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Phono Preamp Project

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Graham Slee View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 09 Feb 2017 at 7:26am
I think I have photocopies of the JLH article, and remember building something along those lines. I remember him waffling on about the "new" NE5532 (or 4) being superior to a transistor design Shocked and he was a proponent of two stage equalisation. The stage had an unbuffered HF filter, and if you built the headphone amp section and switched it in, the time constant changed because of the headphone amp input resistance.

Probably coincidence but the NE5532/4 had the same output configuration as his now-famous class A amplifier design. Perhaps he was "in the know"? Unfortunately the class A amplifier distortion measured high, whereas the NE5532/4 distortion was extremely good.

The same output stage gets used in TTL logic as a high speed switch.

Total capacitive load on the cartridge is traditionally 200pf and early "hi-fi" decks used high capacitance screened cable - the MM only became popular in the late 60s - before that it was ceramics which were good into 600pf. Perhaps the reason for not including an input cap. Cellphones and radio cabs made preamp input capacitance advisable, and with Hi-Fi turntables using low capacitance arm cable, the requirement could be shared.

With frequency being the inverse of 2pi times the square root of inductance times capacitance, it can be seen that with a generic MMs inductance of half an Henry, the tuned frequency is 15915.49Hz. High fidelity at the time was something like 35-15000 c/s, so that gave the right answer. 47k was found to damp the Q of the LC, and so the "ideal" cartridge load is 47k and 200pf (plus or minus a bit isn't going to be a game changer).
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Post Options Post Options   Thanks (0) Thanks(0)   Quote peterb Quote  Post ReplyReply Direct Link To This Post Posted: 09 Feb 2017 at 11:01am
Thanks Graham. To save you rumaging around for the article I have uploaded the schematic here.
In the article JLH said that after listening tests he changed from a 47k load to 39k as it sounded better!
However for a long time I have enjoyed the music played through it and his 30W Mosfet power amp. 
It is now a bit long in the tooth and I wanted to get a little more up to date so have built a new pre and power amp with remote control, DAC and PI/HiFiBerry DAC+ modules. What it didn't have was an integrated phono stage and adding one has brought me to some of my current problems! 
However searching for alternative brought me to this site!


Peter
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Dual 505-1, Cyrus CD T, DIY 80W MosFet amp and PreAmp, 2xKEF 103.2
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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 Feb 2017 at 10:29am
Oh, that's a different preamp to the one I was thinking of. I see this one uses his Lineac idea. It's a "virtual earth" input and the impedance where input impedance meets feedback impedance would ideally be zero, but this won't be. Therefore the load seen by the cartridge will be mostly 39k, but partly the bit you can't see at the virtual earth end, and it might add up to 47k, but I don't feel inclined to go into its workings right now (or ever...).

There are also other problems with virtual earths (or summing nodes as the same idea is used for mixers) that are greater than "zero" in that the feedback will not be accurate either. To obtain a better zero requires tons of voltage gain, and is why op-amps are invariably used.

For anyone trying to see the image the above link may not work in firefox, because for some reason it removed the colon after https, and I had to type it back in on the address line.


Edited by Graham Slee - 10 Feb 2017 at 10:30am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Paperweight Quote  Post ReplyReply Direct Link To This Post Posted: 15 Feb 2017 at 9:40pm
Ordered a roll of Kester 275 flux Sn96.5Au3Cu0.5 after our earlier discussion. It actually solders up nicely with little objectionable fumes. Once the joints are cleaned with 91% alcohol, they look pretty good. The only downside is that once it gets soldered, going back to rework a joint makes it go dry and ugly if you don't add more solder/flux.
 
I also ordered several new bits for the soldering iron. I chose several with a 1.6mm wide chisel tip. They lay down the heat and solder to the joint very well. I made sure to get two extra in case the flux eats through their plating. I can't remember how long I had the last bit on the tip for. They last forever with lead solder.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 15 Feb 2017 at 9:54pm
You're better sticking to leaded solder which is legal for non-commercial use and for repairing products of pre-2006 (pre RoHS) manufacture. Such legislation applies to all electrical and electronics products within or entering Europe, and may differ elsewhere. There is only one lead-free solder that works: 99C with 309, period.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 16 Feb 2017 at 8:19pm
My amateur opinion is the same. I went back to leaded solder as it is easy to use and sounds much better than several lead-free ones I tried.
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 Paperweight Quote  Post ReplyReply Direct Link To This Post Posted: 16 Feb 2017 at 9:26pm
It was just an experiment. You hear one person say it's awful. Another thinks it sounds good. Listening to the difference in solder joints might be a good hobby for the insane.
 
The US won't have to worry about banning lead anytime soon. The conservative nutters in office might just do away with environmental standards altogether. It hurts business when they can't just dump their waste in the water supply.
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