Audio interface with standalone preamp mode

[carlo]

I'm interested to know what audio interfaces out there can be used as standalone analogue preamps without a computer. Of course it's main use will be with a Mac, but I'd like to have the option.
Quite a lot ( Motu m2 and m4, Audient EVO etc.) can be powered by a usb battery or adapter and work standalone, but their use is meant to be connected to monitors for rehearsals and the like. So the signal passes through ADC, then DAC and finally to the output with no possibility to skip the digital stage. But when connected to the line inputs of a field recorder ( to have more gain or searching better preamp quality) the signal will the pass through the internal ADC of the recorder; probably not the best way to go....
As far as I know the cheap Behringer UMC204HD and 404 offer this true option by taking the signal off the insert socket right after the preamp stage and plugging the other end of the cable in the recorder's input. There are some obscure points though ( to me at least) that I'd like to make clear.
1) I've read around that Behringer's outputs are very weak; inserts seem to be some -20 db....will it vanish the job done by the preamps and lead to crank the unwanted gain on the recorder?
2) it seems the headphone out is a bit weak...will it be possible while setting the Behringer as input in the Computer to select another USB out where I'll plug a DAC/amp and listen from there?
3) It's been reported from Behringer that only the 4 channel UMC404 may be used standalone, as it can be powered by a dedicated adapter which the 2 channel version doesn't have. Now, given that the 204 only has USB powering, why seems not possible to power it through the USB port with an adapter or a 5v battery and run it like other interface do?
4) Should the 4 channel option be the way to go, the Phantom power on for all channel at a time will produce any unwanted noise or hum with only two mics plugged in, thus leaving the other two powered but without any load?
Both the Tascam US2x2 and the recent US2x2HR claim to be usable standalone, but I just see a couple of outputs on the back, which gives me the idea that the signal will go digital like the Motu and co.
I beg your pardon for all these questions and I'll be grateful to you all to figure out what the best solution will be. Should you convince me that, even after an ADC+DAC+ recorder's ACD the signal is still great I'll be perfectly fine with a Motu or an Audient. They seems to be good quality, portable and with a nice headphone output.
Thank you really to all

 

[L5730]

Yes, the Behringer UMC204HD and UMC404HD have unbalanced inserts which are send/return on TRS jacks. Post pre-amp send and return pre-ADC, so in use as inserts you'd have to watch output levels of whatever outboard gear was used else one could overload the ADC.
But yes, you could use the interfaces pre-amps and send the unbalanced signal to another device if you so wished. All in the analogue domain.

Many interfaces have an Input/Playback control knob which allows for zero-latency* monitoring of the source input signal, as well as a mix with what is playing from the connected computer.

* Zero-latency.
Here's the sticky bit. If this is true zero latency, then the input pre-amp output voltage is being tapped off in the analogue domain and being fed directly to this control knob. Zero-latency, except for the tiniest delay in the analogue signal being turned into a voltage and running down a few inches of wire.
What a lot of devices call zero-latency is not this. It's actually the pre-amp output through the ADC, maybe some DSP Mixer and then to the DAC. It's usually very low latency, but is perceptible if the volume of the live in-the-room sound and the DAC output are similar. Once hears a slight combing effect. Had this on an old M-Audio 2496 PCI interface with it's "zero-latency direct monitoring".

I think RME devices are ADC>DSP Mixer>DAC but are very fast.
Higher end Motu devices seem similar.

There really doesn't seem to be all that many that allow for tapping the analogue signal straight off the pre-amp. You'd have to look for Inserts.

You are probably better off looking for a standalone all analogue pre-amp. Depending on how many channels, it shouldn't be too expensive. If you managed to get one with an ADC built in, it could probably be hooked up to an audio interface at a later date, to add additional analogue input channels.
However, if it's not absolute zero-latency that is your concern, I'd imagine that most interfaces that measure well here would be fine even if the signal does go ADC>DSP Mixer>DAC, would the difference be noticeable in a level-matched blind test? Doubtful.

 

[carlo]

Yes, the Behringer UMC204HD and UMC404HD have unbalanced inserts which are send/return on TRS jacks. Post pre-amp send and return pre-ADC, so in use as inserts you'd have to watch output levels of whatever outboard gear was used else one could overload the ADC.
But yes, you could use the interfaces pre-amps and send the unbalanced signal to another device if you so wished. All in the analogue domain.

Many interfaces have an Input/Playback control knob which allows for zero-latency* monitoring of the source input signal, as well as a mix with what is playing from the connected computer.

* Zero-latency.
Here's the sticky bit. If this is true zero latency, then the input pre-amp output voltage is being tapped off in the analogue domain and being fed directly to this control knob. Zero-latency, except for the tiniest delay in the analogue signal being turned into a voltage and running down a few inches of wire.
What a lot of devices call zero-latency is not this. It's actually the pre-amp output through the ADC, maybe some DSP Mixer and then to the DAC. It's usually very low latency, but is perceptible if the volume of the live in-the-room sound and the DAC output are similar. Once hears a slight combing effect. Had this on an old M-Audio 2496 PCI interface with it's "zero-latency direct monitoring".

I think RME devices are ADC>DSP Mixer>DAC but are very fast.
Higher end Motu devices seem similar.

There really doesn't seem to be all that many that allow for tapping the analogue signal straight off the pre-amp. You'd have to look for Inserts.

You are probably better off looking for a standalone all analogue pre-amp. Depending on how many channels, it shouldn't be too expensive. If you managed to get one with an ADC built in, it could probably be hooked up to an audio interface at a later date, to add additional analogue input channels.
However, if it's not absolute zero-latency that is your concern, I'd imagine that most interfaces that measure well here would be fine even if the signal does go ADC>DSP Mixer>DAC, would the difference be noticeable in a level-matched blind test? Doubtful.

@L5730 thank you so much for your detailed response. Of course a dedicate preamp will still be better than a "side use" of an audio interface. I'm a bit struggling to understand all tech stuff but from what I get from your answer the main concern is not the degradation of sound quality through digital stages but is mostly related to latency, which would be theoretically absent using inserts out ( this would act as a standalone analogue preamp I guess).
Latency is not an issue because I'm not planning to overdub anything but just plain recording. It could maybe a potential issue when using two mics plugged into the interface + two other mics straight into the recorder in order to record four tracks? According to what you've explained it seems I could have problems with the ADC/DAC route in the Motu or similar while much less with Behringer - inserts out. But, on the other side, probably buying a cheap Behringer is not the real preamp-upgrade. Probably the best bet could be a 4 channel interface ( like the new Audient EVO 8); whatever the latency the 4 inputs of the recorder would be all equally treated.
Thanks

 

[L5730]

If latency is not an issue then it comes down to the quality of the pre-amps themselves as well as the ADC and DAC quality.
In the case of those two Behringer's, the ability to take the unbalanced output from the pre-amps might be nice, but in quality terms, it's probably not as good as the pre-amp>ADC>DAC path of the Motu M2/M4.

If you wanted to record 4 microphones, and had an interface witch 4 channel recording but it only had 2 mic pre-amps (eg. Motu M4), you would have to source 2 more mic pre-amps from elsewhere. In this scenario, it would be helpful to have a little latency as possible on the extra pre-amps so as not to cause those tracks to be out of alignment with the 2 tracks from the internet mic pres.
Ah, another point on this is that there could be a very small sync drift if the two devices clocks are not synchronised. The latency issue is easily fixed in a DAW after recording (just drag the tracks forwards or backwards in time), but a ADC>DAC could possibly add some drift slowing down or speeding up the pitch, ever so slightly, compared to the other device.

Devices with ADAT input allow for expansion by adding additional mic pre-amps from an ADC. Something like the Focusrite Scarlet OctoPre has 8 mic pre-amps and 2x ADAT optical outputs. One could record 8 (at 48kHz*) mic pre-amps from the OctoPre as well as whatever internal pre-amps were present on an interface.
*ADAT is a funny thing, where there is only so much bandwidth, so the higher the sample rate the less channels one can send down the light-pipe.
8ch @ 48kHz, 4ch @ 96 kHz, 2ch @ 192kHz. The OctoPre in my example has 2x ADAT ports so if the interface has multiple ADAT inputs, there is room to use higher sampling rates and more channels. But this is all way off topic at this point

Devices with 4 mic pre-amps ensures that there are no sync issues between the tracks. Latency in itself is dependant on how the device was designed. We're not even getting into latency of monitoring 'through the box' (ADC>DSP Mixer>DAW>DSP Mixer>DAC) which has multiple factors.


What was it that you wanted to achieve? Perhaps there is an easy solution, or at least quirky useable one.

 

[carlo]

If latency is not an issue then it comes down to the quality of the pre-amps themselves as well as the ADC and DAC quality.
In the case of those two Behringer's, the ability to take the unbalanced output from the pre-amps might be nice, but in quality terms, it's probably not as good as the pre-amp>ADC>DAC path of the Motu M2/M4.

If you wanted to record 4 microphones, and had an interface witch 4 channel recording but it only had 2 mic pre-amps (eg. Motu M4), you would have to source 2 more mic pre-amps from elsewhere. In this scenario, it would be helpful to have a little latency as possible on the extra pre-amps so as not to cause those tracks to be out of alignment with the 2 tracks from the internet mic pres.
Ah, another point on this is that there could be a very small sync drift if the two devices clocks are not synchronised. The latency issue is easily fixed in a DAW after recording (just drag the tracks forwards or backwards in time), but a ADC>DAC could possibly add some drift slowing down or speeding up the pitch, ever so slightly, compared to the other device.

Devices with ADAT input allow for expansion by adding additional mic pre-amps from an ADC. Something like the Focusrite Scarlet OctoPre has 8 mic pre-amps and 2x ADAT optical outputs. One could record 8 (at 48kHz*) mic pre-amps from the OctoPre as well as whatever internal pre-amps were present on an interface.
*ADAT is a funny thing, where there is only so much bandwidth, so the higher the sample rate the less channels one can send down the light-pipe.
8ch @ 48kHz, 4ch @ 96 kHz, 2ch @ 192kHz. The OctoPre in my example has 2x ADAT ports so if the interface has multiple ADAT inputs, there is room to use higher sampling rates and more channels. But this is all way off topic at this point

Devices with 4 mic pre-amps ensures that there are no sync issues between the tracks. Latency in itself is dependant on how the device was designed. We're not even getting into latency of monitoring 'through the box' (ADC>DSP Mixer>DAW>DSP Mixer>DAC) which has multiple factors.


What was it that you wanted to achieve? Perhaps there is an easy solution, or at least quirky useable one.

Thank you very much for spending your time in answering my question; it has been really helpful. I just wanted to improve a bit the performance of my Zoom h6 recorder, but looking at a good 4 channel preamp seems a waste of money as in any case I'won't totally skip the recorder's internals and for the money I would spend I'd be better off buying a Sound Devices ( which maybe I'll do in the future). I had considered also a couple of inline preamps like fethead phantom but somebody here explained in details that I wouldn't get any noise improvement out of them. I'm somewhat not ready now for the buy once and forget thing....
The audio interface idea came to my mind just because with a moderate cash you can get a decent piece of gear, mostly to use properly with a pc and occasionally take in the field to feed a recorder. You have cleared all my questions: should I use a 2 channel interface at the purpose better limit the whole thing to 2 channels. If want more I'll consider a 4 channel thing but don't mix different feeds due to the latency potential issue. Thanks again!

 

[L5730]

I said:

Ah, another point on this is that there could be a very small sync drift if the two devices clocks are not synchronised. The latency issue is easily fixed in a DAW after recording (just drag the tracks forwards or backwards in time), but a ADC>DAC could possibly add some drift slowing down or speeding up the pitch, ever so slightly, compared to the other device.

I didn't completely think that through. Sorry, that is incorrect.
What I was thinking is that there would be sync drift between the two devices, but there wouldn't as the ADC and DAC in the device you'd use for extra pre-amps would be synced to it's own internal clock, and thus in time with it's self and the analogue world. If it didn't you'd have people singing and then hearing themselves back at a different pitch or with dropouts. No, the ADC>DAC route would be in correct pitch/speed from analogue input to analogue output.
If you then took the analogue output and recorded it with another analogue source on another interface, there would not be a sync drift, but the latency caused by ADC>DSP>DAC would still be there (it'd be small, very small, but measurable).

Sorry for that misinformation.

So, yeah you can just use another interface as a pair of standalone pre-amps (as long as it works in standalone mode). The ADC>DSP>DAC would add some very small latency, but really it'd be fine. The question you asked in the first post was whether the audio quality would be there. Whilst a direct feed from the pre-amp would be preferable to the additional noise introduced from the ADC and DAC, it's going to be pretty low with these well measuring interfaces.

Would an ART Dual Pre sound any better or worse than a Motu M2 ADC>DAC recorded into another interface? I'd probably say the Motu would be better, but don't see any test results of the ART device.
There certainly doesn't seem be a whole lot of dual channel pre-amps and two single channel pre-amps for anything close to the price of an interface. Kind of makes the interfaces very good value for money.

 

[carlo]

I said:


There certainly doesn't seem be a whole lot of dual channel pre-amps and two single channel pre-amps for anything close to the price of an interface. Kind of makes the interfaces very good value for money.

This was my thought also...for a decent preamp you need to spend extra money compared to interfaces. At the end the potential latency issue, if any, would be completely overcome by avoiding to add channels to the recorder some coming from the interface and some directly plugged. Something like the recently release Evo 8 could probably do the job with four channels at a good price point.

 

[dasdoing]

personaly I would take one with SPDIF-out and conect it digitaly to a recorder

 

[pier25]

Sorry for the necro but I'm looking into this as I need some extra preamps for my current interface.

I found that most Tascam interfaces have a standalone mode which routes the preamps to the line outputs. I confirmed with Tascam's support the audio signal remains fully analog and never goes through ADC unless the interface is connected to USB (so not using the standalone mode).

Their preamps are really clean. Their Ultra HDDA preamps have a noise around -130dBu EIN confirmed by Julian Krause on Youtube.