High gain opamp based preamp for low output MC cartridges.

[wyn palmer]

In 2013 Hypnotoad designed the excellent AD797MCpre which has been built and used by numerous DIYers over the years.
This takes that original design and adds various refinements to improve noise, overload characteristics and RIAA compliance, and to remove the 2.2uF coupling cap by including a DC servo function.
It also adds the circuitry needed to extend the original phono stage into a completely functional preamp with a record output, stepped attenuator volume control, >200kHz bandwidth and unconditional load stability while maintaining DC coupling throughout.
The preamp also performs well with a SMPS, eliminating the need for either batteries or transformers, even with an IEC socket being added to the case.
The phono stage has 63dB of gain and harmonic distortion at 1v output which is unmeasurable.
ALSO: the design has a modified BOM for a MM cartridge with a reduced gain of c. 43dB.
Schematics are provided, together with parts lists and a board which can be purchased. The board enables either a phono stage only or a complete preamp to be built.
Simulated RIAA nominal compliance is +/-10mdB and measured as less than +/- 50mdB, 20Hz-20kHz. Expected DC offset at the record output is <1mv, and <2.5mv after the 6dB output buffer.
The design has been constructed and measured, and it performs, with a SMPS, as expected, with low noise levels dominated by mains hum and spurs, due to the SMPS switching, at 100dB or more below a 1v output level.
The output sounds quiet, smooth, and detailed, and very listenable to, with a nominal 230uv @5cm/sec 1kHz cartridge.
Due to the constraints of the slightly unusual architecture chosen, the gain is fixed and cannot be readily changed, but the add-on warp filter board provides switchable 10dB of gain if needed.
Together with the preamp board there is a secondary board which holds the extra power supply decoupling components for use with the SMPS.
The boards are designed by another Audiokarma contributor- HalfApt- and his enthusiastic efforts are essential to this project.

The Drop box info (below) includes build notes (post #560).
https://www.dropbox.com/sh/4iwr55ql8ynmfnx/AADX2-izp3fJig0ZrnkxcJwTa?dl=

Below are extra links to the BOM for the warp filter and headphone add on boards. Later I'll add more information on the new add-ons.
Mouser cart for headphone amp:

https://www.mouser.com/ProjectManag...jectGUID=f6485585-7fff-45e5-8a39-0c3592d2fc62

Mouser cart for warp filter:

https://www.mouser.com/ProjectManag...jectGUID=c1b708b2-2d04-4880-911d-fe925d064ddf

Mouser carts for the v2.0 boards.

Pin strips for adapters
https://www.mouser.com/ProjectManag...jectGUID=96cd444a-da59-42b2-a14b-10d611a18d04

SMPS filter
https://www.mouser.com/ProjectManag...jectGUID=97f8516d-fe9f-49bc-bd16-4eca06a8cfb5

RIAA MM single specific parts
https://www.mouser.com/ProjectManag...jectGUID=9206d8aa-7d5d-472e-9b02-139b75a9e7ba

RIAA MM parallel specific parts
https://www.mouser.com/ProjectManag...jectGUID=baba71ac-4abd-46d5-b41b-e56b176f07a7

RIAA MC single specific parts
https://www.mouser.com/ProjectManag...jectGUID=e3dd6792-b6bb-4f33-8e07-28e7c6664064

RIAA MC parallel specific parts
https://www.mouser.com/ProjectManag...jectGUID=bd0ce5e8-f10e-4d43-a500-d2f1804f83dd

RIAA Common parts
https://www.mouser.com/ProjectManag...jectGUID=657da618-4f53-482d-9959-aa5992b8ab6f


I/O parts
https://www.mouser.com/ProjectManag...jectGUID=e104e74b-5a86-40c7-a96b-9891bc37ff63

Line-buffer parts
https://www.mouser.com/ProjectManag...jectGUID=18d257fd-9b8e-496f-ad41-f64b6d65c5ff

Warp Main board parts
https://www.mouser.com/ProjectManag...jectGUID=55D79F3B-C5C9-48A4-95FB-3FA59F420457

 

[wyn palmer]

First prototype preamp. Still with coupling caps.

Measured results for prototype preamp.
-0.9dBV output- corresponds to 0.6mv input at 1kHz.
81dB S/N A weighted- corresponds to 73dB at 0.25mv input.
60Hz 74dB below the 0.25mv input level.
No measurable distortion.
S/N was simulated for a -9dBv output at 67dB unweighted and 72dB A weighted.
This is for an output level 8dB lower than the measurement, so if corrected to a 8dB higher level the result was simulated to be 75dB unweighted and 80dB A weighted.
The weighted result is almost exact, the unweighted is off by 3dB due to the hum, but A weighting attenuates the LF to correct for hearing sensitivities, so the part is essentially exactly as simulated as far as the ear is concerned.

 

[wyn palmer]

The first view of the prototype board...

 

[wyn palmer]

Some simulation results with a signal with RIAA pre-emphasis applied.


63dB gain, 18mdB p-p error from ideal RIAA, 20-20kHz.

10Hz-1MHz. Excellent HF roll off, no peaking anywhere. Very nice phase response.

 

[mkane]

Interested

 

[wyn palmer]

Maximum output of the phono stage is 9v rms, or about +19dBv.
Nominal output level -9dBv, so >=28dB overload at the output at all frequencies...
Input stage has >=28dB overload at all frequencies following the RIAA curve up to c. 150KHz.
No measurable TIM for frequencies up to 50kHz, although I'm refining my testing.

 

[wyn palmer]

Interested

Is there any detail that you would like to know?
I'll post a parts list in a while. There's some refinement of actual component choices still to come.

 

[wyn palmer]

For those who are interested - a full set of schematics of the preamp- but not the supply.

 

[mkane]

I can't make heads or tails of a schematic. I can DIY.

 

[wyn palmer]

I can't make heads or tails of a schematic. I can DIY.

Then I'll try to make the instructions as detailed as I can, but in any case feel free to ask.
The schematic and parts list agree on numbering, with the left channel components equal to the right channel with 100 added.
I will start by listing the right channel components for the actual Phono RIAA/gain stage, including the offset correction loop, then the output stages.
The Phono RIAA/gain stage can be built separately without populating the output stages. Unlike the original AD797MCPRE the lack of a coupling cap makes it able to operate into stages that have quite low input impedances- less than 2k ohms.

 

[mkane]

Excellent wyn. I've built quite a few of Sachins phono stages along with quite a few amp kits.

 

[AvFan]

Interested too.

• Does this preamp allow easy changes to the cartridge resistance load?
• What is the capacitance of the preamp? Can it be easily changed by the user?
• Does this continue to use the AD797 or a different opamp(s)?

 

[wyn palmer]

It has pins that can be wired up to an external switch that will permit that. My own version has a two position switch for the cap and a two position switch for the resistor, but there can be as many positions as you wish. There is no explicit location on the board for a series of caps/resistors.
I viewed having built in options for capacitors/resistors as being impractical as there is no way of determining what is a suitably limited set of choices. For example the cartridge I use, the Miyajima Madake, is best loaded in a 100 ohms/0.68uF load according to Miyajima-san, but I like to load mine with 60 ohms/0.47uF. and there are many cartridges that have very different opinions on what the load should be, ranging from a few tens of ohms to several hundred ohms (or even 47k!) and capacitances from 100s of pFs to >1uF, which for a polypropylene film type can be very large.
The input device that I use is the AD797, but there are other choices that are suitable- the LT115 comes to mind. The output amp could, for example, be an OPA1611 and the buffer amps OPA1612. The DC feedback amps are a TL052ACP.

 

[wyn palmer]

It has pins that can be wired up to an external switch that will permit that. My own version has a two position switch for the cap and a two position switch for the resistor, but there can be as many positions as you wish. There is no explicit location on the board for a series of caps/resistors.
I viewed this as being impractical.. For example the cartridge I use, the Miyajima Madake is best loaded in a 100 ohms/0.68uF load according to Miyajima-san, but I like to load mine with 60 ohms/0.47uF. and there are many cartridges that have very different opinions on what the load should be.
The input device that I use is the AD797, but there are other choices that are suitable- the LT115 comes to mind. The output amp could, for example, be an OPA1611 and the buffer amps OPA1612. The DC feedback amps are an AD712. (It seems that Scott Wurcer is featured heavily in this design).

I just tried out a LT1115/OPA1611 combo and other than the noise being about 1dB lower I don't see any difference either in measurements or in simulation, so that will be my combination of choice
The opa1611/1612 will have to be mounted on a carrier, but the new combination is still cheaper than any of the alternatives.
I will post a parts list today.

 

[wyn palmer]

Is anyone actually interested in a circuit description?

 

[mkane]

You bet.

 

[wyn palmer]

The parts list and circuit description will be delayed by a few days as changes are having to be made to accommodate problems in laying out the board and the addition of new features.
The circuit has been changed a bit to alleviate the problems and may change more as the board evolves, although this is unlikely.
We are adding provisions for easy changing of cartridge load components (as was requested), and doing a cost and size reduction by lowering the number of 1% tolerance capacitors and resistors, minimizing the quantity of caps that are not 5mm lead spacing, and reducing the electrolytic max voltages to 25 and 35 volts from 35 and 50 volts, which better matches the max. voltage rails of the LME49710/20 which is limited to +/-17v, and the recommended voltage of +/-15v from the recommended SMPS- the Meanwell PD2515 from Mouser.

The circuit changes have not altered the performance in a significant way.

 

[mkane]

 

[electrode101]

For those who are interested - a full set of schematics of the preamp- but not the supply.

I dunno. Maybe it's because it's Monday; when I open the file I only get text, no schematic... what program are you using for this?

Thanks.

jD

 

[wyn palmer]

I dunno. Maybe it's because it's Monday; when I open the file I only get text, no schematic... what program are you using for this?

Thanks.

jD

You have to remove the .txt extension and leave the .asc.
You need to add the .txt extension to allow the file to be uploaded to AK.
The latest schematics are a little bit different, and they will be posted once the board is complete.