First foray into power ampism

[Waddycall]

H

 

having read a few threads on here recently about power amps, pre amps and rack gear I decided to have a go at using my bass pod xt live as a preamp into a power amp. I bought a used QSC RMX850a off eBay and a cheap rack sleeve to put it in. It’s too deep for a standard 2u rack case.

Mrs W is out today so I’ve had the chance to play loud bass and experiment with it. I’d previously put a lot of time into tinkering with amp models in the bass pod.

so far I’m over the moon. Keeping all the tine shaping in the bass pod seems to make more sense than having eq controls everywhere. I like the simplicity of that and it allows me to focus on just the bass and the bas pod. 

I think the combination of my Ray34 which I’ve fitted Delano hybrid pickups to (passive jazz and active Ray) strung with cobalt flats into the bass pod power amp and VK210 is going to be awesome. Can’t wait to try it out with the band in just over a week. Probably some initial frustration followed by lots of tweaking.

anyway thought it might be of interest.

cheers

 

[BassmanPaul]

Don't blow your speaker cabinet!!!

 

Don't get tempted to get 'every last watt' out of the power amp by bridging it. Run it in parallel mono with a cab on each channel.  if you only have one cabinet, run it on one channel and turn down the other.

 

Use SpeakOn cables and not 1/4" Phone plugs. The 1/4" plugs can't safely handle the power delivered by the amp.

Edited March 5, 2022 by BassmanPaul

[Waddycall]

Just now, BassmanPaul said:

Don't blow your speaker cabinet!!!

 

Don't get tempted to get 'every last watt' out of the power amp by bridging it. Run it in parallel mono with a cab on each channel.  if you only have one cabinet, run it on one channel and turn down the other.

Haha - funny you should say that. It’s bridged! The output from the bass pod is pretty low so having it run on one channel isn’t loud enough (I can have it on full in our detached house) and is no way near loud enough to keep up with a drummer. If I use my Hartke Ha3500 with this cab (250w into 8 ohm) is much louder. According to the specs it’ll put just under 600w into an 8 ohm cab in bridge mode. The power handling of the cab according to the VK website is 1200W which seems a bit optimistic but I feel I’m pretty safe unless anyone thinks otherwise? I did think about putting a channel strip between the pod and amp to bring the line level up but that would add weight and complexity I don’t want. I also don’t want to overload one amp channel and make it clip.

[BassmanPaul]

So you do bridge. It's not that powerful an amp but it is still more than capable of damage. 

 

Do you know that some amps require a specially wired speaker cable for Bridge mode?  In your case the Speakon connected to Amp1 output has to be wired +1 and +2 at the amp end and +1 and -1 at the cabinet end. This cable is not reversible and it's two ends should be clearly marked Amp and Speaker.

 

Edited March 5, 2022 by BassmanPaul

[Waddycall]

1 minute ago, BassmanPaul said:

So you do bridge it. It's not that powerful an amp but it is still more than capable of damage. 

 

Do you know that some amps require a specially wired speaker cable for Bridge mode. In your case the Speakon connected to Amp! output gas to be wired +1 and +2 at the amp end and +1 and -1 at the cabinet end. This cable is not reversible and it's two ends should be clearly marked Amp and Speaker.

 

Cheers Paul, yes it’s currently connected with binding posts to jack as it’s only being used at very low power in the house. I have an NL4 plug on order to modify one end of one of my speakon cables. I ordered a red one to avoid mixing up the ends! That will be sorted before it gets used in anger 👍

[Waddycall]

By the way power amps are a new thing to me so grateful of the comments 👍

[BassmanPaul]

Using the binding posts you have to connect your cable to the two red posts and ignore the black ones.

 

In Bridge, one amp channel is fed a signal 180 degrees from the other. Thus when one output goes positive the other goes negative the same amount. In essence one amp drives the cabinets positive pole whilst the other feeds it's negative. Thus twice the voltage is developed across the cabinet.

 

How are you connecting the amp to the Pod?

[Waddycall]

Yeah I have the two reds connected to the cable. The output from the pod is unbalanced 1/4” into an unbalanced 1/4” in on the amp. It’s a shame there’s no balanced XLR out on the pod.

[BassmanPaul]

If the Pod had an XLR it's a pound for a penny that it would have been a low output suitable for connecting to a PA. It would have been quieter that the phone jack. You must remember that many 'on the floor' devices are designed to plug into the front of a bass amp and so have a much lower voltage level than a power amp needs.

[Waddycall]

There’s a switch on the back of the bass pod to select the output level “line” or “amp”. I have it set to line so pretty much getting all it’ll give although no doubt the power amp could take more. Can’t see what else I can do without adding more components, weight etc

[BassmanPaul]

Are you turning the QSC's level controls to full on?

[Waddycall]

9 hours ago, BassmanPaul said:

Are you turning the QSC's level controls to full on?

As per the manual I have the channel 2 gain at 0 and use the channel 1 gain to set the output as the output level pot on the pod is a bit scratchy (cleaner/lube on the way). I have read a few comments that the gain control on the amp should be set and left but don’t understand the relevance when there’s a single relatively low level input source that’s not having it’s signal mixed and monitored remotely.

[Waddycall]

I just remembered something I read in the manual for my previous amp - an Ashdown RM 800 evo II. It always operates in bridge mode with an output of 800w into 4 ohm. My power amp in bridge mode puts 830w into 4 ohms in bridge mode so to me seems very similar.

[agedhorse]

On the amp, the channel 1 input sensitivity control should be set all the way up (fully clockwise). 

[Waddycall]

9 minutes ago, agedhorse said:

On the amp, the channel 1 input sensitivity control should be set all the way up (fully clockwise). 

Yeah, I keep hearing that and continue to fail to understand it. As it’s an input attenuator I don’t see the difference between turning that down and turning down the output attenuator of the bass pod other than doing it “wrong” saves chucking out 600w of scratchy pot noise (pod pot is noisy) through my cab. 

I also don’t understand how it differs from using the master volume pot on a standard bass amp as surely that is also an attenuator for the input into the power stage?

I’m interested in understanding this though.

[Waddycall]

Here’s what the manual has to say about the gain controls -

[Bunion]

23 hours ago, Waddycall said:

The output from the bass pod is pretty low so having it run on one channel isn’t loud enough (I can have it on full in our detached house) and is no way near loud enough to keep up with a drummer

Have you looked at the volume settings in the XT they might need resetting 

[Waddycall]

26 minutes ago, Bunion said:

Have you looked at the volume settings in the XT they might need resetting 

Yep, all tweaked. I believe I’ve got it as good as I can get it without clipping and I’ve evened out the level of all the patches I’m using. The factory presets are a nightmare for differing levels!

[Dan Dare]

1 hour ago, Waddycall said:

Yeah, I keep hearing that and continue to fail to understand it. As it’s an input attenuator I don’t see the difference between turning that down and turning down the output attenuator of the bass pod other than doing it “wrong” saves chucking out 600w of scratchy pot noise (pod pot is noisy) through my cab. 

I also don’t understand how it differs from using the master volume pot on a standard bass amp as surely that is also an attenuator for the input into the power stage?

 

I agree with you. On a power amp, the gain controls do not have any amplifying function. That's the job of the preamp, which a power amp doesn't have. They merely attenuate, reducing the signal level received from the preamp, mixing desk or whatever is feeding it. So it appears to make little difference whether you attenuate the signal on the way out of the pre or on the way into the power amp. The gains on a power amp are most useful where you are using a mixer and need to drive it hard enough to get the best signal to noise ratio with low output mic's.

[fretmeister]

I used to do this with a Yamaha P5000S power amp and a PodXT Rack.

 

Smegging loud it was, and very versatile.

[MacDaddy]

On 05/03/2022 at 19:26, Waddycall said:

By the way power amps are a new thing to me so grateful of the comments 👍

 

It seems to be a bit more complicated than I would have thought.

I'm still not sure what is meant by 'bridging' and 'parrallel'?

[Waddycall]

4 hours ago, MacDaddy said:

 

It seems to be a bit more complicated than I would have thought.

I'm still not sure what is meant by 'bridging' and 'parrallel'?

Haha ha well hopefully I’ve learnt something -

bridged = 1 input through both channels to 1 output effectively operates as 1 amp

paralellel = 1 input, through two independant channels to two independant outputs effectively two amps with the same input.

[Downunderwonder]

Parallel describes situations additional to dual amplifiers driving discrete cabs off one signal.

 

Parallel is also when two cabs are daisy chained to one amp.

 

Often there are parallel outputs provided on amps for the purpose of not having to daisy chain cabs. The cabs are a parallel load.

[agedhorse]

Parallel amp mode is 2 separate channels driven by the same signal of the same polarity. This requires a speaker on each amp channel.

 

Bridged amp mode is 2 amp channels, driven by a common signal with opposing polarity. A single speaker load is placed between the speaker outputs of the two channels.