Topic: DIY preamp

[Thomas]

        

I have been looking at doing this for a while now, and have come up with some information that i think will build a pretty good preamp. looking at the Jensen Transformer webpage (http://www.comtran.com/) I found a section on schematics.

I think AS016:Standard Microphone Preamp Input Circuitry would be a good choice for the input stage, it looks fairly simple. Beyond that I think I want to use one of their designs that incorporates the JT-16-A transformer (http://www.comtran.com/datashts/16a.pdf), because on the data sheet it says "FOR ULTRA-LOW NOISE AMPLIFIERS", and "Our highest performance mic input transformer", both of these descriptions make it sound like a good choice.

Using the JT-16-A leaves me with three choices for the amplifier stage:
AS018 JT-16-A in Single Stage Low-Noise IC Mic Preamp
AS019 JT-16-A in Single Stage Mic Preamp with Two DC Servos
AS083 Famous Twin-Servo 990 Mic Preamp Basic Circuit

The third of those listed says that it is not a project for beginners, so I am going to discard that idea for now, but of the other two i don't know which one will be better. I may not be correct on this but in ASO19 I think the purpose of the dc servos is to eliminate noise as the gain is changed, which would be important if the gain isn't fixed, but im not set on fixed or variable gain yet so that might help make the decision.

Does anyone know why either of these options would be better than the other?

Has anyone ever tried a DIY preamp before? successes, failures?

Thanks in advance for any advice.

[tadjblack]

I got no help for you but here's a +T for ambition
You go boy!

[chase]

i also have been looking into making a preamp of my own.  do you have any schematics in particular that you were thinking about?  you may want to consider throwing the INA103/163 behind that transformer for the main part of the gain stage.  or if you prefer, you could replicate the inside of the INA103 as i did for the gain stage here.  





hopefully this circuit will work as a clapper to turn on and off a light.

[Thomas]

""i also have been looking into making a preamp of my own.  do you have any schematics in particular that you were thinking about?""
http://www.comtran.com/as/as016.pdf
http://www.comtran.com/as/as018.pdf
The first is the input stage the second is the amplifier.  

I would like to replace the op amps with discreet transformers, i do not know exactly what this means, but i have heard this is a good thing to do.  Also i have heard that the grace units use Burr Brown chips, is the INA103/163 one of the ones that grace uses?  the schematics listed above use analog devices op amps.  If you would be interested in pooling our knowledge to help build this that would be great.  I can get some of the parts for very cheap from the EE supply store here on campus.  LMK

[chase]

The V2 uses the INA103 and the V3 uses the INA163.  Unfortunately the INA163 is a surface mount only so I think it is out of the question.  I am not sure how one would go about making a discreet tranformer preamp because according to the datasheet for the JT-16-A, it's maximum gain is only 6.0dB.  Check out the datasheet for the INA103 (http://www-s.ti.com/sc/ds/ina103.pdf).  They also have a basic schematic for supplying phantom power and a switchable 20dB pad.  The one thing that concerns me with the as018.pdf schematic is that it says there can be up to 15mV DC offset at the output which could be bad.  Also if you check out that same INA103 schematic, there is an opamp at the output of the INA103 chip to correct for DC offset.  We could possibly implement this sort of offset correction into the as018.pdf schematic but i am not sure exactly how.  I think we should combine parts of both because the transformers would add low end and then the INA103 or combination of opamps could provide lots of clean gain.  You might want to poke around the Oade boards, I think there is some useful info on there.  I would but I have to get to lab now, then off to STS9 in Rochester.

[Thomas]

""The one thing that concerns me with the as018.pdf schematic is that it says there can be up to 15mV DC offset at the output which could be bad.""
I was also looking at the AS019 which has twin DC servos which keep the output constant as gain is adjusted.  I decided however (to keep things as simple as possible) to design the pre set at a fixed gain (similar to the oade M118, and M148), or make the gain adjustable through internal switches, and adjust levels on the sbm1.  I dont know though, i think im going to draw this circuit in PSPICE and simulate it to see what happens.

""You might want to poke around the Oade boards, I think there is some useful info on there.  I would but I have to get to lab now, then off to STS9 in Rochester. ""
Yeah i think ive read every post that has anything to do with preamps, When you get back from STS9 look in the DIY section of his new forum at the post i put up yesterday.

[chase]

I read the posts over at Oade and it looks promising.  I think using discreet transistors will be much more difficult than using opamps or an INA103, but if you want a sound similar to an M148 than this is the way to go.  Personally, I don't have enough experience with transistors to start designing a complex circuit using them.  That's not to say we can't figure it out, I'm sure Doug could even suggest specific transistors that would work best.  As far as having it a fixed gain pre, I don't really see the need for that.  It is very simple to just use the same method that Grace uses in the V2 with the rotary type switch and the trim knob.  I know that variable resistors are noisier and a problem might arise with clicks.  Even so, it would make more sense to have a rotary switch on the outside since you are already designing it to do the same thing on the inside but with jumpers.  Just a thought.  Let me know how the simulations go.

[Tim]

I've tried to talk a few of the CO tapers/engineers into building a preamp for a while now...

best of luck to you guys, very cool idea!

+T to you both

[F.O.Bean]

i also have been looking into making a preamp of my own.  do you have any schematics in particular that you were thinking about?  you may want to consider throwing the INA103/163 behind that transformer for the main part of the gain stage.  or if you prefer, you could replicate the inside of the INA103 as i did for the gain stage here.  





hopefully this circuit will work as a clapper to turn on and off a light.

honestly, that schematic looks smooth as butter, looks like it wouldnt take too long to make!!

only some caps/resistors/transistors/clocks

[Thomas]

Man i have never looked that closely at the V2.  I thought the knobs on the front were continuous gain.  that incremented gain with trim is pretty cool  How much of the circuitry (in the AS018 design)  would we have to change around the op-amp to accomodate the INA103?

[chase]

After looking at the diagram further i think we could get rid of R7 which adjusts for the possible 15mV offset voltage and replace that whole stage right out of the as016 and AD797JN with the INA103.  Instead of having an adjustable offset screw I think it would be better to just run some tests once the unit is built and use the INA103's offset correction to fix the offset once and for all.  See Figure 3 in the INA103 datasheet.  The offset is simply corrected by placing a resistor from pin 3 to 4, depending on the amount it is offset  This will definitely need to be tested using an oscilloscope once the circuit is built to verify.  Again in Figure 3 it shows that the gain is set by R_g.  We could have a gain pot or stepped gain at this point.  It shouldn't be too hard to figure out using the equation [Gain dB = 20*log(1 + (6k/R_g))]  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.  I am very excited about this thus far but unfortunately I am pretty much broke until May.  I had to pay 2 months rent plus security deposit for our house next year on top of paying rent for the place I am in now.  I know the Jensen transformers are not cheap either, ~$100, plus the other parts for the circuit.  

edit:  The next thing to add is a good set of VU meters and theoretically we will have quite the pre.

[nic]

+T to you guys for the geek/nerd talk!

[Thomas]

Yeah I definitely like the idea of grace style gain, VU meters are a must but they are very easy to work into the system, I will not be ready to invest in this for a few months either.   Where are you in school chase?  What is your field of study, could you get the hookup through school on any of the parts?

[chase]

I am currently at the University of Buffalo studying EE.  In terms of the size of our school, the EE program is pretty small, 220 out of 24,000.  Unfortunately I don't think we can get any sort of hookup on campus.  For all of our projects we have to order the parts ourselves online.  I assume you are also in EE but where?

[Thomas]

I'm at Auburn University in the wireless engineering program, a new program under EE.  Im pretty sure it is the first wireless degree in the country, it is the same thing as EE just with a slight emphasis on E-mag and RF.  Auburn has a pretty big EE program, im not sure how many students.  Im grew up in Auburn, and did the community college thing for a few years then transferred to AU.  I think you may be a little further along than me in the EE program, im just finishing circuit analysis, and digital logic, which are the first in the series of EE classes here.  
Anyway, what signal should I use to test the preamp circuit when simulating to best represent a mic signal?  I was thinking just a voltage varying sine wave, but what amplitude, shouldn't it be mV?