Topic: gain modding yer dmic-20

[dr.ph0b]

was doing one this weekend for someone, so i decided to take some pics along the way....

open the dmic20 up, slide out the board and locate the 2 internal gain pots..


they are located at R65 and R66


flip the board over and locate the solder points. you will need a de-solder tool for this.


after de-soldering, you should be able to remove the pots with a small pair of needle-nose pliers or a pair of electronic tweezers.



you should be left with nice clean holes on R65 and R66..


use some good 3 conductor wire, and prepare the ends with a fine coat of solder.



solder your wires to the board, making sure the colors are consistant in each hole. its best to pull the wires thru the back as far as possible. then solder the back enuf to secure the wires. clip off the exess the same height as the other soldered compenents, and add another drop of solder.



make sure to use matching rated potentiometers. they are 5K ohm, .25w   Calrad is pretty much the only manufacturer you will find easily.


i clipped off the little solder contacts sticking out sideways, and soldered the wires directly in the holes. this is the easiest and most compact way to do it. this is the right channel pot. i mounted then with the contacts on the side facing away from eachother. given the way i have the wires soldered to the board, the wiring of this pot is backwards.  so youll have to flip the white and the bare wires.


drill holes the front faceplate. i have a template, but its best to trace out the outline of the pots the way you can fit them the best, and measure out were to drill the holes. drill a guide hole with a tiny bit first. you dont really have much chances to redo this part. the very first one i ever did was pretty bad, but i coaxed GP into sending me a new faceplate  

use whatever knobs suit your fancy.  here are 2 options:




a good way to test it hook up everything with mics and all in a quite room, put your deck in record and slap some headphones on and crank the gain dip switches up as high as possible, and move the knobs around, shake the unit, etc to see if you hear any static or unusual noises. its best to do this before and after your mod, so you know if you have introduced a problem.

NOTE: this doesnt replace the use of the dip switches, it just gives you the option to fine tune the gain in-between the 5db increments that the dip switches give you.

[Scooter]

VERRY NICE!!  I was hoping somebody would post on how to to this!  +T

[cwfen]

Are those chips right near the location of the gain pots the op amps? If so, they look like they are just mounted in removable jacks instead of hard wired in there... would it be easy/ possible to trade out the op amp chips to get a different flavor a la "warm mod", "presence mod", etc?

**edit- I answered my own question, in part... It is the amplifier chip, not sure about change out options, yet...

http://pdf.searchdatasheets.com/pdf/874/874.pdf

[cwfen]

Well, one option I found is to upgrade the amplifier chips from the ssm2017 (now discontinued and somewhat maligned) to the ssm2019... the 2019 is supposed to have improved characteristics and is a direct replacement, pin for pin... it would cost around $10 bucks to get the parts from digikey or someplace similar... might be interesting! Just pop out the old, and pop in the new...

[Todd R]

The TI/Burr Brown INA217 is also a drop in replacement for the Analog Devices SSM2017 (the actual 8-pin thru-hole dip part number you'd need for the DMic20 is INA217AIP).  I've got some to put into Josh Simpson's DMic.  Once I get it from him, and hopefully fix it's other problem, we'll put in the INA217 and see how it sounds.  Oh yeah, Josh if you're reading this, I haven't told you, but we're going to mod your DMic.   

Really, considering how easy it is to drop in some replacement parts for the pretty crappy sounding SSM2017 (IME) in the DMic20, more people should try it out.

Todd, +T for the step-by-step above.  Now you need to sit down and figure out which of the resistors in the gain array by the dip switches you have to change out to get the gain steps to be 10db, not 5db.     With that change and with your gain pot mod, the new DMic gain structure would be the same as the V3, which I love.

[cwfen]

That sounds pretty cool with the TI/Burr Brown chip... be sure to post the results here when you get around to doing the mod... I would definitely be interested in hearing how that sounds! Any luck on figuring out the gain structure issues? That would be an interesting mod, as well...

[dr.ph0b]

the resistors for the gain are right behind the dip switches. not that would be a nice 3hr tedious job replacing them all!

todd: if u have any extra of those TI plugin ic's u can get rid of let me know.. i looked at digikey and they are backordered. havent looked elsewere yet..

[Todd R]

the resistors for the gain are right behind the dip switches. not that would be a nice 3hr tedious job replacing them all!

todd: if u have any extra of those TI plugin ic's u can get rid of let me know.. i looked at digikey and they are backordered. havent looked elsewere yet..

Well, I guess theoretically, you would only have to replace half of them--and continue to use every other one that represent the 10 db steps you want to keep.     Ok, just shooting for the moon I guess, just having the added gain pots is a very nice improvement. 

As to the parts--go to the TI website (www.TI.com) and look up the part you want, INA217AIP.  On the page for that part, it will give you the option of getting free samples.  Just do that--you'll need to fill out a profile for yourself and make up an appropriate title for yourself--design engineer, application engineer, whatever.  You can get up to 4 samples of the INA217 (or pretty much any part, as long as you're not talking about expensive microprocessors or something), which'll be enough to do two mods.  I used to sell semiconductors for Texas Instruments, I gave away all kinds of free samples.  Really, it's just easier for them and their distributors to just give the shit away when you only want a quantity of two.  I don't know exactly how other semiconductor manufacturers have their systems set up, but you should be able to easily get samples from other companies in much the same way if you only want a few to try out a mod.  So you should be able to get a couple free samples of the Analog Devices SSM2019 if you wanted to do a comparison--which is easy enough with the DIP socket provided on the DMic20 board.

At any rate, I'll have a couple extra of the INA217 that I hoped to keep for another project.  But if you can't get some samples from TI let me know and I'll send my extras out to you.

[cwfen]

I wonder if it would be feasible to replace the 8 position dip switch set up for gain with a rotary switch with fewer positions to sort of combine the gain settings of the original 5dB per switch into 10dB per setting... not sure how the PC board is set up, or how you would do it, exactly, but to a layman, it sounds "doable"!?

[leegeddy]

>> You can get up to 4 samples of the INA217

todd:
 
while back i called up TI and they sent me 10 INA217 chips as samples.

marc

[Swampy]

Hey, so I ordered the 25-395s to do the gain mod. Could someone point me in the direction of the wire I will need, and also where I can find cheap knobs. Thanks!

[eman]

I did the "gain mod" and also installed a two level rotary switch to move the signal to each resistor in the array. It seems most use the method of adding switches pressed right to left instead of depressing just one, but this ends up with almost the same resistance.
I know, sloppy work, and I replaced my RatShack iron with a nicer one later on. I also put the smoke back in that my friend released at 10KLF last summer (reversed battery polarity- doh!), put in the IC's that Todd gave me for my birthday last year, and look forward to testing them out. Thanks again Todd.



The switch came from the Mouser catalog. You can open it up and move the limit pins to use only the positions that you need. I think there are 12 positions on 2 channels.