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Inexpensive IC based phono preamp for LOMC
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Author:  gofar99 [ 08 Feb 2014, 09:56 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Hi Mark, maybe. Changing the resistor will alter the gain. However all OP amps have a property called gain-bandwidth product. Basically a relation be frequency and gain. The higher the frequency the lower the gain. Now since we want to cut the treble it may be possible. Worth a try. Worst that can happen is it won't work. My sense is that anything past 60db is pushing it though. In the NAB settings the treble is not cut and plateaus above about 2K HZ so there may actually be enough gain in the 2134 to do this. When I did a Bode plot on it, the response was flat out to nearly 30K in the NAB position and the gain there is at least 40 db. With RIAA equalization the gain should actually be less at the high end. At least that is what the theory says. The IC is rather stable at all gain levels so you should not have any funky things happen. So a qualified maybe. Let us know what happens.

Good listening
Bruce

Author:  mwhouston [ 08 Feb 2014, 18:19 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Thanks Bruce. Last night I had a classical night for seven others at my place. It was a vinyl night with recordings made in the early 60s. Not really the best to show off what the Mimic can do. What I found was the lead violin sounded great but the backing orchestra a bit of a mush. It is like they gave the violinist one mic and the orchestra had to share one mic.

The night was really about the music and not about the gear. Fortunately the lead violinist was breath-takingly good causing a few "wows" and sighs from the listeners. Mimic allowed us to follow the violinist and not miss a note. I have two tube phono preamps to assemble but for some time now and until they are complete Mimic will remain in my system.

Author:  gofar99 [ 08 Feb 2014, 20:14 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Hi, The Mimic was one of the nearly throw to gather projects I needed to test something and was quite a surprise that it worked so well. The more I tested and listened the better it was. The Groove is better, but the difference in complexity and cost makes the Mimic a really nice alternative. BTW we are working on a kit for it as there have been inquiries asking about it from folks who don't want to roll their own.

Good listening
Bruce

Author:  mwhouston [ 08 Feb 2014, 20:56 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

gofar99 wrote:
Hi, The Mimic was one of the nearly throw to gather projects I needed to test something and was quite a surprise that it worked so well. The more I tested and listened the better it was. The Groove is better, but the difference in complexity and cost makes the Mimic a really nice alternative. BTW we are working on a kit for it as there have been inquiries asking about it from folks who don't want to roll their own.

Good listening
Bruce

I think a PCB would be good start. PCB and bag of parts with instructions even better. And maybe two price points: high-end parts and econo parts for the newbie. You don't need me to advise.

I build on prototype brd. I have been using for years. Not for the newbie. I thought of making up some assembled brds. to pass on to those who want to try but are not up to parts assembly level. You may offer assembled brds two. Your PS is good. I like the battery idea.

Author:  gofar99 [ 09 Feb 2014, 08:41 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Hi, A PCB for sure. Likely an assembled version as well. I use quality parts now and skimping would probably not cut the cost much. All Silmac, Vissahy, Burr-Brown, gold jacks and so on. It turns out in most of the kits and such that the largest costs are in the PCBs (I like 3 ounce copper on premium board material) and the cases. The PS is different for sure. But it allows use on any power source that can deliver between 9 and 18 volts DC. Ripple and regulation are not required. The only draw back is the bit with some switching supplies. If they are too lightly loaded (as in the project) they tend to get noisy and cause problems.

Good listening
Bruce

Author:  Suncalc [ 09 Feb 2014, 11:37 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

gofar99 wrote:
The only draw back is the bit with some switching supplies. If they are too lightly loaded (as in the project) they tend to get noisy and cause problems.
Add a parallel load resistor across the switcher output. This way you can ensure sufficient current for a stable control loop. It's doesn't have to draw a lot of current. Just enough to allow reliable startup and steady state control loop stability.

It's a trick I use all the time when cobbling together test equipment from available resources.

Author:  kitchi [ 09 Feb 2014, 14:49 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

what size of a load resistor would you suggest Suncalc?

Author:  mwhouston [ 09 Feb 2014, 16:33 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Suncalc, good suggestion and easy to implement. Bruce, 3oz copper? I heard of 2oz but 3?

Author:  Suncalc [ 09 Feb 2014, 18:45 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

kitchi wrote:
what size of a load resistor would you suggest Suncalc?
In general, about 10% of the switcher's rated output is enough to ensure stable startup and operation.

Author:  steezebe [ 10 Feb 2014, 23:42 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

I am under the impression that the goal of a phono pre is to adjust for the RIAA curve; wouldn't I want my main preamp to provide or manage signal gain? Why would I want any gain from a phono preamp?

I feel my analog ignorance is showing.

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