Powering speakers with Headphone amp

[LifeTiltz]

Hello guys! Im a huge fan of this site and Im still a little n00b.
I have looked for information on powering speakers with a headphone amp but i couldn't find any info on it. I bought a pair of KEF R300 and I listen to them in a small room or on my desk. I have a 30£ AVR and have been looking for the most the most bang for the buck way to power them with a decent speced front end. I also currently do not have a headphone amp, but a decent one would relatively cheap
Would the shared ground be a negative in such a setup(where lots of volume isnt needed)? Is there are drawbacks that I dont see?
I would love a conversation on this. (Sorry if its a n00b question. hope you guys are staying safe and happy)

 

[Frank Dernie]

Hello guys! Im a huge fan of this site and Im still a little n00b.
I have looked for information on powering speakers with a headphone amp but i couldn't find any info on it. I bought a pair of KEF R300 and I listen to them in a small room or on my desk. I have a 30£ AVR and have been looking for the most the most bang for the buck way to power them with a decent speced front end. I also currently do not have a headphone amp, but a decent one would relatively cheap
Would the shared ground be a negative in such a setup(where lots of volume isnt needed)? Is there are drawbacks that I dont see?
I would love a conversation on this. (Sorry if its a n00b question. hope you guys are staying safe and happy)

Nowhere near enough power and even a low output impedance on a headphone amp will be so high on speakers it alters their frequency response.
So no
A reasonable power amp tends to be a lot more expensive than low level stuff since the power supply and power transistors are much more expensive.

 

[LifeTiltz]

Thanks for the insight.
I just checked and a schiit Magni 3 has an output impedance of 0.6 Ohm and my kefs are 8ohm. It would do 1.7 watt into 33 ohm. I know it doesnt translate going to 8ohm.
Which numbers would be ok that it would be worth a try? Thanks again frank

 

[NTK]

The power rating of the Magni 3+ is 2.8 W at 16 ohms and 2.4 W at 32 ohms, so it is current limited at 16 ohms and below. Thus you can expect at most 1.4 W at 8 ohms and 0.7 W at 4 ohms.

However, you are most likely more limited by heat dissipation because the Magni 3+, having no heat sink, is not designed to output a sustained level of current needed to drive speakers to normal levels. I don't know (since it is a discrete design) if the Magni 3+ has any built-in thermal protection.

 

[Frank Dernie]

Thanks for the insight.
I just checked and a schiit Magni 3 has an output impedance of 0.6 Ohm and my kefs are 8ohm. It would do 1.7 watt into 33 ohm. I know it doesnt translate going to 8ohm.
Which numbers would be ok that it would be worth a try? Thanks again frank

I would forget headphone amps altogether for speakers as @NTK points out they don't have enough heat sinking.
The minimum recommended power from KEF is 25 watts.

 

[LifeTiltz]

But heat dissipation with a little DIY wouldn't be that hard, would it?
The only reason Im so interested in this weird DIY fun is because a Purifi 1ET400A would be as clean but is like 1K.
If the Magni 3+ were to die due to too much heat it wouldnt necessarily take the speakers with it, would it?
While they do recommend 25watts, if i were to listen to them nearfield i doubt i need that much wattage.
Im just really curious IF it had enough power to listen to it nearfield would it sound good (if the output impedance was low)?

 

[Frank Dernie]

While they do recommend 25watts,

They recommend 25 to 120 watts. My guess 25 watts is for near field listening already.
I am not familiar with the design so considering heat sinking is not something I could do.
Headphone amplifiers are more like preamplifiers than real amplifiers in terms of requirement and spec.

 

[JeffS7444]

Perhaps this would be a better all-around choice:
http://www.tpdz.net/productinfo/398300.html

 

[sejarzo]

...Headphone amplifiers are more like preamplifiers than real amplifiers in terms of requirement and spec.

Which is exactly why the Magni has preamp outputs and not speaker outputs.

 

[RayDunzl]

I have looked for information on powering speakers with a headphone amp

You could probably use these with a headphone amp:

https://www.amazon.com/Sony-SRS-P7-Passive-Speaker-Mini-Plug/dp/B00005MOZG

 

[LifeTiltz]

Damn those Sony’s look good!
Thanks for the link Jeff but the dac and amp performance isn’t that great.
Sejarzo, I don’t understand what you mean, sadly I am ignorant of how the magnapans work. Could you elaborate or link me something?
thanks guys for indulging me sadly my curiosity isn’t quenched yet

 

[solderdude]

Post #4 (@NTK ) gave you the correct answer.
1.4W in 8 Ohm. Of course you can drive a speaker with it but the amp will overheat as it is not designed to do this... provide that much current while having a lot of voltage over its output devices.
The output stage will dissipate MUCH more power than the speakers will and the SOA (Safe Operating Area) will be breached resulting in failure.
There is no heatsinking other than some copper on the PCB which is not enough in this particular case.
Not immediately but it will when playing a bit louder for a few minutes.

You are free to try though.

 

[LifeTiltz]

Post #4 (@NTK ) gave you the correct answer.
1.4W in 8 Ohm. Of course you can drive a speaker with it but the amp will overheat as it is not designed to do this... provide that much current while having a lot of voltage over its output devices.
The output stage will dissipate MUCH more power than the speakers will and the SOA (Safe Operating Area) will be breached resulting in failure.
There is no heatsinking other than some copper on the PCB which is not enough in this particular case.
Not immediately but it will when playing a bit louder for a few minutes.

You are free to try though.

Thank you for the in-depth answer!
More of a hypothetical question: If it was designed where it could play 8ohm @1.4 watt without dying, would it measure well? Would it having shared ground affect it adversely?

 

[solderdude]

It would measure less well than at 32 Ohm. Distortion will be somewhat higher for sure but will still be far under audibile limits.
It is very easy for an amplifier to supply a nice voltage with hardly any current drawn. It is more difficult to do so with higher currents (lower impedances).

When it were designed to drive lower impedances and higher currents the PCB design would be different as well, not only would there have been more heatsinking. Other output devices would have been used with a much larger SOA.
Schiit did design such headphone amps. The Jotunheim R for instance.

 

[LifeTiltz]

Thanks I think I understand a little more. I’ll go do some reading about this.

 

[LifeTiltz]

Ok back with more dumb questions!
Would adding 8ohm resistors to the speaker, bringing it to 16ohm, alleviate that stress on the amp (or going to 32ohm).
It seems the Magni 3+ could do 2.8 watts into 16ohm, would that translate to a speaker plus X ohm resistor (or a few in parallel)?

 

[RayDunzl]

Would adding 8ohm resistors to the speaker, bringing it to 16ohm, alleviate that stress on the amp (or going to 32ohm).

Yes, but would reduce the volume at the speaker.

The voltage from the amplifier would be divided (and the available power divided) among the resistors and speaker.

---

Also, parallel resistors create a lower resistance than any one of the paralleled resistances.

 

[Cahudson42]

OP, you are located in UK? US? Or?

 

[FrantzM]

Don’t. Waste of time. Sorry for being so blunt.

 

[sejarzo]

Thanks I think I understand a little more. I’ll go do some reading about this.

I would suggest some basic reading about electricity, and how voltage (V), resistance (R), current (I - capital i), and power (P) are related. Here's how I (a non-electrical engineer) think about it.

V = I * R (volts equal amps times ohms)

P = I * V, so that means P = I * I * R (power in watts equals amps squared times ohms)

Headphones might require only about 320 milliwatts of peak power to play very loudly. 320 milliwatts = 0.3 watts. If the headphone has 32 ohm impedance, it requires 0.1 amp (or 100 milliamps) current to reach that volume level. The voltage required to do that is about 3.2 volts. That's why you can drive headphones or IEMs to decent levels with a headphone amp powered by a USB port capable of no more than 500 milliamps and volts.

Full size speakers might require 75 watts of power to play loudly. 75 watts of power at 8 ohms impedance requires a tiny bit more than 3 amps of current, and 3 amps of current across 8 ohms requires 24 volts.

In other words, a typical speaker needs 30 times as much current and about 8 times as much voltage as a headphone...or 240 times as much power.

That's why traditional speaker power amplifiers have such large transformers and capacitors in them, to provide power to drive the speakers in top of the power that transistors waste (the reason for big heatsinks on most traditional power amps.)

Consider how large and heavy a typical 12 volt car battery is. You could put 10 typical NiMH AA cells that typically are 1.2 volt in series and get 12 volts. If that worked, why would cars drag around a heavy battery that might cost $150 instead of a handful of AA cells that cost less than $20? It's all about current required to provide the power needed to get the job done.