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Old 20th December 2014, 06:34 AM   #1
Zekk is offline Zekk  Singapore
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Default Why Is My Single Chassis #26 PreAmp So Stable!

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Guys.... For years, I was scarce-off by this talk of oscillation and instability associate with #26 linestage, without regulated filament supply, multiple chassis and so forth.

Kevin's talk of using a bench supply for the filaments jerk me up from my slumber to realise that I can use mine without resorting to invest further.
I've this Laboratory Power Supply ( 2 channels @ 30v@ 3A each) for charging lithium battery for my eBike.

So, I built one channel, linestage and B+ power supply all on a single chassis, just for a listen ..... expecting all hell to break loose with high pitch whistling, humming and heavy thumpping of my woofer!

Guess what, it was dead silent, sounds good and superb wholesome bass!

From the pics above, it was lest than 8 ins. away from my 80 watts monobloc power amp. Equally close is my CD player and the bench supply.

I have drafted the schematic as built . PLEASE advice what I should do and undo for it to sound better. THANKS!

Wishing All You Diyers, Merry X'mas & A Great New Year Ahead,

Cheers!

Zekk
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Old 20th December 2014, 06:52 AM   #2
Zekk is offline Zekk  Singapore
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Old 20th December 2014, 06:58 AM   #3
Zekk is offline Zekk  Singapore
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Old 21st December 2014, 01:58 AM   #4
Zekk is offline Zekk  Singapore
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PUZZLED!!!
So am I! Referring to that 370 pages #26 thread, oscillation and humming were the main obstacle to a great sounding linestage.

I'm a rebel and always like to do things against the grain. The simplest approach before going on the defensive.

Cushion on sockets, plate choke not too near to tube, don't combine linestage and B+ on one chassis, power supply not too near speakers nor power amp were all non compliants in my built.

The whole setup is just 12 ins away from my 101dB 604-E speaker!

Or is it all because of my regulated Matrix Laboratory Power Supply?
Or perhaps, because of the double insulated cotton/ silk WE solid core wire used throughout !?


Nevertheless, I'm going ahead to ordering Rod Coleman's filament supply to simplified my built.

Am too excited to share my thoughs!

Thanks For Watching.

Zekk
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Old 21st December 2014, 03:13 AM   #5
kevinkr is offline kevinkr  United States
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Perhaps you just did a good job putting it together. The Lundahl plate load chokes were a great choice as they are less sensitive to external fields than some others I have tried, and you have also oriented them in a different plane from the power transformer which is a further help. ST glass 26 are significantly less microphonic in general than globe types. What sort of sockets did you use? (I noticed that teflon sockets oddly enough help with microphony transmitted through the chassis.)

The power transformer seems reasonably well removed physically from your chokes, and no question the supply design provides good ripple isolation. I use Rod Coleman CCS in some of my projects and they work very well.

How sensitive are your speakers and what is their LF cutoff? I use shield cans and need to with speakers with an LF cut off at 35Hz and 100dB efficiency.
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Old 21st December 2014, 09:25 AM   #6
Zekk is offline Zekk  Singapore
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Hello Kevin! You're my inspiration and it was your ground work that set my #26 PreAmp aloft.

As mentioned, my speakers are Altec 604-E Duplex of 101 dB sensivity.

I love those balloon valves and that's is exactly what I'm using.
Believes sockets are not Teflon but modern hard plastic.

Best part is, I don't even bother to isolate, cushion nor suspend it above /below the top plate.

But to be fair, if you tap the glass with a test pen, there's little microphonic but not on the top plate.

My power amp's 7F7 and 7N7 were much worse!

Ya... I've contact Rod to purchasing his regulators.

Happy Holiday to All.

Zekk
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Old 21st December 2014, 11:56 AM   #7
316a is offline 316a  England
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I have played with a lot of DHT and the issues reported regarding hum etc can mostly be eliminated by implementing the filament supply correctly as well as placement of the power supply iron . Too high a filament voltage is usually the cause of microphony which does not seem to be an issue with the ST type 26 , although I have had no luck with globes . I doubt oscillation will manifest itself with a valve with such low gm as the 26 no matter how bad the layout is

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Old 22nd December 2014, 10:04 AM   #8
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Quote:
Originally Posted by Zekk View Post
Hello Kevin! You're my inspiration and it was your ground work that set my #26 PreAmp aloft.

As mentioned, my speakers are Altec 604-E Duplex of 101 dB sensivity.

I love those balloon valves and that's is exactly what I'm using.
Believes sockets are not Teflon but modern hard plastic.

Best part is, I don't even bother to isolate, cushion nor suspend it above /below the top plate.

But to be fair, if you tap the glass with a test pen, there's little microphonic but not on the top plate.

My power amp's 7F7 and 7N7 were much worse!

Ya... I've contact Rod to purchasing his regulators.

Happy Holiday to All.

Zekk
Zekk, Your layout work looks to be carefully considered, and neatly implemented - this will make a big difference to noise pick-up problems.

The lab supply must be pretty good - often they are too noisy for something like filament bias. But the lab supply will still try to null-out the music signal across the filament, because the filament is now in its voltage-feedback loop. You will hear the improvement in sound when you try purpose-designed current-driven heating, which addresses the problem. I responded to your message, but the email was rejected - please send your new email address.
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Old 22nd December 2014, 11:48 AM   #9
Zekk is offline Zekk  Singapore
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Thanks Rod! You've mail.

I'm a newbie and as far as IC is concern, I don't have a success story to tell.
So, I'll be bugging you for guildance after board is populated.

BTW, my Laboratory Power Supply can be set to constant voltage or constant current....... which is best?

Thanks!

Zekk
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Old 22nd December 2014, 05:05 PM   #10
kevinkr is offline kevinkr  United States
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Try both, I'd expect constant current mode to be better somewhat better. Measure the voltage across the filament to make sure you are in the ball park of 1.5V across the filament - some variation in voltage measured due to contact resistance is expected.

I have to my chagrin found dht filament heating to be quite sensitive to the quality of the filament supply and not in just obvious areas like ripple, voltage stability, and source impedance.. The wrong or badly designed filament supply can result in really bad sound.
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