What is the difference between a "DI box", such as the RNDI, and a "Desktop Preamp"?

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What is the difference between a "DI box", such as the RNDI, and a "Desktop Preamp"?

Post by Dolmetscher007 »

I have always been on a budget. So I've never owned a dedicated DI Box, nor a dedicated Preamp or Channel Strip. I have always been forced to use whatever preamps were baked into the Focusrite Scarlett USB audio interface I've owned for 11+ years.

I have recently become a little gear-curious, so I have begun to examine my incoming signal from my electric guitar, all the way through my DAW (Logic Pro X) and back through my headphones or near-field monitors. I don't want to go down the rabbit hole of talking about recording guitar by placing a mic in front of an actual amp. So, I am talking about DI recording only here.

At present, my guitar signal comes straight out of my Telecaster, and goes right into the Focusrite XLR/Instrument combo jack preamp, through the ADC, in into my DAW through USB. If I owned a high-end DI box such as the Rupert Never RNDI box, my guitar signal would first go into the RNDI, which boosts the signal and does it's "Rupert Neve voodoo magic," but then the signal would still go into the Focusrite pre-amp, ADC, and into my DAW over USB. All of that makes sense to me.

Where I get a little hazy is, there are a ton of "Desktop Preamps" that have hit the market lately, and as far as I can tell... these preamps would also function as a signal coloring/booster, but still would require the ADC of an Audio Interface to get the signal into my DAW. So while I am sure the circuits are quite different between a Desktop Preamp and a DI box, they seem to serve the same function. And then there is another new hot-ticket-item out there called the "Cloudlifter" which also just seems to be a Pre-Preamp Preamp for boosting weak dynamic microphone signals by +25db so the audio interface preamp doesn't have to push so hard to boost the signal to desirable levels.

My concern is that all the Pre-Preamp boosting magic still has to go through the circuitry inside my less-than-amazing focusrite USB interface. I've never seen an audio interface that was JUST an ADC, but if there is such a bare-bones ADC audio interface that would take the signal straight out of the DI-Box/Preamp and just handle the analog to digital conversion and shooting it over to the DAW through USB... wouldn't that be better than having the signal leave a signal coloring/booster... and have to still go through, yet another redundant preamp circuit such as the Focusrite?
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Re: What is the difference between a "DI box", such as the RNDI, and a "Desktop Preamp"?

Post by Sam Spoons »

The preamps in your Focusrite (and most similar quality interfaces) are pretty impressive, quiet and transparent and not designed to alter the signal passing through them so, when using a 'colour' preamp in front, they should not affect the sound significantly (and likely not at all in real life as long as your gain structure is within prudent limits). Many multi digital fx units like the Line 6 Helix and Headrush Gigboard act as a USB interface too allowing you to bypass the Focusrite but I suspect you are after some 'analogue magic' so that solution nay not be for you.
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Re: What is the difference between a "DI box", such as the RNDI, and a "Desktop Preamp"?

Post by blinddrew »

Which Scarlett do you have? Some have line inputs as well so you could take the line-level output of an external unit without needing to go through the pre-amp circuitry.
Not, as Sam says, that there's anything wrong with Scarlett pre-amps (he says, looking at his 8i6 ;) ).
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Re: What is the difference between a "DI box", such as the RNDI, and a "Desktop Preamp"?

Post by Watchmaker »

It's all about electrical signal levels and input impedance afaict. I'll say it incorrectly so the smarties can sort it out properly.

(edit: as compared to a mic signal) A guitar signal is rather low current and needs a high impedance destination. A DI converts that signal into a mic level signal which requires much lower impedance and allows the mixer or interface to process that input through the same circuitry as a mic. (edit: line level signals are significantly higher current than either mic or pickup signals and need to be treated accordingly within the preamp and processing circuits)

The Scarlett interface has a DI effectively built in with the instrument input having a high impedance so the signal is received at the preamp correctly. All interfaces are essentially one or more preamps built into a unit that also does AD conversion.

The preamp itself simply raises the voltage of the signal for downstream processing or AD conversion. Preamps can be designed to either add color or be transparent.

A signal booster like the cloudlifter has a slightly different job and boosts a mic level signal to overcome low sensitivity mics so the preamp dynamic range can be used to bring a hot enough signal into the preamp so it can then boost that signal to a usable level and is usually powered via phantom power from the interface. Kind of pre preamp. preamps all have a range of level boost they can give. A small dynamic range preamp combined with a low sensitivity mic needs a bit of oomph, thus the cloudlifter, I have two SE dynamites for my ribbons mics, but they are a bit characterful

EDIT: to the question about all this preamp fluffery, you need to process a signal through some sort of preamp prior to conversion as converting only makes the analog signal a digital representation. To have a usable digital record, you need to have a good signal at the moment of conversion, thus you need preamps as signals are all over the place in a wide variety of ways.

another edit: it's amazing how easy it is to leave things out or say half a thing thi king you said the right thing...
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Re: What is the difference between a "DI box", such as the RNDI, and a "Desktop Preamp"?

Post by Dolmetscher007 »

So... am I understanding you guys correctly?

Preface: I will be using imaginary values for the sake of simplification.

1. Electronic Instruments (Keyboards), Microphones, and Guitars all output an AC signal.
2. The signal that each of these things generates is at a different electrical "strength.".
3. Guitars put out a signal of 25 "units" of strength.
4. Some weaker microphone signals are a little stronger at 50 "units" of strength.
5. Some stronger microphones put out 75 "units".
6. Electronic Instrument signals are more like 100 "units" of strength.

In order for a signal to be strong enough to record in any meaningful way, it must be at at least 100 "units" or strength (i.e. Line Level.)

So...
* A DI box, such as the RNDI box will take a guitar signal in at 25 units, and boost it up to 100 (Line Level)
* The Cloudlifter, takes a weak microphone's 50 unit signal, and boost it up to 75.
* A preamp takes a microphone signal from 75 up to 100.
* A keyboard needs to preamp, as it is already at 100 (line level).

If this is all accurate so far... then I still have a hard time understanding what "high impedance" means. I've seen Hi-z and Low-Z on gear before, but never really understood if it was referring to the output signal... or the input jack.
For example, @Watchmaker, you said... "A guitar signal is rather low current and needs a high impedance destination." So... this seems to contradict logic.

The signal is weak... so we need to feed it into a piece of equipment that is more difficult to flow through. (???)

Also... I have seen microphones labeled, "High-Z" mics. It seems to me that a microphone is putting OUT a signal, so why would anyone describe a signal generating device as having a high impedance? It seems like the input the signal is going into would be described as high or low impedance.

<Head explodes from processing too many words.>
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Re: What is the difference between a "DI box", such as the RNDI, and a "Desktop Preamp"?

Post by Mike Stranks »

Dolmetscher007 wrote: Fri Jan 14, 2022 6:50 pm So... am I understanding you guys correctly?

Preface: I will be using imaginary values for the sake of simplification.

1. Electronic Instruments (Keyboards), Microphones, and Guitars all output an AC signal.
2. The signal that each of these things generates is at a different electrical "strength.".
3. Guitars put out a signal of 25 "units" of strength.
4. Some weaker microphone signals are a little stronger at 50 "units" of strength.
5. Some stronger microphones put out 75 "units".
6. Electronic Instrument signals are more like 100 "units" of strength.

In order for a signal to be strong enough to record in any meaningful way, it must be at at least 100 "units" or strength (i.e. Line Level.)

So...
* A DI box, such as the RNDI box will take a guitar signal in at 25 units, and boost it up to 100 (Line Level)
* The Cloudlifter, takes a weak microphone's 50 unit signal, and boost it up to 75.
* A preamp takes a microphone signal from 75 up to 100.
* A keyboard needs to preamp, as it is already at 100 (line level).

If this is all accurate so far... then I still have a hard time understanding what "high impedance" means. I've seen Hi-z and Low-Z on gear before, but never really understood if it was referring to the output signal... or the input jack.
For example, @Watchmaker, you said... "A guitar signal is rather low current and needs a high impedance destination." So... this seems to contradict logic.

The signal is weak... so we need to feed it into a piece of equipment that is more difficult to flow through. (???)

Also... I have seen microphones labeled, "High-Z" mics. It seems to me that a microphone is putting OUT a signal, so why would anyone describe a signal generating device as having a high impedance? It seems like the input the signal is going into would be described as high or low impedance.

<Head explodes from processing too many words.>

Your numbers are way out of whack, but you've got the general principle, But it would really help if you got yourself to using the proper terminology/units... the strength of the signal - as you describe it - is measured in millivolts. I won't start breaking your 'as example' figures down to something nearer true millivolt figures... it'll only confuse at this stage.

But one thing you've misunderstood is the purpose of a DI box. Most often - but not always - used in live-sound the purpose is to take an unbalanced line-level signal that needs to be 'looking at' an impedance of something in excess of 100k ohms - usually higher than that - and through a transformer and/or electronic circuitry converting that to a balanced mic level signal to 'look at' an impedance of about 2.5k ohms plus.

High-Z (impedance mics) are a throwback to the good old days when circuits were designed differently. They were usually at 50k ohms. My first mic (in 1971!) was a dual impedance 600 ohm/50k ohm mic where you altered the impedance by connecting the cable to different pins. I haven't seen a new high impedance mic for many many years... the 'standard' impedance these days is around 200 ohms.

These is very high-level and I've smoothed-off some corners, but I hope it helps a bit.
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Re: What is the difference between a "DI box", such as the RNDI, and a "Desktop Preamp"?

Post by Watchmaker »

It's all quite confusing. I studied electronics and my head still explodes...I also find it nearly impossible to discuss because my understanding is quite rudimentary.

One has to get to grips with Ohm's law: I=V/R written here as Current (I) is equal to Voltage (V) divided by Resistance (R), also stated as voltage is equal to current times resistance. This relationship is always true.

It is the different mechanical properties of microphones and guitar pickups that dictate the properties of the preamp requisite to achieve the necessary post preamp levels. The preamp is solving for an expected output. The Guitar, mic or keyboard are each delivering a different sized input (in terms of both resistance and current).

Basically, Hi-Z and Lo-Z signals both need to be "converted" by the preamp into line level signals. A line level signal, in its simplest form, needs no additional preamplification.

Again, it is the transductance of mechanical energy into electrical signals that dictate what's happening. The preamp needs to "see" and the transducer also needs to "see" an equivalent impedance in order to generate the requisite performance of the preamp. In the old days, preamps didn't have built in DIs so if you wanted to get an unamplified signal into your mixing desk - usually bass guitar - you needed some way to convert the signal such that the preamp saw that at mic level. Nowadays, it's very common to have the Hi Z option built in. It's just a small circuit in front of the actual preamp.

Edit: Mike is mainly a live sound guy and is totally on point with that use case. His post makes me aware of a simple difference maybe you haven't grokked. Pickups are High impedance or Hi-Z which basically means the circuit is high resistance (technically incorrect but good for basic understanding) Generally this is between 500k and 1M ohms. A mic is low Z generally between say 200 and 3000 Ohms (larger range but it's the relative difference between 3k and 1M that's important. Even if these signals have the same current, Ohm's Law indicates that they have different voltages - by an order of magnitude and it is this difference that's in play.

another edit: preamps are historically designed for mic level and line level inputs. It is the Hi-Z feature which is "new" and causes all the rukus. On an old mixing desk line levels bypassed the preamp and went straight to the processing (EQ, compression) then to the fader then to the buss. The preamp is what made mic level usable in the processing circuits.
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Re: What is the difference between a "DI box", such as the RNDI, and a "Desktop Preamp"?

Post by shufflebeat »

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Re: What is the difference between a "DI box", such as the RNDI, and a "Desktop Preamp"?

Post by Hugh Robjohns »

I haven't the inclination or energy right now to wade through correcting all the misunderstandings above... but reading between the lines of your original post it appears you want something in the way of a new preamp to add some colour to your guitar and are concerned that routing it through your elderly interface would degrade the signal.

In reality, that's very unlikely to be an issue... and if you fancy using an RNDI, go for it. It will be fine.

But if it does bother you, you can avoid the potential problem completely by finding a preamp with its own built-in ADC and connect it to the interface via a digital format (usually S/PDIF for a single or two-channel preamp, or ADAT for a four or eight-channel one).

Digital signals pass straight through the interface into the computer unmolested so only the preamp and its (more modern) converter defines the quality.

So... assuming your interface has a digital input, try looking for a colourful preamp with an instrument input and a digital output.
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Re: What is the difference between a "DI box", such as the RNDI, and a "Desktop Preamp"?

Post by Hugh Robjohns »

Circling back to this now I have a little more time...

Dolmetscher007 wrote: Fri Jan 14, 2022 5:12 pmSo while I am sure the circuits are quite different between a Desktop Preamp and a DI box, they seem to serve the same function.

Not really.

A standard DI box accepts an unbalanced instrument level (nominally around -20dBu), presenting it with a very high impedance load which is requires, and converts it to a balanced mic level (nominally around -50dBu) which can then be routed over long cables (if necessary) to a mic preamp to raise up to line level. Most DI boxes also provide 'galvanic isolation' between the instrument and destination to avoid ground loops and afford a degree of protection against electrical faults in the mains power system.

A 'desktop preamp' (or any preamp, for that matter) typically accepts a balanced mic level (nominally -50dBu) and amplifies it up to provide a balanced line level (nominally +4dBu). Many will also accept an unbalanced instrument level input as an alternative to the mic input.

And then there is another new hot-ticket-item out there called the "Cloudlifter" which also just seems to be a Pre-Preamp Preamp for boosting weak dynamic microphone signals by +25db so the audio interface preamp doesn't have to push so hard to boost the signal to desirable levels.

Yes, exactly that. An inline low-gain booster to assist the main preamp with weak microphone signals. Most are phantom powered but don't pass phantom on to the input, so can only be used with dynamic mics.

My concern is that all the Pre-Preamp boosting magic still has to go through the circuitry inside my less-than-amazing focusrite USB interface.

While this is true, most interface preamps add vanishingly small amounts of distortion or noise to line level sources, and the converters -- even in old preamps -- are usually a very long way from being the weakest link in the signal chain. That honour almost always goes to the room acoustics, the mics, and/or the way the mics are being used!

I've never seen an audio interface that was JUST an ADC...

They definitely exist... you need to look harder! :D

...if there is such a bare-bones ADC audio interface that would ... just handle the analog to digital conversion and shooting it over to the DAW through USB... wouldn't that be better than having the signal ... still go through, yet another redundant preamp circuit such as the Focusrite?

Theoretically, yes. In practice, in a home project studio, the difference is usually insignificantly small and not worth getting bothered about.

Watchmaker wrote: Fri Jan 14, 2022 6:08 pmA guitar signal is rather low current and needs a high impedance destination.

The current is largely irrelevant. Most modern audio circuitry is voltage-based and all out references to signal levels are based on their voltage (usually specified in terms of dBu or dBV). Maximum voltage is always generated when feeding a high impedance load -- and we typically use a destination load which has 10 times the input impedance of the source's own output impedance, although the exact value is not important provided the ratio is in the right ball park.

Mic level can range anywhere from -70dBu to -30dBu (although it can exceed 0dBu in some circumstances). The typical output impedance of a mic is 150 Ohms, and the typical input impedance of a mic preamp is 1.5 to 5k Ohms.

Instrument level is typically -30dBu to 0dBu. A guitar pickups can have an output impedance of several kilo Ohms, and typical guitar amps present an input impedance of 500K to 1M Ohm.

Line level can range anywhere from -8dBu (-10dBV) to +4dBu, with potential peaks up to +24dBu. Most line level equipment has an output impedance of a few hundred ohms or less, but typical line inputs are known as 'bridging inputs' and have an input impedance of 10k Ohms or more.

The idea here is that in the professional world its sometimes necessary to feed multiple line-level devices from the same source, and since additional destinations significantly lower the overall input impedance seen by the source its necessary to start with a much higher value than strictly necessary in a 1:1 connection.

Dolmetscher007 wrote: Fri Jan 14, 2022 6:50 pmPreface: I will be using imaginary values for the sake of simplification.

There's no need when a perfectly workable real system of values indicating signal levels exists already! :lol:

1. Electronic Instruments (Keyboards), Microphones, and Guitars all output an AC signal.

Everything does. Audio alternates in polarity!

2. The signal that each of these things generates is at a different electrical "strength.".

True -- see above.

3. Guitars put out a signal of 25 "units" of strength.
4. Some weaker microphone signals are a little stronger at 50 "units" of strength.
5. Some stronger microphones put out 75 "units".
6. Electronic Instrument signals are more like 100 "units" of strength.

As a general rule, the quietest signals are from mics, then instruments and then line level sources. But the ranges of each of these overlap, depending on the specifics. it's quite possible to get line-level signals from a kickdrum mic, and mic level signals from a line-level source during the quiet bits of a track! Electronic instruments generally have a higher level than electric guitars, but not always -- some guitar pickups (or active basses) can be extremely loud!

In order for a signal to be strong enough to record in any meaningful way, it must be ... Line Level.

Usually, yes, but there are recorders that are quite happy to record directly from mic level sources (like most portable recorders, in fact!).

So... A DI box, such as the RNDI box will take a guitar signal in at 25 units, and boost it up to 100 (Line Level)

No, it actually attenuates the guitar signal (by about 10dB) and presents a balanced mic level signal at the back to feed a mic preamp.

The Cloudlifter, takes a weak microphone's 50 unit signal, and boost it up to 75.

Yes, but in real numbers it's expecting an input of, say, -60dBu and delivers an output at -35dBu.

A preamp takes a microphone signal from 75 up to 100.

Typically, a mic preamp would accept anything between -60 and -10dBu and raise the level to a nominal +4dBu line output.

A keyboard needs to preamp, as it is already at 100 (line level).

Some keyboards can deliver line level (or something close to it), but vintage designs tend to be quieter and nearer to a guitar-level signal (because they were designed to be plugged into guitar amps), and most are also unbalanced, of course.

I still have a hard time understanding what "high impedance" means. I've seen Hi-z and Low-Z on gear before, but never really understood if it was referring to the output signal... or the input jack.

Its normally referring to the input socket, and the load impedance seen by the device you plan to plug into it. Guitars generally need to see a high-impedance load (as mentioned above), so you'll see references to hi-Z on instrument inputs.

The term Low Impedance is generally only used in connection with microphones or with the outputs of electronic devices.

For example, @Watchmaker, you said... "A guitar signal is rather low current and needs a high impedance destination." So... this seems to contradict logic.

It doesn't contradict logic, but to understand why you need to understand the basics of electronic circuits.

A guitar generates a low output current because of its high source impedance. To extract the greatest signal voltage (which is what the amplifier needs) the source current has to complete its circuit through a very high destination impedance. It's based on Ohms Law which, in this case, implies that the signal voltage (V) we want in the destination = signal current (I) from the source x destination impedance (R): V=IR

Also... I have seen microphones labeled, "High-Z" mics. It seems to me that a microphone is putting OUT a signal, so why would anyone describe a signal generating device as having a high impedance?

It's telling you that that microphone is designed to work with a high-impedance input -- like on a guitar amp. They are relatively rare these days but dates from the 60s and 70s (before portable PAs became affordable to pub bands) when it was common to plug vocal mics into guitar amps!

Watchmaker wrote: Fri Jan 14, 2022 8:19 pmBasically, Hi-Z and Lo-Z signals both need to be "converted" by the preamp into line level signals.

Signals don't have an impedance. Signals are simply a varying voltage. Only sources and destinations have impedances. In the vast majority of cases, sources have a low output impedance and destinations have a high(ish) input impedance, and we don't generally care what those impedances are as they are not critical to the transfer of signal between devices. The only exception is with electric guitars which really insist on seeing a very high input impedance.

Lots of words... but I hope they bring some clarity to the confusion and misunderstanding permeating the earlier thread.
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