Great River MP-2 , info, schematics & discussions

[riggler]

Hi all,

I've figured the load resistance of each card to be in the neighborhood of 350R based on Dan's comment of a current draw of 40mA per card.
Will I be okay with my 25-0-25 transformer? I compute that the cards will see about 34V from my PSU, which will be of the design pictured in page 23 of this thread.

How did I come to that conclusion?
50V out of the transformer.
Lose 1.4 volts due to the bridge rectifier -- 48.6 volts.
Smoothing caps increase the voltage by a factor of 1.4. -- 68V
Center tap splits it in half. -- +/-34V

350R load drawing 80mA (2 cards to power) = 28V

Will the regs be okay with this or do I need to pad it down with some power resistors? If so, can anyone confirm that my methodology is correct?

To compute power resistors to get down to the desired couple-volts-over-24-volts, lets say 26, do I need to compute my drop from 34V(unloaded) or from 28V(loaded)?

Sorry if I am making this too complicated but I really want to learn the math.

 

[slowbass]

Hello,

I've just finished a mp-2 and after a bit of troubleshooting, it works nicely ! I'm really happy and ready to record.
My little problem was with the Hi-Z jack wiring.
I based myself on the useful pdf in this thread "MP2 wiring v2" but it didn't work perfectly for me. It's written to connect the input jack to the PCB in this order (J4) :

1 GRD
2 +
3 -

which I understood to be Sleeve / Tip / Ring (or am I wrong ?)
I had to reverse tip and ring to make the preamp work (and to short + and - when no jack was inserted in the plug as mentionned in another thread).
Have I miswired something or is the diagram wrong ?

Anyway, maybe that will help someone...

Bye !

 

[muffy1975]

OK team,

i can't find any info about how to wire the output transformer.

can someone post the method and picture if they have time. It's an undocumented part of the build.

I'm i the UK and want to order a carnhill 1+1:1+1 trannie ( cos they are local ). Is this the same as a 1:1 trannie?

Thanks for any help or direction

Michael

XOX :'(

 

[muffy1975]

Hi gang,

Can someone post a picture of the front of an original mp-2? I can't seem to find any online.
Liking my pres so much that I have started to hate my DIY front panel and what a better image for the pres

Michael

 

[riggler]

FYI -- The appropriate crimpers for the Molex KK 2759 terminals in the parts kit:

Molex 2262A
Sargent 3135CT
Sargent 1026-CT
Sargent 1028-CT

 

[okgb]

there are points marked bal and unbal
[ almost the same thing save an isloation resister ]
on the board . be sure of the polarity of the transformer leads
[ one side of the primary to ground ]
As i've said many times , works and sounds quite fine
with no output Xformer if you like

 

[Ptownkid]

I usually just solder the wires to the pins and crimp with a pair of needle nose...

 

[riggler]

Can I get away with a Jensen JT-11-DMCF output transformer? It's the only thing I can get to fit in my ParMetal 1U case...

http://www.jensen-transformers.com/datashts/11dmcf.pdf

On the output header:

Pin 1: To Ground Buss (Or should it go to XLR pin 1?)
Pin 2: (20R) To Transformer Primary (Brown)
Pin 3: To Ground Buss (Or should it go right to the 1/4" jack - sleeve terminal?)
Pin 4: (Wire link where 10R was) To Unbalanced Out + 1/4" Tip

On the transformer
(Brown): + from output header pin 2
(Red): - to Ground Buss
(Orange): To XLR Pin 2 +
(Yellow): To XLR Pin 3 -

Should output XLR pin 1 be from ground buss or from card output? Currently ALL xlr's I have pin 1 strapped to chassis ground.

Am I doing this right?
Thanks!

 

[speiris]

Hello all.

I want to make the peak LED comparator circuit for the MP2.

Can I just modify the circuit from the PSU Schematic to work only with one channel instead of the 4 channels supported in the provided schematic?
If so is there anything special I need to do or can I just "chop" off the extra LM339A part of the circuit?

I attempted this last night but the overload LED was constantly lit.  This was the case when the comparator circuit was connected to the preamp board and also when it was not connected to the preamp board.
Mind you I still don't have an input transformer connected and nothing is connected to the preamp other than power of course.  Apart from that the board is complete (excluding of the 1/4" Hi-Z input socket and the ELMA rotary switch).

The comparators are being powered by the voltage regulation circuit found in the PSU schematic (approx +/-14.65V).

Must I build the circuit to support 4 channels (identical to the schematic) in order for the peak LED to function correctly, even if the PSU is used to power only a single MP2 channel?

Any help would be much appreciated. 
Sriyan.

 

[Dan Kennedy]

You can certainly build a reduced version, but you need to keep the
voltage reference parts that are common to larger version.

Can you show a schematic of what you built?

 

[okgb]

For the d.i.   input check that tip & ring are not reversed
if mic input works then the switch is making connection
and 1/4 doesn't work then it breaks to the wrong point
[ going to xfmrs secondary instead of preamp input ]

I'd think any 1:1 output would do , it's just isolation
but high nickle sounds like a nice option [ Let's ask Dan ]
on my Quad i use the first two pres with the jensen outs
and the 2nd two pres unbalenced , partly cause that's the way they're
hooked to the XLRs and there are no xfmrs on the 2nd pair output
I use them this way for allot of classical recordings often through 100' of snake or so
Drum overheads , anything really also makes a good bass d.i.

Riggler , if you use a heavy peice of copper along the xlr's & 1/4"s
this will be your ground buss [ connecting to pin 1 at the same time

 

[speiris]

Hi,

Just wondering if using a Lundahl LL1538 for the input transfomer is ok (as opposed to the LL1538XL)?

Thanks.

 

[vgdb]

Hi,

Just wondering if using a Lundahl LL1538 for the input transfomer is ok (as opposed to the LL1538XL)?

Thanks.

Hello Speiris,

I asked the same question to my local Lundahl dealer here in France. His answer was that the two transformers do not sound exactly the same. The LL1538XL version should sound "a little thicker, with a little more of material". So I did command two other 1538XL...


Hope this helps,


Vincent

 

[okgb]

The xl has a little more bass handling ability  ,
not sure if you'd notice in " average use "  BUT
they are the ones i used and am very happy with
[ i wouldn't want to mess with happiness ]

 

[vgdb]

they are the ones i used and am very happy with
[ i wouldn't want to mess with happiness ]

I forgot to say I already used two of those 1538XL with a lot of pleasure too... I bought two others because I am now building the quad preamp version, so you can imagine I am very happy with these transfos and with this preamp - thanks to all who helped and to Dan once again!

Vincent GdB

 

[speiris]

You can certainly build a reduced version, but you need to keep the
voltage reference parts that are common to larger version.

Can you show a schematic of what you built?

Hi Dan,

Attached is a copy of the schematic which I've used to build the overload circuit.  It's the same as your original PSU schematic, with some connections not made.

I'm using an LM339-N quad comparator.   From its datasheet it seems to be interchangeable with the LM339-A.

Since my last post I added R14 and R17 to the mix.  This seems to have done some good.   I've plugged in my guitar to the Hi-Z input and managed to get the overload LED to light up.
However when it does turn on it stays on.  Then when another overload occurs it turns off again.   Sending a very loud signal into Hi-Z seems to toggle the LED's on/off status.
By tapping my guitar pickup I'm able to turn the LED on.  Another loud tap and it turns off.   Tap... on again and so forth.
There is no mic input transformer connected yet but I'm getting proper sound out of the Hi-Z.

Is there something I've missed?  Any help would be greatly appreciated.

regards
Sriyan

PS.  I read somewhere that the LED lights up at -3db?  Is this value correct?

 

[pacemaker]

Hi Guys,
i just received boards i bought from another member,
and i'm trying to get all the documentation on this project,
i couldn't download Frank's wiring diagram,it seems to be gone
can anybody send me a link or send it by email ?
i'm also looking at a psu PCB,
does anybody sell them or is there a pdf document so etching can be possible ?
or maybe i can use peterc PSU , just have to verify if it can handle +/-24v
Thanks for your help,
Francois

 

[Dan Kennedy]

Hi Sriyan,

R13, 14, 16, and R17 are essential, they make the voltage references for the comparators.

Three things are different yet from my circuit, which is admittedly crude but there are hundreds out there.

A bypass cap across the comparator, 10 or 100nF as close as possible to the package. Comparators are
notoriously twitchy, tie them down.

The input tie down resistor for the signal input to the comparator. This isn't strictly needed if the connection
is made directly to the audio board, but it stops the LED's from coming on if the input is disconnected, the comparator
needs to be held down so the input won't drift around.

Third, tie down unused parts in the package. Both inputs of each unused comparator should be tied somewhere, the
parts are unpredictable when floating. Ground (0V) is convenient.

 

[speiris]

Hi Sriyan,

R13, 14, 16, and R17 are essential, they make the voltage references for the comparators.

....

Hi Dan.

Thanks a lot for that information.  It seems to be working now!

Just two more things if you don't mind:

1)  I tried to measure the reference voltages at their respective comparator inputs (PINS 4 & 7 with respect to 0V) and I measured -1.8V (at PIN 4) and approx. 9V (at PIN 7).   Does this seem right to you?  Am I measuring at the correct points?    

2)  I have a multimeter which can display voltages in dB.   If I wanted to measure the INPUT signal dB level would it be correct to put the multimeter into AC mode and measure the voltage on the board's signal-wire with respect to 0V?  What dB level was the overload LED on the MP2 designed to light up at?

Thanks again for taking the time.

regards,
Sriyan

 

[pacemaker]

hi Guys,
i could finally download frank's wiring diagram,
do know why but his website is very very long to appear in my browser,
sometimes not showing at all.....
and i'm a bit confused about the polarity impedance switches wiring ,
can anybody enlight me with a small explanation ?

while looking at Dan's post about cheaper Edcor transformers
but not well shielded, 
i saw on Edcor website ,the MXL5cs mic input transformer :
http://www.edcorusa.com/Products/ShowProduct.aspx?ID=352
this one seems to be good in this circuit ,am i right ?

i'm still trying to understand the power supply,
and it seems that if don't use the overload circuit,
i can use PeterC green PSU for this project,
any clue  ?

sorry for being such a noob......
Francois