Adding line in to AN7465S Multiplexor

[clipper]

Hi all.

This is my first post (except for my intro) here so I hope I'm in the right place.

I guess mine is a common project, adding an auxiliary stereo input to a car radio / cassette player.

The stereo in question is a Blaupunkt ACR 3231 which has a very simple tape head reader. From reading up on-line I already knew that adding the line in RCA jacks to the tape head L&R channel wiring was not going to work but I tried it out anyway... and sure enough, the sound quality was very poor, and the volume of the auxiliary audio feed (in this case my smart-phone) needed to be very low for the sound from the stereo to be even intelligable.

Investigating more, I was able to identify the pre-amp chipset as a 6458DS, and I found the schematic on-line.

Having identified the output pins of the pre-amp, I thought that soldering the RCA positive channel wires to these pins would be the answer, so that my auxiliary audio was not passing through the pre-amp. Although the sound was better and the volume more reasonable, there were still problems however.

Firstly, the sound quality was OK with only the RCA positive wires connected, but too low to be useful, but as soon as the negative was connected to ground, the auxiliary line in audio became subject to a large amount of interference from the radio feed, which in theory was isolated by having operated the micro-switch which swapped the output audio from radio to cassette. When there is a quiet moment in the audio input through the RCA jacks, the radio can clearly be heard in the background the auxiliary input audio is simply being plaed over a reduced power radio feed.
The overall output volume is better though.

It seems then, that I need to move my RCA connections, from the output side of the 6458DS pre-amp and possibly directly to the input of the Multiplexor. which is an AN7465S. But this is where my capabilities have run out...

I cannot trace a direct circuit from the pre-amp to the AN7465S, and I do not understand the schematic of the IC well enough to know which pins may be the input from the tale head pre-amp.

Can anyone help?

 

[73's de Edd]

Sir clipper . . . . .


At the circuitry position and the level that you were injecting your audio . . .along with the impedance mismatch . . . you basically had yourself a quasi-audio square wave generator.

Consult the info below on your unit to find the two 22K load resistors and my mark ups of the RED circle for the RIGHT channel input and the BLUE for the LEFT channel inputs.

These are the points that the tape post amplifier amd the radio outputs feed into the input of the power amplifier and should agree with your desired input level if you make connection there, my choice would be through a 1ufd poly @ 50V for both channels.

I don't know your type of connector interfacing, but use shielded wire for both channels.

BRACE YOURSELF . . . .
THIS . . . .IS . . . .IT . . . . .

73's de Edd

.....

 

[clipper]

Sir clipper . . . . .

No need to use my full title, nice as it is....

At the circuitry position and the level that you were injecting your audio . . .along with the impedance mismatch . . . you basically had yourself a quasi-audio square wave generator.

OK, I wish I understood what you are telling me here... maybe you can explain ?(time permitting).

Consult the info below on your unit to find the two 22K load resistors and my mark ups of the RED circle for the RIGHT channel input and the BLUE for the LEFT channel inputs.

These are the points that the tape post amplifier amd the radio outputs feed into the input of the power amplifier and should agree with your desired input level if you make connection there, my choice would be through a 1ufd poly @ 50V for both channels.

Your solution works very well (as I am sure you were already aware!). At the moment I have directly connected as I am not sure what the benefit of the capacitors would be... maybe you could explain the benefit to me?

I don't know your type of connector interfacing, but use shielded wire for both channels.

BRACE YOURSELF . . . .
THIS . . . .IS . . . .IT . . . . .

73's de Edd

.....

 

[clipper]

So with the sound quality now at a level that I am happy with (thanks to 73's Edd), I am now facing another "problem".

The selector switch marked "CASS RADIO" is a simple reed switch activated originally by inserting a cassette into the deck and de-activated upon ejecting it.

I planned to use this switch to select the audio output between the new line in (phone) and the existing radio. But, what I now notice is that grounding the signal pin / wire to switch to radio, does not cut the audio signal from the line in / cassette feed.

Of course, this was not a problem in the original unit because when the radio was activated, the cassette had been ejected and thee was no other posible audio from that input, but with my phone, it means that I have to stop any media reproduction on order for it to not be played in combination with the radio, and, even if I do stop any media reproduction, any noticiation sounds will be played over the radio.

Is there any way to change this situation, so that the new line in audio can be totally isolated from the radio audio?

 

[clipper]

I am still struggling with this and it is causing me a big problem.

If leave the signal wire which connects to one pole of the "cassette / radio" selector switch disconnected, I get sound from the radio by default, and when I connect a source to the new line in (connected as per 73's Edd's post), the amp output automatically switches to the auxiliary source.
Similarly, if I stop audio reproduction on the auxiliary source device, the stereo automatically defaults to radio.

But, given that my intention is to connect a Bluetooth module to the line in, this means that I cannot leave the module permanently connected to the line in because this is detected as if it were an audio input and therefore inhibits the radio.

Can anyone give me an idea of how I can physically switch between the radio and auxiliary line in feeds without too much reconfiguration of the circuitry?

 

[73's de Edd]

Sir clipper . . . . .

So the purpose here was to permit you using two RCA plugs into the system so that you can input music into the car radios audio and speaker system from your Iphone.
And as you have figured it out so far, if there is no tape in the unit, the tape post amp preamp LA6458 is muted so that no tape derived audio can come from its pins 7 amd 1 and get over to the BLUE and PINK inputs of the audio amplifier section of the system, thereby letting the Iphone music play unhindered.
Otherwise, the RADIO audio is channeled right down to the BLUE and PINK inputs of the audio amplifier section of the system.
As it stands now, I guess that you have female RCA receptacles installed to accept the Iphone audio, but your radio audio is also present with your phones audio . . . unless the radio is tuned off from being tuned to a station on FM or AM..
And for your comprehension of the radio audio signal path info . . . . your radio audio comes in from pins 12 and 13 of the AN 7465 and routes down to the BLUE and PINK audio connections.

And if I was previously reading your present situation properly, how would you want to mute the radio, as I feel that you have no problem if a tape is being in the unit and stopped . . .as far as mixed audio bleed thru . . . .correct ?

73's de Edd
.....

 

[clipper]

Ahahhh!, now I understand more! (not all, but more at least!)

Thanks for the response, I hope my additional comments below shed some light on my vague plans and hopeless tribulations....

Sir clipper . . . . .

So the purpose here was to permit you using two RCA plugs into the system so that you can input music into the car radios audio and speaker system from your Iphone.
And as you have figured it out so far, if there is no tape in the unit, the tape post amp preamp LA6458 is muted so that no tape derived audio can come from its pins 7 amd 1 and get over to the BLUE and PINK inputs of the audio amplifier section of the system, thereby letting the Iphone music play unhindered.
Otherwise, the RADIO audio is channeled right down to the BLUE and PINK inputs of the audio amplifier section of the system.
As it stands now, I guess that you have female RCA receptacles installed to accept the Iphone audio, but your radio audio is also present with your phones audio . . . unless the radio is tuned off from being tuned to a station on FM or AM..

More or less, this is correct. I have wired in a 3.5mm jack rather than two RCA plugs, but of course that is irelevant (I think).

As for the unit having a tape in it or not, the tape deck mechanism has been removed, but before removing it I thought I had fully understood the few electrical connections that it had; namely:
1) the tape reader head (removed and not required)
2) the tape drive motor (removed and not required)
3) the "cass / radio" selector switch, which I retained, thinking that I could use it to manually select between radio and the auxiliary device audio.

My assumption was basically that as the unit detected a cassette my means of physically operating the switch, I could use this switch with any other physical operation to have the same effect. But it seems that it is not so simple..

And for your comprehension of the radio audio signal path info . . . . your radio audio comes in from pins 12 and 13 of the AN 7465 and routes down to the BLUE and PINK audio connections.

OK, this is now clear. So, based on this, the selector which I refer to above as "cass / radio" switch actually cannot now "switch" between the two feeds, correct?

And if I was previously reading your present situation properly, how would you want to mute the radio, as I feel that you have no problem if a tape is being in the unit and stopped . . .as far as mixed audio bleed thru . . . .correct ?

As above, rather than just "mute the radio" I want to exclusively select the auxiliary device feed or the radio feed.

As it stands there is not any "bleed thru" because when there is an audio signal coming in from the auxiliary device, the radio is fully muted, so no problem there.

But, if I want to listen to the radio, I cannot leave the auxiliary device connected because as soon as it produces any audio output (such as a notification of a message) this is taken as a "priority signal" for some reason and the radio becomes muted whilst the notification sound is played through the power amp.

By the way, when I activate the "cass / radio" switch (which grounds the signal wire form pin 20) it mutes all inputs, both radio and auxiliary. This is confusing to me.

Perhaps the solution is to forget the existing "cass / radio" switch and use a doble pole switch to isolate the two auxiliary audio channels from their connection points, this would allow radio audio to be totally isolated correct?

 

[clipper]

So, I went ahead with my plan to isolate the aux input by two microswitches. And it seems to work very well. The switches are operated by what was the Fast Forward button on the cassette deck so its quite a neat adaptation.

Isolated, I get uninterrupted radio sound (as you would expect), and with the aux in connected, any sound produced by the phone is carried through the amp, cuting the radio feed for as long as the aux in feed is present.

This is actually really good for using the phone's GPS becasue I can be listening to the radio and the Google lady interupts the radio to tell me where to go!!

BUT, and there's always a big "but"... I am getting terrible ground loop interference when using the phone as aux in sound.

Could this be because of connecting the common negative audio to the ground of the car stereo? Should I have left the common negative "floating"?

Or is it simply due to the different earthing potentials of the phone and radio?