another passive eq?

[synthi]

I`ve got a couple of RFT Kleinstudiogerat mixers and finally I`ve found the schemos for them.
The eq section seems to be passive, so the schemo would be interesting:

I`ve got 4 of theese eqs, they are thinking to be inserted in the preamps, the bad thing is that you can only use the "prasenz" presence or "hohen" trebble one at time since they share the switch, Do you think they can be moded for using separate controls at time? algo there is a easy way for having "Q"?

Thanks!

Synthi.

 

[Samuel Groner]

No answer for your Q, but I would very interested in the entire schemo. Any chance?

Thanks
Samuel

 

[synthi]

Yes Samuel, no prob.
I`ll be in travel for holidays (Spanish Semana santa) until next saturday 26th, then I`ll scan and post the full schemos here.

Synthi.

-edit- I`ve had some spare time with the scanner here :wink: so here is the preamp schemo until next week :razz:

 

[synthi]

I still need to test the modules, but the first impression is that sound good but a bit noisy, maybe because the transistors used?
Comments are welcome!

Synthi.

 

[Samuel Groner]

Well, that's good service! Thanks a lot!
Samuel

 

[mig27]

I also have an RFT "Studiokleingerät" at home.

The entire console is a real sleeper!
Mine has been serviced by a knowledgeable broadcasting technician.
He had to replace a couple of transistors to bring the noise levels back to specs. Restoring this console is a very rewarding project - you won't be disappointed!

RFT also built larger equalizer modules for studio use, the W732c.
Incredible design, all point to point, huge inductors, germanium - my favourite next to the Neumann PEV modules.

Will post some pictures of the guts later on - be prepared for some porn...


Michael

 

[synthi]

mig27: Good news!

I was thinking that the mixer couldn`t be bad...

Best Regards,

synthi.

 

[chriss]

Hi,

I only know the 'Kleinstudiogerät' KSG244. This one also has a passive eq, called 'FB80' (Filter-Baustein = Filter-Unit). I don't know how much this is a relative to the FE625, but the filters here are high-shelve (cut and boost from 1,25khz), low-shelve (cut and boost to 480hz) and the 'Präsenz' (presence) is a boost only with a bell characteristic at a fixed freq. of 3khz. Presence-pot has nothing to do with High-shelve-pot.
Another difference is the fb80 has no in-/out-transformers and has asymmetrical input and output.
I would guess it's quite similar with your unit. But it would be interesting anyway to have the option to alter the freq. of the bell-boost. I think it should be possible to do this with changing the values of the r's and c's around the coil. Unfortunately I couldn't open the bigger version of your schematics to compare it to the FB80's.

chris

 

[jensenmann]

A good mod might be changing to better transistors. Old eastgerman transistors are famous for being crappy
Jens

 

[chriss]

good idea, jens. also for the geffel microphones. here is a list with exchange-transistors:

http://www.elektron-bbs.de/elektronik/tabellen/ddr/transis.htm

chris

 

[jensenmann]

Chriss, vielen Dank! Genialer Link. Jetzt kann ich meine Gefells angehen - schon wieder ein Projekt mehr.
:thumb:
Jens

 

[chriss]

freut mich. würde mich auch über entsprechende erfahrungsberichte freuen, da ich gerade 5 582's auf XLR/48V umrüsten möchte. den mechanischen part habe ich zwar schon erledigt, aber die ultimative spannungsregelung habe ich noch nicht gefunden...

christian

not to exclude the other:
nice. i'd like to know about the modifications jens is about to do to the gefells, since i am about to modify my gefells to xlr and 48V phantom. i've just done the mechanical part yet. the perfect voltage-regulator in smd-style isn't found so far...

chris

 

[synthi]

I`m Back!

thanks for the info chriss,

I have another RFT mixer with FB80s and FB81s (parametric) eqs. check:

http://www.groupdiy.com/index.php?topic=1395&highlight=rft

I`ll be posting more scans for the schemos along the week.

synthi.

 

[jensenmann]

ChrisS, my mv691 already have standard europ. transistors and obviously I am too stupid to open my MV692s because I couldn´t pull the enclosure over the switches. Any idea (except more force) how to do that?
Jens

 

[synthi]

:roll: I just remember I own another "RTF" mic preamp in my vault, is a tube mic preamp VEB FUNKWERK LEIPZIG model MV 4056, does anyone have a schemo?

Thanks,

synthi

 

[chriss]

jens,

you need to lift up the two plastic-covers from the housing. this is best done with a very small screwdriver that you shove under the caps. then you need to untighten the screw and you can pull off the tube. but be careful because the two labels for -10dB and lowcut fall off now, so be sure to notice which of them belongs to which switch. (strange sounding sentence...).
for the mv691: did you change the transistor or was it done at gefell? and more interesting: what type of transistor is it? does it sound noticably better? less noise? i'm curious!

chris

 

[jensenmann]

I bought the MV691s (pair) from a guy who had this mod done by a former MG tech. I never compared it to the original version. Noise should be improved and the pair is stereomatched (he said). Let me know if you´re interested in the tech´s adress. The BC179B is still inside, one transistor is labelled C10 C06. This might be the SC206C (but I don´t know anything about eastgerman transistors). On the solderingside of the PCB are two SMD devices each with three pins but no labeling. Maybe these are the voltage regulator and the third transistor. Obviously the K<pi>303B has been replaced by one of these SMDs.

My MV692s have all the original transistors inside. The voltage regulator is a IC called IS 29-25. If modding requires smd-soldering then it´s pain in the arse and I´m not gonna do this. Anyway the PCB is held by a plastic-entity which covers the entire solderingside. Thus I cannot tell you what´s going on there.

And now it´s time for :sam: and breakfast
Jens

 

[chriss]

the two smd's are probably the same as the gefell-mod for P48. it's a 12v-zener-diode and a pnp-transistor. they are part of the voltage regulator.
if the name of the tech is thiersch, i have his address.
the IS 29-25 is not a voltage-regulator, it is an operational-amplifier. maybe exchanging this one can improve noisefloor. are your mv692s modded for P48-supply? this is usually done with a small 5-part-smd-pcb next to the transformer.
the transistor in the mv692 that is important for noise-behaviour is the KP303. i don't know what type this could be replaced with and if anything else in the circuit needs to be changed then.

chris

 

[jensenmann]

Yep, Thiersch, that´s the guy.
The MV692s are P48s. Do you have the schem of the P48 version? My schem is the 12V version. I´d love to take a look into this one :grin:

Jens

 

[chriss]

No, I just have the 12V-schematics too. Maybe they keep the 48V-version as a secret to earn the money from the mod's. Wouuld be o.k., since the Gefell-guys are really nice and helpfull and their prices are very, very fair.

...