anyone know anything about this cheap 8 channel mic pre?

[tmbg]

wonder if it's got a decent circuit that could benefit from better op amps and caps, or at least be a good chassis and power supply

8 channel mic pre

 

[NewYorkDave]

There was a whole thread, several pages long, about that very product. I'm not having much luck finding it right now, though.

 

[tmbg]

manual claims it's designed around the NJM2068, which I know nothing about, hooray!

 

[NewYorkDave]

Wait... I found it. (Man, I love this messageboard, but the search function is pretty hit-or-miss).

http://www.groupdiy.com/index.php?topic=344

 

[PRR]

SM-8 thread

It is a half-assed design. Based on good concepts, but fouled-up in details. Probably OK box and power supply. Possibly a fine amp with several critical changes, but maybe not worth all that work.

 

[SonsOfThunder]

Hit the brakes, man...Back up!

We discussed the PR-8. The PR8mkII has several things making it different! Not to say that it so largely different, but different nonetheless. I don't have the schem here with me now, but I can send it out tomorrow.

I think this does use the 2068 which is actually a good opamp IMO. The gain is done differently and the output section is not cross-coupled like the PR8. More like the Green on the output section.

HTH!
Charlie

 

[wiz1der]

interested.

 

[SonsOfThunder]

Ya, sorry I haven't posted the schem yet, it has been a crazy week, last week and this one too.

I need to redraw the schem as is is quite confusing in its current form. I'll try sometime this week. I think the design of the mkII will be more to the liking of those hear.

Peace!
Charlie

 

[wiz1der]

Found some info on the op amps in this thing.

www.njr.co.jp/pdf/ae/ae04022.pdf

I dont know what I'm looking at, but maybe just a simple swap out of these things will do some good?

 

[Carl_Huff]

Did I miss the schemo????

 

[SonsOfThunder]

Ran into a snag with redrawing the schem. There is a missing connection in the middle of the circuit and there is a pot that is marked as ganged, but I don't think it is. Don't have a unit to compare it with :sad: otherwise, I could have it done.

I have an email in to them asking for a clarification. Sorry for my delay.

Charlie

 

[adrianh]

look for posts on SM8 it is the single rack space one.

Not a bad design for under USD$12.00 a channel.
If you can do better than $12 knock your self out.

Yes PRR is right.

If you want a chalenge then this is it.
I will have this thing sounding better than my Focusrite Octipre's when I am done.

Will the Octipre be cheaper. Yes I am afraid so.
I am going to open the hood and throw money into a
PR8 and yes it will be very good.
Why because I want too.
I look at it as I bought a finished case and power supply for $99
now I got to put the guts in the box.

 

[wiz1der]

hey adrian, what exactly are u planning?

would this work better on the mkII? I heard there was better stuff in there, and it's 2u.

 

[adrianh]

I need to use one rack space 8 channel designs
for my studio.

on the SP8;
Upgrade all caps to Panasonic FC series or better.
Heatsink phantom regulator.
Remove all the op-amps and throw them away.
Replace the input opamp with better Burr Brown part
Remove the complete output board and throw it away.
Not sure how I am going to solve the power supply issue now.
Add a FET source follower driving a Lundahl output transformer
to replace all that crosscoupled output op-amp stuff.
Solder wire to front panel XLR and run then out the back to pigtail XLR's

The design will be much simplier diff amp input, 2 stages of op-amp, fet follower, transformer.

Just remember I will have 3 times the cost of the mic pre in output transformers so that will show you how crazy I am.
I am going to love this challenge.

 

[wiz1der]

wow!! sounds interesting. I just bought one off ebay on friday for 60.00

will you post progress and pics as you go along? or better yet, a parts list?

I think I may want to follow along with you, but i am a newbie.

 

[adrianh]

wiz1der;

I will do that.
It may take me several weeks to start as I am up to my
neck in cable and bantam patch bays at the studio.

The simple stuff just follow right along.
It will be what 99% of the guys here would do to upgrade the unit.
The output transformer mod is more radical.
I am going to mod the unit in stages.
i.e. CH1 stock
CH2 cap mods
CH3 cap mods plus op-amp upgrade.
CH4 cap mods plus op-amp upgrade plus .........
you get the picture.
After that is done I will let sonics guide me along.
One of the channels will give a sonic payoff vs. the dollars spent.
Then I will have my answer.

Cool buy for $60

 

[SonsOfThunder]

You guys are pretty good at coming up with new names for products... so far we have the SM8 and the SP8 :razz:

I thought the thread started on the PR8mk2:

then it moved back in time to the PR8:

or if you prefer the original graphics:

So what are we talkin' 'bout here?
Still waiting on a response about the schem.

 

[wiz1der]

[quote author="wiz1der"]wow!! sounds interesting. I just bought one off ebay on friday for 60.00 [/quote]

Too good to be true. I got it yesterday (PR8) , plugged it in and....... NOTHING.

No power whatsoever. All the screws were loose like the guy was inside it. I guess this will be good to get my feet wet in DIY!

I did notice that on the AC power connector, there are only 2 of the 3 leads that have wires connected. Should all 3 of these go to something?

there are 2 xformers, I guess one for 110 and one for 220?

I didn't see any fuses anywhere. I DO have a multimeter, any suggestions on where i should start poking around?

Thanks

 

[SonsOfThunder]

@ wiz1der:

Hey, there should be a fuseholder integrated in the IEC connector. It opens like a little drawer. Take the power cord out and you should see a notch where you can pry the "drawer" outward.

Have any of the chips been changed? You might have a bad modification job or just a bad 3-terminal regulator on your hands. Check for shorts on the TO-220 parts marked "7815" and "7915" (on heatsinks near the transformers). There is another regulator for the 48V (with no heatsink), check it too with a multimeter! Get some extra fuses and after you stick the first one in, you might try disconnecting the four pole connector that goes to the front PCB.

There should be a ground wire to the chassis on one of the power inlet terminals.

One trafo supplies +/- 15V, the other is 48V phantom...

Post again if none of this helps. You should be able to get this thing running again. There isn't that much to it!
HTH!

 

[wiz1der]

Hey thanks,

I do have 120v after the inlet at the power switch and 110/220 switch.

I checked the lines coming out of the xformers (2 blue and one black) and (2 white wires ), which I;m guessing should be either the 15v and 48v but i have 0v there.

Can the 15 v tranny be out?

I will check further today after reading your post.