BHL JFET Phono preamp kit!

Minimalist audiophile phono stage using 2sk170 JFETs. Coupling and EQ caps are all Russian paper in oil. High performance. Low budget. Available as a kit including the PCB and all the parts to complete it.

$79 in my eBay listings.

More info

Download the assembly manual

Read a review of this kit at diyaudioprojects.com

There is a build thread over on the Lenco Heaven Forums

Another build at Audiokarma.

power supply options

This circuit should be happy with any supply voltage between 12V and 24V, so feel free to experiment.

I have sold many of these with a simple but high quality switch-mode AC adapter, but the one I used to sell has been discontinued. I have found that some switch mode AC adapters will produce a "motorboating" effect, but they are cheap so if you get this, try another one.

AC adapters are cheap, easy, and usually quiet - but there are certainly better sounding options. One of the best fits for this circuit is to use ~24V worth of batteries in series. Batteries tend to sound really great, are the quietest option by far.

The H-PS-1 power supply from Glass Ware (TubeCad Journal) would be really amazingly good I bet (though I haven't built one). The 18V version would be ideal.

component matching

I match the JFETs to within 1mA Idss. Matched pairs should be used in the same position for right and left channels. Matching only ensures that the gain of the circuit be equal in each channel. The capacitors in the RIAA eq section are matched to 10%. Since these are setting the breakpoint of a low 6dB/octave slope, a 10% change is minor. The resistors in the RIAA circuit are standard values, with 1% tolerance.

capacitor upgrade options

The Surplus Russian paper in oil caps were selected because I personally like the sound of these caps a lot, I find they compliment the JFET sound, and because the price/performance ratio is unapproachable by modern new parts. That said, capacitor sound if often a matter of personal taste, and system synergy, so feel free to experiment. The values that matter are the RIAA EQ section (C1, C2, C3) so fee free to replace the coupling caps with larger or smaller values. The output cap is relatively small at 1uF since the relatively high output impedance of the circuit requires an easy load.

options for getting more gain

I have had several kit builders ask about the options for getting more gain out of the BHL Phono. There are LOTS of options. Gain is determined (approximately) by the ratio of the Drain Resistor (R4 and R11) to the Source Resistor (R3 and R12). R4 if off limits though, because it sets the output impedance of that stage, and changing it would effect the RIAA equalization (which can always be recalculated). For a modest increase in gain, you can reduce the size of R3/R12 or increase the size of R11. Start by roughly doubling/halving them. If you want more gain, you can remove the Source resistors completely, or bypass them with a large (typically electrolytic) capacitor. If you want to get a bit more exotic, you can replace the drain resistor with current sources, or perhaps a current limiting diode (which is simply a preset current source).

Another option to consider is the addition of a stepup transformer in front of the stage, which can improve the impedance matching with many low output cartridges as well. Conveniently, low output cartridges tend to prefer lower impedance loads, so if you need more gain definitely look into stepup transformers since you might be able to get more gain with the added benefit of better impedance matching.

grounding

The circuit board is designed so that the entire bottom copper layer is a ground plane. This should make for the quietest possible circuit with no possibility of ground loops on the board itself. To install this into a chassis, you should use insulated RCA jacks and make sure the board connects to the chassis at only one point, using a wire soldered from the ground pad to a ground lug, which is grounded to the chassis, and is where you connect the ground wire from your turntable. Exotic cartridge/arm grounding schemes may benefit from other ways to do it, but if you have something like that you probably already have equally exotic grounding plans for the phono stage :)

operating points - what to expect

The JFETs used can potentially vary in Idss between 6-12mA. In the circuit, expect somewhere around 10-15mA of current. The batches of JFETs I have received so far have been in the 8-9mA Idss range, but it's possible that future batches will be towards one of the extremes, so don't worry about this too much as long as it sounds right.

more specs

Output impedance = R11 = 3.16k ohms

[jsn at boozhoundlabs dot com]