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PostPosted: 19 Aug 2013, 21:31 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi Everyone, I tidied up the build on the preamp and did add a 12 section dip switch so I can switch loads on the LOMCs I have. The AT-33PTG/II likes to see 200 ohms, the AT-OC9ML/II likes 100 better. I re tested the preamp and on MM it is now at the limits of what I can measure. I was showing a S/N of -101 dbv. Which incidentally matches what I get with the preamp disconnected from the power and input shorted. For LOMC it is in the -96 dbv range. Both settings still provide ruler flat response (within the tolerance of the inverse RIAA filter). It is really neat to see a Bode plot on the scope that is a straight line from 20HZ to 30K. To get it that way though you need to be sure the equalization components are right on spec and you must use an op amp that has both a high gain/bandwidth product (spec) and a high slew rate.

Good listening
Bruce

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PostPosted: 27 Aug 2013, 23:00 
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Hi Everyone, I wrote this up a bit more and sent it off to be a project so if you are interested in a simple good sounding phono preamp keep an eye on that section of projects. In the final version I included the dip switches to change the loading and gain. I use it only with LOMC so an external gain switch was not needed. You can pretty much pick and choose the load resistors for LOMC without causing any particular sonic changes. If you go below about 25 the gain drops a bit though. BTW I am contemplating doing kits on this this as it is so simple and quite good. Easy to build as well.

Good listening
Bruce

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PostPosted: 28 Aug 2013, 03:03 
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Joined: 06 Jun 2008, 18:23
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Location: Australia
Bruce, good work.

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PostPosted: 28 Aug 2013, 18:44 
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Sorry for the hold up guys. Yard work has been keeping me far too busy this summer. I should have the project page posted this weekend.

Cheers

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PostPosted: 01 Sep 2013, 10:29 
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Project page is now available: http://diyaudioprojects.com/Chip/Opamp-Phono-Preamp/

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PostPosted: 01 Sep 2013, 16:09 
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Joined: 14 Feb 2010, 13:13
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Location: Lisbon, Portugal
Great project!

I might give it a try to compare with my LME49720 :)

Bruce, where did you get that nice ground buss?


Cheers!

Miguel


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PostPosted: 21 Oct 2013, 07:47 
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Joined: 05 Jun 2012, 13:10
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Location: Darboy, WI, USA
Bruce,

I'm thinking about building your phono preamp. I've just purchased a Ortofon 2M Red (specs: http://www.ortofon.com/products/cartridges/cartridges-2m-series/2m-red ) to use with my Akai AP-100C turntable.

Would I need to change any of the resistor values for this specific cartridge? Or should I just build to the schematic?

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PostPosted: 22 Oct 2013, 22:14 
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Hi, I would use the existing 47K load for the 2M Red. The output is high enough to run in the MM mode. It ought to sound quite nice. I am currently using it with my Grado Reference Platinum cartridge and it is quite clean. I have used it with an Ortofon OM30 and it was really good with it as well. BTW the reason I switched to the Grado is the wood body matches the color and grain of the Sota turntable it is on. Both cartridges sound pretty much equally great, but in this case I went with the "looks". Actually it was because the Grado uses an elliptical stylus and the OM30 a more radical design. I find that on occasion the elliptical works better on some LPs than my main TT with its AT33PTG/2 (a truly great cartridge only available directly from Asian sources).

If you want to stick to only high output cartridges (those over about 2.5mv) you can eliminate the 51R gain adjust resistor and 220R load resistor as well as the switch. The 2M can work with a wide range of load capacitance and your input cables will likely be sufficient. Thus I would not add additional capacitance. If you really get inspired you can try adding 100pf in parallel with the 47K resistors. Use mica capacitors there if you do that though as they seem to sound better and are extremely stable.

Good listening
Bruce

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PostPosted: 22 Oct 2013, 22:17 
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Hi Miguel, I missed the question .... the ground lug is actually a speaker terminal for use with wall plates. You can also get them separately from lots of sources. I just didn't use the insulating washers. My beef with them is the set screws. They are very soft and you can strip the slot on them rather easily.

Good listening
Bruce

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PostPosted: 23 Oct 2013, 15:26 
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Location: Las Vegas Nevada USA
Unfortunately the latest generation of "super" audio op-amps are available only in surface mount packages that are almost impossible to work with. However, with thin traces, if you can get an 8 pin package it's possible to use them. You'll need a sharp iron, steady hand, and a magnifying glass.

There are some with low impedance imputs that have insanely low noise and distortion. However, the OP134 op-amps have always sounded great to my ears despite their high impedance inputs. I have them in most of my homemade solid state DIY audio projects.

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