Class D amplifiers and speaker impedance. - Page 1
Forum section: electronics / beginners

author: BullyBeef
author_rank: Newbie
timestamp: July 11, 2021, 11:45:55 am
content: I'm not new to electronics, but I am new to Class D amps so I am putting this in the beginner's section. I have a pair of 500watt rms PA subwoofers that need new woofers. The old ones are 3 x 16ohms paralleled so this is about 5.5ohms to the amp. I have the age old problem of being short of cash and I want to use a set of 8ohm woofers I already have. I know that with traditional amplifiers that the speakers have to be the right impedance for the output, but how critical is this with Class D amplifiers? Changing the load from 5.5 ohms to 2.5 ohms will mean the output stage will be putting out more current at full power, but would this be a problem? are class D smart enough to handle this?

author: Benta
author_rank: Super Contributor
timestamp: July 11, 2021, 12:06:50 pm
content: Doesn't matter which class. The question is: can the output stage handle the current? You could argue that the problem is worse with class D, as output voltage swing is the full supply voltage, but the speaker inductance usually takes care of that.

author: Gyro
author_rank: Super Contributor
timestamp: July 11, 2021, 12:49:02 pm
content: Although speaker inductance would also be lower for 3 x 8R against 3 x 16R. Dropping from a 5.5R load to a 2.5R load is pretty drastic. Is thereanyinformation available on the amp modules, or are they completely custom?Without specific data, it's impossible to say without finding out the hard way!

author: themadhippy
author_rank: Super Contributor
timestamp: July 11, 2021, 01:09:06 pm
content: whats the power raiting of the drivers? 2 in series and the third in parallel will get you 5.3 ohms

author: BullyBeef
author_rank: Newbie
timestamp: July 11, 2021, 02:20:26 pm
content: themadhippy - by george you are right. I missed that one - I must be getting old. Thanks.

author: gf
author_rank: Super Contributor
timestamp: July 11, 2021, 02:23:11 pm
content: Quote from: themadhippy on July 11, 2021, 01:09:06 pm... but at the cost of significant power impalance (2/3 + 1/6 + 1/6) among the 3 speakers.

author: themadhippy
author_rank: Super Contributor
timestamp: July 11, 2021, 03:40:15 pm
content: Quoteindeed,and that be why i asked about the power rating of the drivers.

author: fourfathom
author_rank: Super Contributor
timestamp: July 11, 2021, 04:00:06 pm
content: Quote from: themadhippy on July 11, 2021, 03:40:15 pmNo, the power imbalance is in the speakers (drivers). With the series-parallel wiring the speaker in parallel will get twice the voltage as the speakers in series, and so 4x the power. The amplifier won't care, but you won't be able to drive the cabinet to full desired power without blowing the paralleled speaker.

author: themadhippy
author_rank: Super Contributor
timestamp: July 11, 2021, 04:06:33 pm
content: Quoteand if each drivers is rated to suit the total power output of the amp?

author: Gyro
author_rank: Super Contributor
timestamp: July 11, 2021, 04:46:12 pm
content: If all three drivers share a common enclosure volume, driving them at significantly different power levels could cause some interesting effects, cone movement wise.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 11, 2021, 05:53:58 pm
content: Do you want to buy complete amplifiers, or kits, or modules from Ali Ebay?The easiest option is probably to just use an amplifier module for each speaker.Then each module sees a nominal 8 ohm.The amplifier modules themself are relatively cheap. The power supply and the box to put it all in are a significant of the costs.With three amplifier module's you can also use smaller modules, and this probably increases reliability.

author: fourfathom
author_rank: Super Contributor
timestamp: July 11, 2021, 06:01:43 pm
content: Quote from: Gyro on July 11, 2021, 04:46:12 pmYes, this probably would be an issue.And yes, if each driver can by itself take the full amplifier power then there shouldn't be a "blowing the driver" problem.I assume that we are all aware of the problem where amplifier clipping generates enough high-frequency spikes to blow out a tweeter? At least in live music (I play electric bass) we often have an amplifier that is full-power rated well beyond what the speakers can take, in order to stay away from amplifier clipping.

author: stevelup
author_rank: Regular Contributor
timestamp: July 12, 2021, 08:14:58 am
content: You could put two 8ohm drivers in parallel and replace the third with a passive radiator (or just install the third speaker but don't connect it).Weird design... never seen a sub with three speakers before - just out of idle curiosity, what on earth are they?

author: Gyro
author_rank: Super Contributor
timestamp: July 12, 2021, 08:48:38 am
content: Quote from: stevelup on July 12, 2021, 08:14:58 amThe driver will have the same FS as the other two, so might suffer cone flap at that frequency. A lot depends on the cabinet - sealed, ported etc. You could put a short across the unused driver though - or even better, experiment with low value resistors to provide electrically adjustable damping. I've never had the opportunity to play with it, but attaching a passive RC network might have interesting possibilities as a port (or abr) alternative.

author: mikerj
author_rank: Super Contributor
timestamp: July 13, 2021, 07:08:58 am
content: Is there a (passive) crossover built into the speaker? If so the frequency response will be affected by changing the driver impedance.