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Inexpensive IC based phono preamp for LOMC
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Author:  wdecho [ 14 Aug 2018, 12:10 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

I recently bought a new turntable with a built in MM phono pre with an AT-95e cartridge installed, Fluance RT81. A step up from what I was using and as it was shipped sound really good. Having a MC phono cartridge has been on my bucket list and I figured now is time to try one. I purchased the Denon dl110 which is a high output MC cartridge with a 47K load front end The output is 1.6mv if my memory is correct, a little low for a mm preamp but high enough to use. I have listened to it for a week or so with the built in pre and it did have enough gain to adequately drive my setup with the built in mm pre. That being said many in their reviews of said cartridge claim an improvement when used with a mc phono pre so I decided to build a new one of these preamplifiers from scratch. I had used this same phono pre with my old turntable and have enjoyed it over the years. For a normal mc cartridge Bruce used a 51R and 330R in parallel for roughly 47 ohms resistance. At least as close as I have in stock, the actual number is a tad lower. I decided using a 100 ohm resistor for the gain of the op amp would be pretty close to what I desired with this cartridge and it turned out to be about right. I can always change that resistor out and vary the gain. I just built this pre exclusively for this cartridge so I just settled on the 47K resistance on the input. If I were to buy a different mc cartridge with like .3mv output I would just build another one of these preamplifiers being it is so easy to throw together. Anyways I like to build. From a brief listening the top end seems to have more sparkle and the bass appears to be quicker using the same amplifier I listened with last night. Right now I have installed the OPA 2134 but in the past have used the NE 5532 in this pre. Many knock the old NE5532 but I really cannot fault anything about it's sound. Other op amps have a different sound but I would not say better, just different. I am considering trying the LM4562 op amp when I make a new Mouser order. I believe it is an improvement over the phono pre that came with the Fluance RT81 turntable. By the way I love this turntable. Reminds me of the ones of old. I am a happy camper with my new phono pre, mc cartridge and turntable. I still prefer the sound of vinyl over CD's.

Author:  gofar99 [ 14 Aug 2018, 13:12 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Hi, I have DL110 as well as several others and it is a tad lower in output than most that use MM gain settings. The IC preamp can pretty much deliver any gain you might need. For a while I used one with a 500 ohm stereo pot and fixed resistors of 33R (to keep it from going to zero). You can also put a linear 5K pot from left to right inverting inputs with the CT on ground and use it to balance mismatched cartridge channels. Other values for the pot may be OK as well as long as you can adjust the gain of one side and not be so low in total to mess up both channels. I use 47K on the DL110. IMO a pretty good all around cartridge. I have others I prefer more though.

Good listening
Bruce

Author:  mwhouston [ 14 Aug 2018, 17:56 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

I've built quite a number of these mainly for MM and those who have them love them. I've also developed a linear PS using ultra low noise discrete regs which has worked well too. Usually mine are battery powered.

Author:  wdecho [ 16 Aug 2018, 08:36 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

I cut the gain back some more. I had very little control over the volume using a 100r resistor vs the 330r. I am now using 200r ones and they seem to be about just right for this Denon dl110 cartridge.

The idea of installing pots for gain is very interesting especially if I buy, try, more cartridges.

Author:  mwhouston [ 16 Aug 2018, 14:06 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Generally even the best pots are noisy. A way around that may be to use a step attenuator. The ones I use only have two resistors in cct. at any time and gold contacts. They are A$80 each and probably don't down go as low as a 1K so you may not find a suitable one.

Or a resistor wheel could be another good one. Cut the leads down as short as possible. You would need two but they are generally cheap. You could then dial-up the resistors you want and leave them in place.

Author:  gofar99 [ 16 Aug 2018, 15:56 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Hi, I put a set of DIP switches in one. It had several values of the gain and loading. With about 3 resistors with separate switches you can get a lot of possible gain combinations. I seem to recall I used a pair of 330R and a 75 or 68R for gain and a 47K (not switched and always in the circuit), a 400, a 330 and a 180. With parallel combinations you can get all sorts of values.

Good listening
Bruce

Author:  mwhouston [ 16 Aug 2018, 18:27 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

gofar99 wrote:
Hi, I put a set of DIP switches in one. It had several values of the gain and loading. With about 3 resistors with separate switches you can get a lot of possible gain combinations. I seem to recall I used a pair of 330R and a 75 or 68R for gain and a 47K (not switched and always in the circuit), a 400, a 330 and a 180. With parallel combinations you can get all sorts of values.

Good listening
Bruce

Good one Bruce. I had a center off switch on the Heron. With a 220ohm always in and switch other resistors across it to give MC loadings of 220, 100 and 50. Worked well.

Author:  wdecho [ 23 Aug 2018, 06:54 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

I tried 3 different op-amps in this phono preamp, the NE5532, OPA2134 and a LM4562NA. The NE5532 sound is very different from the other two. More mellow sounding and not my favorite with my new moving coil cartridge. The differences between the OPA2134 and LM4562NA are not that noticeably different. It takes time to swap out the op-amps and audio memory can be very short but the differences are very subtle. I am really liking the LM4562NA and it will stay in for now. I found this design which is very similar to this one on the net. I do not know if Bruce has ever seen it or not. http://oi62.tinypic.com/241360l.jpg If not the designer of that one and Bruce came up with very similar values and layout particularly in the feedback network. Bruce's sounds great in my setup with my old ears and is easier to build with a great power supply design. Nice and quite with my 103db horns.

Author:  john_c_kennedy [ 24 Jan 2019, 18:55 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

I built this preamp pretty much following the circuit here: download/file.php?id=6789&mode=view. Except I made a PCB and used 1% 1/8 W metal film through hole resisters and 5% poly caps (except the PS electrolytics).

Since my turntable is a fairly low end Audio Technica AT-LP120-USB with a AT95E MM cartridge I didn't include all the LOMC circuitry. Though the turntable comes with a built in preamp I thought this one sounded much better, but I'm hearing impaired (actually I'm deaf except I have a cochlear implant in one ear) so YMMV.

I also built this to test it with: http://hifisonix.com/wordpress/wp-conte ... e-RIAA.pdf. This is a fairly accurate Inverse RIAA RC network (completely passive) the above link has the details. I used this with ARTA Steps software and a Steinburg UR22MKII USB audio interface to measure the RIAA compliance, and THD of the preamp. The results are shown below. Basically the combination is within about 0.3dB of the RIAA curve (Note the scale of the curves). The THD of the combination is very low also as shown in the graph. The inverse RIAA network makes it easy to test these little preamps because without it the input levels to the preamp are so low it's difficult to test them with a signal generator and scope/DVM alone.

I'm in the process of building this one from TI: http://www.ti.com/lit/an/snoa586d/snoa586d.pdf, which they claim is very accurate also. The app note also contains some interesting info about solid state RIAA preamps too. I'll post the results when I get it completed and tested.

John

Author:  mwhouston [ 24 Jan 2019, 19:24 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Neat build. And great results. I build mine on strip bird. The PS is also on the same board using a resistive network to provide the differentual voltage.

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