| MEGA-amp |
I built two of these chassis to originally house Aleph2's, and they've been sitting for quite some time now. So I've decided to build stereo Aleph5. If all goes well, it should be finished by the weekend:D
-john |
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| Luke |
| good work. I like simple, big amps:-) |
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| MEGA-amp |
ITS ALIVE!!! Worked on first power up..no smoke. Left channel 25mv offset, Right Channel 9mv!!! With one 800Va, 30V secondaries, and CRCLCLC; .22R, 2.2mH, 49KuF, final voltage is at 36v. After 4 hours I could have fallen asleep on the heatsinks, so I guess they pass the 10 second rule and way beyond. It sounds excellent and is also amazingly quiet considering my wiring job is far from adequate. (pics later)
Input network question:
R3, R10 are 68.1K on brian bells board (A30 front end)
R4, R5 are 10K in the Original Aleph 5
I used 22k in both of these spots, I'm running them unbalanced, so should I go with 68.1k, 10k or keep the 22k?
-John
And of course, a big thanks to NP!!!
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| MEGA-amp |
Referring to the above:
R10 - increases input impedance when running unbalanced.(?)
R3 - part of the feedback loop? By my using 22K here, am I lowering feedback? |
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| MEGA-amp |
I tested at +/-20, 36, 46 volts, 46 volts sounds much better. Everything else is stock Aleph 5, with the exception of the removal of R19(221K). The plan is to run it at 36 volts, with 3.5amps DC bias, 0.50R source resistors. With a 42W fet dissipation, I'm sure I'll be OK, but if a mosfet fails, does it send DC to the speaker?
-john |
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| MEGA-amp |
Found the SWEEEET spot!!
I'm STILL a major noob at this, if I burn FETs up, will it send DC to my speaker?? |
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| MEGA-amp |
Pretty much the same as Blue's Supercharged Aleph3 with one extra pair of fets and higher rail voltage.
edit - R13 is removed |
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| MEGA-amp |
To answer my own question because it seems like no one wants to help out. I decided to put it through a torture test. I connected the big 38v 1500va trafo, unhooked the speakers and connected multimeters to the outputs. In between the smoke stack, I was able to see full rail voltage on the meters before I pulled the plug. You learn something new everyday, guess its time to rebuild.
legalese:
I DONT RECOMMEND TRYING THIS, IF YOU DO; YOU DO SO AT YOUR OWN RISK...but you already knew that. |
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| MEGA-amp |
Nothin new here, I've heard that before.:devilr:
The new plan is to run a single 3KVA Variac to a 1500VA trafo to run a single channel. For the times when you don’t want to run the amplifier full tilt, do you run into problems when reducing mains voltage as far as current capability is concerned or any other issues that need to be addressed? If you take a look at the attached excel sheet, you'll see a junction temp of 160C, but a dissipation of 23w per FET. Which one is more important to stay within spec, or is it that you can run the FETS right up to the junction temp as long as dissipation is within 20-30w? For Junction temps; the IRFP240 tops out at 150C, the FQA19N20C (the one i'm using) maxes out at 175C. I'm trying to avoid blowing this one up.
New: |
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| MEGA-amp |
I just purchased a 2000VA 20amp variac. With my 1500va 38sec trafo, I'm going to run my new Aleph with 12 outputs at 5.45 amps at 32 volt rails. I'll be using this variac to drop the mains appropriately of course. Now, i've read that people use variacs to drop the rails on tube amps and this results in the tubes saturating sooner. Do these types of issues affect transistors, and do my current, and VA rating of the amplifier trafo drop while lowering mains voltages. I've read and searched and couldn't come up with anything conclusive.
-john |
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| MEGA-amp |
Am I asking the wrong questions here? This whole thread has been one big conversation with myself
:confused: |
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| jupiterjune |
| quote: | | Am I asking the wrong questions here? This whole thread has been one big conversation with myself |
Gets frustrating, doesn't it? I posed a question here once that I thought was very interesting, and got a reply from only one member (happily, a very good reply).....
I don't know "for sure" the answer to your question, but I THINK that there is no major issue with the mosfets with the lower bias. As far as the trafo -- I have no idea. I assume there is not problems, but some of my assumptions have been completely wrong.
How are those input mosfets working out for you????
JJ |
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| Zen Mod |
| quote: | Originally posted by MEGA-amp
I just purchased a 2000VA 20amp variac. With my 1500va 38sec trafo, I'm going to run my new Aleph with 12 outputs at 5.45 amps at 32 volt rails. I'll be using this variac to drop the mains appropriately of course. Now, i've read that people use variacs to drop the rails on tube amps and this results in the tubes saturating sooner. Do these types of issues affect transistors, and do my current, and VA rating of the amplifier trafo drop while lowering mains voltages. I've read and searched and couldn't come up with anything conclusive.
-john |
there is your answer:
toobz goes in starvation because varying mains voltage fro Pxformer means also varying their heating voltage ,and that's directly related with both toob longevity and also amplifying factor ( in mosfet world known also as xconductance)
there is no reason that you can't do this in reasonable amount with your sand amp.
if you use overkill Pxformer ,with lovering input voltage you'll also lower power factor of that xformer,but- because you need smaller amount of power (presuming that perform all this from that reason) ,there is no any drawback...........
looking at your thread,I really can't see many question........ ;) |
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| MEGA-amp |
Thanks for the help choky. JJ: As for the JFet inputs, I presume your are talking about, Did you notice a "big" difference in sound compared to the 9610's? I have yet to try them; I'm still deciphering choky's Pdf on the subject.
-john |
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| jupiterjune |
John-
Actually, I was looking at your schematic;)
JJ |
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| MEGA-amp |
| the schematic is wrong, they should be labeled IRFP9610, I dont know who drew those up. |
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