WrmZ said:Hi,
I build fet847 mic from schematics and it sounds wonderfull but i got one problem - it there an option to increase the gain of this mic?
Khron said:Are you sure that's the case for the bolded node?
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- Connection or node between capsule backplate and C11.
- Connection between R2 (1G) and backplate (this is the 48V line providing capsule voltage)
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Hmmm... Ok. If the capsule bias is reduced =by half then we lose 6db of signal. When it's @ 48VDC then bringing it up to a "prescribed" 60VDC will provide better signal to noise ratio but not a full 6db more. This is true and 60V is what we use standard in many other mic's. Vintage U87 uses this "48V" method so we know it's a solution that worked fine during that particular run of the mic.Khron said:And besides, 1G + 100nF gives you a 100-second RC constant, which means the capsule bias voltage will only reach its maximum in about five times as long (over 6 minutes)
WrmZ said:One more question about C1 - if i understand it correctly it makes negative feedback - when value of this cap is lower it should give me slightly more gain and bass - it is right?
WrmZ said:the case was about "warming up" mic and waiting for raising capsule voltage - in first test i was recording just after enabling phantom voltage wchich resulted in low gain.
I was not aware of such high value of RC contant - thanks again for clarifying this
jumper 571-1-881545-4 1pc
3-pin header 710-61300311121 1pc
68p test-capsule 810-FG18C0G1H680JNT0 2pcs
2.2k 1% 603-MFR-25FBF52-2K21 10-20pcs, hand-match
SPDT switch 642-TL36W000000 1pc
100n film caps 505-MKS2D031001A00MC 3pcs
1u output cap 505-MKS2C041001FMSSD 1pc
4.7u source cap 647-ULD1H4R7MDD1TD 1pc (option, less bass)
10u caps 667-ECA-1HM100I 2-3pcs
22u cap 667-ECA-1VM220B 1pc (option, more bass; could be used to replace C4 & C6 too)
47k resistors 603-MFR-25FBF52-47K5 2pcs (replacing the 56k R6)
10k 603-MFR-25FBF52-10K 1pc
4.7p C0G feedback 810-FG18C0G2A4R7CNT6 1pc (option)
4p mica feedback 598-CD15CD040DO3F 1pc (option)
10M resistors 279-CFR25J10M 3pc (replacing the 1G R2)
25k trimmer 858-67WR25KLF 1pc
JFET 610-2N3819 1pc
pin turret 534-11218 1pc
1n caps 505-FKS2D011001AKC00 2pcs (replacing the 330p ones)
47u inductors 542-78F470-RC 2pcs
1G resistor 279-RGP0207CHK1G0 1pc (1G R2 unnecessary)
24v zener 512-1N5252BTR 1pc
Khron said:I'll attach the gerbers one of these days, after i "polish" the component silkscreen and export the files, but at least for now, here's my suggestion of a Mouser BOM, with more... shall we say, "down to earth" component choices? :
1.48eu for each (!!!) of the two temporary testing capsule-replacement capacitors; same for each of the two output filter inductors- are you friggin' kidding me???
Ok, deep breath, calming down....
Code:jumper 571-1-881545-4 1pc 3-pin header 710-61300311121 1pc 68p test-capsule 810-FG18C0G1H680JNT0 2pcs 2.2k 1% 603-MFR-25FBF52-2K21 10-20pcs, hand-match SPDT switch 642-TL36W000000 1pc 100n film caps 505-MKS2D031001A00MC 3pcs 1u output cap 505-MKS2C041001FMSSD 1pc 4.7u source cap 647-ULD1H4R7MDD1TD 1pc (option, less bass) 10u caps 667-ECA-1HM100I 2-3pcs 22u cap 667-ECA-1VM220B 1pc (option, more bass; could be used to replace C4 & C6 too) 47k resistors 603-MFR-25FBF52-47K5 2pcs (replacing the 56k R6) 10k 603-MFR-25FBF52-10K 1pc 4.7p C0G feedback 810-FG18C0G2A4R7CNT6 1pc (option) 4p mica feedback 598-CD15CD040DO3F 1pc (option) 10M resistors 279-CFR25J10M 3pc (replacing the 1G R2) 25k trimmer 858-67WR25KLF 1pc JFET 610-2N3819 1pc pin turret 534-11218 1pc 1n caps 505-FKS2D011001AKC00 2pcs (replacing the 330p ones) 47u inductors 542-78F470-RC 2pcs 1G resistor 279-RGP0207CHK1G0 1pc (1G R2 unnecessary) 24v zener 512-1N5252BTR 1pc
0dbfs said:Did you say your PCB is single layer single sided? Will you upload the gerbers and masks so full on DIY'ers can self etch drill and modify further if desired?
-jb
0dbfs said:The backplate, R2 (1G), and C11 all connect to form a node. This is hi-z node for DC signals and low Z for AC signals and should be floating or terminated at a teflon standoff terminal.
Khron said:While taking a look around, i came across the D-U87 thread, and i was quite surprised to see this:
https://cdn.groupbuilder.com/groupdiy/u/39511/58d1402a02187.jpg
I don't see any teflon stand-offs for those 1G resistors, soooo... What's the deal there? 1G resistors, but no high-impedance nodes to worry about?
Khron said:Which Gerber layers / files are required? So i know which ones to export...
... (snip)
I noticed / remembered the length (longest dimension) of the board was 52mm or so, so i moved the top half a couple ticks lower, to fit it within (ie. ever so slightly under) 50mm. Board manufacturers usually have different prices for "up to 5x5cm boards" versus "up to 5x10cm boards" Just a(nother) little cost-saving measure, with the side-effect of getting an even more compact board, and as such, shorter signal trace lengths ;D
wlinart said:I think that we need the full zip file of the gerbers for using with board manufacturers.
For self-etching i thnik the bottom layer would be sufficient, but others with more experience could help better with that aspect.
tip: to upload gerbers on this site you would need to change the extension, to for example .pdf or .png in stead of .zip
Thanks for doing this
Khron said:"Of course" i'd put them all in a zip file - i'd have to be almost insane to attach each file to a separate post ;D
Yes, that limitation's quite annoying, but good thing there are ways to "bend" that rule...
Well, since mine's a single-layer board, i'm guessing (in a physical order):
- bottom soldermask
- bottom copper
- top soldermask(?)
- top silkscreen (component values only, no designators, ie. R3, C6 etc, to not overcrowd the board - it's full of components )
wlinart said:What i actually meant to write was: I think that for using a manufacturer of pcb's, like you suggested, we would need the full gerbers, with all the layers. But of course i could be wrong about that.
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