Telefunken 672s

[volki]

hitchhiker,
which thread was that, about the pads?

axismatt,
great it worked. so you do get a good sount with 2 x 40 ohms? have you tried something like 2 x 600 (to get Z.in of 1K2, which is common in mic pre's); comparison sound-wise? - got to check that myself sometime, would be interesting.

about phantom pwr, you would either need a dc to dc converter to get to +48v properly, or build a dedicated supply. the latter is fairly easy actually.
there are some professional designs (if not top of the line) around which simply use +24v for phantom powering, though. some mic's might not like it, and probably you'll get a degradation in max. spl (maybe in overall sound, even), but if you wanna try for simplicity, you could apply +24v into the mic lines via two matched (>0.4%) 3k4 resistors, and see if you're happy with the result. for that, a little pre-filtering of the supply with, say, 200 R in series and a 220uF cap to ground wouldn't hurt, either.

different suggestions, anyone?

 

[hitchhiker]

volki,
the pads are at the bottom of page one of this thread.

axismatt,
try out Volki's 2x600 ohm suggestion, it should be better. (the pads will
be better then too)

as for phantom power , I once used a circuit that converted the 24vdc
to ac ,then doubled it to 48vac and then rectified it back to 48dc.
I could try to dig it up if you need it.

What I use is a seperate DIY 4 ch unit that I can put in line when I need phantom pwr. That way I don't have to build a phntom supply every time I rack or build a preamp.
( I have mostly tube mics) also,I think most AKG's are fine with lower
voltages as Volki described.

Lance

 

[Guest]

Sorry for resurrectign this thread
But some very important info from Oliver here
http://www.gearslutz.com/board/showthread.php3?s=9a5151f30a244010443f78360937da76&threadid=17301
Apparantly somebody has been sold 672d's as mic preamps - and they are definately NOT...
(poor man)

 

[hitchhiker]

that's interesting history. I have both 672s and 672Ds and yes the 672 is
a bit nicer. They are still very close in character, I guess the same transformers has something to do with that. The 672 has better bottom end and the 672D has an slight edge at aprox 10k. As for headroom, the 672 has not seemed too wonderful for headroom either.

I think the guy who bought them will not regret racking them but they
should be less money than a true 672.

Lance

 

[axismatt]

Hi,
I finished racking my 672s today, with 20dB pads, phase switches, gain pots, and holes for the Phantom Power switches when I figure out how to do them! Anyway they work great and I compared 620R vs 40R resistors on the input Volki. To be honest I prefer the 40R as they were far louder plus brighter- giving the high mid push that's supposed to be associated with 672s. The difference in brightness between the two resistors was greater for an SM57 than for a Royer 121 - is this due to different impedances of the 2 mics? Anyway my two-penneth is that the 40R set up gives a more distinctive sound for these mic pres....

 

[volki]

axismatt, well, "when it sounds right, it is right"! :grin:
the different r's have more impact on dynamic mic's since their complex output impedance varies over frequency and interacts heavily with them.

 

[hitchhiker]

thanks for the report axismat. Maybe I'll just leave mine with the 40r.

I tried the 620r in a 672 that is not yet racked and I wasn't sure I liked
the change either.

cheers,
Lance

 

[moqtev]

Hitchhiker and Aximatt could you explain the 15db pad again for me.....I don't get how you connect the resistors :? For instance when you write 430 ohm resistor on pin 2 and 3....is that pin 2 and 3 on the input xlr (hot and cold) or pin 2&3 on the 672? And where do you place the 200 ohm resistor?....please be very specific, my english must be bad or I'm simply stupid cause I dont understand it

Thanx for your understanding....I will have my brain fixed when I understand this

 

[hitchhiker]

moqtev,

the pin numbers are the XLR, on the bal signals pins 2 & 3 put equal value resitors (in series) and then a resistor between pins 2 & 3 at the preamp end.

This site explains it better.

http://www.uneeda-audio.com/pads/

There are a also a number of good pads shown in the threads. Try a search on input pads, mic pads etc

cheers,
Lance

 

[moqtev]

Aaah thanks Lance - :thumb:

 

[echorec]

Thanks for all the great info in this thread!

I just finished a couple of these and put them in a butchered 12 unit rack. Not super pretty but a handy little German lunchbox.

I used 40 ohms on the input (also tried 750 but preferred the sound with 40). I used 2.2k in series with a 250k potentiometer.

I love the sound of these. What would be the best way (without making a pad) to get minimum gain down about 5 dB? Should I make the input R slightly higher (maybe 80?) or maybe lower the R in series with the potentiometer?

Thanks.

 

[eliya]

sorry to wake up this thread.
I recently bought two modules of v672, I already have a psu built for the modules, and now my dilemma is what input impedance should I use.
I see that some people use high imp and some people use low imp, each one for a different use.

I was thinking, why not to use a dual pot as the input impedance and then I have different input impedances for different applications. or maybe im just way off?

one more thing, in the second post, they said that pins 7&8 are the input pins. is that so?
tnx,
eliya.

 

[rodabod]

[quote author="eliya"]my dilemma is what input impedance should I use.
I see that some people use high imp and some people use low imp, each one for a different use.[/quote]

For mic inputs, 1.2K is a good suggestion for input impedance. It depends on what sort of mics you wish to use.

I was thinking, why not to use a dual pot as the input impedance and then I have different input impedances for different applications. or maybe im just way off?

The problem here is CMRR. The two potentiometers will not "track" well together enough and therefore will not be well matched. You need the impedance at the +ve and -ve inputs to be closely matched otherwise your common mode rejection ratio will be poor (it needs to be well balanced).

 

[jensenmann]

[quote="rodabodFor mic inputs, 1.2K is a good suggestion for input impedance. It depends on what sort of mics you wish to use.
The problem here is CMRR. The two potentiometers will not "track" well together enough and therefore will not be well matched. You need the impedance at the +ve and -ve inputs to be closely matched otherwise your common mode rejection ratio will be poor (it needs to be well balanced).[/quote]

older german tube mikes like to see something around 2k. It´s the beauty in the V672 preamp design that you can decide yourself about the input impedance. Why not make it switchable?

:sam:

 

[rodabod]

[quote author="jensenmann"] Why not make it switchable?
[/quote]

Yes - I just emailed Eliya and suggested adding a DPDT switch and two pairs of resistors.

These preamps sound pretty cool...

 

[eliya]

Thanks again guys,
A guy from my work place who's racking up some of these modules, suggested 750R load for dynamics and condensers. What would you use for ribbons? Im thinking of having a switch so I can use these pre's with all kinds of mics. what input impedance would you use? he suggested a minimum of 2k.

eliya.

 

[eliya]

One more question(sorry for being such a nag..). I've seen some people connecting resistors between legs 1&3 and 2&4 to create the input impedance, and the signal itself goes to legs 7&8. Others connect both input signal and resistors to legs 1&2, so, what's the difference? which is better?
thanks,
eliya.

 

[jensenmann]

It´s exactly the same. Input is 7+8 which is internally fed to 3+4. From there you go through the inputresistors to 1+2 which is the primary of the input iron.
You could otherwise go directly through the inputresistors to pin 1+2. There´s no difference. Did you check the pdf at kbi´s site? Page one has the pinout where you can easily see it.
:sam:

 

[eliya]

Thanks Guys!

Looking at the manual, I figured this out.
thanks again!
eliya

 

[sea mouse]

I have racked a couple of these modules recently and am keen to know if anyone's figured out how to bypass the 15kHz broadcast filter.

I'm a total newbie and don't know what an inductor does and have no way to test it but people mention L201 or L102 and I'm just hoping that someone can give me an idiot's guide to bypassing this filter somehow.

cheers.