ioaudio
Well-known member
as some of you might know, i was lucky enough to find a bunch of transformer lams originating from the seventies, the stuff used for the transformers of the classic neumann tube microphones.
i wound a couple of BV8 copies for U47 (clones), for people who run the transformer with the rare VF14 (EF14).
meanwhile i was working on a new headamp, using a tube which is more commonly available, the ef86 (pf86), rewinding the transformers primary (and secondary) while varying the stack-height, setting the bias, the value of the couling cap etc to "emulate" the sonic qualities of the original u47 headamp.
i designed a pcb layout to fit many mic housings (i used the cheapest one from thomann link , a nice 3-layer mesh grille+psu & cable & shock mount)
edit: re: housing: disclaimer: this is not a beautiful mic.
the top pcb connects to the main pcb and will be soldered together.
pcb height is 74mm (+ height of the tube socket&parts), diameter >=45mm
the entire high Z part is build point-to-point upon the top pcb, polarisation voltages are fed through glass tubes. the tube socket is a hardwired type to ensure high insulation for the grid connection.
the "stiffened" biasing is done with the heater supply.
0,22F goldcaps are used for additional filtering along with 2,2R* resistor.
the 1µF and 0,68µF filmcaps are mounted on the back at the pcb side under the tube.
MK7 microphone schematics with measured voltages for both the ef86 and pf86:
(click pic to enlarge)
thanks to axel/mad.ax for finding - pin number (g2/g3--pin8/pin1) error fixed.
preliminary checklist:
[some rs-part numbers]
1 x cheapo mic or mic housing + psu (used mic in the pics = http://www.thomann.de/at/the_tbone_sct700_roehrenmikrofon.htm)
1 x M7 or K47 capsule + holder
1 x ioaudio MK7 pcb
1 x ioaudio BV7 transformer
1 x ef86 or pf86
1 x noval tube socket chassis mount 18mm mounting hole
2 x M3 (4-40) screws/nuts to mount tube socket
1 x 1µF 250V filmcap tubular [117-987]
1 x 0,33µF 250V filmcap tubular - (0,47µF or 0,68µF, coupling cap)
1 x 47nF 160V film
1 x 10nF 160V styroflex
1 x 1nF 160V styroflex
2 x goldcap 0,22F 5,5V - upright standing version [377-372]
2 x 100 Megaohm 0,25W [247-7834]
1 x 68 Megaohm 0,25W [247-7806]
1 x 33 Megaohm 0,25W
1 x 1 Megaohm 0,25W *updated, marked as 1,5Mohm on the pcb
1 x 910 Kiloohm 0,25W *updated, marked as 1,2Mohm on the pcb
1 x 100 Kiloohm - plate resistor
1 x 20 Kilohm 1W
1 x 2,2 Kiloohm 0,25W
1 x 6,8 or 4,7 OHM 1W (marked 3,9R*)
1 x 2,2 Ohm 1W
MK7 PSU
a psu from scratch, all passive.
adjust voltages on test.
suggested choke for usa = triad C-40X @ alliedelec.com
Pattern selector :
1 x rotary switch 3x1 (lorlin or similar)
1 x 10µF 250V electrolyte
3 x 100 Kiloohm 0,5W
1 x 51 Kiloohm 0,5W
PSU mod:
*note that some users (mostly form the US) reported that this mod does not work completely hum-free when used in fixed bias.
the result seems to depend on mains voltage and the varying quality of the transformer in the original psu.
schematics for modifying the stock generic psu, again with measured voltages for both the ef86 and pf86, with the psu from sct700
preliminary checklist:
Heater upgrade:
2 x 10.000µF/10V
2 x 4.700µF/16V
1 x 22µF/16V
1 x 2,2 Ohm 1W
1 x 3,3 Ohm 1W
1 x 240 Ohm 0,25W
1 x 910 Ohm 0,25W
1 x trim pot 500 Ohm
2 x 1N4002
1 x LM317
--------------------------------------
thanks to greg and brad for the bom&word document:
http://www.twin-x.com/groupdiy/albums/userpics/MK7_max_s_mic.xls
http://www.twin-x.com/groupdiy/albums/userpics/Max_s_Mic_thread.rtf
IMPORTANT:
do not un-mount the transformer, do not tighten/loosen it´s screws/nuts.
be careful when soldering the wires to A+, it´s a bit tight there.
get rid of the stock innards.
solder a 10cm wire for the pattern on pin 3 of the xlr, use insulation
mount the rectangular pcb
insert the round pcb into the slot
check for 90° angle and for the correct position
solder one rectangular pin on both pcbs
check position & angle again, re-heat the soldered pin and adjust if nescessary
solder all pins
insert the glass tubes and fix them with a drop of glue
mount the chassis-tube socket, insert the solder lugs on top.
check for a good electrical connection from the pcb to the socket, use a tooth lock washer.
solder the parts on the rectangular pcb, check polarisation of the gold-/super-caps.
the 100M resistors are fed through the glasstubes as shown.
on the round pcb, use the solder lugs as ground connections for the 68Mohm, 3,9R* and 10nF instead of the associated holes in the pcb.
in this mic housing this additional clearance is needed to fit the slots.
alternative self bias wiring
i wound a couple of BV8 copies for U47 (clones), for people who run the transformer with the rare VF14 (EF14).
meanwhile i was working on a new headamp, using a tube which is more commonly available, the ef86 (pf86), rewinding the transformers primary (and secondary) while varying the stack-height, setting the bias, the value of the couling cap etc to "emulate" the sonic qualities of the original u47 headamp.
i designed a pcb layout to fit many mic housings (i used the cheapest one from thomann link , a nice 3-layer mesh grille+psu & cable & shock mount)
edit: re: housing: disclaimer: this is not a beautiful mic.
the top pcb connects to the main pcb and will be soldered together.
pcb height is 74mm (+ height of the tube socket&parts), diameter >=45mm
the entire high Z part is build point-to-point upon the top pcb, polarisation voltages are fed through glass tubes. the tube socket is a hardwired type to ensure high insulation for the grid connection.
the "stiffened" biasing is done with the heater supply.
0,22F goldcaps are used for additional filtering along with 2,2R* resistor.
the 1µF and 0,68µF filmcaps are mounted on the back at the pcb side under the tube.
MK7 microphone schematics with measured voltages for both the ef86 and pf86:
(click pic to enlarge)
thanks to axel/mad.ax for finding - pin number (g2/g3--pin8/pin1) error fixed.
preliminary checklist:
[some rs-part numbers]
1 x cheapo mic or mic housing + psu (used mic in the pics = http://www.thomann.de/at/the_tbone_sct700_roehrenmikrofon.htm)
1 x M7 or K47 capsule + holder
1 x ioaudio MK7 pcb
1 x ioaudio BV7 transformer
1 x ef86 or pf86
1 x noval tube socket chassis mount 18mm mounting hole
2 x M3 (4-40) screws/nuts to mount tube socket
1 x 1µF 250V filmcap tubular [117-987]
1 x 0,33µF 250V filmcap tubular - (0,47µF or 0,68µF, coupling cap)
1 x 47nF 160V film
1 x 10nF 160V styroflex
1 x 1nF 160V styroflex
2 x goldcap 0,22F 5,5V - upright standing version [377-372]
2 x 100 Megaohm 0,25W [247-7834]
1 x 68 Megaohm 0,25W [247-7806]
1 x 33 Megaohm 0,25W
1 x 1 Megaohm 0,25W *updated, marked as 1,5Mohm on the pcb
1 x 910 Kiloohm 0,25W *updated, marked as 1,2Mohm on the pcb
1 x 100 Kiloohm - plate resistor
1 x 20 Kilohm 1W
1 x 2,2 Kiloohm 0,25W
1 x 6,8 or 4,7 OHM 1W (marked 3,9R*)
1 x 2,2 Ohm 1W
MK7 PSU
a psu from scratch, all passive.
adjust voltages on test.
suggested choke for usa = triad C-40X @ alliedelec.com
Pattern selector :
1 x rotary switch 3x1 (lorlin or similar)
1 x 10µF 250V electrolyte
3 x 100 Kiloohm 0,5W
1 x 51 Kiloohm 0,5W
PSU mod:
*note that some users (mostly form the US) reported that this mod does not work completely hum-free when used in fixed bias.
the result seems to depend on mains voltage and the varying quality of the transformer in the original psu.
schematics for modifying the stock generic psu, again with measured voltages for both the ef86 and pf86, with the psu from sct700
preliminary checklist:
Heater upgrade:
2 x 10.000µF/10V
2 x 4.700µF/16V
1 x 22µF/16V
1 x 2,2 Ohm 1W
1 x 3,3 Ohm 1W
1 x 240 Ohm 0,25W
1 x 910 Ohm 0,25W
1 x trim pot 500 Ohm
2 x 1N4002
1 x LM317
--------------------------------------
thanks to greg and brad for the bom&word document:
http://www.twin-x.com/groupdiy/albums/userpics/MK7_max_s_mic.xls
http://www.twin-x.com/groupdiy/albums/userpics/Max_s_Mic_thread.rtf
IMPORTANT:
do not un-mount the transformer, do not tighten/loosen it´s screws/nuts.
be careful when soldering the wires to A+, it´s a bit tight there.
get rid of the stock innards.
solder a 10cm wire for the pattern on pin 3 of the xlr, use insulation
mount the rectangular pcb
insert the round pcb into the slot
check for 90° angle and for the correct position
solder one rectangular pin on both pcbs
check position & angle again, re-heat the soldered pin and adjust if nescessary
solder all pins
insert the glass tubes and fix them with a drop of glue
mount the chassis-tube socket, insert the solder lugs on top.
check for a good electrical connection from the pcb to the socket, use a tooth lock washer.
solder the parts on the rectangular pcb, check polarisation of the gold-/super-caps.
the 100M resistors are fed through the glasstubes as shown.
on the round pcb, use the solder lugs as ground connections for the 68Mohm, 3,9R* and 10nF instead of the associated holes in the pcb.
in this mic housing this additional clearance is needed to fit the slots.
alternative self bias wiring
