Telefunken 672s

Hi,
I bought a couple of 672 modules a while back with a view to casing them up and adding the usual mic-pre features (gain, phantom power, phase etc). Unfortunately all attempts to translate the German schematics have failed! I know there are some mods which need to be done to get more gain etc but don't know what they are - can anyone help? Also will any 24V 1A power supply work for these?
Sorry if this has been asked before
Thanks
Matt

do a search at rec.autio.pro or rec.audio.tech, several useful info will come up.

hi Matt,

pin 31 +24v

pins 7 & 8 are hi and lo in (bal in)

40 ohms across 1 & 3

40 ohms across 2 & 4

pins 10 & 9 , 2K2 in series with 250K pot for gain setting (works reverse)

ground pins 30 29 21 17 13

jump 27 & 24

jump 23 & 22

output bal 19 & 20

out other transformer bal 25 & 26

there are other output configurations using the dual out xformers
but I've not got around to checking them out.

It's been my experience that 672s are best with input pads, I started out using the sure line pads .

good luck,

Lance

i have 672's, but never used them as a mic pre, so if will you allow me some questions...

20 ohms across 1 & 3
20 ohms across 2 & 4

don't the specs say that minimum input Z has to be 80 ohms or something? so you'd need 2x40 on those pins?
and what about loading the mic down? most mic's (nominal Z.out =150 to 200 ohms) expect load of min. 5 times their Z.out.
i mean, if you do work with your configuration all the time and get good results, i'm not gonna doubt your info! i'd just like some explanation, to see if i got anything wrong from the docs...

pins 10 & 9 , 2K2 in series with 250K pot for gain setting (works reverse)

that gives you which gain range? something between 34 and 60db?

I'm currently using my V672's with two 1k2 resistors on the input.
feedback loop resistor is a 500k, nothing in parallel.

this set-up gives me a decent input impedance (2.4 k) and just enough gain to use a sennheiser 421 on vocals with this preamp.
obviously, there's largely enough gain for condenser mics on any application.

has anyone here tried to set-up a V672 with variable input impedance like on the GT vi-pre? maybe a nice option for the use with ribbon mics...


michael

..............I'm currently using my V672's with two 1k2 resistors on the input. feedback loop resistor is a 500k, nothing in parallel.

Are the 2 x 1K2 important? I`ve just racked a pair of V672/2 without these but I noticed on the diagram Raffafred sent me that it had these.

I used a pot in the the feedback loop to give variable gain.

rob ,

the V672 features what telefunken calls "0 ohm technology".
so according to them, you need some external input resistors when using a V672 as mic pre.

mind you - I never tried my V672's without these input resistors.
but I wouldn't be surprised if your modules are on the dark side sonically, because of the low input impedance.

have you measured the frequency response?
my modules are -0.5dB @ 16kHz the way they're configured now.

cheers,


michael

michael

Thanks for that info. I`ll give it a try. I guess the input Z is just the value of the resistors that are used in series. Presumably If you wanted a lower Z for ribbon mics you just lower the value of the resistors.

Are the V676a also zero ohm technology. Should I be using input resistors with them as well ?

hi Volki,

I think you are right about the 40 ohms across 1&3, 2&4.my mistake.

I edited the post.

thanks,

Lance

@ Rob:

The V676a has a "regular" mic input xformer and doesn't need external resistors.
According to the data sheet, the input impedance is 2k.
Pretty decent mic pre, BTW. Lots of air in a good way.

cheers,


michael

do a search at rec.autio.pro or rec.audio.tech, several useful info will come up.

Click to expand...

Thanks for this but I'm not finding anything useful via google - do you have links or specific web addresses please volki?

Thanks for the info Mig27

here's the most useful info i got from a search at rec.autio.pro:

"I recently racked up a pair of them for somebody. I went by a german schematic I got from somebody here, I think it was Will. Although it was in german, the drawing was fairly self-explanatory. You need to apply the input signal to pins one and two; 24VDC to pin 31, ground pins 29 & 30; and the output can come from either pins 19 & 20 or pins 25 & 26. But you also have to jumper a few pins to get those output terminals to work. The output transformers are jumperable for different impedances. If you have the schematic I have, these jumpers are shown on the right hand side. If not, they're pins 24 & 24; 22-23; and 17 & 21. For gain control, you need to connect some kind of resistor between pins nine and ten. A short circuit here would be minimum gain, which IIRC was about 40-some DB. An open circuit gives maximum gain, close to 80dB. An audio-taper pot would work well there, but you get some LF noise at the bottom of the range. A resistor in series could prevent this but also raise the minimum gain. A stepped switch would be a very nice way to do it. I didn't get to listen to these preamps as much as I would have liked, but they seemd to sound good. A hell of a lot of gain available too. Nice big transformers. "

"I don't have the schematics in front of me but I guess Ullysses covered most of the gozintas and gozouttas in his post. I think the V672 module has unbalanced and balanced outputs - if it's a 672-2 it has a pair of balanced outs. If you don't wire the jumpers it will sound bandwidth limited; I thought that had to do with configuring the transformers series vs. parallel but my brother's the bench tech. We didn't have any trouble with noise at the lower end of a pots range; I think the schematics say 600 but we used 500 audio taper pots and it worked fine. Hopefully the capacitors don't need to be replaced on your module. You may want to use a metal enclosure for your module, I have noticed that our V672's benefited considerable at really high gain levels from the sheilding a metal enclosure provides. Some European dealers are selling little pigtail rigs with XLRs and a connector for a power supply to the multipins - I wonder how quiet those things really are. In any event use the best sheilded cable you can find. Welcome to the "big transformer" sound. <g> I think you can toss around terms like "discreet" and "class A" now as well IIRC.... "

other than that, i don't recall any specific links. what else do you want to know?

Wow, thats some great replies, many thanks!

So basically, all I need is a 24V DC supply, some (standard 1/4 W?) resistors and a couple of 250k (log?) pots and I'm good to go??

Excuse my lack of knowledge but I assume a simple switch to reverse pins 7 and 8 would act as a Phase switch?

Lance what are 'sure' line pads and where can I get them please?

Finally(!) is it easy to put a Phantom Power option on?

Thanks again
Matt

hi Matt,

the Shure line pad is# A15AS. 15,20&25 dB choices 1000 imp in & 150 out.
the 15 dB in the Shure is done as so;

pin 3 -----430ohm ------

( across 2&3 after ) 200ohm

pin2 ------430ohm--------

I've experimented with my own pads and one I like to use often is

240

( across 2&3 after)51

240

I have others that I think work with certain mics.

cheers,
Lance

first off: my last post was actually a quote of info i found at rec.audio.pro! somehow the line where i wrote this vanished...?:roll:

[quote author="axismatt"]Wow, thats some great replies, many thanks!

So basically, all I need is a 24V DC supply, some (standard 1/4 W?) resistors and a couple of 250k (log?) pots and I'm good to go??
[/quote]

some ppl use linear pots in parallel with a resistor btwn pins 9&10, some use audio taper pots... best get some different pots & see which you like best. doesn't cost too much anyway.

as for the 2 identical r's at the input (3&4)you want the 1/4w resistors to be closely matched (1%) in order not to degrade your common mode rejection ratio. metal film r's are best for least noise.

Excuse my lack of knowledge but I assume a simple switch to reverse pins 7 and 8 would act as a Phase switch?

Click to expand...

for reliability over time, many recommend gold plated switches for that. another way is to swith the phase at the o/p, signal levels are higher and thus less critical there.

Lance what are 'sure' line pads and where can I get them please?

Finally(!) is it easy to put a Phantom Power option on?

Click to expand...

just apply 48v via two 6k81 r's (1% or less) to the signal lines. before that, use a cap (e.g. 220uf) to ground and a resistor (e.g. 200ohms) in series to the 48v supply. do a search on phantom pwr here, on kev's diy page at ro, or in other forums. lots of info for you there.

while we're talking about it:
does anyone know how to wire a V672 for the use as DI-box?
I mean with really high Z on the input?

maybe it makes sense to inject the signal of say a bass guitar right after the input xmormer?

would this strategy be the same for a V72?
likely not...

any hints are warmly welcome.
cheers.


michael

I've heard that you can get a great bass DI with a V72 by injecting

the signal at the first tube.

I don't know what the best method is for a 672.Maybe if you started
a new thread with that title? It would be an interesting read; what the
super-techs opinions are for this type of app. with single ended transistor
mic and line amps.

cheers,

Lance

hitchhiker / volki,
Just thought I'd let you know, I bought a 24V, 0.8A switch mode power supply (amazingly tiny!) and hooked it all up as advised in your posts with the 40R res and the 250k pot etc etc...and it worked a treat! So thanks,as a rank amateur I really appreciate your help. Now I need to get a case and install it all 'properly'....
Is it fairly easy to get a 48V phantom supply from my 24V power supply?

Hitchhiker, I also tried your pads on my Altecs (just by putting the resistor 'bridge' in between the XLR and the input spade connectors-is this what you meant?) which worked fine too - I got a 15dB-ish drop from the 430/200R combination and about 20dB drop from the 240/52R.

Cheers
Matt

axismatt,

happy to help...yes the pads go in line before the input of the pre.

cheers,
Lance