Akai MPC3000 Output Boost Mod

Thread Starter

fknchino

Joined Apr 18, 2017
3
Hi guys!
This is my first post in the forum and I'm very happy to have stumbled across this resourceful site/forum.
In short, I'm an Audio-head and I've been lightly delving into the world of DIY/electronics.
That being said I'm very new and I still have lots to learn... So I'm asking for a small favour..

I have an old Akai sampler (mid 90's) that I use regularly.
I figured that maybe if I swapped a few resistors I could make the output stage louder.
Although I am handy with a soldering iron, I really wouldn't have a clue as to which/what part to use or if this would actually work.

If anyone can help me or give me some advice I would be grateful..

http://www.vintagesynth.com/akai/mpc3000.php
http://www.synfo.nl/servicemanuals/Akai/MPC3000_SERVICE_MANUAL.pdf

Might be a piece of cake for some of you guys ;)
Cheers
 

Thread Starter

fknchino

Joined Apr 18, 2017
3
Hello,

It looks like the output stage is made around IC113, IC114 is a separate output for the phones:

View attachment 124974

Bertus
Hi Bertus, thank you for the reply.
Since I had to re-write the post twice (Silly mistake) I forgot to mention I was actually at the 8 Individual Outs ( 8 INDV ) section which is under that board.
The resistors are fixed, no pots to adjust the gain.
I'll make an edit of my O.P and make sure I get the details correct.
Appreciated.
 

bertus

Joined Apr 5, 2008
21,850
Hello,

Each channel of the mixer output stages uses a double opamp:

MPC3000_mixer_output_stage.png

The opamps 313 to 320 are used in those stages.
As the output is made in a kind of balanced way, every time two resistors need to be changed.
In the shown stage the resistors R397 and R405 (15K each) are setting the gain of this stage.
Raising the value will make the gain higher.

Bertus
 

Thread Starter

fknchino

Joined Apr 18, 2017
3
Hello,

Each channel of the mixer output stages uses a double opamp:

View attachment 124977

The opamps 313 to 320 are used in those stages.
As the output is made in a kind of balanced way, every time two resistors need to be changed.
In the shown stage the resistors R397 and R405 (15K each) are setting the gain of this stage.
Raising the value will make the gain higher.

Bertus
Thank you, Bertus
I have a few more (newbie) questions..

I know it's not that simple but, since you mentioned a 'kind of balanced way' is it possible to make these outputs balanced?

Long story short, the unbalanced outputs are supposed to be going into a balanced db25 input.. as you can imagine I first had problems trying to wire up 8 channel 1/4" TRS connector > DB25, both grounded on either side.. phase, ground hum, you name it.

So now my plan was to cut off the negatives on the connector side and get the output as close to +4dBu to compensate, hence the resistor values..

What resistor value should I use to get a +6dB output? Do I replace and raise the value of the resistors on both opamps? 313/320 e.g 15k --> 30k?

Also lastly. I have a strange problem on channel two, i've linked you a sound bite below.
https://soundcloud.com/fknchino/808-mpc-indv-out-2

The sound is meant to be a clean bass drum, but the sound is distorting in a weird way.
I'm hoping that it's not to do with the D/A as all other channels are fine.
If you could suggest a way to troubleshoot I would very much appreciate it.

Thanks again.
 

bertus

Joined Apr 5, 2008
21,850
Hello,

2dB is about a factor of 1.6.
When I use the 1.6 factor the 15 K would become about 24 K.
See the attached PDF for decibel calculations.

Bertus
 

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