QSC 1850hd & Sansamp RBI questions

[Davi]

Im looking for some advice regarding my QSC RMX 1850HD. Im running a Sansamp RBI preamp ( which I like the sound of) into the QSC and daisy chaining 2 x 8 ohm cabs out of channel 1 to get a 4ohm load, I have the filter switches set to 50hz and also the switches set to parallel, the clip LED comes on quite frequently, I have both channels on the QSC set to full and the pre at around 12 o'clock. I also tried the bridged mono mode with the switches set to parallel, channel 2 filters and limiter off the but wasnt impressed by how it sounded. Am I doing something wrong with the way ive set it up ?

 

[bgavin]

The output voltage of the RBI has been measured at 3.6v
Note: this is a BALANCED XLR type output.
Ensure you are properly cabled.

The RMX input sensitivity is 1.16v.
If you are cranking the RBI, this is the cause of the clipping LED.

You have the switches set to Parallel, so it is pointless to daisy chain a single channel.
This RMX can drive an 8-ohm load to 96 volts in bridge mode.
In parallel mode, it outputs 53.7v or 360 watts into an 8 ohm load.
The vast majority of speaker cabs can be driven into cone over excursion with this amount of power.

I would check the polarity of your speaker cabs.
Use a 9v battery and rig a test cable.
Check BOTH cabinets in the identical manner of battery orientation.
The driver cones should all move either In or Out direction.
Any deviation will literally kill your sound.

 

[agedhorse]

Did you use a bridge speaker cable when using bridge mode?

What speakers are you using? That amp can destroy a lot of speakers in bridge mode.

 

[Davi]

You have the switches set to Parallel, so it is pointless to daisy chain a single channel.
This RMX can drive an 8-ohm load to 96 volts in bridge mode.
In parallel mode, it outputs 53.7v or 360 watts into an 8 ohm load.
The vast majority of speaker cabs can be driven into cone over excursion with this amount of power.

Wouldnt having the 2 cabs daisy chained bring the load to 4ohm and 600watts or am I missing something ?

 

[Davi]

Did you use a bridge speaker cable when using bridge mode?

What speakers are you using? That amp can destroy a lot of speakers in bridge mode.

I used jack to speakon from the pre to amp.
I have an Ashdown 4x10 600watt and an Ashdown 1x15 600watt both 8ohms

 

[Davi]

Did you use a bridge speaker cable when using bridge mode?

It was a standard speakon to jack.
What would happen if I used a 2 pole speakon instead of a 4 pole?

 

[agedhorse]

Check your amp, IIRC, true bridge operation requires a SpeakOn plug wired differently (1+/2+).

I would suggest running the amp in parallel mode, one cabinet per channel. That's just about optimum power-wise.

Also verify that all drivers in the cabinets are wired properly and in phase.

 

[Analogeezer]

The output voltage of the RBI has been measured at 3.6v
Note: this is a BALANCED XLR type output.
Ensure you are properly cabled.

The RMX input sensitivity is 1.16v.
If you are cranking the RBI, this is the cause of the clipping LED.

You have the switches set to Parallel, so it is pointless to daisy chain a single channel.
This RMX can drive an 8-ohm load to 96 volts in bridge mode.
In parallel mode, it outputs 53.7v or 360 watts into an 8 ohm load.
The vast majority of speaker cabs can be driven into cone over excursion with this amount of power.

I would check the polarity of your speaker cabs.
Use a 9v battery and rig a test cable.
Check BOTH cabinets in the identical manner of battery orientation.
The driver cones should all move either In or Out direction.
Any deviation will literally kill your sound.

Good advice and I would suggest using the 1/4" output on the RBI since it should present a much lower level to the input of the amp.

Also - not clear why you want to use one channel, or bridge the amp.

Bridging used to be a technique from back in the day when power amps were not all that powerful. Sure there are some uses for it today still but for your application I would just set the amp to parallel input like you have done and use each channel for each amp channel.

What kind of cabinets are we talking about here?

Analogeezer

 

[mike buzzetti]

I would suggest running the amp in parallel mode, one cabinet per channel. That's just about optimum power-wise.

Also verify that all drivers in the cabinets are wired properly and in phase.

this

 

[Zooberwerx]

Good advice and I would suggest using the 1/4" output on the RBI since it should present a much lower level to the input of the amp.

Also - not clear why you want to use one channel, or bridge the amp.

Bridging used to be a technique from back in the day when power amps were not all that powerful. Sure there are some uses for it today still but for your application I would just set the amp to parallel input like you have done and use each channel for each amp channel.

What kind of cabinets are we talking about here?

Analogeezer

Yes, use the RBI's 1/4" Sansamp output to the amp's 1/4" input, not the XLR's. Save those for sending signal to the board or as a pass-thru at the amp.

I ran a whole bunch of RMX's over the years. I would back-off the attenuators to 37% (~10:30 position) as a starting point...got this from dbx. It's really easy to clip the amps with the attenuators fully clockwise plus degradation of the signal / noise ratio.

Riis

 

[agedhorse]

Yes, use the RBI's 1/4" Sansamp output to the amp's 1/4" input, not the XLR's. Save those for sending signal to the board or as a pass-thru at the amp.

I ran a whole bunch of RMX's over the years. I would back-off the attenuators to 37% (~10:30 position) as a starting point...got this from dbx. It's really easy to clip the amps with the attenuators fully clockwise plus degradation of the signal / noise ratio.

Riis

Note that the MOL of the SansAmp is about +14dBu whereas the typical console or xover output it +26dBu. Because of this, the input attenuators on the amp should by turned down a maximum of 6dB in order to retain adequate drive levels. The 12dB lower MOL on the SansAmp (and most other pedal based preamp products) is the limiting factor here.

 

[Davi]

Thanks for all your advice, I will try the suggestions posted and get back to you

 

[Zooberwerx]

Thanks for all your advice, I will try the suggestions posted and get back to you

Hope you get this solved. I've used a similar rig for years...RBI, RPM, and now an original SansAmp silver-face utilizing the 1/4" (+4dB) output. I gave up on the RMX amps and now use the QSC PLX1602 which has the same DIP switch array. Play around with the 30 / 50 hz filter, too. The 50 hz filter actually rolls-in a slight "bump" @ ~100 hz. I run mine at 30 hz but employ a HPF filter upstream.

Riis

 

[Davi]

Hope you get this solved. I've used a similar rig for years...RBI, RPM, and now an original SansAmp silver-face utilizing the 1/4" (+4dB) output. I gave up on the RMX amps and now use the QSC PLX1602 which has the same DIP switch array. Play around with the 30 / 50 hz filter, too. The 50 hz filter actually rolls-in a slight "bump" @ ~100 hz. I run mine at 30 hz but employ a HPF filter upstream.

Riis

Thanks, im thinking of trying a combination of the Ampeg SVP-CL pre with the RBI, maybe ill find my perfect sound
Got the 50hz filter on at the moment, seem to tighten the low end up in a really good way.
What do you like about the PLX over the RMX ? Ive never tried a PLX

 

[Zooberwerx]

Thanks, im thinking of trying a combination of the Ampeg SVP-CL pre with the RBI, maybe ill find my perfect sound
Got the 50hz filter on at the moment, seem to tighten the low end up in a really good way.
What do you like about the PLX over the RMX ? Ive never tried a PLX

Depending on the DOM or modifications, The SVP-CL may very well have a lower-than-expected output. Still a great Ampeg, my favorite over & above the SVP Pro. For the time being, I'd concentrate solely on the RBI / RMX and figure out why the heck it's clipping as described.

Nothing wrong with the RMX series other than being heavier-than-hell. Sold a few years back but needed a "beater" for a bass rig and the PLX was readily available...I didn't put much thought in the process. I still have a 1450 in a rack box downstairs...think I swiped it from my bandmate, Tim. Come to think of it, we also have a whole slew of RMX commercial theatre-grade amps that were donated when they tore-down a local multiplex. They're older technology but great amps provided we're dealing with permanent installs.

Riis

 

[Analogeezer]

Thanks, im thinking of trying a combination of the Ampeg SVP-CL pre with the RBI, maybe ill find my perfect sound
Got the 50hz filter on at the moment, seem to tighten the low end up in a really good way.
What do you like about the PLX over the RMX ? Ive never tried a PLX

Well the PLX is way lighter for starters; 21 pounds vs. probably 45 or more for your 1850HD. Also shallower so they fit into shallower racks.

I've never experienced this business with clipping a RMX, I have two in a PA rack (which I can also use to drive a giant biamped bass rig with six cabinets).

I have hit them with a hot output from a mixer (including drums, bass, a full band), both balanced and unbalanced and I send the output of my Ampeg SVP-Pro through a Drawmer DL-241 compressor (mainly for the gates) as part of the monster bass rig and never had this issue.

I just looked at my RBI manual and there is no spec for the output of the "Sansamp" 1/4" jack, but there is an output level switch but that only impacts the XLR output.

If you are using that to feed the power amp, try selecting the -20 dB setting.

But I would also try the 1/4" Sansamp output into the QSC as well.

Analogeezer

 

[agedhorse]

No, the -20dB setting I'm not appropriate for driving a power amp.