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-   -   LC Audio Dual Mono RIAA - Light MC Headamp (http://www.diyaudio.com/forums/analogue-source/210506-lc-audio-dual-mono-riaa-light-mc-headamp.html)

Algar_emi 10th April 2012 07:51 PM

LC Audio Dual Mono RIAA - Light MC Headamp
 
2 Attachment(s)
Hi. I built from scratch, using only the LC Audio own schematics and public domain PCB pictures, an home made replica of their Dual Mono RIAA intriguing Light supplied MC Headamp and phono section, see http://www.lcaudio.com/index.php?page=8. I'm successful and very happy with the result. The schematics are free to use for DIY only, as noted on the website. Don't ask for my own PCB design, buy the original or design your own.

I don't pretend this is exactly the same board. For one thing I didn't know the exact parts type, and I could only guest, or extract what I can from the PCB pictures. First, good news the published LC Audio schematics are correct, we never know. The published circuits are working beautifully.

For the MC Headamp section, I bought directly from LC Audio the needed low noise transistors 2SD786 and 2SB737. I found the rest of the parts myself. I was lucky to have in stock most of the Rifa and Evox film caps needed and found the Philips electrolytic 220uF caps in a surplus store. Here pictures of my board, not bad ;)

One of the errors I made in my prototype board is I didn't know that the AD743 I was getting was a SOIC, but wide case, not the normal small one. I had to mount these IC soldered on just on side (the one with most connection) and use two small wires to connect the other side two pins. Worked just fine.

Algar_emi 10th April 2012 08:05 PM

2 Attachment(s)
The RIAA curve correction is right on and goes up to 100Khz. I got this first measurements:

MC Headamp Section. It is just incredible, a lot of gain, no noise at all, perfect supply insulation using the Light/Photocell supply. I can't wait to hear it...
Gain 30dB
Noise, lower than my HP analyzer, < -76dB, S/n ratio specs is 88dBA

MM RIAA Section
RIAA curve measured almost perfect up to 100Khz, see results. Low frequency errors are probably due to my setup.
Gain 52dB (at 1Khz)
Output DC offset, measured 0V

I'm planning is using the LC Audio Low Noise regs to supply the preamp. I made my own too :D

I should be able to connect all the preamp and have a listening session soon.

knutn 10th April 2012 08:30 PM

I think LC Audio should give credit to Dr. Marshall Leach and his memory for this design (and not pretend its their own).
The design is still found at the Georgia Tech site: Moving Coil Cartridge Head Amps

AVWERK 10th April 2012 10:55 PM

Another nice build! Let us know how it compares wirh your other fine phono stages you have built over the years.

Keep up the great work!

Regards
David

Algar_emi 10th April 2012 11:06 PM

Indeed, the MC section is inspired by Mr. Leach design. However, it is different in its implementation, transistor choice and its light supply, insuring very low noise, critical it seems for this circuit. The original article can be seen at http://users.ece.gatech.edu/mleach/headamp/

Algar_emi 14th April 2012 03:54 PM

1 Attachment(s)
Prototype with dual mono LC Low Noise regulator and 10Vdc unregulated light bulbs supplies. I'm using 12V,80ma small light bulbs but supply them at lower 10V to extend their life. Test had shown that the photodiode supply voltage is the same if I supply the lamp at 12 or 10V.

Algar_emi 15th April 2012 09:53 PM

Sound. Spent the all weekend comparing phono preamp. In the one I tested, in order of preference (from Best to less good):

SVP-2 own phono section, the top of the top again :D
LC Audio RIAA
Audio Research PH5
Pearl 2

Not bad ;) The LC Audio preamp has excellent timing, high gain, low noise. It is very revealing, crystal highs, but lacking bass I would say.

This is where the SVP-2 beats all the other preamp that I have. It has a tightly matched RIAA network and it shows. I may try to tweak the LC-Audio. It thinks it beats the PH5 that is a little dark to my taste. The LC Audio has more sparkles.

All in all the LC phono stage is very good, and better than the Pearl 2, as far as I can tell. It is more alive, more transparent and has a lot of gain, with very low noise. Not bad for such a little preamp. Recommended.

AVWERK 15th April 2012 11:44 PM

Sylvain
What cartridge and arm are you using BTW?

Regards
David

Joachim Gerhard 15th April 2012 11:46 PM

The 0.1uF cap is much too small. That gives a rather dry bass. I would go for 1uF.

Algar_emi 16th April 2012 12:42 AM

1 Attachment(s)
Test Setup:

AirTight ATC-2 preamp
Aleph30 (DIY)
Michell Tecnodec (with upgrade supply) with Rega RB300 (rewired with Cardas wire)
Benz Micro Ace-Low cartridge
Axiom M3Ti speakers (with a little sub to complement...)

My fault. I double checked the preamp, found a RIAA curve calculator and got these results with the circuit values:

318uS: 299uS (Pole 1 532.3Hz, -31.8Hz error)
75uS: 75.00uS (Zero1 2122Hz, 0 error...)
318uS: 314uS (Zero 2 50.6Hz, -0.6Hz error)

This is very close, and an almost perfect fit to the optimal RIAA. Look at the included curve and see how the nominal and calculated are close to each other. It is difficult to do better without using split EQ (not the case here but this is what it is used in the SVP-2). There was something else.

Found it, my fault :o
Inverted one of the input channel wire. So both channels were in reverse phase! No wonder the sound was thin :yawn:

Put everything in order and listen to it. WOW, great!
Really my best preamp so far, except for my beloved SVP-2, but much cheaper :p
Now it is very recommended!


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