A music coming from component.

[wuidtsams]

Dear all,

I recently assembled a 2A3 SE amp. When running in the amp, I used the notebook to play MP3 as the signal source, and connected two 8 ohm, 5W resistor to each output. When I adjusted the volume control into the maximum, there is the small noise coming from the chassis, the MP3 music. My questions are the following:

1. Is the music coming from the components, such as resistor or some kinds of inductance load, transformer?
2. Shall I find it out to change the component? Is it necessary?

Thanks for your time in responding this in advance.

 

[SY]

My suspicions would go to any largish coupling caps (especially "designer" caps which often are rather loosely wound) and the output transformer. Try isolating the components mechanically from the chassis- rubber grommets will work for the transformer, some soft hot-melt glue may help with the caps.

 

[Sch3mat1c]

Pffbt, coupling caps don't have ANY current or voltage through or across them (aside from the DC bias voltage). It's the OPT, especially under clipping when harmonics are more easily heard. Also more easy to hear if the load is disconnected, which is bad mojo at maximum output..

Tim

 

[cogsncogs]

connected two 8 ohm, 5W resistor to each output. When I adjusted the volume control into the maximum, there is the small noise coming from the chassis, the MP3 music.

I would think so! Why would you connect resistors to the outputs and crank it all the way up in the first place? The so called "small noise" you are hearing is coming from the OPT's as is to be expected!

Wayne

 

[Sch3mat1c]

Why would you connect resistors to the outputs and crank it all the way up in the first place?

Um....testing?

Even more fun when you have 1uF across it.

Tim

 

[wuidtsams]

A small noise.

Dear all,

Thanks for your response.

I just attached the photo. The two OPT locates at the upper small compartment, and all other component including the Power Transformer locates in the lower chassis. And the noise comes from the lower chassis.

According to your comments, I will add the rubber gasket between the mounting surface and the transformer.

The reason to run the amp this way is because I just placed the amp in my office. Since the amp was being completed not long time ago, I think for the new mechine it would be better to run it as more as possible. And I also want to know whether it's stable for long time operation. So that I connect two resistors for each output(because in the office) to run the mechine whole day long.

Thanks for all your kind help again....

 

[cogsncogs]

According to your comments, I will add the rubber gasket between the mounting surface and the transformer.

Under normal listening conditions, i.e. not listening to resistors cranked all the way up IMO that would be totally unnecessary. I can duplicate the same on my amp or any tube amp at those conditions! Sooo, don't worry about it! It's normal! You won't hear it! Rubber gaskets won't stop the sound you are hearing at THOSE conditions anyway. Plus it's not a good idea, you need that heat transfer (and electrical) from the OPT to chassis.

Um....testing?
Even more fun when you have 1uF across it.

Yeah maybe! And a scope too!

Cheers
Wayne

 

[Geek]

Pffbt, coupling caps don't have ANY current or voltage through or across them (aside from the DC bias voltage).

Haven't you ever heard a big ol' disc ceramic cap act like a little piezo speaker? The old ones did this quite well. Never heard it in the new ones though.

 

[Sch3mat1c]

Ah, I had a 0.01uF 500V disc on the breadboard once that sang. As I recall t'was the resivoir cap on a flyback supply I was working on; the sound was parasitic oscillations coupled by the PSU rail (oops).

But the whole purpose of a coupling cap is to pass miniscule amounts of current with little voltage change across it, hence it's never going to make a noise.

Tim

 

[wuidtsams]

thanks for all.

Dear all,

Thanks for your kind response. It seems that there is lots of problem never heard of for me, such as ceramic cap. At first, I am afraid that there may be the error in some components.

Thanks for all your kind response. I decided to re-assemble the amp again, not because of the small noise, but for the poor wiring.

Maybe after the re-assemble, I will report to you whether the small noise still exists. Thanks for all.

Charlie

 

[resident]

@sch3mat1c

Even more fun when you have 1uF across it.

In parallel with resistor?What kind of cap?
And which is the reason to put a 1uF cap?
I'm intersted to know about this.I didn't know it.I was doing my measurements only with a resistor.

 

[Sch3mat1c]

To test the response and stability of an amplifier, you connect a cap across the load resistor and note response, ringing, oscillations, etc.

Some can handle just the cap, I haven't had any iron good enough to tackle this though. I don't really care since no speaker actually looks like a lone capacitor anyway.

The idea being to watch the output waveform on a scope in response to various input, especially a square wave but also a sine sweep is useful (to note frequency maxima, cutoff, etc.).

Tim

 

[resident]

I still can’t understand why to use a cap.You can see response,ringing,oscillations without a cap,too.

 

[Sch3mat1c]

No, you can't. It changes the response. An amp can easily have no ringing, not even overshoot with a pure resistive load and suddenly oscillate on its own when a 1uF cap is connected.

Tim

 

[resident]

hmmmm,
and I was sooo happy that my new amp has a responce of 10Hz-40KHz (-3dB).
I'll test it again with the cap!What kind of cap to use?
The cap is used cause speakers have a capacitive load too?Why 1uF and not a different value?

 

[Sch3mat1c]

Some speakers, like peizo tweeters, can present a good capacitance that drives amps nuts.

You can use anything from .047 to 4.7uF or more. Depends on how much effect you want.

Such things really only cause a problem on NFB amps because it increases the phase shift, increasing oscillation potential. For an open-loop amp, you'll probably just see a rounding of the leading edge and maybe some ringing (from the OPT and capacitor acting together).

Tim