These notes are a formatted & spell-checked version of Igors notes - nothing more!
There are a couple of options to choose from when building the He69.
- Insert switch, or just on/bypass switch?
If you go for the former option, you'll need an SPDT on-off-on toggle switch (reference:
bypass-xlr-pre) and to populate the TO-PRE component on the board with a 2.54mm header.
If no, then you can use an SPDT on-on switch and leave off the header.
discrete opamp (e.g. 2520) with a Jensen 990 footprint. You also need to populate
R1A, R2A, R3A, R4A, R5A, C1A, C3A. You do not need IC1, CBP1 or CBP2
2. For the IC input, you need a THAT1246 chip and 2 capacitors (CBP1, CBP2). The
chip and the capacitors go in the area that would have been reserved for an input
transformer. You do not need the LL7101 or the input DOA (INP_DOA). Nor do you
need the associated resistors and capacitors.
Output transformer. The LL1524 is the output transformer specified, but it can be hard to come by.
The LL1517 seems to be more readily available in the UK from canford audio and Igor has stated
this as an acceptable replacement. Be careful – you may need to mount the LL1517 in reverse (if
anyone can verify this please get in touch), so make sure you have checked the datasheets
The output transformer can be wired as 1:2 or 1:1. For 1:1 wiring, R12 and R15 should have a ratio
of 25/5.1. The exact value depends on the impedance the output amplifier needs to see (see the
schematic). Igor gives example values of 24K9 for R12 and 5K1 for R15 and states for the same
amp as used in the F76 (Igors own design) he used 2K49 ad 511R.
For 1:1 wiring, R12 and R15 need a ratio of 50:5.1.
R14 should set the output impedance and needs to be determined from the transformer datasheet.
Igor states as 2-3K being OK.
R13 and C10 are the zobel network to reduce trasformer ringing. Values for these are often provided
on the transformer datasheet (if they are required). 150-300R ad 4.7-10nF should be fine for either
of the Lundahl transformers.
The value of C7 depends on the value of the feedback resistor (R12) and amp used. If R12=~47K,
22pF should be fine. If R12 = ~2K5, 330pF should be fine.
Note that although there is a place holder for R10 on the PCB, it is not installed
All resistors should be 0.25W, metal film types with 1% tolerance.
Electrolytic capacitors. C5 and C8 should be a mximum of 8mm diameter. The rest of the
electrolytic capacitors should be a maximum of 10mm diameter. All should be rated for 25-35V.
Typical brands to look out for would be Nichicon, Philips, BC, Panasonic.
Other capacitors are polyester film types, 5% tolerance, 5-7.5mm lead spacing, 5mm width, 50-
C6 should be 7-10mm lead spacing.