DIY Audio Projects Forum

K301 RIAA amp from Oatley
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Author:  Gio [ 20 Jun 2011, 18:39 ]
Post subject:  Re: K301 RIAA amp from Oatley

I love the job you did with the foam damping!


Author:  Les [ 21 Jun 2011, 08:04 ]
Post subject:  Re: K301 RIAA amp from Oatley

Gio wrote:
I love the job you did with the foam damping!

Thanks Gio!

Author:  Les [ 12 Aug 2011, 13:33 ]
Post subject:  Re: K301 RIAA amp from Oatley

I've got several hours playing time on my K300, now. I am very happy with it, I found I have made an error in my layout - no biggy. I should have laid out my build opposite from what I did. I discovered that my small remaining hum was in fact bieng induced from external sources, namely transformers of the other equipment. Like I said, no biggy, just flip the entire thing over on its back - no hum! Clean sound even at high volumes!

All that said, while listening last night I had an interesting yet undesired anomaly occur with the K300. After being on for about 5 minutes, it began to sing quite loudly and in only one channel. It was not affected by any external influence. It stayed at the same level regardless of signal input or even thumping on the case - didn't matter. I turned it off for several seconds, and then turned it back on and it played normally the rest of the evening. It's kinda left me scratching my head, though. I wasn't playing any music that had any kind of boomy bass or thumping, so I can't imagine this was vibration induced. Especially in light of the fact after resetting the K300 and replaying the same track at the same volume, the K300 stayed quiet. :confused: This wasn't feedback type of singing (squeal), it was the typical "ring" associated with the 6418, but considerably louder and steady.

I have redesigned my power supply (on paper) and as soon as I can work in my shop with out possibility of heat stroke, I will develop the circuit. I am also going to replace the box and change the layout to avoid future hum issues. The current box will get re-purposed in a bench supply I've been pondering as of late. It's been pushing 100degrees (F) for the past month and my work shop lacks any air-conditioning. My shop has been getting to about 130F - 140F :hot: during the heat of the day and not really cooling off during the evenings. WAY too hot to work! Cooler days coming though! Looking forward to getting back to work on my projects! :up:

Author:  cdc3jj [ 25 Aug 2011, 02:18 ]
Post subject:  Re: K301 RIAA amp from Oatley

Hey Les... I just got the K301 in the mail. Soldered the pcb tonight and all looks good. I'm kind of a noob so I have a few questions. Does it run off a straight wall plug or does it need a wall wart style power supply? Is there a way to turn this up or down? Like a potentiometer wired in some how? I've seen pics in one thread, but it doesn't look like there's a pot so I guess you don't need one? Is that right? I've built some guitar amps and pedals, but everything always has a volume knob of some sort. Last question involves testing the tubes. The diagram shows there's a 9v in the circuit, can I just use a battery for that? Thanks so much for your posts here. I hope I'm not hijacking the thread. If you'd rather email I'm at Would love some help...

Author:  Les [ 25 Aug 2011, 08:40 ]
Post subject:  Re: K301 RIAA amp from Oatley

cdc3jj wrote:
Does it run off a straight wall plug or does it need a wall wart style power supply?

Yes, it uses straight wall power. Follow the instructions provided with the kit for correct mains wiring of the transformer.

cdc3jj wrote:
Is there a way to turn this up or down? Like a potentiometer wired in some how?

It is a phono pre-amp. It is not a power amp. The output of this amp is to be fed in to a power amp, which generally has its own volume control.

cdc3jj wrote:
The diagram shows there's a 9v in the circuit, can I just use a battery for that?

You've misread. The purpose of the 9V and the 820ohm resistor is to set up a simple testing circuit for tube matching, should you want to do that. This IS NOT part of the amplifier circuit. Since the kit includes only 5 tubes, going through the matching test is really pointless. You've got to use 4 of the 5 tubes. I had to have an additional 10 tubes to find a matching set. This kit requires 30VDC, which is provided by the on-board power supply.

cdc3jj wrote:
I hope I'm not hijacking the thread.

Not at all.
The most important piece of advice I can give you about this phono pre-amp (actually 2 pieces) is;
1.) Dampening - This pre-amp has tendacy to be somewhat sensitive to vibration and may/will produce a hi-pitched "ringing". The rubber grommets will help, but depending on the actual performance of the tubes supplied, you may well need to do more.
2.) SHIELD IT! Again, depending on the sensitivity of the tubes you received, this kit can be very sensitive to stray electrical fields. It needs to be mounted, preferably, in an earth grounded metal case. Just be sure to keep the signal grounds isolated from the case (RCA jacks should have isolation from the case).

Please keep in mind this pre-amp is by no means a "high end" pre-amp. However, it does do a very impressive job considering you only paid about $50US for the kit. I use mine regularly in concert with my 10band EQ. The testament to this kit is the fact I get very good response with a minimal curve on the EQ. I have played it with-out the EQ in path and the output quality is very good. I just like a little more bass and a little less mid. :smoking:

Author:  cdc3jj [ 05 Sep 2011, 02:35 ]
Post subject:  Re: K301 RIAA amp from Oatley

Les... Thanks so much for your response. I'm neck deep in home renovations, so I just got a chance today to continue the 301 project. I am building it inside a handbuilt wood box with a plexi bottom. I flipped the LED's upside down so the light is projecting out the bottom through the plexi. I plugged it up and everything lit up as it should.

I haven't plugged it into my receiver yet to check the sound because I don't have the RCA's wired in and I don't know what to do about grounding....

I bought some of the metal tape today to shield the inside of the box and I'm putting a shielded wall between the power section and the tube section. I've got a metal plate from a steel junction box I could put under the power section and solder my power cord ground to it, although I'm not sure that would accomplish anything. Seems like there should be a ground wire coming from the power section to ground the pcb's. I guess I could run a ground wire out the back (like my record player has) but I would prefer to have it all internal. Here's a pic... Image
Hopefully that helps a little to clear it up. Power's coming in from the right side of the photo. Any suggestions you can make will be appreciated. I'm going to try to get the box sanded and stained tomorrow so I can get everything wired up in the next few day. Thanks

Author:  Les [ 06 Sep 2011, 12:08 ]
Post subject:  Re: K301 RIAA amp from Oatley

You have a new and improved version of the K301! I wish mine had come with separate PCBs for power and amp. COOL!

There are 2 types of grounding and they should be kept separate;
1 - Signal grounding
2 - System (safety) grounding & Shielding (Earth ground)

Signal grounds are those related to carrying the signal (AF or RF, in this case AF). You want these kept short as possible. From each signal point on the PCB, use a tightly twisted pair of wire to each jack.

System grounds should only be connected to case. In your case, the system ground from the power cord should be connected to the foil shielding and what ever choice of encasement you're utilizing to shield the power supply.

It is recommended that a 1nF to 100nF X2 type capacitor in parallel with a 100K to 1M resistor be placed between the signal ground and the system ground. This schema provides a safety ground to the signal and can help eliminate EMI/RFI from the signal path.

The ground wire from the turntable needs to be connected to "system ground", not signal ground. NO part of the amplifier or power supply PCB should be directly connected to system ground. The DC ground from the power supply PCB is part of the signal path and therefore is treated AS signal ground.

Author:  blougaville [ 08 Nov 2011, 16:41 ]
Post subject:  Re: K301 RIAA amp from Oatley

Hey, everyone. I am a total noob when it comes to DIY equipment building. I am considering this K301 kit for my first ever build and wanted to see if anyone had some suggestions for me. Should I stick strictly to the kit, or should I dabble in replacing carbon resistors? Should I order extra tubes in hopes to find a few matching pairs? Is there anything else that I should incorporate that isn't included in the kit?

Thanks in advance for any feedback.


Author:  Les [ 09 Nov 2011, 09:49 ]
Post subject:  Re: K301 RIAA amp from Oatley

I would only recommend obtaining extra 6418tubes if you're going to go for a "balanced" kit. Even then there's no guarantee you will be able to obtain 4 equally matched tubes. In the 15 tubes I had (5 from Oatley and 10 from ebay), I only had 4 tubes that were close. I was able to come up with a couple of matched pair, but I was going for quads. The assortment ran the gamut from essentially blown (low conductivity) to 100uA. The four I had chosen were in the ball park of 70uA. I had a couple @ about 80uA and a couple matched at about 45uA. From my experience and looking in hindsight, a lower value may have been better as these 4 I chose are awful sensitive to vibration and outside EMI. The kit now available has a break away power supply, an improvement over the kit I received which did not. I highly recommend putting the power supply in its own fully shielded case. It is recommened a 3-wire AC supply is provided to the power supply. Connect the mains ground to the case. The circuit ground should be connected to AC mains ground via a 150r resistor and 100nF X2 type capacitor in parallel to the resistor. Turntable ground wire to connect directly to case (AC mains ground). All signal wires should be kept short as possible and use either high quality shielded interconnect wire or tightly twisted pair. Remember, no part of the circuit should be connected directly to AC mains ground as doing so could create a ground loop, inserting unwanted hum noise. For best EMI/RFI immunity, the entire project should be placed in a metal case. I have not toyed with any other mods to the kit. It has very good sound the way it is designed. It's a good starter project.

Have fun! :D

Author:  mwhouston [ 09 Nov 2011, 12:59 ]
Post subject:  Re: K301 RIAA amp from Oatley

All Les' tips are good. I have found these small tubes can ring in a metal case and not so much in a plastic case. I line my plastic cases with a roll of bitumanised Al for dampening and sheilding. It comes in wide roll and is easy to work with. Of course put your rubber gromets on the tubes before soldering them in.

Though matching tubes is a good practice it may not make all that much difference here. Still if you have boxes of tubes as I have, why not? I often run mains earth and signal/power altogether and never seem to have a problem but keeping the PS earth isolated with a 100ohm resistor or whatever is easy to do and a good practice.

I have built two of these and both sold very quickly. On an audio club day a few hard core vinyl listeners were very impressed with how good this cheap preamp sounded. You should get good results straight out of the kit. If you upgrade the parts you should look at high-end components. Otherwise you may not hear any difference. I have build a straight kit and heavily modded kit. Both sounded excellent.

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