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Old 25th January 2016, 11:15 PM   #1
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Default Another SIT amp

Hi folks,

Thought I'd chip in with my take on the SIT amp. This one's been going for about two years, but I have finally got it finished this evening after much experimentation, simulation, dead ends and frustration. But it got there in the end and is playing now.

I built a single stage 2SK82 amp similar to Mike's L'Amp way back and that was really nice, but Zout is a bit high. To be fair, not much worse than a 300B amp, but still, I wondered if this could be improved. Being a valve amp type of guy and having read Nelson's SIT Nemesis article, I wondered if output transformers might be the solution. A conversation with Per Lundahl at ETF and I ordered a couple of outputs with a 4:1 ratio.

Gain's a bit low with a 4:1 step-down, so obviously needs some help. I didn't wat to use input transformers, but I had at some point acquired some 2SK79 small signal VFETs and in the spirit of an all VFET amp, decided I wanted to use these as a gain stage. How to load it, though? Lots of sims and tinkering with breadboards, led me to the conclusion that these triode-like devices like flat load lines and hence old school circuits, so it came down to choke loading of some description. In the end, I decided to go all out and use interstage transformers, so basically this has ended up as an old school valve circuit.

The capacitance of the 2SK82 was an issue, however, the bandwidth would be marginal with the 2SK79 driving it directly, but the addition of a cathode follower buffer dircet coupled to the 2SK82 with a split supply solves this nicely and sets bias.

In the spirit of valve amps, I decided to do an LCRC PSU for this, lots of iron and very heavy. I'd originally decided to build it as a single chassis, and ordered one I thought was too large. As ever, only just big enough!

Click the image to open in full size.

Sadly, once I'd built it and crammed everything in. I found out that it there was some residual hum and discovered that it was the PSU chokes coupling to the inter-stages. Argh!

The only real solution was to separate the power supply, so ended up re-working it substnatilaly to get this:

Click the image to open in full size.

And this is the PSU:

Click the image to open in full size.

Took the opportunity to make the PSU LCLC, with the first LC in the PSU box. Well, why not.

Anyway, finally it's quiet and I'm listening to it! Seems to sound ok, so far.
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Old 25th January 2016, 11:38 PM   #2
Zen Mod is offline Zen Mod  Serbia
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Fugly!

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Old 25th January 2016, 11:45 PM   #3
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Fugly!

Meh, as proud parent, I disagree,
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Old 25th January 2016, 11:54 PM   #4
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Meh, as proud parent, I disagree,
That's Zen Mod's attempt of a compliment.
That's about as good as you'll get from him.
Hahahahaha
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Old 26th January 2016, 12:58 AM   #5
wdecho is offline wdecho  United States
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Fugly!

It is a compliment!!!!! Zen Mod's way.
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Old 26th January 2016, 12:59 AM   #6
brianco is offline brianco  Ireland
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....and well deserved too!
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Old 26th January 2016, 08:57 AM   #7
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Hehe, thanks chaps.
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Old 26th January 2016, 09:27 AM   #8
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Quote:
Originally Posted by i_should_coco View Post
Hi folks,
I built a single stage 2SK82 amp similar to Mike's L'Amp way back and that was really nice, ...I didn't wat to use input transformers, but I had at some point acquired some 2SK79 small signal VFETs and in the spirit of an all VFET amp, decided I wanted to use these as a gain stage. How to load it, though? ...In the spirit of valve amps, I decided to do an LCRC PSU for this, lots of iron and very heavy. I'd originally decided to build it as a single chassis, and ordered one I thought was too large. As ever, only just big enough!

Took the opportunity to make the PSU LCLC, with the first LC in the PSU box. Well, why not.

Anyway, finally it's quiet and I'm listening to it! Seems to sound ok, so far.
Fabulous build amplifier congratulations
With all heavy irons inside two mono blocks are perfect solution imho.
Really liked Your Diy in the spirit of valve amps and look forward.Thx
Best regards
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Old 26th January 2016, 09:58 AM   #9
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Yes, I wish I had gone mono now! In it's original integrated from, I could barely lift it, I think it was about 45kg. It's probably still 40kg for the main chassis!
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Old 26th January 2016, 12:46 PM   #10
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Fantastic! Love all that iron
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