[dsavitsk]

I have my new preamp running, and so far is working great (thanks to amb for this.) Anyhow, when it is powered up, it sends a pop through the speakers that shakes the house. Obviously, I can simply turn on the preamp before the power amp to avoid this, but remembering to get these things in the right order is a pain.

so, is this dangerous to my speakers? And, assuming it is not good, is there a reasonably easy way to stop it? Some sort of simple soft start circuit?

The current setup is there is a power switch on the back of the case that controls application of AC to the power supplies. There is then a toggle on the front that controls DC to the circuit (essentailly, it will keep the PS warm, but all the buffer to go into standby mode). Both of them pop, but I would be more interesed in soft starting the DC.

Here's a description of the preamp circuit

Thanks for any suggestions, links, etc.

-d

 

[nikongod]

i solved my various headphone amp's problems with this by adding some more power suply capicitance. how much do you have on the board now?

 

[dsavitsk]

The PS has 2200uF. There are then 2 rail splitters (1 for each channel) and each has ~ 900uF. The switch for the DC is between the PS and the railsplitters.

The rails look like the image (with apologies to tangent) below (with a few small caps as well)

 

[amb]

You can build this simple little circuit which mutes the outputs of the preamp for a short moment after turn-on. This uses FETs to shunt the output to ground, and does not use relays. This circuit is appropriate for your preamp which has 68 ohm series output resistors, and allows this momentary shunting without damage.

This circuit does not mute the output immediately at turn off, but if you don't have a turn-off noise it's not an issue. It could be enhanced further to provide a turn-off muting function too, but would require more parts.

 

[individual6891]

Will that circuit degrade the audio signal during normal use or not?

 

[steinchen]

I'd be interested in the enhanced version to mute power off pops, too. How do I have to alter the circuit for the use with a headamp instead of a preamp ? Is adding 5R resisitors to the outputs enough to limit the current while maintaining the mute effect ?

 

[dsavitsk]

There is no turn off noise, so that's not an issue.

To make sure I understand, should the output connection be a second connection at the output of the buffer? As I understand it, this draws the instantanous current to ground, but after a few moments its resistance increases which blocks any further current draw to ground allowing the signal to go to the power amp?

-d

 

[Porksoda]

Quote:

Originally Posted by individual6891 Will that circuit degrade the audio signal during normal use or not?

It will ad a few pF of output capacitance, but other than that it should act as an open circuit once the FETs turn off.

 

[amb]

Quote:

Originally Posted by individual6891 Will that circuit degrade the audio signal during normal use or not?

It should not degrade the audio signal after the muting period is over, because the FET becomes a very high-impedance load in parallel with the output.

Quote:

Originally Posted by dsavitsk To make sure I understand, should the output connection be a second connection at the output of the buffer? As I understand it, this draws the instantanous current to ground, but after a few moments its resistance increases which blocks any further current draw to ground allowing the signal to go to the power amp?

Yup, that's basically what it is. The critical thing here is power-up timing. The -15V rail that this circuit runs on must power up as quickly (or quicker) than the preamp's rails. Otherwise some turn-on transient will still get through. In your circuit, just use the same negative rail as the preamp. If for some reason there is still a small pop at turn on, I have another trick up my sleeve that involves adding a resistor and a capacitor to each FET drive, but let's not worry about that until you get a chance to try this circuit as is.

Quote:

Originally Posted by steinchen I'd be interested in the enhanced version to mute power off pops, too. How do I have to alter the circuit for the use with a headamp instead of a preamp ? Is adding 5R resisitors to the outputs enough to limit the current while maintaining the mute effect ?

Since a small signal JFET is used to shunt the output to ground during the muting period, the amount of current it could handle is limited and thus the output series resistor needs to be larger than 5 ohms for current limiting. Thus, I don't think this circuit would serve as well for headphone amps because we're trying to keep the output impedance as low as possible. If we change to using a power MOSFET to do the shunting, it would be able to handle the current, but then the amp's output stage might not be happy with that.

Here's an idea for a circuit with both turn-on and turn-off muting. I've not actually experimented with this yet, so you may need to tweak it to get it to work as intended.

 

[Garbz]

The other option is use the same circuit with a relay to cut the outputs. Despite what people say about relays i've yet to hear any noticable (or measurable) effect of running the signal through a relay, and most top of the line amps have output relays as well.

Either way something should be done. THese poweron thumps can easily fry tweeters.

 

[rickcr42]

replace the jfets with a reed relays.

This is a common "upgrade" for CD/DVD players to get the jfet out of the circuit and replace it with a device having only a "in circuit" or "out of circuit" action

 

[dsavitsk]

Quote:

Originally Posted by amb

The parts for this arrived form DigiKey, but would someone mind posting the pinouts for the 2n5486. I looked at the datasheet, but that is not helping me figure out which of these pins is 1, which is 2, and which is 3.

Is that little mark a 1? If so, that's one down ...

Thanks,

-d

 

[MisterX]

 

[skyskraper]

 

[amb]

Quote:

Originally Posted by dsavitsk Is that little mark a 1? If so, that's one down ...

No, that's a little "dot" in the schematic, and denotes the "source" pin. The "gate" pin is the one with the little arrow pointing in, and the "drain" pin is the one that connects to the outputs. See MisterX's diagram for pin assignments.