Topic: DIY preamp

[Todd R]

The only thing I am having difficulty figuring out is how Grace sets up the trim pots between gain steps.  Todd R please step in here and enlighten us  I know you implemented a similar technique in your DMIC-20 mod.  

Sorry, I'm of no help.  The Todd R who did the dmic-20 mod was a different Todd R, from the DC area.  He went by todd r #2 on the oade board, but I can't recall his last name.  I haven't seen how he did the dmic20 mod, but will probably get a chance to look at Simpy's in the near future.

If you're looking for a VU meter, there is another thread that discusses a kit to do this.  Maybe just plugging the kit into a box with the DIY pre might be best.  But otherwise, the kit uses a National Semiconductor LM3916 to run the display.  Check it out on their website, looks like it might be a pretty good part to use to accomplish the meters.  The LM3916 allows the display to be either bar or dot--dot being like the V3 and will save a bunch of power.

-Todd

[goose]

todd r#2 - I believe his last name is Romero, if I recall correctly.  Not sure how to contact him, but he had a website that you may be able to find by searching the oade forums.

[chase]

sorry for the confusion, i just assumed you were the same Todd R, especially after you laid down some good technical knowledge in other threads.

i think a few mV should be fine, i believe most mics output somewhere in the area of 15mV/Pa.  so if we have....

(130dB SPL)(63 Pa/130dB SPL)(15mV/Pa) = 945mV = 0.954V

note this is for the maximum SPL that my mbho's can handle, so at the most they will output nearly 1 volt.  i think 100dB SPL is a better real world estimate though.

(100dB SPL)(2 Pa/100dB SPL)(15mV/Pa) = 30mV = 0.03V

i would say try the simulation with ~30-50mV and see what happens.

btw, cool website for conversions http://www.sengpielaudio.com/calculator-soundlevel.htm

[goose]

If you are interested, todd r#2's email addy (from DatHeads posting) = <todd@routeflap.net>

[Thomas]

thanks for the heads up g00se.  after reading this thread yesterday, i realized i bought an ad500e from todd a while back and i still had that email addy.  i sent him one yesterday and haven't heard back yet, do you think this address current?  I would love to hear how he implemented that gain structure, because i think i have it figured out.

[goose]

I couldn't even give a guess as to whether it is current, Thomas.  

Good luck on the preamp - great idea!

[BobW]

don't forget to get a squarewave in the test for shape.
The op amps you've picked are fast and it shouldn't be too big a deal, Burr-Brown kicks ass.  
Go Team America !

What are you proto'ing with ? SPICE or real wire ?

[Thomas]

I want to put it all into a PSPICE model, but I don't know that PSPICE will have both the chips and transformers i want to use, I want to model it in the program first partially to make sure everything works and partially because i don't have the money to order all of the parts right now (it is going to cost close th $400!!).  I also will have alot more free time on my hands this summer, Auburn doesn't offer labs in the summer, so all i have to worry about is lectures, so I want to actually build it over this summer.   One thing i noticed on the INA103 data sheets is that they said "these chips can replace transformers"  will it create any forseeable problems to use both the jensen transformers and the burr brown chips?

[chase]

i don't think so.  the transformers are adding gain but in a different manner to the op-amps  the transformer is only adding around 6dB of gain, i think.  so you will still need more gain after that.  i think the INA103 can be configured to replace the transformer and we could just use the INA103 but it wouldn't produce the warmth that the transformer will give.  i think this combo will sound really good, especially since we both have mp-2s, which are transformer based, and personally i like the sound with my HOs.

[BobW]

I want to put it all into a PSPICE model, but I don't know that PSPICE will have both the chips and transformers i want to use, I want to model it in the program first partially to make sure everything works and partially because i don't have the money to order all of the parts right now (it is going to cost close th $400!!).  I also will have alot more free time on my hands this summer, Auburn doesn't offer labs in the summer, so all i have to worry about is lectures, so I want to actually build it over this summer.   One thing i noticed on the INA103 data sheets is that they said "these chips can replace transformers"  will it create any forseeable problems to use both the jensen transformers and the burr brown chips?

The part manufacturers often supply modeling data free of charge.
TI has their OpAmps profiled, so does Nippon Devices, D/K on Jensen...

[Thomas]

I just had a thought, i think when i make my preamp instead of mounting the xlr inputs on the front surface of the unit, mount them inside the box near the back with metal tubes that run to the front suface.  this way i could connect the bodies of my mics directly to the mic inputs and they would be encased by the preamp.  i dont know if this makes sense, but think of a preamp with the connections for your actives directly on the front of the box.  i cant wait untill i have some free time to work on this.

[Lee]

I just had a thought, i think when i make my preamp instead of mounting the xlr inputs on the front surface of the unit, mount them inside the box near the back with metal tubes that run to the front suface.  this way i could connect the bodies of my mics directly to the mic inputs and they would be encased by the preamp.  i dont know if this makes sense, but think of a preamp with the connections for your actives directly on the front of the box.  i cant wait untill i have some free time to work on this.

Why not just do away with the mic body entirely?  What I mean is, undo the body casing and take the circuitry out, then hardwire it inside the box where you'd normally connect the xlrs.  Then, mount a connector for your actives on the outside of the box.  Unless this is entirely over my electrical knowledge head (which could definaely be the case), couldn't you hack the ends off your actives and put lemos on there, so they plug into the body circuitry inside the box?

Seems like it would be cool if it worked, but I don't really know shit about the down and dirty technical stuff...

[chase]

that is a really cool idea.  i dunno if i would want to risk taking a mic body apart and then transferring it to the surface itself.  i like the tube idea because it would allow you to change mics and it would shield the mic bodies from the possible electric fields generated inside the pre.  wish i had actives.

[AT853rxwh]

Dunno if you guys saw the post I had.


there is a company called CanaKits that makes a mic pre assembled for round $10.  I have a mic pre that is using them and it sounds pretty decent (wouldn't use it for low level recording situations, as it would require setting the NJB3 at 9+db for recording, but for vocals no amp it worked just fine.)

www.canakits.com

[Thomas]

Man that sounds like a pretty inexpensive way to go.  I wish that was all that it cost to get the parts i want.  

That was exactly what i was thinking at first lee, then i had the thought that i would probably change mics at some point, and i might have trouble getting all my cash out of a pair of bluelinez soldered inside a preamp.  Also if i put the circuitry in side the box, i couldn't ever try different mics with it.  I also really like the idea of all of the inputsoutputs on the front of the box, this way you can set it down in a bag and there will be no pressure on cables.  

On another note my last week of classes started today, exams the week after so ill start working on this soon.