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Di Box Thru PA?

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Re: Di Box Thru PA?

Postby innerchord » Sat May 16, 2020 12:39 am

I'm going to return to the original post and just point out that whatever the guitar pickup goes into first may well have an affect on its tone, and needs to be chosen carefully, especially with acoustic guitar pickups.

Choose something with a nicely-designed high impedance input first.
Radial or LR Baggs are my favourites. You can then do whatever routing you want after that in the knowledge you're starting off with a good signal.

As an aside, I recently caught a YT video testing affordable audio interfaces. The signal losses with some of the top-name interfaces were appalling! It was a good reminder about signal-matching.
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Re: Di Box Thru PA?

Postby Music Wolf » Sat May 16, 2020 7:00 am

Hugh Robjohns wrote:
1LastRide wrote: I am looking at buying a Behringer D120 due to the ground loop hum.

Nooooooooo..... Of all the DI boxes available on the planet, please don't buy that one! You'll only end up buying something else that works properly in a few week's time... ;)

H

I think that we need to restore some balance with regard to the merrits of the DI20.

The DI20 is actually nowhere near as good as Hugh and the others have made out, it is in fact pure evil in a metal casing. Sorry, but I couldn't stand by whilst these DI20-denialists whitewash over the misery and suffering that this audio monster has wrought upon humanity. :madas: :protest:
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Re: Di Box Thru PA?

Postby ef37a » Sat May 16, 2020 8:03 am

innerchord wrote:I'm going to return to the original post and just point out that whatever the guitar pickup goes into first may well have an affect on its tone, and needs to be chosen carefully, especially with acoustic guitar pickups.

Choose something with a nicely-designed high impedance input first.
Radial or LR Baggs are my favourites. You can then do whatever routing you want after that in the knowledge you're starting off with a good signal.

As an aside, I recently caught a YT video testing affordable audio interfaces. The signal losses with some of the top-name interfaces were appalling! It was a good reminder about signal-matching.

That was my first thought. Peizo pickups need to 'see' a very high impedance, 5, better 10 meg Ohms. Such high impedance cannot be achieved from a passive DI and no active one that I know of?

However, the OP's guitar has the HP5* pre amp built in so no hi Z input needed. A passive DI could be used if a very long, balanced mic level feed was desired.

*Always sad to read of otherwise good gear being marred by BS about the superior audio qualities of Gold (higher resistivity than Copper!) and the supposed bad effects of electrolytic capacitors.

Dave.
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Re: Di Box Thru PA?

Postby Sam Spoons » Sat May 16, 2020 11:08 am

Yet another 'anti-D120' post, terrible piece of kit. As Orchid Electronics have been mentions and they make both the Micro DI (£28, UK postage included, and they also post world wide) and the 1:1 Transformer isolation box for £24.

http://orchid-electronics.co.uk/micro.htm

http://orchid-electronics.co.uk/trans.htm
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Re: Di Box Thru PA?

Postby AlecSp » Sat May 16, 2020 2:09 pm

Sam Spoons wrote:Yet another 'anti-D120' post, terrible piece of kit. As Orchid Electronics have been mentions and they make both the Micro DI (£28, UK postage included, and they also post world wide) and the 1:1 Transformer isolation box for £24.
Agreed - on all fronts
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Re: Di Box Thru PA?

Postby 1LastRide » Sat May 16, 2020 7:43 pm

I appreciate the feedback from all of you! I pulled the trigger, like a virgin on his first lap dance, lol. I did order the d120. I'll probably regret it. My apologies for not waiting on responses. You know us Americans tend to jump the gun and not listen to reason. Lol. I will definitely look into the orchid or Art products. Thanks again for the feedback and also Thanks for sharing your knowledge.
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Re: Di Box Thru PA?

Postby Wonks » Sun May 17, 2020 9:07 am

Send it back as soon as you get it if you can and get a better DI box. That's any other DI box basically!
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Re: Di Box Thru PA?

Postby The Elf » Sun May 17, 2020 9:33 am

...or send it to one of our resident electronics experts so they can open one up and determine why they are as bad as they are. In the scheme of things a DI box is electronically simple, so it would be interesting to discover how Behringer could make one that's so poor.
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Re: Di Box Thru PA?

Postby ef37a » Sun May 17, 2020 10:50 am

The Elf wrote:...or send it to one of our resident electronics experts so they can open one up and determine why they are as bad as they are. In the scheme of things a DI box is electronically simple, so it would be interesting to discover how Behringer could make one that's so poor.

Hmm? I have found a schematic and it isn't THAT simple for a DI box Elf!

The first design boob I can see is two 100k resistors in the input path. Not only will they add Johnson noise but with the op amp they feed there is an over all 6dB of attenuation, could be more, I cannot read the value of the first chip's bias R. Then the op amps are the 'four in a pack' TL064s and I seem to recall they are noisier than the audio standard TL072 and that is not that good these days. I would bet if you removed the series Rs and changed the feedback R to give unity gain the noise would be drastically reduced...Hang on! The bloody thing has BALANCED inputs with 100ks in each leg so of COURSE it is going to be noisy!

WTF have they done it that way? The XLR output is driven by the conventional two op amp stages in anti-phase. Ah! I think the balanced input is to enable the box to have a ground lift. The box is probably quite handy if you want to DI at +4dBu levels and higher?

So, rip out all that front end nonsense and use the OP stage to drive a wee transformer to give ground lift and you might end up with a half decent DI? Might as well just buy an Orchid of course!

Now, I did buy one of these and I think I sent it to son? Not sure though so will have a butchers.

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Re: Di Box Thru PA?

Postby Hugh Robjohns » Sun May 17, 2020 11:32 am

ef37a wrote:Hmm? I have found a schematic and it isn't THAT simple for a DI box Elf!

To be fair, it is a dual channel device, so it's really only half as complicated as it looks! ;-)

Image

The first design boob I can see is two 100k resistors in the input path. Not only will they add Johnson noise but with the op amp they feed there is an over all 6dB of attenuation, could be more, I cannot read the value of the first chip's bias R.

All four Rs around the differential input op-amps are 100k. But then there's another 100K in series with the input to form the input attenuator. But they couldn't use significantly lower values here (for the lower Johnson noise) because that would wreck the required high input impedance.

Then the op amps are the 'four in a pack' TL064s and I seem to recall they are noisier than the audio standard TL072 and that is not that good these days.

The specs for the 064 quote an equivalent input noise voltage of 42 nV/√Hz.... which is extremely poor by modern standards (and there are 3 in the signal path!).

For comparison, the TL072 noise voltage is 18 nV/√Hz -- roughly 8dB quieter just for a single dual-op-amp chip by itself! But being a dual rather than a quad package it would introduce circuit layout/size and cost compromises...

The lowest noise JFET opamps I've seen are around 4 nV/√Hz, such as the OP827 (but you only get one in an 8-pin chip and they are a lot more expensive).

I would bet if you removed the series Rs and changed the feedback R to give unity gain the noise would be drastically reduced

...and you'd also lose some of its core functionality! (as well as a higher output level which wouldn't help either).

The bloody thing has BALANCED inputs with 100ks in each leg so of COURSE it is going to be noisy! WTF have they done it that way?

It can indeed work with balanced sources (although I very much doubt its input resistors are matched closely enough to have a decent CMRR figure!), but it is really configured as a differential input specifically so that the input to the op-amp is ground-free, allowing the input ground to be separated from the output ground if required -- as you realised!

It's basically setup to work in the same way as the pseudo-balanced connections from unbalanced sources into balanced inputs that we keep banging on about around here!

So, rip out all that front end nonsense and use the OP stage to drive a wee transformer to give ground lift and you might end up with a half decent DI?

But if you rely on an output transformer for a ground lift feature you would (potentially) lose the ability to power it via phantom! So it ain't quite that simple (there are workarounds, of course). And there are applications where some of that 'front-end nonsense' is actually necessary, of course.

The fundamental problem here is simply that they're using ludicrously noisy op-amps for this application. That and that fact that there are better (but not cheaper) ways to crack this particular nut!

Might as well just buy an Orchid of course!

It would certainly be a lot less noisy and sound better... but they don't provide galvanic isolation either which is still an absolute 'must' for me in many live sound applications.
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Re: Di Box Thru PA?

Postby ef37a » Sun May 17, 2020 11:44 am

Yes Hugh, I realize it is a series of compromises to get the functionality, just bad ones done badly!

And I would not trust the ground lift regime as as safety feature.

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Re: Di Box Thru PA?

Postby Hugh Robjohns » Sun May 17, 2020 11:51 am

Quite.

Here's the BSS133AR DI box schematic. This is only a single-channel box, but in all other respects it matches the DI120s facilities...

The top part of the schematic is the power supply section. The lower part is the audio path and it uses a single op-amp so intrinsically has a much lower noise. It also maintains genuine galvanic isolation with an output transformer.

The print is hard to read but I think the audio op-amp is an MC133178 which is a BJT rather than JFET input type. Equivalent input noise is 7.5 nV/ Hz. https://www.onsemi.com/pub/Collateral/MC33178-D.PDF

Image
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Re: Di Box Thru PA?

Postby Music Wolf » Sun May 17, 2020 11:58 am

I think that the root cause can be seen more clearly in the exploded view

Image

As I said, evil
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Re: Di Box Thru PA?

Postby zenguitar » Sun May 17, 2020 12:36 pm

Well, it is clearly labeled as Specialised Studio Equipment.

Andy :beamup:
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Re: Di Box Thru PA?

Postby shufflebeat » Sun May 17, 2020 1:10 pm

Music Wolf wrote:I think that the root cause can be seen more clearly in the exploded view...

...As I said, evil

Science at it's best!

Great explanation :)
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