Gear Pr0n !!! pics inside

[tk@halmi]

No need to violate intellectual property on this one. The preamp is based on the Cohen circuit (AES 2106). Since I can see not transistors on the board I think they use the SSM2210 for the matched input pair, same as the LM394, but less recognizable in a DIP8 package.

As soon as I hit submit I realized that there are no recognizable FETs visible.

 

[Peter Simonsen]

[quote author="tk@halmi"]With the FET input it should be a variant of the Forssell schemo published on his website.[/quote]

Are you sure about this..I mean ..I belive Fred F does say that these preamps where before he "entered" mil-media ;-)

kind regards

 

[Samuel Groner]

Just like the GML 8900 compressor

A link to that one? Was not able to find it...

Thanks!
Samuel

 

[Peter Simonsen]

[quote author="Samuel Groner"]

Just like the GML 8900 compressor

A link to that one? Was not able to find it...

Thanks!
Samuel[/quote]

PM your mail to me and I´ll send you a couple of pics..;-)

Kind regards

Peter

 

[Category 5]

[quote author="tubejay"][/quote]

I too have been a BIG fan of the MP20/M80 mic preamps. I think they sound great stock. I just picked up an MP20 for $100!!! Did you just dump the AD797's in there, or did you have to modify anything to get them to work right? That's a mod I might be interested to do. Hell, it couldn't get any easier since Presonus sockets the chips.[/quote]

With the AD797s you also have to cut a PCB trace to eliminate one of the capacitors or the preamp will oscillate. I will take some pictures of mine when I get the chance, and you can see where they were. Chad Kelly and Rick Naqvi at Presonus were great about helping me. I am still trying to get ahold of a shematic because there is a new Cinemag transformer that I'd like to fit in there and try. it is much larger, and comes with leads, but the specs are MUCH better than the Jensen part.

$100! I'd pick up another for that price any day of the week! Especially with the potential for modding. I had the Burr Browns in there for a while and loved them, but once I got the AD797s working I was blown away. THe preamps sounds exceptionally clear and neutral. THe transfomer is the only concern I have, and I did pop in the Jensens. THey sound perfectly fine, but the Cinemags are supposed to be quite a bit better. Plus I remember Dan Kennedy saying something about the 11K8-Apc being the dog of Jensens line, with a couple of serious distortion flaws.

Shane

 

[tk@halmi]

[quote author="Category 5"]
I am still trying to get ahold of a shematic because there is a new Cinemag transformer that I'd like to fit in there and try. it is much larger, and comes with leads, but the specs are MUCH better than the Jensen part.Shane[/quote]

The Jensens are a decent upgrade actually.

By the way, you are not restricted to use 1:3.5 or 1:3.8 transformers on the input.

There is a very low noise JFET (2N4392, my favorite FET) follower on the input of the amp so noise-wise ANY transformer with higher turns than 1:3.5 is fine. You could use an LL1538 or anything with 1:5 turns. The added benefit is an extra 3dB to 4dB gain besides better sound.

The only thing Class-A in the entire preamp is that JFET buffer, but most places sell it off as if the entire amp was Class-A. PM me with email addy for schemo.

Tamas

 

[b3groover]

[quote author="Category 5"]
With the AD797s you also have to cut a PCB trace to eliminate one of the capacitors or the preamp will oscillate.[/quote]

Does this go for the M80 as well?

 

[Category 5]

Actually yes. There are a couple of traces that need to be sliced, and a couple of jumpers that need to be added for the AD797s. I am trying to find out where they were. I suppose I can just pull it out of the rack and take it apart and see. I'll need to take some pix anyway. It will be a few days before I get a chance to do that though.

The MP20 and M80 are the same circuit.


I did another session with the flamenco guy tonight. Again, I tried to make use of some of my other pres on his guitar, and honestly...If I don't use the MP20 (keep in mind he had no idea what I am using) he complains that the guitar doesn;t sound like his guitar. "It's not right. Something's missing."

When i use the MP20 he is happy. Go figure! I think the APIs sound pretty dran good too, although there's nothing wrong with using the MP20. I guess it will be the Presonus until the project is done. He didn't like the 312, the 1272, or the Millennia. Same mics too.

Shane

 

[Bauman]

It would be nice to take a look on this MP20/80 circuits... I'm a very curious guy

 

[Peter Simonsen]

Hi Shane,

I´m very interested in what you are saying as I have found some of the same isues as you have..under somewhat different circumstances, but
would you care to talk a little about this ???

I´m constantly on the search on capturing the instrument as it sound in the room...both to me, and the player.

how many different mics did you try out with these different preamps..??? and which mics where you using..???

Thanks and kind regards

Peter

 

[b3groover]

I'm using OPA637s in two channels right now in my M80 but they are oscillating. I've heard OPA627s are supposed to work well, but I haven't tried them.

I'm really curious about the AD797 mod. Have you tried the M20 on drum overheads?

 

[Category 5]

The 637 is only stable in gains above 5dB, but the 627 is unity gain stable. You want to be using the 627 in that circuit. They are essentially the same sound.

The OPA627 is the most common replacement since it basically slots right in and works. I used the 627 for quite a while, but then switched and will never go back.

Presonus did some modifications to mu main board to help in my quest to get the 797 to work right, so I am trying to track down that data for the group. It most definitely did not slot right in as did the OPA627.

Keep in mind that the OPA627 is also an exceptional replacement in this circuit, and you very well may like its' personality more than the 797. I find the AD chip to be more neutral and less colored, while the OPA627 has a nice smooth (warm like a tube) sound to it. Most people love the OPA627 for the MP20, but since I have plenty of higher end options for color i opted for as nuetral tone as possible.

That's what it seems I have got.

Shane

 

[tubejay]

That's a good point, contacting Presonus for help seems like it might be a wise course of action. They in fact encourage messing with new chips in there. That's why they socketed them in the first place. At least that's what I recall from the TapeOp review.

I've had to have a couple pieces of Presonus equipment fixed, including my old MP-20, and they have one of the best service departments in the business. They fixed two out of warrantee pieces of equipment for free. The only thing I ended up paying for was shipping on the way to them. They also put the equipment in new boxes with manuals, and shipped it back to me via FedEx 2 day. Now that's service. Point being, they are VERY helpful, and I bet they'd be willing to help people mod their MP20's for them for use with different chips.

 

[b3groover]

[quote author="Category 5"]The 637 is only stable in gains above 5dB, but the 627 is unity gain stable. You want to be using the 627 in that circuit. They are essentially the same sound.[/quote]

Yeah, I read about the unity gain stable thing after I put the chips in. Silly of me.

I'm looking for a couple channels in my M80 to be very clean, for acoustic guitar and such. Any info on the AD797 mod would be greatly appreciated when you have the time.

In the meantime, I'll pick up some OPA627s and try those.

 

[tk@halmi]

[quote author="Category 5"]The 637 is only stable in gains above 5dB, but the 627 is unity gain stable. You want to be using the 627 in that circuit. They are essentially the same sound.[/quote]

The OAP637 is stable over gains of 5 that translates to 14dB. In my experience, often it takes more than five to make it stable. It is excellent in high fixed gain applications like a phono pre.

The AD797 is really not unity gain stable either. It is a very fussy chip and may oscillate at will. If you run the MP20 at least at 30dB gain it will be stable. Adding very high quality supply bypass capacitors right between ground and power pin helps a little. Getting rid of the socket and soldering the chip to the board directly helps a little.

There is a 100 Ohm resistor in series with a 100pF capacitor between the output and the non-invering input. Higher capacitor values will cut down the frequency response which help because the AD797 has a staggering high BW and a little HF noise can set it off. You don't need a 1MHz bandwith at 26bB gain per stage of the dual servo. You can get all the radio stations in your town though.

There is just no silver bullet with the AD797.

Tamas

 

[Category 5]

I just wanted to mention that I spoke with Chad Kelly at Presonus today about the AD797 and oscillation.

The oscillation problem with my unit was do to Presonus switching from dual discrete input transistors to a single "dual" unit.

Because of the circuit board design there was some very high frequency oscillation, but only in models made beteen 99 and 2000. After that they fixed it in the main board. Mine was oscillating with the 5534s, OPA627s, and AD797s, but I never noticed it.

According to Chad, the 797 chips will work right from the get-go by simply slotting them in. Since the MP20 runs at +/- 18v they will run a little hot, but I haven't had any trouble with mine. You could put a heatsink on em if you're worried, or drop the voltage at the regulators a little bit.

So for those wanting a super clean version of the MP20, the AD797s are a go. Contact Presonus to see if you need the board trace fix. if you do, it's alreday oscillatingf with whatever op-amps you've got in it, albeit out of the audible bandwidth. You will see it on a scope however.

Shane

P.S. My power switch went, and I called for a replacement. Chad sent the part out free of charge, free shipping too. Presonus has some great support.