Car Amp LA4440

[raghs]

Hi All,

I am trying to build an amplifier around LA4440. There is also a preamp, which is feeding the input to the main amp. I am driving my preamp from the RCA output of a car tape.

When the input to the preamp is not connected to the car tape, if I increase the volume of the amplifier I hear loud humming.

If I connect the car tape RCA output to the preamp input and increase the volume of main amp, then there is a loud flutter.

I have been breaking my head on this!
I checked the pot meters and they are properly grounded..
Can anyone tell me what do I need to solve the problems mentioned above??

 

[Perry Babin]

Ground the LA4440 amp and preamp to the chassis of the head unit. Do not let it ground through any other point. Does that reduce the noise?

 

[raghs]

Thanks Perry Babin, Will try & let you know

 

[raghs]

Hi,

I tried what you told, but still there is humming. Also the preamp impedances are not matching, so how can I match them?

 

[Perry Babin]

Was the humming reduced?

Is the hum related to the engine speed?

Can you post a schematic of the circuit you're using?

 

[raghs]

Hi Perry,

I am trying out the circuit using transformers right now and the humming has gone away completely. thanks to your trick - I grounded the amp & preamp to the head unit.

I have a new question for you:
The pre-amp output from my tape is basically a pseudo signal. What they have done is they have put a 22k resistor in series with 1MF electrolytic capacitor to the speaker out and provided a new RCA jack!!

Now when I am feeding this output to my preamp input, the sound is muffled..its not clear. I guess the impedances are not matching? What do I need to do for this??

 

[Perry Babin]

I don't think the problem is an impedance mismatch unless the input of the amplifier has a relatively large RF shunt capacitor. Can you provide a more detailed schematic?

 

[raghs]

Which schematic do you need? The preamp of the circuit which feeds the output to main amp?

Basically I am giving the output of the headunit preamp to the preamp of my circuit, which in turn feeds the output to the main amp..does that make sense??

 

[Perry Babin]

Without knowing the circuit for the la4440 amp, I can't determine why the signal sounds muffled.

 

[raghs]

Hi Perry - I am using the LA4440 in bridged mode, here's the schematic for the same. I got this circuit readymade from the market. The schematic just shows the LA4440 in bridged mode, I dont have the circuit diagram for the preamp & bass-treble module
Basicall the pre-amp is using a chip - HWCT35244 and 2 transistors BC548. I could not find any information or datasheet on HWCT35244

Eagerly awaiting inputs from you

 

[Perry Babin]

The 0.047uf cap on the input is far too much capacitance if you're driving this with a 22k ohm resistor.

 

[raghs]

I dont see any capacitor on the input (Pin 2) in the schematic

Also I observed that when I connected the speaker out of the headunit directly to the amp pin, the sound is loud & clear - but I loose the bass-treble controls. And when I give the RCA out to the preamp, then the sound is still loud - but it is muffled.

 

[Perry Babin]

I'm sorry. I misread the schematic.

I think you'll have to post a complete, accurate schematic of the entire circuit, precisely as you have it to determine why it's muffled.

 

[raghs]

Thanks for you valuable time Perry, I really appreciate your help. But, as I told before, I got this circuit readymade from the market..hence I dont have the schematics.

But all I know is that the preamp has a shielded wire which we are supposed to connect to tape head (deck mechanism), so I assume the input impedance would be approx 200 ohms.

I want to drive the preamp + AMP using my car tape RCA out (pre-amp) and this is a Pyramid make 1704MP radio

 

[raghs]

Forgot to add - my car tape (Pyramid 1704 MP) does not have a tape mechanism or CD player..it just supports radio, USB pendrive, aux input & MMC/SD card.

 

[Perry Babin]

If it was to be driven by a tape head, there will be significant equalization (needed for the tape head to produce a flat frequency response).

You may want to replace the preamp you have. You could use an IC like the following:
http://sigma.octopart.com/89270/datasheet/Philips-TDA1524AN.pdf

 

[raghs]

Could you suggest me some other Chips, better than LA4440, which can be used for car amp and will have around 30Watts output? I should be able to run it on 12V car battery..
MOSFETs also welcome

 

[Perry Babin]

With only 12v, you won't see an improvement with other ICs unless they can drive a lower impedance load. With 12v and a 4 ohm load, you won't be able to get more than 20 watts.

 

[raghs]

I guess I am ok with 20Watts RMS, and my LA4440 in bridged mode can deliver upto 19W RMS to an 8Ohm load.

The preamp, which I am using is based on LA3161. I have the following questions for you:

1) What would be the impedance & output voltage of a cassette tape head?
2) Can I connect my preamp (LA3161) to the volume control of my car tape (Pyramid 1704MP)?? Schematic of the car tape attached.

Kindly advice.

 

[Perry Babin]

You don't need to preamp IC. The signal you have is enough to drive the LA4440.