SM Pro PR8 Mic Preamps

[dejacky]

It was something I was hearing in the track..and the clip led would come on..The pr8 clip led is not a good measuret though in my opinion since clipping still can occur and not be noticed in the LED since it might be very faint.

What's really clipping the worst is the kick drum. I use an AT3035 condenser and it captures the sound it very well...better than any of my other mics. My biggest issue with the PR8 is I only like the sounds I get with the gain turned up...it really adds some "fullness" that is not just "loudness." Those mods sounds interesting and Thank you . I will try to compare that to using a pad to see which sounds better for me. cheers.

 

[BYacey]

C11, C12, C13, C15 - These are all 470 pf on the schematic I was looking at. Unless you are in a severe RF field, these can be reduced to around 15 to 50 pf. It's nice to have the top end to capture fast rise transients.

 

[SonsOfThunder]

[quote author="BYacey"]C11, C12, C13, C15[/quote] Bill, C13 and C15 are the top end "killers". Without those and C14, the freq response was flat (-1dB) out to about 90kHz even with the 4558 still in place. You could probably drop the value of C11 and 12, but I'm not sure that would change the "sound". That's worth a try though.

Gotta get some work done now...

 

[BYacey]

C14 isn't an unreasonable value. If the preamp is stable with some 15 pf caps for C13 and C15, leave it run high, IMHO.

 

[GaryMedia]

I've read through the threads here and I'm ready to order the parts to upgrade my SmPro PR8 preamp.

I'm planning to use the Analog Devices OP275 op-amps and change C13/C15 to 15pF ceramic capacitors. I will probably change C14 to 22pF.

If C10 and C5 really exist, I plan to replace them with a single 47uF non-polarized capacitor.

Also, while I'm in there, it seems that a 16dB pad switch (making Q1/Q2 unity gain) might be useful for those drum miking problems. What value would work for replacing R19 with another resistor (switchable via a rear-mounted SPDT pin switch)?

Also, is there a way to reduce the output impedance of Q1/Q2 to give lower noise?
Thanks

 

[dejacky]

GaryMedia,
this sounds like an awesome idea! I'd like to mimick what you do after your done. Please tell us the differences in sound it made. Thanks.

-dejacky :grin:

 

[Guest]

Hey Alan, Do you know anything about the design of the Dynamix/Seck desks? They would have been one of your main rivals around the late 70's. Soundcraft bought them out in the 80's and used their designs.

I've been trying to get a circuit for a Dynamix 32/8 for a while now? I was told that it was designed by an ex Neve employee, they were built in the same town but it all could be stories. The desks sound very neve-ish though and the eq is excellent. Do you know anything?

 

[SonsOfThunder]

[quote author="GaryMedia"]I'm planning to use the Analog Devices OP275 op-amps and change C13/C15 to 15pF ceramic capacitors. I will probably change C14 to 22pF.[/quote]I think that the ckt is more likely to pick up noise with the ceramic caps in there, but you can certainly try it.

If C10 and C5 really exist, I plan to replace them with a single 47uF non-polarized capacitor.

Interested to see if your unit has the same stuff as mine. If your units are REALLY new, you may find that C13/14/15 are gone since they are supposed to implement that mod in production. :thumb: You should be able to get close to the same low end with a single 22uF bipolar, btw.

What value would work for replacing R19 with another resistor (switchable via a rear-mounted SPDT pin switch)?

this would be answered in PRR's comment:

[quote author="PRR"]But you can try this:

First, clip one end of R28. That gives you another 5dB headroom at the input.

Find R26, a 100K next to U2.B. Tack a 10K resistor across it. Gain will be 20dB lower. This will allow you to set the Gain higher, and keep the noise low.[/quote]You can try it, though I have not.

Also, is there a way to reduce the output impedance of Q1/Q2 to give lower noise?

I think you will find that this is not the major source of noise in the amp. PRR addresses this in one of his comments.

HTH!

 

[GaryMedia]

Thanks for the thoughtful answers.

I think I'll scale back some of my initial ambitions in order to assure a more pleasing outcome.

If I don't use 15pF ceramic capacitors for C13/C15, what do you recommend for using there in concert with the OP275 ?

I won't go exotic with the polyethylene-terephthalate or tantalum non-polarized capacitors in my original plan for replacing C5/C10 if they're there. I think I'll go thru the board and install Nichicon low-ESR electrolytic caps anywhere they appear in the signal path, but use just one cap to replace C5/C10.

It seems from your reply that if I snip R28, then that whole R19/C100/R28 series is out of the circuit and the overall gain drops by 5dB. I think I will do that that, and then install a hi/low gain switch that parallels a 10K-ohm resistor across R26 as needed to drop the gain.

Do you still recommend following thru with the wiring and value changes of R10/R12 to more precisely balance the outputs?

Sorry for the large spate of questions, but I might buy two more PR8 boxes and do these mods to flesh out my MOTU 24i portable recording rig.

 

[SonsOfThunder]

GaryMedia:

I have not tried OP275, though it is a fav OA as well. As I have said before, I used nothing at C13/14/15 with OPA2134 opamps which are about the same speed as the OP275. OP275 is not s prone to oscillations and such so I would try it without the caps and look for any probs with a scope.

Sounds like a good plan on the electro cap replacement. Another thought would be to replace all resistors with metal film.

A DPDT switch should accomplish the gain switching plan. I have not tried this though, but I am very interested to hear how this works.

@dejacky: did you try these changes yet? Did they work out or no?

I wouldn't worry so much about changing R10/12. I understand the thoughts but I wouldn't let this get in the way of making music. :thumb:

You might consider buying the PR8mkII. It does the gain differently and is probably more quiet, though I haven't actually played with one. I have a schemo that needs redrawing and will post that in another thread that was already started. I spend too much time talking about this instead of "doing".

Peace!

 

[wiz1der]

[quote author="SonsOfThunder"]GaryMedia:

You might consider buying the PR8mkII. It does the gain differently and is probably more quiet, though I haven't actually played with one. I have a schemo that needs redrawing and will post that in another thread that was already started. I spend too much time talking about this instead of "doing".

Peace![/quote]


"Get on it!!"


just kidding. After reading all of this, I was ready to go buy a pr8, but if the mkII is waaay better for just another 100.00, I may go that way.

 

[SonsOfThunder]

[quote author="wiz1der"]"Get on it!!"[/quote] Thanks for the "encouragement" :razz:

if the mkII is waaay better for just another 100.00, I may go that way.

I wouldn't say "waaaay better" but it should be more quiet at mid to low gains because they do the gain between the collectors of the Q's. I am told it uses 2068 OAs which are pretty good for the $.

If someone gets one, I'd like to see pics of the internals. :thumb:

Need more hours in the day.

 

[wiz1der]

[quote author="wiz1der"]WHEWWWW!!!!

Thanks for hangin in there guys! Even a new guy trying to jump in and help! i LOVE THIS PLACE!

Anyway, After reading you guys' responses, I didn't want to give up so I went back to the unit and really started seperating wires, following them and there were a couple brittle connections, I gues from transit? I hooked them back up, and IT WORKS!!

:sam:

now ON WITH THE UPGRADES!!

First of all i want to remove C13/14/15, and replace with jumpers, as was discussed earlier. I will do that on 1 channel to see (hear) th difference.

Being new, I see them in the schematic, but I am not confident enough to just get started, so maybe someone can help me identify these parts?

My best guess is:

http://groupdiy.twin-x.com/displayimage.php?pid=115&fullsize=1



Can someone confirm? I will also go purchase 16 sockets to prepare for my OPA 2134's[/quote]








[quote author="SonsOfThunder"]Wiz,



Your pic is much better than the ones I took a while back. They were so bad I didn't want to post them. Can you take a pic of the o/p board too and I'll upload them at work and put locators on the whole thing. :thumb:

Thanks!
Charlie[/quote]

here you go!!!


output board:

http://groupdiy.twin-x.com/displayimage.php?pid=113&fullsize=1

power supply section:

http://groupdiy.twin-x.com/displayimage.php?pid=112&fullsize=1

input board:

http://groupdiy.twin-x.com/displayimage.php?pid=115&fullsize=1

Also, could you mark off the infamous (unneccesary?) c5 and c10? Should these be a different value? or just removed?

 

[wiz1der]

Bizump.

 

[SonsOfThunder]

Cool! Now we got pix marking the ceramic caps that need to be removed. I was going to mark some stuff up today but was absolutely swamped at work and I can't do it on my home computer.

C5 is the electrolytic on the O/P borad there next to the 2-pin plug. You can replace it with a short. You only want to remove either C5 OR C10, but not both.

HTH!
Charlie

 

[wiz1der]

OK we've got a problem. I socketed both boards and :

channel one - STOCK

WORKS

Channel two - OPA2604

WORKS (sounds about the same)

Channel three - Stock opamp without C13,14,15

DOESN"T WORK!! (yes I put jumpers in)

Channel four - OPA 2604, without C13,14,15

DOESN'T WORK!!


All outputs work (I crossed jacks from working input to output 3 and 4,and they do work). I looked at solder, reflowed connections, checked continuity acroos jumpers, still nothing.

Maybe I will try to put caps back in and see what happens.

 

[SonsOfThunder]

Channel three - Stock opamp without C13,14,15 - DOESN"T WORK!! (yes I put jumpers in)

Why did you put in jumpers? You have effectively killed the gain or shorted the input of both sections of U2.

I am presently:
1) Not surprised it doesn't work...
2) Surprised you put those in...
3) Thinking you might be upset with me for something I did not say....

Could you show me where it was ever mentioned to replace these caps with a short? I cannot find that in the thread.

Not to worry though, I think if you remove those shorts, it should work again fine.

HTH!
Charlie

 

[wiz1der]

wax on, wax off.....


not upset at all!! I am LEARNING!!

Will remove jumoers now and try again.


EDIT:

OK it works, I am just running some music through a sampler, but I cannot say I hear a big difference in any of the configurations.

There sure is a lot of gain on here though. all the way down (off), and I get full gain at about 1 or two notches.

 

[SonsOfThunder]

Wiz,

Glad you got 'er going again with no damage and that you are learning. Now, show me 'paint house'. :thumb:

The biggest diff you should here removing the caps is going to e in the highest freq range... "Air", some people call it. Try a microphone.

HTH!

 

[GaryMedia]

I've successfully installed sockets in my PR8, and replaced all the 4558's with NE5532's. On a whim I decided against using the OP275's as I had originally intended. Now that I have sockets, I can easily test opamp options.

While I'm waiting for my shipment of Nichicon 47uF low-ESR capacitors for the 2nd phase of this upgrade, I'm entertaining the idea of removing the output board, and replacing with a set of OEP A262-A2E tranformers.

I think I can make a new output board that is the same size at the original one, but would simply mount the transformers and jumpers to a new set of TRS jacks on the rear panel. I'm unsure about how I should wire the 1+1:2+2 output transformer, and whether a capacitor should be maintained in the circuit between the output of U2-B and the input side of the tranformer.

What do you think?