Standard headphones on a bridged amp

[chilinski]

On my motorcycle I have a small bridged amp listed at 35RMS driving two 3-inch speakers. Above about 60 mph, there's not enough volume to hear the speakers. Also, at times, I'd like to listen to music/radio/books through my helmet headset. Is there a way to use standard headphones on a bridged amp without having to rewire both sides of the headphones so they wouldn't have to share ground? Since we're talking motorcycle here, I'm not tremendously concerned about high quality audio reproduction. While searching the "Great Void" of the internet I saw a suggestion for using 1:1 output transformers on each leg of the speaker outputs, and that would then allow me to use standard headphones with a shared ground on the headset side and avoid having to rewire headphones. I suspect I'd need some sort of attenuation, too, going to the headphones...and probably a switch or two to change from headphone to speaker. Is this way too involved and complicated? Is there an easier answer? Maybe in the end, it would be easier to have a separate headphone amp along with the amp for the speakers. Any suggestions?

 

[marcbarker]

Yeah, basically wire the headphone L & R.... to Amp outputs L+ & R+ respectively, via 100R attenuations. Couple the headphone common to ground via a capacitor, say between 100 uF to 1000 uF. Cap polarity negative ground.

Don't bother with a switch, just have a headphone socket

 

[chilinski]

Let me make sure I understand. I should forget the transformer, wire the signal side of the headphones directly (with resistors) to the R+ and L+ speaker outputs from the amp. Wire the headphone common ground to bike chassis ground (with capacitor). What do you mean by "cap polarity negative ground?" I think you mean to basically ignore the negative side of the amp for purposes of the headphones, but it could also be a way of saying to tie it to chassis ground through a capacitor.

For purposes of education, would you explain the capacitor on the ground side of the headphones does? Why would one choose a low number (100) versus the high number (1000)?

 

[mneary]

He's saying ignore the amp negative side for the purpose of the headphones. Connect the + side of the capacitor to the barrel (ground side) of the headphone jack and the - side of the capacitor to the bike chassis or the radio - line.

The choice between the low number (low value) capacitor and high is a trade between size and bass response. I would go for the smaller capacitor.

The first time you plug it in, do not be wearing the headphones. There will be a very loud CLICK.

 

[Hero999]

I think this is a bad idea because there's a serious risk of damaging your hearing. At high speeds the wind and engine noise is already too loud to be safe for long journeys and wearing headphones with music loud enough to drown this out is bound to result in hearing damage.

Headphones - Wikipedia, the free encyclopedia

Using headphones at a sufficiently high volume level can cause temporary or permanent hearing impairment or deafness due to an effect called "masking." The headphone volume has to compete with the background noise, especially in excessively loud places such as subway stations, aircraft, and large crowds. This leads to the disappearance of the normal pain associated with higher levels of volumes.

There's also the risk that you won't be able to hear important sounds such as emergency vehicles or a car horn.

Other risks arise from the reduced awareness of external sounds—some jurisdictions regulate the use of headphones while driving vehicles, usually limiting the use of earphones to a single ear. The complete isolation from outside noise can be a hazard in itself, as a user could miss the sound of a car horn and walk into traffic with fatal consequences. Losing situational awareness can also lead to theft, particularly in busy environments where bumping into another person would be ignored, e.g., subway stations.

 

[audioguru]

In Canada most motorcycles do not have any mufflers. They make such a racket that their drivers must be deaf and do not hear how much noise they are making.

 

[Hero999]

All of the bikes I've have had, had a silencer otherwise they wouldn't have been legal.

Two stroke engines are noisy anyway but I've only had four strokes which aren't that noisy.

I think two strokes should be banned, except for hedge strimmers because they're too noisy and dirty.

 

[audioguru]

Most motorcycles here are 4-stroke. They go "blat, blat, blat, etc" without mufflers (silencers).
Some diesel trucks also do not have mufflers (roar).

 

[tcmtech]

There's also the risk that you won't be able to hear important sounds such as emergency vehicles or a car horn.

Dont they use those bright blinking and flashing lights on the emergency vehicles just for that reason?

As far as wind noise on a motor cycle it just depends on the bike aerodynamics. Some have a calm bubble of air that the rider is in.

As far as the head set wiring, you guys have it back wards. The capacitors go in line with the + speaker wires and the head set common gets tied directly to the radio ground. The other way you have poor stereo separation and a low but possible chance that if the head set wires managed to get pulled out and shorted to ground you could burn the radio amplifier IC's out from shorting a floating voltage source to ground. The capacitors will prevent that from happening.

I would skip the attenuation resistors myself and go directly to the speaker + lines them selves with a 100 -300 uf 25 volt capacitor for each channel.

And If you think it will be to loud just use that volume control thing. I hear they are included free in the design of radios just in case someone would want a lower listening volume.

What a person does to themselves is their business and not yours. If you drink, smoke, do drugs or what ever shut the hell up about someone else's potential for hearing loss. Get off your soap box and deal with your self abusive tendencies first.

If he has the wrap around head sets or built in helmet sets like some of the motor cycle operators around here have for person to person communications the volume will not to be loud.

And some of us have the loud exhaust on our bikes, vehicles, Semi's just so we can piss off the old grumps who think everything is too loud.

 

[Hero999]

Dont they use those bright blinking and flashing lights on the emergency vehicles just for that reason?

Tell me exactly how you're supposed to see those round a corner?

What a person does to themselves is their business and not yours. If you drink, smoke, do drugs or what ever shut the hell up about someone else's potential for hearing loss. Get off your soap box and deal with your self abusive tendencies first.

I completely agree with you, what he does is his business. However I do feel duty bound to warn him of the dangers of using headphones whilst riding.

I know people go on all the time about the dangers of headphones and most of it is unfounded. Someone listening to headphones in a quiet place such as a library is unlikely to select a loud enough volume to cause any damage, it's the masking effect in noisy environments that causes the danger.

 

[tcmtech]

Tell me exactly how you're supposed to see those round a corner?

Get to the corner and look both ways! Realistically one of you is going to get to the corner first and see the other. Emergency vehicle drivers cant just blindly drive through intersections without looking for traffic too!

If your in a some what obstructed view area most people have enough common sense to pay more attention. Until something happens to me I generally dont worry or concern myself with it to any degree. Its just wasted effort and stress for no reason.

He builds it, he operates it, he pays the fine for misuse. All in all its not my problem.

 

[chilinski]

I really only wanted the answer to the question about how to wire the headphones. I didn't want to set off an "acrimonium."

As of right now, I have two suggestions: 1) resistors on the + wires going to the headphones and a capacitor on the headphone common to ground and b) capacitors on the + and no resistors. These seem to be quite different to me...I guess I should just take a breadboard out to the bike and try it both ways and see what works and figure it out for myself. There's no pulpit out there, so at least it will be quiet.

 

[marcbarker]

What about if you are DEAF anyway? Deaf people can still drive. They lip-read things like traffic signals and blue flashing lights. And when I used to travel home from a Motorhead gig, having spent the evening with my head inside the bass bin and still not loud enough, I could still ride my GT750 2-stroke motorbike OK (it was strangely quiet that night). OK, yeah so the next few days I kept picking up the phone and no one was there, because of the ringing in my ears, but my hearing came back!

 

[marcbarker]

1. The capacitors go in line with the + speaker wires and the head set common gets tied directly to the radio ground.

2. The other [marcbaker suggested] way you have poor stereo separation

3. if the head set wires managed to get pulled out and shorted to ground you could burn the radio amplifier IC's out from shorting a floating voltage source to ground. The capacitors will prevent that from happening.

4. I would skip the attenuation resistors myself and go directly to the speaker + lines them selves with a 100 -300 uf 25 volt capacitor for each channel.

5. And If you think it will be to loud just use that volume control thing. I hear they are included free in the design of radios just in case someone would want a lower listening volume.

I'm going to take issue with the above

1. yes, that's another valid way of doing it. If so, you require 2 capacitors, not one.

2 Please tell us how do you come to that conclusion? Maybe you know something I don't, if so I'm interested.

3. There's a 100 Ohm in series don't forget. How many ohms is the 'short-circuit' worth please? Again I'm interested.

4. Yes, you could omit the resistors and save a few grams weight and cents cost, but the resistors prevent overloading the amplifier (if a short circuit), prevent overloading headphones if volume accidently set to max, and another subtle reason, it improves the signal / noise ratio when listening at low volume, or in other words the noise floor is more noticeable with headphones, so the attenuators push the noise floor down.

5. I agree!


PS about the "loud click". Yes there will be a click, but the 100 Ohms prevents the headphones from popping. Even if you connected the headphone to the power supply, it'd still be quieter than a drummer hears hitting a snare drum. Yes, I have tried it. Both.

To sum up, I'm suggesting:

Run a wire from L+ output, via 100 Ohms 1 Watt, to headphone socket L tag.
Run a wire from R+ output, via 100 Ohms 1 Watt, to headphone socket R tag.
Run a wire from ground to - of a capactor. + of cap to headphone socket Com tag.

the choice of capacitor value is not that critical. It's like making cake mixture, if you prefer less bass, use a lower uF value, if you want to hear the switch-on click to full decibels, then make the value large. I'd go for a 220 uF, because I like the number.

 

[tcmtech]

My point was that being that the system is assumed to be a floating ground you would want the two capacitors on the input sides because without them there is a fair chance you will have an unbalanced voltage differential from one amplifier positive output to the other.

I have worked with car audio systems that used floating ground amplifier IC's for over 20 years now and I have seen many of those type of amplifiers with a good volt or more difference between the different amplifier channels. That is the left channel floating point voltage is a volt or more above or below the right side. Given the small wattage capacity of head phones passing only a volt or so of DC through them can damage the voice coils.

Also if the common conductor of the headphone leads gets touched/ pinched to any part of the motorcycle frame both left and right head phone speakers get raised to the floating point voltage and fry. Given a 100 ohm resistor in series with an assumed 32 ohm head set speaker will pass about 53 milliamps or .37 watts at 7 volts it doesn't sound like much but considering many headphone speakers are only rated at several tens of milliwatts getting 90 milliwatts of DC passed though them may be too much for them to take.
Hence the reasons for capacitive coupling the two sides independently.

As for the inline resistors I dont care if they are there or not. In portable stuff I had years ago with head phone outputs the attenuation resistors were the first to go! Why run the volume at 10 and with much clipping distortion when running without the resistors would give the same volume at 4 and with no clipping. Personal preference mostly but liking to have good sound quality at fair volume was my thing.

Just from practical experience I have come to know that capacitive coupling any type of circuit to a DC voltage should be done on the floating side and not the point considered to be the common side of the system.

As far as the on off popping it just happens regardless of where the capacitor is with type of system set up. Just dont have the head phones on before you turn the radio on or off. And dumping a 100- 300 uf capacitor through then is not that loud any way.

As far as I know most of the small portable entertainment devices still use a capacitive coupled amplifier system to drive the head phones. (mine do any way) And I am pretty sure that on/off pop is not whats making kids loose their hearing!

 

[marcbarker]

I've worked with Bridge Amps too, since early 1980's. Yes, there can be a difference of voltage between L+ and R+, though I'd be suprised if it was as high as 2 V.

However, the good thing about the 100 Ohm attenuators, is that with the 2 V / 200 Ohm, means = 10 mA total current (=0.16 V on each phone, assuming they're 32 ohm). It would bias the headphone transducers, but I think you'll agree it'd be unnoticeable. And certainly nowhere near "1 volt".

However, without the 100 Ohm attenuation, with the L & R connected directly to to L+ & R+ (with or without via DC blocking capacitors), remember there is an overloading problem if volume was already set high at the time phones plugged in. The phones might pop if plugged (probably wouldn't).

As for where the capacitor should go, if the ground common of the headphone was scratching against earth, there would be a loud crackling in the headphones and the user will likely switch off. If the series attenuators are present, I assert the phones won't be damaged. (I'm suggesting attenuators to reduce risk of overloading if human error in volume setting).

Who's talking about "running the amplifier at 10 and much clipping distortion"? Not me. Running the volume at "normal" setting, there is a 'normal' amount of distortion. However, with speakers pressed against either side of your head, it requires attenuators, because without attenuators, and with speakers pressed either side of your head, you are more likely to hear the amplifier's noise floor. And the 'switch-on pop' is going to be unattenuated too. Besides, the amplifier distortion isn't going to be noticably less at lower than normal setting, if anything it will be higher THD (x-over distortion for example). Also there is ergonomics, when set to normal volume and you plug in headphones, you expect 'normal' sound level. Maybe leaving out the attenuators will improve the amplifier's damping performance on the headphones? I don't think headphones benefit from damping do they?

With a 10,000 uF capacitor there would be a double pop that is quite loud. With a 0.1 uF, it would be a switch-on click. Neither seems to be a practical risk to hearing.

To sum up, I'm suggesting:

Run a wire from L+ output, via 100 Ohms 1 Watt [and a capacitor], to headphone socket L tag.
Run a wire from R+ output, via 100 Ohms 1 Watt [and a capacitor], to headphone socket R tag.
Run a wire from ground to headphone socket Com tag.

 

[tcmtech]

You stated your reasons and I stated mine. To be honest I have had no reason to need to build headphone based anything since my late teens for what its worth. And over the years I have gotten out of doing any audio work for anyone but myself simply because of all of the misinformed, uneducated and self riotous nuts that are associated with it.

I have learned to not even try to get involved with most audio threads here just for the simple reason there are too many experts that know they have it all figured out.
For some odd reason I screwed up and popped my two bits in on this one.

It gets old and pointless to attempt to argue with them and I dont have the want, need or care to waste my time on pointless run around.

I was simply stating what I have seen and found and why I chose the method I prefer to use. I dont think either is a wrong way or a right way but rather just a preference. Each may have an advantage over the other in certain aspects but ultimately I really dont care what gets done.

 

[marcbarker]

There's always a different way to do something, what's right for someone, is less than right for someone else, it just shows the diversity of engineering don't it Chances are if the positions were reversed and you had said "it's ok to have unblocked DC on the headphones", I'd be suggesting "needs blocking caps". I've added blocking caps by the way.

Yeah audio is so excessively 'purist'. The audiophiles used to buy all this fancy equipment, gold-plated wires, $500 speaker cables, clean the stylus after every LP, carefully measure and place the speaker separation distance etc., But the funny thing is in the recording studio where they create the music, they use loudspeakers or headphones as microphones, severely overload VU meters, send vocals through guitar effects pedals, and generally misuse the equipment to distort the sound.

 

[chilinski]

I'm almost afraid to mention this because it might start something else. But here it goes:

The amplifier is buried in the bike and its volume control is inaccessible. Volume is controlled on the mp3 player feeding into the amp. The volume on the amp is set fairly high...for the sake of education, let's assume it's at max.

Does that change the suggestions any?

 

[marcbarker]

I agree now with what tcmtech says, have capacitors inline with each phone.

If you choose an attenuator, make it variable? (i.e. a "speaker volume control") It would add extra complication, but might be value added.

Inside just about every car audio equiment (as far as I know, Tcmtech, is this right?), there is a front-back speaker fader control knob, which is a variable air-cooled wire-wound resistor attentuator in series with either the front or back speakers, depending which way you turn the knob.