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SMSL SU-8 + JBL 305Ps

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#1
My SMSL SU-8 DAC just arrived... I have it connected to my set of JBL 305Ps using XLR cables. USB input. MusicBee with WASAPI in exclusive mode.

My question is: what input sensitivity do I have to use for the JBLs? +4dBu or -10dBv? Bear in mind that the DAC output is 2Vrms for RCA and 4Vrms for XLR.

- I first tried +4dBu (for "professional" equipment), the speakers sound fine, however the max volume seems rather low for their rating (41W+41W).
- I then tried with -10dBv (for "consumer-grade" equipment) and max volume seems OK, I think I don't hear any distortion.

By the way, how can I correlate the DAC maximum XLR output (4Vrms) to the speaker max input voltage (cannot see this in the manual) and the input sensitivity settings?

Any ideas/guidance will be greatly appreciated...
 

pos

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#2
Hello @Odysseus,

According to their datasheet, the 305P will produce 108dB SPL peaks for a +6dBV input in -10dBV mode, and +20.3dBu in +4dBu mode.

6dBV is 8.2dBu, 2Vrms
20.3dBu is 8Vrms

So if you use the XLR outputs of your DAC, with their 4Vrms/14dBu and use the -10dBV setting you will exceed the 305P's maximum peak capacity at 0dBFS, and might reach that point with volume set to 0dB with some programs. In that case you will need to keep volume at -6dB maximum.
Alternatively, if you use the +4dBu setting, maximum level you will be able to achieve at 0dBFS will be 102dB SPL.

Is is up to you to choose. I think I would choose the second option myself, as 102dB SPL is already pretty loud for nearfield monitoring, and probably already outside of the comfort zone for the drivers, especially when used without a subwoofer to relieve the small 5" woofer from LF duty.

You might also use the 8.2dBu RCA output of your DAC (preferably with pseudo differential RCA/XLR cables) and enjoy a perfect gain matching with 0dBFS giving the maximum specified 108dB SPL (and accompanying distortion :D) with the -10dBV setting...

EDIT: you can also of course use the attenuator on the back of the LSR 305 to chop those 6dB when using the -10dBV setting with the XLR output, but you will have to guess or measure what those 1-10 markings refer to in term of dB attenuation...
Matching is also not guarantied with these kind of pots when not used at their maximum position.
There is also the risk of saturating the input stage in -10dBV setting, depending on the topology.
 
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#3
Hello @Odysseus,

According to their datasheet, the 305P will produce 108dB SPL peaks for a +6dBV input in -10dBV mode, and +20.3dBu in +4dBu mode.

6dBV is 8.2dBu, 2Vrms
20.3dBu is 8Vrms

So if you use the XLR outputs of your DAC, with their 4Vrms/14dBu and use the -10dBV setting you will exceed the 305P's maximum peak capacity at 0dBFS, and might reach that point with volume set to 0dB with some programs. In that case you will need to keep volume at -6dB maximum.
Alternatively, if you use the +4dBu setting, maximum level you will be able to achieve at 0dBFS will be 102dB SPL.

Is is up to you to choose. I think I would choose the second option myself, as 102dB SPL is already pretty loud for nearfield monitoring, and probably already outside of the comfort zone for the drivers, especially when used without a subwoofer to relieve the small 5" woofer from LF duty.

You might also use the 8.2dBu RCA output of your DAC (preferably with pseudo differential RCA/XLR cables) and enjoy a perfect gain matching with 0dBFS giving the maximum specified 108dB SPL (and accompanying distortion :D) with the -10dBV setting...

EDIT: you can also of course use the attenuator on the back of the LSR 305 to chop those 6dB when using the -10dBV setting with the XLR output, but you will have to guess or measure what those 1-10 markings refer to in term of dB attenuation...
Matching is also not guarantied with these kind of pots when not used at their maximum position.
There is also the risk of saturating the input stage in -10dBV setting, depending on the topology.
Thanks! Much appreciated!

A couple of questions:
- how did you calculate the 102dB SPL for +4dBu at 0dBFS?
- what do you mean by saturating the input stage (assuming that I use the -10dBV setting and use the volume control on the speaker)?

As I would like to stick to XLR to XLR, I guess the safest thing to do is go for the +4dBu setting...

(By the way, I generally use this: http://www.sengpielaudio.com/calculator-db-volt.htm)
 

pos

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#4
You're welcome.

how did you calculate the 102dB SPL for +4dBu at 0dBFS?
4Vrms is 14dBu.
So if 20.3dBu gives you 108dB SPL then 14dBu will give you 102dB SPL.

what do you mean by saturating the input stage (assuming that I use the -10dBV setting and use the volume control on the speaker)?
It depends on how the input stage is implemented.
 
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#5
Just FYI, the SU-8 is very much overkill for the LSR 305s. The JBL 3-series uses a Cirrus Logic CS5341 ADC on the input and a STA350B "power DAC". All inputs are converted to digital and then back to analog and neither the ADC nor DAC in the 305s are exceptional. JBL should have included a digital input to avoid the double conversion inherent in the design.
 
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#6
By the way, best way to connect the SMSL SU-8 and the JDS Atom to the JBL 305Ps? If I do SU-8 to 305Ps using LXR, and SU-8 to Atom using RCA, how do I switch the signal between XLR and RCA on the SU-8 (i.e. between speaker and headphones)?
 

andreasmaaan

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#7
By the way, best way to connect the SMSL SU-8 and the JDS Atom to the JBL 305Ps? If I do SU-8 to 305Ps using LXR, and SU-8 to Atom using RCA, how do I switch the signal between XLR and RCA on the SU-8 (i.e. between speaker and headphones)?
According to the manual, there doesn't seem to be an option to switch between or turn off outputs. This would mean you'd have to do this manually, using the switches on the Atom or the JBLs.

Perhaps someone who owns the unit can confirm?
 
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#8
According to the manual, there doesn't seem to be an option to switch between or turn off outputs. This would mean you'd have to do this manually, using the switches on the Atom or the JBLs.

Perhaps someone who owns the unit can confirm?
So, maybe use the JDS Atom and the pre-amp? And have the 305Ps connected via RCA?
 

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