(Resolved)how NOT overdriving headphones from a high power speaker output stage? - Page 1
Forum section: electronics / beginners

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: August 21, 2023, 12:18:14 pm
content: Is there a way to limit the drive power into headphones from a speaker power amp?The typical resistor in series would ofc cause some wackyness because of the high impedance, so is there another way?

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 21, 2023, 12:37:42 pm
content: You may go from simplest to more complicated ways. For example, if you don't like a simple ballast resistor you may try a voltage divider, e.g. 91 R to 10 R or else values (it depends).

author: JohanH
author_rank: Frequent Contributor
timestamp: August 21, 2023, 12:41:46 pm
content: https://sound-au.com/project100.htm

author: CaptDon
author_rank: Super Contributor
timestamp: August 21, 2023, 01:21:37 pm
content: Google search perhaps??? As the other poster said a resistive voltage divider consisting of a higher value resistor feeding a lower value resistor which is in parallel with the headphones. That is a standard circuit used in almost every home hifi with headphone outputs. Surprised you didn't already have a solution based on your thousands of experiments already. B.T.W. if the lower value resistor across the headphones is one half the headphone impedance or less then the weird impedance vs. frequency of the headphones will be of very little consequence to the fidelity of the attenuator circuit. Google up L-Pad circuits also.

author: jonpaul
author_rank: Super Contributor
timestamp: August 21, 2023, 01:39:03 pm
content: Constant Zo T or Pi resistive fader, avail in 600 Ohm, 50 Ohm, etc.j

author: jonpaul
author_rank: Super Contributor
timestamp: August 21, 2023, 02:15:44 pm
content: Headphone make model? Zo? High Z, low Z, 600 Ohm?j

author: madires
author_rank: Super Contributor
timestamp: August 21, 2023, 02:38:05 pm
content: Add a headphones amp? Driving headphones with a high power amp and a series resistor is a waste of energy.

author: CaptDon
author_rank: Super Contributor
timestamp: August 21, 2023, 03:00:30 pm
content: Why is high output impedance a problem??? Tube amp and afraid of voltage spikes on peaks?? Please tell me how you plan to solve any impedance matching to present a lower impedance load on your amplifier without adding a load resistor? Yes, a resistive divider CAN show your amplifier a good impedance match AND drive the headphones at a lower level. Your question is just stupid. Are you going to run the amplifier at 50 watts of output and then drive headphones from it? The power has to go somewhere. Maybe explain why you can't just use a simple resistive divider??? I have solid state kilowatt amplifiers that work perfectly into loads of several hundred ohms. If you are afraid of spikes or load mismatch effecting a tube amp then you are still going to have to load it with something. Give us some idea of what you are actually doing, what size amp, how much output typically, can't you just turn the damn thing down, How about a typical 120vac to 6/12/24vac transformer to step down both the impedance and voltage to the lower level you want for your headphones........oops, but the load represented to your amplifier will still appear high so there is that nasty resistor again.

author: TimFox
author_rank: Super Contributor
timestamp: August 21, 2023, 03:33:41 pm
content: Quote from: ELS122 on August 21, 2023, 02:27:03 pmHow low a source impedance is required?What is the actual DC resistance of the headphones?How much power is required, and how much is too much?Maybe you should just use an op-amp instead of a high-powered amplifier.

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: August 21, 2023, 04:12:46 pm
content: Hi ELS122,use a transformer, of course wound with pure silver-wire at full-moon.Have fun

author: JohanH
author_rank: Frequent Contributor
timestamp: August 21, 2023, 04:26:38 pm
content: If anyone bothers to read the article in the link I posted above, there is a solution for low impedance headphones. Although it wastes some power in the parallel resistor. However, I doubt there is any better solution except using a dedicated headphone amplifier.

author: CaptDon
author_rank: Super Contributor
timestamp: August 21, 2023, 06:30:03 pm
content: It doesn't matter if you don't like the several similar answers already given, but resistors are the answer and that is no gaslight. Get handy with Spice, pick an input impedance for the H, L, or T pad to make your amplifier happy, pick a resistance in parallel to your headphones to normalize their so stated hard to drive variable impedance (probably 1 ohm would be good since you have ample power to waste) and then calculate the series resistor to provide the amount of attenuation you need. Nearly every hifi does it this way. They may have 50 watts per channel and when you plug in the headphones the headphone jack disconnects the speakers and drives the headphones from a resistive ladder. You still haven't stated why a high impedance at the input to the attenuator is a bad thing. When driven at a modest output level below the design maximum most any amplifier can work from the design output impedance clear up to a complete open circuit. The combination of negative feedback and damping factor should make any good amplifier capable of tolerating decreasing load / increasing resistance across the output. Any way you look at it you're going to be heating resistors if you need to maintain a load impedance on the amplifier output. "If you don't know the answer"....several here have offered answers that you don't seem to like.

author: CaptDon
author_rank: Super Contributor
timestamp: August 21, 2023, 06:37:04 pm
content: Your headphones 'must be driven from a low impedance source'. That one ohm resistor in parallel with your headphones assures they will be seeing a low impedance source!!! And the series resistor will set the desired attenuation level. The impedance shown to your amplifier (as long as it is above the minimum recommended) is meaningless. It won't matter if the attenuator input is high impedance or in this case high resistance.

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 21, 2023, 06:55:08 pm
content: Quote from: ELS122 link=topic=389710.msg5022460#msg5022460Resistor divider as for me is not 'a plain resistor' design, but ok.Seems you don't like compromises. So you just need a proper headphone amplifier. And you may try a transformer-based solution too.

author: TimFox
author_rank: Super Contributor
timestamp: August 21, 2023, 07:16:24 pm
content: Quote from: CaptDon on August 21, 2023, 06:37:04 pmA low impedance source is not a trivial requirement, since there are high-impedance headphones that can work from a high or medium impedance sources.(Around WW II, 2000 ohm "dynamic" headphones, similar to a traditional telephone transducer, were common with vacuum-tube receivers.)Note that the DC resistance of a headphone is a guide to what, quantitatively, constitutes a high impedance (plate of a 12AT7) or low impedance (op amp output).If proper electroacoustic design requires a low source impedance for damping, the internal resistance of the load will always be there.Once the source impedance is smaller than that value, diminishing returns sets in for reducing the source impedance to obtain damping.Pulling numbers out of the air, if a nominal 600 ohm load (including acoustic loading) has 200 ohms of internal resistance, there will be negligible difference between a true 20 ohm source impedance and a lower 1 ohm source impedance.(Of course, if your normal power amplifier is designed for an 8 ohm load, its source impedance will be much lower than 8 ohms.)For a voltage divider, you want to present a reasonably high resistance to the amplifier and a lower resistance to the headphones; amplifiers often have a minimum usable load impedance, and using that for one of the resistors in the divider will minimize the wasted power therein.

author: Manul
author_rank: Super Contributor
timestamp: August 21, 2023, 07:21:57 pm
content: I just thought I will add a very clear example of resistor divider impedance.

author: macboy
author_rank: Super Contributor
timestamp: August 21, 2023, 07:28:31 pm
content: Quote from: ELS122 on August 21, 2023, 02:03:11 pmThe output impedance of a resistive divider is the parallel combination of the high and low side of the divider. So if the low side has a relatively small value then the overall impedance is even lower.So for example 1k on the high side and 47 ohm in the bottom, you get 22:1 attenuation and less than 45 ohm output impedance. Many headphones actually assume a 100ohm output impedance and will have their best performance with that. So then 1k and 110 ohm is an option.

author: Zero999
author_rank: Super Contributor
timestamp: August 21, 2023, 08:13:03 pm
content: Quote from: Roehrenonkel on August 21, 2023, 04:12:46 pmI might get a bit of criticism for suggesting this. A small mains transformer will work quite well. The fact the primary is being driven at a much lower voltage means it'll work all the way down to 20Hz with no problem and I've found most work well high enough for audio.

author: magic
author_rank: Super Contributor
timestamp: August 21, 2023, 08:17:40 pm
content: Quote from: ELS122 on August 21, 2023, 02:27:03 pmWhat the hell is that thing? I can't find anything about it...Anyway, not many choices:- voltage divider, for low output impedance it may burn plenty of power but the amp probably drives 8Ω so what- transformer, a suitable audio rated one may not be easy to find, but perhaps any mains transformer could work(I agree with Zero000 that it's unlikely to saturate)- turn the volume waaaayyy down, protect the headphone with a thyristor-based overvoltage crowbar

author: switchabl
author_rank: Frequent Contributor
timestamp: August 21, 2023, 08:18:40 pm
content: Quote from: macboy on August 21, 2023, 07:28:31 pmDo you have a (modern) example for this? I'm really curious about this, since I have occasionally heard claims that there are headphones specifically designed for high impedance amps. But I have never been able to actually find one (let alone many).On the amplifier side, the industry consensus mostly seems to be that you should target <1 ohm output impedance (even on pro audio products which often used to have 20-30 ohms). This is maybe not surprising given how common 32 ohm (nominal) headphones, suitable for portable players, have become.

author: magic
author_rank: Super Contributor
timestamp: August 21, 2023, 08:26:11 pm
content: It's a myth based on some ancient standard that no manufacturer has ever admitted to caring about.A majority of headphones made in this century are driven from a single Li-ion cell in practice, maybe with a voltage doubler if you are lucky (for a negative rail, a cheaper and smaller solution than electrolytic coupling caps). In this environment, not much headroom is available for large output resistors so sources are understandably low-Z and headphones have to live with this reality or sound like crap.

author: RFDx
author_rank: Regular Contributor
timestamp: August 21, 2023, 11:55:00 pm
content: Quote from: ELS122 on August 21, 2023, 02:03:11 pmYou should explain why that is, preferably without any audiophool BS.Quote from: ELS122 on August 21, 2023, 02:27:03 pmNothing to be found.Quote from: ELS122 on August 21, 2023, 02:27:03 pmWhy 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?

author: EPAIII
author_rank: Super Contributor
timestamp: August 22, 2023, 08:00:00 am
content: 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.

author: Zero999
author_rank: Super Contributor
timestamp: August 22, 2023, 09:45:42 am
content: Quote from: RFDx on August 21, 2023, 11:55:00 pmUsing low value resisors would result in unacceptable power dissipation.

author: magic
author_rank: Super Contributor
timestamp: August 22, 2023, 12:51:59 pm
content: 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).

author: wasedadoc
author_rank: Super Contributor
timestamp: August 22, 2023, 01:49:04 pm
content: Quote from: magic on August 22, 2023, 12:51:59 pmProblem 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.

author: RFDx
author_rank: Regular Contributor
timestamp: August 22, 2023, 01:54:13 pm
content: Quote from: ELS122 on August 22, 2023, 10:41:53 amYes, the same audiophile that drives his headphones with a power amplifier for speakers. No power wasted there I guess.Quote from: ELS122 on August 22, 2023, 10:41:53 amWhat the hell are you talking about?

author: TimFox
author_rank: Super Contributor
timestamp: August 22, 2023, 01:59:38 pm
content: 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.

author: Peabody
author_rank: Super Contributor
timestamp: August 22, 2023, 02:02:26 pm
content: Here's a headphone amp module using the TPA6120. Does anyone have experince with that part?https://www.amazon.com/dp/B07QMWML2V/

author: magic
author_rank: Super Contributor
timestamp: August 22, 2023, 02:21:47 pm
content: 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.

author: donlisms
author_rank: Frequent Contributor
timestamp: August 22, 2023, 02:27:32 pm
content: 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.

author: CaptDon
author_rank: Super Contributor
timestamp: August 22, 2023, 06:30:25 pm
content: 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!!

author: BeBuLamar
author_rank: Super Contributor
timestamp: August 22, 2023, 07:16:53 pm
content: I use a 600 ohms headphones connected to a power amplifier designed for 8 ohm speaker it won't over power the headphones.

author: TimFox
author_rank: Super Contributor
timestamp: August 22, 2023, 07:26:14 pm
content: 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.

author: TimFox
author_rank: Super Contributor
timestamp: August 22, 2023, 09:16:43 pm
content: 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.

author: bdunham7
author_rank: Super Contributor
timestamp: August 22, 2023, 11:04:28 pm
content: Quote from: ELS122 on August 22, 2023, 09:11:02 pmDedicated 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.

author: Zero999
author_rank: Super Contributor
timestamp: August 25, 2023, 09:49:38 am
content: Quote from: bdunham7 on August 22, 2023, 11:04:28 pmIt 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.