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Phono Preamp Project

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 10 Jan 2010 at 11:05pm
Originally posted by RobW RobW wrote:

Am trying hard to keep up but at every turn I find myself having to check my compass. Is there a good primer out there for the basics? I'm a mechanical engineer and have this annoying habit of trying to look at this subject through that filter ... electronics seems a bit like alchemy. Advice appreciated - I'm a late blooming enthusiast.


The Art Of Electronics by Horowitz and Hill, Cambridge publishers, although expensive (mine cost 60 GBP about 15 years ago) should help greatly. The best place to start is right at the beginning of the book where you will discover the authors taking an intuitive approach. I too started out in mechanical engineering but moved into electronics at a gradual pace.

Edit: At one point in my career my head of department (there were just two designers: he and me) was actually an industrial chemist...  during that time we designed quite a lot for the BBC world service...  so you stand a good chance Thumbs%20Up


Edited by Graham Slee - 10 Jan 2010 at 11:08pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 11 Jan 2010 at 10:42am
Thinking about what I just said above, this sort of thing can be tackled in the Audio Projects Magazine!

After several months of reading text books on transistors in my youth, the real breakthrough for me came by reading one of the 1970's Everyday Electronics magazine teach-ins.

I reckon a six issue course (bi-monthlies) on understanding electronics (or electronics made easy) PLUS at least one really useful audio project in each issue, must be worth 60 GBP on the year? Especially considering The Art of Electronics costs the same! (well, it used to)

We could start with the fundamentals of Ohms law and work upwards - and have fun learning! Big%20smile
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Post Options Post Options   Thanks (0) Thanks(0)   Quote RobW Quote  Post ReplyReply Direct Link To This Post Posted: 11 Jan 2010 at 7:11pm

I reckon a six issue course (bi-monthlies) on understanding electronics (or electronics made easy) PLUS at least one really useful audio project in each issue, must be worth 60 GBP on the year? Especially considering The Art of Electronics costs the same! (well, it used to)

We could start with the fundamentals of Ohms law and work upwards
[/QUOTE]
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Hmmm ... are you considering spearheading this Wink or suggesting I look for something?
I already read the introduction to The Art of Electronics (on-line) and am interested enough to have reserved the book at the local University library. 
Can we spawn a new thread on this subject?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 11 Jan 2010 at 8:45pm
Please feel free to do so Smile
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Post Options Post Options   Thanks (0) Thanks(0)   Quote RobW Quote  Post ReplyReply Direct Link To This Post Posted: 12 Jan 2010 at 6:22am
Originally posted by Graham Slee Graham Slee wrote:

Please feel free to do so Smile


New topic spawned in DIY Audio Questions and Answers.
A Route to Learning the Art?

Edited by RobW - 12 Jan 2010 at 6:24am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 12 Jan 2010 at 8:24pm
Now for the bit you've all been waiting for.......................



Not quite a working schematic yet, but the bones of one.



The chart shows the RIAA response it results in (green curve), the phase response (red curve), and the input (blue curve).

The input is applied across the 47k resistor and is then filtered by the 1.5k resistor and 220pf capacitor, and the blue curve is the result. Now, for a moving magnet input the blue curve is purely academic as its inductance dominates and the 1.5k isn't really "seen", therefore its load is simply 47k in parallel with 220pf. The curve will fall off far sooner than the 400kHz -3dB point (approx.). The reason for the 1.5k resistor becomes apparent when you consider its use with a high output moving coil cartridge which has much - much lower inductance and equivalent series resistance. The curve needs to fall earlier than the response of the amp stage - the input slew-rate can't be faster than the output slew-rate or distortion would be the result. In fact, it's a good idea to try and hit 1/10th the output slew rate which just about guarantees avoidance of this type of slew induced distortion.

As you can see, I inserted a 3k resistor in series with the 2.2nf cap. Initially I reckoned on 470 Ohms, but slew-rate is Volts per Time, so we needed more gain at high frequencies to get that rule of thumb 10, or nearly 10, in the voltage domain as well as bandwidth domain.

What it does however, is curtail the RIAA fall-off around 25kHz, and to be faithfull to the RIAA reproduction curve, we have to turn it back down such that it keeps falling at roughly 6dB per octave (20dB per decade). This is handled by the output filter (820R and 4.7nf).

The response curve is within 0.3dB of the required RIAA curve using the common values shown - the 6.6nf cap being 3 x 2.2nf caps in parallel. Apart from the 3k and the 36k, all resistors are found in the E12 value series, meaning the constructor has a choice of carbon film or metal film resistors. In the case of the 3k and 36k resistors, these would have to be metal film to obtain the required value.

In real life (so far we're in simulation land) this circuit may need some tweaking and that's why we have the required test gear. At least we have the starting values to be able to do a prototype. At this point what I now need do is grab hold of one of our old surplus Era Gold or Reflex boards which have most of the pads this design requires and build a trial circuit.

The eagle-eyed will have spotted the improvement in gain margin (bottom-right of plot) to 30dB, which is down to the 3k resistor (it doesn't always work like this for all op-amps).

More to follow - stay tuned.

Edit: You may also have noticed the 4MHz 0dB crossing? If slew rate = 0.3 x m x Ft, and the constant "m" we found is 8, then 0.3 x 8 x 4(MHz) = 9.6V/uS, which, if my memory serves me well, is what we said we wanted. Good old 3k resistor! Wink


Edited by Graham Slee - 12 Jan 2010 at 8:35pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 13 Jan 2010 at 9:44am
A PCB, case, and components kit will be offered at diy-audio-kits.com, plus we offer to build and test the Genera for all those not blessed with DIY skills - watch out for updates here.



So here we have it, our values built on a surplus earlier version of the Gram Amp 2 SE PCB, complete with reject back panel, and it will go into a reject SE case.

There's quite a few other components that aren't on our schematic above: these I worked out as I went along, and I'll be showing you how, but this is just to get the bones of the design together so I can test it.

As this board was designed for a dual op-amp, I'm going to need an adaptor board to mount the two LF356 single op-amps. Here I will use the little adaptor board I designed for the Elevator EXP and the Gram Amp 3 Fanfare. It's a bit like the "brown-dog" adaptor I've seen on "hot modding" forums.

But seeing this board is pinned-out for a dual op-amp, I thought it would be fun to try a TLO72 op-amp. This is an op-amp I remember sounded particularly good in this kind of application (as mentioned before in this topic), and so we may well end up with a generic design where we can start out "simple" and mod upwards as we go.

So, off I go back to the simulator to see the results of using a TLO72. Stay tuned...


Edited by Graham Slee - 17 Jan 2010 at 4:27pm
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