6418 Tube (Valve) RIAA Phono Preamplifier Kit
6418 Valve (Tube) RIAA Phono Preamplifier Kit
Last year I built and reviewed the low cost Oatley Electronics K272 Tube Stereo Headphone Amplifier Kit. In conversation with Oatley Electronics (located in New South Wales Australia), they told me that the kit became very popular and sold pretty much all over the world. The little headphone amplifier kit was based around new old stock Raytheon JAN 6418 sub-miniature pentode valves (tubes). Using the same JAN 6418 valves Oatley Electronics has released an inexpensive ($47AU, May 2010) tube based stereo RIAA preamplifier kit for moving magnet (MM) cartridges, and they were kind enough to send me a couple of kits.
Photograph 1: 6418 Sub-miniature Tube RIAA Phono Preamplifier Kit
The printed circuit board (PCB) is double sided, solder masked and silk screened. Almost the entire top layer of the PCB is grounded to provide shielding. To keep the kit price low, the passive components are very basic. A mix of metal film and carbon resistors, polyester, ceramic and electrolytic caps. Polyester caps are generally not used in audio circuits because some say they do not sound good. Of course one can choose to upgrade the passive parts. The contents of the phono RIAA preamp kit are shown below. A full parts list of the kit contents is included in the Oatley Electronics K282 Kit Instructions (PDF - 646kB).
Photograph 2: Oatley Electronics K282 Tube Based Stereo RIAA Preamplifier Kit Contents
The external power supply is a small 5V switch-mode power supply (SMPS) with a universal outlet adapter that can be used with American, European, Australasian and UK power outlets (100 - 240VAC). The circuit uses a MC34063A switched mode power supply integrated circuit (IC) to convert the 5V external supply to 30V. The switching frequency is set at around 100kHz. The phono preamp circuit is a two stage affair with each valve stage buffered by a 2SK170 Field Effect Transistor (FET). The new old stock (NOS) JAN 6418 tubes are used in pentode mode. The schematic of the tube phono preamp kit is shown in Figure 1. Please note that this circuit is © Oatley Electronics and permission to host the schematic on this site has been provided by Oatley Electronics. A high resolution image of the schematic is available in the kit instructions.
Figure 1: Oatley Electronics K282 Tube Stereo RIAA Preamp Kit Schematic
(Schematic © Oatley Electronics)
The kit is fairly complete, however the builder will need to supply input and output connectors (typically RCA) and most will likely want to build the kit into some sort of enclosure. A completed kit using only the supplied parts is shown below.
Photograph 3: Completed Oatley Electronics K282 Tube Based Stereo RIAA Preamplifier Kit
Valve (Tube) RIAA Phono Preamp Kit Construction
Following the instructions supplied with the kit, assembly is very straight forward, but you will require basic soldering skills. The inductor will need to be wound (14 turns) and full details for constructing the inductor are included in the instructions. The JAN 6418 valves are very microphonic. Two tight fitting rubber grommets (supplied with the kit) on each valve will help reduce microphonics significantly. Place the grommets on the tubes prior to soldering them to the PCB. There are no special tools that are required, but you will need the basics, such as a soldering iron, wire cutters and wire strippers. For my frist phono kit build, I used single strand fine copper wire (~26Ga) for the signal wiring. Usually with sensitive and high gain circuits one would use shielded wire as hook-up wire. If you used unshielded wire like I did, tightly twist the signal and signal ground wires (I used an electric drill) to keep unwanted noise out of the wiring. For the internal power connections I used multi-strand wire (~22Ga) which I twisted together. The on / off control is a single-pole, single-throw (SPST) swtich connected to the 5V supply with a faint blue LED to indicated power. The input and output connections are gold plated RCA jacks.
Photograph 4: 6418 Tube / FET Stereo RIAA Preamplifier Kit Construction
I decided to enclose the phono preamp kit into a medium size cast aluminum case. I always paint these cases and this time would be no exception. Usually I paint in a gloss black or red. The paint I use is epoxy enamel and though it takes a week to dry hard it forms a very tough coat. Prior to painting I thoroughly sand the exterior of the case and use an acid etching primer (rust primer). But for this build I wanted a totally different finish altogether. So I went with the Hammered Copper look (using an enamel rust paint). It turned out to be the "bad paint job" look but it is different. Maybe not that bad.
Photograph 5: Finished 6418 Tube RIAA Preamp Kit - Hammered Copper Finish
6418 Sub-miniature Pentode Tube (Valve) Matching
The small JAN 6418 sub-miniature pentode valves are shockingly unmatched relative to one another. The kit instructions include a simple method using a 9V battery, a current meter (a digital multimeter can be used) and a 820 ohm resistor (provided with the kit) which enables you to match the valves.
Figure 2: Sub-miniature 6418 Pentode Tube Matching Circuit Schematic
By noting the current flow through the valve, the 6418 valves can be measured for matching. You do not need four closely matched valves for the kit. What you do need is a close match of total valve current flow in both channels. The current in individual tubes can vary wildly between 50uA and 170uA. I had fourteen valves to work with and no two valves even came close to each other. But what I was able to do was pick four valves, in two combinations of two, which gave me close to equal gain in both channels. Less than about 10% difference between channels cannot be noticed so don't worry too much about perfect balance here. I used the valves with the higher gain in the first stage and valves with the lower gain for the second stage. The kit comes with one extra valve to facilitate better matching.
Valve (Tube) Microphonics
These sub-mini valves will ring like a church bell on Sunday morning so be sure to use at least two grommets on each valve. The grommets should be placed on the valves prior to soldering them to the PCB (the valve pins can break). When soldering the valves to the PCB, try make the leads as short as possible to help keep the valves from swaying. Also before I screw the enclosure lid down I place a thin layer of Dacron (synthetic polyester fill) over the entire circuit which helps to hold the valves steady. This definitely helps as I have tried it with and without the poly fill. The polyester fill will also help reduce any resonance or audible echoing within the case further helping with microphonics. Don't worry about the valves melting the poly fill as they hardly get warm. Use a good set of rubber feet to de-coupling the preamp from the surface it sits on. Other forms of damping and decoupling such as placing the preamp on a slab of marble or granite may further help reduce mechanical noise.
Special Edition MkII JAN 6418 Phono Preamp
Since completing and enjoying the phono preamp kit for many hours I decided to build another. This time I would use polypropylene capacitors and use a solid state DC power supply. Originally I wanted all the power supply components in the case but due to hum induced by the 24V AC wall wart I moved the rectifier portion to the back of the wall wart. This meant that only DC was entering the case. A double Pi filter network was used. That is small cap - small resistor - large cap - large resistor - small cap (C-R-C-R-C).
Figure 4: Special Edition MkII JAN 6418 Phono Preamp Power Supply Schematic
The Special Edition MkII JAN 6418 Phono Preamp is shown below. I also used a piece of copper clad board for additional shielding.
Photograph 6: Special Edition MkII JAN 6418 Phono Preamp PCB
Photograph 7: Special Edition MkII JAN 6418 Phono Preamplifier Kit
Listening Impressions - Oatley K282 RIAA Phono Preamp Kit
This is an unusual RIAA phono preamp kit that uses a strange mix of parts and ideas, but one not scared to deliver exceptionally good music at a cheap price ($47AU). For the novice DIYer wanting to get into analogue music or those who just likes kits in general, this phono kit will tick many boxes: Valves, ICs, SMPS, FETs, RIAA.
Photograph 8: Special Edition MkII JAN6418 Phono Preamplifier
I have managed to get in a few hours of listening with the SE MkII version. I'm not convinced it is better than the basic kit version. Sonically, both builds appear to be equal. There will be those who just can't stand the thought of polyester capacitors, carbon resistors and the two SMPS. I would say whichever way you go on to build the kit, you will not be disappointed. In a blind listening test the basic kit outperformed two Cambridge phono preamps and a Hafler phono preamp in front of six very critical listeners of the Melbourne Audio Club, across three different turntables and cartridges. Impressive!
Mark Houston, DIY Convener, Melbourne Audio Club
UPDATE July 2011: Oatley Electronics has discontinued the K282 kit and replaced it with an updated version, the K301. For more information about the new K310 kit, see the K301 RIAA tube preamp kit thread on the forum.