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parafeed headphone amplifier


Zener Diode Tweak -
NOTE: V1.02 of the Torpedo PCB incorporates the zener diode tweak directly in the PCB layout. Tombstoning the diodes as shown in this detail is no long necessary!

The number one tweak that should be done with every Torpedo is the Zener Diode tweak. This consists of replacing the LED's at D12 and D14 with a 6.8V Zener diode (1N5235). The diode band should be on the "+" side (the pad closest to the outside edge of the PCB) of the D12 and D14 positions. Since these positions were originally intended for LEDs, the pad spacing is not wide enough to allow the Zener to fit flush to the PCB. Instead, mount the Zeners in the vertical, "tombstone" position, with the "free" lead rolled over to the other pad (the "-" pad). A detail is shown below:

Note that the value of R4 and R5 are adjusted to ˜400R each. This mod increases the Power Rejection Ratio (PRR) of the Constant Current Supplies (CCS) for the tubes. It reduces hum and noticeably increases the dynamics of the Torpedo. The mod is so important that it is now the standard and is included in the base BOM and Torpedo kits and builds. Using the choke instead of R2 has similar advantages and is also now recommended as standard in the BOM, kits, and builds.

Other Tweaks -
There are a few tweaks that you can experiment with on the TORPEDO!. The primary tweak is in the tubes themselves. The variance in tubes, even among the same type, is well documented. Add to that the very long list of suitable equivalents for the 6J6 (see the TORPEDO Tubes page) and you can be tube rolling for a very long time!

Parafeed Capacitors -
These are the capacitors that are used to block DC from the output transformers. 4.7uf at 300V is the minimum recommendation. You can go down to 3.9uf, but don't go lower on the voltage. The tubes are basically running with close to 200V at B+, but this can vary quite a bit and a 200V cap will likely blow up sooner than later.

That still leaves quite a selection of capacitors to choose from - all the way from film to even electrolytics are suitable. We've tried to pick what we think are the best quality and most economical on the TORPEDO! BOM. However, you are encouraged to try others, if you want.
A small example of the wide variety of parafeed caps picked by your fellow DIY-ers:

Here is a set of files that represent a dimensioned drawing of the parafeed capacitor area on the TORPEDO! PCB:
TORPEDO! Parafeed Capacitor Dimensions
(right-click to download the following):
* Highest resolution PDF (best):
* Highest resolution GIF (not to scale):
* AutoCAD DWG (R2010 compatible):

file last changed:Saturday, July 19, 2014 1:44:13 PM
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