Notes on the Radford ZD-22 preamplifier.

Esmond Pitt

These units have measured THD < 0.01% hence 'ZD' which stands for 'zero distortion'. Produced in the mid-1970s and sold for UKP220, or about double the price of a Quad 33 for example.

This preamplifier uses unusually high V+ voltages of 50V (phono board) and 60V (line board), so as to use high-gain transistors and get low distortion out of them due to flattening the gain curve. It also uses constant-current sources in both the phono and line boards to provide high collector impedances without reducing the current available as a large collector resistor load would do, again to increase the gain and reduce distortion. As a result of all this, it is able to deliver a rather useless 10V of output but that's only a side-effect. It also has separate gain and volume controls, which is one solution to the age-old issue of whether to put the volume control early in the chain to maximize headroom or late in the chain to minimize noise.

These things are exceptionally well built and marked for disassembly/reassembly, the only problem I encountered being lifting tracks on the PCB.

PSU

Replace all electrolytics and zeners, and adjust to 70VDC.

PHONO BOARD

Replace all electrolytics. Change R1 to 39k, R2 to 56k, and C1 to 180-220pF, to give a modern input impedance of 47k || 220pF.

LINE BOARD

The only schematic I've been able to find covered board revision 3357/2/A but the unit I had for repair was 3357/3/A which is only slightly different - see below.

There are several errors in this schematic:

The 70V doesn't enter on the RHS by the output stage, although it should; it enters on the LHS by the input stage as you can see on the layout diagram.

C41 isn't on the RHS either, it is between the input stage and the tone control stage as per the layout diagram: in this case this is correct.

The only thing between the 60V that drives the board and the 70V supply is R57 in 3357/2/A or R115 in 3357/3/A - check it!

TS8 in the parts list is of course a 2N2905 like TS13. The ZTX109B is unobtainable nowadays but a suitable equivalent is the BC546B and it's nice and cheap too. I would replace all these. C41 is underrated at 220uF/63V for a component that has 60V across it, or more if R57/R113 go low - replace with 220uF (or more) /100V. Replace all the other electrolytics while you're at it, especially C32 and C35 which can cause very odd things to happen if they go leaky - on mine it caused a VLF motorboating behaviour at about 0.1Hz as C35 charged up and then leaked alternately, causing the voltages all through the board to vary by about 5 volts over this cycle.

In 3357/3/A R45 is replaced by a 6.2V diode to provide a more constant bias to the constant-current source TS9, and there is a power regulator arrangement at the bottom left of the board which curiously isn't fitted to all units: instead, R115 and a shorting link across C/E of TS31 are present. If you have one of these, fit the regulator circuit, it will improve things - there is some signal visible on the 60V rail otherwise. I don't know what the component values are for the missing regulator circuit, but a better arrangement is to use an LM317 in place of the regulator transistor TS31 (remove the C/E shorting link!); replace R115 with a link; add R113 of 1K; add 220R 1W between the LHS of R113 and the LHS of R32; and add 100nF electrolytic across the 220R. This gives about 57.5V of regulated voltage. Do not fit C43 or R114 if you make this modification.

The tone control stage is a conventional Baxandall layout.