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Inexpensive IC based phono preamp for LOMC
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Author:  mwhouston [ 24 Jan 2019, 19:31 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

This is a two stage preamp using the LM833. I also built another using Bruces cct. again using the 833.

http://retro-thermionic.blogspot.com/20 ... -chip.html

But I feel the chip is a bit dull and the OPA2134 better. But some may like the 833s flatness.

Author:  mwhouston [ 24 Jan 2019, 19:33 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Here is the one using Bruces cct. 833.

http://retro-thermionic.blogspot.com/20 ... 833mc.html

Author:  john_c_kennedy [ 25 Jan 2019, 13:15 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

mwhouston wrote:
This is a two stage preamp using the LM833. I also built another using Bruces cct. again using the 833.

http://retro-thermionic.blogspot.com/20 ... -chip.html

But I feel the chip is a bit dull and the OPA2134 better. But some may like the 833s flatness.


Yes, it's 2 stages but the app note explains why they use the 2 stage design. However I personally think the OPA2134 is a better opamp spec. wise because of the FET inputs it has very low bias current and offset voltages, and I think it would work in the 2 stage TI circuit without modification,so I may try it and compare.

Author:  mwhouston [ 25 Jan 2019, 17:31 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

One reason I tried the 833s is I had four from other kits where I replaced the chip.

The other reason is I wanted to try an all bipolar chip, no fets in the audio cct.

Author:  Felix [ 25 Jan 2019, 23:48 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

G'day all, I have built a few of this design and subjectively it is as sonically good as any of my other more complex designs. I prefer to use the OPA2134 with my versions as for some reason the OPA2134 have the lowest hiss level! Regards, Felix.

Author:  john_c_kennedy [ 28 Jan 2019, 14:42 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Ok I built the TI circuit mentioned in the post above. As discussed this is a 2 stage circuit and uses a TI LM833 audio opamp. I also made a PCB shown in the pictures below (no case yet). This PCB and the one I made for Bruce's circuit are single sided but use isolation routing so both boards have an extensive ground plane so noise should be minimal. I also tested this preamp with my inverse RIAA network using the ARTA Steps software, the results are also shown below. This preamp is more accurate following the RIAA curve than Bruce's circuit about 0.015 dB error vs 0.03 dB error. However, the THD is worse than Bruce's circuit. but still pretty good. The THD I measured is also about 2 orders of magnitude worse than TI claims in their app note, and I'm a bit puzzled buy that? The TI labs probably have much, much better test equipment than I do.

I also tested the TI circuit with the OPA2134, and got about the same results as with the LM833 (see below). Overall I like the simplicity of Bruce's single stage circuit and I'm not sure the slightly worse RIAA compliance of Bruce's circuit would make much difference to most listeners? Both circuits would probably benefit from using tighter tolerance caps if you can afford them.

John

Author:  ILoveHiFi [ 28 Jan 2019, 23:04 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Could be fake op amps if you didn't buy from genuine retailers like rs components, element14

Author:  john_c_kennedy [ 30 Jan 2019, 15:32 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

ILoveHiFi wrote:
Could be fake op amps if you didn't buy from genuine retailers like rs components, element14


All the parts were purchased from Mouser, so I'm pretty sure they are genuine.

Author:  gofar99 [ 30 Jan 2019, 16:17 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Hi, Perhaps your test gear is better than mine. I really want to meet anyone who could hear a 0.015 db variation in anything. Very few can hear deviations 100 times that great. With typical components having 1-2% tolerance I find the level of deviation you indicate is below what I have measured on examples of the preamps I have tested here. Use of such components would indicate a much greater range. Most of the ones I have tested seem to be about 0.2 to 0.3 db off at the extreme frequency points (20 and 20K HZ). This would be expected from commercially available components and tolerance values. I designed this preamp to return a high level of performance at a budget friendly cost. I doubt that really higher tolerance or cost components would make much difference. From the feed back I get it seems to be a mission accomplished. 8-)

Good listening
Bruce

Author:  john_c_kennedy [ 30 Jan 2019, 19:39 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Sorry the RIAA complinace errors I last posted should have been 0.3dB for Bruce's circuit and 0.15dB for the TI circuit. Also my ARTA system gain was uncalibrated so here are the corrected compliance and distortion graphs, though the compliance and distortion didn't change.

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