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Inexpensive IC based phono preamp for LOMC
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Author:  sampleaccurate [ 23 Oct 2013, 15:45 ]
Post subject:  The EASIEST mic/line/phonostage preamp IC

There is a specialized microphone preamp IC called the INA217 that has phenominal specs and uses a single resistor to set the gain:
http://www.ti.com/lit/ds/sbos247b/sbos247b.pdf
I used this in the schematic shown in the datasheet for a microphone preamp and it works like a charm. I'm sure it could be used in an RIAA EQ too as a phono stage. 66dB of gain is available without compromising bandwidth from a single chip with very few external parts. Even the CMR is over 100dB!

Author:  gofar99 [ 23 Oct 2013, 17:15 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Hi Stephen, I noticed that as well. I do use SMT devices, but the good old dips are easier to use and since I have on occasion cooked one during the development process having the ability to use sockets is rather handy. The only thing I might have done differently in the preamp is to use two individual op amps that have adjustments for zero dc output. I still would have used a cap there as they could drift, but it would possibly make the output even more symmetrical at higher voltage output levels. I didn't see it as much of an issue as the preamp can easily do +/- 5-6 volts and virtually no cartridge that is used on the correct gain setting will cause anywhere near that much output. Plus the gain in the preamp is easily adjusted. Just swap out the existing resistors for ones to obtain the gain needed. I suggest you not go for more gain than the 51R can deliver as my testing indicates that it will begin to cause frequency deviations from the RIAA curve. But for those folks with a HOMC that puts out less than say 2.0mv it would be reasonable to use something on the order of 220R or 240R in place of the 330R. For HOMCs as well the input load resistors can be changed to suit the cartridge used. In my final version of this preamp I put a pair of 8 position dip switches that allowed for adjustments for gain and loading.

Good listening
Bruce

Author:  sampleaccurate [ 28 Oct 2013, 16:43 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Bruce,

The good news is this chip comes in a standard size DIP - I have several.

http://www.ti.com/lit/ds/sbos247b/sbos247b.pdf

Author:  gofar99 [ 28 Oct 2013, 17:17 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Hi Stephen, It looks good for lots of things. I can see using it in a differential mode right off the cartridge. The problem there would be how to tie in the equalization. If you used a conventional feed back loop it seems it should upset the gain. A funny thought on that ...suppose the equalization was in place of the gain resistor? I wonder if non-linear components could go there?

Good listening
Bruce

Author:  mwhouston [ 30 Oct 2013, 06:00 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

After seeing my friend's (Dave) gorgeous new TT (see image) I wanted one. But though the bank could stretch to it I have no where to put it. The Rega is light and compact and works where it has to live. But we took the chance to listen to some different phono preamps, a Cambridge and Boozhound labs included. With the rest of Dave's very expensive gear the Cambridge sounded the best with his Sure V15 MKIII and new stylus. BUT!!

It got me fired up to make a Heran Special. I had been thinking about it for some time. When I got home from Dave's if sorted the caps and resistors to build myself one and using some tricks I'd picked up from previous OPAMP builds hope to build the lot on a tiny piece of prototype brd. Hard to believe but I had every component for the OPAMP module bar two caps which I picked up at work today. I did need some other parts like RCA etc. but the cct. brd. I could almost complete.

I intend to run mine on 4 X 9V batteries and use the simple CMoy resistive network to provide +/- 18V. A cast Al case has been drilled, prepared and top coated and in a few days time, dry enough to assembled. In the meantime I can assembled the heart of this baby.

Attachment:
image.jpg
Pro-Ject RPM 10.1 Evolution

Author:  gofar99 [ 30 Oct 2013, 10:39 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Hi Mark, Cool. Hope you like it. The MM side seems better to me than the LOMC. But then it may have something to do with the cartridges I use. MM are Grado Reference Platinum, OM30, Denon DL110, LOMC are AT-33PTG/II, AT-OC9ML/II, Benz Micro MC Gold. BTW they all sound better - or at least to my taste with the Lundhal SUTs in front of the preamp as opposed to direct connection. It sounds better than it should for the number of parts and simplicity of the design. Then again, I like simplicity. It doesn't have the potential to mess things up as much as complicated circuits do.

Good listening
Bruce

Author:  mwhouston [ 01 Nov 2013, 03:50 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Of the parts I had one main ingredient was the 2n7 caps required as part of the equalisation network. I would have loved to use some micas or something exotic. At work I have MKTs. I know they are not the most sonically omnipotent but I do understand they are very accurate when it comes to capacity. I guess that's a trade-off.

Author:  gofar99 [ 01 Nov 2013, 09:04 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Hi Mark, I built one with mica caps in parallel (1000 and 680pf) and one with Kemit polys (I believe they are MKS... they say MMK on them). These are the little square ones (not the ceramics that look like them). I really could not tell the difference in sound. The output cap was a Wima MKS.

Good listening
Bruce

Author:  sampleaccurate [ 01 Nov 2013, 12:09 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

gofar99 wrote:
Hi Stephen, It looks good for lots of things. I can see using it in a differential mode right off the cartridge. The problem there would be how to tie in the equalization. If you used a conventional feed back loop it seems it should upset the gain. A funny thought on that ...suppose the equalization was in place of the gain resistor? I wonder if non-linear components could go there?

That's an interesting thought. I suppose it could be attempted to see if it would work. If you're willing to put two op-amps in the circuit this chip could be used to amplify a cartridge, followed by an RIAA EQ circuit, then buffered by an OPA134 for example so as not to load the passive EQ and provide a low impedance output.

But I agree, this chip could have many uses. I use it to amplify a ribbon microphone that has a very low impedance output but a very low amplitude signal and it works very well. The ribbon in the mic has an output impedance of about 1 ohm, but a step up transformer inside the mic steps that up to something around 150 ohms with a higher voltage that can be amplified by active circuitry. This chip adds very little noise when amplifying low impedance, low voltage signals.

Author:  mwhouston [ 01 Nov 2013, 18:12 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

gofar99 wrote:
Hi Mark, I built one with mica caps in parallel (1000 and 680pf) and one with Kemit polys (I believe they are MKS... they say MMK on them). These are the little square ones (not the ceramics that look like them). I really could not tell the difference in sound. The output cap was a Wima MKS.

My MFTs are little square ones too. I like to snub the diff. PS and usually just use a green Mylar. But I happen to have an MKT in 0.1uf so it will use it instead. My output cap is a Solen and the 10nf a Solent also. It would have been nice to have Solen right through. The PS caps are low ESR types.

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