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Old 15th July 2017, 05:24 PM   #2601
needtubes is offline needtubes  United States
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For the lower FET, I'd think you would want to vary Vgs for minimal DC offset on each output terminal. How much you need will depend on the current you chose through the CCS.
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Old 16th July 2017, 04:40 PM   #2602
metalman is offline metalman  Canada
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Originally Posted by Rodeodave View Post
Also, about the McMillan resistors (R46/R47), is there any sense in trying to omit them? Or are they most definitely needed to insure low and steady DC offset at the outputs?
The McMillan resistors are there to reduce (not eliminate) absolute offset. It will have no impact on relative offset. How much they are needed depends on how closely matched everything is, including resistors etc... Better matching yields reduced offset, both relative and absolute. I went to great pains to match everything, and it had real benefits. At turn on, without the McMillan resistors I get an absolute offset of 7 volts that quickly drops below 1 volts after a few minutes, and stabilizes less than 0.1V afte 30 minutes. With the 10k McMillan resistors the absolute offset starts at 3-4 volts, drops below 0.7 volts within a minute, and stabilizes at less tha 0.1V after 30 minutes. With 10k resistors the audible difference is pretty negligible, if you really listen carefully you can make out a slight difference. In the end I took them out and run without the, as I have my mosfets heavily heatsinked and don't have to worry about thermally overloading them during warm up.
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Old 16th December 2017, 10:28 AM   #2603
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A 10K value seems to too high to create the offset stabilization desired. If you don't mind the increased dissipation a value <100R should do better to stabilize this amp. It does look like your amp doesn't need it but did you ever try lower resistance there?
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Old 16th December 2017, 01:18 PM   #2604
wuffwaff is offline wuffwaff  Netherlands
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Hi,

the lower the resistance the lower the sound quality. I tried 33k without problems but.....
Matching everything does only help for one value of the mains voltage. If this varies by a few volts during the day your absolute offset will vary too.
Since mains voltage seem to vary a lot where I live I tried a DC-Servo which solves the problem.

William
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Old 15th January 2018, 08:06 AM   #2605
neno70 is offline neno70
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Default PSU

hi, I bought from ebay (Jims) pcb x Aleph X100w. X the psu that type I have to achieve; on the net I found this design, can be good, there are others, can you advise me? thank you
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Old 16th January 2018, 01:09 AM   #2606
Eric is offline Eric  United States
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Aleph-X builder's thread.
The nice part about the Aleph-X amps is that you can customize their output power to suit your requirements. Have a look at my Aleph-X page (linked in my signature). It contains a protracted start-to-finish resource for building this amp.
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Old 18th January 2018, 06:23 AM   #2607
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Quote:
Originally Posted by Eric View Post
The nice part about the Aleph-X amps is that you can customize their output power to suit your requirements. Have a look at my Aleph-X page (linked in my signature). It contains a protracted start-to-finish resource for building this amp.
hi, I looked at your page .. great work, congratulations. X the PSU, I already own 16 capacitors 47000uf 63v and 2.2 mh inductors on an aleph 2, if I'm not mistaken I can reuse these without having to test on Aleph X, without having to buy those 1.8 mh, right? , already in my aleph 2 I have replaced the rectification bridges with rurg3060, I think it's okay, right? one last thing; in your aleph x have you had problems with DC output on the speakers? thank you
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Old 18th January 2018, 12:32 PM   #2608
Eric is offline Eric  United States
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Aleph-X builder's thread.
Sounds like you have a great power supply already! The Rurg3060 looks good with ratings like 30A and 600v. I just used generic rectifier bridges with similar ratings and didn't worry too much about it.

The Aleph-X amps have an interesting characteristic with DC offset. Because this amp is balanced, you need to pay attention to both absolute DC offset (as measured from speaker terminal to chassis ground) and relative DC offset (measured across the speaker terminals).

My absolute DC offset has always been rock steady at zero volts. This is what is most important - it is what the speaker sees.

Relative DC offset is what you need to play with as it is affected by temperature, construction technique, choice of value for R46/R47. Everyone needed to play around with this set of variables in order to achieve lowest relative DC offset behavior in their build. Relative DC offset starts off high when the amp is first powered up, then falls as the amp warms up. A high DC offset reduces the available power output while the amp warms up. You usually adjust this so it is at zero after the amp is fully warmed up. Then, people tweak trying to get the relative offset lower when the amp is cold, or try to get it to fall to zero faster as the amp warms up.
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Last edited by Eric; 18th January 2018 at 12:41 PM.
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Old 18th January 2018, 01:33 PM   #2609
wuffwaff is offline wuffwaff  Netherlands
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Hi Eric,

I suppose you meant to say relative offset was rock steady? The absolutel offset is what is changing with temperature and a few other things including mains voltage.

Cheers,

William
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Old 18th January 2018, 02:15 PM   #2610
Eric is offline Eric  United States
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Aleph-X builder's thread.
D'oh! You are correct, William! Still use these amps several times a week - just don't think about them that much anymore...

Here is the corrected version of above:
My relative DC offset has always been rock steady at zero volts. This is what is most important - it is what the speaker sees.

Absolute DC offset is what you need to play with as it is affected by temperature, construction technique, choice of value for R46/R47. Everyone needed to play around with this set of variables in order to achieve lowest absolute DC offset behavior in their build. Absolute DC offset starts off high when the amp is first powered up, then falls as the amp warms up. A high DC offset reduces the available power output while the amp warms up. You usually adjust this so it is at zero after the amp is fully warmed up (1-2 hrs). Then, people tweak trying to get the absolute offset lower when the amp is cold, or try to get it to fall to zero faster as the amp warms up.
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