Sansamp RBI into QSC 1850HD amp

[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 ?

[Downunderwonder]

Read the fine manual!

 

Not too many power amps bridge into 4 ohms so I would knock that off until you have RTFM.

 

Parallel is for 1 cab on each channel, Mono is both cabs on just the one channel.

 

Should be much of a muchness until you get to the bleeding edge of headroom exhaustion. I wouldn't like to guess what your cabs can handle or what the amp can put out. Most likely cabs give out before amp.

 

Bring up the output of the RBI until the QSC indicates. Then bring up the QSC output. Should get pretty damn loud.

Edited February 11, 2022 by Downunderwonder

[Jack]

If something really is awry my guess would be the link between pre and power. PA power amps need quite a bit of voltage to hit their stride, not sure the Sansamp can provide. 

 

However, it might just be that 600W isn't a great deal depending on the cabs that's attached and your desired tone goals. 600W with 2 8x10s, grindy rock pick sounds will take your head off. 600W into 2 modest 1x12 cabs when you're aiming for thick reggae won't be enough. 

 

It could be both. 

[agedhorse]

The power amp will not show any indication with the input sensitivity controls down. The metering looks at the signal post sensitivity control.

 

Unless he has a bridging cable that is wired differently than a standard SpeakOn, it will not be in bridge mode regardless of what the switch position is. That might have saved his speakers under the circumstances. This amp will bridge into 4 ohms, but it's more than enough power to destroy the speakers (based on what he posted on TalkBass. Best way to operate his rig is one cabinet per channel in dual mode IMO (and IME)

Edited February 11, 2022 by agedhorse

[Davi]

The cabs are Ashdown 1x15 600w and 4x10 600w 8 ohms each. 

Running in parallel 1 cab per channel would put out around 380w each at 8ohms ? I find im struggling at this level within a band situation. I was under the impression that daisy chaining 2 cabs into the one channel would bring the load to 4ohms and around 600w, ive since learned that this isnt the case with the QSC 1850hd. It seems from what im reading that bridging the amp would be the best way to gain more headroom and reduce clipping with the correctly wired speakon lead.

[Downunderwonder]

It would be very easy for the bridged amp to go all Django Unchained on your cabs.

 

360w a side really should be making plenty of noise. I have zero confidence in a 600w rating on a 15. I tend to discount those by 50% before they start to strain. So the 360w per channel is ample.

 

[agedhorse]

In parallel, you have 720 watts split evenly between 2 cabinets. 2 cabinets off of a single channel is 600 watts split evenly between 2 cabinets.

 

I agree with Downunder, very few 15" drivers suitable for bass guitar are cable of 600 watts RMS for very long.  Looking at their website, their largest bass cabinet (CL-115 large) is rated at 500 watts RMS. 

 

The other question is how the cabinets are rated. If those are thermal ratings, than you should be aware that mechanical ratings are generally less then thermal ratings. Often, it's impossible to know what the mechanical ratings are until it's too late.

 

I think 360 watts per cabinet is a reasonable power level, and say the power handing is 500 watts, the difference between 360 and 500 watts is about 1dB.  Since there are no sensitivity specs on their website, there's no way to calculate the net effect on maximum SPL.

[Downunderwonder]

Reading between the lines it sounds like the previous bridge mono didn't happen so it must have been two cabs on one channel only? Still should have raised the roof. Makes me wonder what could be wrong.

 

Dip switches?

Protection mode from wrong cable?

Gain stages up the wazoo?

User expectations?

[Davi]

I was originally running a Ashdown ABM IV 600 head into the cabs, it had plenty of grunt, I just didnt get along with the way it sounded. 

I then went with an Ampeg SVP-Cl preamp into the QSC 1850hd and loved the sound but struggled with the low output if the preamp.

The sansamp sounds pretty good and more output than the Ampeg but not as much power as the Ashdown and lots of LED clipping from the QSC.

 

[Downunderwonder]

Just checking, you are using the 'Sansamp' XLR out from the back of the RBI set to full ( 0dB) output and turning up the XLR on the front panel?

[Jack]

18 hours ago, Davi said:

I was under the impression that daisy chaining 2 cabs into the one channel would bring the load to 4ohms and around 600w, ive since learned that this isnt the case with the QSC 1850hd.

That is in fact the case. 

 

Given that your cabs are big and sensible I go back to my first guess: preamp/poweramp mismatch. 

 

Extremely cursory reading (please somebody correct me) suggests that the Sansamp has a 0dBu main output and the QSC is expecting +4dBu, meaning it's roughly the same as a ~250w amp now that we're 4dB down from max output. 

Edited February 12, 2022 by Jack

[Downunderwonder]

Two pro grade preamps not giving you what you should expect is either user error or your amp isn't operating to spec. 600w is a lot of power, same same as your Ashdown.

[3below]

1 hour ago, Jack said:

That is in fact the case. 

 

Given that your cabs are big and sensible I go back to my first guess: preamp/poweramp mismatch. 

 

Extremely cursory reading (please somebody correct me) suggests that the Sansamp has a 0dBu main output and the QSC is expecting +4dBu, meaning it's roughly the same as a ~250w amp now that we're 4dB down from max output. 

 

Looking at the manuals (and from experience from sometimes using a similar setup) I think you are on the money.  Sansamp has 0dBu output, QSC requires +3.5dBU input for max power at 8Ohms (the 2450 is +4dBu, next column in the manual in small print).

 

0dBu means the Sansamp maximum output will be 0.775V

3.5dBu means that the QSC requires 1.16V to give the maximum output (as stated in the manual)

 

Thus on paper the Sansamp is only providing about 66% of the maximum input required, hence the low power output.

 

However.....   in a 2015 Basschat thread about the same issue  https://www.basschat.co.uk/topic/272772-best-power-amp-for-tech-21-sansamp-rbi-preamp/  Tech21NYC stated

 

".....we have been through this numerous times on the Talk Bass forum years back. We even measured our RBI's output to duplicate Crown's test specs. These are the results:

With a -10dB 1kz sine wave (which is what Crown uses for the specs) (.3V rms which is an instrument level signal) and all the EQ controls on the RBI set flat, output level at 3 o'clock and the blend off, the output voltage of the RBI is 3.6 V rms........."

 

So I am now left wondering (or getting the test kit out).

 

 

[Bunion]

I looked into qsc myself with an SWR IOD (.775v) only way I found around it from searches was have the qsc modified or put a Rane eq in between them 

Edited February 12, 2022 by Bunion

[Bill Fitzmaurice]

Way too much talk about watts here. I suspect 'I want to get all the watts out of my amp' is why the OP is bridging into a parallel load. Don't do that. Even if the cabs were identical that wouldn't be the best idea, and since they're not each should be driven with their own amp channel. As for the level matching, if the Sans amp is made to feed a bass amp or PA console it probably lacks the necessary voltage swing to drive a power amp to full output.

[Downunderwonder]

5 minutes ago, 3below said:

 

Looking at the manuals (and from experience from sometimes using a similar setup) I think you are on the money.  Sansamp has 0dBu output, QSC requires +3.5dBU input for max power at 8Ohms (the 2450 is +4dBu, next column in the manual in small print).

 

0dBu means the Sansamp maximum output will be 0.775V

3.5dBu means that the QSC requires 1.16V to give the maximum output (as stated in the manual)

 

Thus on paper the Sansamp is only providing about 66% of the maximum input required, hence the low power output.

 

However.....   in a 2015 Basschat thread about the same issue  https://www.basschat.co.uk/topic/272772-best-power-amp-for-tech-21-sansamp-rbi-preamp/  Tech21NYC stated

 

".....we have been through this numerous times on the Talk Bass forum years back. We even measured our RBI's output to duplicate Crown's test specs. These are the results:

With a -10dB 1kz sine wave (which is what Crown uses for the specs) (.3V rms which is an instrument level signal) and all the EQ controls on the RBI set flat, output level at 3 o'clock and the blend off, the output voltage of the RBI is 3.6 V rms........."

 

So I am now left wondering (or getting the test kit out).

 

 

Mixing up dB, dBV and dBU is always going to confuse. I always figured the 0dB on the back of the RBI was so the -20dB pad would have a reference.

 

My RBI had no problem spinning up an old NZ PA amp or a Crown XTI.

 

Afaik they were both standard 1.4V sensitivity input, being 0.7v per leg on a balanced cable. So that would be another potential dog to put down. Is OP running a balanced cable (or shorting half his signal)?

[Downunderwonder]

21 minutes ago, 3below said:

getting the test kit out

Marvellous plan.

[Downunderwonder]

2 hours ago, Jack said:

please somebody correct me

Ok. NEWS FLASH. The QSC manual says it has input sensitivity of 1.15V RMS.

 

You might not be able to overdrive it with a BDDI but RBI no problemo.

 

Starting to wonder if one of OP's speakons is messed up or the 15 already got blown up.

[Davi]

4 hours ago, Downunderwonder said:

Just checking, you are using the 'Sansamp' XLR out from the back of the RBI set to full ( 0dB) output and turning up the XLR on the front panel?

Ive been using an unbalanced jack to speakon sansamp out

[Downunderwonder]

Just now, Davi said:

Ive been using an unbalanced jack to speakon sansamp out

Not using correct terms. Speakon is for speakers!

 

Should be a balanced cable, either TRS or XLR from Sansamp to QSC.

 

If you are lighting up the QSC overload light with a clean signal, that's your lot.

 

Output to the cabs should be Speakons. If you have changed anything in the speaker cables from your old amp that would be a good place to look for a muckup.

[Davi]

8 minutes ago, Downunderwonder said:

Not using correct terms. Speakon is for speakers!

 

Should be a balanced cable, either TRS or XLR from Sansamp to QSC.

 

If you are lighting up the QSC overload light with a clean signal, that's your lot.

 

Output to the cabs should be Speakons. If you have changed anything in the speaker cables from your old amp that would be a good place to look for a muckup.

Sorry I meant to say im using an unbalanced jack from the sansamp output 

Edited February 12, 2022 by Davi

[Downunderwonder]

On 11/02/2022 at 23:24, Davi said:

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.

Checking again, this condition was with one cab on each channel? That is the recommended setup. Did you check sound was coming out of both cabs?

[Davi]

38 minutes ago, Downunderwonder said:

Checking again, this condition was with one cab on each channel? That is the recommended setup. Did you check sound was coming out of both cabs?

I had the 2 cabs daisy chained out of 1 channel. I was under the impression that id get 600watts at 4 ohms and running 1 cab per channel would give me 360 watts at 8ohms, ive since learned that this was incorrect and the wrong way to set up.

Both cabs are working. When I get the chance I will try running 1 cab per channel in parallel with clip limiter switch on and use an unbalanced jack from the sansamp out. 

Edited February 13, 2022 by Davi

[Downunderwonder]

18 minutes ago, Davi said:

I had the 2 cabs daisy chained out of 1 channel. I was under the impression that id get 600watts at 4 ohms and running 1 cab per channel would give me 360 watts at 8ohms, ive since learned that this was incorrect and the wrong way to set up.

Both cabs are working. When I get the chance I will try running 1 cab per channel in parallel with clip limiter switch on and use an unbalanced jack from the sansamp out. 

Potentially half your signal then went nowhere causing the one amp channel to be under driven but I would have thought the amp would be smarter and mirror a mono input to both channels in that mode. Putting one cab on each channel will soon tell.

[Davi]

19 minutes ago, Downunderwonder said:

Potentially half your signal then went nowhere causing the one amp channel to be under driven but I would have thought the amp would be smarter and mirror a mono input to both channels in that mode. Putting one cab on each channel will soon tell.

I noticed both of the channel clip LED lights came on