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"el escorpion" line stage, plus 10W B+ flyback supply & mute circuit
"el escorpion" line stage, plus 10W B+ flyback supply & mute circuit
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Old 1st September 2015, 10:24 PM   #1
pmillett is offline pmillett  United States
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Default "el escorpion" line stage, plus 10W B+ flyback supply & mute circuit

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So a friend gave me this cigar box. I had to do something with it...

It is a line stage using ECC99 dual triodes. It runs from a 12V external power supply.

I designed a non-isolated flyback converter (switching power supply) to generate a 200V B+ supply for the circuit. There's been some discussion about such a thing here lately, so here's a working example (albeit low power).

Also, I built a muting relay circuit to keep the outputs off until the tubes warmed up, to eliminate the big "thump" you can otherwise get when the output caps charge up as the tubes heat.

All the details are on my website Pete Millett's DIY Audio pages.

Pete
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Old 1st September 2015, 10:53 PM   #2
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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"Also, I built a muting relay circuit to keep the outputs off until the tubes warmed up, to eliminate the big "thump" you can otherwise get when the output caps charge up as the tubes heat."
What thump? The valve warm up takes up to 15 Seconds. No need.
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Old 1st September 2015, 11:57 PM   #3
pmillett is offline pmillett  United States
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Quote:
Originally Posted by JonSnell Electronic View Post
"Also, I built a muting relay circuit to keep the outputs off until the tubes warmed up, to eliminate the big "thump" you can otherwise get when the output caps charge up as the tubes heat."
What thump? The valve warm up takes up to 15 Seconds. No need.
In a perfect world, perhaps.

Try connecting a preamp like this to a solid state subwoofer amp. There is a thump... Though it is worse with a resistive plate load or dissimilar triodes.

It's particularly nasty if you power off then back on while the tubes are still warm. Or worse yet if the mains flickers off and on.
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Old 2nd September 2015, 02:32 PM   #4
pmillett is offline pmillett  United States
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Default Thump

The first scope shot shows the line stage output and B+ at startup with cold tubes. You can see a ~1V output thump about 10 seconds after power on. It is low frequency, so it may be acceptable if your power amp has limited LF response. But it makes a subwoofer go nuts.

The second plot shows what happens if you power on with warm tubes (turned off for 10 seconds, then back on). A 20V pulse. That will pop your woofer...

That is why a muting delay circuit is a good idea.

Pete
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Old 3rd September 2015, 06:41 AM   #5
Cassiel is offline Cassiel  Libya
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Quote:
That will pop your woofer...

Yep, Klipschorn's woofer went off axis, needed replacement. My tube preamp also killed a set of mini monitors via a chip amp. Not bad for a small preamp - killing big boys much taller than him.

You really need a mute circuit or, if you're lazy, a mute switch.

BTW so little comments....sexy name, sexy preamp, me not understand. Oh my, Trump is gonna win. Hehe.
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Old 3rd September 2015, 07:35 AM   #6
M Gregg is offline M Gregg  England
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I also have a relay mute,

On my Aikido line stage because its fed from an inverter running app 300V via Maida reg fed from a laptop PSU.

And the thump is very real. I have mute on power up and power down.
Power down is because the inverter winds down after power off so I don't get any strange noises etc.
Nice to see an inverter for tubes..

I had to put a filter on the B+ to remove the switching ripple.

I did power down mute by just using a normally closed relay grounding the outputs and powered the relay from a double pole switch one pole fed the PSU the other fed the relay so the PSU didn't hold the relay up after power off.

The use of an "off the shelf transformer" is a big plus its one of the problems with building a SMPS for tubes. They can be noisy when you wind them and even potting has little effect to keep them quiet.


Regards
M. Gregg

Last edited by M Gregg; 3rd September 2015 at 08:03 AM.
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