Apr 30, 2011 at 8:48 PM
- Feb 25, 2007
- Reaction score
- Feb 25, 2007
-- 5/13/2011: revised the schematic (added transformer wire colors) and IEM sensitivity requirement (117dB/mW, impedance preferably >18 ohm).
** list of suitable IEM's for this amp:
*1964-D (22 ohm, 120db/mW)
*1964-Q (46 ohm, 118db/mW)
*Earsonic SM2 (16 ohm, 119db/mW, the impedance is a little low but should still be ok)
*Earsonic SM3 (34 ohm, 122db/mW)
*JH5 (21 ohm, 119db/mW)
*JH13 (28 ohm, 119db/mW)
*JH16 (28 ohm, 118db.mW)
*Ortofon, E-Q5 (40 ohm, 117db/mW)
*Ortofon E-Q7 (31 ohm, 118db/mW)
*Shure SE535 (36 ohm, 119db/mW)
*UE 11 pro (18 ohm, 119db/mW)
*UE 7 pro (18 ohm, 124db/mW)
*UE 5 pro (21 ohm, 119db/mw)
*UE triple-fi 10 (32 ohm, 117db/mW)
*Weston AC2 (27 ohm, 119db/mW)
*Westone ES2 (27 ohm, 119db/mW)
*Westone ES3 (25 ohm, 124db/mW)
*Westone ES3x (56 ohm, 124db/mW)
*Westone ES5 (20 ohm, 120db/mW)
*Westone UM2 (27 ohm, 119db/mW)
*Westone UM3x (56 ohm, 124db/mW)
*Westone W1 (27 ohm, 122db/mW)
*Westone W2 (33 ohm, 117db/mW)
*Westone W4 (31 ohm, 117db/mW)
Objectives of the design:
-- a good sounding yet super simple amp that works well with (but only with) high sensitivity IEM's such as UM3X, SM3. The phones sensitivity needs to be >=117dB/mW.
-- also works as an active LOD cap-box/pre-amp for an iMod/DiyMod. The circuit must have input high impedance so low value DC blocking input caps can be used, making it actually affordable to use super exotic boutique caps. (instead of the more common 2.2uf~4.7uf value in an iMod dock, this only need 0.22~0.47uf, so even the V-caps are within our reach).
-- Low voltage, safe for casual DIY and casual desktop use. Build cost to be within a couple hundred $$, a few hundred $ on the maaaaaaaxxxxxxx-ed version.
-- Most importantly, the design shall utilize the mystical Stax transformers from a Stax SRD7 transformer box to (hopefully) gain that "Staxious" sound. So if this design become popular, large # of SRD7 transformer boxes on the used market will be hunted down and gutted to build these little evil creatures. Hopefully this will trigger the global extinction of SRD7, how about that! Wahahahahahahahahahaha........ Ok, I might need some more pratice on my evil laugh.
The amp circuit is simple, actually, super simple. A traditional circuit, nothing new.
*6GM8 as the (only) tube, fix biased to about 0.8V (by a pair of power shockey diodes in series). idle current at about 1.5mA, the Stax transformers are p/p type but signal swing and dc current is so low they can handle that with no problem.
-- filament supply uses a 7806 reg and FM caps;
-- B+ uses a ~5mA CCS to load a string of LED's forming a 20V shunt regulator; reservoir tank only needs to be 20uf or so.
picture showing the filament and CCS module ( I somehow decided to use three jFED in paralle, instead of just one, they should work the same).