elskardio

Best method to protect Signal Generator from DC voltage?
« on: March 12, 2017, 11:06:34 AM »
Hi everyone,

I've been using my signal generator recently to bias some tube microphones. I'm always careful but would like some extra protection to make sure I don't feed 120V dc to my generator's output  :P
I've tested with a simple capacitor in the signal path and it didn't seem to affect my readings. Is this the proper method?
What do you guys use?

Thanks for your input
Gabriel


JohnRoberts

Re: Best method to protect Signal Generator from DC voltage?
« Reply #1 on: March 12, 2017, 06:33:27 PM »
Yes a DC blocking capacitor will isolate the DC. If you use a polar electrolytic  capacitor so be sure to orient the polarity in the right direction (+ voltage side goes to + lead of cap). And make sure the capacitor has enough stand-off voltage so the dielectric will not break down in use.

If driving high impedances you might be able to use a better film capacitor, but for general use an electrolytic caps should not introduce audible distortion.

JR
John Roberts
http://circularscience.com
Tune it, or don't play it...

elskardio

Re: Best method to protect Signal Generator from DC voltage?
« Reply #2 on: March 12, 2017, 07:54:54 PM »
Thanks JR

Yes a DC blocking capacitor will isolate the DC. If you use a polar electrolytic  capacitor so be sure to orient the polarity in the right direction (+ voltage side goes to + lead of cap). And make sure the capacitor has enough stand-off voltage so the dielectric will not break down in use.

If driving high impedances you might be able to use a better film capacitor, but for general use an electrolytic caps should not introduce audible distortion.

JR

alexc

Re: Best method to protect Signal Generator from DC voltage?
« Reply #3 on: March 13, 2017, 05:19:35 PM »
AND remember to discharge the cap after use! 

You can store a painful amount of charge in a several uF cap when the charging voltage  exceeds 50Vdc or so!

For tube circuits with HV, make sure that cap is discharged before finishing up with it  :)
I ping therefore I am

JohnRoberts

Re: Best method to protect Signal Generator from DC voltage?
« Reply #4 on: March 14, 2017, 01:04:09 PM »
AND remember to discharge the cap after use! 

You can store a painful amount of charge in a several uF cap when the charging voltage  exceeds 50Vdc or so!

For tube circuits with HV, make sure that cap is discharged before finishing up with it  :)
In fact be careful when discharging the cap. Imagine if it is charged up to 120V DC and you short it to ground. That will drive the other end of the cap 120V negative.  :o :o  So discharge slowly through a large resistor.

This is a known problem with mic preamp phantom blocking caps when charged to 48V and inadvertently shorted in an old school patch bay.  Mic preamps generally add clamp diodes but the current spike can be amps.

JR
John Roberts
http://circularscience.com
Tune it, or don't play it...

ruffrecords

Re: Best method to protect Signal Generator from DC voltage?
« Reply #5 on: March 14, 2017, 01:20:14 PM »
Why not use a transformer?

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

gyraf

Re: Best method to protect Signal Generator from DC voltage?
« Reply #6 on: March 15, 2017, 01:46:28 AM »
+1

I have a 600:600 transformer dedicated to this.

Jakob E.
..note to self: don't let Harman run your company..

ruffrecords

Re: Best method to protect Signal Generator from DC voltage?
« Reply #7 on: March 15, 2017, 02:52:20 PM »
+1

I have a 600:600 transformer dedicated to this.

Jakob E.

I do the same to protect the signed gen from Phantom Power when testing mic pretty inputs.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

elskardio

Re: Best method to protect Signal Generator from DC voltage?
« Reply #8 on: March 16, 2017, 11:18:23 AM »
Great idea!

In bonus I get a balanced signal from my generator  :)

Thanks

Why not use a transformer?

Cheers

Ian

joaquins

Re: Best method to protect Signal Generator from DC voltage?
« Reply #9 on: March 22, 2017, 08:11:13 PM »
Poor thing that output transformer with DC on it, or poor thing the thing trying to drive DC into that winding, depending who wins... Use a cap at the output of the transformer, and some resistor to dampen the resonance.

JS
If I don't know how it works, I prefer don't turn it on.


ruffrecords

Re: Best method to protect Signal Generator from DC voltage?
« Reply #10 on: March 23, 2017, 05:16:21 AM »
Poor thing that output transformer with DC on it, or poor thing the thing trying to drive DC into that winding, depending who wins... Use a cap at the output of the transformer, and some resistor to dampen the resonance.

JS

With correctly wired Phantom there is no dc thru the transformer.  I use one because I once connected my signature gen to a mic input with an incorrectly wired Phantom power. This resulted in the  blowing up of its output op amp. The transformer prevents this.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

joaquins

Re: Best method to protect Signal Generator from DC voltage?
« Reply #11 on: March 23, 2017, 10:58:12 PM »
With correctly wired Phantom there is no dc thru the transformer.  I use one because I once connected my signature gen to a mic input with an incorrectly wired Phantom power. This resulted in the  blowing up of its output op amp. The transformer prevents this.

Cheers

Ian

With CM DC as phantom is OK, I was thinking in NM DC, as your miss wired phantom, a miss probing or just a circuit that has DC.

JS
If I don't know how it works, I prefer don't turn it on.

gyraf

Re: Best method to protect Signal Generator from DC voltage?
« Reply #12 on: March 27, 2017, 06:54:00 AM »
I have not (yet) managed to destroy, or even upset, the 600:200R transformer I wired to my oscillator output sometime in 2004....

Jakob E.
..note to self: don't let Harman run your company..

chipss36

Re: Best method to protect Signal Generator from DC voltage?
« Reply #13 on: February 22, 2018, 11:10:57 AM »
Necro post I know, anyone care to tell me exactly what transformers would work best for this?  Would like to start  looking at mic amps, plotting de emphasis eq in a u87 clone , and a transformerless curcit, using different or no
de emphasis caps,  to look at the “q”  and study how the filter works, but don’t want to send polor voltage to my arb signal gen.

Re: Best method to protect Signal Generator from DC voltage?
« Reply #14 on: February 22, 2018, 08:58:10 PM »
I don't think there's a generic answer to this question, regardless of whether you use a transformer or a coupling cap. In both cases, you have to design your DC isolation carefully considering the exact circuit that you're attaching it to, also with a mind of what you're trying to test and what signals you expect to use.

This week, I'm using a home-built isolator box to test biased electrolytics for impedance and distortion. I drive the cap from my generator through a 100Ω resistor, so I need low impedance coupling. I decided to use 48x 10µF metallized polypropylenes since they are very linear and they can handle 200V of either polarity. The DC bias I'm applying to the cap under test gets filtered from the PSU to the cap, but that filter also has to include the 480µF polypropylene cap, since it too gets charged up by the bias voltage. The constraint on this DC bias voltage filter is thus the settling time of the entire system: the cap under test, the DC coupling cap, and the DC bias voltage RC filter.

The whole system works well, but it's a very thoroughly engineered complete system - there's little generic about it, and probably little other than the basic concept that would apply to other applications.

For simplicity, I'd suggest a simple coupling cap, preferably film if you can get one large enough for your use, and do some calculations to see how large it needs to be to get the required frequency response in the circuit you want to test. Thanks to solar power inverters, you can get some enormous polypropylenes now, but you probably don't need hundreds of µF (or... maybe you do?) The circuit details matter here: a U87 has some extremely high impedance nodes and some very low impedance nodes - it all depends on what you're testing, where you're injecting your test signal, and the series impedance you're using to inject the signal.