First time diy - mic pre - diyAudio
Go Back   Home > Forums > Design & Build > Parts
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Parts Where to get, and how to make the best bits. PCB's, caps, transformers, etc.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Thread Tools Search this Thread
Old 12th December 2012, 05:55 PM   #1
m0rtuss is offline m0rtuss  Croatia
diyAudio Member
Join Date: Sep 2010
Default First time diy - mic pre

Hi dear diyaudio members, hope you'll help me.

I'm trying to build a mic preamp based on INA217 chip. It's a pretty standard preamp (did only few modifications to the original schematic that I found in spec sheet).

This is original schematic:

Click the image to open in full size.

I added polarity inverse switch and pad -20dB switch.

Currently i'm working on a PCB in Eagle, so my question is about wire width. I'm planning to do a photo procedure for pcb, so what is the wire width with which I could work? Now it's set up to 0.016mm.

This is my current version of PCB (on edges you could see I started rewiring with wider lines).

Click the image to open in full size.

Thank you,

  Reply With Quote
Old 14th December 2012, 10:49 PM   #2
diyAudio Member
The Space Egg Corp's Avatar
Join Date: Sep 2010
Location: London & Miami
Blog Entries: 13
Hi Petar

Thicker the better !
You might also consider implementing some kind of ground-plane...
...or well considered 0v earthing paths.

R1 & R2 are best, VERY well matched.
Also R4 & R5

C1 & C2 I would dump the polarised caps; and go for a 10uF plastic type.
They are will need much more room to fit them if you decide to do so...
...much better sound...VERY low signal loss & distortion from the low mic signal.
Also, no micro small 'voltage leaks' from the large 48v phantom power.

Also, the 1N4148 diodes are not really needed...
...and may, if omitted, result in a marginally better noise performance.

I guess you're doing other boards, for the 48 phantom & dual 15v PSU's.



Last edited by The Space Egg Corp; 14th December 2012 at 10:53 PM.
  Reply With Quote
Old 18th December 2012, 09:42 PM   #3
diyAudio Member
tetrageist's Avatar
Join Date: Nov 2012
Location: Berkeley, CA
Send a message via Skype™ to tetrageist
Hi there, bit of a stupid question but I'm trying my best to learn without school at the moment. It looks like R4 and R5 are only there to serve as pull-down resistors to the diodes; assuming they were removed and the end user (me, hopefully) took extra care not to damage the rest of the device, would they still be necessary? I don't see a need for them otherwise, unless the signal itself needs to be attenuated (? guessing that is what the resistors would do) before it is amplified.

Either way, thanks for the original post. Looking forward to recording some interesting sounds with this pre.
  Reply With Quote
Old 18th December 2012, 09:51 PM   #4
DF96 is online now DF96  England
diyAudio Member
Join Date: May 2007
I'm not familiar with the chip used, but my guess is that R4 and R5 are bias resistors for the INA217. The diodes appear to be for over-voltage clamping, so normally will be reverse biased and so not conducting.
  Reply With Quote
Old 19th December 2012, 12:45 PM   #5
diyAudio Member
The Space Egg Corp's Avatar
Join Date: Sep 2010
Location: London & Miami
Blog Entries: 13
The diodes can be omitted.
  Reply With Quote
Old 19th December 2012, 02:04 PM   #6
diyAudio Member
Join Date: Jan 2008
Diodes are protection.

R4/R5 in conjunction with C1/C2 determine form input high pass filter. R4/R5 also effectively set input impedance.

Virtually all powered microphones also have blocking capacitors for isolation. These are effectively in series with mic-pre caps, and raise corner frequency of high pass filter.

This is case where bigger is better. Phase/transient response of LF signals will be badly smeared by using 10uf instead of 47uf:

Frequency/Phase for 10u v 47u with 2200ohm resistor:

40Hz 10u v 47u with 2r2 FR and phase.gif

The -3dB points may seem quite low relative to music signals, but this is what these two filters do to 40Hz square wave:

40Hz 10u v 47u with 2r2.gif

Bigger input capacitor reduces waveform remodeling.


  Reply With Quote
Old 19th December 2012, 02:07 PM   #7
EssB is offline EssB  United Kingdom
diyAudio Member
Join Date: Jan 2009
Read note 2 under Absolute Maximum Ratings in the INA217 datasheet:
- Input terminals are diode-clamped to the power-supply
rails. Input signals that can swing more than 0.5V
beyond the supply rails should be current limited to 10mA
or less.

You have no input current limiting in that schematic and 48V phantom power switching will generate large voltage pulses at the INA217 input terminals, so the input clamping diodes are essential.

Some mic pres use back to back zeners to protect the inputs.
See text under fig.2 on this page 48V Phantom Feed Supply for Microphones

Even with that level of protection I still managed to pop a 5532 in a mic pre yesterday, but I was being stupid with it !
  Reply With Quote
Old 20th December 2012, 01:27 AM   #8
Minion is offline Minion  Canada
diyAudio Member
Join Date: May 2006
Location: Cowican Bay , vancouver island
I have built several of these mic pre"s using the INA217 so ill just post a few of my observations and experiences .....
The 4148 diodes can be omitted if you allways remember never to plug in a mic with phantom power already turned on , they are protection diodes for hot plugging .....
r4 & r5 set the input impedance , R1 & R2 are for current limiting for the phantom power and should be well matched from best CMRR performance .....
C1 & C2 are DC phantom blockers and I had good performance using good quality Polarized cap though I used 100uF caps as opposed to 47uf and bypassed them with a 0.1uf film cap .....
If you want to go minimalistic you can also drop the OPA134 DC servo part of the circuit but you will have to add a output cap to the circuit , I used a 16v 470uF cap and put a 1k resistor to ground after the cap and had fairly good performance ......

I have several of these mic preamps in my home studio (both with all features and minimalistic) and they sound very good ......

  Reply With Quote
Old 20th December 2012, 03:28 AM   #9
diyAudio Member
Join Date: Jan 2008
"That Corporation" 1200 series data sheet is a good read. Chip looks nice too.
  Reply With Quote
Old 20th December 2012, 01:40 PM   #10
diyAudio Member
The Space Egg Corp's Avatar
Join Date: Sep 2010
Location: London & Miami
Blog Entries: 13
EXACTLY ! Minion !

Microphones should NEVER be 'hot plugged'

The diodes can be omitted.

& as said earlier, the phantom power resistors...
...MUST be well matched.
I'd bung in a couple of wire-wounds here, from a matched bunch.
( just my take ! )



I've had good results with 10uF input-caps in a similar situation...
...but hey...I just 'listen to 'em'...
...the sim & scope of course...'tell it how it is'.
  Reply With Quote


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Similar Threads
Thread Thread Starter Forum Replies Last Post
Mic Pre / ADC pindrop Digital Source 1 22nd July 2012 04:11 PM
Phono pre-amp, TT PSU, mic pre-amp build log ptheskil Analogue Source 0 7th May 2012 02:55 PM
$5 mic pre rockstudio Instruments and Amps 11 12th March 2007 06:39 PM
DIY mic pre list tym cornell Instruments and Amps 0 10th February 2006 11:26 PM
mic + pre a007udio Multi-Way 3 10th February 2005 02:27 PM

New To Site? Need Help?

All times are GMT. The time now is 07:46 PM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2