| Electronics > Beginners |
| Replace tape block with a bluetooth receiver in a car radio |
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| den:
Hi everyone, I am trying to "improve" my original car radio (Blaupunkt, 2001, installed in an Audi A6 as Chorus radio) by adding Bluetooth. Bluetooth receiver is just a small device with a loudspeaker, which can play music, that's all. The radio has only 3 inputs - Radio, CD changer and Tape (no Aux). I decided to replace tape with Bluetooth. Important thing - I have full schematics of this radio (attached screenshot of the tape preamp, full document here). But still cannot get it to work normally. What I did up now is - connected Bluetooth receiver to a 5 V rail in the radio and connected loudspeaker outputs to tape COM and input (either L_NORM or R_NORM) through capacitors (you can see there is 4.3 V bias voltage there, so I guess I need capacitors). It kind of works - I can hear music playing, but very quietly, even at full volume. And also - it works actually with only COM connected to one of the loudspeaker outputs, the other input does not matter. Any ideas/suggestions? Why does it work the way it does and how to get it going correctly? Thanks in advance, Den |
| IanMacdonald:
The inputs will be very sensitive at ~ 1mV and have a massive bass boost/treble cut. The sensitivity could by overcome just be using an attenuator (with good quality metal film resistors for low noise) but the 'equalisation' would be better handled by defeating it. https://www.iasa-web.org/tc04/magnetic-tapes-replay-equalisation I suspect the RC networks near the chip inputs are the equalisation, but without chip details it's a bit hard to say what would be needed to give a level response and less sensitivity. Other issue is that the tape mechanism has Enable and Data lines. it's likely that it signals to the processor that a tape is inserted and running, in which case audio is switched over. Without the right signals the output may simply be ignored. This might just be a logic level, or a pulse train. Hard to say as the manual doesn't give much detail on this. |
| CJay:
Post a ,link to the full schematic but I suspect the operation of the tape deck is being monitored and the radio mutes unless it can see signals from teh deck to say it's moving and in operation. Quick and dirty proof would be to put in a tape with the L_Norm and R_Norm from the tape deck disconnected so you can feed in your signal. |
| BrianHG:
I mute out, or cut the tape head preamp. & usually feed my auxiliary audio either mixed. or switched A/B through the line level at the volume knob. Though, this was with an old mechanical volume radio. No preamp hiss and flat audio and if I was using the switch, I can leave the radio wherever it was, like an FM radio & quickly switch on and off my auxiliary. Say you BT receiver has an LED for when passing sound, use that guy for the auto switching... |
| den:
To make it clear - I actually connected the tape block to the first connector to ensure that everything is enabled. Audioprocessor gets a correct initialization from master MC, selecting input 2, which is tape (tested with scope). The problem is - there IS sound, but it is too quiet, even with max volume. I will take a look at the posted link tomorrow, but it actually looks like I am lacking knowledge (apart from other things) of how actually tape stuff work. And I still don't get why it works with one signal wire only. |
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