(Resolved)how NOT overdriving headphones from a high power speaker output stage?

[EPAIII]

There are simple design equations and even tables that can be used to construct pads (two or more resistors) that will have the desired loss in db while maintaining an impedance match. They can be found in many of the paper back books sold by multiple sources. For both impedance matching and a desired loss in db the approach is to combine what is called a minimum loss pad which converts the impedance with a constant impedance pad with the remaining amount of loss. I have constructed a number of pads for professional/broadcast audio using these techniques and have never seen any distortion introduced by them. Keeping the impedance matched on both sides of the pad is the key to this. You can't just throw in arbitrary resistor values and hope for the best.

One such paper back book that I have used for many years is the "Allied Electronics Data Handbook", Eugene Carrington, Allied Radio Corp., fourth edition, December 1963. It has a section with tables and a number of different pad configurations.

On the other hand, it is not overly difficult to derive the equations yourself if you can draw a pad with enough elements to account for the number of unknowns. I have done it on more than one occasion when my book was not at hand. An H or T (three resistors) pad will generally suffice. Problems can pop up if the impedance matching is extreme while the desired loss is small as the equations may not have a real solution without adding an element with gain.

[Zero999]

You could have super low value resistor dividers but I don't have to explain why that's not right.

You should explain why that is, preferably without any audiophool BS.

Using low value resisors would result in unacceptable power dissipation.

[ELS122]

You could have super low value resistor dividers but I don't have to explain why that's not right.

You should explain why that is, preferably without any audiophool BS.

They specifically require a low impedance driver

Why is that? Is the impedance of the headphones all over the place and the frequency response of the headphones actually terrible without a low-Z driver/amplifier?

Well only an audiophile would put resistors in their amp that waste like 20 watts of heat just to drive some headphones.
ResistorDissipationWatts = V^2 / Resistance.

So you're saying I should design amps that expect only the perfect high end gear to be connected to it... Isn't THAT the audiophool mindset? Get super overbuilt audio cables because you're using one of the worst designed overpriced tube amps that pick up noise more than the actual signal.

TimFox and Zero999 already answered it.

[magic]

I think most people assumed you want an external box between a power amp and headphones. In such case burning 20W would only be a matter of getting enough heatsinking.

If you are designing or modifying an amplifier inside then there is another option: clamp output stage base voltages to ± few volts near ground (assuming the conventional topology with unity gain output stage).

[wasedadoc]

I think most people assumed you want an external box between a power amp and headphones. In such case burning 20W would only be a matter of getting enough heatsinking.

If you are designing or modifying an amplifier inside then there is another option: clamp output stage base voltages to ± few volts near ground (assuming the conventional topology with unity gain output stage).

Problem with that solution is that you are then operating the ampliifer in a region where the ratio of wanted audio to unwanted crossover distortion, hum and noise becomes smaller.

[RFDx]

Well only an audiophile would put resistors in their amp that waste like 20 watts of heat just to drive some headphones.
ResistorDissipationWatts = V^2 / Resistance.

Yes, the same audiophile that drives his headphones with a power amplifier for speakers. No power wasted there I guess.

So you're saying I should design amps that expect only the perfect high end gear to be connected to it... Isn't THAT the audiophool mindset? Get super overbuilt audio cables because you're using one of the worst designed overpriced tube amps that pick up noise more than the actual signal.

What the hell are you talking about?

[TimFox]

Re: maximum load resistance for amplifier.
This is more important for a transformer-coupled tube amplifier than for most solid-state amplifiers.
With a very high load (or open circuit) and too much drive to the amplifier, you can get excessively high voltage swings on the plates into high transformer-coupled impedance.
Most AB solid-state outputs are glorified emitter followers and can tolerate reasonably high load impedance.
To damage the solid-state output, you need a short circuit.

[Peabody]

Here's a headphone amp module using the TPA6120.  Does anyone have experince with that part?

https://www.amazon.com/dp/B07QMWML2V/

[magic]

I guess it works unless they screwed something up on the board.
I have this chip on my soundcard and what can I say, it works.

It was originally a DSL line driver, so it's unstable with capacitive load, hence the datasheet calls for 10Ω output isolation resistors, which seem to be included on this PCB. This means minimum Zout is 10Ω, unless someone somewhere came up with a DIY air coil or some other substitute that works. The chip is well known to DIY-ers.

Low frequency THD is not as good as most solid state headphone amps (likely thermal feedback in the chip) but likely nonaudible anyway. I don't remember details, sorry.

[donlisms]

Is there any reason you need to drive the amp at high power while you are using headphones?

The only circumstance where I can think of a reason to do that is an electric guitar amp being pushed into the desired kind of distortion.  Since we're already wasting huge quantities of power in such a case - or at least a few watts - and the point is to *make* it sound "bad", but in a good way, quibbling over the impedance issues or power list in a resistive attenuator is not worth much effort. Various devices allow such crazy tube watts to feed a line-level input in a sound system, or phones, I suppose.

Otherwise, turn the amp down, so you don't have to drop so much power to begin with. You (usually) don't need to drive an amp to full power to make it work right.  And in many cases, unless you're going for phoolery, that might be sufficient.  You would, of course, start with the signal at zero, and work your way up from there (carefully).  (The fact is, most people would be shocked at how little power they're actually using when they listen through speakers.)

If that's not sufficiently controllable, like the gain is just so low that the knob is twitchy, use a resistive divider to take care of the remaining TooMuchNess.  The personal effort in this case would be to study attenuators, starting with what's already been said here.

If the idea is that you don't want to blow up your headphones when you switch over from speakers, you need a protection circuit, not an attenuator.

[CaptDon]

Hey Gilligan, remember when the professor was designing and building his own high power amplifiers from scratch and then added a dedicated 2 watt low output impedance amplifier just for the headphone circuit driven by the same signal feeding the input to the big amp and then asked Ginger and Mary Ann how to keep from overdriving headphones from the high power output??? Yeah, me neither. And he built the whole thing with just Boat parts, Olives, Sea Shells and Excrement and combined the first letters to form BOSE. Then Mr. and Mrs. Howell bought in to it to give it snob appeal!!

[BeBuLamar]

I use a 600 ohms headphones connected to a power amplifier designed for 8 ohm speaker it won't over power the headphones.

[TimFox]

So long as the power amplifier is stable and happy with the higher load impedance, the same voltage applied to 600 ohms makes 1/75 the power of that voltage applied to 8 ohms.

[ELS122]

So long as the power amplifier is stable and happy with the higher load impedance, the same voltage applied to 600 ohms makes 1/75 the power of that voltage applied to 8 ohms.

Well ofc, but not a lot of headphones are 600 ohms, most are in the 30 ohm range. So driving like 5 watts into headphones doesn't seem like a good idea.

Maybe in a class G amp, if you disable the high voltage operation you could limit the power greatly and reduce the max output to a more reasonable watt or so. But that relies on some headphone connector switch and those are known to fail often.
So in the end either using a bucking transformer or just making a dedicated op amp headphone amp as you suggested seems most reasonable.

[TimFox]

I don't remember your answering the question about the impedance of your headphones.
Nor what load impedance your "high power" amplifier can tolerate.
Nor what voltage or power your headphones can tolerate.

[bdunham7]

So in the end either using a bucking transformer or just making a dedicated op amp headphone amp as you suggested seems most reasonable.

Dedicated amplifier might be the way to go, but I'm not sure why a transformer is a better solution than a simple resistive divider.  It's not as if you need to dissipate huge amounts of power.  It depends on the amp and headphones, but for the average stereo receiver and 32R headphones, just using a 16R in series with a 1R or 2R resistor should work.  Any solid state and most tube amplifiers will be fine driving a 16R resistor and you can choose the bottom divider such that the amplifier only needs to output a few watts to drive the headphones to full volume.  Actually with most modern solid-state amplifiers and headphones you'd be just fine with 32R and 4R or something like that.

[Zero999]

So in the end either using a bucking transformer or just making a dedicated op amp headphone amp as you suggested seems most reasonable.

Dedicated amplifier might be the way to go, but I'm not sure why a transformer is a better solution than a simple resistive divider.

It should have a lower impedance and therefore better damping factor, but I doubt that's any reason to use a transformer over some resistors, unless it's battery powered and the aim is to save power.

There would be one instance when a headphone transformer is mandatory, an old glassware amplifier with a live chassis. Even if it does have a speaker transformer, it probably won't have sufficient separation from the mains to be safe.