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Radi0kit-RK318 - MM tube phono preamp
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Author:  mwhouston [ 29 May 2015, 02:22 ]
Post subject:  Radi0kit-RK318 - MM tube phono preamp

I bought two of these kits last year and today assembled one. I went with the high-end parts, Dales, Wima and Cornell Dubillier. The PCB (single side) is Ok but not stunning, masked, tinned and screened. All from the land of octopus and Ouzo.

Assembly took about 1.5hrs taking it easy. The brd. is quite flexible with regard to how you can connect to it for power etc. JJ Tubes are a nice touch but of course there are better around. Long way to go -> PS, enclosure etc. Should add the instruction booklet is quite good. They have gone to a lot of effort to make the assembly as easy as possible. Lots of good stuff in the booklet which is 8 double-sided pages long. Comes in a really nice smart little box. For about AU$100 good value.

Attachment:
cct. brd.jpg

Author:  mwhouston [ 12 Jun 2015, 21:58 ]
Post subject:  Re: Radi0kit-RK318 - MM tube phono preamp

A possible layout to fit in 1U enclosure.
Attachment:
Layout.jpg

Author:  mwhouston [ 02 Jul 2015, 07:12 ]
Post subject:  Re: Radi0kit-RK318 - MM tube phono preamp

Finished construction today and had a brief listen. The enclosure lid needs a few big holes for the tubes to poke through. The sound is on the dark side but it needs 50 hours plus to burn in. I like he sound though but I know it I'll change.

Attachment:
image.jpg

Author:  gofar99 [ 03 Jul 2015, 21:50 ]
Post subject:  Re: Radi0kit-RK318 - MM tube phono preamp

If it is on the dark side then listen only to things like Darth Vader.... on a more serious note is there a schematic and how is the equalization done? I am always curious on how others have attacked to problem of gain and EQ. My personal preference is for passive EQ, but units with active EQ can be quieter. With passive I can only get to a S/N of about -95dbv. Active I can get almost 10 more db.

I was wondering how the tubes would fit in a 1U....holes of course will work. JJ tubes are not particularly sensitive to EMI so it ought to be OK. I personally would leave enough room for tube shields though.

Good listening
Bruce

Author:  mwhouston [ 03 Jul 2015, 22:11 ]
Post subject:  Re: Radi0kit-RK318 - MM tube phono preamp

I may have found the issue the output impedance of the tube phono preamp is 45K. My BoZ has a 25K pot upfront. I'm about to swap back my 6N7reamp with and put impedance of 250K.

My ICs are only 105pf so I can't see these being a problem.

Author:  mwhouston [ 05 Jul 2015, 20:29 ]
Post subject:  Re: Radi0kit-RK318 - MM tube phono preamp

I made some ICs with only 23pf capacitance. Sounds better. The high output impedance of the MM stage would cause a roll off of HF starting at about 5K with leads of higHer capacitance. With these leads roll off doesn't start until 22K.

On another matter. When testing the voltages around the preamp one plate voltage is 20V higher than the other three. I have tried different tubes and swapping the two tubes. Always the same plate reads 152V while the others 132V. I have visually checked the value and placing of every component six times. And have read the value of every component, with a meter, twice. There is nothing wrong with the construction or components. I have even check for dry joints, none.

Anybody got any clues?

Author:  mwhouston [ 06 Jul 2015, 00:28 ]
Post subject:  Re: Radi0kit-RK318 - MM tube phono preamp

Haven't found the varied voltage problem but found why the preamp sounded dark. One cap in each channel was in the wrong place. It could go in two ways but you could not see this unless you looked underneath the brd. I had to rotrate the brd. 180 to allow easier hole drilling for the tubes and noticed the caps was in the wrong two (of three) holes. The cap is part of the EQ cct. so that explains the darkness. All finished now except for decal which is being engraved. With top plate on, no hum, dead quiet. The whole enclosure is earthed to mains.
Attachment:
Rear.jpg
Attachment:
Top.jpg
Attachment:
Front.jpg

Author:  mwhouston [ 08 Jul 2015, 07:54 ]
Post subject:  Re: Radi0kit-RK318 - MM tube phono preamp

Very happy with the overall sound of the MM tube preamp. He is a final image with the retro-thermionic decal.
Attachment:
Front.jpg

Author:  mwhouston [ 30 Jul 2015, 06:55 ]
Post subject:  Re: Radi0kit-RK318 - MM tube phono preamp

This preamp has a lot of gain and when you add it to my new 180W Tripath power amp and two stage 12AX7 preamp I can barely turn the volume knob up. I cut away Ck on the last stage of the preamp to lower the gain but this did little. I was going to add some resistors to lower the signal level going into the preamp but then this adds to the output impedance.

Then I had an epiphany. I happen to have two JAN-GE 12AU7s (NOS) the type I have been using on my hybrid "Parley" tube and SS preamps. Nice tubes but not in the clear yet. Sure enough dropping in the "U" drop the gain to now a very workable level. Not only that but pots don't work well at extremes of their range. Now they work nicely toward the center of their range.

But the JJ tubes supplied have a very short plate, about 1cm. Great for low microphonics. My GEs have a long plate. I just ordered the top of the range Psvane 12AU7-TII gold pins. These have (at least in the photo) a short plate. And where I have Gold Lion 12AX7s in my current preamp I will swap them to the GEs when the Psvanes arrive. So less gain in the phono preamp and preamp. And a better tube.

The GE 12AU7s have a softer tone to the 12AX7s. OK for me but I can see why some like the crisper "X7"s.

http://psvanetube.com/wordpress/tag/12au7/

http://psvanetube.com/wordpress/store/p ... or-single/

Author:  gofar99 [ 30 Jul 2015, 08:37 ]
Post subject:  Re: Radi0kit-RK318 - MM tube phono preamp

Hi Mark, Yes the 12AU7s will drop in a circuit wired for 12AX7s and often work. The problem I have found is that the components are not optimal for 12AU7s. The different anode impedance, current ratings and Mu will often cause the 12AU7 to operate outside the best linear and low distortion range. If you can check the distortion level you may find you are OK. Once in a while the values for parts match up. I have played around with this and found the distortion to more than double and the head room to be seriously reduced and usually asymmetrical. Swapping like this is often done in guitar amps...12AX7, 12AT7, 12AU7, 5751 etc will all swap and since they want distortion it works fine. For hi-fi though I fond it not to usually be a good thing.

Good listening
Bruce

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