After lot´s of fight with hum... HERE is my new DIY MB1

This one almost killed me. Channels one and two are MB1 as we know it. Channels 3 and 4 are another circuit, very close, but not the same. Also a 12ax7 at the input and paralell 12au7 as cathode followers at the output...

This one almost won me. Two big EI power transformers in one box. Lots of hum at first. Now it´s way down, better than my first MB1. Oh, and don´t forget that big choke...

Well, cheap cmponents again. OEP and BOGEN for input transformers and EDCOR 10k/600 output transformers.

Regulated heater and unregulated, choked B+.

complete schematics to follow soon.

http://photobucket.com/albums/a363/Bodanga/

password : groupdiy

OH, and thanks for the help again!!! :thumb: :sam: :guinness: :razz:

Nice project Rafa- that front panel looks great. How are you finding those Edcors? I've never played with them- I can't find a supplier in the UK!

Have you done the back-to-back thing with the mains transformers? It can be quite tricky getting things low-hum, but I see you've moved the transformers about to get the optimum orientation.

Are you happy with the sound? And what's the difference between the two channels as far as sound is concerned?

Nice and detailed pics too :thumb:

Mark

Nice! :thumb:

Nice one Rafa!

Any particular meaning in the word "Bodanga"? Its a cool sounding word anyway.

Peace!
Charlie

No particular meaning. Just kind of a nickname... Some guys used to call me like this in the past :razz:

Well, EDCOR sounds very very OK to me. It´s not like a linx audio at all, and inductance is way lower, but good enought for general use.

Tomorrow I´ll take this to the studio, so I´ll have a better figure of how does it compares with other preamps, including UA, Langevin Tube, Langevin Transistor, Pultec, DIY Pultec MB1 (My old one, with UTC and LynxAudio trafos), DIY neve 1272, etc...

I can't see it. :sad:
asked for a login and password.

Hey, CJ. Log in and password are in my first message in this thread...


Log In: Bodanga
Password: groupdiy

Good job Rafa.
:thumb:

chrissugar

Super box Rafa!
:thumb: :thumb:

Bodanga sounds cool btw!
keep us posted about your findings compared to the other pres please,

Cheers,

Tony

Hey, just came from the studio and I´m really happy with the unit. I only had time to compare it with the Langevin 5116b, my NEVE clone 1272, a strange moded Pultec RIAA box and a soundcraft GHOST mixr for reference...

Well, at least for piano recordings with TELEFUNKEN ELA-M 251 Tube mic and Yamaha Grand Piano, that´s what we´ve found...

We started with the Ghost, and it really SUCK... Donkey balls. Nothing else to say about it. :? It´s like playing piano in a cardpaper box, if you know what I mean :wink: ... It really SUCKS!!!

Then to the NEVE clone with OEP inputs and LynxAudio outputs. Well it performed 100 times better than the Ghost thing. Much more open sound, more attach, more clean high end, yet not bright. Very musical overall, with nice and big bass also.

Then the four channel Pultec MB1 box you see in this thread...

The OEP channels with the original MB1 circuit performed 10 times better than the NEVE, in this source, with this mic, and this situation... I´m finding out I´m a tube guy after all this DIY :grin:. The whole piano was bigger, with a very pleasant sound. It´s more smooth yet with lot´s of attack and fast response. It´s all much more "together". The harmonics interact better. It just seems more tight...The chords in the mids interacts better with the bass and the bass interacts better with the highs. The final result is much more interesting to my ears than the NEVE clone preamp.

The other channels, with a different circuit was close, but not so good. It sounds really close, but overall, the sound is just a little more messed up. The highs are also not that beaultifull. The input transformers are the BOGEN black ones and he circuit is also a two triodes of a 12ax7 for voltage gain and a parallel 12au7 cathode follower at the output. Still much better han the NEVE clone. I think I got this scheme here in the forum as a Peavey micpreamp MOD, by Bluebird, if I reacll right. It does sounds good, but original MB1 circuit sounded better this time.

I´m using all polypropylene caps in this bogen transformers channels, and in the MB1/OEP channels, I´m using old film and foil polyester at the interstage and oil and paper (bypassed with 100pF micas) at the output.

He had another tube preamp there. It was an original Pultec RIAA preamp, with the big beauty K241D input transformer at the input, like the original Pultec MB1. It was modified by an old Engener here, and it has a transformerless output stage. It´s probably a whyte cathode follower after the original MB1 circuit, but I cannot say for sure. I don´t know the circuit inside this box. Well, it´s not worth talking about this one. Both my channels just clearly blows it away . We were just not impressed with it. Better than the Ghost, for sure, but not at all what I would expect from a preamp with a PEERLESS K241D at the input. :? I liked my NEVE clone better than this one. Can´t say why, as I´m using OEP transformer in the NEVE :?: . But anyway, I bought this strange Pultecbox home, so I´ll check what´s up with this thing... Oh, and it had tube rectifiers, but so did my first MB1 clone, and it sounds great.

Then to the langevin Tube 5116b, with regulated SolidState PSU. Well, I must say that my new MB1 channels of my four channel box gets very very close to this. Even with OEP and EDCOR transformers... But the Langevin, with a push pull circuit and all those Western trafos sounded a little better. The sound wasn´t very very diferent overall, but there is a quality, an openess in the highs that my preamps didn´t show. It has more space and clearness in the highs.

Tomorrow I´ll try my old MB1 with UTC at the input and Lynx Audio at the output against my new box. I´ll also try the UA 610 and maybe, if we have the time, a SolidState Langevin box and the cheap $50.00 dual NTP preamp...

I´ll post more if this kind of review interests you.

how do the Edcors compare to the OEP's?

EDCORs are on all outputs. OEPs are on two inputs. Bogen are on two inputs. No EDCORs at the inputs, and no OEPs at the outputs, so can´t compare those...

But EDCORs sounds pretty good to me on the outputs.

One thing a lot of builders overlook is the magnetic coupling between the power transformer and input / output / interstage transformers. Many reputable manufactureres used to put the transformer on an octal plug that would insert into a mating socket chassis mounted. The octal socket is fastened with a big clip allowing it to rotate in place on the chassis. This way, after the unit is up and running, the transformer can be rotated to minimize stray magnetic coupling to the input. Something to keep in mind for future projects.

Yeah, bill. Thanks for the tip.

That´s why I like those one-fixing-point-only transformers, like the beyers and HAUFE. But I´ve messed arround with the power transformers a lot and now hum is very very low.

Regarding the "old Pultec RIAA amp", that's probably the PC-10. It's two MB-1 circuits in a box with the phono equalizer circuit inserted between them. I would hope that the equalization was defeated if it were being used as a mic preamp!

I'll be gosh darn, Bill. So that's what those arrows are on the outside of the DuKane x-former mean. Only in this case, the actual wires inside rotate, so they tell you how far you can go before they snap.

You got it CJ! they would allow some slack on the socket wires to allow for turning. The transformers are marked Do not turn past this point. This prevents winching up the wires under the chassis. There isn't any need to turn more than 180 degrees anyway.

[quote author="NewYorkDave"]Regarding the "old Pultec RIAA amp", that's probably the PC-10. It's two MB-1 circuits in a box with the phono equalizer circuit inserted between them. I would hope that the equalization was defeated if it were being used as a mic preamp![/quote]

Yes, dave, the original circuits was a PC10RA. Off coarse there is no RIAA equalization anymore. It was moded for mic pre use... But the guy who meoded it doesn´t simply took the RIAA away, like anyone would do... There´s some strange things he did there. There is no output transformer and there is an overall feedback added, going from output to the input transformer secondary windings... Can´t say why someone would do this??? :roll:

As it sounds bad as it is, the studio guy just said "take it and build something with this... There was two of those units in the studio. He moded one channel of RIAA into two channels of mic preamp. Now there´s two channels in each of the boxes, for a total of four channels. One channel in each box uses the original MB1 transformer, the Peerless K241D. The other channel uses a UHER transformer. both units are the same now, with a K241D in channel A and UHER in channel B. I´ll just rebuild one of the boxes to original MB1 schem, as it has a PEERLESS K241D at the input. This way I´ll have the chance to hear a real MB1. There is also a 15k/600 transformer that was originally on this box and the guy also gave me this, so I´ll use it at the output. It´s not a TRIAD HS50 as used in the original MB1, but looks and tests very good. Should be a very close model.

Well, sometimes pics says more than many many words...








In the last pic you can see the Big Peerless input and the small UHER input...

OK, some more pics. Here you can see the front panel, with provisions for phantom power and level control for two channels:



And here you can see the input transformer pins, with the strange mod, with bad quality electrolytics as coupling caps before the transformer... :?


It will be a pleasure to rip it all off and redo he MB1 original schem in this one. :twisted: Then I will be able to compare with my early UTC/lynx version and the OEP/EDCOR version in this thread.

I can´t believe how good the OEP/EDCOR sounds. Those are cheap trafos! The hell with puritans. :green: