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General Discussions => The Lab => Topic started by: xefe on April 15, 2009, 10:11:49 AM

Title: mixer preamp mods
Post by: xefe 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)
http://img145.imageshack.us/img145/416/preamp.jpg (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!
Title: Re: mixer preamp mods
Post by: hakanloob 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.
Title: Re: mixer preamp mods
Post by: keefaz 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
Title: Re: mixer preamp mods
Post by: pstamler 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

Title: Re: mixer preamp mods
Post by: oragall 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 ???

Title: Re: mixer preamp mods
Post by: xefe 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?
Title: Re: mixer preamp mods
Post by: xefe on April 28, 2009, 08:44:41 AM
 ???
Title: Re: mixer preamp mods
Post by: tv 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?
Title: Re: mixer preamp mods
Post by: xefe 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?
Title: Re: mixer preamp mods
Post by: Samuel Groner 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
Title: Re: mixer preamp mods
Post by: xefe 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?
Title: Re: mixer preamp mods
Post by: MartyMart 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.