Amp to Power Passive Subwoofer

sublemonal

Novice Member
Hi all,

Firstly please accept my apologies, I am a complete novice when it comes to all things audio which is why I'm here. Please be gentle with my lack of knowledge and my undoubted, basic, 'obvious to most' questions.

In short, I am looking to connect a laptop (MacBook Pro) through an amp to power a single (one) passive subwoofer. The purpose is not for playing great sounding music but to simply play a variety of different low-level tones using tone generator software on the Mac through the passive sub.

After having spent many hours trying to get my head around how to match amps with subs and finding it surprisingly difficult to find matching combinations I'm currently looking at the following pairing.

Amp - Nobsound G2 PRO HiFi Subwoofer /Full-Frequency Mono Digital Power Amplifier 300W - This is a 300W, 2-8Ω, Mono, Full Frequency Power Amp
Sub - Punch 10" P2 4-Ohm DVC Subwoofer | Rockford Fosgate ® - This is a 300W, 4Ω, DVC Sub

Yes, it is a very basic amp but for my purposes, so long as it powers the amp properly, that is all that matters.

The specs on the amp are a little scarce, so my question is basically will the pairing work correctly? (I've emailed the amp maker but have not received a reply as yet, hence my question here).

My assumption is that if I wired the pair in parallel then I would have a 2Ω load on the amp and if I wired in series, an 8Ω load on the amp. Bearing in mind the amp spec suggests 300w*1 2Ω and that 'the speaker matches impedance 2-8Ω' is this going to work without any problems?

I'd appreciate any help or advice and mostly a simple layman's explanation as to whether I've got this right or am way off the mark and if the latter what I need to be looking for?

Many thanks
 

noiseboy72

Distinguished Member
That amplifier is not 300W. The power supply is only rated at 160W (32V x 5A), so allowing for amplifier efficiency, the absolute maximum power output is likely to be closer to 125W.

Other than that, it will drive the sub OK. You do not state how loud or what level of distortion is acceptable to you. It is likely that you will be getting upwards of 2-5% distortion at higher levels with that amp and it really won't be that loud. On a par with a mid range home sub, so not exactly shaking the building to its foundations!

To gain maximum power, you would use the minimum impedance the amplifier can cope with. If it is rated for 2 Ohms, then this should be fine.

You may be better with a much higher specc'd plate amp, as this is more likely to deliver the stated power. You are less likely to damage the amp or in extreme cases the speaker if it is not running so hard or producing so much distortion.
 

sublemonal

Novice Member
Hi noiseboy72 and thanks for the feedback. So my first point of confusion (due to my complete lack of knowledge) is why is the amp advertised as 300W, as in 300W*1 2Ω if it is, in reality, closer to 125W? Is this just mis advertising?

Does 'Max 300W output power and equipped with 32V/5A large power supply, enough to drive 320W passive speakers or subwoofers' not mean what it say's on the tin, as in, is it the V/A combination that determines the true power of the amp? What V/A rating should I be looking for?

All I'm basically trying to find is an inexpensive amp/passive subwoofer combination, it doesn't need to shake the building to its foundations, just play various tones at varying Hz through the sub and it doesn't need to be a 300W setup, just a combination that matches which for some reason seems less easy to find that I'd hoped.

Is there anywhere in particular that I should look as I seem to have exhausted Google?
 

noiseboy72

Distinguished Member
Power calculation is quite simple. Watts = Volts x Amps. In this case, the power supply is advertised as [email protected], so 32x5 is 160 = 160 Watts. "Max 300W" could mean anything. It could be that it will achieve it for a fraction of a second - or for slightly longer but with huge distortion, who can tell!

You should really look for the RMS figure. Look at this amp: Dayton Audio SPA250 250 Watt Subwoofer Plate Amplifier: Amazon.co.uk: Electronics It is rated at 250W and you can see it is physically larger and needs quite a lot of heat sinking to keep it cool. These are all clues that it will provide a sustained 250W - or at least a figure closer to it!

There's loads of cheap Chinese clone amplifiers out there. Virtually none of them are reliably advertised or will get close to the power outputs they state. I had a set of PC powered speakers that were about 10W RMS, but advertised as 1250W PMPO! It was simply not credible, but it didn't stop them printing it on the box!

If you only need modest levels, the amp you link to will be fine. Without knowing what you are trying to achieve, it's impossible to say if the amp or speaker will provide the levels you are looking for.

Please also remember that the power handling of the speaker is partly defined by the enclosure you put it in. The speaker needs to be in a correctly size enclosure, tuned to the speaker parameters to provide some physical air resistance to prevent over excursion of the cone. If you just screw it to a piece of wood or put it in any old case, the power handling may reduce quite significantly.
 

sublemonal

Novice Member
That makes sense and staggering that companies are able to get away with false advertising (to the less knowledgable), not that I'm particularly surprised but I'm sure it must break all sorts of advertising rules and regs.

I assume with the Dayton Audio SPA250 it wouldn't drive the 300W sub I linked to? My understanding (rightly or wrongly, I suspect the latter) is that the Amp and Sub Wattage needs to match. I think Dayton Audio possibly make a 250W and I know Rockford Fosgate definitely have a 250W Sub, so would I need to match the two were I to look at the SPA250?

I do have an old Technics Amp, an SU-V40, which is 2 Channel at 50W per Channel and I had hoped to find a decent, reasonably priced 100W Sub to use with it but have failed to find anything apart from one Dayton Audio Sub that could work but maybe not design-wise.

The Sub won't be sitting in an enclosure, I'm using it purely for photographic purposes, photographing invisible sound waves, the idea is to feed varying sine wave tones through the sub which will have a membrane over the top and to photograph the effect those tones have though liquid vibration by using liquids sitting on top of the membrane. I should have mentioned that in my original post perhaps though I was hoping finding a matching amp and sub would be relatively straightforward.
 

John7

Well-known Member
IMHO you would be better off buying a powered subwoofer from the outset. What you are proposing will require a suitable enclosure for your woofer and a filtering circuit/choke to cut out the high frequency content from the amp and an amp that can provide high current to the speaker.

If you're not after earth shattering bass, there are plenty of budget, dedicated subwoofers on the market or you could go for one of the more well know "hi-Fi" ones (BK Electronics for example).


 
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noiseboy72

Distinguished Member
Sounds fun what you are doing.

1 fundamental that I think will help. The wattage of the amp and speaker does not need to match, they just need to be reasonably close.

A 250W amp will drive a speaker in the range of 200W to 500W without issue. A 50W amp will drive a speaker from 30W to 100W. Quite honestly, a 50W amp will drive a speaker of 1000W, but it just won't get the best from it!

Matching amplifiers is about trying to avoid damaging either the speaker by massively overdriving it - so a 1000W amplifier and a 50W speaker or under-driving it - a 50W amp and and 100W speaker and causing so much distortion that the speaker fails or the amp is damaged.

I suggest that you start with your old amplifier and the speaker you are looking at. You may find that it will drive it to the levels you need to see the effect. I just had a quick play here using my sub and a 50W amp and it was moving a good 3-5mm with a simple sine wave. Using complex harmonics and seeing the resulting patterns should be pretty cool! Are you suspending an elastic membrane above the speaker rather than attaching to it? That would be my suggestion, as otherwise you may see more cone artefacts than actual vibrations. Be aware that you might need to avoid certain frequencies, as the cone may resonate and give unexpected results.
 

sublemonal

Novice Member
That is 'extremely' helpful advice, thank you. I think my main error to date has been trying to find a perfect match wattage wise, which possibly explains why I've struggled so much, but knowing they need to be close but not necessarily match exactly makes a world of difference and I completely understand the logic of not over or underdriving the speaker.

I might, as you suggest, start with my old amp and see how well that works in practice. I'm basically 'constructing' a cymatics rig, so there will likely be a couple of things in play, one stretching a membrane over the sub with liquids placed on top of the membrane launching themselves into the air at certain frequencies generated by the tone generator and a second one will be placing a liquid in a petri dish resting or taped to the membrane of the sub to capture the symmetrical wave patterns that are generated in the respective liquid at certain frequencies. There may also be the effect of sound waves on ferrofluid to add to the mix, which is really fascinating.

Many thanks for your help and advice, I feel I'm on a much better footing now and I've learnt something along the way, so I'm feeling a lot happier about mixing and matching now :)
 

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