LCR Phono Stage

sfox7076 · « **on:** 19 Apr 2017, 02:47 pm »

I have been obsessed with LCR Inductance-capacitance-resistance) circuits to do a passive RIAA stage since I first heard one. I don't remember who made it, but I think it was the Allnic and I heard it at an audio show in 2008 or so. It floored me more than any other phono stage I had ever heard. The sound was natural and neutral. There was zero noise from the surface of the record, and the timbre and tonal accuracy was something that I had never heard coming from digital. I immediately wanted both a turntable and an LCR phono stage.

I went home and hit the net looking for information on LCR stages. There was little written about them and fewer made. The main source for them was Allnic and Ypsilon it seemed. I was reading info about Ypsilon releasing the VPS-100. Well, it came out with a price tag of $30k (plus the step-up transformers needed to go from .2mv up to the level where the 26db of gain from the LCR would work). Yes, it used fancy Siemens C3G tubes I had not heard of (a tube I would later learn to like a lot when wired as a triode for use as a driver tube) and LCR modules that were hand wound, but it cost more than a Subaru Outback...

I also looked into those made by Tango of Japan, but they were running close to $2000 for the transformers, were hard to find, were a hard load to drive at 600 Ohms and the capacitors of the LCR network were partially housed inside the transformer covers. That last part upset me because of all of the times I have or have heard of things needing the capacitors replaced. I am not saying that would happen to the Tango units, but it made me worried. I put the project to the side and listened to my turntable with varying degrees of joy for a few years.

In 2014-5 or so, I got the DIY bug. I started researching amps and other similar stuff. Not sure why, but I did. I met a few people on here, on diy audio and had a chance conversation with Dave Slagle, a transformer guru that lives two miles from where I grew up. I suddenly remembered my LCR love. I discussed that with him and his thoughts on the circuit. I got busy with work and it once again lay fallow. To my extreme luck, a person who lurks on Dave's forum was kind enough to talk to me about doing a full tube version of the LCR phono stage. He even had a schematic. 3 tube stages with an LCR sandwiched between stage 2 and 3. The first stage is 2 6c45s, then a 6dj8 and then a 6H30 (or the el cheapo 6N6P) That design would get me to 60-70db of gain. No step up. It would also be based on a 7k loan instead of a 600 ohm load. Well, sign me up. I ordered the LCR modules from Dave Slagle, got some caps, and went to work.

I worked up the unit, but it was not doing what it was supposed to. It hummed despite having the unit contain only DC on the filaments of the tubes. I was in the infancy of DIY, but was lucky enough to have dealt with a master builder in Mass. He took on the project's layout and, well, all I can say is that he is brilliant. He is an unbelievable builder. he hardest part was convincing him to even look at it as it is outside his normal realm. He looked at the schematic, redid the layout and put it both the power supply and the signal unit in steel boxes. He could not get it to be quiet and, in disgust, he shipped it back to me. I opened it back up, redid some of the wiring, redid the umbilical cord, and went to town looking for issues. I found an error that I had made in the wiring that had just continued over time. Once that was done, and I tweaked some resistors, I had 60db+ of gain and the sound that I had been lusting for for 8-9 years. After using the reverse RIAA stage, it's within 1.0 db from 20hz to 50khz, and within .25db from 35hz to 50khz (there is a 1-.75db bump from 20-30hz because of the internal resonance of a choke. I actually like the bass bump slightly).

The cost was a whole lot cheaper than the Allnic and the Ypsillon stuff, and I learned a ton on the way. Without further adieu, the LCR:

Similar schematic to mine from Naz, the man that also designed the one I use (it is slightly different to mine, but I cannot post the one I have without his permission):

Scale is a little off here, but you should get the picture.

Two changes from this version are a new umbilical (triple shielded and twisted pairs), and I also switched the last set of inductors (300Hy) to a pair of Dave Slagle's inductors. I thought I was having minor issues with the others I had. It was my wiring error. Woops.

Shawn

JakeJ · « **Reply #1 on:** 19 Apr 2017, 03:44 pm »

Wow! Nice work Shawn. It really looks like a pretty simple and nice open layout.

It appears there is a knob on one end (I'll call that the "front" since the RCA connectors are on the opposite face) what does that do?

Also the schematic is hard to read, could you post a larger screenshot that is more legible?

Thanks for posting your DIY phono stage project!

Jake

sfox7076 · « **Reply #2 on:** 19 Apr 2017, 05:14 pm »

It's a 50k pot that attenuates the circuit. It is R33 in the schematic (shown as a fixed resistor in the schematic. More resistance, more signal voltage goes to the tube. Less resistance more voltage shunts to ground. Schematic should be fixed, but you will need to open it in a new tab and zoom in a bit to see it. The site seems to have a limit on pixel size.

GregC · « **Reply #3 on:** 19 Apr 2017, 06:23 pm »

You mentioned this stage on one of my posts great job!

JakeJ · « **Reply #4 on:** 19 Apr 2017, 11:54 pm »

OK, thanks for the better schematic image. Now I realize it is for one channel.

Can I ask what you have invested?

sfox7076 · « **Reply #5 on:** 20 Apr 2017, 01:28 pm »

I went for fancy caps, so the price went up there. If you use normal caps or the Russian teflons, you would be much cheaper than where I ended up. I was dumb at the be. I have about $2000 into it, but a large part of that is because I had to do and redo it a few times... The large expense in this is really the inductors. The power supply is cheap if you have the transformers or you use a toroid. The resistors are all stuff most of us have laying around. The inductors are about $100-$150 each from Slagle and there are 4 of them. I am sure someone else like heyboer could wind them for 1/4 the price, though no guaranty of performance there. If anyone wants to try this venture, I have 300hy chokes (a lot of them) I am happy to send along for shipping. Other than that, you can use 9 pin sockets and just about any kind of wire you want. The two issues I ran into are, don't make a wiring error and shield the crap out of the power cable. Also, consider having the power come in far away from the RCAs in/out.

Shawn

JakeJ · « **Reply #6 on:** 20 Apr 2017, 01:44 pm »

That's a darn sight better than $30K! Congrats on the finished project.

Steve · « **Reply #7 on:** 21 Apr 2017, 01:10 am »

Got three questions sfox.

1) How quiet is the 6C45 tube. Best judgement is ok. Maybe compare to SS phono stages as reference if possible.

2) How does the 6C45 sound? Again I know that is subjective and mixed with other tubes, but being Sovtek I have to wonder.
On the other hand, if it is truly as linear I have heard, it must sound pretty descent.

3) Does anyone else manufacture the 6C45 besides reflector?

Thanks and Cheers
Steve

sfox7076 · « **Reply #8 on:** 21 Apr 2017, 04:02 am »

It is hard because there are two other tubes on each channel. I also opted for 6c45s made in 1981 with OTK stamps. Not new. I got them for $10 each. They are reflector, but from time gone by.

The stage is quieter than any phono stage I have used. I like it a lot better than the high end stages I have used. I tried the Arion, the musical surroundings higher end stage, nova II as well. I even had a vista here. I also tried each of these with hashimoto step ups. No contest. This is as quiet (LCR also naturally kills tone arm resonance). I have had no oscillating from it, though I did put small suppression beads on the grids and made sure I used every pin. No hum unless volume is way up and my ear is on the tweeter. I think that is actually an rca cable issue.

It sounds open, airy and full of detail. My friend who has the tango LCR modules (Oliver of the 45 luv thread) is coming Sunday to hear it. I will pass on his impressions.

BobRex · « **Reply #9 on:** 21 Apr 2017, 01:22 pm »

Quote from: sfox7076 on 21 Apr 2017, 04:02 am

(LCR also naturally kills tone arm resonance).

Hmmm, this is the first time I've heard of this. How does this work?

sfox7076 · « **Reply #10 on:** 21 Apr 2017, 02:05 pm »

Paraphrased from Dave Slagle: One of the nice touches of using the LCR filter is that there is a 12dB per octave slope below 20hz which gets tonearm resonance and rumble out of the picture.

JohnR · « **Reply #11 on:** 21 Apr 2017, 02:08 pm »

Nice build

Steve · « **Reply #12 on:** 21 Apr 2017, 09:34 pm »

Quote from: sfox7076 on 21 Apr 2017, 04:02 am

It is hard because there are two other tubes on each channel. I also opted for 6c45s made in 1981 with OTK stamps. Not new. I got them for $10 each. They are reflector, but from time gone by.

The stage is quieter than any phono stage I have used. I like it a lot better than the high end stages I have used. I tried the Arion, the musical surroundings higher end stage, nova II as well. I even had a vista here. I also tried each of these with hashimoto step ups. No contest. This is as quiet (LCR also naturally kills tone arm resonance). I have had no oscillating from it, though I did put small suppression beads on the grids and made sure I used every pin. No hum unless volume is way up and my ear is on the tweeter. I think that is actually an rca cable issue.

It sounds open, airy and full of detail. My friend who has the tango LCR modules (Oliver of the 45 luv thread) is coming Sunday to hear it. I will pass on his impressions.

Thanks sfox, much appreciated and look forward to more info. I asked about the noise since there is higher gain, thus more chance for hiss/noise. However, the 6C45 family with its extremely high transconductance and Mu offers a chance for extremely quite performance.

I know with the 416B/C tube, transconductance of 50,000/75,000 respectively, Mu of 225, that one could obtain the noise level of a parametric amplifier at 432mhz. So I was hoping for the same performance from the 6C45, which seems to be the case from your description. Unfortunately, with the 416B/C, a very very small bias causes a short from grid to cathode, so audio application appears to be out of the question. The image below is for a 416D version from a DIYer.

Thanks again for the information and looking forward to more, sfox.
Steve

sfox7076 · « **Reply #13 on:** 22 Apr 2017, 03:21 am »

Those are pretty ugly curves. I have traced on my Utracer and the 6c45 was much better. I have some 416s somewhere. Lighthouse tubes where sockets are unobtainable. I have seen at least one phono preamp built with the 416. I have also been told that used tubes are subject to shorts due to how they were used by bell, but I have no idea. Next project may be a two stage direct heated hybrid stage. CX-340 tubes gyrator loaded with a follower. Maybe 841 instead.

Shawn

Steve · « **Reply #14 on:** 22 Apr 2017, 04:23 am »

Quote from: sfox7076 on 22 Apr 2017, 03:21 am

Those are pretty ugly curves. I have traced on my Utracer and the 6c45 was much better. I have some 416s somewhere. Lighthouse tubes where sockets are unobtainable. I have seen at least one phono preamp built with the 416. I have also been told that used tubes are subject to shorts due to how they were used by bell, but I have no idea. Next project may be a two stage direct heated hybrid stage. CX-340 tubes gyrator loaded with a follower. Maybe 841 instead.

Shawn

The DIYer's curves are suspect for sure. Being it is a Bell Lab's tube, I am sure its distortion is quite low. The B version used glass separating the grid from plate and cathode for approx 10,000 hrs life. The C version uses beryllium oxide plus other improvements for double the life since the tube was used extensively in repeater stations etc. I when I tested the tube in College lab, when -1 volt was applied to the grid, the grid shorted to the cathode. The grid "wire" is only 0,0003" dia and only 0,0006" from the cathode. So one must be very very careful using this tube. (I used to have the complete data on the tube.)

Here is a photo of the tube for newbies. The grid is the "center ring with threads."

Cheers
Steve

sfox7076 · « **Reply #15 on:** 26 Apr 2017, 02:29 pm »

Having spent more and more time listening and rolling the tubes in the final stage, it is clear to me and Oliver that the 6N6P is better in this circuit than the venerable 6H30 (80s vintage). It's actually starker than I imagined. The 6N6Ps are so much more open and have a lot more presence in the room than the 6H30s. I really love this phono preamp. My next project is a restore of a New London 901A. A different beast with no audio use, but fun nonetheless.

sfox7076 · « **Reply #16 on:** 3 May 2017, 02:36 pm »

Finally got permission. Here is my schematic (One channel of the preamp). Note I use the 6N6P instead of the 6H30. That required me to reference/bias the 6.3v heater winding to the B+ through a voltage divider. The heater scheme is not on here (just DC 6.3v @ 4.5A because of all the tubes) and the B+ (CLCLC power supply). The unit is dead quiet.

Shawn

sfox7076 · « **Reply #17 on:** 3 May 2017, 02:38 pm »

Quote from: Steve on 22 Apr 2017, 04:23 am

The DIYer's curves are suspect for sure. Being it is a Bell Lab's tube, I am sure its distortion is quite low. The B version used glass separating the grid from plate and cathode for approx 10,000 hrs life. The C version uses beryllium oxide plus other improvements for double the life since the tube was used extensively in repeater stations etc. I when I tested the tube in College lab, when -1 volt was applied to the grid, the grid shorted to the cathode. The grid "wire" is only 0,0003" dia and only 0,0006" from the cathode. So one must be very very careful using this tube. (I used to have the complete data on the tube.)

Here is a photo of the tube for newbies. The grid is the "center ring with threads."

Cheers
Steve

As an aside, I have a schematic for a 416C to a 300B amp from the same gentleman. I am not building it, but it is cool.

d888sp4 · « **Reply #18 on:** 21 Jul 2017, 01:23 pm »

Quote from: sfox7076 on 3 May 2017, 02:36 pm

Finally got permission. Here is my schematic (One channel of the preamp). Note I use the 6N6P instead of the 6H30. That required me to reference/bias the 6.3v heater winding to the B+ through a voltage divider. The heater scheme is not on here (just DC 6.3v @ 4.5A because of all the tubes) and the B+ (CLCLC power supply). The unit is dead quiet.

Shawn

Hi,
very nice job !
Please, can you upload .asc file so I try to look into LTSpice ?
I'm also working to a pre phono LCR circuit diagram I'd like to compare the performance of my schema with yours.
I'm trying to avoid the use of capacitors on the signal path (transformers coupling).
When I'm satisfied with the performance I'll publish the scheme here.

Thanks.

Alberto

BHD · « **Reply #19 on:** 27 Jul 2017, 07:20 pm »

That's a very cool build! I wanted to try an LCR phono stage so I bought the Lounge Audio and was absolutely floored by the sound quality for the price and it absolutely slaughtered the Clearaudio I was using before. I'm going to be building the Pete Millett LR phono stage to replace it as soon as I can get the parts together.