Preamp Circuits

Last updated 23/5/2001.

Here are various schematics (thanks to Jon Nicoll for drawing them) which will make it easier to find your way around the circuits that you might be tweaking.

Moving Coil Phono Circuit

Here is the classic Naim phono circuit taken from the Mark 3 boards.

Moving Coil NA323 Phono Board...
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A few component changes were made over the years, with slight layout changes. The modern Mark 5 boards have a few changes; R2=4K3, R3=100K, R17=200K, R14=82K. The loading is done by R1 and C9. The one shown is the S type i.e. 470R, 6n8. The K type is 560R, 1n0.

Note that the gain can easily be adjusted (I've tried changing it by a factor 2 with no problem) by changing out R13 (220R). For example putting in a 110R in its place will double the gain, which may be useful if you are trying to match the subjective output from LP and line inputs (particularly in combination with lowering of the line stage gain... see below). Personally I have found that doubling the gain can give the phono stage more 'grip' and slam with low output moving coils (eg, the original Linn Arkiv).

Preamp Circuits

Here are the unity gain buffer stage and line output stage (and relay circuit), for the 42 and 62 preamps.

42/62 Line circuits...
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Note that on the unity gain buffer stage (left circuit) the output 10uF cap with associated 100k resistor (as on the input) got left off by mistake. Will glue it back on at some point....

The line stage in the 72 preamp had a couple of minor changes. R16 (470R) and C6 (470pF) changed to 10k and 100pF respectively. Also, in the 72, there is a 5.6 pF cap across the collector-base of the Q8 transistor.

The gain is set by R22 (12k) and R13 (1k), and you can adjust these somewhat (for example if the line input volume is miles louder than the phono volume, you could change R22 to 6k to more or less half the gain). As a very effective tweak, these two resistors should be of the highest quality i.e. Vishay bulk foils (and yes, in this application they are definitely worth the money).