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Inexpensive IC based phono preamp for LOMC
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Author:  gofar99 [ 06 Jan 2014, 20:49 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Hi, Not at all. It was designed to give a high level of performance with minimal complexity and cost. Not competition for the big $$ guys. In a side by side comparison with my "Groove" there is a very distinct performance difference, but then I would expect that as the Groove play in a much more stratified region and is a lot more complex and costly. I do prefer the MIMIC sound over my Moon LP3 and it was in the $500 range. I find it to analytical with the MIMIC more musical. Individual preferences and results may vary naturally.

Third one is in the photo. The S/N has finally done it, in MM I can not measure it as it is buried in the shop noise floor around -100dbv. Response is the same 10HZ to 30K on the money to within about +/- 0.3 db. It is as good as my reverse RIAA filter is. With a LOMC setting (100 ohms and high gain) it just sneaks into my noise measuring levels at -98dbv. If nothing else IC based phono preamps with active equalization are really quiet.


Good listening
Bruce
Attachment:
Phono Preamp 3.jpg

Author:  mwhouston [ 06 Jan 2014, 21:41 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Bruce looks neat. You have just reminded me mine needs an earth lug. I have not needed it for my system but where I was testing it yesterday we had to rig-up an earth with alligator clips. Simple to fix.

Author:  gofar99 [ 06 Jan 2014, 22:01 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Hi, Yeah, I need one here all the time. We have radiant heat tapes in the ceilings and all sorts of stuff that generates EMI. I have to periodically sweep the house with an AM/shortwave radio to find the new ones. I suppose in a way that is good as I automatically include a number of EMI filters and concepts in my designs. Probably half the folks have no need of them, then again the other half would be distraught without them.

Good listening
Bruce

Author:  mwhouston [ 07 Jan 2014, 02:45 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

gofar99 wrote:
Hi, Yeah, I need one here all the time. We have radiant heat tapes in the ceilings and all sorts of stuff that generates EMI. I have to periodically sweep the house with an AM/shortwave radio to find the new ones. I suppose in a way that is good as I automatically include a number of EMI filters and concepts in my designs. Probably half the folks have no need of them, then again the other half would be distraught without them.

One thing I have included on all my preamps, phono preamps, MC pre-preamps and headphone drivers\preamps is ferrite beads http://www.jaycar.com.au/productView.asp?ID=LF1250 on the input wires inside the preamp. I lace fine wire warp twice through a very small bead. In the image they look bigger than what they are. In some cases in also include them on the ground or common wire and on the output wires. They are so cheap and the task is easily to perform why wouldn't you do it??? Also my much loved bitumanised heavy Al foil goes a long way to RFI\EMI suppression and resonance control.

Author:  gofar99 [ 07 Jan 2014, 08:38 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

A good thought with the IC ones in particular. Thanks. It hasn't proved to be an issue with the tube ones. I always use metal cases on the phono preamps for EMI/shielding.

Good listening
Bruce

Author:  gofar99 [ 12 Jan 2014, 14:00 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Hi, A small update. In the schematic it shows a type X2 capacitor between the case and the signal ground. This is important for things powered from the AC mains. Since this one is wall wart powered it can use a poly 0.1uf capacitor in that location.

Good listening
Bruce

Author:  steezebe [ 04 Feb 2014, 21:55 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Howdy all. This is my first attempt at building a phono pre-amp and I've got a couple questions about the grounding for the design. What do I do with the ground coming from my TT vs all of the other grounds? Can I send them to Mains ground? (I'm a digital guy exploring the analog world, and I don't understand why a TT has a ground in the first place...)

I build all the power cords and interconnects for my system, and since I can't stand wall warts, I plan on using the MeanWell PD-25 AC-DC PS internal to the preamp (which I will filter if necessary), and use a standard 120/60 plug with a ground pin. My idea is to bus all circuit grounds to earth, but I'm not sure if that is a good design or not?

Author:  gofar99 [ 05 Feb 2014, 09:30 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Hi, I'll give it a go again, when I check on the PS you want to use in place of the WW my text went into OZ. The preamp is a generally easy build. The ground wire from the turntable is a shield. It should be connected to the metal parts of the TT and goes to the metal case of the preamp. It reduces hum and sensitivity to EMI. It should not carry signal or power. The actual signal ground in the preamp is not connected to the case directly. It does connect via the 120R and .1uf capacitor. This provides sufficient isolation and maintains the integrity of the shield. There are other ways to do the same thing, but this has worked in my projects and commercial gear for a number of years. It is part of my goal to insure that (a) the gear is safe and (b) that I can maintain a S/N of at least -90dbv all the way to the speakers.

The thought behind using an AC to DC converter to power the preamp is fine...... however. If you use one that puts out the symmetrical +/-12 VDC with equal impedance it ought to work well. If you are thinking of using one to drive the DC to DC converter on the board it is not so good. I have discovered that even when well filtered the two will interact to some extent and you will get noise products in the signal output. This is also true of the WWs. Digital switching ones will tend to do the same. I went to RF chokes and filter network to see if you can get them to behave well and the success is not as good as I wanted. The thing to remember is that the OP amp is delivering about 60 db of gain over much of the frequency range and any noise will be amplified by a similar amount depending on frequency. It is something I discovered after the project was posted. My recommendation for best S/N is to use a WW transformer supply (needs only about 200ma) to drive the on board DC to DC converter. With that arrangement it is easy to get over 90 db S/N on MM (closer to 100 in all of mine) and around that in (90) in LOMC and as I use them NAB (reel to reel tape).

Good listening
Bruce

Author:  gofar99 [ 06 Feb 2014, 21:30 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Hi, Just in case someone didn't read the whole last reply......it is important to use a non-switching (doesn't matter if it is regulated or not) wall wart. Switching ones can interact with the DC to DC converter and produce artifacts. I had one do this and it was initially baffling. Batteries will work as well as the current drain is quite small typically under 10ma at 12V. The low drain is what I suspect caused the interaction as the switching wall wart was functioning below its minimum load.

Good listening
Bruce

Author:  mwhouston [ 08 Feb 2014, 02:09 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Bruce can the gain be boosted to 70db? Is it just lowering the 50R to say 45R?

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