V72 Siemens Telefunken Module + Racking questions

[Seditionary]

traded a pair of preamps for this v72 with a nice rack --

the channel sounds a little thin though, missing low end.  do you think i need to recap it? 

let me know if you guys have any suggestions / comments about this... and if you suggest i try and add some things to this rack!  i know some people have attenuators in their racks, but it is already pretty quiet... i guess these guys never really work with dynamic mics, yeah (super quiet with a 57 plugged in)?  i know they are meant to be used with tube mics, which i'm sure send a louder signal.

anyhow -- anyone with expertise in the v72 realm, please let me know how to make this guy get its low end back

thanks y'all  8)

 

[craptical]

The V72 (stock) has internal power supply, to be fed 220VAC directly. If there is an external power unit a starting point would be to open it up and check what happened to the original PSU. BE CAREFUL, POTENTIALLY LETHAL VOLTAGES PRESENT EVEN AFTER SHUT DOWN AND CHORDS REMOVED! So leave to an expert if you feel the least bit unsure.

The unit is supposed to give you +34dB's of gain, not adjustable, so less means there's a problem somewhere.

Best,
Dave

 

[Seditionary]

here are some photos of the guts.  they used to have a power supply internally?  i thought they needed to be powered by the rack (or console).  the transformers and circuit board on the rack are a power step down construction for our 110v in the states.

i know a tech had looked at the module before and cranked the gain up on it a bit.  just lately, apparently, he brought the gain back down on it...

here are some shots of the guts.  man, this thing is beautiful!!!!!  looks like it has the original caps from germany still inside it.  the transformer and everything are amazing looking.  those huge caps on the back definitely don't look friendly to the touch

thanks again for your time, y'all ! ! !

 

[craptical]

Another thought: The little circuit board might be for additional 48V phantom power for mics. In that case you should see +48VDC on these wires and also 220VAC for module power from another set of wires coming from the big trafo and going directly to the Tuchel plug on the back of the V72. There is a recent thread regarding (possibly phantom power) input/transformer related problems here:

http://www.groupdiy.com/index.php?topic=48285.msg610786#msg610786

 

[Seditionary]

hmm... maybe.  i don't think the circuit board is for phantom power as the guy who had it before said it didn't do phantom power, but you might be right.  i don't see +48VDC on the wires... but i can switch the big transformer from 110v to 220v.  are you saying if i switch it to 220v, it is basically running the unit at phantom power?  i have it set to 110v. 

the power cable (plug) goes into those plastic holdsters, which connect the power cable (plug) lines to the small transformer and the big transformer.  the big transformer only has one out, which is a stereo cable going directly to the bottom pins of the tuchel plug.  out of the circuit board, there is a stereo cable going to the top pins of the tuchel plug.  the middle tuchel plugs are wired to the input and output xlr connections. 

let me know if this helps or if you want more photos of that if it isn't clear.

thanks so much for your time !

 

[Seditionary]

any other ideas of what that circuit board might be for if not for 48V?  they said it was for changing the power from 220 to 110. 

should i try running the big transformer at 220 instead of 110? 

with that article you linked to... are you saying you think the input transformer on the unit could be bad?

thanks

 

[Seditionary]

you know what... i just received the unit and i hadn't really put it through the paces.

i was doing tests with one speaker on while talking into a mic.

just put on a pair of good cans.  tested the preamp next to another preamp that was operating at 35db. 

they are about the same volume!  i think what i was perceiving as "not a lot of low end" was actually just the fact that the mic is so quiet at 34 db... being a dynamic mic.  compared to another preamp (a neve clone)... this v72 is much smoother.  in comparison, it sounds great   and i think the low end is there.  crazy what low volumes and a bit of room phase (and your mind playing tricks on you) can do to a signal

 

[gemini86]

Good to hear it's working for you!

Given that these units want to see 220v AC at their power inputs, the big transformer is going to be a 110 to 220 step up. No idea what that little supply is for unless they replaced the heater wiring for DC power.

 

[Seditionary]

being a step up... is it strange that you can run the big transformer at 220v or 110v though?  there is a little switch on the big transformer (you can see the little black switch between the two screws on the big transformer (in the last photo on my original post))... does it makes sense that the switch is set to 110v?  should i try setting it to 220v?  is it possible that with the big transformer running at 110v... that the circuit board and the old school little transformer with the lead cut is the step up to 220?  it is sounding good though (with the big transformer switch set at 110v)... just curious to learn more about this rack... and whether i could hurt the input transformer or components on that external circuit board by switching the switch on that big transformer to 220v? 

thanks for your time and good tidings

 

[gemini86]

The track is probably made to run in two different Mains countries,  110 vs 220. Most older PC power supplies have this switch as well.

Best way to find out what is going on is to measure voltages.

 

[Seditionary]

cool -- so maybe i should keep it set to 110v... the plug has an adapter on it, switching it from euro to usa style... i'm guessing i can bring this rack to europe, take off the plug adapter and set the big transformer to 220v and still be good to record

 

[mjk]

The large transformer is an isolation transformer with step-up capability. Obviously the transformer is set correctly because the unit is working. Please don't try setting it to 220 and then plugging it into 120V.

By "you should see +48", I think it was meant for you to try to measure the voltage on the red/black pair of wires coming from the power supply to see if there is 48VDC present. It sure looks like a home made phantom power supply. Is there a pair of 6.8K ohm resistors in series with the red and black wires? Is that a TL783 in the TO-220 package?

The V72 has input connections on the bottom, output in the middle, and power on the top. Maybe you should post a closeup photo of the connections to the tuchel. Something in your description doesn't quite add up...

The more subtle issue is that the V72 likes 220 volts, not 240 volts. If you just step up the common 120 VAC US line voltage with a 1:2 ratio you will get 240 volts or even a little more more if you use an oversized transformer. It really should be dropped down to 220V somehow. If it's a step-down transformer wired backward as a step-up it may be close. I would definitely measure the AC voltage going to the module and make sure it's not too high.

 

[Seditionary]

cool -- i will do some tests... as i'm a newbie when testing power supplies (and don't want to get shocked)... just making sure this is safe -- you are saying to power on the rack, set my dmm to V / DCV and touch the probes to the red and black wires connected to the tuchel? 

it might be a step down wired up backwards... as the guy who had it before said it was a step down... would you recommend taking the circuit board out if it is 48/phantom?  i heard that can mess up in the input transformer over time.

let me know if the test i described is safe... if so, will report back to the amount of power flowing to the unit.

cheers and thanks !  it sounds great though   just need to compress the signal when using a dynamic to get the volume a bit louder... it is right about 34 db though.

 

[Seditionary]

man, i love our other preamps... but comparing it to our other pres for vocals... i think this will be the go to for vocals in most situations... don't want to mess with it too much as it sounds beautiful.  if it is to protect the channel / transformer though, will definitely mod it.  and who knows if it'll make it sound even better.  as it is though, it sounds incredible... wow... these preamps really aren't overhyped... kind of like how i felt when i first visited the redwood forest in california... everyone talks about it, but it definitely isn't overhyped.  beautiful.  ha.

 

[mhuss]

Wow, that is very clean. I have a pair here I've been working on, one is mostly original, the other has had most of the resistors and caps replaced.  ???

The one that is working is pretty flat, frequency wise, not thin at all, and seem to work fine using either the original power supply or the new one I put in my rack. Sadly, I'm having problems with one input transformer.

In the stock configuration, there is a 1 or 2 uF cap between the two halves of the input winding to block DC, so obviously, it was designed to deal with some DC on the input. However, some here have said the old frail windings don't like DC much. In the one I have with the input xfmr problem, someone had removed the cap and jumper the two primary windings together directly. 

That little board and transformer does look suspiciously like a 48v lash-up.

--mark

 

[Jean Clochet]

In the stock configuration, there is a 1 or 2 uF cap between the two halves of the input winding to block DC,

Many years ago I owned a REDD17 desk for a while.
The 1uF cap you're referring to is inbetween the windings and isn't really for blocking DC but is there to act as a rumble filter/low cut.
The cap is brought out to pins 2a & 2b on the rear tuchel connector so you can short it out if you want/need.  In the REDD17 desk, the V72's used as mic amps had an additional cap strapped across pins 2a & b (it was a .25uF for 1.25uF total value) to lower the frequency of cut.  But on the amps used as the mix and output amps, the cap was shorted out.

Just so you know