Standard headphones on a bridged amp

On the older factory systems your would be exactly right. They only used a two channel amplifier and just changed the resistance from 0 to about 50 ohms in line with either the front or back sets of speakers to get the fading effect.
The modern ones use an independent amplifier for each set now.

Assuming the gain is set fairly high you could just use a dual 1K potentiometer (give or take) in series with headphone speaker lines. This would give you a wide range of adjustably.
But yes ultimatly there are loads of ways to do this. Some simple some more complex.

Car audio amps do not use a high power rheostat in series with the speakers as a fader. It would throw away too much power. Instead they use a variable attenuator at the inputs of the amplifiers.

I think it's better to use separate capacitors for each headphone because the point about DC between the channels makes sense.

As far as attenuator resistors is concerned I'm not sure: adding resistors would ruin the damping factor, yet omitting them could result in disaster if there's a short circuit. Most modern amplifiers are protected anyway so I'd be tempted to omit the attenuator resistors, if the amplifier isn't protected then I'd consider it to be mandatory.

Two points:

One - DC offset - if you've got 2V difference between the outputs on a bridged amp, then the amp is seriously faulty. Particularly on a car amp, where you've only got 12V to start with - but even on a multi-hundred watt PA amp, the difference should be WELL under 1V (usually it's only a few mV, and I wouldn't like to see it any higher than 100mV).

Two - use two capacitors, either feeding each live side of the headphones, or (to prevent clicks) two in series across the supply, with the bottom of the headphones connected to their mid-point.

I think 'DC offset up to 2 V' only applies to L-R case.

One thing is for sure, blocking caps are essential.


Are we all agreed to this below for a "Mk3 design"?

Amp L+ to +end of cap1. -end of cap1 (via attenuator if used ) to phone skt L

Amp R+ to +end of cap2. -end of cap2 (via attenuator if used) to phone skt R

Cap3+ to supply (e.g. "Antenna Out" pin on radio) Cap3- to Cap4- & to phone socket common
Cap4- to ground

I tried the resistor/capacitor combination today (between bouts of fixing horse fencing...electric and high tensile) and it seemed to work just fine.

I had thought about the volume control, but there's only so much space to mount things on a motorcycle, and knobs are generally hard to operate...you almost always turn it too far because of vibrations, bumps, etc. I built a digital push button volume control from a DS1802 (using a TLE2426 rail splitter to get + and - 2.5 volts to feed the chip), but it doesn't seem to work right everywhere. For example, sitting between the MP3 player and my CMoy amp, it works great. Between the MP3 player and the amp it does little until the final step when the volume jumps dramatically. At some point you just gotta say, "I'm just not smart enough to figure all this stuff out."

So I think I shall be content with the caps and resistors. I am hugely appreciative of all the help. If you ever need any advice on how to dig a posthole, I'm your man.

Car audio amps do not use a high power rheostat in series with the speakers as a fader.

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Whats your mailing address? I will send you a bunch of them from several different manufactures head units.
Ford, Chevy, Chrysler and about every other brand made between 1970 and 1990 had them! I have seen them in a number of foreign made vehicle head units from that era as well.

If you ever need any advice on how to dig a posthole, I'm your man.

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I got that coverd too.
I help plant about 400 -600 seedling trees a year and a tractor mounted three point post hole digger almost makes it fun!
Three people can easiy do 100 trees an hour! Even in the toughtest sod and hard packed ground!
The post hole bit tears that stuff into fine mulch in seconds! I would never plant a small tree with anything less!

In the olden days cars used many old parts (big high power rheostats).
Lately cars use an amplifier for each speaker and a high resistance low power potentiometer to adjust input levels or use digital switching.

audioguru said:

In the olden days cars used many old parts (big high power rheostats).
Lately cars use an amplifier for each speaker and a high resistance low power potentiometer to adjust input levels or use digital switching.

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Silicon being cheaper than rheostats nowdays. I guess one day in the future it'll be one single-channel amp that does all 4 channels, via a switching network.

tcmtech said:

I got that coverd too.
I help plant about 400 -600 seedling trees a year and a tractor mounted three point post hole digger almost makes it fun!
Three people can easiy do 100 trees an hour! Even in the toughtest sod and hard packed ground!
The post hole bit tears that stuff into fine mulch in seconds! I would never plant a small tree with anything less!

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Auger!? One hundred an hour?! What a luxury...that's living. I'm talking good old Virginia red clay and a digging bar and post hole digger. I'm at the foot of Great North Mountain so that means rocks mixed in with the clay. Several years back, I helped my neighbor dig some holes with a tractor-mounted augur. He sheared three bolts on three holes. When I was good at digging holes by hand, I could do three (3) an hour. That was cruisin'. I dug 75 by hand one week for a horse fence. On the last day, it was raining while I was digging. That was good, because no one could tell I was crying.

I live about 2km from a big river.
I have big flat stones in my yard the same as at the river.

What do you do when your post-hole-digger hits a pretty big stone?
One stone in my backyard is about 5m in diameter and 1m thick.

tcmtech said:

I got that coverd too.
I help plant about 400 -600 seedling trees a year and a tractor mounted three point post hole digger almost makes it fun!
Three people can easiy do 100 trees an hour! Even in the toughtest sod and hard packed ground!
The post hole bit tears that stuff into fine mulch in seconds! I would never plant a small tree with anything less!

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Wouldn't it be easier to just sew the seed?

Sew more than you need (to account for losses), water them and watch them grow.

What do you do when your post-hole-digger hits a pretty big stone?

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Move over a little bit!

When I was good at digging holes by hand, I could do three (3) an hour. That was cruisin'. I dug 75 by hand one week for a horse fence. On the last day, it was raining while I was digging. That was good, because no one could tell I was crying.

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Work smarter not harder and dont be such a cheap ass! spend a day or two setting up a tractor and auger system and you will never regret it. Whats not crying in the rain over holes in the dirt worth to you?
For me it was worth about $500 and three days of my time and I ,and my friends and neighbors, will never need to do it again in my life!

Actually I designed the three point system for the one tractor we use and it has a power down capability that can pick the back of the tractor off the ground! I also have carbide and high abrasion grade steel cutters on the auger bits. With the push of a lever I can, if absolutely needed, drill though most rock!
But when planting trees it not necessary so just moving over a foot or two gets around the rock.
Being where I live is mostly glacial loam soil we dont have many big rocks that cant be dug up or easily moved so it rare to have to deal with them to any great degree.

Wouldn't it be easier to just sew the seed?

Sew more than you need (to account for losses), water them and watch them grow.

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They are my dads trees. He likes planting those stupid pencil sized ones that take 20 years to get tall enough for any one to see them. (if they dont die first, which most do.)
I have enough brains to just use a machine to dig up a tree and transplant it! I am working on a design for my own tree moving truck I hope to build in a year or two so I can just pull a tree out from some place else and just move it.
Why wait 20 years to get a 20 foot tall tree when I can just spend a few thousand dollars and and a week building a machine and then have all of the 20 foot high trees I want just by moving them!

Lol, just noticed I had the wrong spelling, I meant sow, not sew. ;D.