xefe

mixer preamp mods
« on: April 15, 2009, 10:11:49 AM »
Hi!

First time poster! Love this forum!

Here is the schematic for the preamp section of my Phonic MR1843x mixer.



http://img145.imageshack.us/img145/416/preamp.jpg

Can somebody tell me what are the obvious (if any) procedures to improve the sound of the mixer preamp? I only use the preamp section as i use the insert jack as direct out to a Terratec Ews88 mt. Opamps, transistor, resistor or capacitor swapping or adding/removing?
Waste of time? bad design?

Thanks everyone!
« Last Edit: April 30, 2009, 11:44:43 AM by xefe »


Re: mixer preamp mods
« Reply #1 on: April 15, 2009, 06:39:59 PM »
10uf coupling caps seem kind of small, especially since there are quite a few of them. I could be wrong. What would the 3db point be for the low pass filter formed by c105a and r114a? I'm terrible at math.

keefaz

Re: mixer preamp mods
« Reply #2 on: April 15, 2009, 07:18:40 PM »
Welcome  :)
c105a and r114a form a hi pass filter (or low cut) if I am not mistaken, -3db point is at 3.38Hz freq

pstamler

Re: mixer preamp mods
« Reply #3 on: April 16, 2009, 12:43:09 AM »
C116 could stand to be bigger. As it stands, at maximum gain the low-frequency cutoff is about 30Hz. (But will you be using it at maximum gain?)

The opamp could probably stand to be upgraded.

The big problem, though, is that the common-mode rejection is probably pretty poor. That would require fairly extensive redesign.

Peace,
Paul


oragall

Re: mixer preamp mods
« Reply #4 on: April 16, 2009, 12:44:48 PM »

The big problem, though, is that the common-mode rejection is probably pretty poor. That would require fairly extensive redesign.



can you explain  the common mode rejection ???


xefe

Re: mixer preamp mods
« Reply #5 on: April 19, 2009, 10:20:23 AM »
Do you think that changing c101a and c102a to 100uf/63v, c105a and c104a to 47uf/25v, c117a and c121a to 100uf and c116a to 1000uf will do something for me, at least in theory?

The pre's are quiet, just a bit harsh and thin sounding, and sterile.

Can i completely remove c104a and c105a?
« Last Edit: April 19, 2009, 10:23:44 AM by xefe »

xefe

Re: mixer preamp mods
« Reply #6 on: April 28, 2009, 08:44:41 AM »
 ???

tv

Re: mixer preamp mods
« Reply #7 on: April 28, 2009, 08:53:54 AM »
You _can't_ remove C104a and C105a. From the schematic, I think that 47uF would be a little low for them - try 100uF?
If you sprinkle when you tinkle, please be neat and wipe the seat.

xefe

Re: mixer preamp mods
« Reply #8 on: April 28, 2009, 11:15:55 AM »
You _can't_ remove C104a and C105a. From the schematic, I think that 47uF would be a little low for them - try 100uF?

But isn't phantom power decoupling already being made with c101a and c102a? c104a and c105a look redundant to me... Can you explain?

Samuel Groner

    Z├╝rich, Switzerland
  • Posts: 2935
Re: mixer preamp mods
« Reply #9 on: April 28, 2009, 11:28:32 AM »
Indeed redundant, except that perhaps the PAD might click a bit due to bias currents flowing through it. Shouldn't hurt too much though I'd say.

Samuel


xefe

Re: mixer preamp mods
« Reply #10 on: May 01, 2009, 06:45:33 AM »
Already changed c102 and c101 with 47uf/63v (100uf did not fit in the space), bypassed c104 and c105 with shunts (to remove them from the signal path) and changed c116 to 1000uf/16v. I can already hear improvement.

Im with the mixer open just for one mor day! I will need it done by tomorrow. Any last ideas?

Any ceramic cap that i can remove to open the frequency response?

MartyMart

Re: mixer preamp mods
« Reply #11 on: May 01, 2009, 06:56:38 AM »
The 10n caps at the opamp power supplies to ground look a bit small, usually these are 100n caps.
They should be fitted as close as possible to the opamps.

MM.
"Success is the ability to go from one failure to another with no loss of enthusiasm"


 

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