Telefunken Preamp and Phantom Power
- Thread starter jejor
- Start date
Help Support GroupDIY Audio Forum:
This is a bit of an issue after Oliver at Tabfunkenwerk claimed the input transformers were not up to having the phantom switched on and off in the long term, due to back EMF. Something to do with the type of enamel used on the copper wire in the transformers deteriorating with age.
Many people just use a regular phantom supply with there telefunken modules & have not experienced problems. Maybe since this is a problem that occurs with repeated switching of the phantom on & off over several years.
There was a thread to design a phantom supply that ramped up gently & not cause failure of the transformers, but I`m not sure a completed circuit was finished.
I`m not entirely sure what conclusion you should draw from this information, maybe someone else can remembeer the outcome.
Many people just use a regular phantom supply with there telefunken modules & have not experienced problems. Maybe since this is a problem that occurs with repeated switching of the phantom on & off over several years.
There was a thread to design a phantom supply that ramped up gently & not cause failure of the transformers, but I`m not sure a completed circuit was finished.
I`m not entirely sure what conclusion you should draw from this information, maybe someone else can remembeer the outcome.
rafafredd
Well-known member
yes, there was a completed ramped phantom power circuit circulating, but I can´t find it right now. It´s somewhere in the lab...
NewYorkDave
Well-known member
If there is no current flowing through the coil, there is no back EMF. If both ends of the coil are at the same potential at all times, then no current flows.
obviously, the considerations are supposed to apply for the tube designs only (v72, v76 etc.). i haven't seen this statement in connection with the later SS designs (three digital numberings).
dave, in that post oliver did give reasons why there still could be a problem as phantom is switched on or off, but i forgot what it was exactly.
dave, in that post oliver did give reasons why there still could be a problem as phantom is switched on or off, but i forgot what it was exactly.
Dave,
I do see the plain logic behind your statement, but like Volki said someone did post a logical reason why there would be a back EMF. I just can`t remember what it was, & this is what Oliver stated was causing the insulation breakdown of the windings.
It may have been something to do with a very small difference in the 6k8 resistor values can be magnified in certain winding structures to a significant level to be a problem. But, I`m not sure
Volki,
Are your sure this wasn`t in connection with the 3 digit numbers as well? I thought it was ?
Rafa,
If you can find the ramped phantom circuit I`d like a copy to look at.
i did save ollie's explanation on my HD, just found it. quote:
"Theoretically this shouldn't pose a problem for a none center taped transformer, but in practice there is always the capacitive property; primary to core or the Faraday shield, then also the primary windings to secondary windings etc. These will become charged if 48V DC is applied and the transformer (standard tube input x-former and tube mic outputs have ratios from 1 to 10 and even higher) acts like a ignition coil creating approx. 1 to 2KV!! on the transformers secondary. Mic pres with no damping resistor like the V76 (input transformer has 1 to 30 ratio) can even create up to 3KV!!!! and this is enough for a millisecond energy burst that is powerful enough to burn microscopic holes into the insulation.
It is necessary to keep in mind that in the 1940/50 manufacturers only guaranteed the PVC coated magnet wire for approx. 24V AC, which is why every single layer was insolated with additional material like oilpaper, etc.
Therefore when you dump 48V into a input or output transformer you engage in a slow process of killing your historic transformer.
DC blocking capacitors do not help at all because the time required to charge the capacitive property is still too much, and every time the microphone gets disconnected while the phantom power is on, the entire energy of those caps is dumped charging the windings of the transformer and pre-magnetizes the core.
If you want to use phantom power then it is necessary to use a ramped 48V supply with a time constance of more than 5sec.
This will take care of the problem posed by this energy burst, simply by having a slower time constance than the capacitve property of the transformercan charge.
But still there is a slight chance that something will get burned in the process, as you are dealing with historic/vintage parts, that had a maximum shelf life of aprox. 25 years. "
so the problem obviously is the insulation properties of the xformer wire coating in conjunction with the high step-op ratio used with tube amp's. the SS amps in question were designed about 10 to 20 years later, after better insulation materials had become available, plus input xformer ratios for SS usually are no higher than about 1:3 or 1:4.
"Theoretically this shouldn't pose a problem for a none center taped transformer, but in practice there is always the capacitive property; primary to core or the Faraday shield, then also the primary windings to secondary windings etc. These will become charged if 48V DC is applied and the transformer (standard tube input x-former and tube mic outputs have ratios from 1 to 10 and even higher) acts like a ignition coil creating approx. 1 to 2KV!! on the transformers secondary. Mic pres with no damping resistor like the V76 (input transformer has 1 to 30 ratio) can even create up to 3KV!!!! and this is enough for a millisecond energy burst that is powerful enough to burn microscopic holes into the insulation.
It is necessary to keep in mind that in the 1940/50 manufacturers only guaranteed the PVC coated magnet wire for approx. 24V AC, which is why every single layer was insolated with additional material like oilpaper, etc.
Therefore when you dump 48V into a input or output transformer you engage in a slow process of killing your historic transformer.
DC blocking capacitors do not help at all because the time required to charge the capacitive property is still too much, and every time the microphone gets disconnected while the phantom power is on, the entire energy of those caps is dumped charging the windings of the transformer and pre-magnetizes the core.
If you want to use phantom power then it is necessary to use a ramped 48V supply with a time constance of more than 5sec.
This will take care of the problem posed by this energy burst, simply by having a slower time constance than the capacitve property of the transformercan charge.
But still there is a slight chance that something will get burned in the process, as you are dealing with historic/vintage parts, that had a maximum shelf life of aprox. 25 years. "
so the problem obviously is the insulation properties of the xformer wire coating in conjunction with the high step-op ratio used with tube amp's. the SS amps in question were designed about 10 to 20 years later, after better insulation materials had become available, plus input xformer ratios for SS usually are no higher than about 1:3 or 1:4.
rafafredd
Well-known member
http://www.groupdiy.com/index.php?topic=3079
http://www.diyfactory.com/projects/softstartphantom/softstartphantom.htm
Kev, can you include the softstart phantom project at the DIYFactory index page? It´s not there... It was a pain to find it.
http://www.diyfactory.com/projects/softstartphantom/softstartphantom.htm
Kev, can you include the softstart phantom project at the DIYFactory index page? It´s not there... It was a pain to find it.
Make note of this sentence
Oliver A wrote
" Mic pres with no damping resistor like the V76 (input transformer has 1 to 30 ratio) can even create up to 3KV!!!!"
The no damping resistor part
A good thing to read about would be kettering ignition systems. This page looks good.
http://users.mrbean.net.au/~rover/electron.htm
Off topic but about caps.
Funny thing in the early 80 there was an aftemarket motocycle company that sold high quailty caps (condensers) that were film caps IIRC. I was looking into that for my GS750 I ended up using a electronic hall effect system with different coils Dyna? Another thing that interested me with caps
Oliver A wrote
" Mic pres with no damping resistor like the V76 (input transformer has 1 to 30 ratio) can even create up to 3KV!!!!"
The no damping resistor part
A good thing to read about would be kettering ignition systems. This page looks good.
http://users.mrbean.net.au/~rover/electron.htm
Off topic but about caps.
Funny thing in the early 80 there was an aftemarket motocycle company that sold high quailty caps (condensers) that were film caps IIRC. I was looking into that for my GS750 I ended up using a electronic hall effect system with different coils Dyna? Another thing that interested me with caps
Thanks Volki, Rafa & Gus.
In Olivers quote it says about transformers from the 40`s or 50`s but the V72 was made way passed this point.
In Olivers quote it says about transformers from the 40`s or 50`s but the V72 was made way passed this point.
rafafredd
Well-known member
I think the real problem is the high turns ratio of the V76 input transformer. It reflects awful high voltage transients at the secondary when phantom is switched on and off at the primary...
With a 1:30, you have 48v times 30 = 1500v. But maybe this could really go as high as 3Kv?
But, where wold this damping resistor go? Would just a resistor shunting the secndary to ground make things better here (not that it would be possible on a V76...)?
With a 1:30, you have 48v times 30 = 1500v. But maybe this could really go as high as 3Kv?
But, where wold this damping resistor go? Would just a resistor shunting the secndary to ground make things better here (not that it would be possible on a V76...)?
