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Inexpensive IC based phono preamp for LOMC
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Author:  Dr.Turner [ 16 Nov 2014, 15:17 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Dear Bruce,
that clarified, another question sprouts naturally...why the bypass is 0,1uF on the +12V and 0,01uF on the -12V?

Thanks for your time, being a noob I would like to understand and learn, other than building an stunning preamp, of course :)

Later,
Dr.Turner

Author:  gofar99 [ 16 Nov 2014, 16:06 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Hi There are days that you just can't get anything right. The correct value is 0.1uf in both cases. See the attached, revised , revised and hopefully correct schematic below

Good listening
Bruce
Attachment:
LOMC IC Phono NAB Preamp November 17 2014B.jpg

Author:  Dr.Turner [ 17 Nov 2014, 03:01 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Dear Bruce,
now it make sense even to a noob like me!

So, if I got it right, I can use the 0,1uF Panasonic X2 metallized polyester film capacitor all through the power supply board (a total of four will then be used). Correct?

Later,
Dr.Turner

Author:  gofar99 [ 17 Nov 2014, 10:29 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Hi, 0.1uf is fine in all the locations. They all don't need to be X2 types, but there is no harm if they are. I often use X2s in places they don't need to be as I by them in bulk. The only place you need X2 caps is when they connect to any part of the AC mains including the ground portion. Most country's electrical codes require them there. X2 capacitors have a few differences from standard ones. Primarily they are rated for AC voltages as well as DC.

Good listening
Bruce

Author:  Felix [ 28 Nov 2014, 15:37 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

G'day all, long time lurker and I've finally just got around to officially registering. I do a lot of general DIY stuff mostly in audio and Amateur (Ham) Radio, with antennas a specialty. Anyway just a quick work to offer some praise and thanks for this design.

To my ears it is the best sounding DIY phono stage that I've ever heard and it just seems perfect for the OPA2134 dual op amp. It has became my reference DIY phono stage and sounds tremendous teamed with my OEM stylus Shure M97xE moving magnet phono cartridge.

As I only use moving magnet cartridges, I did not include any of the additional circuitry for moving coil cartridges and when I'm listening I simply cannot believe that such a simple RIAA preamplifier circuit can sound so utterly superb and 'musical' but this one does. Thank you Bruce Heran. Regards, Felix.

Author:  mwhouston [ 28 Nov 2014, 18:04 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

I'm back on mine because my other phono preamp, usually in my main system, is out on appro.

Have to agree there is nothing wrong and everything right with this little wonder. Just listen to two Opera box sets over the last few days. Absolutely enjoyed it. I must add a load changer to mine. My Ortofon likes 50ohm load, currently 100ohm.

Author:  Felix [ 28 Nov 2014, 18:20 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

G'day mate, yes an amazing little circuit it is, and it seems quite uncritical although when I first built it, it oscillated like crazy but eventually realised that I did not include any .1 uf supply bypass capacitors on the supply rails. Once installed, it was perfectly stable. I am using my old existing ESP P05B power supply to power it and maybe to leads were a bit long!

I also included some low value resistors in the output in the interests of providing some isolation from following stage capacitance. They are probably not needed but I included them, 'just in case'. Regards, Felix.

Author:  gofar99 [ 28 Nov 2014, 20:27 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Hi, It is part of a submission I made to Audio Express for publication next year (it will be the third I have had published). The device in the pending article adds a second stage as a line preamp and headphone amp for ones that can use up to about 5-10ma drive. That is the limit for the IC. Works great with my Sennheisers and Koss phones. I attached the schematic and some photos photos. One is a "deluxe" version and the other a compact one. Both are electrically the same. The deluxe has input selection and a better pot. Anyone who wants to build a full feature preamp with phono / R-R tape capabilities can build one that really sings for for a very low cost.

Good Listening
Bruce
Attachment:
LOMC IC Phono NAB Preamp Revision August 8 2014.jpg
Attachment:
Deluxe Front View.jpg
Attachment:
Deluxe inside no cover.jpg
Attachment:
Mini Front 2014.jpg
Attachment:
Mini inside 2014.jpg
Attachment:
50K 8 volt square.png

Author:  mwhouston [ 28 Nov 2014, 23:17 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Bruce do you have a graph of how well it tracks the RIAA curve. Also have you thought of using Bursons discrete OPAMPS in a build? I have.

Author:  gofar99 [ 29 Nov 2014, 08:43 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Hi Mark, I did do the test....but didn't keep the scope shots. They were so nearly flat lines from about 25 HZ to 22K that it was in my opinion too close to the tolerance of the inverse RIAA network and signal generator to be useful. As I recall there was a tiny (about 0.05db) rise at around 2K and the extremes at 20HZ and 20K were down about 0.2 dB. This is like I said though within the range of tolerance for the inverse RIAA and signal generator. Based on how OP amps work I suspect that the preamp in this case at least as accurate as the test gear. When I get a chance I'll give one a test again and keep the images.

As a budget project I didn't consider the Bursons for use in it. In reality any OP amp should work. The OPAs are really a bargain when used in audio projects. Cheap and excellent performance.

Good listening
Bruce

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