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Old 2nd April 2013, 11:44 PM   #321
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Default BA-1 pics

Here's my recently completed SemiSouth BA-1 monoblocks. A pair of matched R100's at the gain position, and matched 240's as current sources in each monoblock. 22.5V rails, 3A bias current... that's the max these heatsinks can take. They are definitely room heaters that happen to make music. Everything point to point wired in the output stage, bias circuit on perfboard. Papa forgot the power on LED so I replaced that zener diode at the voltage divider with a mini CCS string of 3 green LED's and a 2SK117GR (~4 mA) with an RC in front of that... just for more isolation.

Front end is separate with it's own shunt regulated supply (Salas DCB1 minus the buffer part that I cut off). Sorry for the dark picture, it's all sinister you know... (maybe Darth Vader would approve) The supply is loaded up with a bunch of yellow LED's that glow through the perforated metal top. With my source turned up and paused, I hear nothing with my ear nearly touching the speaker cones so it's all quiet as the grave. The Hammond transformer powering this does have a low level mechanical buzz I'm not happy with, may end up changing that out.

Sound is very good, and getting better as everything settles in. It definitely is better after warming up, also. Will be interesting to see how this compares to the F5 I lived with this last year or so. I'll see if I can round up a couple of internal shots.
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File Type: png BA-1.png (334.8 KB, 549 views)
File Type: jpg BA-1 FE.jpg (149.0 KB, 521 views)
File Type: jpg BA-1 front.jpg (174.8 KB, 525 views)
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Old 3rd April 2013, 12:19 AM   #322
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Default power supply

400VA Antek's, Rifa PEH200's (80,000 uF in each), Juma's cap multiplier from his "F5 with 2SK2013...", manual soft start
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Old 3rd April 2013, 12:31 AM   #323
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Default the heatsink

Outputs secured by a fat piece of aluminum bar stock. Source resistors are .34 ohms and I've got about .52V across each. Output offset is very low and stable.
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Old 3rd April 2013, 12:42 AM   #324
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Fugly!

but , better to put it

F
u
g
l
y
!

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Old 27th April 2013, 11:38 PM   #325
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Default update

I might be talking to myself in this thread but anyhow....

After more listening I discovered a problem on certain tracks, centered somewhere around the upper mids / low treble. Just a harsh, static like type of distortion at times. But very noticeable on track 6 of the Stereophile Test CD2, a Flute Sonata, very harsh / grating sound. I've heard this disc many times (every time I change something with my system) and knew I had to try and figure out what was going on.

Searching the forums I found Generg's "BA2 J2 Monster Clone" or something and ZM talked about the LTP current and "letting it bleed" when driving these SS JFETS. Well I tossed out the original voltage divider circuit and built a CCS out of a pair of K170's chosen at 10mA each. 500r pot at the bottom to ground with a 220r resistor in parallel which gives an adjustment range of 0-3V. My parts need about 1.94V or so. 680r's on top to keep the jfet dissipation in check. And I figured "bleeding" 20mA should be more than enough. It completely cured that problem. Need more time on these but things are getting real interesting now...
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Old 28th April 2013, 06:43 AM   #326
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schm wouldn't harm

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Old 28th April 2013, 06:08 PM   #327
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Default problem

Was going to post a schematic but I've run into a problem. Amp #1 has been stable for several days with the circuit above. Amp #2 was looking good on the bench then suddenly the output offset became unstable during warm-up. As if it's oscillating or something. Out of frustration I've thrown together a two transistor current source with thermally bonded BC556's on a little piece of veroboard. 100r load resistor and a 50r trimmer on top. So again this is setup for ~20mA of current. It looks like figure 6a here: Current Sources, Sinks and Mirrors in Audio Only difference is I've added a .1uF cap emitter to collector of Q3.

If this thing runs stable for several hours at full operating temp I'll throw it back in the system to make sure it still sounds OK. More later....
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Old 28th April 2013, 06:26 PM   #328
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I'm often using that sort of CCS - ( right one on linked pic)

probably because even I can calculate values for it

Click the image to open in full size.


brake R4 in two 4K7 , connect their node via 1-10uF to positive rail

also - base resistors of some 330-1K can't harm
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Last edited by Zen Mod; 28th April 2013 at 06:29 PM.
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Old 4th May 2013, 04:22 PM   #329
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Hey! looky at all the new-fangled Pass stuff since I've been...well, somewhere else! Unbelievable..
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Old 18th May 2013, 03:37 AM   #330
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Latest incarnation, and maybe last, of this Semisouth bias circuit ... DN2540 cascode (TO-92's, $1.24 @ Mouser) per Walt Jung's Current Regulator's Revisited article in AudioXpress. Rload=100r, Rset=200r trimmer, 300r carbon gate stoppers right at gate pins of the 2540's. 8k series resistor from the top of Rload to the SS gate bus bar, 220r gate stoppers at the gate pins of the power JFETs.

Maybe what I've done here is absolute overkill silly-ness, but it's simple and cheap, and sounds fantastic. Along with a couple of other small changes elsewhere, this is the best sound I've achieved yet. I'm not missing that F5... (PV1.4 = 1.26 X 104 ??? )
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