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Gear / Technical Help => Battery Boxes, Preamps, Mixers, ADCs, and Processors => Topic started by: Thomas on April 07, 2004, 02:43:58 PM
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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.
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I got no help for you but here's a +T for ambition
You go boy!
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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.
(http://www.acsu.buffalo.edu/~cjbanna/circuit.JPG)
hopefully this circuit will work as a clapper to turn on and off a light.
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""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
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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.
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""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.
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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.
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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
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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.
(http://www.acsu.buffalo.edu/~cjbanna/circuit.JPG)
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
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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 ;D How much of the circuitry (in the AS018 design) would we have to change around the op-amp to accomodate the INA103?
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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.
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+T to you guys for the geek/nerd talk! :)
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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?
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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?
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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?
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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
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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.
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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 (http://www.sengpielaudio.com/calculator-soundlevel.htm)
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If you are interested, todd r#2's email addy (from DatHeads posting) = <todd@routeflap.net>
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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.
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I couldn't even give a guess as to whether it is current, Thomas.
Good luck on the preamp - great idea!
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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 ?
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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?
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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.
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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...
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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. ;D
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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. ;D
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...
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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.
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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
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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.
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Well I got my sp-preamp and it seems a bit more detailed than the cheapo canakit. Course when I didn't have the soundpro I thought it had lots o detail... DAMN THIS HOBBY!!! :P
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Gentelmen
this DIY preamp sounds like an interesting undertaking.
I think you'll manage to interface the transformer and the INA103 just fine.
Looking at the 018 schematic, my suggestion is to lift the grounded leg of the transformer and connect the "center" of the transformer to ground (ground the middle of the transformer "output-short" by using two resistors (50% of single res value) and two caps (200% of single cap value) for RN, CN and RL mounted symmetrically around ground.
Then there is no DC flowing in the secondary winding either.
Then the INA (which has bjt inputs) sees the same source impedance
and potential on both inputs. And voila - the resulting offset might just be
of academic interest. (And you can rip out the entire branch with R3, R5 C3 R6 etc).
Now you have a nice, truly balanced input signals for the INA.
I don't think the PSpice will yield that much. The transformer is a known (spec sheet wise -not wrt to spice modeling). The INA is well described.
The true unknowns are really the stray elements C and L, ground planes or lack of such, bypass caps .. None of which are easily captured for modeling. And the pulse/freq responce can be manipulated by the transformer's RN CN & RL load.
But do a very carefull _noise_ analysis. The INA want's the lowest source
impedance. Can you provide that?
I am not sure why you need all that gain. And I not sure why you don't just reproduce
the original circuit using the AD797.
But heck, I am all for experimenting! And I will suggest you build the circuit first with cheapo xformers from Radio Shack just to get going.
JK
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hey jk
welcome to the forum, i have drooled over your products for a long time. thanks for the advice, i am a beginner to all of this, and have just started to get into my major classes where we really start to do design. i have been thinking of building the one in the schematic lately since this is my first shot at building. Should i just buy a transformer from radioshack that is a 4:1 ratio? i think i read that the jensen one is like 600:150?? im not sure though. could i just replace the cheap transformers for the jensens when i was satisfied with the rest of the circuit? One thing that has me sort of apprehensive about this is the $ of the jensen parts, if i could try it with no risk of damaging expensive parts, i would have no worries. thanks again for the advice & +t.
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Hi Thomas!
Thanks! Great forum this taperssection with lots of useful information on a slew of topics.
Yes, I think that making a "beta" of this mic-pre using a cheapo transformer is the proper way of testing your
ideas. It will bring out the weaknesses in the current
design, allow you to refine the circuitry and establish if it's worth to build the real thing.
All this without much investment on your part. The winding ratio of the cheap transformer is not important. It's role is just to present a balanced signal to the INA. (Some issues of noise etc exist but that's very much of secondary interest right now).
One more thing, the INA163 uses current feedback so you might be forced to reconsider my advice of just balancing the input impedances and ignore the DC offset....
Regards and best
Jon
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After taking this project to one of my professors and asking him for advice, it has become abundantly clear that i need one more class under my belt to do this design competently. I took digital electronics this summer, (p-n junction, transistors, logic families, fabrication, and implementation), and am currently enrolled in the second part of that class analog electronics (amplifiers,???) so after this semester i would like to design from scratch using the same basic layout of the jensen designs. i also would like to try my hand at deisgning my own chips because we have a chip fab here on campus, and my DigElec professor is in charge of it ;D Maybe a preamp on a chip kind of like the DPA thin film amplifiers that enable the small size of their mics.
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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.
(http://www.acsu.buffalo.edu/~cjbanna/circuit.JPG)
hopefully this circuit will work as a clapper to turn on and off a light.
Alright, that whole page scares me, especially the "J-K Flipflop" you wrote on the top. Those things are pure evil. One more reason why I'm CS and not CompE or EE. Props for attempting a DIY preamp though.
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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.
Alright, that whole page scares me, especially the "J-K Flipflop" you wrote on the top. Those things are pure evil. One more reason why I'm CS and not CompE or EE. Props for attempting a DIY preamp though.
I thought it was a Schmitt Trigger.............I suck at solid state !
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Alright, that whole page scares me, especially the "J-K Flipflop" you wrote on the top. Those things are pure evil. One more reason why I'm CS and not CompE or EE. Props for attempting a DIY preamp though.
that whole page gets me moist (pardon my french). the exact reason why im EE not CS. well im not actually EE im WE but with a specialization in hardware, which is great because i don't have to take any power classes ;D
power= ??? ::) ???
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And how about this one: http://sound.westhost.com/project66.htm ..15V isn't ideal, though.