Pete Millet "LR" Phono pre amp

This has been discussed in a couple of areas so I thought it deserved its own page. I think @TheloniuSnoop is about to go all in on this project as well.

The Pete Millet LR phono pre amp can be found here: http://www.pmillett.com/LR_phono.html
Lots of information, a couple of different setups shown and a power point showing measurements, etc

The main idea behind the design centers around removing all capacitors in the phono eq section of the amp. To do this the design calls for Cine-mag inductors. I also built the power supply that was recommended - pretty straight forward, easy to build. Cine-mags have a couple week lead time so order these first!

Total cost was about $350 with the inductors costing $120, $60 of boards from P Millet, $40 ebay case, and $100 of parts from Mouser / Digi key.

I have to refrain from making too many sound impressions yet, as I need to chase down some background noise. Kinda sounds like the seashore, pretty low but distracting at quiet parts. Sound is very clear and resolving, more solid state sounding than my Franken TC750. Will have to chase down the background noise, listen more and report back.

I know T-Snoop was going to have step up transformers, seperate cases for PS, impedience switches etc. Would love to see the build so I can learn from you!

I am still pretty new to building projects, but things have gone well so far. I need to learn how to clean up the interior, plan things out a bit better, and find a better looking case.

My build:

Internals


Rear panel with lighted power switch


LR Phono on top. Franken TC-750 below

 

Cool! I just started one and will let you know how it goes.

 

Awesome - I am sure your build will be much more elegant that mine. Looking forward to see what you do with it.

 

"seashore" noise would usually indicate oscillation, though it is hard to imagine that's what is going on here. Perhaps look for a cold solder joint or possibly the proximity of the power transformer to the circuitry.

 

Just an FYI, but Dave at Landfall Systems (Pete's case mfr) confirms that he gets "ocean noise" on his L-R when the case lid is off. So, it sounds like a shielding issue.

Pete went so far as to use "EMI feedthrough filters" to feed the PS wiring through the case divider in the Landfall case. I never knew such things existed - also called "resin sealed bolt-in filters."

 

Awesome thanks Tom and Doug - this gives me something to chase down. I was going to try and hook it up to a battery to see if the problem was on the power board or the pre amp side. EMI Filters are new to me as well, let me dig in and see what I can find. I've been putting up with the noise as it isn't that bad and I have another pre that has been taking up most of my listening time.

 

Well, I have good news and bad news about my LR build.

Good news is it's done.

Bad news is I have a bad opamp in U1 and can't get the DC offset voltage any lower than 3.5VDC in the left channel.

Gonna have to order another one and try again. Here's some pix:

 

Very nice! I'm still waiting on some things to start on mine.

 

Success! My unit is silent as the grave at normal listening levels. I only get a tiny bit of hum when I turn up the volume at idle to
'say goodbye to your eardrums' levels. The only anomaly is the somewhat pronounced clicking I get when changing the volume.
My pot is a Khozmo shunt-style stepper. A grounding issue perhaps, but if that's the only screwball thing to deal with, then I'm
not going to snivel.

I'm only on my second album atm, but I'm liking what I'm hearing so far.

BTW, you'll notice that I've followed Pete Milletts lead and didn't ground the PS to earth.
That's because I HATE EARTH GROUND IN A PHONO PREAMP. NOTHING GOOD EVER
COMES OF USING EARTH GROUND.
OK, I'm calmer now. I won't yell again. I promise.

 

WANT

 

I suppose I should 'splain what everything on the back panel does.

There are inputs for both MC (upper RCA pair) and MM (lower pair) cart's. The MC inputs connect to Cinemag CM-1254 SUT's, which have
dual coils. The 4PDT switch selects between 1:20 and 1:40 ratios. Here's the diagram: http://cinemag.biz/phono/PDF/CM-1254 Hookup.pdf
The lower switch selects MC or MM. The two vertical RCA's on the far left are output.

I'm running a Denon 103R and prefer the 1:20 setting. I must say that this pre is quite impressive. I haven't tried it with an MM cart yet.
I do have a Shure V15VxMR, but am too busy enjoying the Denon to change.

 

Yeah, with respect to Pete's build, I'm planning on "doing as he did, not as he said."

 

Oops! See below.

 

Thought I'd drop in and show my sockets to roll MC load resistors.



I found the value calculator here: http://www.hagtech.com/loading.html

 

Having my first listen with the load resistance at the Denon 103R minimum 40 ohms. Wow! I'm not hearing anything I don't like.
This thing is a keeper. Can't wait to try other loads, although I can't imagine that it could get much better.

 

Well, I finally finished mine, but I really had to wrestle with it. No "ocean noise," but there is a hum. Thankfully, it's beyond the level of peak listening volume, so it's not much of a problem right now. I suspect it's caused by DC offset on the outputs that are bedeviling me something fierce. I spent all day yesterday trouble-shooting it and finally narrowed it down to a bad opamp.

I get -14.95VDC and +15.65VDC off of the power supply. For awhile, I suspected that was the cause of the offset, but all my measurements at the resistors and regulators are within the specs on the LM2941/2991 datasheets. So, I started trouble-shooting the LR board. No matter how much I racked the trimpot, I could not get one channel below about 1.2V offset on the output, way too high. I finally decided it had to be the opamps in that channel. I swapped the first two (starting at the input) from one channel to the other. Wow - the offset followed the opamp. The offset, while moved to the other channel, was still too high IMHO to operate the preamp. Turns out that I have no spares of AD797's (on order, now). Since I couldn't replace it, I thought maybe if I swapped it with the one further down in the circuit (on the same channel), its offset trouble would have less effect. That seemed to work. Now, I have about 0.2V offset on both channels.

Things still bug me, though, because every time I zero out the offset, when I turn the amp back on the offset is back up to nearly 1/2 V. Pete had told me that swapping out the R5 and R7 resistors could change the gain enough to use MC cartridges - as opposed to input transformers. So, I worked up a DPDT switch to change between 10K and 100K resistors at R5 and R7. Unfortunately, even with a 1/2V offset, that gets multiplied by 10 when flipping the switch - offset is in whole digits of Volts. I guess I'm lucky that I don't have a MC cartridge right now.

Anyway, what I can listen to right now sounds really, really good. Very smooth with no harshness and a very analog-rich sound signature. Hopefully, some new AD797's will whip it into even better shape.

I'd post some pics, but it's looking really ugly right now. I tried to purchase a power switch like Pete's, but the stupid ebay seller sent me a 16mm switch instead of a 19mm one. I have a big ugly O-ring behind the switch to keep it from falling through the front plate. Again, I have more of those on order, too. I'll post again with some pics when it's ship-shape.

 

Well, I just compared the hum to my brand-new Hagtech Bugle. The Bugle has louder hum and it starts earlier on the volume travel than the Millett LR. I guess I'm just not used to trouble-shooting a phono pre-amp and the kind of signal/gain these things have.

The LR definitely has a smoother feel to it. The Bugle is a lot more "in your face." It seems like the LR has just as much detail, but it's more refined.

@TheloniuSnoop - what the heck is going on with these opamps? I noticed you had the very same trouble. I've never seen opamps do that. Bad, yes, but have one work in spite of the inherent offset? Weird.

 

I never tried my LR with the bad opamp. Too chicken. I ordered two new ones and the first one I tried worked, so the second one went to my stash. Perhaps there was a run of marginal opamps, and the LR has a very small tolerance for the 797's.

I concur on your evaluation of the sound. Detail without the pain. Excellent bass with great dynamics as well.

 

I was too excited to turn this thing on and start listening. Besides, once I swapped positions with the "bad" opamp, I was able to get the offset down, so I figured there was little I could do to damage it at that point.

I'm hearing a lot more stuff than before - bass that felt like it was hitting me in the chest with my HD800's. More detail, too. I guess it took a bit of burn-in and finding the right vinyl.

EDIT:


Quick pic - I'll post more details in a day or two.

 

I guess the best advice we could give a prospective builder would be to double the opamp order to be on the safe side.

IMHO, the LR would scale quite well as one moves up the vinyl ladder. This is going to sound quite cliche, but I'm
rediscovering my vinyl collection with this amp. I'm not looking to upgrade in even the distant future. Done and done.