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Old 2nd December 2014, 12:34 AM   #1
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Default Kofi's Uniamp

Hey all,

It's been a while, but I'm getting back into the audio game again with a Uniamp from Pete Millet's wonderful website.

I am building this for a friend so I'm trying for a showroom quality build here, but given my track record, some blemishes may arise which we will call "character" from here out, OK?

So, I've attached my BOM for your review. I didn't think too long about it, I just ordered what I thought was right, putting some extra cash into the coupling caps and some key resistors and keeping the other parts somewhat down to earth. I have been known to over-buy in some areas, but I tried to reel it in here where I felt I could.

Transformers will be Edcor (10 week lead time!) with the recommended Sowters pulling the phase-splitting duty. I ordered some pre-made chassis here. These looked cool in the photo and should be plenty big enough for each monoblock. I'll report on these once I have them in hand (maybe late this week or early next).

I have some questions about the power supply that I'll pose in a subsequent post. Let me know if you think the BOM is wacky in some way.

Here we go!

Kofi
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File Type: pdf UNIAMP2_bom (version 1).pdf (72.0 KB, 257 views)
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Old 2nd December 2014, 12:47 AM   #2
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So, regarding the PSU. Per Pete's calculations, the KT88s will run at 78mA with a couple of 6SN7s for voltage amplification. i called that about 100mA and ran some Duncan PSUD2 simulations with the 400-0-400 transformer and a 5U4-GB rectifier.

I can't get anywhere near the 370V B+ that Pete references. With a 47uF cap input, I get about a 460V B+. Changing to a choke input, I get about a 330V B+. I've attached shots from PSUD for review.

I've been married long enough to know that this is somehow my fault. What am I doing wrong?

Any guidance would be appreciated.

Kofi
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File Type: png Uniamp Cap Input.png (132.7 KB, 886 views)
File Type: png Uniamp Choke Input.png (122.1 KB, 872 views)
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Old 2nd December 2014, 02:23 AM   #3
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Have you tried shrinking that input cap down? Start at 1u and see what happens. 47u seems large for a 5u4 anyway.
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Old 2nd December 2014, 02:39 AM   #4
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Quote:
Originally Posted by dsavitsk View Post
Have you tried shrinking that input cap down? Start at 1u and see what happens. 47u seems large for a 5u4 anyway.
Thanks!

1uF gets me to about 350V, 2uF gets me about 410V, 1.5uF gets me about 386V. Looks like maybe I just need to play with the lower values until I lock in the right B+.

Alright! Progress! Thanks so much!

Kofi!
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Old 2nd December 2014, 01:03 PM   #5
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Hi, You can put the choke Rdc at 60 ohm, as per Edcor specs, not so much a difference, but... a small 47ohm in front of the 1.5uF can help too
Cheers
J.
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Old 2nd December 2014, 02:18 PM   #6
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Thanks, Mosquito. I'm unable to model that in PSUD as the application claims that would be an 'illegal combination'. I'll take it that I can try that in practice, however.

Anyone who has built a Uniamp care to chime in? How did you implement the PSU?

Kofi
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Old 2nd December 2014, 02:55 PM   #7
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this
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Old 2nd December 2014, 09:13 PM   #8
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Aha! Yes! I was just able to model it in PSUD as well. Looks like I needed to back out the C filter before adding the RC.

Thanks!

I'll need to order a few small caps and resistors so I can fiddle with the B+.

Has anyone had any experience engineering this run the KT88s a bit hotter? Pete advised that a B+ of 450V with a cathode resistor of 550R might get about 40W out, as opposed to the 26W I would get based on the 370V and 400R resistor that Pete is using.

Kofi
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Old 14th December 2014, 01:28 PM   #9
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Hey! Parts have arrived!

Everything is here now with the exception of the Edcor transformers (power, choke, OPT), which will hopefully ship next week.

Check out the photos...
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File Type: jpg 20141214_092133.jpg (733.5 KB, 655 views)
File Type: jpg 20141214_092120.jpg (650.4 KB, 150 views)
File Type: jpg 20141209_203249.jpg (344.5 KB, 174 views)
File Type: jpg 20141209_202808.jpg (822.7 KB, 193 views)
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Old 14th December 2014, 01:33 PM   #10
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So, if you take a look at the Sowter 3575C transformer, you'll notice that there is a mark next to one of the pins. Based on the pinout on the Sowter website (see below), I'm thinking this mark is identifying pin #2, the connection to the transformer core / case.

Does that seem right?
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