SM Pro PR8 Mic Preamps

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SonsOfThunder

Well-known member
Joined
Jun 11, 2004
Messages
1,144
Location
Mid Coast (Oklahoma)
Hi All!

I have been lurking here (and at the previous site) for several months now. Haven't registered cause I didn't think I had anything to say of value to this point. But let me start by saying that CJ and Gus are my heroes! My SOP is generally to find things that work "ok" but have some flaw, find the flaw and "remove" it. More m.o.d. than d.i.y. perhaps, though I have a couple of projects that I'd like to share when (if) I get them done.

There was a post "previously" talking about the Nady 8 ch mic pre and finally the one named above. I have two of the SM and have measured, traced, schemo'd, modified, and remeasured and finally used these. While they are not a set of TRUEs or something, I think they are a very usable piece.

I have been in contact with the gent who did the original design work and he has been very conversational about mods. He is currently checking them out himself.

I think you guys would be surprised to see the schemo and its similarity to another project that is very familiar and respected around here. At any rate, I'd assume that some would be interested to have a decent 8 gang mic pre for USD$99 and a few minutes of yer time clipping out a handful of parts.

Could I get someone to host the schemo and perhaps some pics?

Peace! and thanks to all for furthering my "continuing education".

Charlie
 
Hi Peter!

Thanks for your kind offer to host. The Jpg for the schemo is 97k, there are six graphs at ~7k each. I have no pics yet, but I can get those if anyone would like to see.

Thanks!
Charlie
 
Just send them along, those filesizes are nothing. Pics ? Nice, yes please.

Would be interesting to discuss this box, but let's have a look at the schematic first.

Bye,

Peter
 
Thanks.

That's the unmodded schematic ?

I'm not sure if I understand R19. You sure about the stuff around Q2 ?

So no gain-pot but a volume-pot after the first stage. The second stage (U2B) seems to do most of gain (still not sure about the first section).
Did you ran into eventual overload problems ?


Bye,

Peter
 
Thanks for your generous offer Peter, I saw Andy's reply first so I sent them to him...now we can all share! :grin:

I left out the PS, the Phantom Pwr, and the Peak LED... No big surprises there anyway. (need a "yawn" emoticon here!)

I just figured out that I forgot to send some response graphs to Andy...they are in .wmf format. (insert "doh" emoticon here!) I hope he can convert them to JPEG...my machine will convert 3 of them but not the other two. These will show the gain range, the stock freq response, stock THD, and then freq resp and THD after removing some components. You can see from the schemo that it uses NJM4558 instead of the 4580, if you were wondering about this.

I chose 33mV for an input voltage kind of arbitrarily. Perhaps someone knows a better input value to simulate a typical mic output? Anyway the gain was cranked up til I got +4dBu output and then the measurements were taken. Stock you'll see that the unit had some significant rolloff (2.5dB) at 20kHz. I'll show you guys how to get it to -1dB at 96kHz, though I just realized that my graph line doesn't show up in the graph I sent Andy. I'll make some new ones...hopefully in direct JPEG format this time!

btw, I have two of these. One has just one channel with the "removal" mod, the remainder are stock. My other one is in constant use in my studio with this mod and OPA2134 throughout. So I can make any measurement that anyone would like to see.

Comments?

Peace!
Charlie
 
Peter,

"beep, beep, beep" Can you hear that? Its my "back up" beeper. I had not double-checked the schemo. There are no reference designators on the PCB, all of the numbers, I just made up! I had thought the configuration was rather strange myself. I did mean to recheck it but it has been some time since I first drew it and I forgot!
:sad: :oops:
So! I am working on the corrections. I'll have that done this weekend.
The remainder of your assessment is right on. There are no problems with overload that I have experienced. There might however be better methods of controlling the gain (ala the Green Pre and others). I can certainly test for that.

Btw, this means we'll have to have a pic of the PCB for me to tell you what to remove...I've been working on that already as well.

Sorry for my mistake holding up this conversation...See this is why I have hesitated to post anything here!

Peace!
Charlie
 
Q1 and Q2 forms a diferential circuit, and U2A seems like a balanced to unbalanced converter, then a volume control, then a gain stage formed by U2B (gain of 50=100K/2K) which then goes to a unbalanced to balanced coverter circuit formed by U1A and U1B.

I also see 2 supply voltages, +/-12V and +/-15V.

I wonder why Q1 and Q2 are needed? It seems you can connect pin 2 of U2A directly to pin3 XLR (with the C7 coupling cap) and pin 3 of U2A to pin2 of XLR?

Also, the signal from XLRpin3 can travel via C7, C12/R19 straight into pin2 of U2A. Isn't it?

Am I reading this wrong?
 
Sorry I took so long with this guys...

Here's the JPGs of Charlie's that I was able to convert:

gain_range.jpg


thd_output_1k.jpg


thd_output_1k_(mod_b).jpg



Later...
 
Andy, Thanks for posting those. The first graph shows the gain range and you can see the high freq rolloff, -3dB @ 20k...bleh! From the second graph you can see that the THD is actually reduced a bit from just removing the "extra" parts.

Peter, I sent a revised schemo to Andy that has now been double-checked. Its a bit difficult to trace this thing out with out the ref designators...plus its a single-sided PCB and some of the routing is a little crazy. Again my apologies.

Owel, Check the new schemo when its up, and you'll see its basically like the Green (et al) in the front. I like the balanced out config on this better. Its similar to a circuit shown in the app notes for the OP275. Again, I'm busted for not paying more attention on the supply voltages. Its +/-15V (7815/7915 regs). I don't fully understand why all of the pre's use a discrete diff pair either...even the SSL 9k does this also. Perhaps PRR or DK or someone will enlighten us.

So I post my first schemo and get busted on the mistakes and I didn't get my "newbie welcome" from Kev, that I was looking forward to. :sad:

Btw, I asked SM Pro if they were building these for Nady...and the reply... basically "Nady saw ours and ripped us off". So I'm not sure how close the Nady is to this.

Peace!
Charlie
 
Thanks for the added info.

why of all the pre's use a discrete diff pair

You have no control over the input stage of an opamp;
you can't really tweak it (apart from choosing another type).

Putting discretes in front of an opamp is done to improve noise-specs.
Larger emitters, higher currents. (There's more to it, but you'll get the idea. The old place had a good thread about this, complete with matching for source-imped. etc)


For an amplifier which needs to amplify a lot (like a micpre)
the common approach is to give the first stage as much gain as possible
(without overloading) and optimize it for noise.

In this manner you've brought the signal
up to a healthy level and less can go wrong from from there.

Later,

Peter
 
>So I post my first schemo and get busted on the mistakes and I didn't get my "newbie welcome" from Kev, that I was looking forward to.

No, we're not busting you. It's just an interesting discussion. I remember the SM Pro and Nady being discussed in the old board... i.e. salvaging the 1u rack case and throwing the board away.

Thanks for posting the schem and charts.

Just curious though if SMPro is up in arms that their schematic is posted here? Or do they know we're of the "higher" variety and will not ripoff their pre??? We'd rather rip a Neve or API :)
 
A similar transistor arrangement can be seen here;

http://sound.westhost.com/project66.htm

It's widely used in Mackie & Behringer mixers :wink:
 
And then this: if it seems a nice box for upgrading
(or giving it a new PCB altogether), where can we get it here
in Europe ?!?

Last time I looked they don't ship outside the US. (Perhaps they
don't have any left, could also be end of story).

Anyone seen a seller in Europe ?


Bye,

Peter
 
from NYD:
The very first thing I'd do, is get those junky-ass 4558s out of there.

Right, ban them to a tubescreamer :wink:
I thought the manual 'promised' that there were '4580's in this unit ?
(Uli B.'s opamp-answer on Greg M.'s 4560...)

Thanks for the updated schematic, especially with R19,28 & C100 present everything seems to fall into place now. Strange they didn't make one of these resistors (like R19) a gain-pot - perhaps not because of the reverse-log requirement ? The presence of the low valued R28 seems to indicate this was an original intention but didn't completely make it.

I don't have an idea how much the linearization of the input stage (by adding two more discretes like in 'the others') would improve the performance of this circuit.

Bye,

Peter
 
It sure wouldn't cost much money to buy one to play with.

Sorry for repeating myself, but anyone seen these available in Europe ?

Inside-pics would be interesting - like if it's enough room to add PCBs or that everything is better removed and a completely new interior could be added.

Peter
 

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