an odd amp question

[Farview]

If you run your small crappy amp turned up loud with the speaker disconnected, you will likely soon have a small crappy amp with a blown output transformer.

Transistor amps don't have output transformers. It depends on the amp whether or not it will be a problem.

 

[little guy]

Transistor amps don't have output transformers. It depends on the amp whether or not it will be a problem.

how do i tell if i have an output transformer???

 

[little guy]

it's a Dean Markley K-20

here's a pdf manual of my amp

 

[Farview]

how do i tell if i have an output transformer???

If it's a tube amp, you have an output transformer. You don't, but as arcaxis said, you will blow your output transistors if you run them with no load.

 

[little guy]

If it's a tube amp, you have an output transformer. You don't, but as arcaxis said, you will blow your output transistors if you run them with no load.

is there a way to tell if i've already blown them???

 

[Farview]

Your amp won't work and (in the case of a tube amp) smoke will be pouring out of it. Or a fuse will blow, much less spectacular, but has the same effect.

 

[ggunn]

If it's a tube amp, you have an output transformer. You don't, but as arcaxis said, you will blow your output transistors if you run them with no load.

Interesting. Do you know the mechanism for that? If I remember my bipolar t'sistor theory correctly, if there is no emitter (output) current (and there can't be if the circuit is open), then there can't be any base (input) current. If the output t'sistors are FET's, then the gate current is effectively zero no matter what the load. I know how high output current blows t'sistors (gate rupture or thermal runaway), but I don't know a mechanism for an open circuit failure.

As I learned in this forum (with a little outside reading) a while back, it is the energy storage in the inductance of the output transformer that causes voltage spikes on the primary if the secondary is open, and that's what damages tube amps with output transformers. If the output transistors are directly coupled to the speaker outputs and there is no transformer in the circuit, what causes the transistors to blow with no load?

Please forgive my geekiness; I'm an engineer and I just can't help myself... ;^)

 

[boingoman]

Interesting. Do you know the mechanism for that? If I remember my bipolar t'sistor theory correctly, if there is no emitter (output) current (and there can't be if the circuit is open), then there can't be any base (input) current. If the output t'sistors are FET's, then the gate current is effectively zero no matter what the load. I know how high output current blows t'sistors (gate rupture or thermal runaway), but I don't know a mechanism for an open circuit failure.

As I learned in this forum (with a little outside reading) a while back, it is the energy storage in the inductance of the output transformer that causes voltage spikes on the primary if the secondary is open, and that's what damages tube amps with output transformers. If the output transistors are directly coupled to the speaker outputs and there is no transformer in the circuit, what causes the transistors to blow with no load?

Please forgive my geekiness; I'm an engineer and I just can't help myself... ;^)

ggun,
I enjoyed that discussion, and asked the same guy about this. Again, some of it I didn't understand, but may point you towards something.

He said it used to be much more common in older designs, not so much in newer op-amp designs. A lot of old designs used a driver transformer, which resulted in the same instability that the output trannie in a tube amp does. Any input signal drove the amp to full power, and both transistors in a pair would start to conduct. He said something about them being connected through ground, and when both conducted they essentially saw a short circuit, even though there was no load. Make sense? It may to you.

Improvements in the placement of the feedback loop have also eliminated many of the instabilities of an unloaded transistor amp, according to my geek friend. He said instability can result from phase shift both from the transformer and feedback loop, and that modern solid state designs have their feedback loop derived from a point with minimum phase shift. He also said most modern amps have small load resistors, and that almost any load will keep them from oscillating.

So his answer was, anything transistor made in the last twenty-five years or so won't be affected by running with no load.

 

[ggunn]

ggun,
I enjoyed that discussion, and asked the same guy about this. Again, some of it I didn't understand, but may point you towards something.

He said it used to be much more common in older designs, not so much in newer op-amp designs. A lot of old designs used a driver transformer, which resulted in the same instability that the output trannie in a tube amp does. Any input signal drove the amp to full power, and both transistors in a pair would start to conduct. He said something about them being connected through ground, and when both conducted they essentially saw a short circuit, even though there was no load. Make sense? It may to you.

Improvements in the placement of the feedback loop have also eliminated many of the instabilities of an unloaded transistor amp, according to my geek friend. He said instability can result from phase shift both from the transformer and feedback loop, and that modern solid state designs have their feedback loop derived from a point with minimum phase shift. He also said most modern amps have small load resistors, and that almost any load will keep them from oscillating.

So his answer was, anything transistor made in the last twenty-five years or so won't be affected by running with no load.

Thanks, boingoman, though I'm a little disappointed that you got straight to the answer. Don't you want to 'aggle? ;^)

 

[little guy]

so could someone, in plain english, let me know what i should get from this thread???

 

[boingoman]

so could someone, in plain english, let me know what i should get from this thread???

LOL.

Your sound will probaly be better if you mic the amp.
You probably won't hurt it by running it without a speaker, but why take the chance.

 

[little guy]

sweet!! thanks for the help everyone!! i'm gonna hook the speaker back up and mic it!!