DIY Audio Projects Forum

K301 RIAA amp from Oatley
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Author:  Les [ 26 May 2011, 12:12 ]
Post subject:  Re: K301 RIAA amp from Oatley

If you'll notice in the pictures, all of the tubes have been fitted with rubber grommets. While this does go along way to damping the ring noise, it is still there. I have tested the foam I have with my little headphone amp built a little while back. It works, plain and simple. The best part is that this foam didn't cost me a penny. It was refuse from the company I work that was used to pack PCBs from repair depots. You can't beat free! :-D

As far as filamentary voltage on these little jewels, I have taken these down to the point just above cut-off and they still want to ring - of course when they cut-off, they don't ring, or do much else for that matter. I have heard rumors that biasing the tube a certain way will eliminate the ring, but I haven't found any solid facts or methods. According to Paul of Oatley Electronics, these little tubes are used in Audio-Technica AT3060 microphones. If that is so, they would have had to find the way to quiet these tubes. I would love to get my hands on one of those mics to dissect and analyze what they've done.

Author:  Gio [ 27 May 2011, 12:26 ]
Post subject:  Re: K301 RIAA amp from Oatley

The cold weld product looks great. There are plenty of videos available. Not sure how I never heard of this stuff before, it looks great.

Les, how will you be finishing the chassis?

Author:  Les [ 27 May 2011, 16:00 ]
Post subject:  Re: K301 RIAA amp from Oatley

J.B. Weld is a great product. Once it fully cures, it can be sanded, primed and painted. I've even used it to repair a car radiator!

The case got a good sanding last night and the corners look a heck of a lot better. The overlap seam is now completely hidden. :up: I didn't work very long last night as I am suffering a stupidity accident from this past week-end. I managed to upset a rider-mower and it landed - up side down - on my legs. Nothing broken, but bruised and some deep tissue injury. I was stupid and rode too high on a bank behind my house and rolled the mower. Sound fun? IT AIN'T! So please, don't try this at home! :down:
Gio wrote:
Les, how will you be finishing the chassis?

I'm glad you asked ;-)
This week-end I will be priming the case, 2 coats, and then finished with 2 coats gloss black paint. Then final assembly! If everything goes right, I should have the whole thing done by Monday evening (05/30). Nothing fancy, I'm a terrible artist. If some-one would like to machine me a nice K301 emblem for gratis, I would accept it and proudly display it. ;-) Up front, nothing but a nice toggle switch that has a translucent toggle, lit by a deep blue LED. The back will, of course, have the IEC receptacle and an RCA jack pad made from acrylic (it's nice and think @ about 4mm thickness), and the phono grounding stud.

Author:  Gio [ 29 May 2011, 20:00 ]
Post subject:  Re: K301 RIAA amp from Oatley

Hi Les,
All I have been doing is sanding with fine grit paper with palm sander. Touch up with steel wool as required. Good cleaning with degreaser. One coat of Tremclad semi-gloss rust paint. One thick coat with a disposable foam paint brush. A second coat has been problematic and one coat looks pretty good. I'd like to get a better finish, but i don't want to use spray. So I'm looking for tips.

Author:  Les [ 30 May 2011, 21:52 ]
Post subject:  Re: K301 RIAA amp from Oatley

Gio wrote:
I'd like to get a better finish, but i don't want to use spray. So I'm looking for tips.

I just followed the instructions on the can. I prefer spray paint simply because my brush painting skills really blow. ;)

Build is complete! It is in the cabinet and listening impressions to follow!

Saturday evening, after a day of hardware shopping (lawn care stuff), I put on the primer - 2 coats, then the gloss black paint - 2 coats. I used automotive quality primer and paint and it turned out really nice. I let the whole thing cure over night.


Sunday - assembly.

completed on bench.jpg

completed on bench rear.jpg

I performed some simple tests on the bench. With case completed and base plate installed, I monitored the output. The inputs were open. The scope displayed showed no hum and I could not distinguish any other noise coming from the amp, however, having the scope input at such low setting, it picks up all the background interference in my shop (florescent lamps). I tapped on the case and inspite of the deadening foam, the little tubes registered the thump and there was a small amount of ring that damped very quickly. Just a note, I had to remove the rubber grommets from the tubes as they were not helpful. With the grommets in place, the tubes rang easily, in spite of the foam. Removing the grommets allowed the foam to do its thing. Sorry, I don't have any pictures of the traces. It was late, I was tired, my leg was aching. I just wanted to get it done. Having satisfied myself that all was working correctly, I brought the unit in to the house and connected it to my stereo.

I was listening to "Canadian Sunset" for this picture. The EQ is actually out, so what you see is the actual representation of the pre amp's output. I know, not much to really see since there's no audio you can hear and it's not a video. SO, I will attempt quantify momentarily.
in rack.jpg

First, I connected the amp to the stereo inputs only. Powered on the pre-amp - soft thump and very brief ring as the voltage climbed to full. Stereo volume at that point was about 1/4th full. No audible hum at that point, so I pushed the volume up to about 3/4 full and just picked up a slight hum. To be sure, I switched off the pre, the hum abated. Ok, so I have a slight amount of 60hz. I don't play my stereo at that level anyway, but I will later redesign the PS to a full wave - later. While the pre was off, and resetting the stereo volume to the prior setting, I connected my BSR Quanta500. Pre amp on, phono to play position (it has a muting function when not in play) and...... uh oh, a plainly audible raspy 60hz buzz. On the EQ display, it shows up at 60hz (minimal signal level) and then at the upper end of the range more substantially. I disconnected the phono and tried an alternate source - no noise. There is something not quite right between this amp and the phono. The noise was not present in the SS amp I was using. When playing music, if the music is loud enough, the buzz is covered, but it prevalent in quieter passages. I still have some kinks to iron out, obviously. Moving on and mentally tuning out the noise, I was amazed at the over all sound quality of the amp.

Here's what you've been waiting for....

I played several tracks from one of my favorite instrumental album sets. RCA's "Great Instrumental Hits - 3 Album Set". This set includes popular songs like "Canadian Sunset", "Midnight in Moscow", "Cast Your Fate in to the Wind", etc. I selected several tracks and listened to them with EQ out. To watch the display, I saw no real difference between what the SS pre amp produced and what this pre-amp produces. Generally strong through the mid ranges while rolling off the ends. Of course, as we know, there is more to sound than its frequencies, right? There is a very noticeable difference in the tonal qualities of the produced sound. While the SS pre-amp tonal quality was lack luster, almost flat - lifeless, the tonal quality of the K301 was considerably brighter, more lively. Cymbal taps and clashes were bright and crisp and piano was lively. Everything was just better detail. In spite of the buzz that was prevalent in quiet passages, what was there sounded very nice, I'm sure it would have been better if it had been clean of any buzz. Was I happy to just leave the EQ out? Nah. Bringing the EQ in, I was able to reform the sound to a more pleasant (to my ears) reproduction. Is this a bad thing, no, my preferences like heavier bass, slightly attenuated mid and only slightly emphasized highs, but unlike the SS pre-amp, I was able to keep those changes more close to "flat".

As a first time DIY project, this kit holds promise. As it stands, the DIYer has to get creative to find ways to tame the JAN6418's tendency to ring and the on-board PS really shouldn't be for any phono amp, especially one with such sensitivity. Outside of the case, this pre-amp picks up all interference. Properly encased and shielded, it does a very nice job. Also, eventhough Paul of Oatley Electronics says you should not need to worry about spares, I would recommend that if you can do so cheaply ( I bought 10 extra through eBay for about $2 (US) each), do so. Out of the 5 tubes provided, I only had one pair nearly matched. It took going through the remaining 10 to get another pair that matched the first pair. Is matching important? Well, do you like an imbalanced audio production? I was able to get 4 matched tubes. Did I have to have 4 matched, no. But you should have at least 2 matched pair.

Over all, I give this kit a solid 3 out of 5 stars.
- JAN6418s ring with this kit, badly, if not muffed. Rubber grommets do help, but just not enough. Dense, sound deadening foam closely encasing these tubes does do a better job.
- While Paul of Oatley Electronics tried to provide a kit that was more preferred by some by having a non-SMPS supply, having that supply on board was a bad idea. Again, the JAN6418s are way too sensitive. A better option would be to provide the kit with a break-away PS so the PS could be shielded from the amp.
+ It is easily built. The PCB is well laid out and component placement clearly marked
+ It performs well when properly shielded and those tubes are tamed
+ Runs on 120VAC or 240VAC.

:!: This kit assumes the builder has a good grasp on electronics fundamentals... how to read resistor color codes (or how to use an ohm meter), good soldering skills and understanding of schematic layout. It is a 2 sided PCB, so be careful as removing erroneously placed components can be a challenge. While not having the elevated voltages typically prevalent with tubes, it does use mains power and therefore due care MUST be exercised.

Author:  Gio [ 02 Jun 2011, 12:44 ]
Post subject:  Re: K301 RIAA amp from Oatley

Hey Les, thanks for sharing your findings. :thumbsup:

The finish looks good. I can attest that the grommets alone help plenty with damping the tubes. Interesting that with the foam they do not help, so it is one or the other.

Good results and I am with you on the damping. To get good results with these inexpensive kits one really needs to spend a good amount of time to build a good enclosure that shields and damps the tube.


Author:  Les [ 03 Jun 2011, 08:47 ]
Post subject:  Re: K301 RIAA amp from Oatley

Thanks Gio!
Gio wrote:
I can attest that the grommets alone help plenty with damping the tubes

The grommets were not as effective as the foam. I suspect the reason the combination of the 2 didn't work was because the grommets prevented the foam from making a good tight contact with the tube. With the foam alone, the entire surface of the tube is in direct contact. This foam is anti-static, so no worries of static build up.

Any thoughts regarding my noise?
What I've done so far (and has helped, but not solved):
- route the signal cables as far away from the power cord as possible
- wrapped the ground wire around the TT power cord and fastened that ground to earth grounding
- reoriented the TT's power plug for least amount of noise (it's non-polarized)
I haven't examined the solid-state RS amp yet to see what was done.I don't have any type of coupling between the signal ground and the case/earth ground. Would a capacitor help, do you think? I'm planning on working some more on the issue this weekend and I'll be redesigning the PS next week to a full-wave type. Since I will be on vacation, that will give me plenty of time to work on it and make some refinements. I am happy with the kit and if I can smash that irritating buzz that is being injected from the TT, I will be even more happy. It'll be a nice hold-over kit until I can get started on and complete my "ultimate" compactron stereo PP amp, which will have an incorporated phono-pre.


Author:  Gio [ 06 Jun 2011, 21:10 ]
Post subject:  Re: K301 RIAA amp from Oatley

Les wrote:
- wrapped the ground wire around the TT power cord and fastened that ground to earth grounding

I don't have any type of coupling between the signal ground and the case/earth ground. Would a capacitor help, do you think?

Hey Les, Not sure I follow. Which ground wire did you wrap around the power cord? The ground wire from the the TT ground to the preamp ground?

You could try connecting the signal ground to the power ground through a ~150R in parallel with ~0.1uF Type X2 cap. Like Bruce does: ... PP-Preamp/


Author:  Les [ 06 Jun 2011, 23:09 ]
Post subject:  K301 RIAA amp from Oatley-COMPLETE

Gio wrote:
Hey Les, Not sure I follow. Which ground wire did you wrap around the power cord? The ground wire from the the TT ground to the preamp ground?

The ground wire from the TT, wrapping around the TT power cord, then tied to the case/earth ground of the pre.

> I found the buzz. The phono signal cable from the TT had become noise prone. I disconnected the phono end of the cable from the TT and the buzz became a full fledged power hum. For a shielded cable, this should not happen. I have some extra shielded cables and replaced the cable. Noise is almost completely gone. I also did some re-routing of the tiny signal leads bringing the pick-up signals to the internal phono jacks of the TT, which also helped. I'll consider the addition of the resistor and cap between case and sig grounds of the pre. At this point, the remaining buzz is buried in the back ground noise when playing an album and is noticeable only when the stylus is off the album. My EQ was able to filter that out by fully cutting (10db) the 16khz freq. which also cuts a large part of the hiss from album play. It's not noticeable with the EQ out, but that results in a flat response that, while sounding very good for a flat response, is just too flat for me.I encountered an interesting phenomenon that has happened only once before occurred while trying a new cable. The first new cable was acting microphonic. You could tap, or move, the cable and generate noise. I have run in to this but once before. This is a phenomenon that can occur in a shielded cable whose impedance causes a certain mismatch. Fortunately, I had another that was good. This "microphonic" cable may be ok for line level signals, but not good for phono or other types of very small signal pre-amps. Anyway, I can now say that I am very impressed and happy with the overall performance of this little amp.

At this point, I am done with this project. I may make improvements at some time later. I have other projects waiting in the wings that I wish to move on to; most in particularly, my FireFly amp.


Author:  Les [ 19 Jun 2011, 00:42 ]
Post subject:  Re: K301 RIAA amp from Oatley

Since I cracked my unit open to make some measurements to aid AJ in another post regarding the K301, I went ahead and added the suggested capacitor/resistor pair. That didn't change any thing. No noticeable difference in background noise what so ever. :| While I had the cover off, I took a few more pics, since my previous set of pics didn't show completed inside and view of the glow from the tubes (actually the lighting affects of the LEDs).

Complete wiring, prior to pad "encasement".

With padding

And the "glow" - ooooooh

Since the unit has several hours of operation on it now, I checked the condition of the voltage of the PS, and check for heating issues. Everything is as should be. No fluctuation of VDC (rock stable at 30VDC) and clean. No signs of over heating. Me -> HAPPY :D

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