Power Amp Question

[Bleat]

So I have just got 2-channel 400 watt per side digital power amp, and the plan is to get 2x 8ohm cabs and daisy chain them for a 4ohm load at 400watts. The question being are power amps ok to run just the one side without speakers plugged into the other channel? 

I am guessing that this is something they are happy to do?  I do have a manual for it, but it makes no mention of using just one side of the amp.  

[Dood]

Yes, no problem, just make sure you plug in to just one input and, if the option is available, set the amplifier to Stereo. (There is sometimes an option called 'Parallel' or 'dual mono' that sends one input to both amplifier channels). Not that it will make any difference, but I also like to make sure the channel I am not using is turned all the way down too. 

Edited December 22, 2020 by Dood

[Dood]

Oh and of course, using the dual mono option later means that you can strap another couple of 8 ohm cabinets to the other channel and unleash hell at your next gig  provide useful monitoring for your drummer by turning the extra cabinets round to face them.

Edited December 22, 2020 by Dood

[Bill Fitzmaurice]

I'd run dual mono. There's no advantage to running one side only.

[Bigguy2017]

Yes, run as dual mono or in bridge mode mono, if the amp does this.

[Bleat]

Thanks for the comments!

The amp (Soundtech PS-802) can only run in either Stereo or Bridge mode @ 800 Watts  8 ohms only..no dual mono function. I think that bridging would require something pretty big, powerful (and expensive) cab wise, and actually far more power than I would ever really realistically use anyway. There is of course practical transport and handling / loading into car to consider, so I am opting for a smaller, easier to handle solution  where I could use either one or both more compact cabs.  I *could* run a cab off each channel at 230watts per side 8ohm , but I would be messing with 2 volume controls and ideally looking for the same cab type anyway.

I will clarify the current signal chain:

Bass> Line 6 Pod Pro which doesn't *seem* to have  left/right channels out but either a *modelled* out and DI out?. Maybe someone knows more than I do about that though?  So taking just one output > graphic EQ channel 1> Power amp channel 1> to 1x8ohm cab linked to another cab of same type.

 

[Bleat]

Ok I think I have sorted my own stupidity here.

The Pod manual is not very clear with all the different labelling of connections. (or maybe too complicated for my brain to understand properly) I found my spare leads out and have managed to connect up both pod outputs into channels 1+2 of graphic eq, then to left and right channel of my audio interface. Everything is working as I thought it should.

It looks like using both channels / volumes of power amp for bass cabs. That should give me 460 Watts total to play with.

[agedhorse]

With that amp, which I know  very well, you are much better off running dual mono.

[Lfalex v1.1]

As previously stated, try bridging it.

I did it with a QSC PLX 1202 (so about 600w bridged into 8 Ohms, 1200 into 4 Ohms)

I (on occasion) used it with an old Trace 2x10 with 2x80w OEM Trace (Celestion) drivers. 

You'd think that 600Wrms into a cab rated at 160w would cook it, but it didn't. It got way too loud (in rehearsal) before there was any evidence of my pushing it too hard.

Does the amp have a high pass filter? I ran the QSCs one at 30Hz. Prevents power being wasted on trying to force the loudspeaker system to reproduce low frequencies it simply can't.

[mytilini]

Related question (if I may) to the many, many minds greater than mine!  I've a TC Electronic BG250-210 combo, with a pre/post switchable D.I. output built-in.  Normally, that feeds the FOH mixer only, with me using the combo's speakers as my onstage backline.  Big gig next year, in which I need much more on-stage volume (TWO drummers!).  I've no other preamp signal output or extension cab options within that combo.

My thought is to feed the combo's inbuilt D.I. output, as the single input to a Behringer Ultra-DI D120 active 1 in-to-2 out D.I. box.  Effectively, to use the tonal set-up from the BG250, to input into a power amp.  I plan to connect one channel of the D120 via an output mic lead into the mixer.  The other channel's output would link a mic lead into the parallel-mode input of a spare Cerwin-Vega! CV-2800 P.A. power amp that I have.  That power amp puts out 600W. continuous, per channel @ 8 Ohms.   I'm going to hook up two 8 Ohm bass guitar cabs, one per channel.  The cab's are:

1). a JBL 1x15" loaded with a 600W. continuous Eminence Kappa speaker.

2). an Ashdown 4x10", giving a total continuous power handling capacity of 600W.

Question please - is this feasible?  Will a D.I. box's output have sufficient sensitivity to "drive" the input of this typical stereo P.A. power amp, or would I need to insert some other form of preamp into the signal chain?

Thanks in advance for any advice or help you can give and may I wish you all as happy a Christmas as possible in the current circumstances! 

[Dood]

My answer above was based on you wanting a 4 Ohm load and the amplifier bridging at 8 ohms minimum.

[Jack]

1 hour ago, mytilini said:

Related question (if I may) to the many, many minds greater than mine!  I've a TC Electronic BG250-210 combo, with a pre/post switchable D.I. output built-in.  Normally, that feeds the FOH mixer only, with me using the combo's speakers as my onstage backline.  Big gig next year, in which I need much more on-stage volume (TWO drummers!).  I've no other preamp signal output or extension cab options within that combo.

My thought is to feed the combo's inbuilt D.I. output, as the single input to a Behringer Ultra-DI D120 active 1 in-to-2 out D.I. box.  Effectively, to use the tonal set-up from the BG250, to input into a power amp.  I plan to connect one channel of the D120 via an output mic lead into the mixer.  The other channel's output would link a mic lead into the parallel-mode input of a spare Cerwin-Vega! CV-2800 P.A. power amp that I have.  That power amp puts out 600W. continuous, per channel @ 8 Ohms.   I'm going to hook up two 8 Ohm bass guitar cabs, one per channel.  The cab's are:

1). a JBL 1x15" loaded with a 600W. continuous Eminence Kappa speaker.

2). an Ashdown 4x10", giving a total continuous power handling capacity of 600W.

Question please - is this feasible?  Will a D.I. box's output have sufficient sensitivity to "drive" the input of this typical stereo P.A. power amp, or would I need to insert some other form of preamp into the signal chain?

Thanks in advance for any advice or help you can give and may I wish you all as happy a Christmas as possible in the current circumstances! 

Hi! Generally speaking, it's considered good practice to start a separate thread. But hey, it's Christmas!

 

How annoying that that combo doesn't have an effects loop, which is really what you need to do this. I think that your DI plan will technically work. But technically is the worst kind of work. I think that, as you have mentioned, you'll run into a problem whereby the DI will output a mic-level signal and the power amp will be expecting a line-level signal. Now, 1200W into 2 large cabs will undoubtedly be pretty loud regardless, but I don't think that you'll be able to drive the amp anywhere near hard enough to achieve full output. You might find you brought all that gear and it's not actually that loud.

 

Some suggestions:

1. Try it before hand, it might be fine.
2. If not, can you DI into the PA as normal, then have the power amp being fed by an aux on the desk? This way you'd essentially be using the amp and cabs as a dedicated, bass-only monitor.
3. If you HAVE to buy something new, several cheap preamps can lift mic level to line level. I've used a Behringer MIC100 (£35 new, less used) and a Rolls MP13 to do just this. Then you'd put the preamp between your DI and power amp.

[Bleat]

3 hours ago, Lfalex v1.1 said:

As previously stated, try bridging it.

I did it with a QSC PLX 1202 (so about 600w bridged into 8 Ohms, 1200 into 4 Ohms)

I (on occasion) used it with an old Trace 2x10 with 2x80w OEM Trace (Celestion) drivers. 

You'd think that 600Wrms into a cab rated at 160w would cook it, but it didn't. It got way too loud (in rehearsal) before there was any evidence of my pushing it too hard.

Does the amp have a high pass filter? I ran the QSCs one at 30Hz. Prevents power being wasted on trying to force the loudspeaker system to reproduce low frequencies it simply can't.

Thanks for the comment Lfalex , I may consider this if I can source the right cab (at the right price, and weight !) for my needs. The power amp doesn't have a hi pass filter just a switch for Stereo or Bridge Mode + in/out connections. It was the out sockets on the Pod and the crappy manual explanation that had me confused hence the original question! Now I have Left and Right outs running I can at least either use both amp channels or bridge it.

[BassmanPaul]

If your power amp is Class D it's certainly NOT digital. D was just the next letter assigned to a next developed amplifier topology.

It's analogue. 

Edited December 23, 2020 by BassmanPaul

[BreadBin]

3 minutes ago, BassmanPaul said:

If your power amp is Class D it's certainly NOT digital. D was just the next letter assigned to a next developed amplifier topology.

It's analogue. 

Yes, class D is just a different type of power supply. Lighter and more efficient, but the subject of many debates especially with regard to heft 

[mytilini]

2 hours ago, Jack said:

Hi! Generally speaking, it's considered good practice to start a separate thread. But hey, it's Christmas!

 

How annoying that that combo doesn't have an effects loop, which is really what you need to do this. I think that your DI plan will technically work. But technically is the worst kind of work. I think that, as you have mentioned, you'll run into a problem whereby the DI will output a mic-level signal and the power amp will be expecting a line-level signal. Now, 1200W into 2 large cabs will undoubtedly be pretty loud regardless, but I don't think that you'll be able to drive the amp anywhere near hard enough to achieve full output. You might find you brought all that gear and it's not actually that loud.

 

Some suggestions:

1. Try it before hand, it might be fine.
2. If not, can you DI into the PA as normal, then have the power amp being fed by an aux on the desk? This way you'd essentially be using the amp and cabs as a dedicated, bass-only monitor.
3. If you HAVE to buy something new, several cheap preamps can lift mic level to line level. I've used a Behringer MIC100 (£35 new, less used) and a Rolls MP13 to do just this. Then you'd put the preamp between your DI and power amp.

@Jack- thanks so much for your reply - sorry, didn't mean to ride in on the coat tails, but it did seem a similar issue - apologies to all; no offence intended!  Thanks for your suggestions:

1). yep, 1st. thing is to try the combo's D.I. out straight in the power amp's input - if that works, I can split the D.I. out using something as inexpensive as a Behringer Ultra-DI D120 to feed mic level to both desk & power amp simultaneously.

2). sadly, that aux input option doesn't fly at the venue I'll be playing; I have to sort this out locally on the stage.

3). never knew that such mic level-line level units existed - I'll check out your suggestions, thank you!

Finally, a thought.  I have an old (1993), but fully-working DOD TR3M effects pedal, which has stereo TS jack outputs on it.  I could plug the bass into that, then send the link-through branch into the combo, feeding the other directly into the power amp.  That way, I could firstly add one of these MIC100 units to that branch.  Sensible or hogwash...?!

[BassmanPaul]

45 minutes ago, BreadBin said:

Yes, class D is just a different type of power supply. Lighter and more efficient, but the subject of many debates especially with regard to heft 

Actually it's a different type of amplifier. Often the power supply will be a Switch Mode design to cut down weight.

[Bleat]

The manual says it is a "digital switching mode power supply" that "makes high powered performance with extremely low weight possible"  

On the front it says "digital power source" . It's 1U and about 8kg.

 

 

[Lfalex v1.1]

Compare and contrast that with the aforementioned QSC PLX 1202.

It's class A/B (very little in class A before heads determinedly into B, though)

2u high. Weight? A lot. 15kg+

[Jack]

1 hour ago, mytilini said:

3). never knew that such mic level-line level units existed - I'll check out your suggestions, thank you!

Just preamps, man. The Behringer ones are about as small and cheap as you'll get. One of my all-time favorite 'amps' was a nice Presonus Studio Channel preamp into a QSC PLX1602 power amp, all in a 3-space rack. You could plug anything into it. Bass, guitar, keys, mp3 player, pedal boards, anything and it would just make it really, really, extremely loud. My pedal board, into that rack, into a Dr Bass 2460 was my frfr rig before I knew what an frfr rig was.

 

1 hour ago, mytilini said:

Finally, a thought.  I have an old (1993), but fully-working DOD TR3M effects pedal, which has stereo TS jack outputs on it.  I could plug the bass into that, then send the link-through branch into the combo, feeding the other directly into the power amp.  That way, I could firstly add one of these MIC100 units to that branch.  Sensible or hogwash...?!

It makes sense but it doesn't really matter how you split the signal. If you already have the DI you might as well use that, if not then use what you've got. Gear is great for gear's sake but if this is going to be a one-off then it's best to just use what you've got.

 

Personally I'd be asking exactly what the combo is bringing to this situation. It's not like you'll hear it if it's next to the stack, so you're probably only really using the preamp and tone controls. Do you have any other way of getting those? Preamp pedal? A little mixer? Another amp with an effects loop?

 

Edited December 23, 2020 by Jack

[mytilini]

1 hour ago, Jack said:

Just preamps, man. The Behringer ones are about as small and cheap as you'll get. One of my all-time favorite 'amps' was a nice Presonus Studio Channel preamp into a QSC PLX1602 power amp, all in a 3-space rack. You could plug anything into it. Bass, guitar, keys, mp3 player, pedal boards, anything and it would just make it really, really, extremely loud. My pedal board, into that rack, into a Dr Bass 2460 was my frfr rig before I knew what an frfr rig was.

 

It makes sense but it doesn't really matter how you split the signal. If you already have the DI you might as well use that, if not then use what you've got. Gear is great for gear's sake but if this is going to be a one-off then it's best to just use what you've got.

 

Personally I'd be asking exactly what the combo is bringing to this situation. It's not like you'll hear it if it's next to the stack, so you're probably only really using the preamp and tone controls. Do you have any other way of getting those? Preamp pedal? A little mixer? Another amp with an effects loop?

 

Agree, but the entire reason I bought the BG250 was that it had no effects loops, no line outs, an inbuilt tuner, no extension cabinet i.e. all I wanted in just one box.  Having done a bit more reading, I'm almost certainly gonna get the D120 to allow post-fade export of the tone-shaped signal to both the desk and that power amp at the same time.  LIght, quick simple and it'll cost me less than £20 from Amazon to do this - I already have top-grade mic leads from when I used to do the P.A. for my old band too.  Am gonna skip the TR3M altogether.

[agedhorse]

On 23/12/2020 at 05:07, Lfalex v1.1 said:

As previously stated, try bridging it.

I did it with a QSC PLX 1202 (so about 600w bridged into 8 Ohms, 1200 into 4 Ohms)

I (on occasion) used it with an old Trace 2x10 with 2x80w OEM Trace (Celestion) drivers. 

You'd think that 600Wrms into a cab rated at 160w would cook it, but it didn't. It got way too loud (in rehearsal) before there was any evidence of my pushing it too hard.

Does the amp have a high pass filter? I ran the QSCs one at 30Hz. Prevents power being wasted on trying to force the loudspeaker system to reproduce low frequencies it simply can't.

If you understood that this amp is NOT rated for a 4 ohm load in bridge mode, you wouldn't recommend something dangerous to the health of the amp. Bad advice.

 

[agedhorse]

On 23/12/2020 at 06:07, mytilini said:

Related question (if I may) to the many, many minds greater than mine!  I've a TC Electronic BG250-210 combo, with a pre/post switchable D.I. output built-in.  Normally, that feeds the FOH mixer only, with me using the combo's speakers as my onstage backline.  Big gig next year, in which I need much more on-stage volume (TWO drummers!).  I've no other preamp signal output or extension cab options within that combo.

My thought is to feed the combo's inbuilt D.I. output, as the single input to a Behringer Ultra-DI D120 active 1 in-to-2 out D.I. box.  Effectively, to use the tonal set-up from the BG250, to input into a power amp.  I plan to connect one channel of the D120 via an output mic lead into the mixer.  The other channel's output would link a mic lead into the parallel-mode input of a spare Cerwin-Vega! CV-2800 P.A. power amp that I have.  That power amp puts out 600W. continuous, per channel @ 8 Ohms.   I'm going to hook up two 8 Ohm bass guitar cabs, one per channel.  The cab's are:

1). a JBL 1x15" loaded with a 600W. continuous Eminence Kappa speaker.

2). an Ashdown 4x10", giving a total continuous power handling capacity of 600W.

Question please - is this feasible?  Will a D.I. box's output have sufficient sensitivity to "drive" the input of this typical stereo P.A. power amp, or would I need to insert some other form of preamp into the signal chain?

Thanks in advance for any advice or help you can give and may I wish you all as happy a Christmas as possible in the current circumstances! 

Except that the DI out will not be able to drive the amp to rated output unless it's full line level (ie. nominal +4dBu)

[agedhorse]

On 23/12/2020 at 10:10, BreadBin said:

Yes, class D is just a different type of power supply. Lighter and more efficient, but the subject of many debates especially with regard to heft 

No sir, class D has nothing whatsoever to do with  the power supply. It has to do with the topology of the power amp only.

[agedhorse]

On 23/12/2020 at 13:15, mytilini said:

Agree, but the entire reason I bought the BG250 was that it had no effects loops, no line outs, an inbuilt tuner, no extension cabinet i.e. all I wanted in just one box.  Having done a bit more reading, I'm almost certainly gonna get the D120 to allow post-fade export of the tone-shaped signal to both the desk and that power amp at the same time.  LIght, quick simple and it'll cost me less than £20 from Amazon to do this - I already have top-grade mic leads from when I used to do the P.A. for my old band too.  Am gonna skip the TR3M altogether.

The balanced output clips at +2dB peak (-1dBu RMS) maximum, not enough for driving a power amp to rated output in the real world. You will need +4dBu (minimum) plus at least 6dB of headroom to make up for losses relative to real world implementation.