Is this Design practicable?

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Hi Guys!
I have been browsing this forum several times and found many useful tips, hints and knowledge. :)

Now it's time for my first post, my question is simple for the experienced users here I think.
It's a simple microphone preamp but I want to make it as good as possible
so I'm open to all suggestions regarding improvement of the circuit.

So far it looks like this:

An externally hosted image should be here but it was not working when we last tested it.


Oh, and don't mind my crappy drawing please ;)
 
the left hand half of the LTP is trying to drive zero impedance.
-IN is a virtual earth.

You must set up the opamp as a balanced input opamp with gain set by the 4 resistors not the two you have shown.

For the LTP to present a balanced load to the mic, the load on the LTP must also be balanced.
 
Boy this is way complicated for 2011. Op amps are so fast and quiet now I think you could replace all the discrete transistor stuff with a differential input stage of 1/2 of your output amp. The output op amp stage looks okay. 5532's are okay but ST33078 or JRC4560 or (4562 looks better but I haven't tried it) are quieter and faster. This schematic may just do you fine. http://www.diyaudio.com/forums/analog-line-level/177393-diy-dj-mixer.html The second drawing is the mike input parts. Make sure PS is locally bypassed (3 cm or closer) by .1 uf disk caps and the feedback on every stage has a 20 pf cap around it, to prevent oscillation, because these new op amps are really fast. Cheap, though, $.60. The commercial mixers I've seen on here use 4558 amps which are way noisier than these new ones but require no disc caps, so they save money at every socket.
 
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@indianajo
I wanted my input stage as noise-free as possible thus the non-ic approach.
I also know that there are chips that contain similar circuits but I designed it around parts that are already in my shelves.
The 5532 might be an old chip but TI still uses it's circuitry in some of their latest audio-IC's, many commercial designs still use it, so I think it's absolutely sufficient here.
But let's not get into another op-amp discussion. :)

Anyhow, thanks alot for your input!

Regarding your Link: Mr. Elliott has a very similar design on his website, surprise! :D

An externally hosted image should be here but it was not working when we last tested it.
 
Please do us a favor, build this discrete unit then try the link design with two halves of a 4562, see which is quieter. My 33078 upgraded disco mixer is near the performance of my legendary PAS2 tube preamp, which has been upgraded with new metal film resistors over 100k and new plastic film caps. The disco mixer has a tiny bit more hum, but hum is the enclosure's fault, probably, not the op amps. (Has a stainless steel front panel not keeping out magnetic fields).
 
Thanks Max. Don't want to hijack the thread, full details on the hum war on the disco mixer here:http://www.diyaudio.com/forums/anal...performance.html?highlight=disco+mixer+mid-fi A magnet does not stick to the disco mixer front panel, but it sure looks good. It is star grounded with the case the center of the star, the magnetic phono headshell grounded to the mixer case, the disco mixer case grounded through a 3rd wire (2 pin power plug) to the power strip. The RCA jacks are isolated from the steel back panel by rubber o-rings, ground at the op amp. Any of those not exactly that way, hum is increased. Turning down all inputs doesn't decrease hum. If either radio or turntable power cord is turned over (non polarized) the hum is increased. Moving the power transformer & switch out of the mixer 2 m away made the biggest improvement. You have to stand 20 cm from the speaker to hear the hum, so I'm not working on it more right now. But nobody is going to pay $1000 for one of these. Putting 4558 op amps back in makes enough hiss to hide the hum, that's how low it is. Looked at fairchild 5532 op amp data sheet, it has good input noise numbers, but not as good as 33078, 4560, 4562. Lot slower then all those too, so power supply bypass cap and feedback resistor bypass cap not as important on 5532. I'm listening on horn speakers to top octave piano + occasional brush cymbal, so any HF intermodulation distortion will show up. I like the 33078 and next time I put an order in I'll try 4562- they should plug right in the dip package.
 
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