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Old 13th March 2009, 07:27 PM   #581
daly41k is offline daly41k  United States
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Default aleph1.2 to AX100

GL, Oh I found the problem, it seems I missed soldered some resistors feeding the MSP18 transistor setting the bias for the output current sources. In fact I did that when I first built the amp. I wondered why this amp ran hotter than the other mono amp but it sounded good so I left it. I had some outputs running at 700ma bias. They are now all back down to between 550 to 600ma and the amp runs much cooler.

Yes I have been listening all winter-sounds very good indeed. DC offset at the outputs is a respectable 80ma. However my absolute DC offset takes a long time to settle down. It slowly goes down from about +/- 15 volts to 3 volts after warm up and slowly goes down from there. I never really checked to see how low it goes as I don't worry about this anymore. I noticed in mine anyway that the more DC offset at the outputs that I have the faster the absolute DC offset goes down.

I think the MPS18 transistors get warm and things change a bit. I also don't have any heat sinks on the Jfets nor do I have them cemented together. So perhaps the heat is an issue.

I did drill some extra cooling holes in the top of the amp ffor better convection.

I haven't had the chance to run them full range yet as they are being used above 2500k in a bi amped system but I will hook up full range into my old Watkins WE-1's to check them out in the near future. Good reliable and excellent design. Thanks for all your help. Now if I could only get rid of the input caps???????????

saludos, dave
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Old 9th April 2009, 08:55 AM   #582
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Hello

Ill just jump in and ask.

How does this look?

Cheers
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Old 9th April 2009, 09:02 AM   #583
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update
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Old 18th April 2009, 12:01 PM   #584
daly41k is offline daly41k  United States
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Default aleph 1.2 to xa100

audiodan, have you built this circuit? Did you get any dc on the inputs? Looks good, i can give it a try next time I get the energy to take apart my amps. I have tried mine now full range and they sound great. dave
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Old 18th April 2009, 08:23 PM   #585
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Hello Dave

Quote:
Originally posted by audiodan Im sure I read somewhere that Pass Labs was selling 2SJ109BL We are not. Rumor has it that Linear Systems is preparing a complement to the LSK389.
Havent made the change over yet, doing some research on NP response to the 2sj109.

As for the DC, didnt have a problem with dc on the original schematic.

Cheers
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Old 20th April 2009, 12:20 PM   #586
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I know it was certainly covered before in details, but just want to be sure to buy the correct (and expensive) transformer. For the Aleph-X, for about 100W (in 8 ohms), using a CLC filter, what would be the correct 750VA transformer voltage to order?

I had a 30Vac secondary before, but it was way too high and I had to use multiple CRCRC filtering and a lot of wasted heat to get a decent transistor power dissipation.

I guest that a 24V to 22V secondary would be a better match.

Any recommendations are welcome.
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Old 22nd April 2009, 09:12 AM   #587
daly41k is offline daly41k  United States
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Default ax

I just use the formula ac voltage x 1.41 to get the dc voltage after recitication for a full wave bridge. Going backwards +/-30vdc/1.4= +/-21.4volts ac before loading down.

You will lose a few volts across the diode bridge and depending on the resistance of the Inductor a few volts there as well. My iron core inductor was about .3ohms. This times class a current of 6 amps = .3 x 6 (IxR=V)=1.8volts. This would mean that you would need 30 volts dc + 1.8 volts + .7(volts diode drop x 2) or approx 33 volts dc before loading down /1.4 =24volts ac secondaries which can provide 30 amps (24 volts ac x 30 = 750). You can also use the formula's in Nelsons articles for designing power suppliers based on Class A operation to arrive at the transformer size. I have assumed you know already that you need 750va. dave
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Old 22nd April 2009, 04:29 PM   #588
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Hi,

dont forget it is a bridged amp so the voltage need not be that high. Im using a 800VA /18V transformer with a clc (torobar 2,2mH) filter and about 7A bias for 22,2V DC.

Power is 102 Watts @ 8 Ohms

William
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Old 3rd July 2009, 09:32 AM   #589
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just done the change overs to my ax-100 and got ax100j up and running.

-With my ax100 and using irfp9610 relative offset was 5 mv, now using 2sj74 relative offset in balanced mode is 80 mV and non balanced is 300 mV but with nothing connected to input this is 4 V and drops very fast as amp heats up. but only in non balanced mode.

I have no decouple caps at input.

Abolute offset starts about 10 v and drops quickly into mV range.

Am using same schematic as gl.
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Old 4th July 2009, 10:58 PM   #590
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Very happy!

got relative DC down to 14 mV on both balanced and non balanced outputs, with no cap on input.

All I done was just keep swapping Jfets around in jig until required results.

resistor 31 reads 4.575
resistor 32 reads 4.584

Could have got these closer but DC gets worse.

Running at 8A
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