Home » R/E/P » Dave Hecht - Master Tech » Telefunken V72 broadcast filter 'mod'
| Telefunken V72 broadcast filter 'mod' [message #339011] |
Mon, 05 May 2008 05:46  |
 RMoore
Messages: 3064
Registered: May 2004
Location: Netherlands (ex-Vancouver... |
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Does anyone have info, tips, schematics, advice as to how to perform a removal/bypass of the broadcast filter which is apparently stock in the circuitry of non-'S' (studio) version V72 units?
Off the top of my head the so-called broadcast filter provides a roll-off above 15k and I apparently low end as well.. (?)
I have been trying to find this out for eons.. I think its as simple as bypassing a resistor or two..
Or maybe I am hopelessly misguided & it involves different circuitry / xformers etc
If anyone can steer me in the right direction on this - big thanks in advance!
Best Regards,
Ryan M
Excerpt from article on Oliver Archut / TAB reissues of V72:
The V78M is a single-channel, 1U racked preamp designed for budget studios interested in a mic preamp of the highest quality. It is sonically identical to the V72S, the studio version of the famous V72, which in its day was the standard preamp for German broadcast. The V72 rolls off at 15 kHz and 40 Hz to meet the requirements of FM broadcast, while the “S,” or studio version, is full range at 20 Hz - 20 kHz.
http://www.proaudioreview.com/pages/s.0026/t.7006.html
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http://www.myspace.com/twilightcircus
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| Re: Telefunken V72 broadcast filter 'mod' [message #339115 is a reply to message #339011 ] |
Mon, 05 May 2008 13:03  |
amorris
Messages: 454
Registered: June 2004
Location: florida |
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I always thought that the transformer limited the bandwidth and a bypass cap was used to go around the input transformers loading.?
?
I did see an internet write up on how to do it though. a few years ago.
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| Re: Telefunken V72 broadcast filter 'mod' [message #339133 is a reply to message #339011 ] |
Mon, 05 May 2008 14:12 |
 J Hinson
Messages: 84
Registered: December 2007 |
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| RMoore wrote on Mon, 05 May 2008 03:46 |
Excerpt from article on Oliver Archut / TAB reissues of V72:
The V78M is a single-channel, 1U racked preamp designed for budget studios interested in a mic preamp of the highest quality. It is sonically identical to the V72S, the studio version of the famous V72, which in its day was the standard preamp for German broadcast. The V72 rolls off at 15 kHz and 40 Hz to meet the requirements of FM broadcast, while the ?S,? or studio version, is full range at 20 Hz - 20 kHz.
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Not to stir things up but: None of the original EMI V72S amps I have measured had anything different about them over a standard V72 save for an increased amount of gain. Likewise, the EMI documentation I have supports this.
Indeed, the first units that were used at EMI were actually converted 'inhouse' from standard V72 amps. The mod consisted of changing out one resistor for two resistors.
Possession is 9/10ths of the problem.
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| Re: Telefunken V72 broadcast filter 'mod' [message #339486 is a reply to message #339473 ] |
Tue, 06 May 2008 22:36 |
J Hinson
Messages: 84
Registered: December 2007 |
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| Oliver Archut wrote on Tue, 06 May 2008 19:14 |
The broadcast V72 has a different input x-former than the S version, the difference inside the x-former is mainly with placements of the coils. If you measure the x-former V72vs. V72s you get different cut off points. Also the plate inductor was different.
Best regards,
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Hello Oliver,
Thanks for the response.
Interesting. Maybe that's the case for Siemens and Halske V72s units then but it isn't the case for V72s units that were originally installed in the Abbey Road REDD desks. These units have the exact same iron as standard V72 amps (yes i've actually taken an EMI V72s input transformer apart out of necessesity) and were modified inhouse from standard V72 units.
The REDD paperwork I have also indicates that the extra 6dB gain modification on EMI modified V72s units is differently realized than on a Siemens V72s unit.
In the EMI V72s', the cathode of the first EF804S is split into two resistors and the feedback (-tve and +tve) taken from the mid point. But written as a side note on the REDD schematic, it states:
"Later V72s, modified by Siemens and Halske, have cathode circuit of EF804S unaltered (i.e. same as V72) but with 50pF (39) removed & resistor 80K (18) changed to 160K."
Edit - Additional info regarding the rumble filter in EMI V72s':
In the positions within the REDD desks where the rumble was required, an additional .25uF capacitor was soldered across pins 2a and 2b of the mating tuchel connectors in the racks. This was then in parallel with the V72s' internal 1uF and lowered the frequency of the filter. Not all amplifiers had this filter engaged, some positions within the desk had pins 2a and 2b shorted together which, of course, takes the filter out of the circuit.
Cheers,
J.H.
Possession is 9/10ths of the problem.
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| Re: Telefunken V72 broadcast filter 'mod' [message #339500 is a reply to message #339497 ] |
Wed, 07 May 2008 00:40 |
J Hinson
Messages: 84
Registered: December 2007 |
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| Oliver Archut wrote on Tue, 06 May 2008 21:33 |
Hello John,
I have the original blueprints for all V72 ever made, there are several drawings for the EMI versions that they got the V72 direct from Siemens. There are several revisions done to the 300 or so units, but the input x-former is different in all of them compared to the broadcast version.
Best regards,
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Hey Oliver,
We may have to agree to disagree to a certain extent. 
I can only go by EMI documents and actual V72s amps I have on hand or have seen.
I will concur that the amps I looked at from a REDD17 that was NOT an Abbey Road desk had this "Later V72s, modified by Siemens and Halske..." business going on. In other words, the 80K feedback was a 160K feedback.
Not so on other amps I examined from an A.R. REDD desk which were indeed modified in house. The V1 1K25 cathode resistor had been changed out for two black 'Welwyn' brand high stability resistors. EMI mostly used this brand of resistor for inhouse manufacturing, Siemens, never. Upon close scrutiny of two particular amps, the 'S' part of the front panel engraving was also etched after the fact. I have copies of the original 1958 frequency plots and blueprints for these particular units. It is a set of internal EMI documents detailing how to do the modification and what to expect after it's done.
Now, I'm not saying that all V72s supplied to EMI were standard units converted inhouse. There were quite a number of REDD desks made that used the V72 and a number of those desks were also for other EMI facilities. Weren't the V72s' also used in decca desks?
So, what I ascertain from this is: you have details of 300 V72S amplifiers and they all have different input transformers from a standard V72. I have details of at least a desks worth of V72S amplifiers that used the same transformer as a V72. Therefore, we know that there were, at one time, at least 330ish V72S amplifiers in this world.
Thanks for the info, it was worth getting out of bed today 
J.H.
Possession is 9/10ths of the problem.
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| Re: Telefunken V72 broadcast filter 'mod' [message #339526 is a reply to message #339011 ] |
Wed, 07 May 2008 07:54 |
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Oliver Archut
Messages: 610
Registered: April 2004 |
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Hello John,
agreeing to disagree, that is fine with me.
You are right about the modification with the cathode resistor, but the standard S units were identical in schematic with the IRT version, but input x-former has the same differences than the V76s vs. standard V76 that limits the low end at 20 vs. the 40Hz. Tthe 2uF cap needs to be larger at the S version to get the full low end. When you took some inputs apart you surely will have seen the other stock number on the x-former, they look nearly identical but winding scheme and core material is different. The stock number it is either under the Mu-Metal can or inside the can, but I also have seen some that do not have a number at all.
The EMI mod were due to the need to have 40dB gain make up as well as making up for the 160Ohm resistor to get to 200 Ohm.
EMI requested at one time a fully modified V72 with 200 Ohm output, and more standard British) tubes like EF86 or 6BR7,but those ones were never build.
The very early V72S that were build used EF40 tubes and used a different winding style for the inputs, EMI got some of them, at least EMI Cologne/CLG as well EMI in Italy.
I have no idea about Decca desk in the UK, but most of the Teldec desk (Decca Germany) used mainly V76S and V77.
Best regards,
Oliver Archut
www.tab-funkenwerk.com
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| Re: Telefunken V72 broadcast filter 'mod' [message #339613 is a reply to message #339526 ] |
Wed, 07 May 2008 14:30 |
J Hinson
Messages: 84
Registered: December 2007 |
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| Oliver Archut wrote on Wed, 07 May 2008 05:54 |
Hello John,
agreeing to disagree, that is fine with me.
...but input x-former has the same differences than the V76s vs. standard V76 that limits the low end at 20 vs. the 40Hz. The 2uF cap needs to be larger at the S version to get the full low end.
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Are you referring to the 2uF that couples to the output transformer or did you mean to write 1uF and it's the one in series with the input transformer windings?
If so then I have an EMI document which shows the effect of various values externally paralled across the exsisting 1uf. Values up to 3uF are shown but 1.25uF (1uf internal, .25uf external) is what was implemented. Larger values than this, while lowering the filter cutoff, caused a hump at the extreme low end. Maybe it doesn't on the different transformer you are talking about but it did for these amplifiers in this particular desk.
| Oliver Archut wrote on Wed, 07 May 2008 05:54 |
When you took some inputs apart you surely will have seen the other stock number on the x-former, they look nearly identical but winding scheme and core material is different. The stock number it is either under the Mu-Metal can or inside the can, but I also have seen some that do not have a number at all.
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Do you mean part number when you say "stock number"?
Anyway, there were two "S" input transformers that I took apart quite some years ago as they were blown. After them sitting idle in a box for ages, I ended up sending one of them to C.J. from Prodigy Pro about 3 years ago as he likes this sort of thing. I believe there was a full blown autopsy of it with pictures and everything, It might still be online actually. The other transformer I still have. It has the same winding scheme as every other V72 input I've seen. Tell me what I'm supposed to see for a stock/part number if it's different.
| Oliver Archut wrote on Wed, 07 May 2008 05:54 |
The EMI mod were due to the need to have 40dB gain make up as well as making up for the 160Ohm resistor to get to 200 Ohm.
EMI requested at one time a fully modified V72 with 200 Ohm output, and more standard British) tubes like EF86 or 6BR7,but those ones were never build.
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I see you've read Brian and Kevins book. That info regarding the building out of impedance to 200 ohms with a 168 ohm resistor came from me.
Interestingly, I have a desk blueprint where EMI had neglected to consider the effects of this and the amp gains shown are incorrect. The drawing is a preliminary one for REDD.37 desk serial numbers 58121A and 58121B.
I believe the current thoughts might be that the desk that ended up in Kingsway Hall (serial # 58070A) along with the desk that went to Pathe Marconi (don't know the serial number) may have been built by C.L.G. but that the desks for A.R.S. (58121A & 58121B) were built in house. Certainly seems that way from information I have.
Anyway, thanks again for the info. The schematic you posted isn't the same one I have. While me posting scans of some of these EMI documents would settle that there were at least some V72 units that became "S" after leaving Siemens (specifically ones used in A.R.S. desks), I'm not inclined to do so.
Again, I do not doubt that lots of them (300 or so by your reckoning) have a different input transformer.
Thanks again and best regards,
J.H.
Possession is 9/10ths of the problem.
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| Re: Telefunken V72 broadcast filter 'mod' [message #340098 is a reply to message #339011 ] |
Fri, 09 May 2008 14:23 |
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Oliver Archut
Messages: 610
Registered: April 2004 |
Gold Member |
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Hello John,
indeed the picture of the x-former you send to CJ is a standard V72 input with the BV#191 (BV=Bauvorschrift->Building instruction), then there are two minor different S version, the early one and the later one with the BV191/1 and BV192/4 number.
Main difference is the (static)shielding, winding scheme as well as the core material.
The BV# is placed on every input transformer either inside or outside on the Mu Metal shield. Siemens wrote them with ink on the silver wrapping, so if it is oxidized there is no way to see it. Telefunken and later Siemens x-former had them glued on the bottom of the Mu-Metal can, all Haufe x-former had the BV number as well glued either on the side or the bottom of the can.
The hump you are referring to is when the x-former core is getting saturated due to the core material used in the standard units, EMI also strapped sometimes the inputs with he" proper" static value, increasing the bump most of the time than making it better.
The S version was also shipped like the standard V72 with an internal 1uF for 40Hz cut, but with a external additional 1uF cap the cut of is 20Hz and bridged the S version goes down to 10Hz, depending on the source impedance. The interesting question is now how does the standard ax-former got into an S version in the first place? Quite strange.
Brian and Kevin send me book as a thank you to help them out with info on the hard to get german data like the Hiller mics, AEG/Telefunken tape machines and of course the V7x info, but I have not had to much time to read in it yet, for that what I saw it looks great.
The EMI/CLG resistor solution is quite a staple of EMI, most Telefunken/Siemens engineers in the old days smiled about that way of doing it, given the fact that the overall impedance is complex/dynamic and can not be forced by just an resistor. But hey it worked for them.
I looked last night to see if I find more of the EMI/CLG documents I have, when Brian and Kevin wrote their book I just moved here to Kansas and not all boxes were unpacked, yesterday I found the some EMI/CLC drawings, mostly for CLG consoles but the EMI V72S and V76S drawing as well as some REDD console hook ups I dug up too. The V72S drawings are identical to the Siemens.
EMI got quite quite exited a few years back about their old stuff, they filled a bunch of new trademarks on their old stuff to protect it for digital/plug in use.
Best regards,
Oliver
Attachment: emi72.jpg
(Size: 432.49KB, Downloaded 132 time(s))
Oliver Archut
www.tab-funkenwerk.com
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| Re: Telefunken V72 broadcast filter 'mod' [message #340113 is a reply to message #340098 ] |
Fri, 09 May 2008 15:09 |
J Hinson
Messages: 84
Registered: December 2007 |
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| Oliver Archut wrote on Fri, 09 May 2008 12:23 |
indeed the picture of the x-former you send to CJ is a standard V72 input with the BV#191 (BV=Bauvorschrift->Building instruction)
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The interesting question is now how does the standard ax-former got into an S version in the first place? Quite strange.
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Exactly!
The other transformer I have is identical to the one CJ pulled apart. Both came from 'S' modules.
Given that there are EMI documents detailing the conversion of a standard amplifier to an 'S' amplifier (this is hinted at in the lower left of the latest schematic you posted), and also the fact that I have seen credible evidence of this conversion being done in-house, one could maybe assume:
1/ A.R.S./EMI were not aware of any difference in the input transformers
or
2/ A.R.S./EMI did not think the difference was worth considering.
or even
3/ A.R.S./EMI actually preferred the standard issue transformer. (probably the least likely scenario)
Would there have been much difference in response between the two if the rumble filter were engaged, albeit at a slightly lower frequency due to a total input C of 1.25uF?
FWIW and as best as I recall without actually looking, all amplifiers utilized as mic amps in the REDD desks had the filter engaged. Inter and Output amps did not.
| Oliver Archut wrote on Fri, 09 May 2008 12:23 |
EMI got quite quite exited a few years back about their old stuff, they filled a bunch of new trademarks on their old stuff to protect it for digital/plug in use.
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Yes I'm somewhat aware of that.
I held one of those trademarks myself at the time. I let them have it back as they have a bit more money than I do with which to pay lawyers ; )
Cheers,
J.H.
Possession is 9/10ths of the problem.
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| Re: Telefunken V72 broadcast filter 'mod' [message #340206 is a reply to message #339011 ] |
Sat, 10 May 2008 06:15 |
jensenmann
Messages: 24
Registered: April 2007
Location: Karlsruhe, Germany |
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Regarding hipass filter the question is answered. Put another cap with values mentioned above parallel to the 1uF cap which is in series with the two input transformer windings - or even short the original cap. But this cap was not ment to be a broadcast filter. It is used to prevent the input transformer core from saturation.
The broadcast stations had their hipass filters in front of the transmitter poweramps. Hipass filtering in any instance before the transmitterstations was not intended.
The modules after the tube era had lowpass filters on board - well, some of them. (this can?t be generalized). Often the inhouse techs of these stations modified their units for their own purpose. But the tube era stuff had no lowpassfilters onboard - at least I don?t know of them.
Jens
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