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Inexpensive IC based phono preamp for LOMC
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Author:  mwhouston [ 04 May 2016, 07:18 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Efficiency of the Jubilees?

Author:  wdecho [ 04 May 2016, 07:45 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Not exactly sure but way up there, probably 105db. Too lazy to research. They are a commercial Klipsch product. Big, very big.

Author:  wdecho [ 06 May 2016, 09:03 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

I have a friend that I talked into DIY who has built 2 Firstwatt clones. He has never built from a schematic yet. Would he achieve about the same results with your Poddwatt DMB Series 1 kit you have for sale? I believe he might try to build a complete kit project with some persuasion. He recently purchased a pair of Klipsch Jubilee speakers so wattage is no problem.

I am not sure Bruce has seen this question so I am asking again.

Author:  hughtmccullough [ 10 Feb 2017, 08:20 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

I realise I am coming a bit late to the party but if Bruce is still around I would like to understand better the noise measurements. I don't see how the S/N could be 100dB as both the cartridge and the OPA2134 will generate more than that.

Cartridges of course vary but when you consider the thermal noise they generate, and the source resistance and inductance, it is difficult to see how a MM cartridge S/N could be much better than 85dB, even after the RIAA eq. which attenuates the high frequency noise quite a bit. That is with a perfect, zero noise op amp.

The OPA2134 has a noise voltage of 8nV/rt.Hz. Again the RIAA eq. attenuates the high frequency noise. However, even taking a 2kHz noise bandwidth, the op amp will generate about 360nV of noise. This is optimistic as there will still be a fair bit of high frequency noise get through and, also, at frequencies below 500Hz the noise is actually amplified. This noise is added directly to the cartridge output which is usually around 5mV. This gives a S/N ratio of around 85dB for the op amp. Therefore the overall S/N couldn't be much better than about 82dB.

I was a bit confused that the units you used for S/N were dBv as opposed to dB. dBv is a voltage level and not a ratio. It also refers to a signal feeding a 600 ohm load. If you meant dBV that is still a voltage level, simply referred to 1V. I wonder is that the source of the confusion.

I realise that none of this is all that important when your ears tell you that it is quiet but I just like to understand things and get a reference point for theoretical figures vs. real experience. I may of course be wrong and, if that is the case, I would like to learn where.

Author:  gofar99 [ 10 Feb 2017, 18:28 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Hi, Welcome to the forum. Confusion with S/N is a rather common thing. There are several ways to determine it and my experience is that they seldom agree. I use dBV reference to 1 volt RMS as it is what my DSO provides. For that measurement it does not need to reference any impedance. I do find that curious as it is possible to measure voltages at high and low load impedances and I would expect to come up with different results. Anyhow, I use 47K and 100K most of the time unless stated differently. For consistency I use 1000 HZ as the test point. For some gear, notably tube related I will also do the frequency range of 20HZ to 100HZ as it is where I find the most noise.

I do not find any fault with your thinking or numbers.....what I do find is that what I measure does not always reflect what the math would indicate. It can go either way, better or worse. There are some things that do relate and folks do need to take into account for phono preamps. First the best S/N you can get on a vinyl disk according to sources I have encountered (I'll have to take this on faith as I can not verify it) is in the 65-70 db range. I don't recall whether it was db voltage or other method. This value relates to the way the disks are made and how the pick up will at optimum settings recover the information. I seem to recall it is somewhat frequency related (seems logical) but do not recall the particulars. ICs have as you noted a native S/N. I find that in circuits that involve a significant amount of negative feed back the actual in circuit value (of the entire circuit) can be better. A possible consequence of bandwidth limiting. Response below 20HZ and above 20KHZ does not factor much into audio gear. Noise out there can problems with some amps with very low frequency response and some sub woofer set ups, but folks with such systems are not likely to use $100 phono preamps. Remember this is an "inexpensive " preamp. Phono preamps by their very nature with the extreme frequency slope are IMO the most difficult things to measure. The way I find best is to look at a spectral analysis display of the output (limited to the audio range). Setting a cursor on the peaks usually provides a good value of how far below it is with respect to the reference in my DSO of 1 volt. There is no input to the preamp and the load is 47 or 100K. I find that the results for the inexpensive one to agree with others that have published specs. I use a Sim Audio Moon LP3 for a reference. It is supposed to be at -106 for MM and 96 for LOMC. This is the range where I find them under test. Now a disclaimer of sorts. Anything below that range is getting into the residual noise of my gear and I don't report values there.

Good questions/comments.

Good listening

Author:  Felix [ 10 Feb 2017, 18:39 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

G'day all, interesting stuff and good comments Bruce. Having built that phono preamp I am impressed at how quiet it is, compared to others claiming to be 'quiet'. The use of the OPA2134 is interesting as it is slightly noisy/hissy in some circuits, but is very quiet in Bruce's design. Regards, Felix.

Author:  rrowley [ 06 Apr 2017, 09:08 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

I'm building the preamp as a tape head amplifier and would like to use it for other speeds/EQ as well as the provided NAB curves for 7 1/2 and 15 ips.
For the NAB 15 and 7 1/2 ips eq I've gathered it 's 3180 uS(50Hz) to 50 uS(3180) and uses the following values R1 330K,R2 4.7K and C1 10nf so I need to learn how to calculate the values for the other speeds which are..
15 ips CCIR 7956 uS(20 Hz) to 35 uS(4545 Hz)
3 3/4 NAB 3180 uS(50Hz) to 90 uS(1768 Hz)
1 7/8 NAB 3180 uS(50Hz) to 120 uS(1325 Hz)

I Imagine there's some heavy maths involved here but is there a program or book I could use?

Author:  gofar99 [ 06 Apr 2017, 21:49 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Hi, I'll have to look to see where I got the equations. Since I only needed 7 1/2 ips I didn't bother with the others. I use just 2 tracks at that speed in only one direction. I find that if you use both directions (it is a 4 track machine) there is always slight bleed through of really intense bass. Enough so I can see it on a scope. It is possible that the heads are at fault, but since it works so well otherwise I will leave it alone. Also since I scored some new unopened cases of BASF there is no shortage of raw tape.

Good listening

Author:  rrowley [ 07 Apr 2017, 18:09 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

That would be great Bruce.I think the 3 3/4 and 1 7/8 are very similar to the 7 1/2 NAB Curve but 15 ips ccir has two filters.That's what most of my stuff is recorded on.There's no end of information on RIAA curves but not a lot for tape curves.
Funny thing you should mention scoring unused BASF.I've got 40 reels of sealed LP35 here :-)

Author:  gofar99 [ 07 Apr 2017, 21:50 ]
Post subject:  Re: Inexpensive IC based phono preamp for LOMC

Hi, I have not had time to research it tapes are EMTEC SM468. They have the BASF symbol on them. Factory sealed boxes in German.

Good listening

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