Audio interface line input level

[mattroe]

Hi all!

I have bought a new focusrite scarlet 8i6 (3rd gen) interface, I'm really happy with it except for one thing - the line inputs seem really quiet.

I tested with a bass-station 2 set to put out its maximum level and connected from the line output, via unbalanced guitar cable to the 8i6 line input. The signal peaks at around -14db when I record it in audition. The signal is clean, and the recording is good. But when monitoring it's very weak in comparison to the daw output, or the preamp inputs on the front.

I didn't have this problem with a first gen 8i6, and then I checked the settings, the line inputs on that have a 'high gain' setting that I was using to boost the signal. But the new version of the 8i6 doesn't have this as an option.

I can work around the problem by monitoring through the daw, and boosting the level there. It sounds fine, but that means having to load up the daw every time I want to play my synths, which is irritating, plus it adds a little latency (not much, I'm getting good latency with the interface).

I'm not sure where to go from here. Getting 4 preamps to boost line level signals seems like overkill, I really just want a kind of fixed gain boost (about 12db would be great), preferably passive (I'm running out of power sockets), if that's even possible. Could that be diy'd?

I don't have much of a budget at all.

I thought about returning it and getting a behringer umc1820, which would give me preamps on every input. But I originally chose the 8i6 over that because of the behringer's limited routing options, and because you can't seem to mute inputs individually. The 8i6 gives a lot of control over that side of things.

Tldr - is there a cheap, preferably passive way of raising a line level signal by around 12db or thereabouts?

Any replies appreciated.

Thanks,

Matt

[ef37a]

Hi Matt and welcome.
I Have looked at the line input spec' for the 8i6 and it is +18dBu 'maximum' presumably that level produces ~ 0dBFS? Focusrite in the (rather distant!) past got some stick for poor headroom on certain products so I suspect they have erred on the side of caution here? +18dBu is a level you could expect from many mic preamps and even quite low end mixers.

The Bass Station typically gives no figures for its line output voltage that I can see (what is WRONG with synth makers? Why the secrecy?) but I have found in the past that kbds generally are pretty weak sisters and rarely deliver more than -10dBV (0.3I6V) often less.

You would think someone would make a USB powered box with a clutch of NE5532 op amps in it and that 12dB gain you want and an output of +22dBu? Not that I can ever find.

The lack of power outlets is easily fixed with a 'diss strip'. You can't be THAT cash strapped!

Dave.

[Drew Stephenson]

-14dB is a perfectly healthy input level, normal recommendation round these parts is to record with a signal bouncing along around -18dB and peaking around -12 to -10dB.
If this is being drowned out by other things, can you turn your other sources down and raise your master gain instead?

[ef37a]

-14dB is a perfectly healthy input level, normal recommendation round these parts is to record with a signal bouncing along around -18dB and peaking around -12 to -10dB.
If this is being drowned out by other things, can you turn your other sources down and raise your master gain instead?

Drew, Matt says his line inputs PEAK to -14dB (FS one assumes in Audition? We have to be careful to 'label' our decibabbles otherwise we are up a current of excrement without means of propulsion!) That means his average level is probably -24dBFS and if he brings all other sources down to the same level (which will of course do no harm at 24bits) he might not have enough gain left in the system for real time monitoring? Especially on headphones.

I did look on line for a simple booster with about 12dB of gain but could only find rubbish (probably) made for the ICE market and expensive kit for the hi fi tweaks. It really is not rocket science to make! Cannot see why no one has done so?

Dave.

[Hugh Robjohns]

Given that the output of the synth is unbalanced and close to nominal domestic line level (-10dBV), and you need to feed a balanced line input operating at nominal Pro level (+4dBu), the obvious solution is to use a domestic/pro interface box like the ART Cleanbox Pro.

https://artproaudio.com/product/cleanbo ... converter/

Other brands do similar converters, but I have first-hand experience of the ART and can recommend it as good value for money.

These devices typically accept two channels of unbalanced -10dBV inputs and deliver balanced +4dBu outputs (so boost the signal by 12dB).

They are intended for interfacing consumer equipment, like a cassette recorder, with a professional mixing console and so most also have the corresponding return path to convert balanced pro outputs to unbalanced consumer inputs too.

In your case you can use the unbalanced to balanced side to make your synth outputs more compatible with the line level inputs of your interface.

[merlyn]

Tldr - is there a cheap, preferably passive way of raising a line level signal by around 12db or thereabouts?

No, passive circuits can't boost a signal. A passive circuit can only attenuate a signal. Boosting a signal requires power, otherwise you have invented a free energy machine and solved the energy crisis. Consequently the recommended Cleanbox requires a power supply.

[Hugh Robjohns]

No, passive circuits can't boost a signal. A passive circuit can only attenuate a signal.

A step-up transformer looks passive to me....

...as used to boost the signal levels from ribbon and moving-coil mic diaphragms, moving coil pickups, API preamp outputs and many other applications.

There are practical limitations, obviously, but it is a passive solution that can boost signal levels -- by stepping up the signal voltage, which is what we need in most audio applications.

A transformer can't boost the overall power, though, as that would contravene the laws of physics as you say.

For the OP, while you could use passive transformers to step-up the signal voltage by 12dB, the cost for decent quality line-level transformers would be highly prohibitive. An active gain stage, such as the Cleanbox Pro, is a more sensible and affordable solution.

[merlyn]

I note with interest you didn't recommend a step-up transformer for this application.

12dB is multiplying by 4. So the number of turns in the secondary is 4 times the primary. The impedance goes up by the square of the turns, so the impedance of the secondary is 16 times that of the primary. Say the primary is 10k, the secondary is 160k. If the input of the 8i6 is 10k, most of the signal stays on the secondary, so there is no boost.

You knew that ...

[Hugh Robjohns]

No, I didn't recommend transformers to the OP. I did say there were practical limitations...

The fixed level line inputs in the 8i6 have an impedance of 44k, apparently, but your argument is entirely valid. Step-up transformers tend to be used with (relatively) high-impedance inputs for the practical reasons you state.

Nevertheless, there are passive ways of increasing signal levels if the circumstances can be made favourable.

[ef37a]

I had not realized that Art made an active booster though I am of course familiar with their transformer devices.

Yes, a transformer could be used to give a 12dB boost and assuming the B S2 has a low enough source resistance (not that we will ever find THAT out!) the 44k Focusrite input would not load it unduly.

Problem of course is that transformers need to be quite expensive if they are not to degrade audio performance and even a fairly low 1:4 step up ratio would probably require a Zobel network on the secondary to smooth the extreme top end...and you can't do THAT unless you know the characteristics of the load.

Dave.

[Mike Stranks]

Thanks for flagging that Hugh.

I, too, hadn't realised exactly what that gizmo from ART does - probably because I'd not needed to look for anything like it...

But I do now... so thanks!

[Hugh Robjohns]

The Elf champions it for interfacing guitar pedals (etc) with pro interfaces/consoles, and for re-amping, too. It is quite a handy unit to have around. And no transformers involved...

[merlyn]

Even with the 8i6 having an input impedance of 44k the transformer I described is an attenuator. The voltage in the secondary is 4 times the voltage in the primary.

VS = 4*VP

If we call what the 8i6 sees V8i6 the output impedance of the transformer and the input impedance of the 8i6 act as a potential divider

V8i6 = (44/(44+160))*4*VP = 0.86*VP

If we do it right and make the output impedance of the transformer 1/10 of the input impedance of the 8i6 that gives us an impedance of 4.4k in the secondary, which is 275 in the primary. We've simply moved the problem to the primary as 275 will be lower than the output impedance of the Bass Station.

If someone wants to juggle the numbers and come up with a scenario where a transformer can provide a boost, please do.

[ef37a]

Merlyn, transformers are not potential dividers.
As the first example that I found look at the Sowter 3483, a "600 Ohms to 10k Ohms".
That is near as jazz 1:4 and gives a loss of only 0.3dBu when loaded with 10k and with a source resistance of 40 Ohms. Now, the source R of the Bass Station will not be that low but it could easily be 100-120 Ohms. But the load is 4. times 10k so I submit that the transformer WILL give a decent chunk of the desired 12dB boost.

This does not of course get around the fact that transformers used in isolation (Boom! Boom!), without being specifically 'tweaked' to the actual circuits will always give a bit of a ragged response but a voltage boost they certainly will give!
Otherwise how have all those valve pre amps worked for the last near century?

Dave.

[Hugh Robjohns]

As I said:

The fixed level line inputs in the 8i6 have an impedance of 44k, apparently, but your argument is entirely valid. Step-up transformers tend to be used with (relatively) high-impedance inputs for the practical reasons you state

I think we are all agreed, even without the sums, that a step-up transformer wouldn't be appropriate for the OP's specific application, but there are applications where step-up transformers can be -- and routinely are -- used to boost signal levels passively.

Again, as I said, dynamic mics, especially ribbons, rely on it. As do some moving-coil pickup preamps. And quite a few outboard processors and mixing consoles rely on step-up output transformers to provide the required pro output level (usually 1:2 ratios, for a 6dB boost).

So it is incorrect to claim "passive circuits can't boost a signal". They can, in certain correctly-engineered circumstances, which I feel is an important educational and practical point which warranted correction.

[mattroe]

Wow, Thanks for all the replies! Much appreciated!

Ef37a - yep, you hit the nail on the head, it's when headphone monitoring that I have the problem. With my speakers (ae22's) I can turn everything else down a bit and crank the speakers, but on headphones (hd650) it starts to get too quiet. And I live in a one bed flat, so headphones are often my only option.

Hugh - you found the solution! That's exactly the sort of thing i'd been looking for, but I kept getting home stereo type results that didn't quite fit because I didn't know quite what to search for. Cheers!

Two cleanbox pro's should sort me out. I have a rat's nest of a set up based around a guitar, bass, bass-station, a bunch of cheap pedals and 5 axoloti's which are synth/fx/drum machine/sequencers/loopers/samplers, depending on what i've got loaded in them. They put out a similar level to the bass-station, maybe even quieter, so they should benefit too.

Regarding the rest of the interface, I'm really happy with it, it sounds ace, and recordings are clean. I thought the 'air' setting would be a gimmick, but it seems to really open up my muddy-high-gain-humbucker strat.

The passive solution was a bit of a stretch, but if you don't ask... Thanks all for the science part, it's interesting, though i will have to re read to fully understand.

Cheers,

Matt