Desperate Help Needed

[Sam_sky]

Hi,

I have a very old Amplifier (Stereo) powered by two LA4058 IC's

as per the spec - it says it is a 100W Amp - has a circuit more or less like this

• http://fixmag.ru/download/1259351172_sp-011.pdf
• Refer Page number 5 for the circuit

but the difference in my circuit is that - it uses the same circuit twice meaning - I have the similar circuit with two LA4058 IC's

Guess the POWER AMP with each IC is to power Right and Left channel's ---------> not sure

- The power source is 13.6V AC transformer

- Connects a 4 ohms speaker and 4 ohms tweeter

- I use it for my guitar ( connected via a processor).

Issue:

• Every thing was working well as desired - issue popped up after I decided to move the amp complete unit to a speaker box
• (like one unit where it will have Speaker and amp in same speaker box) - like a guitar amplifier setup.

- When I connect the speaker to the Right channel - all I hear a distortion buzz.. ( increases when I increase the volume )

- when I connect the Speaker to the left Channel - I get a duh duh duh sound ( Think that is distortion but with lesser strength) ( increases when I increase the volume )

Trouble shooting done:

1. Checked for the earthing or common for the Transformer and power amp and wiring connections

2. When I disconnect my Pre amp from Power Amp. I get no sound from the speaker - unsure if the speaker getting the signal

but When I power ON.. I dont hear or feel the Speaker cone move..

- Unsure if I can connect the CD player or guitar directly to the Power Amp to check the problem is with power Amp or Preamp.

Also the LED for Both channels does not glow - 

Unsure why the noise is coming - basically the setup is not working

Please help..

Sam

 

[audioguru2]

Nobody makes an LA4058. You got the numbers mixed up, the schematic shows a Sanyo LA4508.

Your 13.6V transformer must feed the rectifiers on the Power Part of the schematic. Maybe you fed the transformer wrongly to the amplifier instead of to the rectifiers? Then probably many parts are destroyed. Your 13.6V transformer will produce a peak of 19.2V and the rectifiers reduce it to +17.2V.

With a 17V supply the LA4508 produces about 6W at clipping into 4 ohms per channel. Who told you 100W? It has fairly high distortion at low and high frequencies and it cuts frequencies above only 5kHz.

Your amplifier probably uses one LA4508 for left woofer and left tweeter and the second LA4508 is probably used for right woofer and right tweeter. 

 

[Sam_sky]

My bad it was LA4508.. 

I managed to sort out the issue.. but there are other problems which I would tell below..

Basically the power Amp seems to power up two channels separately - Right and left and it works fine. 

Isolated the issue onto the preamp.. but it proved to a issue with the input switch (or I think so).. sound ok now with the mind distortion..

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There is one more issue from the beginning (even before I could dismantle) - the sound is getting distorted - this is not like it doing this on high volume.. more like a guitar over drive sound.. (Please note that the Amp was working fine with the clean sound with the same circuit previously).

The preamp uses 3 transistor 547b and does not have any overdrive or distortion circuit..

- I need to check sound without the preamp check the quality of the sound.. 

- I have checked with one 4ohm speaker and a tweeter (connected in parallel) and without the tweeter - still the same

My question..

1. Why does this it distort - (it is like a nice over drive sound)..(sounds great when I play my guitar with distortion effect) - but the cleans notes are affected..

2. If it is the preamp - what component should I check to avoid the clipping? (Bass and treble set to low and high - does not have an effect on distortion)

Sam

 

[audioguru2]

The output of a preamp will produce clipping distortion if the input level is too high. The pickup from an electric guitar produces a level much higher than an FM tuner, a CD player or an MP3 player. Most preamps for a guitar pickup use a Jfet because of its very high input impedance with a gain of only about 1.4 times.   

 

[Sam_sky]

The output of a preamp will produce clipping distortion if the input level is too high. The pickup from an electric guitar produces a level much higher than an FM tuner, a CD player or an MP3 player. Most preamps for a guitar pickup use a Jfet because of its very high input impedance with a gain of only about 1.4 times.   

Actually I tried the a Cell phone headphones out thro this at very low volume .. still the same..

I will connect directly and update you and check the result..

Only concern I have is - because of this distortion, is it bad for the speaker or the AMP..?

* without the preamp see if this occurs..

 

[audioguru2]

Distortion can blow out a tweeter because distortion has strong high frequency harmonics.

Try feeding an input directly to the power amplifier without connecting the preamp.

 

[Sam_sky]

Distortion can blow out a tweeter because distortion has strong high frequency harmonics.

Try feeding an input directly to the power amplifier without connecting the preamp.

Thank you...

Yes - I tried bypassing and it sound fine..

(Think the preamp is messed up - might put a switch to alternate between preamp and direct in power amp in)

Because the overdrive sound from preamp is nice to hear (Unsure if the preamp blow out soon).. 

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Audio Guru you are awesome.. This is not the first time you have helped me - previously you have helped with a project with TL072 - on board Guitar Preamp

and came out with flying colors.. and I took that to a professional guitar guy and he failed after preparing 1000's PCB's with the same circuit.

but I used a General PCB and came beautifully well & with your suggestions..  ^-^

--------------------

Guess you can do lots of freelancing earn with all your suggestions .. I use my Excel automation on Fiverr dot com as a hobby and get decent money.

Professionals like you could earn 1000K+ in a month within 3 to 4 months of experience

Well I'm not advertising the freelancing site.. just telling..

Take care

Sam

 

[audioguru2]

Thanks for the freelancing job recommendation but I am a 70 years old government worker (I am retired with a government pension and some stocks and bonds) so I do not need more income. I do whatever I want whenever I want.  

 

[Sam_sky]

Thanks for the freelancing job recommendation but I am a 70 years old government worker (I am retired with a government pension and some stocks and bonds) so I do not need more income. I do whatever I want whenever I want.  

Awesome..