Adding Input/Output Transformers to my Art Pro MPA II

[deepmojoman]

Hi all. I have decided that I want to adI/O transformers to my Art Pro MPA II. But, there are impediments that I'll need help with:

1. Currently there are no schematics of the MPA II anywhere on the internet, and . . .
2. Tony at Moddefaction says that he feels that if you added an input trafo then that would cause the Variable Impedance function to operate incorrectly since any trafo you added to this circuit would be of a fixed impedance.

As far as impediment #1 goes, thanks to a post on this forum started by KingKorg I was able to get my hands on the schematic for the MPA Digital, dated 2003. I looked inside my MPA II, and I'm confident I'll be able to work with the schematic from '03, as I'm pretty confident that not a lot has changed about the the Variable Impedance circuit.

Which brings us to Impediment #2. I spoke to Dave Garren-owner of Cinemag, and I told him what I was thinking about doing (I'm thinking of the CMMI-5C for the Input trafo, for a nicely charactered sound), and he said some things that I will need some insight on:

1. Replace the 5K dual pot currently controlling the Variable Impedance circuit with a new pot (he suggested somewhere near the value of around 18K) . . .
2. And also to add one resistor apiece to each leg of the pot going to ground, so as to prevent the transformer from shorting out.

He suggested that I do these things once I asked him whether or not simply adding the CMMI-5C (3750 ohms impedance on the secondary) would just simply add that impedance to the maximum 2.7k ohm impedance you get from the Impedance knob. I wasn't quite clear of his answer but he nicely subscribed those two suggestions for my dream circuit.

So this is where I will need some help from you guys:

1. Does anyone out there have any idea of what putting an input trafo right in front of a new pot with a value different from the stock 5K do to the Variable Impedance circuit (like: will it just add it's secondary impedance to whatever impedance is dialed up on with the Impedance pot, or is it a much more complicated process)?
2. Does anybody have any idea what value of pot I should be looking for?
3. Finally, does anybody have any idea of the value I should use for the two resistors that I will include between the pot's negative leg and ground?

I have included the '03 schematic of the input section of the MPA, so that you can know what I'm talking about.

Thanks so much.

 

[deepmojoman]

Hi all. Any thoughts on this? I'm really in need of assistance on this

 

[user 37518]

Why do you want to add transformers ? the design of the amp is really made to be transformless you have multiple transistors at the input for low noise and the preamp works in a double balanced configuration. It is fine as it is.

 

[moamps]

Input transformer has ratio 1:5 so you should increase the pot and R247 25 times in order to keep the same input impedance.  So dual 100k or single 50k pot and 3k9 for R247 would be fine. Doing that, you will probably need to change some other parts too, like capacitor C219, R250, 251  etc.
Could you post output schematic too?

BTW, I agree with user 37518, there isn't much sense to mod this preamp that way except it is very "boring" and "not involving". Did you tried another tube?

 

[Ricardus]

I'm pretty sure Revive Audio has a mod to add transformers, so I know there are people doing it.

 

[emrr]

the ROI is not good.  Use as-is or sell, build one of the many kits here. 

 

[deepmojoman]

First off, thanks for all the replies: they are all informative in some way.

Why do you want to add transformers ? the design of the amp is really made to be transformless you have multiple transistors at the input for low noise and the preamp works in a double balanced configuration. It is fine as it is.

Well, I'd like to add another option to color the sound, in a way that transformers do. I was thinking of adding a switch (if possible) to simply bring this new circuit in and out of the signal chain, like Warm Audio does with their TB12. But, given what Moamps suggests, it seems like this might be just a little out of the realm of possibility (speaking about the switch, I mean).

So the short answer is: I love transformers.

Could you post output schematic too?

Done (though again bear in mind that this will be the schematic for the Pro MPA, NOT the schematic for my unit: the later MPA II. I have no idea how much the new model differs from the earlier model, though I am banking on it not differing too much. From the looks of a pic of the Digital MPA, it looks basically just like my box, only with some added digital capabillities. Beyond that they look like they have identical abilities.)

And thanks for your input-it looks great.

the ROI is not good.  Use as-is or sell, build one of the many kits here. 

I get what you're saying. But the ROI for me is simply to do it! I'm a tinkerer. If it doesn't add anything of value in my estimation, then I'll simply reverse the mod. But I love learning about new circuits. It's really that simple (oh, and : have I already mentioned that I love Transformers? In Preamps?)

 

[emrr]

There’s no reason not to put transformers in a box with connectors and patch them for color, here or wherever.  No need to be terribly precious about it. 

 

[Winston OBoogie]

No need to be terribly precious about it.

Haha    I like.

Maybe graft a transformer on the output with something along the lines of Rupert Neve's 'silk' circuit whereby an adjustable voltage can be applied to the primary to saturate the core to various degrees.

 

[deepmojoman]

There’s no reason not to put transformers in a box with connectors and patch them for color, here or wherever.  No need to be terribly precious about it.

But don't you think that an Input and an Output transformer, wired in a box so that you can patch them with the Input trafo before and the Output trafo after the preamps ins/outs will sound quite a bit different than an Input/Output couple wired into the circuit, interacting so differently with impedance and gain staging?

That's been done of course, and I've read many people's different impressions on their effectiveness. Was about to build one for a while, but I could never get over my sniggling doubts that I'd be doing much of anything beyond tying an extra input jack in front of a preamp jack, and tying an extra output jack in front of the preamp's output.

 

[shot]

You are trying to have all-in-one preamp. What will be the next move? To try to bypass tube circuit with solid state?
Have you considered that there's a possibility you mess up the preamp completely when doing a lot of modding? It happens. Some stupid mistake like broken trace, lifted component leg or wire, solder drip or whatever. And if you do, you'll need some serious troubleshooting skills to make it work again even in the original state.

I think you came to a point where you need another preamp, or more than one new preamp, to give you more choices when recording. This is normal. That's why we all have gizmos in our racks growing each year!

You'll spend money to do this just to see what will happen. Don't do it. Just go for another preamp. Brand name or DIY, whatever!

 

[emrr]

But don't you think that an Input and an Output transformer, wired in a box so that you can patch them with the Input trafo before and the Output trafo after the preamps ins/outs will sound quite a bit different than an Input/Output couple wired into the circuit, interacting so differently with impedance and gain staging?

Why would you think it'd react differently?  The relationships will be the same. 

 

[deepmojoman]

You are trying to have all-in-one preamp.

Actually, it’s much more the other way around: I already have so many other preamps-from ISA to Warm Audio to Golden Age- that I feel free to experiment on this one.  It’s just simply fun to me. I guess not for you but it is to me. Used MPA IIs are a dime a dozen-easily replaced if necessary. But I’m still learning in the interim.

Why would you think it'd react differently?  The relationships will be the same. 

Well actually, right after I wrote my reply to you, I was thinking of how hard it might be to saturate the input transformer with such a box with just the source in front of it. Then I though: “Is it any different in any other “pedigree” preamp?” So I checked out the schem for a 1073 and…there’s literally nothing between the mic (or line source) in front of each respective input transformer. Just the source and then the trafo.

So, I didn’t write the reply, and came to the conclusion that I could see no way in which the two different methods would be different, other than the fact that in inserting an input trafo physically into the circuit ALONG WITH adding some RC changes to the circuit would change the impedance relationship in a way that an external box would not.

So, yeah. Still haven’t quite decided against my initial idea though.

Maybe both (though not on the same box). The world could definitely use a good Transformer box.

 

[Ricardus]

Volker makes a 2 channel transformer box for patching with 3 different transformer choices I believe.

 

[emrr]

Maybe both (though not on the same box). The world could definitely use a good Transformer box.

Between factory made and custom order pieces being offered online, there must be 40+ varieties out there currently.