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PostPosted: 21 Jul 2013, 15:04 
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UPDATE (1 September 2013) Project Page: Opamp Based RIAA Phono Preamp for MM and MC Phono Cartridges

Hi Everyone, I didn't want everyone to think I only do tubes (about 95% true though) so I did an IC project. I wanted to see how quiet I could get a preamp for low output moving coil cartridges (LOMC). So with a bit of studying and a bit of bench tweaking I came up with one. Response is as flat as I can measure between 10HZ and 30K IAW the RIAA curve. S/N above 100HZ is -96 dbv. At 20 HZ (worst case) it is -89 dbv. Very very quiet. The gain is about 58-59 db and input impedance is 200 ohms. The gain and input impedance can be adjusted rather easily. The sound is quite nice and the preamp could easily be the only one someone might want.
Attachment:
LOMC IC Phono Preamp July 21, 2013.png
Attachment:
IC Phono Preamp Inside.jpg
Attachment:
IC Phono Preamp Front.jpg
Attachment:
IC Phono Preamp Rear.jpg

Good listening
Bruce


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PostPosted: 21 Jul 2013, 17:03 
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Joined: 14 Feb 2010, 13:13
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Location: Lisbon, Portugal
I´m pretty sure this pre-amplifiers sounds good.

Op-amps have been very improved during these years.

A couple of weeks ago I finished mine based on the LME49720.

It is outstanding and by far the best one I have heard to far! :)


Cheers!


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PostPosted: 21 Jul 2013, 18:29 
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Hi, It actually is rather good, but the tube based Groovewatt with Lundhal SUTs up front is still better. So now I have another piece of gear to use on occasion. I need to stop coming up with ideas .... my storage space for good stuff is running low.

Good listening
Bruce

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PostPosted: 23 Jul 2013, 07:25 
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Bruce: I think you mean the values in blue are for MM not for MC.

I might also add that for something like the Ortofon Rondo Blue (which I have) the load resistor should be 50ohms or 100ohms. 50 ties the the cartridge down hard and produces almost a clinical performance where 100 allows a better musical flow without crushing dynamics.

For most MCs start with a load of X10 of the carts stated impedance plus some. So for a 4ohm MC cart. I would start with about 50ohms and play with anything up to about 200ohms. My Ortofon works well even with 220oms load but I strap down with 50. Time to play.

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PostPosted: 23 Jul 2013, 09:45 
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Hi Mark, True the blue values are for MM. My AT-33PTG uses 200, the AT-OC9 100, the Benz Micro 400. In in my main preamp I allow for values of 20,40,50,100,150,200,300,400,and 600. In a simple one like the one in the text I figure that a diyer might have only one cartridge and would pick the value that matched it and not worry about others. Either that or just add switches (BTW dip switches like in a PC work great for this).

Good listening
Bruce

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PostPosted: 23 Jul 2013, 16:32 
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While you adding DIP switches for loading might as well include being able to switch from MM to MC. Thanks for the schematic. 0.5 db tracking is a close fit. That could vary with component quality. Either way it is nice and tight.

I would probably run mine from batteries. But that's just me.

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PostPosted: 23 Jul 2013, 16:55 
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Hi Mark, Yes you could switch the gain that way too. It is just grounding one or more resistors. I list the fit to the curve as shown as that is the tolerance of the inverse RIAA filter I use on the Bode plot tests. The actual fit may be better as the line on the graph is for all intents and purposes a flat line from 10HZ to past 30K. Since the inverse filter has only a tolerance of +/- 0.25 db that is what I listed. The scope says half that for deviation.

The use of the Meanwell DC to DC converter saves the use of batteries and is quite clean. The noise below 20HZ is likely an artifact of the way I have to measure it. So the -89dbv there is pretty much of no consequence. It could be as low as the rest of the spectrum. There is no noise at the speakers even with my ear on them. At first I though it wasn't working as the LED in it was not glowing (a wire came loose). Putting the needle on the lead in groove made me jump with the loudness. BTW.... if you ever need a really first rate cartridge you should investigate the AT 33PTG/II. It is hard to believe the quality of the sound for right at $500US. I would rank it with ones at twice that price. Clean, detailed, quiet in the grooves, excellent channel separation and a good match for nearly any type of music. Especially good for handling female vocals.

Good listening
Bruce

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PostPosted: 23 Jul 2013, 17:24 
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When I bought my direct drive TT a year or so ago it came with an AT MC cart. The TT is a DJ-Tech which is really an Audiotechnica TT re-bagged. But the TT has a thick steel plate under the patter and most MC carts have huge magnets. The arm would not track due to these issues. So I use the TT for my MM carts. Perfect.

At the time the TT was sold with a AT MC cart and the cart was only $300. A friend (Martin) has the same cart on a better TT. Every time I hear it I am stunned by how good it sounds for $300. It may be the same cart. I do remember it was a AT33???.

I may just build your MC pre.

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PostPosted: 23 Jul 2013, 18:27 
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Hi, Yes the dreaded steel platter syndrome. My re-plinthed and re-armed 701 has a cast non ferrous platter (about 3KG) so all is well. There is another AT-33 series that is more common I believe it is the AT 33ML. Also quite nice. The PTG ones are only available directly from Japan and rather unknown.

The preamp is easy to build and you can adjust the RIAA components to alter the sound a bit. I don't suggest that unless you have a way to verify the matching to the curve. Changing the input load resistor and gain resistor does not seem to alter the response any (thank goodness).

Good listening
Bruce

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PostPosted: 23 Jul 2013, 20:12 
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Bruce: I found the name of the AT craft. It is the ATF3/111. Only $300 here and an excellent performer. Early next month I have organised a classical night at Martin's and it is an all vinyl night. So I and 8 other will be well entertained on the receiving end of this fine (and bloody cheap) cart.

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