Telefunken V 672

[schoko23]

I recently bought a Telefunken V 672 with option for another.

When I first tried to use it as a mic-pre I couldn't get any low end out of it.
I used an EV Re 20 and compared the signal passing the V672 and then into the line in of a Mackie VLZ to the signal going directly to the mic in of the Mackie.

The first problem was that there was some awfull hum when I didn´t put the Signal Ground to the 0-Volt of the Power Suplly altough the manual says that there is no connection beween the housing (Signal GND ) and 0-Volt.

When I connected Signal Ground and 0-Volt, the hum was almost gone, but still the amp sounded quiete noisy.

Another problem was that no low frequencies seemed to be passing the module.

Meanwhile I replaced all caps against new ones, which helped.
But still even the Mackie preamp has a lot more low end.

Has anybody some ideas what the problem could be ?

Has anybody already racked one of these modules?

I'd like to build a rack containing two V672, a 24V DC power supply, a 48V DC power supply for Phantom Power, a phase revese, an instrument in and probably some VU-Meters.

Maybe anyone could come up with schematics.



greetings from Austria

(please excuse my english)

 

[Rob Flinn]

Have you used input resistors. the V672 uses what is called by telefunken "Zero ohm inputs" i.e there is no input impedance. What you do is conect a pair of matched resistors , one to each side of the input & they determine the input impedance. I think I used a pait of 620 ohm resistors for mine. It definately sounds a bit strange without them !!

 

[schoko23]

Yes, I tried different resistors on the Input (2*20Ohm, 2*100Ohm) and a 200kohm poti for the feedback resistor.
Still I don`t get any bass frequencies.

 

[EZ81]

Hi Schoko,
schematics/manuals etc. are at kubarth.de, GDIY, vintage modules.

If you (or anybody else) happen to toast an input transformer, let me know, i bought a few :shock: not knowing about the strange input technology.

The input impedance is only the sum of these two resistors, maybe 200Ohms is still to low?

;Matthias

 

[Rob Flinn]

The input impedance is only the sum of these two resistors, maybe 200Ohms is still to low?

For an average mic 200 does sound low to me. 1200 is better.

 

[q2audio]

A while back I did some testing to come up with a good value for input resistors for V672's. I initially thought 300 ohms (x2)was a good compromise, but then discovered that this caused bandwidth issues at higher gains (the top end rolled off - in a nice way if you are looking for that). I finally decided on 82 ohms (x2) as a good place to be but it needed a 20dB pad (per the Jensen app note) in front of it for loud sources such as guitar. This combination gave me, if I remember correctly, about 6 to 50-ish dB of gain available (using/not using the pad) and it did not seem to load the mic even thought is seems that it should. There must me something going on with reflected impedance from the input stage- I'm not much of a transformer guy so someone else should comment on that. Also, a 250K linear 'gain' pot between pins 9 and 10 is a pretty good compromise as well, but as you get toward past 2:00 (CW) the gain increases very little. I think a 150K pot would have been about perfect if it were a standard value. Q2

 

[matta]

Hmmm,

Went I got my V672?s (These are ANT models, which came after the Original
Telefunkens) I had a similar issue to it passing sound, but all top end, no
low end at all, and only at High Gains was I getting anything (I can?t
remember off hand what resistors I used).

My issue turned out to be a bad Electrocap. After swopping it out it was
restored to full health, it turned out that the cap was shorting to ground.

You mentioned you replaced the caps, did you DO all the lytics, including
the Tantalums?

If working correctly you should have more than enough bass response, these
pres are lovely and have a warm low end, I use my pair on kick and snare and
love the ?retro? vibe it gives the kits, esp. on some Jazzy styles of music.

Cheers

Matt

 

[schoko23]

First of all thanx for your replies!

Meanwhile I got an old user manual in german.

The gain control when you use the 672 as a micpre is described as following:

gain from 0 to 30db is achieved by using a constant feedback resistor of 2,4kOhm and variable input resistors between 13,4kOhmh (wich would mean 0db gain) and 2*20Ohm (for 35db Gain).

gain from 35db to 65db is achieved by using a constant Input Resistor of 2*20Ohm and a variable feedback resistor between 2,5 and 300kOhm.

In my opinion this could best be done by using a step-switch with three layers. two of the layers for the variable input resistors and one for the variable feedback resistor.
then one more switch to change between the gain modes.

This data I read out of a graph which also shows the noislevel over the gain.

If anyone is interested please let me know.

Of course I know that 40Ohm looks damn low as an input resistor for a mic wit 200Ohm, but they say it in the original telefunken user manual.

I think that the sound of this module is never going to be the same over different gain settings anyway.

But still I don`t get any low freequencies through it, no matter what gain setting.

I already changed all the lytics, the tantalum too, but for testing I raplaced them by cheap caps. Could this be the problem?

Should I replace the MKS too?

I thought they wouldn´t dry out over the years.

 

[q2audio]

I have seen the lack of input resistors cause distortion/saturation of the input transformer but not lack of low end. It sounds like there is a problem in the module somewhere. Do you have an oscilloscope or know anyone who does? You need to track the signal through the module looking at 40Hz and 400Hz alternately and see at what point the problem is happening.