Aha! A new Pres@nus! I have heard some people say that they like this thing as-is, but I have not used one myself, but I have been thinking about getting one to M.O.D! Thanks for the pic! That saves me the work of opening one up to look. :thumb:
Let's step back for a second and take this a bit at a time, if that's okay. Firstly, what is it you find not to your liking about this unit and what are you using it for? dr. foster has given us a lot of advice from his experience, which I'm sure is good. But if you are new, I'd rather have more info before I advised you tearing into a working piece of gear (though its not a lot of $ or a rare item)
Can you tell us the part # of each of those chips? I cannot tell what they might be from the pic, though I'd believe that they are all quad opamps (14 pin SOIC) The resistors appear to be 0805 (size) not much fun to solder without that bent tip on a Metcal iron. I have done it with and without, I'd much rather use the Metcal... Some of those opamps might just be used to drive the meter or other things so some tracing of the circuit might save some time and money. A FET992 driving the meter won't sound better, if you catch my extreme drift. :grin:
[quote author="dr. foster"]Opamps are very sensitive to their power supplies. If the supply is slow and/or has a high output impedance, the opamp's performance will suffer.[/quote]
I agree with this... but first, I notice a lack of (I assume) bypass caps there where the empty footprints are near those chips. (Are they connected to pins 4 and 11?) Its a pretty compact layout, but I might think about adding something there first. They probably left them off because it wasn't "necessary", but its good eng. practice to put them in and might make a good improvement with small effort.
I am not familiar with the OPA404, but I looked up the data sheet. Seems a bit too fast to me (35V/uS) and if you didn't know how to look for oscillation and resolve that issue, I'd advise against them (my opinion!). They are also ~$12 each at Digikey. A more cost effective upgrade might be OPA4134 (spec'd for audio by BB and a pretty good OA, IMHO) ~$4 each. TLE2074 might be another chip to look at.
I'd be careful about removing the O/P caps before I actually checked the DC offset with a voltmeter. Bypassing them with a film cap might be an improvement, if you find you need to keep them in place.
[quote author="dr. foster"]See all of those black electrolytic capacitors near the heatsink? Upgrade those...[/quote]
(Uhh, I don't mean to be picking on the "good dr." here. I hope you won't take offense) Do those caps say "NEC" on them? If so, see if there is a two-letter code on them somewhere. NEC does make
some good stuff and you should be able to find the data sheet out there somewhere. If they are not low ESR/ESL, Nichicon has some great parts, I can't see spending a big chunk of change to mod this thing.
[quote author="dr. foster"]it probably has IR noise out the wazoo.[/quote]
I'm ignorant as to what
IR noise might be...could you enlighten us, please? I'm sure that whatever it is, a dance thru this thing with an o'scope would reveal any objectionable noises or problems. Your eyes (and monitor) must be much better than mine to determine what those diodes are from this pic!
Experimenting with different tubes
might give you more differences than any of these other mods we are discussing here. (And it really easy to change those :green
There are a variety of brands/flavors of 12AX7 out there, 5751 might be a pleasing alternative as well, though it has slightly lower gain (=70 vs. 100 of the 12AX7)
Let us know what you see there. HTH!
Peace!
Charlie