Testing with speaker

[Mark245]

Hey all,

I have a question about testing my new audio amp design. The design is meant to drive an 8 ohm speaker. Right now the only speaker I have is a 4 ohm speaker but I would like to test with 8 ohms. Can I place a 4 ohm resistor in series with the speaker (in between the Zobel network output and the speaker)? This is just for preliminery testing purposes until I find a suitable 8 ohm speaker.

Thanks,

Mark

[trd1587]

Originally Posted by Mark245

Hey all,

I have a question about testing my new audio amp design. The design is meant to drive an 8 ohm speaker. Right now the only speaker I have is a 4 ohm speaker but I would like to test with 8 ohms. Can I place a 4 ohm resistor in series with the speaker (in between the Zobel network output and the speaker)? This is just for preliminery testing purposes until I find a suitable 8 ohm speaker.

Thanks,

Mark

Connect 2 x 4omh speaker in series.

AMP----- +SPK/\SPK- ------ +SPK/\SPK- -----GRND

[Bone]

Yes you can if you don't have another speaker

[AndrewT]

Hi,
almost all of the testing is done without a speaker connected.
Only once you know the amplifier is operating correctly do you then connect a speaker to find out what it sounds like.

You will need an input shorting plug, an output dummy load, a range of small capacitors to add in parallel to that dummy load, a DMM that reads down to full scale 200.0mVdc and 200.0mVac. An AM oscillation detector.
And if you have an oscilloscope you also need a signal source.

[Mark245]

AndrewT,

Actually you have hit on the point of what I am trying to do. I have breadboarded a 20 watt amplifier. I want to make sure it is going to work right before I go to the trouble of making a pc board. The first time I plugged it up, the output stage blew. I replaced the output transistors and fixed the bad connection that I had. Now, when I adjust the pot on Qbias, I get a Vce of 0 volts and 1.3mV across R35 and R36 no matter how I adjust the pot. I read that you want 1.28v Vce on Qbias and 8mV across R35 and R36. Does the load need to be connected to set the bias current? I have checked all transistors and they are Ok. I have also verified all connections.

The circuit schematic is attached.

You mentioned using an AM oscillator detector. What is this used for? To detect if the amp is oscillating? I have an O'scpoe and function generator. What is the correct procedure for testing if the amp works?

Thanks everyone for the responses so far, it is a big help. Any further help will be greatly appreciated.

Mark

[anli]

If the testing aim is a subjective expertise, you can not use 4 ohm series resitor: you will get peak at low freqs as a result of, in fact, using speakers with 4 ohm output resistance amplifier. Two speakers in series is more appropriate way.