Can someone remind me how to do a simple job please?
I have a motorcycle (K1200LT) with loudspeakers front and rear, with independant volume controls. These are stereo fed from a Becker readio cassette unit up front.
I want to provide mono (L+R) feeds into a helmet without stuffing the amplifiers or affecting the loudspeaker output. So do I use a couple of capacitors from the two lives or what? It's so long since I played with this sort of thing.
It would be easier to just wire up in stereo from the front speakers. Is there a reason you can only have mono in the helmet? If so, just running off the left would be easy, and don't play any Queen!
Christian.
use a couple of 220ohm 5 watt resistors from the stereo outputs to a mono headphone output.
Hopefully - but its by no mans a dead cert, the earth side of the stereo outputs is actual bike earth.
If they are not, use 4 x 100ohm resistors - two from each live to 'mono live' and two from each 'other live' to the headphone earth..
Even if not (and probably not, unless it is an ancient low wattage stereo), a single headed output to earth is likely to be acceptable, although there might be some DC component that needs filtering out by a cap.
Christian.
Agreed, but an extra resistor is cheaper than an extra capacitor.
Actually, with amplifiers powered from low voltages, you can't always assume that one side of the speaker is earthed.
In order to get high volume levels, some manufacturers use 'bridged' amplifiers where each terminal of the speaker is driven in anti-phase to the other, so the maximum voltage swing as seen by the speaker is twice that of the battery.
They use two anti-phased amplifiers to drive each speaker, but as the amplifiers can be built for tuppence, that's considered a good trade off.
If using 220 ohm resistors they needn't be anything like 5 watts - even assuming an 'HT' rail of 12 volts and a dead short. Standard 0.6w metal oxide types will be fine.
Because you cant asume the amp isnt bridged output you need to use 4 resistors. Using 2 may kill it.
Amp L- to helmet - Amp R- to helmet - Amp L+ to helmet + Amp R+ to helmet +
via one resistor in each case. 100 ohm probably better than 220 in this case.
NT
The "HT" rail is quite likely to be in excess of 12V, especially if the power output is reasonable (i.e. much in excess of 10W rms/channel).
Christian.
No - the usual way in a car radio is via bridged output amps if more than approx 6 watts. Some earlier designs improved on this with an output transformer.
Normal transformerless outputs from 14 volts into 4 ohms are actually less than 6 watts, though, regardless of ad speak. Bridging those can give approx 3 times the power output still into 4 ohms.
Higher power amps use a switch mode voltage booster to supply the bridged output amps.
NT
Indeed - but not likely on a motorbike?
That is precisely why I said all that.
Fule.
I din;t feel the need to display my experience of bridged audio amplifier design either.
>
Yeah...I just grabbed that from my days of shoving headphone sockets on
100W amps and 400W bridged amps..
Anything is likely on a motorbike. The whole concept of a motorbike is unlikely to start with.
Its probably a GoldStar with a 400W amp in one of the panniers....
As per previous post, it is a BMW K1200LT with factory fitted audio system. It is a Becker radio/cassette unit with dashboard remote control. It feeds 4 speakers and each pair has its own feed.
Well it looks like it will be 4 off resistors then. I have got a Starcom intercom unit coming which has an isolated speaker takeoff point. I have asked the manufacturers if the unit can be switched into mono, which will make it easy.
Many thanks for all the suggestions. I really should have known the answer myself, but old age fuddles the brain!
An extremely good reason!
I tried it with a portable Minidisc player and some tracks sound VERY unusual. It's interesting to note that some vocals seem to be exclusively on one channel.
Not sure about this one, but I believe it is the same or similar as fitted to BMW cars. E36 has some meaning apparently. The manufacturer is Becker.
The E36 is a 3 Series made from approx 90-99. If the radio is an all in one unit - most likely for a motor bike - it will likely have four speaker outputs each of 18 watts, That's been pretty well the standard for some time. Higher power ones usually have external amps, but I don't think this was factory fit on an E36.
Have something to add? Share your thoughts — no account required.
Ask the community — no account required