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Old 29th April 2011, 05:30 PM   #91
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Dan - Glad you've got it quiet. Makes me want to work on mine to get that last little bit of hum out.

Kannan - The benefit of fewer output devices is decreased total gate capacitance, which makes the output stage easier to drive at high frequencies. This may explain some of the sonic differences between the Aleph variants. Whether it's audible with the A75 and it's higher current VAS is subject to testing. I like my 6 pair amp, but haven't tried more devices.
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Old 29th April 2011, 06:21 PM   #92
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My point is about increasing the current to compensate the Class A loss due to lowered output Transistors. yes he can try lowering the outputs and see whether he can hear the differences also try increasing the current slowly. Dan has worked with Class A bias anyway.
He is now having the courage to test new things on his A75. I Wish him luck !
have been reading his
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Old 29th April 2011, 06:30 PM   #93
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It sounds pretty good. Better than before but I would attribute that to the bigger caps. I upgraded those from 20k to 56k. I still think it sounded best when I had the ground / chassis connection right at the -IN between the RCAs but the buzz was alot louder then. I might still try that ground point again through a Thermister or R/D. Right now it's grounded through the RD on the PS board with a 14GA wire directly to the power entry ground. I've always used the 2-stage gain configuration with no feedback. I find that feedback around the output is a litttle too agresive. No feedback is more natural and the soundstage is more open.

So you don't think there's any benefit soundwise to use fewer outputs at a higher bias? I've noticed most of the Aleph and F series amp go with far fewer and was thinking there might be some benefit going that route with the A75 as well.
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Old 29th April 2011, 06:50 PM   #94
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Only point to note is the temperature rise on the transistor case - since this is making it work more in to class A bias
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Old 29th April 2011, 07:13 PM   #95
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It's definitely been an experience. Educational and frustrating. Part of the exercise has been focused on circuit performance and the other on listening. I actually played music professionally for 10 years back in the mid-70-80s and always reference the "live" sound to my music system. Right now I'd say it's very close. My distaste with transistor is it tends to sound too etched and aggressive in the mids and upper end. Being a guitar player I guess would explain that. When it comes to my guitar amps it's either tubes or nothing. I like smooth and liquid with a wide & deep soundstage. I'd say right now with the way it's set up it's about 90% there. The rest of the system consists of Cabbasse 6"mids, Accuton ceramic domes, some NHT12 inch subs. My preamp is also a biamp using a LR filter crossed at about 100hz. I drive the subs with a big Carver. The CD transport is an old Pink Triangle Cardinal which feeds my DAC with a BurrBrown PCM63 via BNC.

Now that the A75 is up and running it's time to start the AJ project. It's looks less complicated than the A75 and should be a fun build. It's been many years since I built anything. Not sure I can manage with only 25w. I'd really like at least 50w/channel.
Dan
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Old 29th April 2011, 07:20 PM   #96
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kannan
Good point. Each transistor would run twice as hot, cutting down on service life. I have plenty of heatsink to handle it but if there's no audible benefit then I'll leave it at 12Qs. Would be nice it lasted another 15 years.
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Old 29th April 2011, 10:18 PM   #97
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Bob,
If you're running just 6 pairs that's 25W each. Are you using fans or had any problems with case temperatures? Or did I misunderstand...6 Ps and 6 Ns. For what's it's worth I have plenty of top end running the with 12 Qs (6P/6N). My ears aren't that sensative to highs anymore so not sure I could tell the difference if it had just 6.

Dan

BTW...thanks for all the help with this project.
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Old 30th April 2011, 02:54 AM   #98
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With fewer devices you could lower the bias a bit to keep the devices in their safe operating area at the cost of lower output in class A, but the amp will still operate quite nicely in class AB to over 75W. As long as you have enough heat sink you can run 25W per device. Just calculate the junction temperature and try to keep it under 100C.

My A75s are not stock and were a bit of a busy first build. I figured that I needed more than 75W because my 75 WPC receiver sounded fuzzy on loud passages. I scaled it up a bit somewhat based on what I could find for surplus power supply parts and ended up with 57V output rails and 65V front end, tweaking some part values and voltage or power ratings to get there. I am at about 1.75A total bias per channel, which should give me about 50W class A and somewhere north of 150W class AB if needed.

That works out to ~16W per device. Heat sinks are 10" vertical 14" deep with 32 1.5" fins. They get to around 50C using NP's touch temperature scale.

Were I to do it again, I would probably build a standard A75, as I rarely exceed 20W peaks.
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