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Author Topic: DIs  (Read 6197 times)

luis Markson

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DIs
« on: July 03, 2011, 04:16:59 am »

I've been reading some manufacturers recommendations for DI selection and wanted to ask a few questions.

I only ever used active DIs, because that is always what is provided, but I am wondering if I should be looking at packing a passive unit...

From what I understand, active DIs were invented so that when the signal is split to feed both the amp and the PA, the loading caused by everything on the other end of the stage box did not effect the output of the instrument. This allows the instrument to retain its true tone and drive the required inputs correctly.

I have read that with modern instruments such as keyboards and active bases, their output can be as high as 18v, potentially overloading the DI box, with the solution being the use of a passive DI.

But what about the pad? Are pads on DIs padding the input or the output?

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Patrick Tracy

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Re: DIs
« Reply #1 on: July 03, 2011, 04:44:07 am »

Pads are on the input to protect the active circuitry or transformer from overload. Many are designed to accept speaker level input. You don't necessarily need to go passive.

luis Markson

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Re: DIs
« Reply #2 on: July 03, 2011, 05:19:59 am »

Pads are on the input to protect the active circuitry or transformer from overload. Many are designed to accept speaker level input. You don't necessarily need to go passive.

So what is the advantage of passive?
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Geoff Doane

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Re: DIs
« Reply #3 on: July 03, 2011, 09:06:13 am »

So what is the advantage of passive?

It gives you transformer isolation, which may eliminate hums that active DIs can't.

A passive DI is also much simpler, and doesn't require phantom power.  In a festival situation, where things might get unplugged by mistake, or could get wet because of the weather (or beer  >:(), not having phantom power running all over the stage is considered by some to be an advantage.

OTOH, cheap passive DIs are probably worse than cheap active ones.

GTD
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Tim McCulloch

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Re: DIs
« Reply #4 on: July 03, 2011, 11:02:17 am »

It gives you transformer isolation, which may eliminate hums that active DIs can't.

A passive DI is also much simpler, and doesn't require phantom power.  In a festival situation, where things might get unplugged by mistake, or could get wet because of the weather (or beer  >:(), not having phantom power running all over the stage is considered by some to be an advantage.

OTOH, cheap passive DIs are probably worse than cheap active ones.

GTD

Some active direct boxes contain transformers (Radial) in a hybrid design, others use a high-impedance bridge circuit (Countryman) only.  Hum "elimination" is a result of lifting the ground (earth) line, and that is not dependent upon a transformer.

As for having "phantom power all over the stage" I fail to see a problem.  Rule #3 of Festivation: Mute every input except MC/house music and every output (monitors, especially) that isn't related to the main PA.

Cheap DIs, whether active or passive, as still cheap.  There are sonic and mechanical/physical compromises used to meet the price point.  It's up to the purchaser to decide if those compromises are acceptable to clients, owner and crew.
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John Roberts {JR}

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Re: DIs
« Reply #5 on: July 03, 2011, 12:31:40 pm »

I've been reading some manufacturers recommendations for DI selection and wanted to ask a few questions.

I only ever used active DIs, because that is always what is provided, but I am wondering if I should be looking at packing a passive unit...

From what I understand, active DIs were invented so that when the signal is split to feed both the amp and the PA, the loading caused by everything on the other end of the stage box did not effect the output of the instrument. This allows the instrument to retain its true tone and drive the required inputs correctly.
Early DIs were all passive (transformer), back when transformers were relatively cheap and good gain stages relatively expensive.

DIs initially served the basic function of interfacing instrument level signals with mic preamp inputs. To optimize this interface they needed to both drop the voltage level from instrument to preamp, and raise the termination impedance from preamp back to the instrument. Both functions that transformers do well. Ground isolation is gravy that transformers provide free, but there was little need for ground isolation, when the instruments being interfaced were bass guitar or Rhodes keyboards involving no power supplies or active electronics.

Modern active DI come in many flavors and range from low end (cheaper than using a actual transformer), to complete free standing preamps, with more bells and whistles than some console inputs. Some have output transformers built in for completely different reasons (isolation) than level/impedance translation.   
Quote

I have read that with modern instruments such as keyboards and active bases, their output can be as high as 18v, potentially overloading the DI box, with the solution being the use of a passive DI.
Passive DI do not automatically deliver high voltage signal handling. In fact a synth with very strong low bass output could very likely saturate many classic passive transformer DI.
Quote


But what about the pad? Are pads on DIs padding the input or the output?
Active DI range from being powered from 9v batteries, to 48V phantom, to independent power supplies. So overload by a source with similar power constraints is one of those "it depends" questions. A guitar with active output (9v preamp built in) could make a hotter output that typical passive pickup**, and potentially overload a wimpy transformer, while a 9V powered active DI should keep up with a 9V powered instrument.  OTOH a +/-15V powered electronic keyboard, could squash many old school DI, while a design engineer (moi) will ask why would a unit with a line cord ever want or need to plug into a mic preamp?   

There is little reason to have a switchable pad on the DI output, so if a pad is present it will likely be on the input. This will conflict with the goal of presenting the highest impedance termination to typical musical instrument sources (and a resistive pad will introduce it's own self noise), while there are dedicated DI for accepting speaker level signals where the self noise of a pad is manageable due to lower impedance resistances available when terminating speaker type signals (note: dedicated speaker DI sometimes include EQ to model cabinet resonances). 

I am apprehensive about offering definitive, "ALL DI do this or that" , there are many options and variants out there, and common practice involves a modern crop of DI that have evolved to interface with the modern crop of audio sources. If these modern DI are well designed, you won't need to think much about it, they will just plug and play. 

Sorry for a wishy washy answer... more of a background history than modern advice.

JR

** Note some modern rare earth magnet pickups can probably make more voltage output passive, than some active battery powered instruments.
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Ned Ward

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Re: DIs
« Reply #6 on: July 03, 2011, 12:51:06 pm »

Luis -

If you're playing at festivals, large clubs and places with great sound support, a good passive or active DI shouldn't make a difference.

Where it does make a difference is if you're doing sound yourself, if the mixers are limited in phantom power, or if phantom availability is "iffy." I have bought all passive DI's to eliminate any issues with phantom, and have been happy.

When I started out, I tried to skimp by just buying the Whirlwind IMP 2 DI's. They worked, but I did notice an upgrade to the sound when I bought the Radial JDI's and their Pro DI.

I think if you budget $100 per DI, you'll find you can get a great sounding passive DI. Radial as well as others make them. I just wouldn't buy the cheapest one out there (kind of like spending less than $100 on a mic)...
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Frank DeWitt

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Re: DIs
« Reply #7 on: July 03, 2011, 05:02:37 pm »

Luis -

When I started out, I tried to skimp by just buying the Whirlwind IMP 2 DI's. They worked, but I did notice an upgrade to the sound when I bought the Radial JDI's and their Pro DI.


Can you hear the difference in sound between the Radial Pro (Radial transformer) and the Radial JDI  (Jensen Transformer)

Frank
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luis Markson

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Re: DIs
« Reply #8 on: July 03, 2011, 08:21:53 pm »

The main rig I work with has a box of DBX D12s so I'm hoping to improve upon those. I worked for many years for a company that used BSS AR-133 exclusively so I don't have much experience with much else.

I'm trying to come up with ways, as a freelance operator, to value add my service. Imediate sonic improvements gained by supplying key hardware seems to be a good first step. I'll have to see if I can find someone with a JDI or J48 for me to a/b against the dbx units. I know in theory it should be chalk and cheese, but I need to hear it for mself.

Here's a question about an upcomming gig:

Singer/songwriter playing accoustic.

Has anyone tried this before:

Keep in mind this idea relys on radial DIs

Git > DI(1) > thru > DI(2) (speaker emulation enabled)
        v                    v
      desk                desk
 
Slight pan left and right.

This idea aims to replicate a 2 mic trick I use on electric guitar to thicken things up.
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Geoff Doane

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Re: DIs
« Reply #9 on: July 03, 2011, 09:31:22 pm »

Some active direct boxes contain transformers (Radial) in a hybrid design, others use a high-impedance bridge circuit (Countryman) only.  Hum "elimination" is a result of lifting the ground (earth) line, and that is not dependent upon a transformer.

As for having "phantom power all over the stage" I fail to see a problem.  Rule #3 of Festivation: Mute every input except MC/house music and every output (monitors, especially) that isn't related to the main PA.

Cheap DIs, whether active or passive, as still cheap.  There are sonic and mechanical/physical compromises used to meet the price point.  It's up to the purchaser to decide if those compromises are acceptable to clients, owner and crew.

I can really only speak to the active Radials, since those are the ones I've poked around inside, but that transformer is not an audio transformer, it is part of a SMPS that converts the phantom power to something more useful for ICs.  Since they run on phantom only, the ground lift switch doesn't disconnect pin 1 at the XLR, so I'm not sure what they lift, but it won't give tens of volts of isolation like a transformer will (not that you should ever need that much).

Also in the case of the Radials, their "economy" Pro48 has essentially the same circuitry as the J48, just fewer switches and features.  In fact, the Pro48's measured audio performance (headroom in particular) is slightly better, and the power LED stays lit all the time.  In this case, the less expensive product doesn't have a performance penalty, although it's not exactly cheap at ~ $100. ;D

The avoidance of phantom power was something that was brought up to me by the owner of another production company, someone that had many national tours and festivals under his belt.  It's not necessarily my point of view, but I thought one worthy of mentioning anyway.  Unused inputs are supposed to be muted, but mistakes happen, at every level of this business.

As for the assertion that a good passive or active won't make a difference, I would agree with that except for the case of very high impedance instrument pickups which want to be bridged into a correspondingly high input impedance.  A passive DI, even a very good one, just can't do that.  You need active circuitry to buffer the pickup and then drive a long mic line to the console.  At what point it makes a difference is certainly debatable, and I haven't done any A/B testing to find out, but when I plug the guitar player into a decent active DI, I know I won't have to worry about the DI being the weak link in the chain.

GTD
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Tim Padrick

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Re: DIs
« Reply #10 on: July 04, 2011, 04:38:52 am »

Can you hear the difference in sound between the Radial Pro (Radial transformer) and the Radial JDI  (Jensen Transformer)

Frank

With a keyboard that has a active output: In a direct A-B, yes.  In actual application, I'd say it depends on the rig.  (But since my A-B was through an old Mackie and a pair of little Eons, the difference may be greater than I think.)
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Geoff Doane

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Re: DIs
« Reply #11 on: July 04, 2011, 08:56:46 am »

Can you hear the difference in sound between the Radial Pro (Radial transformer) and the Radial JDI  (Jensen Transformer)

Frank

With a keyboard that has a active output: In a direct A-B, yes.  In actual application, I'd say it depends on the rig.  (But since my A-B was through an old Mackie and a pair of little Eons, the difference may be greater than I think.)

Can you characterize the difference Tim?  I would guess that the ProDI saturates more easily in the low end, but I haven't done the test.

GTD
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Patrick Tracy

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Re: DIs
« Reply #12 on: July 04, 2011, 01:53:02 pm »

There is little reason to have a switchable pad on the DI output, so if a pad is present it will likely be on the input. This will conflict with the goal of presenting the highest impedance termination to typical musical instrument sources (and a resistive pad will introduce it's own self noise), while there are dedicated DI for accepting speaker level signals where the self noise of a pad is manageable due to lower impedance resistances available when terminating speaker type signals (note: dedicated speaker DI sometimes include EQ to model cabinet resonances). 

Correct me if I'm wrong, but it seems to me that you generally don't need the pad and higher impedance at the same time.

Ned Ward

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Re: DIs
« Reply #13 on: July 04, 2011, 02:39:22 pm »

Luis -

When I started out, I tried to skimp by just buying the Whirlwind IMP 2 DI's. They worked, but I did notice an upgrade to the sound when I bought the Radial JDI's and their Pro DI.


Can you hear the difference in sound between the Radial Pro (Radial transformer) and the Radial JDI  (Jensen Transformer)

Frank


To my amateur ears, no - I'm running either through a 01v-HPR122, or through a VRX setup when we play corporate events. I'm sure Tim and others know better.


On the Pro, I bought the ProD2 as Tim and others on the boards had spoken well of it, and couldn't see plunking down the $300+ for the stereo JDI for my own needs.
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John Roberts {JR}

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Re: DIs
« Reply #14 on: July 04, 2011, 06:02:02 pm »

There is little reason to have a switchable pad on the DI output, so if a pad is present it will likely be on the input. This will conflict with the goal of presenting the highest impedance termination to typical musical instrument sources (and a resistive pad will introduce it's own self noise), while there are dedicated DI for accepting speaker level signals where the self noise of a pad is manageable due to lower impedance resistances available when terminating speaker type signals (note: dedicated speaker DI sometimes include EQ to model cabinet resonances). 

Correct me if I'm wrong, but it seems to me that you generally don't need the pad and higher impedance at the same time.

That's pretty much what I said (or meant)... Certainly termination impedance doesn't matter when connected to a power amp.   It's also possible to use a multi-pole pad switch so you can change both resistor legs of the pad to make a divider with reasonable values and termination.

IMO this is still a little conflicted as passive sources should not need pads, and active sources should not really need DIs or to plug into mic inputs... 

Pretty much a hodge podge product segment, where you need to figure what works for your specific needs.

JR
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Re: DIs
« Reply #14 on: July 04, 2011, 06:02:02 pm »


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