LC Audio Dual Mono RIAA - Light MC Headamp

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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.
 

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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.
 

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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.
 
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 :eek:
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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Hi. I thought I'll share with you step by step how it is a lot of work to build an enclosure for your project. I know that for me that the longest step of any project. I love my LC Audio phono preamp that I decided to install it into a nice enclosure.

As some of you may know I try to recycle old enclosures when I can. In this case it is a satellite TV received that had been collecting dust for so years now.

What is nice about commercial or industrial enclosures is that they are well built, even the cheap commercial ones. They are a strong, squared (important when trying to align metal parts), well designed, and well finished, all for free. With a little work (well a lot...) they can make the foundation of a nice enclosure.

The one I choose had a ventilated top, all steel construction (important for phono preamp shielding) and was just the right size. It has a switching power supply already mounted. I removed it, studied the circuit, cut the board, removed the unneeded parts and modified it to suit my needs. This way I was able to keep the AC entry connector and nice PCB fit. I also modified the bottom, removed the unneeded pcb standoff, and keep the ones I can reused. What is also great about the particular enclosure is that I can remove the rear panel with just two screws.

The top panel fits into a plastic front panel. I discarded the cheap front panel since I was planning the replace it with a nice brushed aluminum one. I had to cut some of the top metal tongue that inserts into the plastic front. The top is steel but still pretty thin, so I could still use my none ferrous bench saw blade using cutting oil. Check the picture, the end result is not bad.
 

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One good way of mounting PCB and parts is to use a sub plate mounted on the bottom with just a few screws. This way you spare the bottom of numerous holes and screw heads. It makes for an easier assembly job too. I like it in particular with a used enclosure. The sub plate matches the weird original mounting pattern then you can mount your own parts where you need it. I installed all my standoff for the PCB on the sub plate. As you can see this is a lot of drilled holes.

Since it is a phono preamp, I planned to have the ac power section shielded from the sensitive phono section. In order to do so I'll have all the big power transformers separated by a steel dividing wall (see in the middle of the case). The further increase the shielding I put mu-metal plate on the transformer sides. This mu-metal, self-adhesive material from 3M is just great. It is used in my Air Tight preamp as well. I know that a separate power supply is even better for noise rejection, but I hate all those little supplies that I have in my racks. If I can have all the parts in one enclosure I'll do it. This preamp was dead silent on the prototype with the transformers mounted close to the PCB. I expect no problem on the final product. We'll see.
 

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