Great River MP-2 , info, schematics & discussions

[tonedude]

Is there a BOM around for the kit that sold here a couple of years ago?

 

[okgb]

Shouldn't be much different than the " official " one
check the documents section.

 

[tonedude]

Shouldn't be much different than the " official " one
check the documents section.

Hmm... I guess I'm missing something, because I don't seem to find a BOM in the doc section?

But I did find this one in the beginning of this thread: http://www.matt-allison.com/diy/greatrivermp2/MP-2 bom.txt

Thx, guess I found what I'm looking for.

 

[tonedude]

A first step...

... two stepped gain switches!  ;D

8)

 

[okgb]

Nice work the original G.R.  MP-2's had a small pcb with smd resisters mounted for the switch,
and although not recommended you could use a 2k pot instead [ or in a pinch ]
I looked and can't find a bom, so I guess that's why there is the one you found, thanks !
[ And matta will send it, so I'll include on the tech doc's page ]

 

[dmp]

Just about finished with mine.  Need to keep an eye out for Jensen outputs to put in for the balanced out, but will try the unbalanced and see how that works.
Trimmed the output on both channels to about 1 mV without any problems.

 

[okgb]

I used an unbalanced out version for a long time until I made a quad , and added Jensen outputs
the difference seemed obvious directly compared but NOT if used individually , still both are nice
options to have . Also the Cinemag output is totally good and a little smaller [ I think ]

 

[tonedude]

Looking at the PSU schematic I notice that the 30VA (?) transformer seems to be good for powering a quad of mp2's, so 15VA should be enough for powering a duo?

I searhed this thread for info about power consumption per MP2 channel but didn't find it, does anybody know how many mA one channel needs?

 

[tonedude]

Ok, found it! 

14va is fine.

The boards draw about 40ma each.

In a quick test with a 22v transformer and a variac to run it up some a 47 ohm resistor in each leg is about
right to drop the resultant dc at the filter caps to about 40 volts, which is fine for the 317/337 pairs.

You'll need 5w rated resistors, they'll get warm.

There are a lot of variables, this is a reasonable place to start.

Put a 20 ohm 2W resistor in the feed to the voltage doubler, this will drop it down some, you might want
to put a load across the filter cap at the output of the doubler as well, maybe 2.2K also 2W. This will help
hold it down too.

 

[tonedude]

I'll try to power them up tomorrow. Keeping my fingers crossed for a smokeless experience...

Now its just the PSU and casing on the to-do list...  8)

 

[Sredna]

Be shure to double and triple check the power connection to the boards!

There was a document out there where the + and - were switched, caused  smoke and
tedious troubleshooting  :-\

Good luck!

 

[tonedude]

Be shure to double and triple check the power connection to the boards!

I even traced the PCB twice to really be sure that the positive side of the PSU did go to the 317... and then when applying power there was.... no smoke. YES!!!

First I checked offset without r46 and it was well below 10 mV on both. Then i did spend some time trying to adjust the offset voltage, I did have to rise the r13 to 51ohm on one of them, to be able to get really close to zero offset when adjusting r46 (offset was a negative value therefore the r13 rise...), but after that I had no problem to get below 0.5 mV offset.

I then hooked them up them up trying to play some bas through the di and both of them did sound great!!!  8)

One question though, is a negative offset a problem or is it not more problematic (as long as its low and close to zero) than a possitive offset?  (I guess its not but thought that I better check...)



Thanx Dan, Greg, Ptown for making this DIY project happen, and thanx to all of you guys contributing to this thread!!!

 

[tonedude]

Testbench... 

 

[tonedude]

Getting closer...

 

[dmp]

My pair has a pretty noticeable buzz when using the unbalanced out. I don't have transformers in yet for a balanced out.
Other than that they work correctly. I was able to null the output to about 1 mV with R46.
I used a copper rail along the back XLRs to a chassis ground point, connected the power ground to this point as well, and the 3rd prong of the power cord.

 

[tonedude]

My pair has a pretty noticeable buzz when using the unbalanced out. I don't have transformers in yet for a balanced out.
Other than that they work correctly. I was able to null the output to about 1 mV with R46.
I used a copper rail along the back XLRs to a chassis ground point, connected the power ground to this point as well, and the 3rd prong of the power cord.

Without being an expert in any way, I can tell you how I did the grounding.

Ground on the PSU goes directly to the power ground on PCB's, and also directly to the copper bar, separate wires. The copper bar then grounds to the chassis, bolted. Ground on all in and outs connects to to the copper bar (... except for the shielding of the transformers, that connects to ground through the PCB's...).

I have input and output transformers, and also the direct out. No buzz or hum on any of the outputs.

/ Mattias

 

[dmp]

OK thanks. I'll put it on a scope and see if I missed anything on the grounding.
It's loud enough that there must be something pretty wrong.
I etched my own rectifier board so maybe that has a problem.

 

[okgb]

It only takes one point to cause a ground loop, I  have a broadband buzz on mine after installing lundhals
on perfboard , I checked and resoldered the perfboard I believe one of the grounds was touching a mounting bolt
and that fixed it . otherwise I have the grounding you describe EVERYTHING  to the copper buss off of the xlr's  board psu to there
as well psu to buss bar , no hum on either output , not sure I chasis grounded that point though. I used one for years with no output xfmr
double the grounding when your fresh and clear headed, good luck .
and check the obvious too , I spent weeks chasing a ground loop in a soldano preamp only to discover it was the isolated input jack
either it moved a millimeter or there was a sliver of metal touching, but similar thing took it off , put it back on , problem solved.

 

[tonedude]

Hmmm, seems as if I wired the LL1524 to be used as a 1:2 transformer (I got the "inspiration" from looking at Igors F76 schematic... ). I figured out that it adds another 6dB to the output, but is there any negative and unwanted side effects with that setup in this context?

- In what ways can 1:2 wiring the LL1524 effect performance of the MP2?

 

[okgb]

What is the difference in impedance to the circuit ? that loading maybe one thing to consider
And it may not be the best gain staging if you're adding more output gain and less from the preamp
in practice it may make no noticeable difference