MP-1 Opamp recommendation

[Loktite]

Hi, I'm planning on doing the V2 noise mod on my MP-1. The instructions call for an OPA2604AP which I can't find on digikey or mouser. Is there a chip that has superseded the OPA2604AP, or an alternative that anyone here would recommend? Also, any recommendations for some simple mods to do while I'm in there? I like the tone and characteristics of the ADA MP-1, but sometimes wish the bottom end was a bit tighter. An MDRT is in the plans for the future, but not right now.

[ramiroelliot]

Hi, I'm planning on doing the V2 noise mod on my MP-1. The instructions call for an OPA2604AP which I can't find on digikey or mouser. Is there a chip that has superseded the OPA2604AP, or an alternative that anyone here would recommend? Also, any recommendations for some simple mods to do while I'm in there? I like the tone and characteristics of the ADA MP-1, but sometimes wish the bottom end was a bit tighter. An MDRT is in the plans for the future, but not right now.

I have the same problem and have not found any specific instructions yet. 

[rnolan]

Hey Loktite, I'll leave it for our tech gurus (MJMP, Gregg, SC etc.) for OP amp recommendations.  IIRC from what Systematic Chaos (SC) did (basically replace everything he could) he found some suitable (and better) substitutes.  He also sourced the earliest transformer, ADA changed them along the way.

But while you are in there (and obviously depends what's already done or not):
Battery Mod
Rear Jack Mod (IIRC you have a 1.38 EPROM so probably Top Switch V1 MP-1).  This mod gives you a standard long shaft switchcraft front input jack (which is the one jack that takes the most wear) and converts the rear jack to inst level (normally line level on early V1 units).
There is a mod to change the bass eq centre freq to be lower. Not sure if that will help make the bottom end "tighter".
MDRT will/does tighten up the bottom end, but it also really opens up the dynamics.  I like it, others prefer the creamy ness (a little compressed?) original sound.
V 2.01 EPROM, more factory patches and Sysex/midi dump load patches to PC

Another thing to  consider (based partly on recent posts by Kazinator) is replacing output jacks (or all of the jacks).

So you can get most bits from MarshallJMP (MJMP Mod Shop)

[rnolan]

Hey ramiroelliot, welcome to the depot .  Maybe do a introduce yourself post and let us know how we can help

[Loktite]

Hey Loktite, I'll leave it for our tech gurus (MJMP, Gregg, SC etc.) for OP amp recommendations.  IIRC from what Systematic Chaos (SC) did (basically replace everything he could) he found some suitable (and better) substitutes.  He also sourced the earliest transformer, ADA changed them along the way.

But while you are in there (and obviously depends what's already done or not):
Battery Mod
Rear Jack Mod (IIRC you have a 1.38 EPROM so probably Top Switch V1 MP-1).  This mod gives you a standard long shaft switchcraft front input jack (which is the one jack that takes the most wear) and converts the rear jack to inst level (normally line level on early V1 units).
There is a mod to change the bass eq centre freq to be lower. Not sure if that will help make the bottom end "tighter".
MDRT will/does tighten up the bottom end, but it also really opens up the dynamics.  I like it, others prefer the creamy ness (a little compressed?) original sound.
V 2.01 EPROM, more factory patches and Sysex/midi dump load patches to PC

Another thing to  consider (based partly on recent posts by Kazinator) is replacing output jacks (or all of the jacks).

So you can get most bits from MarshallJMP (MJMP Mod Shop)

Thanks for the quick response! I'm running it into the effects loop of a tube amp now, and I have the switch set to line level. Is there a reason why I would want to convert it to solely instrument level?

I may just try out a mod 3.666 starting with a mod 3 just to see if I even like the flavor of it. I'll try that out after I quiet things down first. I might as well order a bunch of extra caps and resistors just to play around with. RIP Radio Shack, everything has to be ordered online these days... Anyone know a component shop in the Seattle area?

[Kazinator]

Some 12 years ago, I put something like a dozen TI-branded NE5532 op-amps into my MP-1.
For the quad op-amp in the EQ section, I used an On Semi MC33079.

The unit became significantly quieter and the sound quality opened up.

I didn't put a single socket in for any op-amp; soldered them all down.

I continue to be happy with the mod. I noticed the sound quality improvement right away, as well as the fact how remarkably quiet the unit became. The tube board is wrongly blamed for some of the noise.

An important parameter of an op-amp is the common mode rejection ratio (CMRR): this indicates how well it does not amplify the unwanted common mode. What is that?

Suppose we send exactly the same signal to the + and - inputs of the op amp. This should not be amplified, because there is no difference between the two: the + and - voltages are moving together. However, that together movement, the common-mode signal, does get amplified.

What might appear as common mode is noise: external noise coming in on both the signal conductor and ground line.

This is something that particularly affects single-ended devices like guitar gear.

Crappy op-amps with poor CMRR -> more noise.  Better op-amps with good CMRR -> less noise.

E.g. TL072 has a CMRR of 75 dB. NE5532 is 100 dB. OPA2604A likewise.

This means that the unwanted common mode amplification in a TL072 is 25 dB higher, which is a lot louder.

[rnolan]

Hey Loktite, the rear jack mod converts the rear input to inst level, it's line level on the V1 MP-1s like yours.  The top switch switches the output from inst to line and they stopped doing that on the V2 MP-1s which are all line level out.

The rear input can be handy for patching in a rack setup, e.g. if you have a rack tuner.  But being line level made it not so useful.

When the first MP-1s were made, most people had guitar amps so they had the top switch so you could plug directly into the guitar amp input at inst level.  Also Fx loops weren't generally included in guitar amps back then.  Also stereo guitar oriented power amps weren't really a thing until ADA did them (B200s etc.).  The main changes ADA made to the v2 MP-1s was make the rear input inst level (as the rear jack mod does) and do away with the top switch and make them line level out only.  By this time rack based rigs were becoming more common and rack Fx e.g. the original Alesis Quadverb became available at more affordable prices.  These days we seem to have come full circle and people are buying lots of pedals and running them into guitar amps.

[Kazinator]

It was pretty silly to have a potentiometer controlling the output level and the top switch too. Each thing like this contains a sliding contact, which potentially degrades the tone, so you want as few as possible; certainly nothing redundant. If the MP1 output is too loud for some destination device which expects "instrument level", just use the knob to turn it down.

Speaking of the top switch, I have one and just today, I reflowed its solder joints.

I was debugging a weird tonal difference between OUTA and OUTB. I did a jack replacement on OUTA, but that couldn't be it. I reflowed all the work but it didn't fix the issue.

So I reflowed the U10 and U11 pins and the switch. One of those things did it!

Then, on a roll, I also reflowed the EQ quad op-amp (which I mentioned two posts above). It turns out, it had a cold joint all these years. A whole bunch of nasty top-end is suddenly gone, replaced by very sweet, smooth tone. My external EQ has gotten flatter again. I'm only cutting 1.2 kHz and 1.6 kHz a bit, and rolling off some highs past 6 kHz. Also a light boost on 2.5 kHz and 3.0 kHz. The 2.5 sounded like crap before, I've never ran with it boosted, and used to cut it, even.

Yikes!

If you're swapping op-amps, do a really good job on the soldering. If anything looks even remotely cold or dry, redo it.

BTW, while doing all this today day, I also noticed that the U2 op-amp had dodgy-looking after-market solder work. I have no memory of ever doing anything with that. It is the heart of the voltage control servo for the tube board. Someone who owned this MP-1 before me must have replaced that IC for whatever reason; maybe it got fried. Anyway, I added solder to the dry-looking joints.

[rnolan]

Hey Kaz, do you have any recommendation for the OPA2604AP these guys can't find?

[Loktite]

I'll order some of the NE5532 opamps then! Thanks for the advice, Kaz. I'm fairly handy with a soldering iron so I'll be checking all the joints while I'm in there. Relatively speaking, it's a very serviceable unit compared to some of the things I've fixed.

rnolan, thanks for explaining and clarifying that it is the input jack on the rear that gets changed to inst level. I don't have any more rack stuff at the moment, but when I do start building up my collection (I'm really wanting a quadraverb in the future when I can find one local for a good price). I'll be sure to remember that upgrade.

[Kazinator]

Hey Kaz, do you have any recommendation for the OPA2604AP these guys can't find?

If it has to be Burr-Brown, the obvious just to be that popular OPA2134.

[Loktite]

Hey Kaz, do you have any recommendation for the OPA2604AP these guys can't find?

If it has to be Burr-Brown, the obvious just to be that popular OPA2134.

Any reason to recommend the OPA2134 over the NE5532 otherwise? It does not have to be any particular manufacturer for me.

Edit: I'm just going to order both and see how they sound to my ears.

[rnolan]

Hey Loktite, I have the original QV which is still going strong. They also made a QV plus I believe that some like, I think Max really likes those IIRC.  I haven't looked into it but if the plus works the same but has improved A/D and processing it would be worth considering, it's also a younger bit of gear.  I have a Midiverb4 in my live rack, so a much later unit but it's dumbed down from the QV and, while it sounds fine, I find it limiting.  But depends what you want.

[Loktite]

Hey Loktite, I have the original QV which is still going strong. They also made a QV plus I believe that some like, I think Max really likes those IIRC.  I haven't looked into it but if the plus works the same but has improved A/D and processing it would be worth considering, it's also a younger bit of gear.  I have a Midiverb4 in my live rack, so a much later unit but it's dumbed down from the QV and, while it sounds fine, I find it limiting.  But depends what you want.

I think for my purposes the QV will be great. I've seen some midiverbs and QV2, but my eyes are set on the original. Just have to wait for that sweet combination of having money and something showing up at the right time.

[rnolan]

Hey Loktite, so Max (Iperfungus) likes the QV GT which has an analogue guitar input section as well, but otherwise is the original QV.  Better still he's going to sell one of the 2 he has.  He's in Italy so you'd probably have to change the plug on the power supply (or get a US version). I messaged him about this post so expect him to chime in and be helpful, he always is