zobel network + decouple cap?

Thread Starter

DonLduk

Joined Jun 12, 2017
16
Hello.
I have a 500watt amp and the output stage goes straight to the speakers, like this schematic where the output power stage are with toshiba SC5200 and SA1943.

I want to know if I should add a zobel network and a decoupling capacitor? or just a zobel network? or just the decoupling capacitor? or just the discharge capacitor? I have read a lot of different articles and there are different solutions for an optimal connection, and now I'm confused.

The VCC is +45v and -VCC is -45V, amp wattage is 300W and speaker is 8ohm 1000 Watt 15" woofer.

The schematic is like this:

Screen Shot 2017-11-05 at 10.22.48 PM.png

I would like to request your help to clarify how should I connect the speaker. Right now it is straight and it works ok but there are some pops happening when turned on/off.

Thank you in advance.
 

GopherT

Joined Nov 23, 2012
8,012
Hello.
I have a 500watt amp and the output stage goes straight to the speakers, like this schematic where the output power stage are with toshiba SC5200 and SA1943.

I want to know if I should add a zobel network and a decoupling capacitor? or just a zobel network? or just the decoupling capacitor? or just the discharge capacitor? I have read a lot of different articles and there are different solutions for an optimal connection, and now I'm confused.

The VCC is +45v and -VCC is -45V, amp wattage is 300W and speaker is 8ohm 1000 Watt 15" woofer.

The schematic is like this:

View attachment 138753

I would like to request your help to clarify how should I connect the speaker. Right now it is straight and it works ok but there are some pops happening when turned on/off.

Thank you in advance.
If you have 90V peak-to-peak maximum with +/- 45v rails, then the maximum power output is for a sine wave is 32vRMS 4 amps, into one 8 ohms. Yields about 130 watts.

If you are basing it on a square wave, then I get 45 v into 8 ohms and 5.6 A and 250w but that assumes you are using it for a siren instead of music. How did you calculate 300w?

Why are you using a 50-year old op amp instead of something modern - do you enjoy the hiss from this device?
 

DickCappels

Joined Aug 21, 2008
6,457
Believe it or not, you can go to many large markets in N.E. Thailand and buy high powered audio amplifiers with an LM741 at heart of each stage. Here, as is some other places, the quality of the sound is measured solely by how loud it is.

To answer DonLduk's questions, I would definitely add decoupling capacitors to the power supply rails. Make sure they are rated for several amps of ripple current if you want them to last a long time. The capacitors will keep the ripple current from being drawn from the power supply and help keep the audio signals on the board.

If you were asking about capacitively coupling the signal to the speaker, I probably would because otherwise the failure of a 2¢ component in the amplifier could result in putting the entire 45V across the voice coil. You could also fuse the speaker lead or on the power supply lines to the complimentary pair(s).

If your amplifier can reproduce a square wave nicely without signs of instability like oscillation or excessive ringing you probably would not benefit by adding a Zobel network.
 

Thread Starter

DonLduk

Joined Jun 12, 2017
16
Thank you for your comments. I have replaced the 741 for a 5532 and it sounds much better, day and light. You guys are awesome with your historical knowledge. How could you tell an opamp is old? i guess only from experience.

Now, regarding the connection of the speaker. Sometimes the amp goes "unstable" and after manipulating the tone pots it stards adding noise (like a ringing hum that scalades slowly on volume).

Also, which values should I add to decouple the speaker? will a 0.1uF poliester capacitor enough? to set it in series with one wire of the signal?
 

crutschow

Joined Mar 14, 2008
25,116
How could you tell an opamp is old?
Because it was one of the first IC op amps that was available when I had my first engineering job. ;)
which values should I add to decouple the speaker? will a 0.1uF poliester capacitor enough?
Not nearly.
Due to the low impedance of a speaker you need a very large capacitor if you want response down to 20Hz.
Use at least two 2,000μF, 50V, electrolytic capacitors back-to-back (since they are polarized) in series with the speaker.
 
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