John Linsley Hood "Shunt Feedback" RIAA Pickup Preamp K1500
In the late early 1990s I started using a passive preamp for line level sources in combination with Rotel power amps. Obviously this meant I had a requirement for an RIAA preamp and I settled upon a DIY kit of a JLH design bought from Hart Electronic Kits Ltd, then of Oswestry in the UK. This company was established in the 1950s and provided excellent service for many electronics and hifi hobbyists but unfortunately they stopped trading a few years ago and so these kits and parts are no longer available.
The front panel contains nothing but text and an on-off switch.
On the rear panel we can choose between moving magnet or moving coil pickup. The DIN power socket accepts +/- 9-12 volts as my amplifier is assembled.
Inside the preamp. When I assembled this in 1991 it seemed a good idea to add considerably more capacitance and to use a simple RC low pass filter to make the voltage resistant to external noise. I did not install the (optional) 78L12 / 79L12 regulator chips.
It's a fairly simple op-amp circuit. Tidy good quality single-sided fibre-glass circuit board with silk-screening to show where each part should be installed. The empty positions, 21 and 22, are for the 78L12 and 79L12 regulators which I did not fit, preferring at the time to add extra capacitance instead. The links at 1 and 2 are fitted when the regulators are not used.
Sitting on the shelf underneath the record player. Next to the phono preamp you'll see the power supply - a repurposed Grundy Newbrain power supply which I happened to have hanging around.
The kit came with ample information. It's been 25 years since I assembled this kit (literally half a lifetime for me at the moment) but I remember it being very easy to do. Hart had produced electronics kits for more than 30 years by 1991. The instructions were very well written and extremely easy to follow.
Full list of parts provided.
The circuit diagram, as originally published in the November 1990 edition of "Gramophone". Note that my kit came with an SSM2139 for the IC1 position.
What I use this phono preamp for now
See my Quad 33 / 303 pages for details of this modification
For a few years, this phono preamp languished out of use but I've recently modified my Quad 33 preamp to turn its "disc" input into a line level input and am using the JLH preamp again. It's a revelation. Much more detail than I heard through the Quad preamp. Why ? No magic is required: My record player is quite distant from the amplifier and long cables are not a good idea for phono cartridges because they are susceptible to capacitance effects.
With the JLH phono preamp any electronic noise is well below the level of noise from the LP surface.
Also see my page about the Quad 33 / 303. Itself a great amplifier, but now modified so that I bypass its internal phono pre-amp circuitry and use the superior JLH preamp instead.
Newbrain power supply modification
The Grundy Newbrain power supply which I use to power the phono preamp was originally built to provide a range of voltages to that computer. Amongst them were a +12 V and approximately -11 V supply which I used for some time with the phono preamp. However I eventually modified the power supply by removing the original PCB and installing a piece of veroboard on which I had constructed this power supply:
The transformer is as standard, but I use only two of the pins on its output. The TIP31 transistor is one of the two originally used in the Newbrain's original circuit. I also re-used the bridge rectifier. Other parts came from my stock.
Note that I've disconnected the mains earth from the circuit as this caused hum. It still connects to the chassis of the power supply, including the heatsink on which the TIP31 is mounted (with a non conductive gasket), but all three of the output voltages from this circuit are floating relative to the mains.
There's nothing magical here. It's merely a zener regulated supply with a transistor as an emitter follower. The voltage of the output is that of the zener minus one "diode" drop - actually the base-emitter drop of the TIP31. The 28 V regulated voltage is split by resistors to provide a centre ground connection. The op-amps load their positive and negative rails almost identically and the simple resistive voltage splitter works well enough.
The load on this circuit from the JLH preamp is tiny while the capacitance is large (especially as its in parallel with the increased capacitance as part of the PSU RC filter which I built into the preamp itself). The output voltage is barely affected at all by the load and there's no visible ripple. The 10 K resistor followed by 10 uF capacitor across the zener reduce inrush voltage. It takes about half a second for the output voltage of this circuit to stabilize at its full potential.
JLH's K1450 discrete transistor RIAA preamp design was contemporaneous with this model, the K1500, and was housed in the same case. Details of the K1450 circuit can be found at this link.
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