Hi!
I am about to purchase a car audio system. The usual way were to buy a head unit, front speakers, amplifiers and a sub. I have however a problem with head units. As a source I use an iPod, all my music ripped an available whenever I want. So, I need a head unit with iPod connectivity. What do I get? If not for Alpine IDA series I get a CD drive I would not use. I get a radio I would use once in a month. I get inbuilt amplifiers I don't need with external amplifiers. And I get an iPod interface which is never as good as the original structure of the iPod. What I would use is a volume control and pre-outs.
Well, I could do that...Or at least I have built several pre-amps for home use. My problem is, that I have just discrete, class A preamps built with symmetrical power rails and now I have to make a chip based preamp around the single power supply of my car.
I would like to have suggestion as to which IC I should use for these application.
Thanks in advance.
I am about to purchase a car audio system. The usual way were to buy a head unit, front speakers, amplifiers and a sub. I have however a problem with head units. As a source I use an iPod, all my music ripped an available whenever I want. So, I need a head unit with iPod connectivity. What do I get? If not for Alpine IDA series I get a CD drive I would not use. I get a radio I would use once in a month. I get inbuilt amplifiers I don't need with external amplifiers. And I get an iPod interface which is never as good as the original structure of the iPod. What I would use is a volume control and pre-outs.
Well, I could do that...Or at least I have built several pre-amps for home use. My problem is, that I have just discrete, class A preamps built with symmetrical power rails and now I have to make a chip based preamp around the single power supply of my car.
I would like to have suggestion as to which IC I should use for these application.
Thanks in advance.
When Maxim purchased Dallas Semiconductor they inherited some nice single supply digital potentiometers -- push up/down -- Maxim's website is www.maxim-ic.com You could couple this with a single supply IC from Maxim or TI -- TI has a good working paper on making single supply opamps work.
TI has a series of digital pots with gain -- but you need to interface with a PIC, Stamp AVR etc. I have found that simple bit-bang routines work, but if you're not into micro-controller programming go with the Maxim products.
TI has a series of digital pots with gain -- but you need to interface with a PIC, Stamp AVR etc. I have found that simple bit-bang routines work, but if you're not into micro-controller programming go with the Maxim products.
Hi Banfi,
I'm working on such a circuit at present, for my own needs.
I'm using SSM2135, which is very good sound quality for a car. It can run off a single supply, which in my case will be a linear regulated supply of 8 or 10V from the 12V car supply.
As you say, an analogue potentiometer should be used here, since you just need to trim the level just once to fine-tune the
ipod volume level to the head unit. My circuit will be designed for an adjustable gain of 1-11, which is more than adequate.
I'm documenting it at http://360.yahoo.com/shabaz_yousaf
I'm working on such a circuit at present, for my own needs.
I'm using SSM2135, which is very good sound quality for a car. It can run off a single supply, which in my case will be a linear regulated supply of 8 or 10V from the 12V car supply.
As you say, an analogue potentiometer should be used here, since you just need to trim the level just once to fine-tune the
ipod volume level to the head unit. My circuit will be designed for an adjustable gain of 1-11, which is more than adequate.
I'm documenting it at http://360.yahoo.com/shabaz_yousaf
Did you try to use the headphone jack as preamplifier output? It should have sufficient output swing and you could use the iPod's own volume control. Adapters should be easily available.
If you absolutely need a preamplifier, you can use nearly any op amp (NE5534A, OPA134, ...). All you need is a virtual ground, which you create with a rail splitter. That can be as simple as two resistors or more advanced with an additional transistors (e. g. see single supply application in datasheet for LM3886). Rail-splitters are also available as IC, e. g. TLE2426. With a rail splitter you always need DC blocking capacitors at in- and output.
Or you use the battery-based dual power supply option like on portable headpone amps.
If you absolutely need a preamplifier, you can use nearly any op amp (NE5534A, OPA134, ...). All you need is a virtual ground, which you create with a rail splitter. That can be as simple as two resistors or more advanced with an additional transistors (e. g. see single supply application in datasheet for LM3886). Rail-splitters are also available as IC, e. g. TLE2426. With a rail splitter you always need DC blocking capacitors at in- and output.
Or you use the battery-based dual power supply option like on portable headpone amps.
You could use a dc/dc converter, to get your +/- 15v rails from the 12v supply available. I've used the minmax mau119 (less than $5US) with good results and the slightly larger regulated version with even better results. They are all fairly inexpensive and available here http://www.cdistore.com/minmax/ then you can use any traditional split rail design that you want. Just be careful to get the right input/output filtering on the dc/dc.
The biggest advantage of that over a single supply system is less chance of noise since you have a new isolated ground, just incase your ipod is referenced to the cars chassis ground if your also charging it while you drive.
The biggest advantage of that over a single supply system is less chance of noise since you have a new isolated ground, just incase your ipod is referenced to the cars chassis ground if your also charging it while you drive.
Hi rwaudio,
Absolutely. However, it is possible to go further:
Basically, I used a single supply rail (not +/- rails) to minimise
converter noise for my first stage. This stage increases the
iPod level from its low level (maybe 0.5v) to the desired level.
To ensure no ground noise, we need to ensure that the cable
from this amp to the iPod has very low resistance (easy to
control, because this is in our hands, it is not existing car
wiring). We can achieve it using substantial cable, and/or keeping
the cable length short. We can consider it a single ground point.
However, as you know, this does not prevent noise appearing
on that single ground, because it needs to be connected to
the car ground somewhere, and there is bound to be plenty
of noise as a result. So, my second stage uses a +/-15V supply
as you say.
Since this supply is at the same ground as my first stage,
my supplies will track the 0V+noise by +/-15V. At this stage,
I'm less concerned about converter noise, because my iPod
audio has already been amplified by a factor (I needed very high
voltage levels).
My second stage gives a balanced output.
Its not a perfect solution, but I think its the best compromise
so far, at reasonable cost.
I think this is a good idea, just waiting to prototype it and test it out. If it works, I'll share the diagrams.
Absolutely. However, it is possible to go further:
Basically, I used a single supply rail (not +/- rails) to minimise
converter noise for my first stage. This stage increases the
iPod level from its low level (maybe 0.5v) to the desired level.
To ensure no ground noise, we need to ensure that the cable
from this amp to the iPod has very low resistance (easy to
control, because this is in our hands, it is not existing car
wiring). We can achieve it using substantial cable, and/or keeping
the cable length short. We can consider it a single ground point.
However, as you know, this does not prevent noise appearing
on that single ground, because it needs to be connected to
the car ground somewhere, and there is bound to be plenty
of noise as a result. So, my second stage uses a +/-15V supply
as you say.
Since this supply is at the same ground as my first stage,
my supplies will track the 0V+noise by +/-15V. At this stage,
I'm less concerned about converter noise, because my iPod
audio has already been amplified by a factor (I needed very high
voltage levels).
My second stage gives a balanced output.
Its not a perfect solution, but I think its the best compromise
so far, at reasonable cost.
I think this is a good idea, just waiting to prototype it and test it out. If it works, I'll share the diagrams.
Forgot to mention - the charging circuit is taken from the same point. Basically the single PCB is responsible for charging and for audio amplification. That means being careful with the PCB layout, but
these are things we can control, whereas existing car wiring to
cigarette lighter jacks, etc are hard to control.
these are things we can control, whereas existing car wiring to
cigarette lighter jacks, etc are hard to control.
Hi rwaudio,
Just thought it would be nice to integrate ipod into BMWs, since they have balanced amp inputs by default, at around 11V p-p.
But the circuit contains both a normal and balanced output, so that balanced components only have to be populated on the board if this is needed.
Also, the microcontroller can be used for integrating external controls (e.g. buttons or steering-wheel controls) but again doesn't have to be populated if the ipod touch-screen is sufficient.
The board arrives tomorrow so will prototype it next week hopefully.
I'll keep you updated.
Just thought it would be nice to integrate ipod into BMWs, since they have balanced amp inputs by default, at around 11V p-p.
But the circuit contains both a normal and balanced output, so that balanced components only have to be populated on the board if this is needed.
Also, the microcontroller can be used for integrating external controls (e.g. buttons or steering-wheel controls) but again doesn't have to be populated if the ipod touch-screen is sufficient.
The board arrives tomorrow so will prototype it next week hopefully.
I'll keep you updated.
Well, I get quite good results. I have very low ripple on my supply rails (around 10mV at 100kHz) which ends up giving me approx 60mV ripple on the balanced outputs (the audio can swing 12V p-p). Again, this is at a high frequency, so I don't hear it on the output (I just tested using headphones on the balanced output, which has more than enough power to drive them).
And the frequency response is great - flat across 35Hz-20kHz, and only a slight dip (maybe 1dB) down to 20Hz. Very acceptable.
The only issue, is that when I plug the ipod in, I hear some hiss. The hiss is low, but definitely audible. I find it pretty unacceptable. However, I also hear the hiss on the ipod headphone output.
Any idea if this hiss is normal on an ipod touch? I don't have another one to compare with. Its depressing, because the amplifier has such low noise in comparison.
Perhaps the hiss will be reduced if I make a better ipod cable (I am using some commercial one, which has no screening - I'm picking up no hum however, just hiss).
And the frequency response is great - flat across 35Hz-20kHz, and only a slight dip (maybe 1dB) down to 20Hz. Very acceptable.
The only issue, is that when I plug the ipod in, I hear some hiss. The hiss is low, but definitely audible. I find it pretty unacceptable. However, I also hear the hiss on the ipod headphone output.
Any idea if this hiss is normal on an ipod touch? I don't have another one to compare with. Its depressing, because the amplifier has such low noise in comparison.
Perhaps the hiss will be reduced if I make a better ipod cable (I am using some commercial one, which has no screening - I'm picking up no hum however, just hiss).
I have a co-worker with a 1st gen 8G ipod touch, and after some lengthy but non critical listening I would say it has no hiss on the output. The setup was Touch vs Classic, with 32ohm Alessandro MS1 headphones direct, then through two different headphone amps all using the headphone out. None of the different setups had any hiss. What gain do you have the amplifier set at? (Total of all stages including balanced) I would recommend trying to put the signal into the 2nd stage directly and bypass that first gain stage. My philosophy in audio is KEEP IT SIMPLE, and sofar things are working very well. I would suggest against the extra gain stage at all cost. It's been awhile since I measured the output of my Classic, but I'm pretty sure it was around 1V rms, I could be wrong though. The other thing, is what are you using for your gain stages? Op-amps or discrete? And what kind of op-amps if your using them?
Also 1db down at 20hz is quite a bit, and is often caused by a low value coupling cap, either input/output/between stages. Try raising this value if possible, my headphone amps are flat to 20hz and in theory the 3db down point is below 1hz.
As for the cable, shielding could help since cars are noisy. But when the car is off there should be no noise or atleast very little even without a sheilded cable.
My last suggestion would be to disconnect the ipod charging circuitry if you haven't tried already, and see if the signal is cleaner with the ipod running off it's own battery.
Also 1db down at 20hz is quite a bit, and is often caused by a low value coupling cap, either input/output/between stages. Try raising this value if possible, my headphone amps are flat to 20hz and in theory the 3db down point is below 1hz.
As for the cable, shielding could help since cars are noisy. But when the car is off there should be no noise or atleast very little even without a sheilded cable.
My last suggestion would be to disconnect the ipod charging circuitry if you haven't tried already, and see if the signal is cleaner with the ipod running off it's own battery.
Thanks for the info! (btw you have an interesting website).
I did not get a chance to try all your suggestions today, but I did listen to the ipod with just earphones, and I definitely hear some hiss. I figure my ipod is faulty
It is a gen 1 Ipod touch, running V2.1 software, which is not the most recent.
I get the sound even if no audio is playing. If I connect it to the line amp and power off the ipod fully while it is still plugged in, then the sound reduces.
With the cable disconnected from the ipod end, then I hear no hiss whatsoever from the amp.
These tests were with the charging voltage switched off.
I have stuck the circuit for stage 1 (preamp) here:
http://home.btconnect.com/shabaz/ipod/amp_stage1.png , it is set for a gain of slightly more than 2. I measured the ipod output (with an MP3 of a 0dB sinewave) and the output is 2.5V p-p. So my preamp stage doubles it to around 5.5V. The opamp was SSM2135.
The balanced amp (stage 2) is here:
http://home.btconnect.com/shabaz/ipod/amp_stage2.png
This i.c. (DRV 135) has a hard-coded gain of 2, so the final balanced output is about 11V p-p, which is the level I needed.
Regarding the freq response, I should probably remove the caps from the output of the first stage, they are probably not needed.
It seemed not too bad from 20-35Hz (just visual observation from the scope) but as you say, this is easily fixed.
My pcb is pretty packed though; rev 1 has no room for bigger caps http://home.btconnect.com/shabaz/ipod/board_front.JPG
(the opamps are on the underside).
I did not get a chance to try all your suggestions today, but I did listen to the ipod with just earphones, and I definitely hear some hiss. I figure my ipod is faulty
It is a gen 1 Ipod touch, running V2.1 software, which is not the most recent.I get the sound even if no audio is playing. If I connect it to the line amp and power off the ipod fully while it is still plugged in, then the sound reduces.
With the cable disconnected from the ipod end, then I hear no hiss whatsoever from the amp.
These tests were with the charging voltage switched off.
I have stuck the circuit for stage 1 (preamp) here:
http://home.btconnect.com/shabaz/ipod/amp_stage1.png , it is set for a gain of slightly more than 2. I measured the ipod output (with an MP3 of a 0dB sinewave) and the output is 2.5V p-p. So my preamp stage doubles it to around 5.5V. The opamp was SSM2135.
The balanced amp (stage 2) is here:
http://home.btconnect.com/shabaz/ipod/amp_stage2.png
This i.c. (DRV 135) has a hard-coded gain of 2, so the final balanced output is about 11V p-p, which is the level I needed.
Regarding the freq response, I should probably remove the caps from the output of the first stage, they are probably not needed.
It seemed not too bad from 20-35Hz (just visual observation from the scope) but as you say, this is easily fixed.
My pcb is pretty packed though; rev 1 has no room for bigger caps
http://home.btconnect.com/shabaz/ipod/board_front.JPG(the opamps are on the underside).
That looks like a pretty well made board, very full too.
In your schematics how did you deal with the grounds? IE the 3.9v reference from the first stage is "ground" while the 0v from the balanced stage is also ground. Are these connected? How about the ipod signal ground? Ipod power ground? Car ground? Car amplifier ground? I'm still curious why you chose to use two different power supplies for the two stages? I assume you chose the SSM2135 for it's single supply audio performance, it looks like a good op-amp. But why not run it from the +/- 15v your using for the balanced stage? I haven't tried the DRV135, but I've used the DRV134, and I found the best performance and sound quality by omitting the output capacitors, it may be different in your application though.
How is your control section working? What type of inputs (controls) are you using? Encoder? Push buttons?
I'd like to build a desktop headphone amp with built in ipod dock, however I'm not sure what the best control method would be.
In your schematics how did you deal with the grounds? IE the 3.9v reference from the first stage is "ground" while the 0v from the balanced stage is also ground. Are these connected? How about the ipod signal ground? Ipod power ground? Car ground? Car amplifier ground? I'm still curious why you chose to use two different power supplies for the two stages? I assume you chose the SSM2135 for it's single supply audio performance, it looks like a good op-amp. But why not run it from the +/- 15v your using for the balanced stage? I haven't tried the DRV135, but I've used the DRV134, and I found the best performance and sound quality by omitting the output capacitors, it may be different in your application though.
How is your control section working? What type of inputs (controls) are you using? Encoder? Push buttons?
I'd like to build a desktop headphone amp with built in ipod dock, however I'm not sure what the best control method would be.
Hi,
Yes the 3.9V ground is the same ground plane as the amp stage 1, actually it is a single ground plane for this linear supply and the amp stage 1. Amp stage 2 is driven from the +/-15V switch mode, which has a separate ground plane area (connected at one point near the source on the PCB. The supply then gets filtered, and drives the amp stage 2 which keeps its ground away from the switch mode ground, i.e. keep the switch mode ground separate and connected to the source of the main supply on the pcb. I think this is working, because the +5.2V charging supply (which can be switched off) also operates similarly, and I don't hear any degradation with that switched on or off. Regarding the ipod, it is connected to the pcb, with no grounding anywhere else on the chassis of the car. The pcb has a single chassis ground to one point of the car. So basically it is essential my ipod has an adequate ground from itself to the PCB when its charging. This should use separate wire, but its a cheap cable.. don't think that is the case. Anyway, in my tests I've turned off the charge voltage. I'm pretty sure I've got an ipod fault, since I hear it even with headphones.
The separate supply was used for stage 1, because initially I was worried of getting switch mode noise on the stage 1 supply rails. However the switch mode+filter is generating tiny noise (10mV at 100hz), so its probably not essential to have separate supplies.
Regarding control, the ipod will accept serial commands, so the microcontroller generates this (there is some info on my site regarding the serial format). There is some button input via one of the connectors, or serial input of commands from an external controller. In my case, I will use serial input from the car's existing bus. If you want to implement push button control, a possible method (I used this) is to use PCF8574, which allows many buttons (up to 8) with only a single interrupt line to the microcontroller, it simplifies the code. If you need any info on this, let me know. I am only going to have 3 button inputs (play/pause and skip right and left). Encoder would be harder (difficult to write software for that) plus you would need to use a different mode in the ipod serial commands (where you can retrieve the current ipod song index number, and then send a different index based on the number of pulses from the encoder). The 3-button method is not brilliant but not bad, because the touch screen can still be used for traversing lists. I'm still experimenting with that area actually.. not too experienced in ipod control
Yes the 3.9V ground is the same ground plane as the amp stage 1, actually it is a single ground plane for this linear supply and the amp stage 1. Amp stage 2 is driven from the +/-15V switch mode, which has a separate ground plane area (connected at one point near the source on the PCB. The supply then gets filtered, and drives the amp stage 2 which keeps its ground away from the switch mode ground, i.e. keep the switch mode ground separate and connected to the source of the main supply on the pcb. I think this is working, because the +5.2V charging supply (which can be switched off) also operates similarly, and I don't hear any degradation with that switched on or off. Regarding the ipod, it is connected to the pcb, with no grounding anywhere else on the chassis of the car. The pcb has a single chassis ground to one point of the car. So basically it is essential my ipod has an adequate ground from itself to the PCB when its charging. This should use separate wire, but its a cheap cable.. don't think that is the case. Anyway, in my tests I've turned off the charge voltage. I'm pretty sure I've got an ipod fault, since I hear it even with headphones.
The separate supply was used for stage 1, because initially I was worried of getting switch mode noise on the stage 1 supply rails. However the switch mode+filter is generating tiny noise (10mV at 100hz), so its probably not essential to have separate supplies.
Regarding control, the ipod will accept serial commands, so the microcontroller generates this (there is some info on my site regarding the serial format). There is some button input via one of the connectors, or serial input of commands from an external controller. In my case, I will use serial input from the car's existing bus. If you want to implement push button control, a possible method (I used this) is to use PCF8574, which allows many buttons (up to 8) with only a single interrupt line to the microcontroller, it simplifies the code. If you need any info on this, let me know. I am only going to have 3 button inputs (play/pause and skip right and left). Encoder would be harder (difficult to write software for that) plus you would need to use a different mode in the ipod serial commands (where you can retrieve the current ipod song index number, and then send a different index based on the number of pulses from the encoder). The 3-button method is not brilliant but not bad, because the touch screen can still be used for traversing lists. I'm still experimenting with that area actually.. not too experienced in ipod control
Hi guys first post. I'm absolutely floored by the stuff you guys are doing, although I'm green to DIY audio and it's out of my league for the moment. I thought there would be more wire fabrication and WD-40, as my DIY audio has been thus far, but PCB's and integrated circuits!
Shabaz, thanks for the info. I'd love to see the process of building your board, but yahoo 360 is closed I guess? Please let us know if you have a new address. Also, I don't understand exactly what that's doing.. it's a preamp, but what is the amp? The car stereo? So you're connecting some of the pinout of dock connector to the car stereo serial, so you can change songs via stock buttons?
For some insight on my thoughtprocess for this topic, I have iPod 5.5G with homebrew DIY iMod. Therefore there's the pure line out at the dock with 2 caps in the flow before RCA. I will make flex car mount like Arkon with custom dock. I'll have Alpine PDX-5 amp. I have an HU, but after looking at people's setups on headfi I have come to realize that if I can avoid the HU I'd probably get much better quality sound (less is more) which is deserved by my speakers. The complications I see though are: 1 how to adjust volume (homebrew preamp I guess with remote knob), 2 how to have a subwoofer output and rear output (as opposed to just L/R, crossover?), and 3 how to connect firewire via the car 12v with a switch on/off. I know my car is approx 13.6v on, and it varies, and I know Pin #21 on dock can act as car shutoff switch, and I'm worried about ground loops and terrible noise/interference from something with the firewire charging ipod, ipod playing through preamp, etc. Are these assumptions on the right track? Any tips on how to execute #3 properly? Thanks in advance.
I look forward to learning a good bit by cruising around! If Shabaz's work is standard, I'd better get some books on the subject!! Yeah, do you guys recommend any specific books?
Shabaz, thanks for the info. I'd love to see the process of building your board, but yahoo 360 is closed I guess? Please let us know if you have a new address. Also, I don't understand exactly what that's doing.. it's a preamp, but what is the amp? The car stereo? So you're connecting some of the pinout of dock connector to the car stereo serial, so you can change songs via stock buttons?
For some insight on my thoughtprocess for this topic, I have iPod 5.5G with homebrew DIY iMod. Therefore there's the pure line out at the dock with 2 caps in the flow before RCA. I will make flex car mount like Arkon with custom dock. I'll have Alpine PDX-5 amp. I have an HU, but after looking at people's setups on headfi I have come to realize that if I can avoid the HU I'd probably get much better quality sound (less is more) which is deserved by my speakers. The complications I see though are: 1 how to adjust volume (homebrew preamp I guess with remote knob), 2 how to have a subwoofer output and rear output (as opposed to just L/R, crossover?), and 3 how to connect firewire via the car 12v with a switch on/off. I know my car is approx 13.6v on, and it varies, and I know Pin #21 on dock can act as car shutoff switch, and I'm worried about ground loops and terrible noise/interference from something with the firewire charging ipod, ipod playing through preamp, etc. Are these assumptions on the right track? Any tips on how to execute #3 properly? Thanks in advance.
I look forward to learning a good bit by cruising around! If Shabaz's work is standard, I'd better get some books on the subject!! Yeah, do you guys recommend any specific books?
damn, can't email shabaz to get his attention, too green . I guess I can get to that later. I have decided to USB charge on my custom dock connector, not firewire, which is excessive. I've also discovered that I can do my diyMod (the oversize name for the "audiophile" ipod from headfi) with the capacitors inside the unit, so this means my dock can be compatible with all ipods. Taking that into account, I wanted to make it visitor-friendly, so I am going to modify the USB 5v with a couple of resistors to trick other people's ipod into charging also. This is relevant to 3rd g nano's and 3g+ iphones, and involves the data pins of USB needing voltage. See this link for the pinouts. Anyone care to comment if my usb charge and then RCA out will cause a ground loop?
Also Shabaz, if you do return, you should try a listen with the dock line out from your Touch to see if it's possibly just the headphone jack that's tainted, which I've heard of.
. I guess I can get to that later. I have decided to USB charge on my custom dock connector, not firewire, which is excessive. I've also discovered that I can do my diyMod (the oversize name for the "audiophile" ipod from headfi) with the capacitors inside the unit, so this means my dock can be compatible with all ipods. Taking that into account, I wanted to make it visitor-friendly, so I am going to modify the USB 5v with a couple of resistors to trick other people's ipod into charging also. This is relevant to 3rd g nano's and 3g+ iphones, and involves the data pins of USB needing voltage. See this link for the pinouts. Anyone care to comment if my usb charge and then RCA out will cause a ground loop?Also Shabaz, if you do return, you should try a listen with the dock line out from your Touch to see if it's possibly just the headphone jack that's tainted, which I've heard of.
Hi
Hi willPS,
Sorry for the late reply! I should check more often.
Basically, in the end, I went for this solution:
1. BMW tape deck with Nav screen: cut tape wires from that to the BMW amp
2. Amplify the ipod to BMW levels (which are balanced audio) and then feed into the tape wires into the BMW amp
3. Cut BMW amp speaker wires
4. Use a speaker-to-line level converter connected to the cut wires on the
BMW amp, and feed into the PDXs (I got a firm near me to install the PDX+speakers)
5. Keep a cassette permanently in the tape deck (to fool it into play mode)
This solution allows me to retain existing volume controls, etc, and also to retain CD changer functionality. I just lost tape functionality. (in fact I did not lose it, I used relays, but it is unnecessary).
Regarding sound, to be honest, I think the place where there is most likely for noise to be introduced is the amp between ipod and the tape input to the amp. Certainly there is some harmonic distortion because of the speaker-to-line converter, but that is hard/impossible to hear (for me anyway). I might try to make a converter one day using better transformers (mine is some $20 converter), but I don't have experience in that; would be good to know if anyone has designed a decent (less that $150-$200) converter. The BMW amp introduces very little noise (as far as I can tell (i.e. very hard to hear even when on silent, the volume is turned up extremely high).
If you have a good head unit, you could try to feed it into the line input, perhaps you will get better results. You would not need to do any unbalanced to balanced conversion of course, so that's a bit easier.
Otherwise, if no HU is used, then as you say, some method of volume adjustment is needed.
With the ipod plugged in, I do hear some hiss and electronic noise (no engine whining or anything, just some very faint processing noise from the ipod when selecting songs). It is faint, but it does bother me because I know it is there : ( I think it can be fixed if I use a better connection to ground, but I have not had a chance to do this recently. Also I've noticed my ipod is faulty, I get some hiss on the headphone output too, and all my colleagues claim they never hear it on their ipods.
I did so much research into grounding and loops, and still it is possible to screw up : ) Perhaps I am over-complaining, no-one else has noticed the noise.
Regarding the USB charging, the resistor values are quite critical, the info is here: Shabaz's Blog on Yahoo! (I will move this info somewhere more permanent later).
To switch the charger on/off with the engine, you could use a relay powered from a suitable connection on the car. I'm not sure where that is, the guy who installed the PDXs left me with such a connection to use.
For the charging voltage, you could just purchase a griffin charger and hack that, it is easier than assembling the 5.2v power supply from components.
Hi willPS,
Sorry for the late reply! I should check more often.
Basically, in the end, I went for this solution:
1. BMW tape deck with Nav screen: cut tape wires from that to the BMW amp
2. Amplify the ipod to BMW levels (which are balanced audio) and then feed into the tape wires into the BMW amp
3. Cut BMW amp speaker wires
4. Use a speaker-to-line level converter connected to the cut wires on the
BMW amp, and feed into the PDXs (I got a firm near me to install the PDX+speakers)
5. Keep a cassette permanently in the tape deck (to fool it into play mode)
This solution allows me to retain existing volume controls, etc, and also to retain CD changer functionality. I just lost tape functionality. (in fact I did not lose it, I used relays, but it is unnecessary).
Regarding sound, to be honest, I think the place where there is most likely for noise to be introduced is the amp between ipod and the tape input to the amp. Certainly there is some harmonic distortion because of the speaker-to-line converter, but that is hard/impossible to hear (for me anyway). I might try to make a converter one day using better transformers (mine is some $20 converter), but I don't have experience in that; would be good to know if anyone has designed a decent (less that $150-$200) converter. The BMW amp introduces very little noise (as far as I can tell (i.e. very hard to hear even when on silent, the volume is turned up extremely high).
If you have a good head unit, you could try to feed it into the line input, perhaps you will get better results. You would not need to do any unbalanced to balanced conversion of course, so that's a bit easier.
Otherwise, if no HU is used, then as you say, some method of volume adjustment is needed.
With the ipod plugged in, I do hear some hiss and electronic noise (no engine whining or anything, just some very faint processing noise from the ipod when selecting songs). It is faint, but it does bother me because I know it is there : ( I think it can be fixed if I use a better connection to ground, but I have not had a chance to do this recently. Also I've noticed my ipod is faulty, I get some hiss on the headphone output too, and all my colleagues claim they never hear it on their ipods.
I did so much research into grounding and loops, and still it is possible to screw up : ) Perhaps I am over-complaining, no-one else has noticed the noise.
Regarding the USB charging, the resistor values are quite critical, the info is here: Shabaz's Blog on Yahoo! (I will move this info somewhere more permanent later).
To switch the charger on/off with the engine, you could use a relay powered from a suitable connection on the car. I'm not sure where that is, the guy who installed the PDXs left me with such a connection to use.
For the charging voltage, you could just purchase a griffin charger and hack that, it is easier than assembling the 5.2v power supply from components.
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