I went for a longer flight at the weekend and decided to play some music from my iPhone through the GMA340 but unfortunately the volume levels are too low – it appears the input of the Garmin wants a 1K ohm input level whereas the headphones impedance is only 8 Ohm. I was therefore looking to rig up a pair of impedance matching audio transformers and saw Radio Shack used to offer Part number 273-1380 but these have since been discontinued. Anybody any ideas where I can find equivalents / get some here in Europe? I tried the usual suspects (Maplin, Conrad, ebay, etc) but haven’t found anything. Any ideas?
The value in ohms is the load, not the signal level.
If the iphone doesn’t output enough of a signal, I would investigate why, because there is normally plenty available. I can drive the music input of my PS7000 intercom from just about anything with a 3.5mm headset jack socket, including any phone.
Can an iphone really drive 8 ohm headset speakers? I didn’t think the typical smartphone headset is 8 ohms, though of course that is a common impedance for normal size hifi speakers. I have just measured one smartphone headset and it is about 25 ohms.
What connections are you making? The iphone will be outputting stereo but the intercom input may be only mono, in which case it needs to be done with two resistors.
IIRC the iPhone’s output is for driving 32 ohm headphones. The headphone jack is usually entirely adequate for a line in (and the levels are usually far too high if you turn the volume up, rather than the other way around).
The GMA340 music input is a 460 Ohm load as is looking for 500mV to generate a max output. If your GMA340 has Mod 5 embodied (look at the data plate mod record – should be on all units built from May 2002) then it can have a 20dB gain increase for the music by grounding pin J2-15. This can be hard-wired or use an external switch. If the gain increase is enabled it increases back-ground noise as well!
wigglyamp wrote:
The GMA340 music input is a 460 Ohm load as is looking for 500mV to generate a max output. If your GMA340 has Mod 5 embodied (look at the data plate mod record – should be on all units built from May 2002) then it can have a 20dB gain increase for the music by grounding pin J2-15. This can be hard-wired or use an external switch. If the gain increase is enabled it increases back-ground noise as well!
This is exactly the reason I wanted to use an impedance matching transformer as the GMA340 has Mod 5 embodied but I don’t want to increase the music levels whilst also increasing the background noise.
For all others: I connected an iPhone 6S Plus using a 3.5mm plug to 3.5mm plug cable to the input in the Cirrus (stereo input) but I had to turn up the volume on the phone way up just to be able to barely hear something. I tried with my Galaxy Note Edge – exactly the same.
stevelup wrote:
Try this eBay seller:-
Cheers, just what I wanted :-)
I reckon there is something funny going on here e.g. a shorted output on the Iphone.
The headset output of a normal smartphone should be ample.
What exact connections are you using, Steve6443?
I use an iPhone 4 or iPad with a GMA340 with no problem at all. I do have the 20dB gain strap hard-wired but don’t see any appreciable increase in noise at all. As Peter said, a standard input should work and it looks like you have a specific fault. We’ve installed loads of GMA340s with music and not had any issues.
Peter wrote:
I reckon there is something funny going on here e.g. a shorted output on the Iphone.The headset output of a normal smartphone should be ample.
What exact connections are you using, Steve6443?
I tried numerous devices- for example, I want to use an iPhone 6S Plus using a straight through 3.5mm <→ 3.5mm cable (approx 50cm long) to take the output from the iPhone and plug this into the audio inputs of the Cirrus – I tried both inputs ( Input 1 is in the centre armrest, input 2 is between the rear headset connectors.)
I had to turn the volume levels up pretty high in order to have any reasonable volume in my headphone; looking through the handbook, I didn’t see any method of adjusting the volume of the music hence I switched to an old iPod (80GB Classic) and then to a Samsung Note Edge – in all cases, the volume was extremely low which leads me to believe that the impedance levels need matching which is why I have just ordered 2 × 8 ohm : 1000 ohm impedance matching transformers and will create my own adapter.
For info, I have a portable radio which has a ‘line out’ function as well as a headphone output – connect the line out, volume levels are good. Connect it to the headphone sockets, the music level is low…..
