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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: 19 Feb 2019 at 5:46am
We'd need 136 billion viewings before Google put this in its results...

(unlike others....)
That none should be able to buy or sell without a smartphone and the knowledge in how to use apps
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 19 Feb 2019 at 9:12pm
It's a pity as they'd learn a lot from this thread just as I did. Thank you again.

Jon

Open mind and ears whilst owning GSP Genera, Accession M, Accession MC, Elevator EXP, Solo ULDE, 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 DogBox Quote  Post ReplyReply Direct Link To This Post Posted: 30 Mar 2019 at 4:44am
Originally posted by Fatmangolf Fatmangolf wrote:

....they'd learn a lot from this thread just as I did. Thank you again.

Smile yep! As I think a lot have learned more than Graham might realize..! Because when you build a Genera for yourself and you power up for the first time and there's no flash or puff of smoke and you get to actually 'hear' the proof of what he has been saying...a ridiculous grin comes upon your face that's really hard to get off! And you find yourself playing record after record because it's so much fun listening to your stereo! Or, your "new" stereo... because it never sounded THIS GOOD! 
...and I haven't even finished [but I will, + print it all out for reference] past page 6! 
 Anyone that might be considering having a go at this little box, Go Ahead! Do It! Get yourself a smile like I did.. and even when you come back the next day and do it again - it happens again! Its great! 
Plus he says it will get better in (?) how long? 35 - 40 hrs of being on? better go read the Burn In Chapter on the Forum. See? It's all Here! 
Clap ThankYou! Graham. Best little box I ever built! Now, what's next... 

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Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 06 Jan 2021 at 8:10am
If you ever build the Genera kit and find it has "no-signal" one or both channels, but the DC voltages measure OK, check the values of each channel's R5 and R8.

Easy mistake, but if the 390k ends up where the 390R should be, and vice versa, there won't be any voltage gain.

The colour codes are just one band different: orange, white, black, orange = 390k, but orange, white, black, black = 390R.

My tip: always check the reading on your meter's resistance scales.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Sylvain Quote  Post ReplyReply Direct Link To This Post Posted: 10 Jan 2021 at 12:21pm
Thank You for the update info....NOW any UPGRADE mods tips to turn the design to ''SE' .

Please.....
a tiny bit more gain 
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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 2021 at 4:08pm
The gain is set by R5 and R8, Av = 1 + (R5/R8)

Or, is it?

At mid frequencies it might be set by R6 in parallel with R5 divided by R8 (plus one).

At really high frequencies, R7 in parallel with R6 and R8...

And at frequencies between, the reactances of C3, 4 and 5 get thrown into the pudding.

But whatever the result is, one number is always added, and that number is 1.

So, changing R8 down for more gain (or even up for less gain) alters the relative gain at other frequencies, and therefore the equalisation changes.

Now, how many MC/MM stages have I seen that do exactly that? A simple gain switch changes the value of R8.

Not only is the switch simple, the minds who use it are simple too!
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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 2021 at 9:56am
Many design routines were developed over the years to calculate the correct values for high accuracy RIAA equalisation. None of them are as accurate as they claim to be. They tend to have been developed with a particular gain stage in mind.

They are close enough to start the empirical decision process, where guesstimate values are tried and plotted against known values using an audio analyser. The process is relatively long-winded because changing a value that makes one part of the curve fit usually changes to another part of the curve. The graphic artist, having used a bezier curve tool, might testify to this difficulty!

After days of waste paper recalculations, discarded components, and lifted PCB pads, due to the desoldering and re-soldering process, one might land upon an accuracy of 0.3dB. The empirical process can go on until a higher accuracy is found. Still, then component tolerance might cause disappointment in the next prototype.

The original RIAA specification called for an accuracy of 2dB, but don't go there, or the bored rich with nothing better to do than to buy test gear to criticize can seize the opportunity of "cancellation."

Another way to find the correct values is to simulate the circuit using a simulation program (with integrated circuit enhancement) called SP(ICE), or simply, SPICE.

As John C often says, it would be good to tell it what you want, and it give you the result, but it doesn't work that way. You have to provide the program with what you think is right, and it will tell you the outcome - and if the result is right from the start, you're probably not reading it right!

Just as in real life curve testing, you end up guesstimating the values until they hopefully converge. At least it saves a lot of components and burnt PCBs. You have, however, to eventually build the circuit and stick it on the analyser. Still, most times, the accuracy in real life is good.

I choose to use active EQ because it results in a constant overload margin. In passive EQ, the overload margin narrows with frequency, but if you decide to advertise "the right end," you can hugely impress your customers.

Working out the component values for a passive is also easier. If you have three gain-stages, you can do the high-frequency pole with a resistor and capacitor (a reactive potential divider) with little more than a scientific calculator. Provided you use op-amps as your gain stages, you can assume zero source resistance into your simple filter.

You can be clever and use a discrete circuit instead of op-amps, but then errors creep in. Unless you really know what you're doing, some critic will revel in ousting you, unless the critic happens to be a friend.

It gets more tricky when it comes to the pole and zero required for the 50Hz roll-off, followed by the 500Hz stop frequency. The zero part is the tricky part, and if calculated in a simplistic way to obtain 500Hz, it will be found to be out when run on an analyser.

However, SPICE can rescue the situation. The eventual curve might be close enough to risk paying a university student to test it for you. Then, off you go - join the ever-expanding number of phono stage manufacturers all thinking of the fortunes they can make.

The unfortunate thing about the above is not understanding the out of band consequences. It doesn't really matter if you use active or passive EQ, but you only have to fix one stage if it's active.

The difficulty with any reactive load - which the EQ network is - is its effect on the gain stage's high-frequency stability. That is the reason I set my simulator to 1 terra-hertz for its upper-frequency limit. There is absolutely no way an audio analyser can do much more than 80kHz. However, Neutrik once did a costly unit capable of 300kHz.

Some books explain high-frequency stability better than I can here, but it is to do with phase margin and gain margin. Old scholars and new scholars have ended up in disagreement over the same result only because of how they were taught. At least they can win 50% of the time. If like me, you can see it from both ends, I can guarantee you'll lose 100% of the time!

What I don't lose, however, is the control of high-frequency instability. Those who have commented on the quieter reproduction of clicks and pops have unwittingly testified to that - thank you. Stability was also noted by TNT reviewer Geoff Husband - a total amateur by his own admission - but a true professional when it comes to an open mind. In general, critics fail in their comprehension of the workings of the real world, which extends beyond the audio spectrum.

It should make sense that the apparent reduction in clicks and pops due to caring in the phono stage can and will be undone should the items it feeds feature high-frequency instability.

If any amplifier manufacturer wants an excellent high-frequency instability "sniffer," I suggest the use of vinyl through one of my phono stages. If all quiet, then you can bet on no high-frequency instability. If the clicks and pops are exaggerated, you need to worry.

Now, if you've wondered why few report their findings in unison, then hopefully, the above has explained why? We don't all use the same amplifiers. Here, we could expand the subject of the phono preamp project to the whole system. Perhaps we should.
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