Using a small amp for headphones

[dinglebeer]

Just for fun I was thinking of finding some cheap cans, hacking them up and using them through the speaker output of an amp with low power. I know I'd have to be really careful with the volume but I don't see why it wouldn't work. Has anyone tried this, or would anyone?

 

[jrtrent]

I'm not a do-it-yourselfer, but there is information at the link below on driving headphones from a power amplifier. I can only assume a similar circuit is included in my OmegaStar PAT-5 (old Dynaco pre-amp chassis with newer circuitry by Van Alstine), which uses the power amp output to drive the headphones. I must say the sound betters any regular headphone jack I've used.

http://sound.westhost.com/project100.htm

 

[thefragger]

 

[Markw]

That ESP link seems to provide exactly what you need.

A lot of headphond jacks, particularly in he olden days, were driven off a parallel path to the speakers but they used resistors to pad down the signal to a level more suited for headphones.

 

[Stanga]

I use an Aiwa AA8700 amplifier to drive my K701 and HD650. I have yet to find a headamp that does a better job.

 

[hifi_nut]

Headphones usually require only a small fraction of a watt of power to work properly, hence the "resistors to pad down the signal to a level more suited for headphones" mentioned by Mark W. Impedance matching might also be an issue when feeding a pair of cans straight from the outputs of your amp.

I have this MA-18 Headphone Volume Control by QED, that inserts between amp outputs and speakers, that does a great job of taming down the signal without any apparent degradation. It also switches the speakers off when an Headphone jack is plugged in. I´m not sure, but I think it sports some sort of transformer inside. Guess I´ll have to open it up one of these days.

 

[whoaru99]

Generally speaking, audio circuits aren't impedance matched as they are in say a 75-ohm video circuit.

Usually, the issue with headphone impedance matching is if the source output impedance is too high it can't deliver sufficient output for the headphones.

Something like an audio amplifier output would/should have very low output impedance compared to the headphone so that part really isn't an issue, imo.

 

[roggom]

Exactly, the audio amps are designed for 4 or 8 ohm loads and the circuit linked by JR and hifinut do the job pretty good for matching a range of headphones. Most headphones today are 32 ohm and they need a little more current output which you reciever definately has. If yo ulook in the qed it will probably have 3~5 watt resistors in it to handle the excess.

Higher impedance cans will require a larger voltage swing, but again with standard amps you havent an issue.

Remember to always test with cans off of your head!

If you want to have some fun leave the cans intact and make a CMOY http://tangentsoft.net/audio/cmoy-tutorial/ for around $20 youcan source the parts at rat shack.



In short the headphone jack in stereos are there for convinient listening but beacuse the headphones only need less than a watt of power it is like pulling a bass boat with a semi truck.

 

[TerryO]

I drive my Sennheiser 580's off the speaker terminals of an AKAI reciever. The trick is to assemble a precision voltage divider and have it between the cans and the amp. I matched the resistors in order to keep them within a tolerance of .25%. I can then use the receiver's volume control. Works well and it allows the SS amplifier output to see a constant 8 ohm load regardless of the impedance of the cans.

Most Headphone circuits on receivers just use a series resistor and my experience indicates it doesn't sound nearly as good as taking it off the speaker terminals through a divider circuit.

I bought the receiver at St. Vinny's for $14.95 and spent approximately $10 or so for the project box and parts (used a handful of resistors to get matched pairs). So for $25 I ended up with a dedicated headphone rig that includes a tuner and source selection as well as sounding very good indeed!

Is it better than everything else?
Probably not, but you can spend a lot more and still not beat it by much. If you're on a budget it allows you to get much better headphones than you might have, if you had to purchase a new dedicated HP amp and tuner.

Best Regards,
TerryO

 

[Jon_Logan]

Impedance matching between a power amp and headphones in NOT necessary. The problem you'll have with headphones connected to a power amp is noise floor. That's why you see those 1 or 2 W (~300 ohms) resistors on headphone jacks. It's to form a voltage divider with the headphones themselves. It reduces the output slightly. To worry about impedance matching at audio frequencies, you're gonna need miles of a transmission line to see any eveidence of a reflection or VSWR problem. You see that this series resistor will attenuate differently, depending on the Z/impedance of you phones. 8 ohm headphones will be more attenuated (for a given sensitivity) than
600 ohm "cans". You can run a power amp all day long without a load. Okay, I said it. Let the fur fly

 

[comfycan]

The "headfi" forum is a really good resource for this kind of topic. They have a "DIY" sub-forum also.

Headphones can be a lot of fun. Warning: buying phones and headphone amps can be a lot like audio-crack.

 

[ecluser]

You see that this series resistor will attenuate differently, depending on the Z/impedance of you phones. 8 ohm headphones will be more attenuated (for a given sensitivity) than
600 ohm "cans". You can run a power amp all day long without a load.

You are perfectly right Jon.

More than this, the ~300 Ohm resistor in series with the phones makes the amplifier act much more like a current source than a voltage source. The frequency response of the phone will be modulated by the phone's impedance variations with frequency.

A low wattage amplifier connected directly to the phones may improve the sound.

A minor modification in the feedback branch of the amplifier can lower his voltage gain, and you can safely use an amplifier connected directly to the phones.

 

[Jon_Logan]

More than this, the ~300 Ohm resistor in series with the phones makes the amplifier act much more like a current source than a voltage source. The frequency response of the phone will be modulated by the phone's impedance variations with frequency.

QUOTE]
Ick! I like the idea of getting an amps overall gain closer to the application. Lower gain for headphones.

 

[TerryO]

Impedance matching between a power amp and headphones in NOT necessary. The problem you'll have with headphones connected to a power amp is noise floor. That's why you see those 1 or 2 W (~300 ohms) resistors on headphone jacks. It's to form a voltage divider with the headphones themselves. It reduces the output slightly. To worry about impedance matching at audio frequencies, you're gonna need miles of a transmission line to see any eveidence of a reflection or VSWR problem. You see that this series resistor will attenuate differently, depending on the Z/impedance of you phones. 8 ohm headphones will be more attenuated (for a given sensitivity) than
600 ohm "cans". You can run a power amp all day long without a load. Okay, I said it. Let the fur fly

I guess that what I didn't explain is that the "cans" aren't actually an important part of the 10:1 voltage divider, and I wasn't really concerned about impedance matching as such, for precisely the reasons you mention.
BTW: These are still better that the Cmoy HP amps that I've built in the past.

Best Regards,
TerryO

 

[davidk5]

I have to agree the problem woul be with the Impedance , i have headphones that run anywhere 32 - 300 ohms .
I love the looks & idea of that Qed box , how much is one of those ??

another option is to buy a designated Headphone amp .

 

[Jon_Logan]

I have to agree the problem woul be with the Impedance , i have headphones that run anywhere 32 - 300 ohms .
I love the looks & idea of that Qed box , how much is one of those ??

another option is to buy a designated Headphone amp .

Hi Davidk5, What might be the problem with impedance? ...running 32~300 ohm loads??