pseudo differential input car amplifier schematic - Page 1
Forum section: electronics / projects

author: free_electron
author_rank: Super Contributor
timestamp: December 11, 2013, 02:07:51 am
content: I am looking for schematics of car amplifiers. those big honking power things.most specifically of interest is the input circuitry. i am looking for the schematic of the pseudo differential amplifier.I know there are special chips for that NEC , JRC and some others make thoseNXP does too : TDA8579 is one.problem is the TDA is very hard to find so i am looking for alternate.These chips have a very high common mode rejection and create a balanced ground. the RCA connectors are NOT connected to ground on the amplifier but go through a capacitor to a pin of the chip. so there is no ground loop possible with these.What i am trying to do is to pick off the audio from a cellphone. the problem is that, whenever there is an incoming call there is the abligatory preut preut preut from the GSM burst. using such a pseudo differential chip eliminates that completely.i can brew it myself using to opamps but there are always audible plops and clicks when connecting/disconnecting. the TDA8579 has special circuitry that prevents that. there is an internal charge pump that recalibrates the DC point on the fly. so i'd rather use the parts designed for thatAll i need is partnumbers for a few alternate chips.i used to know one that came in those funny SIL9 packages uPA17--something... but i can't remember. Clarion and MTX used those. I used to have all those schematics....

author: calexanian
author_rank: Super Contributor
timestamp: December 11, 2013, 06:48:46 am
content: http://www.thatcorp.com/datashts/THAT_1200-Series_Datasheet.pdfI have used a That 1200 as a unbalanced input receiver and still maintained a decent CMRR. I have a couple or five the sales rep threw at me at a show a few years ago. They are yours if you want them. You can float the input all over the place and it still behaves.Did looking at all that car audio junk hurt your eyes? Hope you were wearing sun glasses!

author: free_electron
author_rank: Super Contributor
timestamp: December 11, 2013, 11:24:13 pm
content: That comes close but is not exactly what i am looking for.i will design in the TDA8579. they'll have to buy it.

author: dannyf
author_rank: Super Contributor
timestamp: December 11, 2013, 11:36:28 pm
content: QuoteYou probably want to explain what you meant by "pseudo differential amplifier".QuoteIt is not difficult to build an amplifier with high cmrr: most instrumentation opamps do that for example. or you can build your own.QuoteThat can be difficult to deal with in analog. One way would be to detect incoming signal and after some time constant, turn on the output.

author: Marco
author_rank: Super Contributor
timestamp: December 12, 2013, 12:17:56 am
content: Does the TDA8579 use the unbalanced coupling capacitors as a way to detect shifts in or particular noise on the common mode voltage to know when to blank the output? Otherwise I don't see how it would be any better than a fully balanced differential amplifier with high input impedance.

author: TheBay
author_rank: Unknown
timestamp: December 12, 2013, 12:20:26 am
content: I've got Class A/B Car amplifiers and some very rare Class T (Class D Tripath) if you want me to have a look at any i'd be happy to help.

author: free_electron
author_rank: Super Contributor
timestamp: December 12, 2013, 12:29:43 am
content: yep.pseudo differential means there is LEFT , RIGHT and a common return path shared by left and right.So this is not a true differential signal where you would have left / left return , right / right return.Essentially what this chip does is create a nullpoint signal (not the same as ground ! ) that is capacitively coupled onto the shield of the incoming audio cable.The chip then does a difference calculation between this shield signal and left / right.The problem is the following : if you tap into the audio output of a cellphone to bring it into an amplifier you are always screwed with teh GSM trasnmitter. as soon as this sucker starts blasting away the common mode signal injected onto the audio cable is audible in the speakers.By cutting the shield off the ground and connecting it to the nullpoint you can run a difference amplifier on both signals thus rejecting the GSM signal ( which is common mode )The end result is crystal clear audio without the prrrrrt prrrrt prrrrt of the gsm bursts.This TDA8579 chip was specifically designed for that purpose and is part of the reference design audio interface for that fruity company.I was trying to avoid that chip as neither digikey nor mouser carry it but it seems it is sold en masse on ebay.This thing is part of a design i am doing for a new book on PCB design. The project is a mini-hifi system that combines FM radio , audio input, iDevice interface , PC audio card with USB , USB hub , graphics displays , switch mode power amps , high voltage switcher , mechatronics and flex boards.I am trying all kinds of different things in this design from power, high voltage, RF , audio , high speed digital , flex pcb (for the remote control ) , capsense and anything i can think of.Besides being the big democase it has to work well ...

author: dannyf
author_rank: Super Contributor
timestamp: December 12, 2013, 12:43:27 am
content: QuoteEither I have no idea how this "pseudo differential amplifier" differs from a regular "differential amplifier", or the OP wants something less of a fully balanced differential amplifier - not sure why though.QuoteHow would this pseudo differential differ then from a true differential where the two return paths are joined, either at the transmitter or at the receiver?QuoteYou can do the same with a true differential amplifier too.QuoteThat depends on what this "amplifier" is. I would be surprised if it is the case with a true differential balanced amplifier - assuming that the gsm signal is not common mode.I would add that I have never seen a case where gsm signal got into a regular audio amplifier: it would be rare for an audio amplifier not to have substantially attenuated the gsm signal.

author: Marco
author_rank: Super Contributor
timestamp: December 12, 2013, 12:52:47 am
content: Quote from: free_electron on December 12, 2013, 12:29:43 amThat still doesn't quite explain why the decoupling has to be unbalanced.

author: dannyf
author_rank: Super Contributor
timestamp: December 12, 2013, 12:58:13 am
content: The receiver could be providing phantom power to the transmitter / pick-up.Either way, it seems to be a case where problems are created so a solution can be found.

author: calexanian
author_rank: Super Contributor
timestamp: December 12, 2013, 02:17:38 am
content: Vincent. If it proves to be impractical to use that part you may want to ask John Atwood if he knows of anything. He is up on the audio stuff.

author: free_electron
author_rank: Super Contributor
timestamp: December 12, 2013, 02:33:19 am
content: it looks like this is harder to explain.here is the problem :take a simple audio amplifier. in the back there are RCA inputs. take a 3.5mm headphone jack to RCA cable , plug one end in amplifier, plug other end in cell phone audio out. Connect ground terminal from the dock connector also to amplifier chassis ground.Play music on the phone. wait for the phone to start transmitting its GSM pulse ... you will hear PRRRRT PRRRRT PRRRRT in through the music.This is cause by the GSM signal creating a huge common mode burst into the audio inputs through the shielded cable. it is partially caused by a ground loop through the docking connector. The simple shield of an audio rca cable does not stop this powerful signal.The solution is to cut the ground loop and employ differential inputs. that is exactly what that TDA chip does. it creates a nullpoint that is capacitively coupled to the GSM ground coming over the RCA shield. Note that this is ONLY a nullpoint for AC ! it is not the same DC level as chassis ground !This nullpoint is actively created and very low impedant.Then the amplifier takes the AC differential off the shield and the signal. The rejection is better than 80dB. in essence the GSM signal is rejected as a common mode signal ( it is not the RF signal itself you hear but the pulsing of the RF signal creates a click that does fall in the audible band ).This chip works with a simply 5 volts supply ( you don't need a symmetrical supply ). Almost any car radio that has an AUX jack to hook up your cell phone uses this, or a similar, chip as input stage to rejects the GSM crap. There are special audio mixer chips for car applications that have a dedicated channel with the same topology as the 8579.Since my design both powers the iDevice and taps into audio i have the same problem. i hear the GSM pulsing. stick that TDA inbetween ( all you need is 3 caps and that TDA chip , and the crap is gone.I can't dick around creating complex differential ampliefier circuits. it's gotta be affordable. The TDA chip was designed for this purpose and costs 80 cents. problem is neither digikey nor mouser stock it.I know there are other chips besides this NXP chip that were designed for the same purpose. That is what i am after. So my original question stands :if you have schematics for the AUX input stage of a car amplifier or car radio set : gimme. i am after the chip number. Most likely NEC, JRC or Toshiba have alternate parts. It's just that i can't find part numbers as these things are rather specialistic. One peek at car stereo schematics and i will know what i need to know.I know about the TDA part as it is in the reference design for an iDevice interface. I have a Sony, a Panasonic and a JVC mini stereo device that are certified for iDevices and they ALL use that TDA chip as audio input stage.I am just trying to see if there are others as the fruity company does not disclose information unless you are willing to fork over hefty amounts of money ... which i am not willing to do.below an idea of what my design looks like so far. the individual blocks have been tested and work. not shown in this design is the switching power supply, the front panel and cpu board or the remote control.the main board holds a 2.1 channel class T power amplifier delivering 25 watt RMS left and right and 40 watt RMS to the woofer. This thing makes a lot of noise without heat ! Main board has power amps, supply , mixing system with equaliser and tone control as well as a subwoofer active filter and the three class T amplifiers.The vertical board has a USB 2.0 hub , FM radio with RDS , 24 bit stereo Audio DAC ( the PC sees this thing as a 4 channel hub , there is one front and one back user USB port , the Sound card and the iDevice ) as well as a headphone amplifier.the switching power supply can charge both iDevice profiles as well as the USB charging profiles and is always on. a softstart can click in the toroidal transformer for the main amplifier. You can charge usb devices from both front and back usb port as well as the dedicated iDevice channel ( the iDevice channels picks off audio as well as USB )the whole system is under control of a STM32 Cortex processor and has a graphical touch screen interface. a HID control library allows talking to the STM32 through the USB interface. when hooked up to a PC this thing behaves as a standard audio card.The remote control is made as a flex PCB that is folded around a plexi carrier frame holding the coin cell. it features backlit touch buttons ( white led illuminated the plexi that shines through the flex pcb. the buttons are capsense.More details will be coming by mid next year.This is the core project for my book on PCB design. i Designed this as project as it has all kinds of difficult problems you can encounter when doing a board design.

author: calexanian
author_rank: Super Contributor
timestamp: December 12, 2013, 04:33:51 am
content: Have you tried using a very low line impedance? If its looking for a headphone output you may be able to get away with 50 ohms or so and your noise, or hash if you prefer may not be much of an issue. A transformer perhaps? With a low pass filter? This is out of my area of expertise, but it got me thinking.

author: free_electron
author_rank: Super Contributor
timestamp: December 12, 2013, 05:24:19 am
content: you got no control over line out ... it is what it is.filter is no-go as this would impact the normal sound.those bursts are in spikes with a repitition rate of a few hundred hertz zo they are very audible.anyway. TDA is designed in. end of story. too bad i couldn't find an alternate.