adaptors

[CynicalMan]

Recently, I have been looking at using power adaptors (the kind that plug into wall outlets) in my experimentation. The problem is that even new adaptors, measured with a load, seem to put out 3 or 4 volts more than they are supposed to. I tried one in a circuit and I think it might have destroyed the op-amp I was using. Any suggestions or explanations?

 

[blueroomelectronics]

Standard wall adapters are not regulated. Switchmode ones are. What voltage do you need?
Canada eh, whereabouts?

 

[CynicalMan]

I need 9V for guitar effects.

 

[Blueteeth]

A sufficiently talented fool right here....

As Bill rightly suggested, switch mode power supplies are regulated, just not as well as linearly regulated supplies. You have a few options.

1) 9V SMPS DC power supply. Many about, fairly cheap...but..*may* be noisy for audio. I know many claim that SMPS's work at high frequency, but in my experience, they kick out a few spikes slap bang in the middle of the audio spectrum. Mind you, with audio, most things interfere.

2) Standard 9V *Voltage regulated* supply. Will stay at 9V whatever. Cheaper than the above. Don't buy from random hardware stores, get a half decent one,

3) Unregulated 12V DC Adapter -> 7809 regulator. Essentially a DIY version of the above. You'll just need a couple of caps, the regulator, and a heatsink for it.

I'm afraid being a Brit, I can't help you much on sources. The usual suspects of Farnell, Mouser, Digikey spring to mind. And don't forget Sparkfun, and the ever faithful Ebay.

Blueteeth

 

[audioguru]

Most ordinary opamps have a max allowed supply voltage of 36V. 12V from a 9V wall adapter will make an opamp happy.
Which opamp?

 

[CynicalMan]

It was an lm386n in this smokey amp circuit. It previously worked with a 9V battery but with an adaptor (9V 1A, showing an output of 13V), the 386 heated up and afterwards it did the same with a battery.

I find it hard to believe that it's a regulation problem since every adaptor I tested with a load had a voltage of 3 or 4 volts too high.

 

[Nigel Goodwin]

I find it hard to believe that it's a regulation problem since every adaptor I tested with a load had a voltage of 3 or 4 volts too high.

Because all your adaptors were cheap crappy unregulated ones, the specified voltage is at full load, and at lower loads will be considerably higher than specified.

Buy a decent quality regulated one, and it will be fine.

 

[audioguru]

A 9VDC unregulated adapter will have an output voltage of about 13V or more without a load. Its voltage will be 9V (with a lot of ripple) at its rated current. The current of an audio amplifier changes with whatever it is playing.

Your Smokey Amp blew up because it is missing two very important parts that keep it from oscillating. Oscillation makes it get too hot. When it melts inside then it is destroyed. The datasheet for the LM386 shows the two very important parts.

 

[Optikon]

It was an lm386n in this smokey amp circuit. It previously worked with a 9V battery but with an adaptor (9V 1A, showing an output of 13V), the 386 heated up and afterwards it did the same with a battery.

I find it hard to believe that it's a regulation problem since every adaptor I tested with a load had a voltage of 3 or 4 volts too high.

An opamp driving nothing is usually a pretty light load. When you tested your adaptors with a load, was it a significant load or the measly opamp? The unregulated adaptors will get down to their correct output with a significant load. Try full load on them with a dummy power resistor.. if they still show too high, then they are damaged.

 

[audioguru]

The LM386 is a power amp IC not an opamp. It drives an 8 ohm speaker.
Its max output current is a peak of about 400mA.
The max ouput current from an opamp is a peak of only 20mA to 40mA.

 

[CynicalMan]

The load I used for testing was a 10KΩ resistor.

 

[audioguru]

The two missing parts in the Smokey Amp are its load of 10 ohms at very high frequencies where an 8 ohm speaker's inductance causes its impedance to be very high.
When the amplifier oscillates at a very high frequency because of the missing two parts then it gets extremely hot.

 

[Torben]

The two missing parts in the Smokey Amp are its load of 10 ohms at very high frequencies where an 8 ohm speaker's inductance causes its impedance to be very high.
When the amplifier oscillates at a very high frequency because of the missing two parts then it gets extremely hot.

That looks like an RC snubber to me--is that the correct term for it?


Torben

 

[audioguru]

I think the RC low impedance at high frequencies is called a Zobel Network. A Zoble network is used in a passive crossover network for a speaker with a woofer and a tweeter.

 

[Torben]

I think the RC low impedance at high frequencies is called a Zobel Network. A Zoble network is used in a passive crossover network for a speaker with a woofer and a tweeter.

Ah, OK. "Zobel network" turns up some good reading on google. I do see the term "RC snubber" used for it, but from what I'm finding I think maybe "RC snubber" refers more to the network when used to protect against EMF kickback.

Thanks!


Torben

 

[Hero999]

Standard wall adapters are not regulated. Switchmode ones are.

Not all switchmodes are regulated, some cheap and nasty mobile phone chargers aren't regulated. I have a circuit board from one of those and there's no feadback from the secondary to primary and the output voltage is 8V off load and it's specified at 5V.

Your Smokey Amp blew up because it is missing two very important parts that keep it from oscillating. Oscillation makes it get too hot. When it melts inside then it is destroyed. The datasheet for the LM386 shows the two very important parts.

It's called a smokey amp.
What did you expect it not to smoke?