SM Pro PR8 Mic Preamps

[SonsOfThunder]

Gary,

Curious to know what you think of the pre a this point of mod?

I would measure the DC before C10, that should tell you if you can rip it out.

PRR perviously said the output section had a gain of ~1.4, so you could wire the trafo for 1:1 if you can lose a little gain or wire it 1:2 if you need the gain.

HTH!
Charlie

 

[GaryMedia]

I'm back in action from my inaction on this PR8 project. Having installed the NE5532's and clipped out C13/C15, the fact that this thing still passes a signal is very heartening.

I'm getting ambitious now with three intitiatives:
1- replace the signal path electrolytic (10uF and 47uF) capacitors with Low ESR versions.
2- remove the output circuit board and replace it with at set of Cinemag CMOQ-2S transformers
3- replace the existing volume control with another gain-setting plan

Number 3 really is getting to be difficult because I can't find a source for a reverse log pot. It also needs to be a small diameter to fit in the front panel. Perhaps a better way to go is to choose another set of resistor values to go between Q1/Q2 and around U2A for volume management and perhaps a better noise figure.

For example, if I wire the output transformer for 1:1 thereby losing the 2x gain of the U1A/U1B, then I'd like to select resistor values that change the gain plan of each channel...perhaps changing from the original 6x14x2 to 18x10x1 if that won't overload anything in U2A/U2B. Another option is to wire the transformer as 1:2 (if U2B can handle it) and make the box 12x7x2. I need bigtime help in this planning.

I ordered the capacitors today, and I'm waiting for a shipping quote from Cinemag for the transformers.

Hey, I just thought of a 3b approach using a photocell lit by a LED (do photocells go low enough?) varied by a front panel pot to provide the Q1/Q2 gain set control instead of a reverse-log pot. I know...odd and crazy, but interesting...no?

All help and suggestions are appreciated.

 

[SonsOfThunder]

Gary,

I am remiss in updating some things on this and the PR8mk2, which I finally have the correct schemo for, but not the time (or maybe motivation!) to redraw and post at the moment.

Impressions of the unit with 5532 and missing caps? Congratulations on not breaking the thing! :thumb: :thumb: :thumb:

If you do a recap, you should eliminate the redundant cap between the two chips.

Trafos sound like a nice idea, though I don't know anything about the one you spec here. Will it fit in the 1U case?

RLT pots that are small could be an issue. I think someone had posted about this recently. If you can find a pot that will fit, find out the manufact. and give them a shout and see if they make it in RLT and ask if they will sell you a set. Alternately, there are some smaller switches available from Grayhill that might fit. Not many positions, but you could work that out...maybe with an additional high/low range switch?

Photocell atten is not so crazy...this is the way it was done in the UA1108 mic pre, though no one seems to know exactly how it was implemented.

Hope that helps!
Charlie

 

[GaryMedia]

My singer will be in this weekend so I can get some impressions of the PR8 as is at this point. My references will be my Roland MMP-2 preamp (really like its flexibility and functions) and my modified ART TPS with ECC83 tube and MC33079 opamp. I've not yet run vocals thru the ART either.

After sleeping on it, I decided that I'd rather have a weird 'attenuation' knob on the front of my PR8 rather than going thru the trouble of finding an RLT pot. With that in mind, let's just proceed with the idea of the proper gain-setting electrical topolog and forget the conventions of 'clockwise-for-more-gain'.

As the transformer fit, I haven't done the PayPal yet to get them shipped, but according to the spec sheet they're 1.65-inches high and 1.26-inches wide (and 2.81-inches across the mounting flanges). In the 10.25-inch space created by the removed rear board, I could install all eight side-by-side if the 1.65-inch height allows the case to close. Otherwise, I'll have to lay them down in pairs with a mounting rail between them. We'll see when they get here.

Electrically, the plan is to eliminate C10 and replace it with a 100-ohm resistor then go right into the primary on the output transformer. I got the idea from this website: http://sound.westhost.com/project30b.htm

I also have some concerns about so many transformers in close proximity so I did the research and found an inexpensive source for mu-metal just in case there's a problem. I can essentially wrap each transformer in a mu-metal shield.

Unfortunately the case could be so heavy at this point that it would preclude rack mounting. For that reason alone I am extremely interested in getting a PR8 Mk2. I missed one on eBay last week that went for $130. It might be much more prudent to use the Mk2 as the transformer-output box and stop where I am with the existing PR8. I would do the same opamp replacement mods and electrolytic upgrades.

If you're up to providing even a rough draft via PM to me of the Mk2 circuit, I'd put in the work to see if I could finish the drawing. I would relish the option to use the 8 output transfomers that I ordered in the Mk2, and embellish the Mk2 further by populating it with a set of input transfomers. Sanity check pending what's mechanically possible in the Mk2 box.

Thanks for your continued interest.

 

[paddy]

[quote author="PRR"]Even if the cost of C100 is only $1 per box and they only sell 500 boxes, they could have bought an hour of an experienced designer's time for a quick look-over.[/quote]

[quote author="SonsOfThunder"]
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: [/quote]

hmm .. I guess they saved themselves $500 then

 

[SonsOfThunder]

[quote author="paddy"]hmm .. I guess they saved themselves $500 then[/quote]
Ceramic caps like those they took out are maybe a nickel each (USD$0.05) so I think maybe they saved less. I didn't change their design (yet), but after they checked out the mod they mailed me an OC8. Its a pretty surprising box for the money and I have found it pretty useful in my studio.

Peace!
Charlie

 

[paddy]

.. I meant they got a couple of good engineers to look at it for free thereby saving themselves $500 .. at least!

 

[GaryMedia]

Since the modifications so far have been confined to just the removal of C13/C15 and new opamps (NE5532's), I didn't expect such dramatic changes but testing during the weekend went much better than expected.

1 - The noise level of the unit is very low now. There is no audible hiss throughout the volume settings. I verified my impression by recording the output of the PR8 into my MOTU 1224 and 2408-mk2, then checking the noise profile in Adobe Audition. Results were sound-checked with Audio-Technica ATH-40fs headphones and Alesis M1 active MK2 speakers with a 15-inch subwoofer crossed over at 75Hz through a DBX 223XL active crossover.

2- I recorded my singer through the Roland MMP-2 using a Studio Projects B3, and through the PR8 using an ADK A51 type V set up next to one another for simultaneous capture into the MOTU. I'm familiar with the sound of these two mics and it allowed for fast/simple impressions. The results were very good. The PR8 in its current state is easily a match for the MMP-2 in terms of clarity and sparkle ( I hate using these wine-tasting terms to describe sound).

3- Without recording, I had a friend play his acoustic guitar with built-in pickups through a Behringer DI20 running as a splitter to send output to both the PR8 and the modified ART-TPS. As I switched the custom room speakers (I'll describe these at another time) between the two preamps, there were subtle tonal differences in the 200-500Hz range that I think people describe as "roundness". Occassionally we'd perceive a "roundness" advantage in the PR8 over the TPS, but it was never anything that woud push a strong preference either way.

It's very encouraging to see that such a simple change can yield such satisying results. Now that I'm happy with the first steps of the journey, I'm emboldened for more! I ordered the Cinemag transformers on June 24th, and I'm thinking hard about using a gain-setting network between Q1/Q2.

I learned here http://www.geofex.com/Article_Folders/potsecrets/potscret.htm that a reverse-log pot has a curve that can be emulated by properly selecting a resistor across two of its terminals. If someone can guide me with selecting values for the pot and resistors, I'm ready to buy parts!

Concurrently with installing the gain network between Q1/Q2, I guess the output of C8 gets connected directly to R25 (elimnating R24, R27) and the value of R26 gets reduced to something like 10-20K, correct?

I'm also thinking that I'd like to optimize two channels to run ribbon microphones (the Apex210/Nady RSM-2) that I hope to buy through another member of this forum. I suppose that those channels need a very low value resistor between Q1/Q2, and some other values around U2B. All comments appreciated here!

Thanks to all, this is going very well.

 

[wiz1der]

So , would these same types of "snip" and replace upgrades be able to be applied to SM Pro's DI8 direct box? meaning some nice transformers etc?

The DI8's are on sale right now for 99.00 8 di's in a 1u unit WITH a 8 ch line mixer!! I need some di's right now for some synths/drum machines, and was looking at bo's DI, and someone retrofitted one into a whirlwind box, maybe we can get something going on this 8 DI's in 1u thingy!!

 

[SonsOfThunder]

[quote author="wiz1der"]The DI8's are on sale right now for 99.00[/quote]
That's regular price at most places.

Trace out a schem and we can have a looksie, but let's do that in another dedicated thread.

What are your impressions of your modded PR8??

I have been using my PR8 w/OPA2134 and an OC8 compressor for vocals with my modded TCM1050 (SCM1000 capsule) and I am REALLY liking it. I'm sure that my (yet to be built) JLM pre will probably smoke it, but for the money and a little effort, I think it is excellent!

I need to start using my other PR8 with (Signetics!!) 5532s and note any diffs.

Peace!
Charlie

 

[wiz1der]

It's amazing what snipping a few caps and changing opamps can do!! I am really happy i bought this on ebay for 60 bucks, and with some TI samples and a wire cutter, made it sound like 600.00!!


Yes another thread would be proper for the DI

BUT:

I don't know the first thing about tracing schemo's. I thought someone here had a connection at sm pro.

I can take some really up close and personal pics of the board. wait, I haven't bought this unit yet. Maybe I'll go ahead and order one.....

 

[meblumen]

I did a search and dug up this old post. I too want to upgrade my SM Pro unit. Unfortunately I'm not great at reading schematics and aren't 100% of everything I want/need to do. From what I can discern it seems like I need to remove cap 13 and 15 and then have the option of installing sockets so I can upgrade the opamps.

Anyway, I popped open the lid and first thing I notice is hot glue (or something like it) holding together the joints. What's worse is some of it pretty sloppy and is kinda all over the place, is that normal of a cheap device like this or should I be worried?

Ok on to the upgrade. Below is a pic of my unit:

http://i20.photobucket.com/albums/b225/m_blumen12/Things%20Not%20For%20Sale/SMPro.jpg

I know the image quality is not good (and neither are my photography skills) but if possible could you tell me which of the caps to remove?

Secondly, to do this work, is it best to remove the whole board from the chasis, or simply bend up the board after removing the screws? If you are supposed to remove the entire board, how to you remember where all the wires and whatnot go?

 

[SonsOfThunder]

The answer is in the pic on page 5.

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

The hot glue is just there to make sure that none of the plugs come out of the sockets. I promise, you can't hear the diff. It should peel right off if it really bothers ya.

I would NOT recommend bending anything. But you should be able to clip out the caps without removing anything besides the lid. The jumper wire sockets only plug in one way. They are color coded so you could write that all down on a piece of paper before you start or put tape on each wire and label it if you think you need to. Really they all just go in order from one end to the other. The jumper for the power is a 4 pin, so its kinda hard to mess that up.

If you want to change opamps, you need to pull off the knobs on the front and remove the nuts from each pot. The board should slide out then.

If this just looks too hard, maybe you have a friend who can help you out...maybe look over your shoulder while you do this? Two heads are sometimes better than one.

Hope that helps!
Charlie

 

[meblumen]

Thank you for your help. Two last questions, when removing the caps, should I just clip one of the leads or remove the entire cap altogether?

Also has anyone tried to install phase reversal switches on any of their unit's? It looks kinda crowded but it would be a nice feature if it's possible to get it in there. Thanks again for all the help.

 

[SonsOfThunder]

[quote author="meblumen"]should I just clip one of the leads or remove the entire cap altogether?[/quote]

Yep, either...

install phase reversal switches...?

SM Pro did this on the new version. I think its possible, though I haven't done it. I normally use a plug in for phase reverse though...

Peace!
Charlie

 

[wiz1der]

hey Charlie,

Anything with the DI-8 yet?

 

[meblumen]

Sorry one last question. I was reading through all of the posts again and I'm a little confused. The pic has 3 caps circled but people were making reference to only removing 2 caps (13 & 15). I'm assuming those are the two on the ends and 14c is the one in the middle right in front of the opamp. So just to clarify should I remove all 3 caps or just 2 and if only 2 which ones?

 

[meblumen]

Sorry to be a pest but can somebody helpme out please. Thanks in advance.

 

[SonsOfThunder]

[quote author="meblumen"]The pic has 3 caps circled but people were making reference to only removing 2 caps (13 & 15)...So just to clarify should I remove all 3 caps...?[/quote]
Not to sound like a broken record...
[quote author="SonsOfThunder"]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. [/quote]
You can do what you like with your unit. Clip just the two caps on one channel and listen. Clip the third (leaving the lead long enough to solder back or just desolder C14 and replace if you don't like the result.) If I remember correctly, removing only C13 and 15 extended the -1dB bandwidth to something just beyond 20kHz, but to get really flat past 20kHz required the removal of C14 as well.

Hope that helps...
Charlie

 

[SonsOfThunder]

[quote author="wiz1der"]Anything with the DI-8 yet?[/quote]
Any pics of your unit? I think I have the factory schem, but without tracing a unit, I'm not sure I'd trust it. I'll go see what I can find...