HI everyone
I try to build a car amp using DC/DC amp (+/-30v).
And using 78xx serise get +/-15v for preamp.
But when i turn on , it always get a "bomb" sound.
If i turn up the gain in preamp.
The turn on sound get louder too.
I measure the preamp terminal output , when i turn on , it get a pulse.
It seems causing by different charging volacity in +/- voltage.
I have try to slow down the pwm's soft-start. It help not too much.
So , dose anyone have ideal to solve it ??
Or how to mute the amplifier stage when turn on for 1 second??
I have a ideal , when i turn on , i get the preamp terminal output down to gnd using a transistor.
But i have't try it , I will tyr.
I try to build a car amp using DC/DC amp (+/-30v).
And using 78xx serise get +/-15v for preamp.
But when i turn on , it always get a "bomb" sound.
If i turn up the gain in preamp.
The turn on sound get louder too.
I measure the preamp terminal output , when i turn on , it get a pulse.
It seems causing by different charging volacity in +/- voltage.
I have try to slow down the pwm's soft-start. It help not too much.
So , dose anyone have ideal to solve it ??
Or how to mute the amplifier stage when turn on for 1 second??
I have a ideal , when i turn on , i get the preamp terminal output down to gnd using a transistor.
But i have't try it , I will tyr.
A simple small signal bipolar transistor works fine as a mute device, altough there may be better approaches. Remember to apply some negative bias to the base when the circuit is not muted (-5V or so assuming a NPN).
You may also try to find out why the preamp produces such a turn-on transient. In my experience, op-amps powered from +-15V tend to be somewhat quiet at power on.
You may also try to find out why the preamp produces such a turn-on transient. In my experience, op-amps powered from +-15V tend to be somewhat quiet at power on.
I also think the opamp should be quiet at powr on .
Does it cause by voltage rising up and down to +/-15v to effect ??
I try to take off soft-start function.
The output offset at turn on get more .
I will try use a npn transistor to get the input of amp to gnd.
I get a circuit example from member "Sdoom".
It very helpful.
Does it cause by voltage rising up and down to +/-15v to effect ??
I try to take off soft-start function.
The output offset at turn on get more .
I will try use a npn transistor to get the input of amp to gnd.
I get a circuit example from member "Sdoom".
It very helpful.
EVA wrote :
why do u need -ve bias...r u talking abt small signal fet ? It is better to use fet...for minimum distorsion...
With bipolar transistors, a negative voltage is required during unmuted operation in order to prevent the negative going half of the signal from being clamped by the diode formed by the C-B junction (assuming a NPN). Also, a strong enough base drive is required during muted operation since the negative going half of the signal is actually clamped to the base through the CB diode, thus causing the transistor to be biased backwards (C and E reversed) and to show poor current gain.
FETs also have diodes formed from both terminals to gate, and require a low enough negative bias when muted to prevent the signal from being clipped to the output. A high enough load impedance is also required when unmuted to prevent excessive voltage drop across the device that could turn-on of one of the diodes and clamp the Vgs.
FETs also have diodes formed from both terminals to gate, and require a low enough negative bias when muted to prevent the signal from being clipped to the output. A high enough load impedance is also required when unmuted to prevent excessive voltage drop across the device that could turn-on of one of the diodes and clamp the Vgs.
You wrote that you built the amp. If you disconnect the preamp from the amplifier, does the amplifier turn on absolutely silently. If so, then muting the preamp may work. If the amp is also noisy when it switches on, you may want to use a muting relay on the amplifier output. This would prevent any turn on noise from reaching the speakers.
I know that many people don't like to have relays in the signal line but they do the job.
One note... if the object of the muting is to get the amp to switch on with absolutley no pop or tick, you must get the DC offset of the amp as close to zero volts as possible.
I know that many people don't like to have relays in the signal line but they do the job.
One note... if the object of the muting is to get the amp to switch on with absolutley no pop or tick, you must get the DC offset of the amp as close to zero volts as possible.
I was talking about JFETs, as a single MOSFET can't be used to mute an audio signal due to the body diode that would clamp it to the output.
And yes, JFETs work as if there were two internal diodes, one from gate to source and other from gate to drain. It's an inherent feature, not usually mentioned because it's absolutely obvious for anyone unsderstanding the working principle of a JFET. In practice, for any N-channel JFET this means that the gate will be clamped either to G or S terminals as soon as gate voltage is raised a diode drop above them. Thus only zero or negative gate polarisation is useful.
And yes, JFETs work as if there were two internal diodes, one from gate to source and other from gate to drain. It's an inherent feature, not usually mentioned because it's absolutely obvious for anyone unsderstanding the working principle of a JFET. In practice, for any N-channel JFET this means that the gate will be clamped either to G or S terminals as soon as gate voltage is raised a diode drop above them. Thus only zero or negative gate polarisation is useful.
- Status
- Not open for further replies.