Mic Preamp. Adding things and getting ideas. Would love help please.

[Leek]

"color" is an utterly meaningless term when you don't qualify your needs.

50-55db of gain, 48v pgantom, pretty flat frequency response, using discrete or IC's is acceptable.  I want something that imparts a sonic imprint instead of the wire with gain approach. Output attenuation so I can drive the input, yet tune down the output to keep the saturation from the mic transformer.  What else do I need to provide?

 

[Andy Peters]

50-55db of gain, 48v pgantom, pretty flat frequency response, using discrete or IC's is acceptable.  I want something that imparts a sonic imprint instead of the wire with gain approach. Output attenuation so I can drive the input, yet tune down the output to keep the saturation from the mic transformer.  What else do I need to provide?

You haven't defined what sort of "sonic imprint" you want -- that is what I mean by "color."

I would imagine that "color" and "sonic imprint" are at cross purposes with "pretty flat frequency response."

 

[Leek]

You haven't defined what sort of "sonic imprint" you want -- that is what I mean by "color."

I would imagine that "color" and "sonic imprint" are at cross purposes with "pretty flat frequency response."

"Color" is a subjective term and is hard to describe. I don't want something "clean" as in say a GML, Grace or Forssell preamp. I want the 2 and 3rd harmonic distortion from the input transformer and etc.

I'm working on a schematic using the N*** styled input transformer, into a BB OPA134 preamp stage and then a THAT 1646 output stage. The output stage will be a basic THAT 1646 circuit. Here is what I've got so far and I'm open to suggestions..

https://postimg.org/image/av722u8id/

C1 = 0.22uf Film Cap
C2, C3 and C4 = 0.1 Film Cap
R1 and R2 = 10k Carbon Comp

Power supply is a bipolar 15v +/- supply.  I want to add two potentiometers one for gain and one for output attenuation. Then later after hearing how the 1646 output stage sounds, create a Transformer balanced output that's selectable via a switch. So, I can use the 1646/Transformer when I want.

 

[ricardo]

Sigh!

• Go and look up circuits that use that Neve transformer to see how to connect it properly
• You don't need C1
• With R1 = R2 = 10k, the amp has 2x gain & rotten noise ... but so what?
• ensure your carbon comp resistors have been hand carved from solid Unobtainium by Virgins
• READ the 1646 datasheet before you start putting pots in the signal path.
• You'll need something to get your 2nd harm.  Buy it from Ye Olde Unobtainium Shoppe

 

[Leek]

Sigh!

• Go and look up circuits that use that Neve transformer to see how to connect it properly
• You don't need C1
• With R1 = R2 = 10k, the amp has 2x gain & rotten noise ... but so what?
• ensure your carbon comp resistors have been hand carved from solid Unobtainium by Virgins
• READ the 1646 datasheet before you start putting pots in the signal path.
• You'll need something to get your 2nd harm.  Buy it from Ye Olde Unobtainium Shoppe

Sigh.. I'm starting fresh with a THAT 1512 and THAT 1646. I'm starting with all high quality styrene caps, Nichicon electro's and 0.1uf Wima's for decoupling. Apparently discrete isn't the place to jump in and learn. Though, I have learned a few things and it's been fun up until now. It seems I get more smart*** comments then help now. The 1512/1646 seems a good place to start and by using both data sheets seem pretty straight forward to get a working circuit going. When I get that going, I will need help with adding the input/output TX's that I have. Quite possible only the input as the impedance of the opamp is so low driving the TX would require a whole different output design. But, considering my transformers are Chinese knock offs of more sought after TX's the pins aren't exactly the same. So, looking up how an old Mairnair 10468 is wired doesn't really help does it? 

I can't wrap my mind around why so many people here are so keen on keeping "noise" down. If I wanted a noiseless preamp I'd go to Sweetwater.com and buy one. The purpose of this thread was to get a preamp built (even if noisy) and then learn how to get rid of the noise. Add transformers for tonal qualities and learn different construction techniques. Learn how to incorporate discrete and IC's together. Learn the difference in using discrete circuits versus integrated circuits and how each perform differently.

Things I have learned have been beneficial and I'm thankful of that. Like for example, how an IC wants to see both the +/- at output and which ever is more "positive" will more a less over take the other signal and be the only signal at output. In this scenario we use a voltage divider so that the input is biased so that the output see's both voltages evenly and the signal isn't one sided. I learned the difference in an inverting and non-inverting op amp configuration. How to incorporate those. I learned why we use an op amp buffer. For example, an op amp buffer is used because the input is usually a high impedance and then has a very low output impedance. This allows us to chain circuits together without impedance issues.

So, before posting on my thread being negative, take the time to realize I'm trying the best I can. Not all of us are versed in electrical engineering. Some of us started as audio engineers, only ever soldered patchbays and the occasional xlr/trs cable. This is how a lot of us got a start and well, you're being negative about it. This isn't the 70's anymore where engineers learned from watching other engineers or stealing other people ideas. We now have the "internet" where one (should) be able to come and ask others for tips and ideas. Heck, look at Rupert Neve. The 1073 is full of junk components. Tantalums, Electrolytics, horrible spec transformers. Sounds good though don't it? Doesn't stop people from cloning them or spending 5000$ for an original either.

So, before continuously belittling me or trying to push another idea on me. Understand that I'm trying to take what I have and get something going. Then, after I get a circuit working (noisy or not) make it better. There's no need in telling me repeatedly it's going to be noisy when the whole purpose was to get a working circuit together. Because, with care, practice and the added benefit of learning I could eliminate that noise. The problem is, everyone is so negative and doesn't want to help me with what I have. They want to push another idea on me.

So, if we can please stop being so negative and help me get something working that'd be great. I do have BC547's, Pn2907's and BC557 transistors, I have N*** style (same as used in Chameleon Labs and GAP preamps) input/output transformers. I have a bipolar 15v +/- supply. Now, help me build something with that please. Because with the Input/output TX's and transistors noise shouldn't be an issue any more.. I'm also going to build the THAT 1512/1646 also so I don't have so much noise and hopefully everyone stops replying about "noise" now..

 

[ricardo]

It seems I get more smart*** comments then help now.

Err.rh!  Posts #30, 31, 33, 34 is a fully worked out circuit that gives EXACTLY what it is humanly possible to deduce of what you want.

The 1512/1646 seems a good place to start and by using both data sheets seem pretty straight forward to get a working circuit going. When I get that going, I will need help with adding the input/output TX's that I have.

Yes.  You don't need transformers with 1512/1646.  At best you will not degrade their performance.

So, looking up how an old Mairnair 10468 is wired doesn't really help does it?

You wired it up so completely wrong that it might damage certain mikes.

I can't wrap my mind around why so many people here are so keen on keeping "noise" down.

There's noise and there's NOISE.  Your last design has about 34dB more noise than it should.

The problem is, everyone is so negative and doesn't want to help me with what I have. They want to push another idea on me.

Err.rrh!  Post #50 is just trying to get your circuit to work.  I've given up trying to "push ideas at you".

OK.  I might have been a bit tongue in cheek about Virgins & Unobtainium but ALL the rest is pukka.

I do have BC547's, Pn2907's and BC557 transistors, I have N*** style (same as used in Chameleon Labs and GAP preamps) input/output transformers. I have a bipolar 15v +/- supply. Now, help me build something with that please. Because with the Input/output TX's and transistors noise shouldn't be an issue any more..

.. and I've suggested how to use all the bits you have in  #43.  You can use your +/- 15V supply as a 30V supply.

 

[Leek]

Ok ricardo, I now have a working THAT1512/1646 mic preamp. I've breadboarded the preamp based on the THAT data sheets and I hope I have a little better understanding of things. I am now working on a Transformer mic input into a Transistor based based preamp stage and a THAT 1646 output stage. I am a little better at calculating resistor values and etc so hopefully by doing so I have eliminated NOISE and the preamp stage looks ok. I will be using the Neve style input transformer wired in series to feed the circuit. Could you care to look over it and tell me rather or not I'm in the right direction?

I have recently, what I hope perfected a decent preamp. I was hoping you could over look it and give your insight?
I'll provide schematic and transformer link as well. All other parts are commonly used and shouldn't require explanation.

The input stage (balanced) is a Neve style input transformer connected in series. 

Pin 1 = + XLR In
Pin 2 & 3 tied together
Pin 4 = - XLR In
Pin 5 = Ground
Pin 6 = Ground
Pin 7 & 8 tied together
Pin 9 = + signal to preamp stage
Pin 10 = Ground

The preamp stage is based around two low noise BC549 transistors.
Each power supply input (on both the preamp stage and THAT 1646 output stage) are decoupled using 0.1Uf Wima Caps.

The output stage is exactly configured on the THAT 1646 data sheet.

Here is the link for the schematic. (https://postimg.org/image/utc00rutt/)


Hopefully this looks a lot better Ricardo.

 

[L´Andratté]

Bump for a great circuit post by ricardo, that doesn´t get enough credit!
Thanks, just what I was looking for!
https://groupdiy.com/index.php?topic=64069.msg811214#msg811214

 

[user 37518]

The preamp stage is based around two low noise BC549 transistors.
Each power supply input (on both the preamp stage and THAT 1646 output stage) are decoupled using 0.1Uf Wima Caps.

The output stage is exactly configured on the THAT 1646 data sheet.

Here is the link for the schematic. (https://postimg.org/image/utc00rutt/)


Hopefully this looks a lot better Ricardo.

That circuit is not the best, it will have high  nasty distortion since there is no global negative feedback, the first stage with the emmiter resistor bypassed will produce most of the distortion, even at low levels, the second stage has emmiter degeneration but 100 ohm is still very littlle degeneration and will produce significant distortion,  the input impedance of your circuit is around 200 ohms, the 0.1uf capacitor means youll have a hi pass cut off frequency of 7KHz., but that doesnt even matter because you are running the first stage at an emmiter current of 12mA, that means that the transistor is saturated, I dont want to even analyze the second stage, that circuit is a mess.

Have you tried simulating your circuits? it seems to me you are just compiling circuits without knowing how they'll perform or what you are actually doing, in either case you are much better off with ricardo's circuit. A discrete mic pre is not the best if you are a beginner,  specially if you dont even know the most basic configurations, I willl suggest that you follow Ricardo's advice and try using something like the THAT 1510, or use ricardo's circuit if you want something discrete with more distortion.

Sorry I just realized that I replied to a comment which is a year old.

 

[L´Andratté]

Hey that´s a terrible circuit!
I meant the one I linked directly to in my post!

 

[Joechris]

I build Ricardos preamp on a Vero board with a small Beyer 1:5 transformer in, unbal out. It sounds beautifull!!
No wonder they made better records in the early 70s.. So now I cant use the pres on my interface anymore, It
just sound nasty in comparison.. I have to build a few more. Any with Eagle/pcb skills fancy making a gerber..?
Thanks Ricardo for sharing!
Joe

 

[L´Andratté]

Hi Joechris!
That´s very cool!
Maybe give me a week and I make an eagle board.
Did you use all the components per schematic?

I´m thinking about a 2nd switchable stage, for high gain á la Neve...?

I joined the micbuilder group, looking for the doc ricardo mentioned, but couldn´t get any acess yet.

Ricardo doesn´t seem to hang around gdiy at the moment :'(

 

[Joechris]

Maybe give me a week and I make an eagle board.

That would be great!!

Did you use all the components per schematic?

Yes, exept R12 (input trans dummy), and the imp bal components R10, R11 and C4, but those would be nice to have on a PBC.

I´m thinking about a 2nd switchable stage, for high gain á la Neve...?

Well as is it has 56Db of gain with a 1:5 input transformer. Thats fine for an accustic guitar with a ribbon. So for me it has enough
gain. But if you want to drive a 600 ohm input, a buffer with more current drive is needed. But I dont need that since I will only
drive 10k inputs. And I like the simplicity of it... as is.
It would be nice to add a 1k revlog pot for gain(R6), And swithes for pad, 48v and phase on the PCB, so it could hang of those on a front panel. But thats all up to you. 
Space for a mic transformer with pin layout for a few different transformers could be nice,
but its always so anoying that the cool transformers you have in a box in the basement never fits..!

 

[L´Andratté]

Well as is it has 56Db of gain with a 1:5 input transformer. Thats fine for an accustic guitar with a ribbon. So for me it has enough
gain.

Yes, point taken, it´s the silly urge to "improve", one stage it will be.

But if you want to drive a 600 ohm input, a buffer with more current drive is needed. But I dont need that since I will only
drive 10k inputs.

Yes, 22k in my case ;D but Ricardo mentions somewhere above a PN2905 (max. Ic 600mW) could also be used for Q2.

And I like the simplicity of it... as is.

Definitly, almost as simple as my brain! ;D
I usually do all the pre transformer stuff and pots/switches wiring offboard, but it is not much hassle to add that stuff to a pcb.

But don´t get too high expectations and read my bottom line. Strictly...

 

[Joechris]

Well, since you been a strictly amateur for such a long time I have a good feeling about this
Thanks anyway for doing this.

 

[hans a]

Im glad i found this thread.
I was looking to find a mic pre design that i could use to bypass my existing consoles mic preamp (ne5534 with ll1538). But as the console use virtual earth it would be unsuitable to try with this design would it?
I have the book art of electronics so maybe i can find a solution there. Im thinking the virtual earth can be near the rail. But it may be wishful thinking.
Thanks for posting.

 

[abbey road d enfer]

Im glad i found this thread.

Beware this thread does not make much sense now that it has been pruned of some of its most pertinent answers
I would suggest you open a new thread, where you should define your needs/aims/context...

I was looking to find a mic pre design that i could use to bypass my existing consoles mic preamp (ne5534 with ll1538).

Why would you want to do that? 5534+LL1538 is a very respectable combo, as long as it's well implemented.

But as the console use virtual earth it would be unsuitable to try with this design would it?

So what? VE is related to summing amps and bus...
If you mean the console uses a single rail supply with a half-voltage rail, this is not an obstacle. All circuits can easly be adapted to either single or bipolar rails.

 

[hans a]

I guess you have a point. I can try it out on a small test-pcb.
I guess i thought it to be nice to have a couple of different sounding channels. Ive heard complaints on high noise-levels. Its not tied to the input tho, there is a fine adjustent pot tied to a tlo02x chip.

 

[hans a]

The console is using dual rail 22v

 

[abbey road d enfer]

The console is using dual rail 22v

So, what do you mean by the console being virtual earth?