help a newbie tweak a Tube Pre.

i'm new to the diy schiz, but would like to start learning.

i'm hoping there are some mods i can do to make this Pr*sonus preamp sound different (i don't have tone preference, i just want experience), and help me learn a little about electronics in the process.

so if you see anything that could be improved upon, please shout it out. i don't know about electronics, but i follow directions well. and i solder well.

thanks.
here are some pics i took of the beast. and i have no schematic.
www.anicorettenightmare.com/tapeop/PTP1.jpg
www.anicorettenightmare.com/tapeop/PTP2.jpg

Well first, one would really need to see a schematic. Second, It looks like it would be hard to work on with those resistors being miniature. its probably a dual trace board too.

I don't think this would be a good thing to try and make "sound better".

If I were you I would get some kind of mic pre kit like the hamptone. or try making one of the simpler projects around here like the green pre.

good luck.

Yeah, man forget about the BlueTube... Isn´t it a BlueTube?

I have one of this and I can say it´s CRAP CRAP CRAP.
VERY NOISY.

Just sell it and put your money on nice DIY gear.

It might be the newer one channel Pr*sonus tube model.

That's got to be the newer of the Presonus tube preamps. I just finished modifying the EQ3B, and this looks similar in layout and component choices. You won't get a schematic for this--Presonus won't give them out. (I'm not sure why--it's not like this is some revolutionary new design that they've come up with)

You say that you can solder well--do you have any experience with surface mount components? This is what makes modding the Presonus stuff difficult for the average DIYer. I'll bet those quad opamps are Motorola's version of the TL084, and there are a number of quality replacements for it that won't suck too much current. A new favorite of mine (one that I used on the EQB3 for the buffer stage) is Burr Brown's OPA404. It's a FET opamp, and quite fast. While it's not as fast as some other quad opamps I've designed around, it does have a nice, balanced sound.

For the through-hole stuff:

Get rid of those cheap electrolytics in the signal chain--the lavender ones. Use a better-quality upgrade, such as Black Gate, Elna, or Nichicon UPW's (the poor man's audio-grade capacitor) on the input, and DC couple the output--just remove the output caps entirely, and jumper the pads. Chances are, your DC offset is nominal, and the input stage of whatever it is you'll connect this to is AC coupled anyway (meaning that it has capacitors on the input).

Upgrade your power supply. 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.

See all of those black electrolytic capacitors near the heatsink? Upgrade those with a better quality cap (Nichicon UPW is a great choice because they're cheap and good quality) Get rid of those nasty rectifier diodes--this thing probably uses a voltage multiplier of some sort, and it probably has IR noise out the wazoo. Upgrade them with something like a Vishay or General Semiconductor UF4001 instead of the IN400x that came stock with the unit.

What type of regulators is this thing using? If it's a 78xx type, look into upgrading to at least an ST Microelectronics L78xxCV type regulator--I've found that they tend to perform better than regular 78xx types.

Those Ei tubes aren't too bad, although you could probably stand to do better. I don't know what sort of screening process Presonus' manufacturer uses, and some of those Ei's can be awfully noisy.

Good luck!

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

My TubPre unit is all surface mount. They use ST33079 quad chips like the MC33079. I would not bother trying to upgrade it unless you are a surface mount soldering/desoldering expert. The starved plate tube section totally sucks. The only thing left is replacing the input and output capacitors to somethind spendy. Their advertisement of using a dual servo topology and the implication that it does not use capacitors in the signal path is a bit misleading. Isn't it?

The 33079 is actually a decent opamp for audio. Not much to gain by swapping these out, I fear.

Is this a starved-plate design? How high is the plate voltage?

If this is starved-plate, you may be able to upgrade this pre considerably by bypassing the tube!

Jakob E.

whoa. thanks for the help.
Sweetwater sent me this free with purchase of a mic, so there's nothing to lose, and i don't put much stock in it's revival. this is just a fun learning project.

the tube was the only thing i knew how to replace - i got the Ei and an Electro Harmonix one. haven't heard either yet.

please check this picture:
www.anicorettenightmare.com/tapeop/PTP3.jpg
there are 3 chips that i can see:
Chips 1 & 3 read MC33079D PGU337
Chip 2 reads 28CHRKM TL084C (and i believe it has the TexasInstr logo)

Regulators:
I did not recognize any regulators in there (nothing that looks like the ones in textbooks, i mean). tell me what i'm looking for and i'll get a #

Capacitors [near heatsink]:
replace all with better ones?
and they don't read NEC, but REC

Electrolytic Caps:
identified (lavender) [but all appear to be in O/P chain].

Rectifier Diodes:
identified and will be replaced.

as far as DC coupling O/P, removing caps and jumpering pads...i don't know what that means.

upgrade P/S - unless that means buying a new wall wart, i really shouldn't yet. unless you'd be willing to walk me through it.

thanks again for all the input. i can't wait to dig in!! :grin:
and feel free to draw all over those pictures..

the regulators are the three pieces near the electrolytic caps on the left side, atached to a big heatsink...

thanks, rafafredd

a pic of the regulators should anyone need
www.anicorettenightmare.com/tapeop/PTPreg.jpg

they are LM317, LM7815CT, and LM7915CT

thanks again

[quote author="tk@halmi"] Their advertisement of using a dual servo topology and the implication that it does not use capacitors in the signal path is a bit misleading. Isn't it?[/quote]I'd have to agree. If they have servos in there, what's the point of the caps? In that case you should be able to ditch all the electros in the signal path.

One of the techs at work showed me a trick for reading the number on chips that are difficult to read. Put some alcohol on a cotton swab and wipe across the top of the part, wait a couple seconds and when the alcohol starts to evaporate, you'll be able to read it for a few seconds. Simple but amazing.

and they don't read NEC, but REC

Click to expand...

I thought the "N" looked funny...please ignore my comments then. (chant: Nichicon...Nichicon...Nichicon...Nichicon...!)

removing caps and jumpering pads...i don't know what that means.

Click to expand...

Just remove the caps a replace with a wire...clipped resistor leads are good for this.

If this is starved-plate, you may be able to upgrade this pre considerably by bypassing the tube!

Click to expand...

That's certainly another option!

The 33079 is actually a decent opamp for audio. Not much to gain by swapping these out, I fear.

Click to expand...

I'd say that's advice you can trust. :thumb:

HTH...
Charlie

FWIW, if you have some money to toss around, your energy would probably be better spent building something that is *designed* to sound cool, there are a myriad of kits out there to this end. Putting together something like that will garnish you the same learning and experience than trying to mod something with surface mount parts. Plus, when you finish a well designed amplifier, you have something that sounds good and if you chose a proven design, something that can be made to work and something that people can help you with. You could screw around with that presonus thing and spend hours and hours and hours swapping parts and really not see much of a NOTICABLE improvement. I mean, you might get some noticable level of performance upgrade out if it but I would honestly be suprised if you are going to make such a big difference that would be noticable in a mix without significantly redesigning it which is a total waste of time IMO when you can build something like the hamptone jfet preamp which sound cool or the JLM audio pre which if you can afford to use his 99v opamp will absolutely explode your head. Of course if you only have a few bucks you can play around, but I would be suprised if changing a regulator or a capacitor will really make an improvement thats worth making. I get the feeling with these boxes, the flaw is not in the component selection but in the design itself. Take a UA 1176 and go and put all hifi caps in there and stuff like that and I bet you'll hear differences that are pretty noticable but you know, something like this I would suspect to require a redesign much more than a component swap. Ive never futzed around with one, this is all just my speculation, but for the time you'll spend potentially making a parallel jump in sound with it, if you can afford to spend the money on one of several kits available now you can make a real significant upgrade to your audio path while learning at the same time and of course the benefit with a popular kit is there are lots of people building them so we can help you out when questions arise...

dave

Wow, Dave...that seemed like one big sentence...
:grin:
Charlie

soundguy, i surely believe you. this is just for electrical experience, and getting familiar with which parts do what in circuits. and i'd rather learn on this thing than my 1176.

i'd like to graduate to better DIY projects, but right now i don't know if current flows into this end or that end of caps and resistors...i have a preamp i'm willing to kill, and Slone's Guide to Electricity and Electronics.

and thanks for looking out for me (as you've done before on TapeOp).

plus the GreenPre guy says the kits aren't ready yet...