Why didn't the DI box get rid of the hum?

[alexis]

Hi,

What the problem is:
I had a bad hum after introducing a two-pronged (i.e. ungrounded) mic preamp just prior to the XLR leading to the Soundcraft mixer/PA.

The hum got worse when I touched the preamp, worse still when I grasped it firmly (I even heard a song from some random radio station playing through the PA when I did that!) and was variable depending on location on the floor with respect to other cables.

Importantly, there was no hum until that preamp was introduced into the system.

What I did to try and fix the problem (I tested alone, when I was the only one plugged in):
I thought this might have been a 60Hz ground loop so:

1. I ran an unbalanced TS out of the mic preamp into a DI, then an XLR from the DI to the Soundcraft.
- That made no difference in the hum

2. I then pushed in the "Pin-1 Lift" button in on the DI box.
- That also made no difference in the hum

3. With the DI box still in place I used the Soundcraft to Gate out the hum (hard gate at -20 dB).
- This completely got rid of the hum. This -20dB was right at the border of hum vs no hum.

Questions:
1. Was I wrong to think the hum was a ground loop introduced by the ungrounded mic preamp?

2. Was I wrong to try the DI box, or did I use it incorrectly?

3. Is it likely the XLR isn't wired so prong 1 is the ground?

4. Is there a better way to fix this problem?

I'm a little worried that either the gate will chop out some of my vocals during a show when conditions are different (though the vocals were fine when I tested in isolation) or the hum will return at the same gate settings that abolish it when I tested in isolation when everyone else plugs in.

Mic Preamp: Rolls MP13 "Mini-Mic Preamp"

DI: Radial Pro D1

Thank you for any thoughts!

[AlecSp]

My first question is why you're using an external mic pre before what is possibly your ui24r?, though we were unhelpfully left guessing as to your mixer).

What's wrong with the mic pre in your mixer? What happens when you plug your mic directly into the mixer?

I seem to remember a previous thread where you seemed to want to overcomplicate your signal chain...

[alexis]

My first question is why you're using an external mic pre before what is possibly your ui24r?, though we were unhelpfully left guessing as to your mixer).

What's wrong with the mic pre in your mixer? What happens when you plug your mic directly into the mixer?

I seem to remember a previous thread where you seemed to want to overcomplicate your signal chain...

UiR24 yes, but no hum at all without the preamp.

Great questions, thanks, AlecSp, and always looking to make the signal chain better, I can address that below.

If it's possible though (?) I'd really like to focus in this thread on my understanding of the hum, DI box, etc. if that's OK.

Thank you!

[Martin Walker]

Now I've still got a bad cold, so I may not be thinking clearly, but the fact that you have a bad hum from this ungrounded (2-pin PSU) mic preamp that gets WORSE when you touch its casing (and the suspicion of RF pickup) suggests to me that it's not a ground loop, but rather the complete lack of any ground whatsever.

A quick test would be to touch a wire between the mic preamp casing and any other grounded item - if your hum disappears then I suspect you've got a problem with the XLR cable connecting the preamp to the next item in your audio chain.

[Hugh Robjohns]

I had a bad hum after introducing a two-pronged (i.e. ungrounded) mic preamp just prior to the XLR leading to the Soundcraft mixer/PA.

That suggests you've somehow removed a ground reference from the source and/or the preamp's input isn't properly terminated.

The hum got worse when I touched the preamp, worse still when I grasped it firmly (I even heard a song from some random radio station playing through the PA when I did that!) and was variable depending on location on the floor with respect to other cables.

Yes. Case not grounded but connected to the internal audio reference point. You act as an aerial when touching it, injecting whatever E-M fields your body picks up.

Importantly, there was no hum until that preamp was introduced into the system.

Take it out then...

I thought this might have been a 60Hz ground loop...

How can it be a ground loop? You've said there's no mains safety earth connection (two prong mains lead) and the I/O connections are presumably the same. Where's the loop?

1. I ran an unbalanced TS out of the mic preamp into a DI, then an XLR from the DI to the Soundcraft.
- That made no difference in the hum

That suggests the hum is getting into the front of the preamp...

2. I then pushed in the "Pin-1 Lift" button in on the DI box.
- That also made no difference in the hum

Suggesting the hum is getting in before the preamp...

3. With the DI box still in place I used the Soundcraft to Gate out the hum (hard gate at -20 dB).
- This completely got rid of the hum. This -20dB was right at the border of hum vs no hum.

Yes, it would, wouldn't it? Until you start singing/playing and the gate opens... this is not a fix, it just masks the problem.

1. Was I wrong to think the hum was a ground loop introduced by the ungrounded mic preamp?

Evidently, yes.

2. Was I wrong to try the DI box, or did I use it incorrectly?

Always worth experimenting, but it saves a lot of effort to identify the problem and work logically, rather than apply random problem solvers.

3. Is it likely the XLR isn't wired so prong 1 is the ground?

No idea. It can happen a cable tester and/or multimeter are your friend...

4. Is there a better way to fix this problem?

Given that you haven't fixed it, I'd say yes, definitely... but we don't know what the actual problem is yet.

Mic Preamp: Rolls MP13 "Mini-Mic Preamp

Ah... finally, some key info!

I dont know why you're using a cheap preamp on stage when the FOH mixer is full of perfectly capable preamps.

And you haven't said how its connected, or what else it's connected to. Or what you're trying to achieve....

However, with a balanced mic connected via XLR cable, and the balanced preamp connected via the XLR output to a balanced line input on the desk, it should just work. It should get a good ground from the FOH console (which should be grounded) and be hum-free.

You may need to be careful of placing the preamp near other mains-powered gear and cables, and avoid connecting anything else to the preamp (to avoid ground loops).

[AlecSp]

UiR24 yes, but no hum at all without the preamp.

Great questions, thanks, AlecSp, and always looking to make the signal chain better, I can address that below.

If it's possible though (?) I'd really like to focus in this thread on my understanding of the hum, DI box, etc. if that's OK.

Again, why are you using a not very special external pre-amp, rather than just using the pre-amp already in the mixer? Especially when, as you've already found, the introduction of it, not to mention the additional kit you then add, is causing you problems.

As an aside, a miswired XLR cable can certainly pick up radio signals, but still pass the cable tester. Continuity in all the pubs doesn't stop you having used the shield as + or -, which will cause you problems. But not necessarily what you're suffering from here.

[alexis]

Now I've still got a bad cold, so I may not be thinking clearly, but the fact that you have a bad hum from this ungrounded (2-pin PSU) mic preamp that gets WORSE when you touch its casing (and the suspicion of RF pickup) suggests to me that it's not a ground loop, but rather the complete lack of any ground whatsever.

A quick test would be to touch a wire between the mic preamp casing and any other grounded item - if your hum disappears then I suspect you've got a problem with the XLR cable connecting the preamp to the next item in your audio chain.

Thank you, Martin, will do that at the next practice )

[alexis]

Thank you, Hugh and AlecSp.

I reluctantly added a mic preamp because the band member who runs the Soundcraft ui24r couldn't get my mic level high enough even with everything turned up there. My mic output sounded half as loud, or less, than everyone else's. There was no hum present before adding the mic preamp.

Vocal chain prior to introducing mic preamp:
My goal was the ability to adjust loudness of my voice in my monitors (IEMs in my case) independently of feed to FOH; similarly, to adjust loudness of my keys in my monitors (IEMs) independently of feed to FOH.

I couldn't figure out how to do that except as below:

Voice
-Earthworks sr117

-ART SplitComPro mic splitter
- Vocals routed into channel one and channel two of Mackie ProFX12V3+ mixer

- Vox signal in Mackie mixer Channel one routed via aux bus to Mon Out and on to Radial Pro D1 DI box, then to Soundcraft

-Vox in Mackie mixer Channel two routed to IEMs.

The keys are independently routed to IEMs, and also to Soundcraft FOH via mixer sub out 1 and 2 busses.

The reason I reached for the mic preamp is, unhappily, prior to introducing the mic preamp my vocal was noticeably lower volume than when I used other singers' mics. Troubleshooting this we ran the Mackie mixer monitor out directly to the Soundcraft FOH, and it was low volume. I don't quite remember, but iirc adding the DI box may have lowered the volume a bit also.

Again there was no hum without the preamp.

I would have been very happy with the set up if my vocals weren't so low in output.

Vocal chain after introducing mic preamp:
Same as above, except Mackie mixer monitor out routed to mic preamp via TRS-XLR; mic preamp out routed to Radial Pro D1 DI box via unbalanced T/S-T/S; and finally signal routed from Radial Pro D1 DI box via XLRf-XLRm, using the same cable that didn't give hum before introducing the mic preamp. After adding mic preamp: hum galore.

I had a bad hum after introducing a two-pronged (i.e. ungrounded) mic preamp just prior to the XLR leading to the Soundcraft mixer/PA.

That suggests you've somehow removed a ground reference from the source and/or the preamp's input isn't properly terminated.

The hum got worse when I touched the preamp, worse still when I grasped it firmly (I even heard a song from some random radio station playing through the PA when I did that!) and was variable depending on location on the floor with respect to other cables.

Yes. Case not grounded but connected to the internal audio reference point. You act as an aerial when touching it, injecting whatever E-M fields your body picks up.

Importantly, there was no hum until that preamp was introduced into the system.

Take it out then...

I thought this might have been a 60Hz ground loop...

How can it be a ground loop? You've said there's no mains safety earth connection (two prong mains lead) and the I/O connections are presumably the same. Where's the loop?

. Yes, thanks, I was confused.
.
.

1. I ran an unbalanced TS out of the mic preamp into a DI, then an XLR from the DI to the Soundcraft.
- That made no difference in the hum

That suggests the hum is getting into the front of the preamp...

Hugh, can you help me understand this please - do you mean the hum is originating in the preamp unit itself? What does that mean ... what causes the hum? Is it something that a better quality mic-preamp would avoid?
.
.

2. I then pushed in the "Pin-1 Lift" button in on the DI box.
- That also made no difference in the hum

Suggesting the hum is getting in before the preamp...

Same question as immediately above - consequence of using a bad quality preamp?

...

4. Is there a better way to fix this problem?

Given that you haven't fixed it, I'd say yes, definitely... but we don't know what the actual problem is yet.

...
I dont know why you're using a cheap preamp on stage when the FOH mixer is full of perfectly capable preamps.

Explanation as the top of this post.

Not being sure what was causing the hum, I reached for a DI box. Bad on me

...

However, with a balanced mic connected via XLR cable, and the balanced preamp connected via the XLR output to a balanced line input on the desk, it should just work. It should get a good ground from the FOH console (which should be grounded) and be hum-free.

Thanks, that is what I was hoping for.

I hope this info clarifies my set up, and what I'm trying to achieve.

To summarize:
1. I'm using the Mackie monitor out to route my vocals ultimately to the Soundcraft FOH because I couldn't figure out another way to achieve independent IEM/FOH volume control, as described more fully near the top of this post.
2. The Mackie monitor out volume is low, even when maxed out
3. Introducing the Rolls Preamp, as described above, to address #2 introduces a bad hum.

Thank you!

[James Perrett]

Alexis - I only read the first paragraph of your last response but all I can say is that you do seem to love to take the most difficult route possible to solving a problem.

The complexity of your system is crazy!

[AlecSp]

Alexis - I only read the first paragraph of your last response but all I can say is that you do seem to love to take the most difficult route possible to solving a problem.

The complexity of your system is crazy!

Yup, crazy indeed. And a whole pile of previously undisclosed items in the signal chain looking at the specs of the Earthworks sr117, it doesn't look unduly insensitive. But who knows what your Heath Robinson collection of stuff is actually doing.

Just looked back and found a similarly bizarre thread from the OP about kit to bring to an open mic. Same situation, same conclusion:

Thanks guys for the perspective, maybe partially summarized as, "Too complicated dude, you're gonna fail".

Nope, *completely* summarized as, "Too complicated dude, you're gonna fail"

Just plug the mic direct into the ui24r and control your mic on your phone/tablet.

[Hugh Robjohns]

Sorry Alexis, I have to agree with my colleagues here: you have a perfect storm of a lack of knowledge and understanding (both you and the FOH operator), and an overly complex stage setup.

In the pro world, your vocal mic and keyboard would feed the FOH desk directly, and the FOH desk would generate your IEM mix — possibly allowing you to create your own mix remotely via phone or tablet.

However, I understand much of your complexity is because you feel the need to create your own local IEM mix and therefore need local splits of vocal mic and keyboard.

Regarding the low vocal level... The SR117 mic has a higher output than most stage mics (5mV/Pa), so that's not the problem.

From your rig description, you're sending line level outputs from the Mackie to FOH, and so there should be more than enough signal available. You just need to get the gain structures right — at both your end and the FOH.

But sticking a mic preamp on a line output to boost level is patently bonkers.

From a levels point of view it would work between DI box out and FOH in... but it would be so much easier just to boost the level from the mackie into the DI box.

I also don't understand your routing arrangements at all.

Assuming the FOH desk is expecting MIC LEVEL signals, I would configure the system as follows:

Voice > Earthworks sr117 > ART SplitComPro mic splitter.

The splitter's Main Mic Out is the one that passes phantom power and it doesn't really matter if that's FOH or your Mackie — pick whichever you're comfortable with.

But for the sake of this example, let's say the Mackie is providing phantom, so Main Mic Out goes to a channel of your Mackie, (with phantom power) for local IEM monitoring.

Isolated output goes to FOH. This is standard mic level direct from the mic and shouldn't cause any level issues at FOH.

Obviously, use balanced XLR cables for all mic-related connections.

There are various options for the keys.

The easiest would be to connect the left/right outputs into appropriate channels on mackie to feed local IEM, using TS cables.

But divert one channel via the In and Thru sockets of the Radial ProD1 DI box. That will provide a direct split of the keyboard for FOH, with the DI box output being at mic level.

However, this arrangement will only send one channel of the keyboard to FOH (you'd still have stereo in your local IEMs).

You may prefer to send a stereo mix, which you can do via a Mackie Aux send.

Connect keys left/right directly to Mackie channels using TS cables.

Adjust Mackie mic and keyboard channel gains to optimise signal levels in Mackie using the desk meters.

Turn up Aux 1 (prefade) on both keyboard channels ONLY — not on the mic or fx channels.

Turn up Aux 1 master.

Connect Mackie Aux 1 out to Radial DI box input using TS cable..

DI box out is then routed to FOH via XLR as a mic level mono keyboard signal.

Start with the channel aux and master set to unity gain positions (usually 12 or 2 o'clock) to provide nominal line level into the DI box. Then adjust aux master up or down as necessary to keep FOH happy.

Use Mackie faders to balance IEM signal at main outputs. Monitor in usual way as required.

As I said, this assumes FOH is expecting mic levels, which is normal.

I would start with ground lift switches on the DI and mic splitter in their 'not lifted' positions, and only experiment with lifting grounds if you have hum issues with the FOH signals.

If you need to feed line levels to the FOH a different configuration will be required....

[Sam Spoons]

Which Mackie Mixer do you have? Many of the smaller ones have a switchable main output between mic and line level.

Do you run your keyboard in stereo? Many FOH rigs are run in dual mono which makes stereo inputs pointless.

[Hugh Robjohns]

Which Mackie Mixer do you have?

As detailed above: Mackie ProFX12V3+ mixer

Many of the smaller ones have a switchable main output between mic and line level.

Yes... but not this one.

Do you run your keyboard in stereo? Many FOH rigs are run in dual mono which makes stereo inputs pointless.

This was discussed previously. Alexis prefers to monitor the piano in his IEMs in stereo.

I think he also experimented with sending just one channel to FOH (which only has space for a mono signal), but preferred the sound of a summed mono (from both channels) created from his mixer.

[Sam Spoons]

In my defence (quite apart from age related attention span deficit) it is a particularly convoluted thread

[Hugh Robjohns]

In my defence (quite apart from age related attention span deficit) it is a particularly convoluted thread

It certainly is!

[The Elf]

I don't have enough remaining life-force to read through all this, but in trying to help...

Alexis - I really would suggest sitting down and thinking through what you really need your system to do for you. Forget how it is right now, and what gear you have. Just identify what you want the end result to be. Let's take another run at this!

My stage system consists of:
- keyboards into on-stage mixer
- on-stage mixer to FOH
- monitoring from on-stage mixer to my IEMs

I don't see why it should be any more complex than that.

[Hugh Robjohns]

My stage system consists of:
- keyboards into on-stage mixer
- on-stage mixer to FOH
- monitoring from on-stage mixer to my IEMs

To be fair, that's essentially what he has...

I don't see why it should be any more complex than that.

It's slightly more complex because he also needs to share a vocal mic between FOH and his own stage mixer, using reverb fx from his mixer for his IEM.

[The Elf]

It's slightly more complex because he also needs to share a vocal mic between FOH and his own stage mixer, using reverb fx from his mixer for his IEM.

Rather than 'share' the mic I would be looking to provide a separate pre-fader vocal feed from mixer to FOH (which is what I do for a talk mic in my own rig).

I hope I'm not muddying the waters further. What seems to be required is pretty simple!

If I'm not helping here then tell me to shut up!

[Sam Spoons]

I too would split the mic signal to FOH* and to the Mackie as The Elf and Hugh suggest then send the keys to the Mackie in stereo and use a single pre-fade aux to send the summed mono keys to the Radial and on to FOH.

It's still more complex than I think is necessary but after a few years of doing both jobs I think I understand that it's also important for the performer to feel comfortable on stage. If I was doing the PA for them and I had the time I would do my best to find a solution that achieves that end.

* alexis has a mic splitter box I think?

[The Elf]

The only element I added to my rig was a pair of isolators between my mixer and FOH. They were custom built for me by Orchid.

Each box (one for mix and one for stage mic) takes a pair of line inputs and provides *both* mic and line outputs. FOH can choose whichever they like.

[Hugh Robjohns]

I think we all know there are a dozen perfectly valid ways of achieving the desired end goal, using different mixer routings and hardware splitters and DI boxes.

The problem is that neither the FOH operator nor Alexis have much experience or knowledge, and it is also unclear exactly what facilities are available on stage in the way of a snake or tie-lines, and at the mixer.

Add to that Alexis' laudable desire to learn, but without being able to recognise and explain to this remote audience the key details of the various problems he comes across, with updates often taking weeks.

I think if any of us were at the venue we'd have it sorted out in minutes. As it is, it's a challenge...

But, Alexis does have all the appropriate gear already to do what he wants. It's just a case of optimising settings — some apparently being FOH issues beyond Alexis' control.

[Aled Hughes]

While the UI24R doesn’t have an extraordinary amount of gain available, it’s always had plenty for anything I’ve thrown at it (which is quite a few different mics and sources). It’s also quite painless to set up a phone, tablet or computer to control your own mix, and to lock out the controls you don’t need to touch. Wouldn’t this solve all/a lot of the problems?

Could you elaborate more on the problems you’re getting when connecting the mic straight to the Soundcraft? Is the level still low even with the input gain maxed out? At the risk of stating the ridiculous, is the person in charge of the desk aware that there is a separate fader for setting the gain independently of the channel volume? Do you sing extremely quietly? Are you able to find out roughly the input level to the Soundcraft when you’re singing and with the gain fully up?

[alexis]

Thank you, kind folk, for taking the time to offer such helpful comments.

Though there was so much good advice given here, some of the best may have been to question what my stage goals are. I've gladly done that, and I've started a new thread with the hope in mind that starting at the beginning like that, and reviewing the equipment I have available, may be a better approach.

https://www.soundonsound.com/forum/view ... hp?t=93193

Thanks again for all your help!

Added link to new thread - JP

[Hugh Robjohns]

Fair enough. I'm locking this thread to avoid duplication and confusion.