Problem with test wiring for DIY ipad dock

[Cretster]

Hi all - this is a really cool forum you guys seem to have!

I've been interested in electronics probably about 25 years now in terms of pulling things apart and fixing radios etc when I was a kid. Used to make my own etched PCBs for projects but am very rusty on it all these days.

Anyway, I decided to build my own ipad/iphone charging speaker dock which will run the audio from either device into an amp and speakers.

I'm doing some basic wiring on the dock connector but am really baffled as I cannot get a charge through to the phone for some reason, so am hoping for some help. I hope this is the right subforum, but apologies if not!

Usually the phone or pad will charge with +5v from USB. So I am using this as a test source.
This is the dock connector I am using:

Now when I apply voltage from a USB connection (5.7v measured) to the red & black wires I have soldered to the board, the phone does not begin charging. If on the other hand I connect a USB iphone cable directly to the rear socket on the board then the phone starts charging no problem.

I have measured that in both cases, there is the required voltage reaching the relevant pins on the connector at the front, where the phone is connected, so I cannot fathom why it charges when I plug in the proper connector to the board, but not from directly applying voltage to the board, even though both situations give the correct voltage as going into the phone connector.

I'm sure this must be something noobish that I'm doing wrong here, but damned if I can see what, when I can see the required voltage right there on the same pins, but no charge commencing.

If someone could help suggest why this won't work I'd be really grateful!!

BTW - gnd can be found on the metal of the connector, on the solder at the left, the middle pins (15/16), and +5v is measured at the 3rd pin from the right in the group of 4 (pin 23).
This is a theoretical diagram for what should be where, and indeed is, but it's not working for charging somehow.

Many thanks
Jim

 

[timothy48342]

I could be that the iphone tries to connect as a USB device when you give it power through the red and black wires. But because your not giving it access to the data wires of the USB, it detects that there is a problem and refuses to take the power.

You could try giving it 2 more wires, data 1 and data 2.

This is obviously just a guess, but I've heard that some devices are made that way.

-tim

 

[KJ6EAD]

You probably need to put some voltage on USB pins 2 & 3.

http://www.ladyada.net/make/mintyboost/icharge.html

 

[Cretster]

Wow - you guys are quick, thanks!

Sooo, it looks like I need to fab up a little setup with a few resistors if I want to test it like this. Interesting and it explains it perfectly thankyou!

The reality for once this is built is that I'll be supplying charge power from the guts of an ipad 240vac charger anyway, so I guess I just sidestep this test setup and start using that right away, in order to avoid having to make something specially which would later be redundant.

This is the speaker dock at present in case anyone is interested:

It has a dock on top for the phone, and the pad will slide in to dock from the right hand side. The amp controls will be above the ipad. It has an integral subwoofer at the back.

So far so good until I ran into this charge issue!

Once it's all tested properly I will then make it look nicer with hardwood veneer etc.

 

[Cretster]

BTW - just in case anyone else looks at this in future, the USB Data pins on the iphone connector are 25(D-) and 27(D+).

 

[Cretster]

Update time!

I've got it all hooked up now so that both connectors are wired in parallel to an ac charger.
It works successfully so that the pad and the phone charge simultaneously.
There's a caveat though. If the pad is connected first then it charges ok, and the phone does when added second.
If I attach them the opposite way (phone then pad) the ipad states "Not Charging". Presumably it thinks there isn't enough juice left from what the phone is using.

That's ok though. This is with an alleged 2 amp ac charger. With a 1 amp charger I cannot get both to charge at the same time. So I will see if I can get a 3 amp one to see if it removes this selection issue. It's not a biggie though.


The problem I have got now though is audio.
Basically the line out connectors from the two docks are wired in parallel to a single 3.5mm socket on the amp. Like this:

The idea here is that by having the line out from the pad and the phone connected together to the amp at the same time, it means if there is an audio event like an alarm for example on one device but not the other, I won't miss it as the audio will work from both at the same time.

I realise this means music potentially could be played on both at the same time but that is controlled by, well, not doing that.

The problem is that the amp doesn't seem to 'behave' that well with the two sets of signals like this and I don't know how to refine it. To elaborate, if I play music on the ipad, then start music playing on the phone & pause the ipad, it doesn't play correctly from the phone, or rather it's mega quiet for a short while then it becomes louder. Was almost sure I heard crackling from it too but I can't be certain.

Any ideas on how to improve the way this behaves in respect of the audio outputs, without getting too complex?

Cheers!

 

[CocaCola]

Google

passive audio mixer circuit

or

active audio mixer circuit

 

[Cretster]

Perfect - many thanks.

 

[Cretster]

Well I came a cropper last night, frustratingly.

I got the audio working much better after your suggestion (just adding a 4.7k resistor pair to each output at this stage), so I was really pleased that both devices dock and charge correctly, and the audio outputs reasonably well.
Then disaster happened. I've no idea what I have done, and I cannot find any evidence of any damage on the board or to any components, nor any shorts/bad solders etc, but the right channel has suddenly begun producing a very loud offensive hum instead of music coming through it.

I'm not sure if this means the amp is toast or if it's likely to be a simple fault that's easily recitified but it's pretty disappointing to have got to the point where it was more or less ready to install properly into the cabinet, then have something go wrong like this.

I'm afraid my level of electronics ability is not sufficient to know what to start looking at/testing, but if anyone can suggest likely causes I'd be grateful, thanks!

For what it's worth, this is the amp board. I don't have a schematic though:

 

[CocaCola]

Is it the amp that is making the noise? Hopefully it's not the devices as when you 'Y' connected them you very well could have damaged the outputs...

 

[Cretster]

Sorry, I should have worded that better.
The hum comes from the speaker connected to the right channel.
I've swapped the speakersto confirm it's the channel on the amp rather than the speaker itself at fault in any way.

This happens with no input signal applied (or with!), but with the speakers connected. The hum noise does vary in volume according to the volume control position.

I'm trying to remember the precise sequence of events for how this happened but it did actually work correctly after doing the resistor/mixer mod, albeit briefly. I remember thinking it sounded a lot better than before, and observing that the volume increases a little when one or other device is removed from one of the docks.

I may then have done something that's sabotaged it with soldering or suchlike but I can't quite remember if I did anything further like that. It's possible there's a tiny short on the circuit somewhere from soldering but if there is, I can't find it so far.

Anyway, I've ordered a cheap amp from ebay china which looks more or less the same spec/size/approx components, and uses the same dual 12v ac input that the transformer I've got is supplying. Presume it's a pretty generic budget amp design or a variation on a theme. I'm expecting it'll be more or less a like for like piece of kit.

Frustrating that I have to wait now 2-3 weeks for that to arrive but in the meantime I can concentrate on woodwork of the cabinet etc. and the installation of the rest of the wiring to make it neater.

 

[Cretster]

This is what I've ordered as a replacement but I'd still love to figure out what is up with this one if that's feasible without spending tons of time on it.
http://www.ebay.co.uk/itm/220949326118?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

 

[BobK]

I hope you noticed this line:

For use,you need to add matching heatsink to this power amplifier while working.

If you run this without a heat sink it is not going to last long.

Bob

 

[Cretster]

Cheers - don't worry though as I've got assorted heatsinks knocking around, and loads of alloy I can machine if need be.

 

[Rleo6965]

Do you still have shorted 2 outputs of Ipad and Iphone to the 3.5mm jack. You will damaged output of 2 device. You should placed 2 pcs DC coupling capacitor in series to both output wires for left and right channel..So, you need a total of 4 capacitor for 2 device or audio sound source.

 

[Cretster]

Thanks for the info on capacitors. I didn't see that in any of the passive mixer circuits I looked at though - what is the purpose/function of them when used like this?

Hope I'm not being dumb here (probably!) but I don't know what "DC coupling capacitor" is. I know about ceramic ones and electrolytic ones etc, but not heard of those before.
What sort of value would be needed?

 

[Rleo6965]

DC Coupling capacitor will isolate the 2 audio device to avoid damage or shorting it other. The capacitor also act as "ac resistor" for audio frequency. We call it impedance. . Thus audio output would not likely to overheat.
Unlike your your original circuit using direct wiring that short output of 2 device. Maybe 10 mfd / 12V electrolytic capacitor can be use.

It's better if other Senior Member to give exact value of capacitor.

But it still advisable to have a passive mixer and much better an active mixer before the amplifiers.

You can try this simple single transistor circuit for single channel only. You need to build 2 identical for stereo or 2 channel.
http://www.circuitstoday.com/single-transistor-audio-mixer

 

[Cretster]

Thanks for the info again.

I think what I meant to ask last time but worded it badly was more along the lines of what is the benefit/difference to using these capacitors instead of the resistors I've added?

 

[Rleo6965]

You can use resistor or capacitor . But its safer for the capacitor if there is a dc component on output of the 2 device. Since we don't know exact output circuit of the 2 device. Its better on the safe side. Your 2 device might be expensive.

 

[Cretster]

Ahh, I see. Thanks for explaining - I understand now.