TDA7297 cascade channels?

[Frankchie]

I have an inexpensive Chinese TDA7297 module that I want to use to replace the audio output stage of an old car radio.

The problem is that the module does not have enough gain. The radio is mono and the module is stereo so I was wondering if I could cascade the channels to get more gain. Basically I want to connect the output of one channel into the input of the other channel.

Because the inputs are relative to ground to cascade the channels means that one output leg of the ist channel gets grounded via it's connection to the input of the 2nd channel. I'm not sure if this upsets the working of the module. I suppose I could insert a coupling a capacitor in the grounded leg but I'm not sure if that is all that's necessary..

BTW, because the car speakers have one wire grounded the output channel of the module also needs one speaker output leg grounded. So basically both channels would have one output leg grounded. I'm not sure if this complicates things or not.

I don't have a schematic of the module, I only have the TDA7297 data sheet and I have posted the link below.

Anybody have any suggestions?

http://www.radiotechnika.hu/images/TDA7297.pdf

 

[Audioguru]

The left and right amplifiers in the TDA7297 are bridged, so each speaker wire is driven with power with no ground. The outputs have a DC voltage on them which cannot be fed to a grounded speaker. The bridged output power is 15W into an 8 ohm speaker when it is turned up too high producing awful 10% distortion and your car battery is severely overcharging at 16.5V. The bridged power into an 8 ohm speaker when the distortion is not bad is about 6.5W when the car battery is being charged at a normal 14V. If you connect one of its output wires to ground it will blow up.

But you want to use only one wire of one amplifier to drive a grounded speaker which will produce about 2W into 8 ohms which is way too low for a car radio. The TDA7297 is not designed to drive 4 ohm car speakers but you can try one wire from one channel and get about 3.5W which is still a very low power for a car radio. Since the outputs have DC on them then a coupling capacitor must feed the grounded speaker, use 3300uF for a 4 ohm speaker or 1500uf for an 8 ohm speaker.

Really you should connect the TDA7297 amplifiers to 8 ohm speakers as they are designed to do, bridged without the speakers being grounded and use a little stereo preamp to increase the levels if they are too low. The preamp does not increase the maximum output power, it just increases the loudness if it is low.

 

[duke37]

I have not played with these. The output of channel1 is on pins 1 and 2, both centred on half PSU voltage. An output signal could be taken from either of these and fed to channel2 through the 2.2μF capacitor.
The gain could be too high.
Channel1 may need a load resistor p1 to p2 for stability.
Amplifier noise may be a problem.

P1 and P2 are in antiphase so a linear potentiometer connected between them will have zero output at the centre and maximum output at each end.

 

[BobK]

The gain is 32 db or 40 V/V. What are you driving it from, a crystal radio? Anything that can drive a line input is more than sufficient to max out the amp.

Bob

 

[Audioguru]

Since he tried this amplifier to replace the output stage of an old car radio then maybe he overloaded it with a 4 ohm speaker or connected it completely wrong with one speaker wire grounded.

Frankchie,
1) Did you select the car radio signal from its volume control?
2) Was the speaker the required 8 ohms (not the 4 ohms that are usually used in cars)?
3) Did you disconnect the speaker from ground and connect it to the bridged outputs of one channel of the TDA7297?
4) Did the car have a charged 12V battery (some old cars used a 6V battery)?
5) Was the output from the speaker at a low level and was not distorted (not near the maximum rated power)?

 

[Frankchie]

Wow, what great feedback..Very helpful. Thanks everyone.

Let me try answer all the questions.
1. I took the output from the car radio volume control center tap. It's about 50mv P-P.
2. The speaker was 8 ohms, although according to the radio schematic the required speaker is 10 ohms (1972 olds)
3. The speaker and the radio were on my bench and the speaker was not grounded.
4. I used my bench PS for the radio set at12.6 volts. and a 9v wall wart for the module (convenient - plug matched jack on the module). See note 1
5. Yes, the speaker output was moderate and not distorted. But not loud enough for a car.
6. The unused channel was loaded with a 20 ohm resistor.

Note 1: Late last night I decided to try a 2 amp 12v wall wart that measured 17v no load. The module spec says 18v max so I figured it was safe to try. Unfortunately, I got no output and when I reverted to the 9V WW it still did not work (pwr led was on). I gave up and went to bed. I haven't tried again yet, but my guess is that I fried something. If it's fried, I have another one on order. I'll post back as soon as I get back to it.

 

[Frankchie]

I have not played with these. The output of channel1 is on pins 1 and 2, both centred on half PSU voltage. An output signal could be taken from either of these and fed to channel2 through the 2.2μF capacitor.
The gain could be too high.
Channel1 may need a load resistor p1 to p2 for stability.
Amplifier noise may be a problem.

P1 and P2 are in antiphase so a linear potentiometer connected between them will have zero output at the centre and maximum output at each end.

That sounds workable, I may give it a try if my module is still working (see my other post). Good tip about zero output at the center tap.

Thank you.

 

[Audioguru]

A long time ago the center tap on a volume control had a series RC to ground and was used for bass boost. If the volume control was set at the center tap or less then the midrange and high frequencies were reduced by the RC and bass went through without reduction. As the volume was turned up there was less bass boost.

You need about 5 times more gain, not 50 times from the other amplifier. Simply cut out the resistor in series with a capacitor to ground at the center tap of the volume control. You will not miss the bass boost because mono AM radios are used for speech, not music.

 

[Frankchie]

A long time ago the center tap on a volume control had a series RC to ground and was used for bass boost. If the volume control was set at the center tap or less then the midrange and high frequencies were reduced by the RC and bass went through without reduction. As the volume was turned up there was less bass boost.

You need about 5 times more gain, not 50 times from the other amplifier. Simply cut out the resistor in series with a capacitor to ground at the center tap of the volume control. You will not miss the bass boost because mono AM radios are used for speech, not music.

 

[Frankchie]

The module has a gain control that simultaneously adjusts both channels. So by adjusting the gain hopefully that will solve the "too much gain" problem.

Anyway, I just checked the module again and it is dead. Somehow the unregulated 17v level of the 12V wall wart that I substituted must have damaged the module.

I have another module in transit to me so I'll pick things up again when I get it.

 

[Frankchie]

The module has a gain control that simultaneously adjusts both channels. So by adjusting the gain hopefully that will solve the "too much gain" problem.

Anyway, I just checked the module again and it is dead. Somehow the unregulated 17v level of the 12V wall wart that I substituted must have damaged the module.

I have another module in transit to me so I'll pick things up again when I get it.

I should have mentioned that the radio is AM/FM so maybe the bass boost circuit should be left intact.

Thank you

 

[Audioguru]

I should have mentioned that the radio is AM/FM so maybe the bass boost circuit should be left intact.

You said the 1972 car radio was mono. In 1972 I worked for Philips car radio engineering. All the AM/FM car radios we made were stereo, not mono. The cheap AM-only radios were mono.

You said you are taking the audio from the tap on the radio's volume control which has reduced levels of mid and high frequencies. Instead, try taking the audio from the input end of the volume control and it should sound much louder and fine without bass boost if the speaker is half-decent and is enclosed properly. All my AM/FM radios do not need and do not have bass boost. Years ago I added bass boost to my clock radio to allow its little 3" speaker to sound better.