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Gain & Impedance ?

Krusty09

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Jul 3, 2018
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Hello.

I have a question that I am not that familiar with and need some help.

I have a buddy of mine who is a Principle of a School and they have gotten some new equipment for their Auditorium recently and it's pro audio gear and I'm looking to help out a bit like running wires and that kinda off stuff and racking in equipment generally setting it up.

Ok with that said I noticed in manual that the amp has like 3 or so different input gain settings and each of the 4 channels have a PRX412M jbl 8ohm speaker hooked to each one . They also have a beheringer xr18 as their mixer. I guess I'm looking what would be the best gain option to set the amp input at to match the mixer output correctly and the best overall settings for those speakers.

These are the specs. Any help would be great. They will have a big end of school year show and graduation coming up so I am hoping to get it setup the proper way by then so have a nice show .

These will be connected via xlr from mixer to amp direct.

Beheringer xr18

Output Characteristics

Output impedance, XLR, unbal. / bal. 50 Ω / 50 Ω 50 Ω / 50 Ω
Output level, XLR, nom./max. +4 dBu / +16 dBu +4 dBu / +16 dBu
Output impedance, TRS, unbal. / bal. 50 Ω / 50 Ω 50 Ω / 50 Ω
Output level, TRS, nom./max. +4 dBu / +16 dBu +4 dBu / +16 dBu
Output impedance, RCA 1 kΩ —
Output level, RCA, nom./max. +4 dBu / +16 dBu

This is the amp.

Crown Audio CDi 4|600 4-Channel DriveCore Series Power Amplifier (600W)​


The gain input options are +26dbu. +34dbu and +37 dbu


Input SensitivityLine:
0.7 Vrms to 2.5 Vrms / -.8 dBu to 10.2 dBu (at 8 Ohms)
1 Vrms to 3.5 Vrms / 2.2 dBu to 13.2 dBu (at 70 V)
1.4 Vrms to 5 Vrms / 5.2 dBu to 16.2 dBu (at 100 V)Input Impedance10 Kilohms (Balanced)
5 Kilohms (Unbalanced)Max Gain34 dB (at 4 Ohms, at 8 Ohms, at 70 V, at 100 V)Max Input Level+20 dBu

These are the speakers



Thanks for any and all help in advance.
 

DonH56

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"Principal"

XLR and TRS are just different connectors for the same (balanced) output. 50 ohms is a low-impedance drive and using balanced (XLR or TRS) connectors makes it easier to avoid ground loops. The RCA output impedance is a bit high IME but I would not use RCA in this application.

+4 dBu is a standard pro-level output. The gain you need depends somewhat on the levels you set at the mixer and the speakers you are driving. (JBL has a PRX400 series, not sure which model you have.) More gain in the amp will also increase the noise from the mixer so the answer usually is "it depends". You can try all three and see which works best for your venue. I would probably start at 26 dB and see if that is loud enough with enough headroom for the job, and bump up to 34 dB if you need a bit more gain. That said, JBL says the PRX400 speakers were designed to mate with the Crown XTi amps, and in that line the XTi 4002 (650 W output) has 34 dB of voltage gain, so the easiest thing to do would be to set the gain to 34 dB and if that works call it a day.

HTH - Don
 

DVDdoug

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This is called "gain staging".

I would set the amplifier for "maximum desired volume" when the mixer's meters show 0dB. (That might be hard to judge before there is a crowd).

...Because every active device adds some noise, in order to keep the best signal-to-noise ratio, typically you want to keep the strongest signal as possible to the end and then turn-down the power amplifier's gain.
 
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Krusty09

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Jul 3, 2018
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"Principal"

XLR and TRS are just different connectors for the same (balanced) output. 50 ohms is a low-impedance drive and using balanced (XLR or TRS) connectors makes it easier to avoid ground loops. The RCA output impedance is a bit high IME but I would not use RCA in this application.

+4 dBu is a standard pro-level output. The gain you need depends somewhat on the levels you set at the mixer and the speakers you are driving. (JBL has a PRX400 series, not sure which model you have.) More gain in the amp will also increase the noise from the mixer so the answer usually is "it depends". You can try all three and see which works best for your venue. I would probably start at 26 dB and see if that is loud enough with enough headroom for the job, and bump up to 34 dB if you need a bit more gain. That said, JBL says the PRX400 speakers were designed to mate with the Crown XTi amps, and in that line the XTi 4002 (650 W output) has 34 dB of voltage gain, so the easiest thing to do would be to set the gain to 34 dB and if that works call it a day.

HTH - Don
Don,

Thanks for the info and the reply. I will try what you recommend. The model is a typo i guess. It is the JBL PRX 412m. I put a link at the bottom of the original post to the specs but i misspelled in the post. Sorry i was at the 3 overtime Rangers game last night and pretty tired today.
 
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