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SMSL SU-8 + JBL 305Ps

Odysseus

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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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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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Odysseus

Odysseus

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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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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.
 

crberryman

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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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Odysseus

Odysseus

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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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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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Odysseus

Odysseus

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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?
 

Jungstar

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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.

Can you elaborate? This sounds weird and borderline incorrect to my knowledge.... to understand you correctly: If you connect a really good Balanced DAC and sends a balanced signal, it should convert it to digital and to analog again? Are you 100% sure about this? IF there were a digital input, I would agree, then there should be a DAC inside, but I don't think this is the case as there is only line in via XLR or 1/4".
 

crberryman

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Can you elaborate? This sounds weird and borderline incorrect to my knowledge.... to understand you correctly: If you connect a really good Balanced DAC and sends a balanced signal, it should convert it to digital and to analog again? Are you 100% sure about this? IF there were a digital input, I would agree, then there should be a DAC inside, but I don't think this is the case as there is only line in via XLR or 1/4".
Correct, any analog signal via 1/4 or XLR is converted to digital then DSP applied and converted back to analog. Here's the teardown - http://rdimitrov.twistedsanity.net/blog/show.php?entry=JBL LSR305 Teardown and Analysis . The dsp/amp chip used is the STA350BW here is the data sheet - note there is not an analog input. STA350B, STA350BW - STMicroelectronicshttps://www.st.com › resource › datasheet.
 

snowsurfer

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I have that combination and I use the +4dB setting. Yes the max volume is not ear shattering, but even like that I don't go over the 30 volume setting on the SMSL. The volume controls on the back of the JBLs are set to max in this arrangement. As for your Atom question, I had the same doubt, and in the end I went for the THX 789 and ordered some custom made XLR splitters so that I can use balanced connections on everything. Overkill? Very much probably so, but once I "invest" in some new appropriate headphones, my next upgrade will be the Neumann KH120A monitors, so more than overkill it's just future proofing.
 

Jungstar

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Correct, any analog signal via 1/4 or XLR is converted to digital then DSP applied and converted back to analog. Here's the teardown - http://rdimitrov.twistedsanity.net/blog/show.php?entry=JBL LSR305 Teardown and Analysis . The dsp/amp chip used is the STA350BW here is the data sheet - note there is not an analog input. STA350B, STA350BW - STMicroelectronicshttps://www.st.com › resource › datasheet.
It still seems weird to me.... Why DOES everyone talk about DACs and no one talks about powered speakers components??? I get they cannot make adjustments to the sound without making in digital in some way but it seems different that a "normal" DAC/ ADC
 

Ron Texas

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Use the -10 setting. If the volume is low with +4 you are not going to have enough headroom in your DAC when playing at loud volumes. That's what I did.
 
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