essential, and it employs an identical architecture.
I think I covered everything.
 
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By the way @mkane did build it without using a metal box, and others have too. I don't recommend it, but it seems to work adequately.
You should ask him...
 
To clarify the metal box vs. copper foil issue.
The problem is penetration depth. Cu has only 20% greater conductivity than Al and foil is thin compared to the sheet Al that is used. It's not especially resistant to low frequency EM fields as a result. However, there is indeed existence proof that the hum fields are low enough that its unimportant.
In the passive attenuator I used multiple layers of Cu foil glued and soldered together to provide an improvement of >40dB in the hum performance relative to having none, so it can indeed work.
 
AvFan built the enclosures for our builds. On the outside its wood. Inside is a different story. I large case with 2 dividers, 3 compartments. I lined mine with copper foil. Don't think it made a difference. Womped myHherrons ass
 
Ok I just checked I have the v1.3 boards, How can I get the Add on boards and which boards are they?

Is it just the Warp filter and Headphone boards?

Athanasios
 
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