Shroom mic pre

rankot · December 03, 2019, 12:40:58 PM

This is not a stomp box topic, but it's close enough to put it here. Few years ago I've found this interesting schematic on the web:

It is transformerless balanced mic preamp, using two pentodes with common grid circuit, followed by two common cathode triodes. Original is designed around 6AK5 and 12AT7, I wanted to try it with EF86 (or Russian 6J32P) and 12A*7 (or Russian 6N2P). This is the PCB I've designed, comments are welcome! Still untested, but soon will be.

rankot · December 03, 2019, 12:43:07 PM

I've also successfully simulated this using LND150 MOSFETs instead of EF86 tubes.

FUZZZZzzzz · December 04, 2019, 06:32:26 AM

very cool! Ive always wanted to build a couple of mic preamps with tubes. will you also include phantom power?

rankot · December 04, 2019, 06:58:27 AM

I have two inputs here - one with phantom power, one without it. It could be done with a switch, but this input w/o PP also skips input capacitors.

I'm still considering what kind of power to use here - traditional or SMPS.

I also tried this as an all-JFET preamp, and it works, at least in ltspice, but the gain is lower than with tubes:

Code Select


Version 4
SHEET 1 2608 1124
WIRE 736 16 608 16
WIRE 880 16 736 16
WIRE 1120 16 960 16
WIRE 1280 16 1120 16
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SYMBOL cap 1008 400 R180
WINDOW 0 24 56 Left 2
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SYMATTR Value 10µ
SYMBOL cap 1008 608 R180
WINDOW 0 24 56 Left 2
WINDOW 3 24 8 Left 2
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SYMATTR Value 10µ
SYMBOL ind2 1840 400 M0
SYMATTR InstName L4
SYMATTR Value 1
SYMATTR Type ind
SYMBOL ind2 1760 352 M0
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SYMATTR Value 1
SYMATTR Type ind
SYMBOL ind2 1760 464 M0
SYMATTR InstName L6
SYMATTR Value 1
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TEXT 2056 736 Left 2 !.tran 0 .5 .4
TEXT 2056 768 Left 2 !;.ac oct 32 10 100k
TEXT 2088 224 Left 2 !.param Ra 47k
TEXT 2088 320 Left 2 !.param RaT 22k
TEXT 2088 352 Left 2 !.param RcT 82
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TEXT 2056 800 Left 2 !.OPTIONS plotwinsize=0 numdgt=15
TEXT 2056 832 Left 2 !.fourier 1000 V(out)
TEXT 1808 304 Left 2 !K1 L4 L5 L6 1
TEXT 2096 272 Left 2 !;.step param RcT 50 300 50

And this is the tube - based:

Code Select


Version 4
SHEET 1 2172 1124
WIRE 960 -16 752 -16
WIRE 1008 -16 960 -16
WIRE 1120 -16 1088 -16
WIRE 1280 -16 1120 -16
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SYMBOL voltage 112 400 M0
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SYMATTR SpiceLine Rser=600
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SYMBOL Misc\\triode 1120 224 R0
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SYMATTR Value 6N2P
SYMBOL Misc\\tetrode 672 720 R90
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WINDOW 0 0 56 VBottom 2
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WINDOW 3 36 40 Left 2
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SYMBOL cap 416 240 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
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SYMBOL cap 416 672 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C9
SYMATTR Value 10µ
TEXT 1672 704 Left 2 !.include Tube_IM.lib
TEXT 1672 736 Left 2 !.tran 0 .5 .2
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TEXT 1704 192 Left 2 !.param Rk 2k7
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JFET has approx 10 times less amplification than tube based.

rankot · December 04, 2019, 01:29:56 PM

Schematic of JFET version:

rankot · December 04, 2019, 05:20:07 PM

And PCB for JFET version. Comments are welcome!

Marcos - Munky · December 05, 2019, 02:33:17 PM

Both layouts looks very nice, I really liked those curved traces. I didn't checked everything against the schematic, but what I checked seems to be correct.

rankot · December 05, 2019, 06:13:14 PM

What I'd like to do is to add a possibility that this JFET preamp is powered by phantom power, so it can be smaller (it uses only ~2mA), and perhaps made to work without additional power supply. But I have no idea how to do it. Help's welcome.

Marcos - Munky · December 05, 2019, 07:50:58 PM

I don't know how much voltage a jfet can handle, but I know they can handle 24V, which is half of the phantom power voltage. Using a 7824 is not an option, since the max voltage input it can handle is 36V. But since the current is so small, a simple voltage divider probably can do the job. You know, that "vref" voltage dividers you can find on ic based overdrives. A voltage drop of 24V at 2mA will dissipate 48mW, so regular 1/4W resistors can be used.

rankot · December 06, 2019, 04:41:52 AM

I'm aware of that, and those JFET's I plan to use are actually J201. They can handle 40Vds and that can be easily achieved with simple voltage divider (not shown on schematic and PCB). But I was also thinking of use 2SK105, 2SK208 or 2SK209, which can handle 50Vds, so no divider's needed (maybe 48V Zener to protect them?). It's all quite obvious if we use separate power supply.

So, actually, I didn't made a good question - what really bothers me is how to "extract" 48V from phantom powered XLR, use that to power preamp, and then return the signal to the same XLR?

Marcos - Munky · December 06, 2019, 07:52:56 AM

I don't have any experience on phantom power, but is it just a 3 way jack with signal, ground and +48V, correct? If it is, I believe you can just connect R5 (100r resistor that goes to B+) to the xlr lug that have +48V (the one connected to R21 and R22).

rankot · December 06, 2019, 08:10:42 AM

Nope, I don't think it's gonna work like that. We have two outputs, one in opposite AC phase to another, and they 'float' 48V above ground. So we must extract that 'floating' DC voltage somehow and put it where it's needed, without pulling back AC component from the output.

tubegeek · December 06, 2019, 08:13:28 AM

The three connections are: two balanced audio signals (usually identical and 180 degrees opposed) and ground. The non-phantom send has the two audio signals referenced to OV, and the phantom option has them referenced to +48V. There are current limiting resistors in series with the +48 as well.

If you look on Rod Elliott/ESP's pages there's an article on powering electret mics off the phantom power that may be helpful. I've built a similar item but I must confess I do NOT understand how it works.

When I get to my laptop I'll show proper links & schematics, but if you're impatient, start at ESP. The project is titled something like "low cost measurement mic."

Kipper4 · December 06, 2019, 08:20:43 AM

Link to ^

https://sound-au.com/project93.htm

rankot · December 06, 2019, 08:32:48 AM

Saw that at ESP, there's one interesting page here too, but far above my electronics knowledge... $:-\$

http://www.newportacoustics.com/gear/microphones/preamps

Maybe figure 7 from ESP gives the answer? Will try!

tubegeek · December 06, 2019, 09:14:49 AM

Maybe this worth reading? Certainly the THAT Corp. app notes pages are WELL worth a look.

https://www.instructables.com/id/Build-the-Four-Channel-SSM2019-Phantom-Powered-Mic/#step1

rankot · December 06, 2019, 11:09:44 AM

OK, guys, this seems to do the trick (at least a part of it):

However, it can't forward 48V phantom power to the microphone, just 37.6V...

HELP!

Kipper4 · December 06, 2019, 01:58:07 PM

Many electrets and phantom powered Mics will work with much less than 48V.

Still trying to get my head around it.

rankot · December 06, 2019, 02:09:57 PM

ltspice model for phantom powered JFET preamp:

Code Select


Version 4
SHEET 1 3184 1124
WIRE 736 16 608 16
WIRE 880 16 736 16
WIRE 1120 16 960 16
WIRE 1280 16 1120 16
WIRE 736 48 736 16
WIRE 1120 48 1120 16
WIRE -96 80 -96 48
WIRE 16 80 -96 80
WIRE -96 112 -96 80
WIRE 16 112 16 80
WIRE 736 144 736 112
WIRE 1120 144 1120 128
WIRE 1184 144 1120 144
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WIRE 1920 144 1728 144
WIRE 2480 144 1920 144
WIRE 2896 144 2800 144
WIRE 1120 160 1120 144
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WIRE 2720 608 2720 416
WIRE 2720 608 2560 608
WIRE 2768 608 2720 608
WIRE 1920 624 1920 592
WIRE 1968 624 1920 624
WIRE 2560 624 2560 608
WIRE 2720 640 2720 608
WIRE 1728 656 1728 464
WIRE 368 672 368 464
WIRE 992 672 992 608
WIRE 1088 672 1088 608
WIRE 1088 672 992 672
WIRE 1120 672 1088 672
WIRE 1920 672 1920 624
WIRE 1120 688 1120 672
WIRE -176 720 -176 496
WIRE 16 720 16 192
WIRE 16 720 -176 720
WIRE 80 720 16 720
WIRE 224 720 224 608
WIRE 224 720 144 720
WIRE 336 720 224 720
WIRE 496 720 496 624
WIRE 496 720 432 720
WIRE 656 720 496 720
WIRE 816 720 816 608
WIRE 816 720 720 720
WIRE 1072 720 816 720
WIRE 2560 720 2560 704
WIRE 2720 720 2720 704
WIRE 2720 720 2560 720
WIRE 2560 736 2560 720
WIRE 1184 784 1120 784
WIRE 1328 784 1248 784
WIRE 1456 784 1456 688
WIRE 1456 784 1408 784
WIRE 1728 784 1728 736
WIRE 1728 784 1456 784
WIRE 1920 784 1920 736
WIRE 1920 784 1728 784
WIRE 2896 784 2896 144
WIRE 2896 784 1920 784
WIRE 1120 816 1120 784
WIRE 1008 912 960 912
WIRE 1120 912 1120 896
WIRE 1120 912 1088 912
WIRE 1280 912 1280 16
WIRE 1280 912 1120 912
FLAG 736 144 0
FLAG 1488 464 0
FLAG 2560 736 0
FLAG 2480 608 0
FLAG 2640 144 V48
FLAG 1600 464 0
FLAG 1952 448 0
FLAG -96 48 V48
FLAG 1968 288 OUT+
FLAG 960 912 B+
FLAG 2768 608 B+
FLAG 1968 624 OUT-
SYMBOL voltage -176 400 M0
WINDOW 123 24 124 Left 2
WINDOW 39 24 152 Left 2
SYMATTR Value2 AC ac 1.
SYMATTR SpiceLine Rser=600
SYMATTR InstName V1
SYMATTR Value SINE(0 .01 100)
SYMBOL res 208 304 R0
SYMATTR InstName R1
SYMATTR Value {Rk}
SYMBOL res 208 512 R0
SYMATTR InstName R9
SYMATTR Value {Rk}
SYMBOL res 480 256 R0
SYMATTR InstName R3
SYMATTR Value {Ra}
SYMBOL cap 720 48 R0
SYMATTR InstName C1
SYMATTR Value 22µ
SYMBOL res 976 0 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R6
SYMATTR Value 1k
SYMBOL cap 736 208 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C2
SYMATTR Value 100n
SYMBOL res 832 416 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R8
SYMATTR Value 470k
SYMBOL res 1104 32 R0
SYMATTR InstName R10
SYMATTR Value {RaT}
SYMBOL res 1072 416 M180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R11
SYMATTR Value {RcT}
SYMBOL res 480 528 R0
SYMATTR InstName R12
SYMATTR Value {Ra}
SYMBOL res 832 624 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R13
SYMATTR Value 470k
SYMBOL cap 720 704 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C4
SYMATTR Value 100n
SYMBOL res 1072 624 M180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R15
SYMATTR Value {RcT}
SYMBOL res 1104 800 R0
SYMATTR InstName R16
SYMATTR Value {RaT}
SYMBOL res 1136 480 R270
WINDOW 0 32 56 VTop 2
WINDOW 3 0 56 VBottom 2
SYMATTR InstName R17
SYMATTR Value 2k2
SYMBOL res 1104 896 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName Rcurrent
SYMATTR Value 1
SYMBOL cap 1248 128 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C6
SYMATTR Value 470n
SYMBOL cap 1248 768 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C7
SYMATTR Value 470n
SYMBOL res 1424 128 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R14
SYMATTR Value 1k
SYMBOL res 1424 768 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R18
SYMATTR Value 1k
SYMBOL res 1472 704 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R19
SYMATTR Value 470k
SYMBOL res 1472 368 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R20
SYMATTR Value 470k
SYMBOL res 1936 416 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName Load
SYMATTR Value 10k
SYMBOL voltage 1712 464 R90
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName Phantom
SYMATTR Value 48
SYMBOL cap 144 208 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C8
SYMATTR Value 10µ
SYMBOL cap 144 704 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C9
SYMATTR Value 10µ
SYMBOL njf 432 272 M270
SYMATTR InstName J1
SYMATTR Value J201
SYMBOL njf 432 672 R90
SYMATTR InstName J2
SYMATTR Value J201
SYMBOL njf 1072 160 R0
SYMATTR InstName J3
SYMATTR Value J201
SYMBOL njf 1072 784 M180
SYMATTR InstName J4
SYMATTR Value J201
SYMBOL cap 1008 400 R180
WINDOW 0 24 56 Left 2
WINDOW 3 24 8 Left 2
SYMATTR InstName C3
SYMATTR Value 10µ
SYMBOL cap 1008 608 R180
WINDOW 0 24 56 Left 2
WINDOW 3 24 8 Left 2
SYMATTR InstName C10
SYMATTR Value 10µ
SYMBOL npn 2496 304 R0
SYMATTR InstName Q1
SYMATTR Value BC549B
SYMBOL npn 2784 304 M0
SYMATTR InstName Q2
SYMATTR Value BC549B
SYMBOL res 2496 352 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R2
SYMATTR Value 100k
SYMBOL res 2576 720 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R4
SYMATTR Value 470k
SYMBOL res 2784 352 M180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R7
SYMATTR Value 100k
SYMBOL cap 2736 704 R180
WINDOW 0 24 56 Left 2
WINDOW 3 24 8 Left 2
SYMATTR InstName C11
SYMATTR Value 100µ
SYMBOL res 2496 592 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R23
SYMATTR Value 1Meg
SYMBOL cap 2400 560 R180
WINDOW 0 24 56 Left 2
WINDOW 3 24 8 Left 2
SYMATTR InstName C12
SYMATTR Value 10µ
SYMBOL res 1712 160 R0
SYMATTR InstName R24
SYMATTR Value 6k8
SYMBOL res 1744 752 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R25
SYMATTR Value 6k8
SYMBOL cap 1936 240 R180
WINDOW 0 24 56 Left 2
WINDOW 3 24 8 Left 2
SYMATTR InstName C5
SYMATTR Value 10µ
SYMBOL cap 1936 736 R180
WINDOW 0 24 56 Left 2
WINDOW 3 24 8 Left 2
SYMATTR InstName C13
SYMATTR Value 10µ
SYMBOL res 1936 608 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName Load1
SYMATTR Value 10k
SYMBOL res -112 96 R0
SYMATTR InstName R21
SYMATTR Value 6k8
SYMBOL res 0 96 R0
SYMATTR InstName R22
SYMATTR Value 6k8
SYMBOL res 2544 480 R0
SYMATTR InstName R27
SYMATTR Value 10k
TEXT 2056 608 Left 2 !.include Tube_IM.lib
TEXT 2056 640 Left 2 !.tran 0 .5 .4
TEXT 2056 672 Left 2 !;.ac oct 32 10 100k
TEXT 2056 328 Left 2 !.param Ra 150k
TEXT 2056 424 Left 2 !.param RaT 33k
TEXT 2056 456 Left 2 !.param RcT 82
TEXT 2056 576 Left 2 !.include TubeLib.inc
TEXT 2056 296 Left 2 !.param Rk 3k3
TEXT 2056 704 Left 2 !.OPTIONS plotwinsize=0 numdgt=15
TEXT 2056 736 Left 2 !.fourier 1000 V(out)
TEXT 2064 376 Left 2 !;.step param RcT 50 300 50

garcho · December 06, 2019, 04:26:02 PM

Rankot, I've built that mic pre a few times, it's a great utility mic pre, zero complaints. There are a couple other designs he has that I've also built, they are almost all straight ahead and not any more complicated than say a phaser or delay or something like that. I don't think it's above your head, if you want, when I'm back home in a week or two, I can see if I have a layout laying around, or a breadboard diagram.

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Shroom mic pre