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Phono Preamp Pt2: MC

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Graham Slee View Drop Down
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Topic: Phono Preamp Pt2: MC
    Posted: 26 Jun 2012 at 6:42am
Now's as good a time as any to start my ramblings about a DIY moving coil phono preamp.

The thing I've discovered about DIY electronics, both from doing it way back, and being a vendor of it, is that if a project is complicated to make, many will have problems with it.

The Novo headphone amp kit isn't too complicated but if the constructor can't get to grips with how the circuit works, and a transistor pin isn't soldered in, causing other transistors to fail, the constructor is lost and has a struggle to put it right - often needing our help.

The op-amp takes away the problem of a missed soldered joint causing confusing failure. OK, miss one of those eight pins and it's not going to work, but often the cure is to simply solder it back and everything works fine. By using an easily obtainable op-amp, even if the thing is blown, it's easy to effect a repair.

If just one op-amp is used - either a dual for left and right, or two singles - troubleshooting is easier still.

So, can a single op-amp provide all the gain a moving coil phono stage requires? I'll give that a cautious yes.

Because the RIAA magnetic-cartridge-and-record equalisation calls for a falling response from around 50Hz, it tends to follow the same gradient as the open loop response of an op-amp.



As a rule of thumb, all we want is the op-amp curve (red) to be 20dB higher than the RIAA curve (green) at any particular point, save for the really high frequency closure point maybe, and there's probably going to be sufficient negative feedback to make the harmonic distortion and noise acceptable.

However, the chart shows the response of a moving magnet phono stage and we need roughly 20dB more gain than shown here (the green line needs lifting 20dB). So using the op-amp which gave the red curve here, there will only be 15dB, but that op-amp was OK for MM. What we want is an op-amp with the right sort of characteristics needed for moving coil. And I'll discuss that next post.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 26 Jun 2012 at 8:43am
Thanks Graham. This has been eagerly awaited by me and many others.
Jon

Open mind and ears, whilst owning GSP Genera, Accession, Elevator EXP, Solo UL, Proprius amps, Cusat50 cables, Lautus digital cable, Spatia cables and links. and a Majestic DAC.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote KC1 Quote  Post ReplyReply Direct Link To This Post Posted: 27 Jun 2012 at 12:56pm
Nice One Graham, I would definitely relish this as a project...
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 27 Jun 2012 at 8:40pm
So, what op-amp are we going to use?

Let's take a look at the signal levels the input will have to deal with. I guess we need to make this capable of 1mV as a sort of maximum to cover very low output moving iron cartridges too.

Some will remember my discussion that made it clear that the record output doesn't rise with frequency, but the cartridge itself? The output from the record is flat to 500Hz, dips to 2kHz and is then flat to around 25kHz.

When you add both responses together you get the so called RIAA "recording" response everybody is so familiar with... you're not? OK, go and take a refresher here: http://www.gspaudio-community.activeboards.net/phono-preamp-project_topic745.html



You can see the rising response above. At 20kHz the signal hitting the input is 10 times larger than that at 1kHz (the reference frequency where the cartridge output is specified). So 1mV becomes 10mV at 20kHz (ish).

That by the way is RMS volts so the peak is 1.4142 times (the square root of 2), so 10mV RMS becomes 14.142mV. Peak to peak is 28.284mV. So the linearity of a BJT (bipolar junction transistor) op-amp input, which is around 60 to 70mV is OK then?

Just a quick note for those who think an op-amp can take any input, well, that is true in so far as some part of it can handle it. Just think what happens when you exceed 60 to 70mV? The input transistor is saturated. So what's the input driving now? It's driving the next stage - not only that, but it's driving the voltage amp stage (VAS) integrator capacitor. Yes, the negative feedback reduces the distortion to acceptable, or more than acceptable, levels. But at anything above 60 to 70mV p-p, you have slewing distortion because of the difficulty of the cartridge to drive the integrator cap.

Now, here the secret pops out. Vinyl can handle 5 times its rated output. Maybe at 5 times its rated output at 20kHz it would be too much for the stylus to track successfully, but it could be a bit more than 28.284mV p-p. How much more we don't know. But it looks like we're OK if it's only say twice. It's when it's more we have to worry. But all MC phono stages are going to be the same - that's the secret the MC fraternity would prefer you not to know.

If you remember my ramblings about Genera (Pt.1) input linearity, you'll know I went on about clicks and pops and their leading edges being quite sharp and living at very high frequencies - possibly 200kHz. The output of a MM cartridge starts falling somewhere between 16kHz and 24kHz depending on its inductance, but MCs have low inductance so keep on outputting at a rising 20dB per decade beyond this. So at 200kHz the noise from a click is going to have a voltage in the region of say 280mV p-p. Yikes! And this is why MC gives more emphasis to clicks and pops.

We can tame this by giving it a heavy capacitive load to drive, but then we get peaking, and this is why MCs really need loading down resistively.

So straight away we have a compromise. We know at some point the MC cart is going to saturate the input. Could we therefore use an FET input op-amp which has 10 - 20 times more input "linearity" (linearity in an FET op-amp isn't really a straight line - it has a curve, but it's better than saturation). The trouble with FETs is they're too noisy for such a low cartridge output. So we're stuck with a compromise no matter which way we turn.

However, if we were to use a really low output MC cartridge, say 200uV, that shrinks the possible 280mV p-p down to 56mV p-p which is within the BJTs input linearity. It looks like 250uV is going to be the optimum MC cartridge output, if you can get one like that.

The only problem in going too low is you're deeper in the noise of whatever op-amp is chosen. You can go for a really low noise op-amp, but they're dead slow slew rate wise, and we want something musical. You have to play off noise with speed. With MC it's always a compromise.

So, here's where I break off again and I'm off hunting an op-amp or two which should fulfill the above requirements...



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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 28 Jun 2012 at 8:24am
Let's look at noise. Firstly what is the input to our MC stage in decibels? Let's assume that we're going to be using a cartridge that outputs 300uV (micro-volts) at 1kHz and 5cm/sec. I reckoned 250uV may be optimum, but you'll soon see that going too low on MC cartridge output digs it into the noise of the preamp.

0dBu is 775mV so 300uV is a 1/2583th of 775mV. Log of 2583 is 3.412. Multiply that by 20 gives 68dB. Put a minus sign in front to indicate it's below 0dB... -68dB!

So our 300uV cartridge puts out -68dB. What noise can we put up with in ratio to the signal? Is there a standard? The only one I know about was the old IBA (commercial radio) spec requiring at least 58dB. If we use this, by adding the decibels together, we get 126dB.

Equivalent input noise (EIN) is quite useful in predicting results. The above tells us the EIN needs to be -126dB to give us a 58dB signal to noise ratio. Yes, I know that's crap compared to digital sources but we will insist on using MCs that need 68dB of gain, where digital needs virtually none!

Look at EIN again. We need -126dB. Look at a digital audio spec. Do we see -126dB noise? Perhaps -120dB on the "best" DACs?

So -126dB is actually good. The best studio mic channels are -130dB by the way.

Can we achieve -126dB with an op-amp? Quite easily! However, I talked about "speed". There are  op-amps that may do better on noise but have slew rates which are so slow, they'll not communicate any musical emotion at all. There are plenty of these MC phono stages around and you can pay thousands for them...

There are two quite musical op-amps that fit the bill but we're just 2dB out on spec if you can put up with that? -124dB EIN is the best they're going to give.

So what are they? Surprise surprise, one is the once ubiquitous NE5534, and the other is the LM833. Both were designed for audio. The NE5534 is slightly less noisy than the LM833 not that you'd notice - there's only 1dB between them.

The NE5534 is a single op-amp, and the LM833 is a dual. Should we try and get away with using the LM833? It will fit the existing Genera board without an adapter. The NE5534 (2 off ) would need an adapter board to convert the socket to two.

OK, let's start by investigating the LM833 (next post).




Edited by Graham Slee - 28 Jun 2012 at 7:12pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tg Quote  Post ReplyReply Direct Link To This Post Posted: 28 Jun 2012 at 9:39am

Following with much interest, did not follow the remark that vinyl can handle 5 times its rated output.

Do not answer that if it will take the discussion and thought train too far off course.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote franklin Quote  Post ReplyReply Direct Link To This Post Posted: 28 Jun 2012 at 9:50am
Everytime I see NE5534 I feel dramatic or on a "dark side", cursed.
Not that I don't like NE5534, but .....
Good to see we are getting away with something else Big smile


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