Melcor Deconstructed

[Flatpicker]

[quote author="Jonkan"]I tried the same preamp with the 603s before i modded the opamp. With c1=680pf.

Cant say that i reacted the same way about the sound back then.

Also tried my mindprint di-port pres.
Still sounded good, but it was a while ago so i cant be 100% sure about all of this.

Could it be the polystyrene cap in the melcor opamp that has caused this? Should i change back to 1000pf COG/NPO?
/Jonas[/quote]I should have said that it was the polystyrene that hurt the acoustic guitar transients on the 603 - not the entire mod. So yes, it may be what's killing the Melcor transients as well. Switch to a ceramic or better yet a miniture metal film, and see.

It's really funny how out of every mic I modified using a polystyrene between the capsule and FET, only the 603 and the MC319 had transients that were too soft for my taste.

 

[Jonkan]

I could easily fit a regular poly film cap in there, would that be preferrable to a cog/npo ceramic? Is that what you meant by metal film?
/Jonas

 

[Flatpicker]

I was referring to metalized polyester (these come in small packages), but just use the poly film if it'll fit. I?m curious if it puts the ?attack? back into the sound. Let me know how it turns out.

 

[Jonkan]

ok so i tried some more mods.

1) C2 = 47pF, and goes to Q4 (B) instead of Q3 (b)
2) 39k resistor between Q3 (C) and Q5 (B)
3) dual D2
4)C1=polyester film, instead of polystyrene or cgo/npo

About mod 1): What is the difference between this mod, and connecting the c2 cap directly to the negative input? Which would you prefer?

(btw, it works great this way, so i havent really tried the other)
And can we agree on c2=47pf, or should i change back to 10pf?

This mod made the opamp more stable to my ears. Before i had some minor oscillation going on with some gain settings, not anymore.

mod 2)
This mod seems to **** with the lowend-low middle compared to just doing the other mods. Muddies up the sound. So i quickly unsoldered it again. Anyone else tried it yet?

mod3) seems to make the biggest improvement to me, im keeping this.

mod4) So the 1000pf polystyrene cap didnt work out for me, tried a regular poly film cap. Sounds better! Even better than COG/npo to my ears (yes, i compared them directly).

Sadly I managed to destroy something in one of my channels when trying out all of these mods (accident with a wire from the psu). So if you api experts can help me get my channel up and running again, please do.
Details here:

http://www.groupdiy.com/index.php?topic=10041

Please help a newbie out!
/Jonas

 

[rafafredd]

What is the difference between this mod, and connecting the c2 cap directly to the negative input? Which would you prefer?

Please, if you have a working opamp, try the mod with the C2 directly connected to the inverting input. This is how the original was, so it would be good to know if it will work.

 

[rafafredd]

BTW, no polystyrenes on the one I´ve taken apart... All ceramiocs. Maybe COGs, I don´t know...

 

[Jonkan]

Im not touching those opamps again, until i get my ruined channel up and running... :green: I just whish i had installed those millmax pins i bought. Then i wouldnt be in this mess in the first place. I have the whole opamp soldered to the mainboard using diode legs. I didnt realize you had to drill in the pcbs, and as i dont have a drill...well..
/Jonas

 

[louder]

alo jonas
if you used fabio`s boards,you don`t have to drill nothing,if you have the right pin`s and socket`s.
as for the melcors, i used cog`s,and they are superb with no mods.there`s something the way they saturate that i love.when you drive them hard,they sag(?),a thing that i like.
best regards
pedro

 

[Jonkan]

ok, then i must have gotten the wrong pins...They almost fit, but not quite.

Both cog and poly sound fine to me, but that day on that source etc, i preferred poly. Didnt try to saturate the pre much though. Recorded myself playing ac.guitar (badly i might add), and singing through a single mxl603s. Sounded incredible
/jonas

 

[Jonkan]

What value are you guys using for the R2 resistor?

Somone recommended 1k in one of the threads here (cant remember which), instead of 91r.

Is this a good idea, or should i switch back to 91r?


At the moment, im trying rafaredds mod where c2 goes to the - leg. Seems to be working fine!

Should i use c2=10pf with this, or c2=47pf?

Cant decide though which i like better though, c2 to - leg, or the mod peter c purposed.

To many choices for me!

 

[Flatpicker]

[quote author="Jonkan"]Should i use c2=10pf with this, or c2=47pf?[/quote]10pF

Cant decide though which i like better though, c2 to - leg, or the mod peter c purposed.

Can you describe the sonic differences you are hearing?

 

[Jonkan]

I have to do more listening tests, but ill let you all know my impressions!

What about r2?
/Jonas

 

[clintrubber]

Rafa, it was you who traced the 1731, correct ?
Can you recall any thermal coupling of any of the biasing diodes
to the power devices ?

Thanks,

Peter

 

[jeroddumas]

I have built 3 opamps, 2 work fine. The 3rd does not. i have check all values and thaey are fine. It passed audio at first, but it was very low, now it doesn't work at all. Any clues as to where I should start my search?

 

[clintrubber]

[quote author="jeroddumas"]Any clues as to where I should start my search?[/quote]
Check the DC-voltages.

Dunno if there's a 'should be like this'-overview somewhere, but you should try to get an impression about the DC-situation first.

Hey wait, you have two working examples ! Compare the DC-voltages.

 

[rafafredd]

No termal coupling on the originals of any kind. Diodes was just CLOSE, but not touching the actual output transistors. Maybe the compound would help termally coupling them...

 

[dissonantstring]

hi everyone,
sorry for the stupid question, but for the D2 on the 1731 the double diodes (1N4148) should be in series? like so? ---->|------>|----
thanks,
grant

 

[AnalogPackrat]

Grant--that's correct.

 

[Bauman]

yes, put them in series!

:guinness:
Fabio

 

[clintrubber]

[quote author="dissonantstring"]hi everyone,
sorry for the stupid question, but for the D2 on the 1731 the double diodes (1N4148) should be in series? like so? ---->|------>|----
thanks,
grant[/quote]

Yep, in series :thumb:

This increases the quiescent current, well, better formulated: brings it back to the original situation since as the 'original investigators' found out that that D2 was originally 'a type with two diode-drops' i.s.o. just one.