Soft start for a preamp?

[markusA]

I'm about half way thorugh implementing a soft start for my tube-dac.
It's a timer circuit that shorts the outputs to ground...

I'm always looking for better/smarter solutions.
Recently I came across some reference to tube rectifiers being used and functioning like a soft start device?

I'm a tube n0ob so this was all news to me.
How would you solve the soft start issue?
I'm trying to prevent spikes at the output when switching on the tube buffer.

[Doz]

The nature of the tube rectifier is that it won't start rectifying until it is in itself warm, so your HT won't come up until the tube is warm, and it comes up gradually. My tube DAC has a foul SRPP stage which I keep meaning to ditch, but I get no pops when switching on. It uses an EZ80 rectifier.

[markusA]

Using tube rectification sounds like a neat idea. Avoid extra complexity, just add a rectifying tube?
Now, if only I knew how?

Oh, I almost forgot... The PSU's are floating.

[Doz]

Originally Posted by markusA

Using tube rectification sounds like a neat idea. Avoid extra complexity, just add a rectifying tube?
Now, if only I knew how?

Oh, I almost forgot... The PSU's are floating.

What heater voltage do you have?

[jackinnj]

Out of the LM317 datasheet with a twist. By putting an HV PNP transistor across the adjust resistor, and using an RC to slowly turn off the transistor you can use a "Slow MAIDA" regulator -- this one takes about 30 seconds to reach full potential -- about the time for filaments to hot up:



SY will probably tell me it's a profoundly bad idea...

[M Gregg]

Originally Posted by markusA

I'm about half way thorugh implementing a soft start for my tube-dac.
It's a timer circuit that shorts the outputs to ground...

I'm always looking for better/smarter solutions.
Recently I came across some reference to tube rectifiers being used and functioning like a soft start device?

I'm a tube n0ob so this was all news to me.
How would you solve the soft start issue?
I'm trying to prevent spikes at the output when switching on the tube buffer.

Just for interest,

I think the soft start covers three issues:

1. As stated slow turn on of B+

2. Soft start of heater inrush current.

3. Output mute until stabilization of all power.

Regards
M. Gregg

[jackinnj]

You can apply the same LM317 circuit to the tube filaments -- you probably won't need the pass transistor, change the voltage regulator values as recommended by the doctor and cut the C1 capacitor value by 10 -- to 0.1uF

[Wavebourn]

Originally Posted by M Gregg

Just for interest,

I think the soft start covers three issues:

1. As stated slow turn on of B+

2. Soft start of heater inrush current.

3. Output mute until stabilization of all power.

4. Limit inrush current through charging filter capacitors.

For #1 big time constant of RC and a source follower is a good cheap solution. For #2 and #4 wirewound resistor in series with primary of power transformer, shunted by contacts of relay powered from filament voltage rectifier, is a good cheap solution.

#3 is a different story, I would use there FET switches shunted by relay contacts.

[markusA]

#3 is my main issue. Everything else is a bonus.

[tomchr]

Originally Posted by markusA

I'm about half way thorugh implementing a soft start for my tube-dac.
It's a timer circuit that shorts the outputs to ground...

I'm trying to prevent spikes at the output when switching on the tube buffer.

That's not a soft-start circuit. That's a mute circuit.

A soft-start circuit or in-rush limiting circuit limits the surge in current that runs through the mains transformer at turn-on. A tube rectifier with a slow warm-up characteristic may be helpful here.

However, a in-rush limiter will not do much to limit turn-on thumps (fluctuations in the amp output voltage) at turn-on. A mute circuit like the one you describe will be helpful here. There are many ways to do this, but a simple timer circuit that turns on the amp output after, say, 30 seconds is probably the simplest.

~Tom