Great River MP-2 , info, schematics & discussions

[okgb]

Hey Nice Dan  , you should make those . i bet they'd take off !

jp triple check

 

[Dan Kennedy]

When adjusting for R46, I first power the boards up, and see that the
offset is less than about 15mV, and frankly I forget which polarity.

This proves that the FETs are a reasonable match.

Then insert the trimpot in for R46, adjust for zero offset, which you can't
do because just breathing on the board will move it a little, but you can
get it trimmed into less than 1mV pretty easily.

Remove and measure the trimpots, then replace with the closest 1%
resistor you can get.

It's not super critical, the offset won't get any worse, and anything under
5 or 10mV's of offset isn't going to cause any problems.

 

[Sredna]

I soldered a trimmer (multiturn) directly on the PCB.
Its indeed a very sensitive task, breatholding! 


Any objections against that?


/Anders

 

[Cpartipilo]

I think the only objection would be that the trimmer might not be as accurate as a resistor than Dan required in the design.  Look that he specifies a 1% tolerance resistor.
I'm sure there are trimmers that accurate i just don't know if it's worth doing once you know the value of the resistor.

 

[Spendor]

Hi there - I'm using Lundahl 1538XL transformers and want to confirm a few things. There were no 475 ohm resistors supplied with the kit but if using the DI inputs should I have them in? Should I lose the zobel R7 and C20 and replace R6 with 120K? Just wanted to confirm the value of R8 - the schematic reads 562k while the printout and supplied resistor value is 5R62?
I also have a pair of Jensen JT-11-FLCF output transformers - would these be suitable for this preamp?

Thanks!

 

[okgb]

I have a quad made from my first pair with the 1538 xl
which i think are great , Dan had said name recognitian
was part of the jensen choice .
no need for the zobel with the Lundahl , just remove or omit it
one of my pairs has jensen outputs  [ if your's are 1:1 should be fine ]
it's a nice option , seems to add a little extra charactor [ i think ]
but the pre sounds great without it as well .

not sure about your resister , was this from a kit ?

 

[Spendor]

Thanks for the reply - yes this is a kit from PTown. Not sure why the 475 is left out in the kit. When you used the 1538XL did you change R6 value or keep it at 42k2? Still wondering about that resistor discrepancy too....

Cheers

 

[dissonantstring]

hi splendor,
i haven't built my MP-2 yet, but i do believe you'll need the 475R resistor (R9) which should be an easy value to find locally.
the schem shows R8 as 562K.  not sure if you are referring to R45 which is 5R62.
best regards,
grant

 

[Svart]

I just had to drop in and say that Dan is awesome for letting us build this project.  I've been following it for a while but I don't think I'll get around to building this project but that doesn't change the fact that Dan is one awesome guy for letting us do it.

Thanks!

 

[dissonantstring]

I just had to drop in and say that Dan is awesome for letting us build this project.  I've been following it for a while but I don't think I'll get around to building this project but that doesn't change the fact that Dan is one awesome guy for letting us do it.

Thanks!

+1000 on that svart!

 

[Spendor]

Thanks Dissonantstring - you're absolutely right - I was getting them mixed up there. Very close to trying this beast out - lets hope there's no smoke.
Cheers.

 

[okgb]

When i was building my mp-2 and mentioned to Dan
that i was using the Lundahl he had said just to remove the
220pf  [ thus taking r7 out of the circuit as well ] and that is what i did
i would think all others should remain as marked in the schematic
never had any problems with mine this way .

triple check though as there are no more f.e.t.s available
[ unless you can dig around on the internet ]

 

[evm1024]

I've got my MP2 up and running with little drama. The case is a leftover that is not so nice. But it is functional. I also have hum issues and interactions with hum and relay positions. I'll have to go after that in the coming week. I read the prior posts on hum with interest.

I've tried to use the buss wire between the input XLR as the single point ground. The transformers are strapped down but have a layer of plastic for isolation. Nylon bushings for PCB mounting.

THe Toroid has quite a field near it. Perhaps a little shielding there. And perhaps shielding (one side grounded) to be added for DI and transformer wiring.

Anyway, I gather that at this point I should pull pieces out to the chasses to see if I can locate a ground loop and to verify that there is no loop in the wiring harness.

Any comments are most welcome.

Regards, Ethan

 

[Svart]

almost everytime I investigate hum in a device I find that grounding has a loop or grounding between two areas has a high impedance.  Generally I will take a thick wire, 12ga or so, and ground it at the star ground and then probe with it to different ground areas of the boards looking for changes in the hum.  usually I can find an area that kills the hum and then I fix the grounding to that area.  A lot of the time I find that grounding the boards to the chassis(scrape the paint away at these points to get a good ground) helps more than trying to truly isolate and star ground.

Good luck!

 

[3nity]

EVM those switches are so bad i suggest you change for something else.
I have a friend who has built a GR too and had issues with those switches.
Solder lugs start to get loose when solder iron ...

 

[Bluzzi]

That would be me for the bad switches. I was having some weird buzzes depending on the switch combinations. After some tests I determined the switches were bad! In my case I had these switches from a long time ago when I worked for an electronics distributor. They were samples and in retrospect maybe samples were taken from "bad" product that did not pass inspection.

So it does not mean that your problem has same cause but check switches anyway.

Other than that my GR's sound as clean as mountain dew.

Jim

 

[vgdb]

Bonjour à tous,

I am completely new to this post/site, please excuse my poor english (it takes a long time for me to read and write all this litterature, I'm french).
I would like to build my own preamp because of economical reasons and also because I like to understand the inside of things... and I discovered your project very recently.

I have identified three projects on the net that I find very interesting : the Green Preamp, a copy of the tube Pultec MB-1 and I discovered yesterday (by searching green preamp kits in fact) this GR MP-2. I saw that the whole parts for this project are available on the DIY Parts Kits site and I suppose it is connected with this thread? This is a BIG amount of time spared for a newbie like me who doesn't have all the networks established for providing parts and so on  :'( and I would appreciate spending most of my time in understanding and constructing rather than searching parts on the net for my first project...

My question :
- is this DIY project "normally" accessible as a first DIY "big" project (I already constructed magnetic and piezzo mics + buffer preamp for my cello and I know a DIY project is always requiring a huge amount of time and pain!).
- I saw this subject already discussed at the beginning of the post, but I suppose I would not be mistaking if I choose to build the GR MP-2 rather than the Green Preamp  :-\?
- I hope I will have the opportunity to buy the whole preamp kit (I emailed on the site yesterday for availability and contact). Could you tell me if there are "hidden costs" other than the input/output Xfrmrs and PSU and box?

Thanks for taking care of my questions and congratulations to everyone who allows this type of project to exist.

Best regards,

 

[evm1024]

I got some quality time with my preamp tonight. I tried heavy gauge wire to no avail. However, I did notice that when powered off the hum goes away and I'm left with the typical hiss and hash you would expect from a high gain (unterminated input) preamp. This lead me to poke around in the PS. I found the +48 caps to be very sensitive. Touch the can with a vom probe and you get much more hum. THe PS is a standard bridge used to get + and - from a center tapped transformer (off to some caps). Plus the AC legs are run to some electros which feed a voltage doubler that then is regulated down to 51 volts so that the onboard 48v reg can do its job.

This is the JLM circuit minus the + and - regulators.

I'll be looking into the PS as a source of the hum. Perhaps disable the +48 and then use a real JLM regulator board and see what that does. Or perhaps there is a gound loop there.

Regards, ethan

 

[Sredna]

Hi vgdb!

I would say that the MP-2 (in kit form) is a medium level project.

If your soldering skills are ok and you're checkin the components (values)
twice before soldering them, this is a nice project.

Beware that the double FET (the heart of the circuit) is no more easy available,
better get it right the first time... (there is one in the kit of course!)

The only hidden cost I could see from your list would be the ELMA switches.
You can get them from ptownkid (DIY Parts Kits) as well.

Output transformer is optional so you can save some money there...

Good luck,

Anders

 

[diyfanatic]

I'm french

it's ok nobody's perfect...and you're not alone! 

the green pre is really basic to do, even more if its a kit!
the gr2 will cost you more in part (output tx) than the green who is transformerless!

depending on what type of sound you need (clean or colored) you might consider the fet boy too!