Determining output impedance of an audio amp?

[PlainBill]

I have repaired a HDTV that was originally sold with detachable speakers. However, they were missing when I received it. I had assumed that it would be simple to look up the impedance specs on the speakers and make sure the substitutes matched. I should have known better. Neither the Users Manual not the Service Manual gives that information.

Now I'm faced with determining the output impedance of the internal amp. At one time (a LONG time ago) that was easy - 3.2 ohms. Of course, with the advent of Hi-Fi, 8 ohm speakers became common, but 4 and 16 ohm speakers aren't unheard of. I HAVE hooked up a pair of 4 ohm speakers from a defunct 'home theater system' and the amp does work, but don't know if they match the amp?

Any suggestions?

PlainBill

[Th3_uN1Qu3]

Re: Determining utput impedance of an audio amp?

If it doesn't get hot, it's fine. I think you are mistaking terms here though, the output impedance of a solid-state amp is somewhere around 0.1 ohms or even lower, however the actual speaker impedance it can drive is related to the peak current capability of the output transistors.

I'd trace the wires to the amplifier chip and consult its datasheet. I haven't seen a lot of monitors with built-in speakers, but all i've seen so far used 6-ohm speakers. Which means they can do lower than 8 ohm, but not quite there for 4 ohms... But of course, if you don't abuse it chances are it'll work fine for a long long time.

[Dgtech]

Re: Determining utput impedance of an audio amp?

Hey PB, I'm just wondering... Sometimes I get lucky with looking things up on ebay using the model number of the set. If these are detatchable, maybe you can find it there and ask the seller for the specs.

[PlainBill]

Re: Determining utput impedance of an audio amp?

Hey PB, I'm just wondering... Sometimes I get lucky with looking things up on ebay using the model number of the set. If these are detatchable, maybe you can find it there and ask the seller for the specs.

That's a good idea; too bad it didn't pan out. I did have better luck with a different approach, however.

If it doesn't get hot, it's fine. I think you are mistaking terms here though, the output impedance of a solid-state amp is somewhere around 0.1 ohms or even lower, however the actual speaker impedance it can drive is related to the peak current capability of the output transistors.

I'd trace the wires to the amplifier chip and consult its datasheet. I haven't seen a lot of monitors with built-in speakers, but all i've seen so far used 6-ohm speakers. Which means they can do lower than 8 ohm, but not quite there for 4 ohms... But of course, if you don't abuse it chances are it'll work fine for a long long time.

Well, your second idea was a winner. The audio amp IC is a TPA3005D2. According to the datasheet the minimum speaker impedance is 8 ohms with a 12 volt supply. Philips had upped the supply voltage to 16 volts (still within specs) so it looks like using the 4 ohm speakers would have been a BAD IDEA at high volume levels.

Then just to prove that Lady Luck is a practical joker, I was searching for my roll of speaker wire so I could hook up a pair of 8 ohm speakers I had on hand. (I'd been trying to remember where I put it for a couple of days). When I found it, I also found a pair of 16 ohm speakers that worked quite well.

One of the advantages of getting old - I keep finding stuff I never knew I lost!!!

PlainBill

[Th3_uN1Qu3]

Re: Determining utput impedance of an audio amp?

Well, your second idea was a winner. The audio amp IC is a TPA3005D2. According to the datasheet the minimum speaker impedance is 8 ohms with a 12 volt supply. Philips had upped the supply voltage to 16 volts (still within specs) so it looks like using the 4 ohm speakers would have been a BAD IDEA at high volume levels.

Mmm, a class D. That's quite decent. It has thermal protection but no overcurrent protection in sight, so yeah, not a good idea to run it on 4 ohms. In my (albeit limited) experience, thermal protection never actually prevents the device from failing. The TDA2030 series are famous for this, they are over temperature protected... in the datasheet. The only thermal protection that survives a hard short is in the LM317 and 79xx regs, but a prolonged overload still kills those as well.

One of the advantages of getting old - I keep finding stuff I never knew I lost!!!

That's a good one. Have to remember it. Glad you found your answer.