bradb
Well-known member
slick
MK7 - tube mic project
- Thread starter ioaudio
- Start date
Help Support GroupDIY Audio Forum:
bugfight
Well-known member
oooooh nice.[me=bugfight]likey![/me]
Arno said:
Please do explain the body
I am a vandal! It's an old Thiele M4 body, with the original circuit stripped out.
I've supported the PCB by soldering metal ring tags to the ground bus, and then the vertical posts go through this. It's very solid so far. I still need to make a perspex top plate to mount the capsule.
ioaudio
Well-known member
excellent mic body 8)
hazmatstrat
Well-known member
Hi guys,
I just posted a Thiele M4 on the Black Market.
http://www.groupdiy.com/index.php?topic=30981.0
Thanks Happy Holidays.
Jim
I just posted a Thiele M4 on the Black Market.
http://www.groupdiy.com/index.php?topic=30981.0
Thanks Happy Holidays.
Jim
ioaudio
Well-known member
e.oelberg said:
the biasing method needs a super clean heater supply, as already mentioned. i´m not aware of any project here on the forum which needs a supply that clean.
bradb
Well-known member
Hence the big inductor on the heater supply in max's "from scratch" design...
ioaudio
Well-known member
depending on your psu (you didnt specify) you should be able to mod it, without starting from scratch.
Got mine up and running yesterday - what a great project! Many thanks to Max for offering this unique set, a truly great christmas present.
I thought the stock T.bone mic sounded ok, nice detail but very bright, kind of a scooped sound. A screamin' deal for the price with the mic, shockmount, psu and a long cable in an adequate suitcase ( many thanks to all the children who made this for pennies - gotta stop buying this exploitation crap ).
New circuit sounds much more musical, especially noticeable in the smoother high end. I'm using the stock capsule which is only cardioid so I can't check different patterns yet. Mic assembly is a piece of cake with this design, but modding the psu is tight. With a bit of mapping I got all the electrolytics on the tiny board with the diodes and resistors tucked underneath. Pinout for the LM317 is different so I reversed it and had to cut some traces and reroute. Position switch needs to be small to fit this case, a ST3P mini toggle might be the best way to fit it. I moved the transformer and shield back as much as I could to gain about an inch of breathing room.
Voltages come out low and there is a bit of hum in the noise floor (my pal imo finished his the same day and has the same problems). My voltages come in at 141v B+ and 6.8v H+ from the psu for EF86. Cathode bias is good at 1.26v. Going to change a few values to get closer to Max's schematic...
I thought the stock T.bone mic sounded ok, nice detail but very bright, kind of a scooped sound. A screamin' deal for the price with the mic, shockmount, psu and a long cable in an adequate suitcase ( many thanks to all the children who made this for pennies - gotta stop buying this exploitation crap ).
New circuit sounds much more musical, especially noticeable in the smoother high end. I'm using the stock capsule which is only cardioid so I can't check different patterns yet. Mic assembly is a piece of cake with this design, but modding the psu is tight. With a bit of mapping I got all the electrolytics on the tiny board with the diodes and resistors tucked underneath. Pinout for the LM317 is different so I reversed it and had to cut some traces and reroute. Position switch needs to be small to fit this case, a ST3P mini toggle might be the best way to fit it. I moved the transformer and shield back as much as I could to gain about an inch of breathing room.
Voltages come out low and there is a bit of hum in the noise floor (my pal imo finished his the same day and has the same problems). My voltages come in at 141v B+ and 6.8v H+ from the psu for EF86. Cathode bias is good at 1.26v. Going to change a few values to get closer to Max's schematic...
ioaudio
Well-known member
good to hear some progress!
sounds like low mains for the psu. what is your heater voltage directly after the rectifier loaded?
the psu is quite tight, but a lorlin fits. i rebuild everything on stripboard, problem is when you put the electrolyts to close they will probably couple the hum capacitively.
sounds like low mains for the psu. what is your heater voltage directly after the rectifier loaded?
the psu is quite tight, but a lorlin fits. i rebuild everything on stripboard, problem is when you put the electrolyts to close they will probably couple the hum capacitively.
Nicely done Max! I had thoughts of perfboard yesterday, but rat shack isn't open on christmas.
I also suspected the hum coming from the close quarters of those electrolytics. I might eventually build your other psu, but this works well for now.
Wow, I really love the sound of this circuit/transformer. If it sounds this good with the stock capsule I really look forward to a good double sided one.
Cheers,
Paul
I also suspected the hum coming from the close quarters of those electrolytics. I might eventually build your other psu, but this works well for now.
Wow, I really love the sound of this circuit/transformer. If it sounds this good with the stock capsule I really look forward to a good double sided one.
Cheers,
Paul
ioaudio
Well-known member
pH said:
which should is OK.
imo
Well-known member
First of all, thanks to Max for the project. The circuit is a lovely one, and the mic is a great project. I am using a old M7 capsule and it sounds really full and alive with the .47 outcap
Both myself and Paul(PH) ordered the Tbone mic as a donor and used that for the housing for the PCB.
The Power supply mods required a decent amount of surgery to add the necessary changes, but with a bit of dancing it all fits in the box with minor augmentation.
There are two issues with the mic that i was curious to hear input on.
1. The B+ sits at @143 unloaded. I know its within "spec" for a tube circuit, but aren't these two 75v zeners in series? They are getting plenty of current to feed them so they aren't starved. Is anyone else using this circuit, and if so are you getting 150v?
2. We are both getting a pretty substantial hum in the mic. I tried moving the 10000U caps away from the rest of the circuit but nothing changed. Aside from my B+ being slightly low, the heater voltage is a bit low into the mic. I subbed a 1r for the 2.2r and was able to get 5.7 to the heater with 1.3 at the cathode, so it seems in line where it needs to be.
Could the zeners be introducing noise into the circuit? We are entertaining redoing the power supply with the more traditional choke based supply, but it would be nice to figure out what is going on as is.
Input is great. Thanks!
Ian
Both myself and Paul(PH) ordered the Tbone mic as a donor and used that for the housing for the PCB.
The Power supply mods required a decent amount of surgery to add the necessary changes, but with a bit of dancing it all fits in the box with minor augmentation.
There are two issues with the mic that i was curious to hear input on.
1. The B+ sits at @143 unloaded. I know its within "spec" for a tube circuit, but aren't these two 75v zeners in series? They are getting plenty of current to feed them so they aren't starved. Is anyone else using this circuit, and if so are you getting 150v?
2. We are both getting a pretty substantial hum in the mic. I tried moving the 10000U caps away from the rest of the circuit but nothing changed. Aside from my B+ being slightly low, the heater voltage is a bit low into the mic. I subbed a 1r for the 2.2r and was able to get 5.7 to the heater with 1.3 at the cathode, so it seems in line where it needs to be.
Could the zeners be introducing noise into the circuit? We are entertaining redoing the power supply with the more traditional choke based supply, but it would be nice to figure out what is going on as is.
Input is great. Thanks!
Ian
ioaudio
Well-known member
ian,
thanks for the kind words!
>I subbed a 1r for the 2.2r
you have a lower ripple rejection with the smaller resistor.
example (@120Hz):
2.2R + 10000µF = -24.4114 dB
1R + 10000µF = -17.623 dB
we would need more than double capacity for the 1R resistor
1R + 22000µF = -24.4114 dB
calculator:
http://www.pentodepress.com/calculator/RC-ripple-filter.html
>Could the zeners be introducing noise into the circuit?
yes, but that would be hiss, not hum.
>The B+ sits at @143 unloaded.
could you post all voltages loaded (rectifier, first resistor, second...)
the low voltages seem to be the problem in 115vac land.
maybe it would be easier to source a power transformer with higher out voltages.
also the lm317 would profit from a bigger voltage drop for better ripple rejection.
-max
thanks for the kind words!
>I subbed a 1r for the 2.2r
you have a lower ripple rejection with the smaller resistor.
example (@120Hz):
2.2R + 10000µF = -24.4114 dB
1R + 10000µF = -17.623 dB
we would need more than double capacity for the 1R resistor
1R + 22000µF = -24.4114 dB
calculator:
http://www.pentodepress.com/calculator/RC-ripple-filter.html
>Could the zeners be introducing noise into the circuit?
yes, but that would be hiss, not hum.
>The B+ sits at @143 unloaded.
could you post all voltages loaded (rectifier, first resistor, second...)
the low voltages seem to be the problem in 115vac land.
maybe it would be easier to source a power transformer with higher out voltages.
also the lm317 would profit from a bigger voltage drop for better ripple rejection.
-max
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