Maximum input level for guitar D.I.?

[ballstix]

I'm looking to get a portable interface for using amp sims while I'm out of town. I can get the NI Komplete Audio 6 for $199 today, but I'm sort of uncertain about the max input spec: http://www.native-instruments.com/#/en/products/producer/komplete-audio-6/?page=1906

It says for instruments the max level is +8.4 dBu. Does that leave room for high-output pickups to clip it with the preamps at 0? I know this was a problem for the lower end Focusrite interfaces. Othwerwise I can buy an AudioBox 22VSL.

Thanks!

 

[JHA]

8.4 dBu is ~2V RMS and ~2.88V peak. For example EMG81 can produce 4.5V peak signal, which is logically half of 9V battery. That is ~12.3 dBu.

 

[deLuther]

Many operational amplifiers can not output level exceeding half of voltage, usually output is less than that (depending on OPAMP used, rail-2-rail OPAMPs have seldom usage), also under load EMGs have significant battery drain, so battery voltage under load is also reduced from 9 V. So, for example, EMG 85 have slightly more than 2 Vp, SD AHB-1 have slightly less than 3 Vp.
+8.4 dbu is almost like lower end Focusrites, only slightly better, but not enough.
Often interfaces have input levels specified in terms of peak level, not RMS, so safer to suppose that +8.4 dbu equals to 2 V peak, not 2.88 V.

 

[JHA]

Many operational amplifiers can not output level exceeding half of voltage, usually output is less than that (depending on OPAMP used, rail-2-rail OPAMPs have seldom usage), also under load EMGs have significant battery drain, so battery voltage under load is also reduced from 9 V. So, for example, EMG 85 have slightly more than 2 Vp, SD AHB-1 have slightly less than 3 Vp.
+8.4 dbu is almost like lower end Focusrites, only slightly better, but not enough.
Often interfaces have input levels specified in terms of peak level, not RMS, so safer to suppose that +8.4 dbu equals to 2 V peak, not 2.88 V.

Actually no opamp can produce higher AC voltage than half the power supply voltage (battery in this case). You forget that a new unloaded 9V battery is more than 9V, about 9.3-9.4V. According to the EMG datasheet, most actives including the 85 is 4.5V peak, not 2V.

dBu as an unit is referenced to ~0.775V RMS = 0 dBu, so 8.4dBu is ~2V RMS and ~2.88V peak. Unless Focusrite has their own version of the dBu, your interpretation is wrong. You can check it here: http://www.sengpielaudio.com/calculator-db-volt.htm.

 

[ballstix]

Hm, I actually have a Saffire Pro 24 DSP as my main interface right now. For some reason its maximum input level isn't listed in the manual. For the Pro 24 non DSP version, the max is +8 dBu.

Right now I use a 7-string with a BKP Painkiller in the bridge, strings really close to the pickup. I can get it to peak at -11.0 dBFS in my DAW with no gain by really hitting the strings hard. Does that seem to make sense? As far as I'm aware, Painkillers have fairly high output. I also have a 6-string with a JB bridge that seems to be about half as loud.

I guess it'd still be wiser to go with the 22VSL given its +15 dBu max headroom?

 

[deLuther]

JHA, I know that dbu means RMS values, but too often interfaces (not only Focusrite) have levels specified in terms of peak level, I don`t know why, just mention it.
I don`t forget about voltage of fresh battery, but under load voltage reduces, OPAMPs (not rail-2-rail, of course) further lowers output margin (this can be found in datasheets). I measure outputs of my 85 and blackout (with calibrated input with known level) and only got something like 4.5 Vp (not Vpp, peak to peak) on 18 V.
Of course this is not oscilloscope...

ballstix, check the new M-Audio C400/C600, they both have maximum input level of +16 dbV or +18.2 dbu. And there is some rumor that saffires can have lower impedance than 1 MOm...

 

[JHA]

Yeah they could indeed lie about the specs but I would expect a big name like Focusrite not to do it. But who knows? Nowdays a company can get far if the gear looks good. EMG doesn't really specify if it's 4.5V peak-to-peak or peak. In either case it would clip the Saffire input if it can only handle 2V peak. Hmm, I have an old calibrated oscilloscope lying in the closet, I'll see if I can get it to work and I'll measure my EMG81.

Ballstix, Saffire Pro 24 DSP has a 10dB pad in instrument input so it should be able handle +18dBu signal.

 

[deLuther]

Where is Pad on Pro 24/DSP? No images with such button, no settings in drivers for instrument inputs (channels 1-2), both units have +13 db minimal gain on instrument input, which is overkill for many modern pickups (leaving weak singles aside).
Pro 40 on other hand has Pad button, which can be seen on pictures and in manual.

 

[JHA]

On this page http://www.focusrite.com/products/audio_interfaces/saffire_pro_24/specifications/ it mentions the pad three times:

Instrument Inputs
• Frequency Response 20Hz-20KHz +/- 0.1dB
• Maximum Input Level min gain no pad +8dBu
• Maximum Input Level max gain no pad -42dBu
&#8226; THD+N, -1dBFS, min gain, no pad < 0.0015%
&#8226; SNR 105dB "A"
&#8226; DNR 105dB "A"
&#8226; Input Impedance > 1MOhm
&#8226; X-talk < -100dB
&#8226; Adjustable Gain > 50dB
&#8226; Pad Attenuation 10dB

Just assumed it's the same for the "DSP" version. Btw. they mention that they measure their gear to the AES17 standard so we could assume they at least get the dBu right.

No luck with the oscilloscope, that POS must be like 30 years old, gonna throw it away.

 

[ballstix]

There's definitely no pad switch on the Pro 24 or DSP. That specification is pretty unclear. I just sent them an email for clarification.

 

[deLuther]

JHA, it`s funny thing that Focusrite mentions Pad in specifications, but it`s clear from images that no such button exists. There is level setting in drivers, but only for inputs 3-4, not for 1-2.

Because of those "funny" things it is safer to suppose that level can be measured at peak, i.e. have pessimistic estimation (where exists one obvious error can by another)

 

[ballstix]

Here was my email:

Hi! On the product page for the Saffire Pro 24 ( http://www.focusrite.com/products/audio_interfaces/saffire_pro_24/specifications/ ), it is stated under instrument inputs that pad attenuation is 10dB. There is no pad switch on the device, so what does that mean? I am specifically curious about it in relation to the +8 dBu maximum input level. Does this mean that the maximum input level with no gain is actually +18 dBu?

The reply from Focusrite said that I was correct. Seems strange to me considering people are able to clip their Pro 24s with the gain set to 0...

 

[deLuther]

It seems that today many support guys cannot find their asses without a flashlight

I think that better to figure it yourself, using interface output as signal generator, just send sinusoidal signal and measure RMS with multimeter to get actual level, then do some loopback test.

 

[JHA]

The most accurate way to test inputs would be to use a calibrated signal generator. If you use the soundcard out+multimeter method remember that many cheaper multimeters don't measure over 1kHz signals.

 

[deLuther]

JHA, yes, I know, I`m usually prefer testing on 50 Hz sine or not more than 1 kHz. Of course, signal generator will be precise tool.