Understanding speaker/amp impedance.

[admin_exported]

Hi all,

I'm pretty new to this hi-fi game, and I'm just having a bit of confusion over impedance.

Ive just bought a used Pure DMX-50 dab mini hi-fi as a second system, and would like to get a pair of Q-acoustic 2020i to use with it.

My main concern is the speakers are 4-6ohm, and the amp is rated at 8ohm. This won't be a 'go loud' set-up, so should this be safe?

I've never paid too much attention to this as my main system (Mordaunt Short MS25i pearls and Yamaha a-420 amp) are both rated at 8ohm.

Any advice would be great!

Chris.

 

[RobinKidderminster]

It will be fine

 

[shafesk]

It'll be fine, most amps nowadays handle difficult loads and even 4 ohms is not too hard. Just make sure you meet atleast the minimum power required by your speaker. FWIW I can comfortably drive 4ohm Dynaudios with my 18 watt amp.

 

[Benedict_Arnold]

Not an electrical engineer, nor an electronics expert by any means. But, from what I remember sleeping off hangovers in the electrical engineering lectures they made us mechanical engineering undergraduates attend, maximum power transfer is obtained when the impedance of the source matches that of the load. Our professor went through some horrendously long mathematical proof of this, but I'm b***red if I can remember it after 25 years.

It strikes me that if your amplifier puts a 1 volt, for argument's sake, voltage across an 8-ohm impedance speaker, the speaker will draw 1/8 of an ampere. if you put 1 volt across a 4 ohm impedance speaker it'll draw 1/4 ampere, i.e. twice as much current. Thus the amplifier may run hotter due to the high current draw resulting from lower impedance speakers.

However:

a) due to the coils used in the speakers themselves, which have electrical reluctances, speaker impedances are impedances, AND not resistances. the impedances are not constant, dropping at higher frequencies;

b) your amplifier should be designed to operate safely at 10 on the volume knob. Since most of us don't listen at that level, the total current you draw shouldn't ever reach the level it would listening at 10 anyway.

c) most integrated amplifiers have thermal cutouts built into them, so even if you did overheat the output phases, the circuitry should protect itself.

Okay, asbestos underwear in place, please flame away 🙂

 

[Benedict_Arnold]

Not an electrical engineer, nor an electronics expert by any means. But, from what I remember sleeping off hangovers in the electrical engineering lectures they made us mechanical engineering undergraduates attend, maximum power transfer is obtained when the impedance of the source matches that of the load. Our professor went through some horrendously long mathematical proof of this, but I'm b***red if I can remember it after 25 years. Matching amplifier and speaker impedance is probably why they recommend they match, therefore.

It strikes me that if your amplifier puts a 1 volt, for argument's sake, DC voltage across an 8-ohm impedance speaker, the speaker will draw 1/8 of an ampere. if you put 1 volt across a 4 ohm impedance speaker it'll draw 1/4 ampere, i.e. twice as much current. Thus the amplifier may run hotter due to the high current draw resulting from lower impedance speakers.

However:

a) due to the coils used in the speakers themselves, which have electrical reluctances, speaker impedances are frequency-dependent impedances, and not constant resistances[. the impedances are not constant, dropping at higher frequencies;

b) your amplifier should be designed to operate safely at 10 on the volume knob. Since most of us don't listen at that level, the total current you draw shouldn't ever reach the level it would listening at 10 anyway.

c) most integrated amplifiers have thermal cutouts built into them, so even if you did overheat the output phases, the circuitry should protect itself.

Okay, asbestos underwear in place, please flame away 🙂

 

[dumbledore]

Strictly if the amplifier is designed for 8ohms speakers will not be safe to use it with 4ohm speakers. This one from Pure as is part of a system with its own speakers may not be intedended to be used with other speakers. Consequently the designer can cut corners and design the amplifier to be strictly 8ohm compatible. In this way it can optimise current and voltage delivery to obtain the 40W rated power with the least cost. An ideal amplifier will go to 80W with a 4 ohm load, but this is unlikely to be the case with this amplifier. You will be lucky if it still delivers 40W and not blowing up in the process. My feeling is that it should be fine to drive the 4ohms speakers as long as you not push it to the limit. The amps designed for 8ohms will certainly run out of current trying to drive a 4 ohm speaker at the limit. When this is the case the distorsion is more nasty then when running out of volatage whch is the normal way to design the amp at a given ohms rating.