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Inexpensive IC based phono preamp for LOMC
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Author:  gofar99 [ 17 Nov 2013, 08:54 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Step Up Transformer for low output moving coil cartridges. If you only use high output ones or MM/MI ones then there is no need. I seem to recall you used Lundhals.

Good Listening
Bruce

Author:  mwhouston [ 17 Nov 2013, 17:30 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

gofar99 wrote:
Step Up Transformer for low output moving coil cartridges. If you only use high output ones or MM/MI ones then there is no need. I seem to recall you used Lundhals.

I got the lundhals for MM to act as dampeners\loading but it never worked. I use my Hiraga bipolar MC stage mostly but also have a few Le Pacific for the MC stage. This is passed on to Boozhound for further amping as MM\RIAA.

Completed the new phono stage last night but I have oscillations and now have to twist ground wires around my hot wires inside the preamp. There are a few straight wires inside carrying signal and feed back and though I have them well separated the preamp doesn't agree.

Author:  gofar99 [ 17 Nov 2013, 20:45 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Hi, A bit OT, but you should try the Lundhals as SUT for one of your low output cartridges and then feed that into your MM phono preamps. I find it quieter and having a really nice sound.

Good listening
Bruce

Author:  mwhouston [ 17 Nov 2013, 20:52 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

gofar99 wrote:
Hi, A bit OT, but you should try the Lundhals as SUT for one of your low output cartridges and then feed that into your MM phono preamps. I find it quieter and having a really nice sound.

I wish you could try my Hiraga? But I know a lot of flat plastic spinners love their SUTs.

Author:  mwhouston [ 20 Nov 2013, 02:54 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

After three days of trying to track down osculations, today, while drive my daughter to the doctors it hit me, the 1nf cap I put on the input were causing the issue. When I got back I cut them away and now all is good. I figured this because every time I pulled the inputs oscillations stopped. With all fairness to Bruce it was not on his original schematic. Currently using MC mode with 50ohm loading.

The sound feels natural. I'm yet to compare to my Hiraga\Boozhound setup but currently I like it. Also it needs some time under the belt. But first impressions are good. A bit hissy but so are my others. How hissy? Yet to compare.

Bruce I hope you don't mind the name. But may mind the colour. Happy to have it going and so compact with so flexibility. I can chose MM or MC and in MC 50 or 100 ohm loading. I'll come back with a better sound eval. after some more listening.
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Author:  gofar99 [ 20 Nov 2013, 22:11 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Hi Mark, the name is fine 8-) . Mine are not hissy so I wonder if your ICs might be noisy. I had one batch of them that was. They also didn't balance at near zero output on a symmetrical power supply. One was nearly at the positive rail voltage. Go figure. An alternative is that just perhaps because of the design I used in the PS it is quieter than some 9 volt batteries. This would not be usual or expected, but batteries can have a rather high impedance on occasion and the PS I used is quite low. Good food for thought. I measured both of the ones I built and they were over -100dbv in MM and over -90 dbv in LOMC. Both levels are inaudible.

Yes the input cap would be likely to create a resonant circuit with the inductance of the cartridge. I seldom use input caps on any project as virtually all source equipment will either have on on the output side or not need one to start with.

Good listening
Bruce

Author:  gofar99 [ 24 Nov 2013, 21:43 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Hi Mark, I retested my preamp and it was 0.4mv and 0.5mv on MM wideband. On the spectrum mode it was better than -95 db above 100HZ and only peaked once at -89 below that. values at LOMC were roughly 5-6 db worse. Only the Simaudio Moon was quieter and not all that much. It was at 0.2 and 0.2mv on LOMC and 0.6/0.7 on LOMC. It is rated at -105 and 95 respectively. Response of the IC preamp is as you would expect ruler flat using an inverse RIAA source. It is as flat as I can measure between 10 HZ and 30K.

The tube ones BTW are about 3-4 DB noisier and nearly as flat from 20HZ to 20K.

Good listening
Bruce

Author:  mwhouston [ 25 Nov 2013, 05:11 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

A guy called Alan from Seattle saw mine and bought it. Gone today! I should at least build another cct. brd. I didn't even get to listen to this for 7 days! Now Matt's 6V6 maybe.

Author:  mwhouston [ 06 Dec 2013, 00:19 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

I'm going to build another "Heran" phono stage. This time not as flexible and a lot cheaper. I should be able to build it for about $30 not including batteries. I have all the parts.

My audio mate, Dave (think 400lb speakers, quad push-pull 300B monoblocks), was disappointed I sold the first in under week before he got a listen. He has an original Sure V15 MKIII with new stylus and the new Pro-Ject RPM 10.1 (I want one) TT.

This time: off the shelf light enclosure, single input on cheaper (still gold) RCAs, only three 9V batteries but still mil-spec chip socket and OPA2134. I'm keeping the ability to switch MC & MM but locking the MC load to 100ohms. This should suite 90% of MC carts.

Author:  mwhouston [ 06 Dec 2013, 00:24 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

gofar99 wrote:
Hi Mark, the name is fine 8-) . Mine are not hissy so I wonder if your ICs might be noisy. I had one batch of them that was. They also didn't balance at near zero output on a symmetrical power supply. One was nearly at the positive rail voltage. Go figure. An alternative is that just perhaps because of the design I used in the PS it is quieter than some 9 volt batteries. This would not be usual or expected, but batteries can have a rather high impedance on occasion and the PS I used is quite low. Good food for thought. I measured both of the ones I built and they were over -100dbv in MM and over -90 dbv in LOMC. Both levels are inaudible.

Yes the input cap would be likely to create a resonant circuit with the inductance of the cartridge. I seldom use input caps on any project as virtually all source equipment will either have on on the output side or not need one to start with.

Good listening
Bruce

Bruce just re-reading: this my audio friend, Dave, bought two stereo chip amp kits from China. All LM3875s (4) were shorted internally and dropping 36V on the output. I got him four chips from work, he dropped them in and now all is fine. Sh!t happens.

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