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Post by deepdark on Sept 24, 2016 14:21:47 GMT -6
50$us + shipping
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Post by deepdark on Sept 24, 2016 10:15:44 GMT -6
Hi guys i worked hard to put this project alive. I would like to know if there is any interest for an LA2A pcb, with build manual, wiring diagrams, dc heater option for V1 as well as ac heater, etc. I can't acces my pictures right now, but you can see the feeler post on my facebook page and let me know on the post your interest. Www.facebook.com/elab.electronicregards
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Post by deepdark on Jan 15, 2016 12:41:24 GMT -6
HI
A friend of mine just got one for nearly nothing. It's a nice little mixer: 16in 2 out, made in the mid 70's. Anyway, This mixer originally came with an external 21V power supply. The one he got had a 12V wall wart solder into it. It works good, but the weird fact is that I made a 21V external power supply to replace the Wall wart, and it does not work. I mean, the pilot led does lit up, there is 21V on the B+ rail, but no sound coming out of the miuxer, while it does sound good with a psu of 12V. I think the only thing that came out with the 21V supply was a faith noise, that's it! I'm scratching my head to understand why, for the life of me, it does work good with nearly half the voltage than originally specified. Does the previous owner modified it to accomodate 12V? Does it have any kkind of jumper to select between 2 B+ voltage? The documents are somewath incomplete so it's hard to really know what's going on lol. Do you think I'm overthinking here and should simply made the power supply at 12V and be happy with that, since we know nothing about the previous owner and what he did to it, other than strapping a wall wart to it loool!??
www.allen-heath.com/ahproducts/pop-mixer/
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Post by deepdark on Sept 30, 2015 14:29:50 GMT -6
Not that impressed to be honnest. Not my go to pre for sure. It's nice to record some upfront, rock vocals, guitars etc. But it's not my choice for some acoustic stuff. I think, if one would make a nice dual tube pre, the schematic on Jensen transformer website would be a nice starting point. I'm planning building it soon.
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Post by deepdark on Sept 23, 2015 6:42:03 GMT -6
I've had succes with Garritan personal orchestra (GPO4). Once you get used to it, it comes pretty easy to work with
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Post by deepdark on Sept 21, 2015 6:29:01 GMT -6
One way I succes recording dreadnought guitar (Taylor) is to put the microphone at the headstock, off axis point down to the microphone body and about 4 to 6 inches of the headstock. That way, I minimise the low end, and accentuate the highs.
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Post by deepdark on Sept 14, 2015 14:58:44 GMT -6
even at 600, it will probably be significant
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Post by deepdark on Sept 14, 2015 14:20:31 GMT -6
I don't plane redisigning and rebiasing the output stage to make it run cooler haha. To quote something i've just read
"another underestimation in the REDD.47 design was that they put off an ungodly amount of heat - which meant having to regulary changing valves"
Don't know how exact this statement is, but it looks to resume what we've just discussed over the thread lol. Weird funk'ng pre, as you said.
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Post by deepdark on Sept 14, 2015 13:49:12 GMT -6
so all that heat, power consumption is related with the way output transformer and overall reverse feedback is handled at the output, and even with a lower ratio, i guess it will still runs hot because of the huge current flow, right?
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Post by deepdark on Sept 14, 2015 13:14:26 GMT -6
I agree it's very odd that the resistor is running hot given the rating. And, even a 5W is enormous for a preamp output stage, especially one rated for +14dBm max output with peak distortion. Theres a lot about that output stage NOT to like, really low headroom, really high power consumption, big heat output, huge drain on PSU. But, it's there in the manual. Weird f'in preamp. Plenty of things do higher output with much less current draw. is it the same reason for the big heat output and output stage running hot?
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Post by deepdark on Sept 14, 2015 13:07:58 GMT -6
I used a Edcor 10K/600ohm, and typically, it will be load by about 10K if used straight into a ad/da.
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Post by deepdark on Sept 14, 2015 12:17:45 GMT -6
power isn't forced from the PSU, it's drawn from the amp. it's gonna draw what it draws. It draws more with high voltage supply, it draws less with lower voltage supply. The PSU could supply 100 amps at 300V, but so long as voltage remains at 300, the amp will draw no more or less than if the PSU only supplied 100mA. This, and also the rated maximum draw and the actual draw of the circuit have to be very different. If a unit draws 30mA while idle it may well draw 50mA for the first millisecond as the caps are filled. Usually the PSU has to handle the current draw of the circuit + 50% though with circuits with a lot of big caps going closer to doubling is safer. Yeah you're right. I always double values to get the right power transformer, to be sure I'm not to close to the limit. My first filter cap is about 1uf/ma and generously round up so I get 47uf in this case, where I could have put something like 22uf, even 33uf. I had calculated about 2W through the psu, I got 5 sections so less than 1/2W / resistor. I put 1W résistors everywhere in the psu. In reality, here we have about 21ma and my power transformer is rated 69ma at the secondary, so I could tripling the initial valu and not exeeding the maximum rated value of the PT.
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Post by deepdark on Sept 14, 2015 12:12:52 GMT -6
power isn't forced from the PSU, it's drawn from the amp. it's gonna draw what it draws. It draws more with high voltage supply, it draws less with lower voltage supply. The PSU could supply 100 amps at 300V, but so long as voltage remains at 300, the amp will draw no more or less than if the PSU only supplied 100mA. You're right. PSU only provide what's needed. I look at it the wrong side. but still, the power remains the same, which lead me to think, as you said, the output is so damn gluttonous for such a task. If really output is about 5W, damn it's quiet big. E88CC must have to be replace often than normal, isn't it?
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Post by deepdark on Sept 14, 2015 8:44:25 GMT -6
Some food for thought. If we look at it from the psu side, we know we have 308V at about 21ma, which is about 6.5W. We know we feed the input and output stages anode. Input, with such a small current consomption, would take no more than about 0.2w, right? Let's say 1w at the input, doesn't it let at least 5W at the output? And the only way for B+ to feed the output is from R11, isn't it? From that point of view, first we can say that the output is ridiculesly high, but also, we can ask ourselves where does all that power go?
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Post by deepdark on Sept 12, 2015 14:52:46 GMT -6
What. Was speaking with higher b+ and heaters is more about the heat of the tubes, and the overall power dissipating through 8k2. Anyway, it's a pretty agressive pre, no real control over the input, it's pretty raw but it's cool.
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Post by deepdark on Sept 12, 2015 14:05:04 GMT -6
Anyway, it works lol.
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Post by deepdark on Sept 12, 2015 10:14:50 GMT -6
You're right. I don't think the resistor being hot a big concern. With 7vac heaters and higher b+, maybe here i'm pushing the tube farther than emi. What do you think?
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Post by deepdark on Sept 12, 2015 9:53:28 GMT -6
And if that output really heat, all that heat must be dissipating somewhere, so at the plate resistor. Right? I don't know why it runs hot. I took every single voltages, vdrop, current. Everything is within spec. Maybe it has a reason why the original psu was regulated. Maybe this particular circuit is sensible to variation in current.
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Post by deepdark on Sept 12, 2015 9:42:38 GMT -6
I didn't know that particular output was rated to run that hot. I suspected first maybe some high freq oscillation, but in testing, i didn't noticed anything. And because the pre works, as expected, i don't really see what's weird here.maybe the fact my b+ is 20v higher is responsible??.
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Post by deepdark on Sept 12, 2015 9:25:51 GMT -6
Oh and i just tested it with a dynamic mic and it sounded quiet good. Nothing weird, gain selector works. No hiss. No burning resistor, etc. Run has expected.
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Post by deepdark on Sept 12, 2015 4:32:50 GMT -6
I just let it run for about 30-45 minutes and no smoke, no odor, by the way.
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Post by deepdark on Sept 11, 2015 19:42:32 GMT -6
Im not using a board,mit's a turret mounted one. Yup, i took voltages drop and current everywhere. Every single voltage and current is spot on. Because of my higher than soec b+, there is about 3,3w passing through the resistor, which is more than half the 5w spec. I should have take a 7w but i decided to take a 10w. When i say it is hot, it is not smoking or smelling. It's just running a little more than warm. In my point of view, 3w of dissipation must remain somwhat hot. Even in a resistor being higher than twice the current rating. 3w remains 3w, but the resistor just run into a safer side. It's my understanding of what is going on. As i said, i measured everything, and it's spot on.
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Post by deepdark on Sept 11, 2015 6:07:09 GMT -6
Just changed the 5w wirewound for a Ohmite 10W and it seems to have cured the trouble. I let it run for about 5 minutes for now and no smoke, nor any fume or burning smell. It remains hot, but it's allright. So, 3.3W through a 5w isn't really optimal XD
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Post by deepdark on Sept 8, 2015 20:26:14 GMT -6
that's exactly what I was thinking. Maybe my resistor isn't the right for the task. Here is what I've just put in there www.vishay.com/docs/28730/acseries.pdfIdeal for pulse operation, ceramic with lacquer coating, etc. etc. etc. Maybe I'm just at the edge of the limit voltage? I got there table and look at it. Limiting voltage is set by the square root of P*R, so square root of 5W * 8200r = 202V (probably the voltages across the resistor), which set it near the edge of the voltage I'm passing through. And if it's really what happened here, then I'll have learned something (actually, plenty of things haha). Anyway I'll just put a 10w in there and live with it. Since all my voltages are right and current are what they are supposed to be, I guess I should be fine, right?
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Post by deepdark on Sept 8, 2015 20:02:04 GMT -6
Curent after R9/jonction of C4 and R8, read 0 (my dmm isn't that precise). I calculated before a drop of 43v through R9 (1M6) which gave a pretty low current, so here it looks to be right, too. I begin to suspect my sanity lol. But seriously, I really don't see what happen here.
Current is right, at least around e88cc. current is spot on before and after 8k2, so how can we eat up such power if current reading is correct? Wire gage? There is no sign of dc after each caps, too.
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