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Old 21st March 2012, 07:25 AM   #31
slomatt is offline slomatt  United States
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Originally Posted by boywonder View Post
Correct. The two grounds are tied together on Brian's PCB and one goes to the binding post and the other to the PS ground, then PS ground to chassis ground stud. My transformers are single 12V secondaries; one is doing the -ive rail and the other is doing the +ive rail, no center taps.
Ah gotcha, I didn't notice in picture that you were using two transformers to get the +/- rails, I assumed they were center tapped.
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Old 21st March 2012, 03:54 PM   #32
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So tell us how it sounds compared to toobz!
I gave it a good listen last night, and it's quite nice. Good imaging, slightly hot/sibilant on the highs...very slight, at least compared to my TubeLabSE 300B. Plenty of power for 95db speakers. Barely any audible hum if you stick your ear next to the cones, at least as low as my tube stuff. I have noticed that the transformers hum very slightly.

I'm running generic 50V bridge rectifiers so I'll probably try some schottky's or FREDs and see if I can get an improvement.
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Old 21st March 2012, 04:08 PM   #33
Atilla is offline Atilla  Norway
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Haha, it's really funny, but my timing is quite similar. I've all parts (!) obtained and the PSUs more or less assembled.

Your build is an inspiration! I guess it's time to get to soldering again.
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Old 21st March 2012, 07:39 PM   #34
ra7 is offline ra7  United States
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Thanks boywonder!

How did you machine that groove in the faceplate? You said you did it at home?
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Old 21st March 2012, 10:20 PM   #35
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How did you machine that groove in the faceplate? You said you did it at home?
With a Bridgeport, a ball end mill and a big-*** 2 flute end mill.......... (not counting the mounting holes, just a drill bit for those)...
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File Type: jpg ball end mill.jpg (153.5 KB, 861 views)
File Type: jpg giant 2 flute.JPG (169.6 KB, 836 views)
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Old 21st March 2012, 10:24 PM   #36
6L6 is offline 6L6  United States
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Aw man, I really want a mill...

Or better yet a neighbor with one.
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Old 21st March 2012, 10:42 PM   #37
ra7 is offline ra7  United States
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Sh**, that is awesome, man!
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Old 29th March 2012, 08:33 AM   #38
slomatt is offline slomatt  United States
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I've installed precision potentiometers in place of R8 to adjust the DC offset and R13 to adjust the bias.

Using R13 I am able to get the bias up to ~1.17v (.550v across .47ohm source resistor) but it won't go any higher than that. This again makes me wonder if R14 and R16 should be 750ohm for the Mini-Aleph.

The DC offset on each channel started out around 70mV, by adjusting R8 I've lowered it to < 10mV on each channel. What is interesting is that each time R8 is adjusted the offset fluctuates wildly and then slowly returns back to close to the original value. It took some fairly large resistance changes to get it stable at < 10mV.

So far I am very impressed with the sound quality. Compared to other amps I own it is if there is a layer of "fuzz" removed that was masking the real signal. Interestingly enough our cats now give the speakers confused looks when they are playing, apparently because they can't locate the source of the sound. They haven't done this with other amp setups, so either the sound is more realistic or possibly the amp is putting out some super-sonic noise that is driving them nuts. The amp is making my speakers faults more noticeable, so in the true DIY audio cycle my next project will likely be a new set of speakers.

The next steps are to finish the enclosures so that the amps and power supply are shielded, currently there is a bit of hiss which I hope goes away when this is done. I also have a set of Vishay MKP capacitors that I'm going to use to bypass C1/C2/C3 to see if that makes difference.

- Matt
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Old 29th March 2012, 03:03 PM   #39
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Nice! Are you going to post a few pics of the finished product?

My bias seems very close to yours IIRC, although I did not try to take it any higher. So if you leave R13 disconnected there is not much increase in bias? My heat sinks get quite warm but I can touch my hand on them indefinitely.
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Old 30th March 2012, 08:03 AM   #40
slomatt is offline slomatt  United States
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Nice! Are you going to post a few pics of the finished product?

My bias seems very close to yours IIRC, although I did not try to take it any higher. So if you leave R13 disconnected there is not much increase in bias? My heat sinks get quite warm but I can touch my hand on them indefinitely.
Definitely, once I do final assembly I'll upload some more pictures. I'm still trying to figure out why the bias maxes out at ~1.17A (not 1.17v as I mistyped previously). My heat sinks are warm but not hot, and I think I could safely run 1.2-1.3A. One option is to lower the resistance of the source and sense resistors, but I'm hesitant to desolder and replace them.

- Matt
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