Is this Design practicable? - diyAudio
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Old 6th February 2011, 07:04 PM   #1
MaxK is offline MaxK  Germany
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Default Is this Design practicable?

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:

Click the image to open in full size.

Oh, and don't mind my crappy drawing please
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Old 6th February 2011, 08:36 PM   #2
MaxK is offline MaxK  Germany
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In my first drawing I forgot some components on the output side, so here's another one.
(Hopefully error-free this time)

Click the image to open in full size.
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Old 8th February 2011, 10:43 AM   #3
AndrewT is offline AndrewT  Scotland
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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.
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Old 9th February 2011, 04:20 AM   #4
Bonsai is offline Bonsai  Taiwan
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Download LTSpice and put your circuit in and play around a bit. There are a lot of strange things going on - I doubt it will work as it stands.
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Old 9th February 2011, 11:38 PM   #5
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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. diy dj mixer 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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Last edited by indianajo; 9th February 2011 at 11:59 PM.
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Old 10th February 2011, 01:37 PM   #6
MaxK is offline MaxK  Germany
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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!

Click the image to open in full size.
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Old 10th February 2011, 04:00 PM   #7
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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).
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Old 10th February 2011, 04:07 PM   #8
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Stainless steel should keep out magnetic fields. It is only steel with a bit of chromium added.
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Old 10th February 2011, 06:38 PM   #9
wrinkle is offline wrinkle  United Kingdom
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Quote:
Originally Posted by Andy5112405 View Post
Stainless steel should keep out magnetic fields. It is only steel with a bit of chromium added.
Just try hanging a magnet onto a stainless steel panel first, then think about it a bit when it falls off.
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Old 10th February 2011, 07:05 PM   #10
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Many grades of stainless steel are magnetic. It is when nickel is added (generally higher grades) that it becomes non-magnetic.

With that said, magnetism is probably not causing the hum. That is most likely grounding.
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