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Start up and shut down noise

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Posted on August 2, 2016 at 10:45:31
Stuben
Audiophile

Posts: 515
Location: Guber Ohio
Joined: December 30, 2005
Kevin,

Been a while since I have posted. I have been working on several PPP and CFSEP projects. I have been able to achieve some good results sonically but I have had some interesting oscillations (weird sounds) on start up and shut down.

My PPP amp employs cathode feedback through the primary of the OPT. I get a weird oscillation on start up when I use old stock EL84s in the output stage but only on certain speaker drivers. This does not happen using new stock EL84s. Interestingly, when the new stock EL84s are used, I get a loud sound that sounds like a slow cork pop on shut down. This does not happen with the old stock EL84s which have issues on startup.

On Cathode follower build, I get a motor boating sound on startup only when I use old stock 6L6 but not with old stock EL34s...

Short of serious trouble shooting with multiple valve complements and much hair pulling, I would like to short the outputs with an "On delay" on startup and short the outputs prior to shut down to cover all scenarios. I see you have delay kit. Could this be implemented to cover most of this?

Stuben

 

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RE: Start up and shut down noise, posted on August 2, 2016 at 16:26:35
KevinC
Manufacturer

Posts: 2897
Location: NC
Joined: April 19, 2001
Hello again!

The time delay kit will pull in or break relay contacts after an N second delay, where N is adjustable by means of changing a resistor value, so it can very well be used to mute the amplifier output until power up is complete.

On the turn-off end, the relay will open/close (depending on which set of contacts you wire the load to) upon power off, but not instantaneously, because the timer power supply capacitance is not zero. It can certainly be reduced from the value supplied with the kit (1000uF) to shorten the time to "release", but it still won't be fast. I have used another method to provide suppression of SE preamp turn-off bump, which is to use a 24VAC relay to short the output when power is removed. AC relays don't seem to be available in voltages less than 24V, although you can get 115 and 230VAC relays, too. With AC power there is no power supply capacitance to delay action.

Kevin Carter
K&K Audio
www.kandkaudio.com

 

RE: Start up and shut down noise, posted on August 3, 2016 at 10:05:41
Stuben
Audiophile

Posts: 515
Location: Guber Ohio
Joined: December 30, 2005
Kevin,

Thanks, I was running the Push Pull build for years in my basement sound room driving a DIY set of speakers with FE207s, a tweeter with a simple baffle correction. Never had any issues other than shut down noise when using a set of Russian EL84 equivalents. I moved the amp up stairs where I have a pair of DIY back loaded horns with FE106, a tweeter and a simple correction. This is when the other problems started...the only real difference is that the basement speakers have a Zobal network..., I love the sound of the amp, it uses LL1660S interstage (my first purchase from K&K). I have a couple of octal relays with 120 VAC coils and a pair of spare sockets in the chassis...might just tie them in with a switch to prove out the concept before trying a kit.

Stuben

 

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