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Author Topic: DC blocker audio cable (to block 2.5v PiP)  (Read 1354 times)

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Offline Chanher

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DC blocker audio cable (to block 2.5v PiP)
« on: October 10, 2023, 08:00:07 PM »
I still want to try to use a Zoom F2 mono recorder to record mono sbd feeds but you can’t turn off the 2.5v PiP that comes out of the 1/8 input. Not wanting to risk any damages of any kind, I’ve been trying to research DC blockers; there do not appear to be any products that fit my needs.

Does anyone here have any knowledge of this or can you recommend a forum that would know about this?

There’s coaxial DC blockers, but for tv antennas etc. I’m considering building a cable. I’ve read about electrolytic capacitors with DC blocking capabilities. There’s concerns of bass rollofs. Has anyone had Ted build something similar? Thanks in advance for any knowledge.
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Offline wforwumbo

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Re: DC blocker audio cable (to block 2.5v PiP)
« Reply #1 on: October 10, 2023, 08:06:22 PM »
The theoretical DC blocker is a high pass filter. You won't get good performance out of anything passive built in to the cable, you'll get far too much attenuation of bass frequencies from passive designs, and you'd have to *shudder* solve the transmission line equations for your cable's RLGC impacts.

Your best bet is to google an active op amp based higher order DC blocking circuit powered off of a USB or 9V battery. If you're handy with a soldering iron, you can build one yourself. Again, you want an active high pass filter, and given that you have two channels you probably want a dual op amp. Something with a corner frequency of 10 Hz should do the trick.
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Offline Chanher

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Re: DC blocker audio cable (to block 2.5v PiP)
« Reply #2 on: October 10, 2023, 09:05:56 PM »
That’s excellent info specific to our hobby, thank you. I was afraid of this. I may have to scrap this idea of the F2 as a mono sbd recorder. Maybe just buck up and buy another F3.

I did come across these XLR phantom blockers. Wouldn’t one of these stop 2.5v? I could always order one of Amazon and test with a multimeter. I’m curious about high pass filtering….

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Offline mjwin

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Re: DC blocker audio cable (to block 2.5v PiP)
« Reply #3 on: October 11, 2023, 01:44:53 PM »
I don't think you'll find an off-the-shelf solution to this. However,  to block the DC pip voltage you really just need a simple capacitor.   I managed to endure the dreadful Zoom website for long enough to check the spec of the Zoom F2, and the input impedance is specified as a minimum of 2k ohms.  In this case you could use a commonly available 10uF electrolytic capacitor  in series with the input from the sound board to block DC. This will, of course, form a high pass filter, but at around 8Hz, I don't think this will cause any audible issues! Any capacitor will do, and one rated at 6.3V or 10V (or more) will be fine.  The + (positive) end should go to the F2.

I'm actually more concerned by the maximum input level of the F2. It's designed for a mic input, and Zoom actually specify the maximum level as -5.5dBu. That's 5.5dB below 0.775V, which works out as 0.412V, which is way less that you will likely get from a soundboard feed. Pro audio levels are typically +4dBu (1.2V), so unless you can vary the level of the SB, the F2 inputs will clip.

You could introduce a series resistor (say 10k) to reduce the levels, but maybe that's getting a bit further into electronics than you wish to go. It's also complicated by the fact that Zoom specifies the input impedance as a minimum of 2k (seems reasonable for plug-in-power), but give no maximum...

Personally, I'd try a 10k resistor and a 10uF capacitor in series & give it a go.  You can easily mount the components in a1/4" jack or XLR plug which goes to the soundboard. Of course you'll need to be reasonably comfortable with soldering electronics stuff, or know someone who is.

Hope this is of some use!

Offline Chanher

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Re: DC blocker audio cable (to block 2.5v PiP)
« Reply #4 on: October 11, 2023, 02:31:22 PM »
I don't think you'll find an off-the-shelf solution to this. However,  to block the DC pip voltage you really just need a simple capacitor.   I managed to endure the dreadful Zoom website for long enough to check the spec of the Zoom F2, and the input impedance is specified as a minimum of 2k ohms.  In this case you could use a commonly available 10uF electrolytic capacitor  in series with the input from the sound board to block DC. This will, of course, form a high pass filter, but at around 8Hz, I don't think this will cause any audible issues! Any capacitor will do, and one rated at 6.3V or 10V (or more) will be fine.  The + (positive) end should go to the F2.

I'm actually more concerned by the maximum input level of the F2. It's designed for a mic input, and Zoom actually specify the maximum level as -5.5dBu. That's 5.5dB below 0.775V, which works out as 0.412V, which is way less that you will likely get from a soundboard feed. Pro audio levels are typically +4dBu (1.2V), so unless you can vary the level of the SB, the F2 inputs will clip.

You could introduce a series resistor (say 10k) to reduce the levels, but maybe that's getting a bit further into electronics than you wish to go. It's also complicated by the fact that Zoom specifies the input impedance as a minimum of 2k (seems reasonable for plug-in-power), but give no maximum...

Personally, I'd try a 10k resistor and a 10uF capacitor in series & give it a go.  You can easily mount the components in a1/4" jack or XLR plug which goes to the soundboard. Of course you'll need to be reasonably comfortable with soldering electronics stuff, or know someone who is.

Hope this is of some use!

Awesome, thank you! I always enjoy trying to find different solutions. (and thanks for tolerating the Zoom website, a chore in itself).

I am also concerned about the max input level; I've read online about people successfully recording out of a professional mixer into a F2 and there are roughly the same amount of people reporting that attenuation is necessary. Numbers are numbers though and I will attenuate.

Since I've always wanted to get into soldering and cable construction, I think this is the perfect time to try it. If any of you advise that this is a bit too advanced for a beginner, I may seek a local electronics professional. I found this youtube video about wiring in a resistor and I'm curious if this method will work for both. I definitely have read that the positive end of a capacitor should go towards the power source.

https://www.youtube.com/watch?v=DXkvTWweacE

Can anyone advise different resistors for audio use? I don't mind doing research so I will also look this up myself. I hate ordering off Amazon but I may end up doing just that...
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Re: DC blocker audio cable (to block 2.5v PiP)
« Reply #5 on: October 11, 2023, 02:47:06 PM »

Mouser or Digikey is where I always bought any components that I needed for projects. If you only need a few items shipping can ding you but they have the best stuff at the best price IME.

When I was doing a lot of cable building - and when my eyesight was lots better than it is now - I built XLR attenuator cables to use between my Grace V3 analog outs and my R4 XLR inputs. You can fit quite a few components into the shell of an xlr connector. Try to find a small profile polarized poly film capacitor that's terminated at each end instead of one that's designed to sit upright on a through hole board.
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Offline mjwin

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Re: DC blocker audio cable (to block 2.5v PiP)
« Reply #6 on: October 11, 2023, 03:24:47 PM »
Being able to solder is a really useful skill, if only to be able to make up simple audio cables and replace broken connectors.
As for your linked video, I'm not too sure about that guy's wire-stripping technique, nor why he's using such thick wire!  You might be better looking for some "how to solder electronics stuff" guides online.   

You don't need a fancy temperature controlled iron for this kind of thing, just something with a fairly small tip (say 1/8" or less). Use thin solder with a flux core that's designed for electronics, not plumbing.  The trick is to coordinate applying the iron, then just enough solder, and then quickly removing the iron.  Just practice on scraps of hook-up wire and offcut component leads. BTW, unlike in the video, most of the time you don't need to wrap the component leads any more than is necessary to get them to stay in place  while you solder, as you don't need great mechanical strength, just a good connection.

For basic audio use, e.g. making up an SB patch cable as discussed,  ordinary 1/4W metal film resistors are fine. You just need components with wire leads, and not the surface-mount variety (same with capacitors). It's a pity we don't have local electronics stores anymore, or even any back-street ham radio suppliers, as they were  great for this kind of stuff. But if you do know anyone locally who's into electronics, that would be a good start. Sorry I can't be of any practical help, as I'm the other side of the pond!

Good luck with your project!
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Offline Chanher

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Re: DC blocker audio cable (to block 2.5v PiP)
« Reply #7 on: October 11, 2023, 04:46:23 PM »
This is beyond helpful, thanks to everyone who shared their knowledge. It might take me a while to get this put together, but I will report my results.
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Offline Sebastian

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Re: DC blocker audio cable (to block 2.5v PiP)
« Reply #8 on: October 14, 2023, 10:37:51 AM »
I don't think you need to block the DC voltage at all as any half-decent SBD will already block DC voltages at its outputs. And we're talking about PIP, which is usually low voltage and almost zero current anyways. You won't be able to do any damage to professional equipment with that.

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Re: DC blocker audio cable (to block 2.5v PiP)
« Reply #9 on: October 14, 2023, 11:04:37 AM »
I don't think you need to block the DC voltage at all as any half-decent SBD will already block DC voltages at its outputs. And we're talking about PIP, which is usually low voltage and almost zero current anyways. You won't be able to do any damage to professional equipment with that.

This is not accurate. All "professional" equipment doesn't have blocking capacitors on every output. PIP voltage could wreak havoc on a circuit that doesn't block it.

You could totally ruin someone's performance by introducing unwanted DC of any sort into the mixer.

The easy solution to this is to use a recorder that can have the PIP turned off.
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Offline Sebastian

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Re: DC blocker audio cable (to block 2.5v PiP)
« Reply #10 on: October 14, 2023, 01:25:16 PM »

This is not accurate. All "professional" equipment doesn't have blocking capacitors on every output. PIP voltage could wreak havoc on a circuit that doesn't block it.

You could totally ruin someone's performance by introducing unwanted DC of any sort into the mixer.

The easy solution to this is to use a recorder that can have the PIP turned off.

Please just name a single professional mixing console built in the last 20-30 years that does not block outside DC voltages. These things are literally designed to be as robust as possible and to take a certain amount of abuse. And accidently applying a voltage to an output (e.g. by connecting an externally powered mic) is something they are all protected against, either via a DC blocking cap or by driving the output to 0V using an opamp. Of course there are always limits to these protection methods, but a 2V PIP voltage will never come close to "wreaking havoc" in any real-world scenario.

But you're right, using the right tool for the job (e.g. a proper recorder that does not output a voltage) would always be the best option.

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Re: DC blocker audio cable (to block 2.5v PiP)
« Reply #11 on: October 14, 2023, 03:41:48 PM »

Sebastian
Please see the link to the article from the other thread the OP started about this scenario and why he created this thread about how he wanted to avoid it. It states very clearly why this is a bad idea and explains exactly why. - https://www.soundonsound.com/forum/viewtopic.php?t=17114 One of the contributors who says "NOT okay" is John Willett who many here may remember as a contributor to the forum.

Here's a quote - I would say the problems he describes could be accurately described as "wreaking havoc".
"For example, plugging such devices into one Mackie SR24-VLZ series mixer I regularly use makes its VU meters sit steadily at mid scale and makes them unusable! This happens if the recorder is plugged into either the tape output RCA jacks or the main output TRS connectors. Strangely, it does NOT happen on the XLR outputs on some versions of this mixer.
I'm guessing it's a weird side effect of some servo balanced output stage design or that the DC is biasing inputs of devices (like the meter) internally bridged to the outputs inside the mixer.
Others, including some small Behringer mixers seem to develop symptoms of DC current flowing in the main output fader - making it scratchy and noisy and also exhibit incorrect VU meter readings.
These symptoms all disappear when cables with properly chosen blocking capacitors installed are used."


You do whatever you want to I'm not here to tell you what to do. However, it is bad practice for recordists to show up at a gig and knowingly send any sort of power into an output on a recording console. Ever.
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Offline capnhook

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Re: DC blocker audio cable (to block 2.5v PiP)
« Reply #12 on: October 14, 2023, 05:18:24 PM »


You do whatever you want to I'm not here to tell you what to do. However, it is bad practice for recordists to show up at a gig and knowingly send any sort of power into an output on a recording console. Ever.



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Offline Chanher

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Re: DC blocker audio cable (to block 2.5v PiP)
« Reply #13 on: October 15, 2023, 02:01:18 PM »
Yea ultimately I'm misusing gear; I bought this with recording soundboard feeds as well as recording older family members talk about family history. My cousin always tries to film them and they HATE it and always clam up and this is so easy to hide.

Unfortunately I didn't read that PiP is always on. I'll email Zoom about a firmware update but it's probably a hardware issue. Ultimately, it is bad practice to send power to someone else's recording console and I don't want to encourage anyone to even risk something like that.

Let's see if I can manage to not fuck this cable up, and if anyone wants one I'll see what I can do. It'll probably end up sounding like shit ha, but maybe I'll learn how to build cables.
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