Gallien Krueger RB800 - Add 8 ohm cab?

astroarbol

astroarbol

Jul 18, 2023
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#1
Hi everyone. Just found a good deal on a GK RB800 head. Been wanting one for a while, so pretty psyched to have one on the way...

I play in a pretty loud 3-piece prog/metallish band. I'm planning on running the RB800 at full range at 300 watts into my Avatar 500 watt 4 ohm 4x10 cab.

QUESTION: The RB800 also lets you run an additional 8 ohm cab from a separate 100w power amp. I'm not particularly interested in experimenting with bi-amping right now, but if I were to run my 4 ohm cab and an additional 8 ohm 2x10 cab at full range, would the extra 100W going through the 2x10 provide a noticeable boost in volume/air movement? Would I need to turn down the master volume on the 4x10 cab to match the limitations of the 2x10 cab?

Any info/advice is appreciated. Thanks!
 
#2
First let's calculate the dB changes that results from the available power:
1-300 = [Log(300) x 10] = ~24.8dB
1-100 = [Log(100) x 10] = 20dB

You can add the calculated dB changes to the 1W/1m sensitivity ratings to determine how loud each cab will play.

Then you can enter the results into an online calculator:
Summing of Correlated Sound Sources / sengpielaudio

Or, if you are comfortable with math, use this formula to calculate the results.

Summing of Correlated Sources.

upload_2023-8-1_18-12-38.png

Where:
LA = Level of speaker A
LB = Level of speaker B​

Let's assume a 4 ohm 410 and 8 ohm 210 that use exactly the same drivers and tuning...in other words the 410 is equivalent to two of the 210s. I consider this the best case scenario.

The Sensitivity ratings are:
410 = 103dB 1W/1m
210 = 100dB 1W/1m
Keep in mind these are made up figures and you need the actual figures of your cabs.

Now we add the sensitivity ratings to the dB change figures we calculated earlier:
LA = ~24.8dB + 103 = ~127.8 (410)
LB = 20 + 100 = 120dB (210)

Wow! That's quite a difference, but let's go ahead and get the mutual summing results so we can answer your question. I'll take the easy way out and use the online calculator:

upload_2023-8-1_18-26-59.png


Correlated Summing of ~127.8 and 120 = ~130.8dB

So you get ~+3dB.

This is a theoretical result and real world results may be slightly less.

Further Exploration:
Let's figure the SPL each driver is making. We can do this based on the assumption that we lose -6dB of mutual coupling each time the number of drivers are cut in half. In this example the voltage remains constant, so the power each speaker is receiving also remains constant.

410 = ~127.8dB.
- If we reduce the # of drivers by 1/2: ~127.8 - 6 = ~121.8dB
- If reduce the # of drivers by 1/2 again: ~121.8 - 6 = ~115.8dB
So each driver in the 410 is making ~115.8dB

210 = 120dB
- If reduce the # of drivers by 1/2": 120 - 6 = 114dB.
So each driver in the 210 is making 114dB.

Each driver in the 410 is ~1.8dB louder then each driver in the 210. Not much of a difference here.

We also know:
Each driver in the 410 is getting 300W/4 = 75W
Each driver in the 210 is getting 100W/2 = 50W

If you calculate the dB difference of 75W and 50W you get:
[Log(75/50) x 10] = ~1.8dB...which checks with the results above.

We could also figure the 1W/1m sensitivity rating of the individual drives based on the assumption that sensitivity goes down -3dB each time the number of drivers are cut in half.

We started with the assumption the 410 is rated 103dB 1W/1m and the 210 is rated 100dB 1W/1m

410 = 103dB 1W/1m
- If we reduce the # of drivers by 1/2: 103 - 3 = 100dB 1W/1m
- If reduce the # of drivers by 1/2 again: 100 - 3 = 97dB 1W/1m
So each driver in the 410 is rated 97dB 1W/1m

210 = 97dB
- If reduce the # of drivers by 1/2": 100 - 3 = 97dB
So each driver in the 210 is rated 97dB 1W/1m

This makes sense because we assumed our cabs use the exact same drivers and tuning.

Let's check the results:

-Calculated SPL of individual drivers in 410 = [Log(75) x 10] + 97 = ~115.8dB
-Calculated SPL of individual drivers in 210 = [Log(50) x 10] + 97 = ~114dB

Same results as above.
 

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#5
O.P.,

Gigged a G.K.800RB into a 4 ohm Ampeg 8x10 cab from the 4 ohm, 300 watt side of the head, set for full range for many, many years.

Sounded GREAT!

Our keyboard player had a 8 ohm, Ampeg, 4x10 cab that I'd run from the 100 watt, 8 ohm side of the head, set for full range, for any outdoor, large stage jobs we did along with the 8x10.

Really sounded GREAT, (as long as both cabs were side by side for the coupling effect.)
(Tried separating cabs, one on each side of the drummer a couple of times, had phase (?) issues.)
 
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#6
You'll probably be just fine with the 800RB and 4x10s.

I used to own the Avatar 2x12 and still have my Avatar 2x10s. They are stupid loud with my GK 700RB-II, the efficiency of that 4x10 cab will probably get you plenty of volume on tap with the 800RB. The mids can be a little wonky with the Avatar cabs (mine are neos, not sure about the other cabs), so you may want to get aggressive with the low and hi mids on your GK head to compensate if you're dropping out of the mix. Once I've gotten things dialed in, the result is usually compliments on the tone of my rig. :bassist:
 
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#7
I run 3 BagEnd S15 cabs with my GK 800RB at times. They are 8 ohms each. 2 are via the 300 watt side for 4 ohms and one is via the 100 watt side at 8 ohms. The three cabs stacked sounds great.

Before I sold it, I ran my Avatar 4x10 8 ohm cab from the 100 watt side as well. Also sounded great.
I don't use the 800RB for biamping. I like the full range sound better.
 
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#8
After I sold my beastly Cerwin Vega B36MF cabinet(biamped monster) I ran the 100 watt amp into a single 15, it was loud and sweet. I believe I recall that the 100 watt side runs at a minimum of 8 ohms, and the 300 a minimum if 4, but check that out
 
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#11
Wasnex said:
First let's calculate the dB changes that results from the available power:
1-300 = [Log(300) x 10] = ~24.8dB
1-100 = [Log(100) x 10] = 20dB

You can add the calculated dB changes to the 1W/1m sensitivity ratings to determine how loud each cab will play.

Then you can enter the results into an online calculator:
Summing of Correlated Sound Sources / sengpielaudio

Or, if you are comfortable with math, use this formula to calculate the results.

Summing of Correlated Sources.

View attachment 5144434
Where:
LA = Level of speaker A
LB = Level of speaker B​

Let's assume a 4 ohm 410 and 8 ohm 210 that use exactly the same drivers and tuning...in other words the 410 is equivalent to two of the 210s. I consider this the best case scenario.

The Sensitivity ratings are:
410 = 103dB 1W/1m
210 = 100dB 1W/1m
Keep in mind these are made up figures and you need the actual figures of your cabs.

Now we add the sensitivity ratings to the dB change figures we calculated earlier:
LA = ~24.8dB + 103 = ~127.8 (410)
LB = 20 + 100 = 120dB (210)

Wow! That's quite a difference, but let's go ahead and get the mutual summing results so we can answer your question. I'll take the easy way out and use the online calculator:

View attachment 5144443

Correlated Summing of ~127.8 and 120 = ~130.8dB

So you get ~+3dB.

This is a theoretical result and real world results may be slightly less.

Further Exploration:
Let's figure the SPL each driver is making. We can do this based on the assumption that we lose -6dB of mutual coupling each time the number of drivers are cut in half. In this example the voltage remains constant, so the power each speaker is receiving also remains constant.

410 = ~127.8dB.
- If we reduce the # of drivers by 1/2: ~127.8 - 6 = ~121.8dB
- If reduce the # of drivers by 1/2 again: ~121.8 - 6 = ~115.8dB
So each driver in the 410 is making ~115.8dB

210 = 120dB
- If reduce the # of drivers by 1/2": 120 - 6 = 114dB.
So each driver in the 210 is making 114dB.

Each driver in the 410 is ~1.8dB louder then each driver in the 210. Not much of a difference here.

We also know:
Each driver in the 410 is getting 300W/4 = 75W
Each driver in the 210 is getting 100W/2 = 50W

If you calculate the dB difference of 75W and 50W you get:
[Log(75/50) x 10] = ~1.8dB...which checks with the results above.

We could also figure the 1W/1m sensitivity rating of the individual drives based on the assumption that sensitivity goes down -3dB each time the number of drivers are cut in half.

We started with the assumption the 410 is rated 103dB 1W/1m and the 210 is rated 100dB 1W/1m

410 = 103dB 1W/1m
- If we reduce the # of drivers by 1/2: 103 - 3 = 100dB 1W/1m
- If reduce the # of drivers by 1/2 again: 100 - 3 = 97dB 1W/1m
So each driver in the 410 is rated 97dB 1W/1m

210 = 97dB
- If reduce the # of drivers by 1/2": 100 - 3 = 97dB
So each driver in the 210 is rated 97dB 1W/1m

This makes sense because we assumed our cabs use the exact same drivers and tuning.

Let's check the results:

-Calculated SPL of individual drivers in 410 = [Log(75) x 10] + 97 = ~115.8dB
-Calculated SPL of individual drivers in 210 = [Log(50) x 10] + 97 = ~114dB

Same results as above.
Click to expand...
The summing calculator is wrong assuming identical drivers in each cabinet.
 
#12
astroarbol said:
Hi everyone. Just found a good deal on a GK RB800 head. Been wanting one for a while, so pretty psyched to have one on the way...

I play in a pretty loud 3-piece prog/metallish band. I'm planning on running the RB800 at full range at 300 watts into my Avatar 500 watt 4 ohm 4x10 cab.

QUESTION: The RB800 also lets you run an additional 8 ohm cab from a separate 100w power amp. I'm not particularly interested in experimenting with bi-amping right now, but if I were to run my 4 ohm cab and an additional 8 ohm 2x10 cab at full range, would the extra 100W going through the 2x10 provide a noticeable boost in volume/air movement? Would I need to turn down the master volume on the 4x10 cab to match the limitations of the 2x10 cab?

Any info/advice is appreciated. Thanks!
Click to expand...
It’ll definitely work fine.
I run a 4ohm Bergantino 410 with my 800RB and it’s plenty.
An additional 210 would give you more volume for sure, and with speakers closer to your ears, which is the most important ingredient imho.
 
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#14
agedhorse said:
The summing calculator is wrong assuming identical drivers in each cabinet.
Click to expand...

Please explain?...or at least provide a hint.

agedhorse said:
Don't run an 8 ohm and 4 ohm cabinet in parallel off of the main power amp in this biamp capable models. The LF (main) amp channel is rated to 4 ohms minimum load.
Click to expand...


This is not what we are discussing. The 800RB can push 4 ohms on the 300W amp and 8 ohms on the 100W amp. It will push both at the same time in either biamp or dual mono.
 
#15
Wasnex said:
Please explain?...or at least provide a hint.
Click to expand...

A +3dB sensitivity only occurs with a doubling of the number of drivers of the same sensitivity. A 210 and 410 where the 210 is 3dB less sensitive, when combined will result in about 1.75dB greater sensitivity under ideal conditions.


Wasnex said:
This is not what we are discussing. The 800RB can push 4 ohms on the 300W amp and 8 ohms on the 100W amp. It will push both at the same time in either biamp or dual mono.
Click to expand...

There has been a lot of information provided, yet no caution about a common misconception or lapse of good judgement where the main amp is the only one that's used and incorrect load combinations get used. I provided the warning so that the OP does not misinterpret what's being said in the most convoluted way possible and damage his amp. I see this sort of thing happen often enough to merit the warning. You don't have to agree with it, but it's for the OP and others who may look at this thread in the years to come.
 
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#17
agedhorse said:
A +3dB sensitivity only occurs with a doubling of the number of drivers of the same sensitivity. A 210 and 410 where the 210 is 3dB less sensitive, when combined will result in about 1.75dB greater sensitivity under ideal conditions.
Click to expand...

Yes AFAIK this is correct...however, the calculator and formula predicts the level increase when coincident sound sources are added; rather than the combined sensitivity.

With two equal coincident sound sources you get approximately ~+6dB theoretical. Double the number of sound sources to four and you get ~+12dB. Increase the number of sound sources again to six and you get ~+15.6dB.

For example:

If you have two speakers making 90dB
90 + 90 -> [90 + ~6 = 96.012dB]*

If you have four speakers making 90dB
90 + 90 + 90 + 90 -> [90 + ~12 = 102.041dB]*

If you have six speakers making 90dB you get + 15.6dB
90 + 90 + 90 + 90 +90 +90 -> [90 + ~15.6 = 105.563dB]*

* These are the figures reported by the calculator.

So the calculator says the level goes up ~3.5dB theoretically when the 210 is added.

In my example we had six speakers rated for 97dB 1W/1m.

Four of the speakers are getting 75W each and making ~115.8dB. This is the 800RB's 300W amp pushing a 410. 300/4 = 75W per driver.

Two of the speakers are getting 50W and making ~114dB. This is the 800RB's 100W amp pushing a 210. 100/2 = 50W per driver.

So we have 115.8 + 115.8 + 115.8 + 115.8 + 114 + 114 = ~130.8dB*

* These are the figures reported by the calculator.

Sensitivity Rating:
410 = 103dB 1W/1m
210 = 100dB 1W/1m

I don't think we can use the ~1.76dB increase in the sensitivity rating because the speakers are not receiving equal voltage. Remember 300W total to the 410 and 100W total to the 210.

410 [Log(300) x 10] + 103 = ~127.8dB
210 [Log(100 x 10] + 100 = 120dB

Per the calculator and formula: Mutual summing of 127.8 and 120 = ~130.8dB...same as above.

If both cabs were powered by a 400W amp.

I believe the combined sensitivity of 100dB and 103dB would be 104.76dB 1W/1m

Calculated SPL of our 410 + 210 connected in parallel to 400W
[Log(400) x 10] + 104.76 = ~130.8dB. The rounded numbers are the same, but there is a small difference without rounding.

Each driver is getting 400/6 = ~66.67W
Calculated SPL of each speaker = [Log(~66.67 x 10] + 97 = ~115.2dB

Previously we indicated with six coincident sound sources you get ~+15.6dB.

So ~115.2 + 15.6 = ~130.8dB [same result]
 
#18
There are 2 different factors going on, sensitivity is one and power increase is the other.

Sensitivity excludes power increase when the same units are used. The SENSITIVITY portion of the doubling of identical coincident sources is theoretically +3dB. Since the 410 and 210 do not result in a doubling but an increase of only 50%, +3dB isn’t going to happen.

The OTHER increase of 3dB (theoretical) is due to the doubling of power, but this is not part of the sensitivity comparison since the unit metrics change from the increase in power.

The use of the term sensitivity implies that for comparison, the unit metrics do not change. Thus, comparing sensitivities needs to be normalized for power differences.
 
#19
agedhorse said:
There are 2 different factors going on, sensitivity is one and power increase is the other.

Sensitivity excludes power increase when the same units are used. The SENSITIVITY portion of the doubling of identical coincident sources is theoretically +3dB. Since the 410 and 210 do not result in a doubling but an increase of only 50%, +3dB isn’t going to happen.

The OTHER increase of 3dB (theoretical) is due to the doubling of power, but this is not part of the sensitivity comparison since the unit metrics change from the increase in power.

The use of the term sensitivity implies that for comparison, the unit metrics do not change. Thus, comparing sensitivities needs to be normalized for power differences.
Click to expand...

The formula does not care about impedance, power, sensitivity etc. All that matters is the SPL of each coincident sound source and resultant summing.

upload_2023-8-3_13-25-55.png


Where:
LA = Level of Sound Source A
LB = Level of Sound Source B
etc
​
Provided Figures for Individual Sound Sources and Results:
115.8 and 115.8 and 115.8 and 115.8 and 114 and 114 = ~130.8dB*​

If ~130.8 is not correct, what is?
 
#20
You are using the term sensitivity incorrectly.

The sources SPL must be defined the same way for the calculator to return the correct numbers.
 
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