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Post by marzukia on Sept 5, 2009 8:18:44 GMT
Hi guys, I'm in the middle of putting together the A.K. version of the SCHA and I've got an issue with the BC550/560 transistors that I bought (They're the B type).
From what I understand I should be matching the hFE on these transistors so that they're similar for each channel. I've been able to get pairs from the ones that I've got but the hFE of the BC550 batch are completely different from the BC560 batch.
Example, all the BC550s are around 280 hFE and the BC560s are around 360 hFE.
Should I just use the BC547/557s that I got with the kit, or are the readings I got above normal?
Any help would be appreciated, this isn't really my first DIY but I'm still totally clueless about a lot of things!
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Post by Deleted on Sept 5, 2009 8:49:22 GMT
marzukia It is unlikely to make a great deal of difference whether you use the BC547/BC557 or the BC550/BC560 that you have. If the HFEs of the supplied BC547/BC557 are lower than the BC550/BC560, or much further apart , I would use the BC550/BC560. They are from the same family of devices, so there will be no problem with which you use in this application. If you intend using a JLH, then you should if possible use the "C" versions ,which are higher gain,in the JLH. Alex
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Post by marzukia on Sept 5, 2009 10:16:18 GMT
Thanks for the advice. The BC547/557s that came with the kit actually measure really close to each other, they're all around 280 hFE.
Also I'll try to get C versions of the BC550/560s for the JLH, but if I can't get any is it acceptable to use BC547/557s?
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Post by Deleted on Sept 5, 2009 11:15:33 GMT
Performance is likely to be degraded a little, but it will work. For the BC550C, you can also use BC548C and BC549C. For the BC 560C you can also use BC558C and BC559C If you can't readily get "C" versions, try getting a few extra and use those with the highest HFE. Alex
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Post by marzukia on Sept 24, 2009 8:07:24 GMT
I bought a few BC550Cs and BC560Cs as you said and put them in the SCHA. I also put together a JLH on protoboard and I'm having problems with getting it to work, specifically R8 on both channels (the 33R ones) keeps getting cooked when I power it up.
Any ideas on what might be doing this? I hope the transistors themselves are fine because I don't have much spare and would have to order some more.
(I'll post pics of the board in a bit)
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Post by Deleted on Sept 24, 2009 8:22:30 GMT
marzukia With the JLH, check especially the orientation of the BC639/BC640. Looking from the flat labelled side, the pins from left to right, are Emitter, Collector, Base. A problem in that area could see the transistor below it in the schematic diagram being turned hard on. This area is isolated from the BC550C/BC560C by the 10uF capacitor. (assuming correct orientation of the capacitor) Alex
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Post by marzukia on Sept 26, 2009 11:22:55 GMT
I checked the BC639/640 with the schematic and they look like they're oriented correctly BUT after spending some time looking over the schematic and the printed boards that everyone's using here it would seem like I got just about everything else wrong. So I ripped out the Toshiba transistors (didn't want to throw them away since I matched them and they worked fine), bought some more caps and started again on a new board. I tried to copy the layout of the proper boards as best as I can but this is the first time I've built anything on protoboard so I still managed to screw up esp. around the BC550C/560Cs (hence the rat's nest of wires in the pic). But despite everything this time it *seems* to runs fine (ie. nothing caught on fire or blew up). Later on I'm gonna go over it as per your fault finding guide. The - and + sides seem to be a bit off, ones 14.9V and the other's 15.1V. But this isn't an issue right? Also, on the Jaycar power supply board the LED power output reads about 21V. I understand it's supposed to be 5V; Is this a result of the PSU mods? Or another problem?
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Post by Deleted on Sept 26, 2009 11:43:17 GMT
marzukia The voltage regulators are never spot on unless you use trimpots for adjustment, and those readings are quite good. The LED connector block will now read around +21V without the LED connected, as it is direct from the filter capacitor via the 10K resistor.( Approximately 15VAC x 1.414 with a full wave rectifier.) The most important voltage readings are at the collectors of the BC550C/BC560C, which can be measured at the 2K7 resistors, and around 1.7 -1.8V WRT earth at the base of the BC639/BC640 . (junction of the 10uF cap and the 120K resistor) Alex
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Post by marzukia on Sept 27, 2009 6:28:19 GMT
I'm putting everything together now and I'll be sure to check those points after I get the power section wired and grounded properly.
Are there any special things I need to do with this amp WRT grounding? Do I need a loop breaker anywhere? I'm using an ABS rackmount enclosure and the toroid, PSU and JLH is mounted on aluminium, and the IEC and amp board is earthed to the aluminium.
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Post by Deleted on Sept 27, 2009 7:00:00 GMT
I'm putting everything together now and I'll be sure to check those points after I get the power section wired and grounded properly. Are there any special things I need to do with this amp WRT grounding? Do I need a loop breaker anywhere? I'm using an ABS rackmount enclosure and the toroid, PSU and JLH is mounted on aluminium, and the IEC and amp board is earthed to the aluminium. Marzukia You don't need a loop breaker. Connect the earth from the IEC mains socket to the aluminium that the toroidal transformer is bolted to. Also use the earth ring, or similar, from an old RCA socket and slip over the front of the potentiometer bush when mounting to the front panel, and wire this back to the aluminium where you have connected the earth from the IEC socket. You can also connect a lead from the vacant earth point on the main PCB , to this earthing point too. Alex
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Post by marzukia on Sept 28, 2009 7:33:32 GMT
Thanks for clearing that up!
Also RE the JLH, I'm not getting any readings that's shown on the guide...
I get:
7.1V on Q1/Q2 bases
10.5V on Q1/Q2 collectors
6.7 on Q1/Q2 emitters
0.7V on Q3 base
0.1V on Q4 base
Really tiny reading on Q4 emitter
13.7V on Q5 collector
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Post by Deleted on Sept 28, 2009 8:41:21 GMT
Thanks for clearing that up! Also RE the JLH, I'm not getting any readings that's shown on the guide... I get: 7.1V on Q1/Q2 bases 10.5V on Q1/Q2 collectors 6.7 on Q1/Q2 emitters 0.7V on Q3 base 0.1V on Q4 base Really tiny reading on Q4 emitter 13.7V on Q5 collector Marzukia Seems you have some major problems. For example, the bases of the BC639/BC640 should read between 1.7 and 1.8V with respect to 0 volts (earth) That is, 3 x base to emitter voltage drops in series.The base voltages of the BC550C/BC560C should be approx. .6V higher than the emitter readings (2K2 resistor) Fresh readings from a new but uninstalled JLH with +-15V in to the CL. BC550C - C=10.8V approx. B=7.35V E=6.73V BC560C - C=10.85V B=7.32V E=6.71V BC639/BC640 Base voltages will be within the range of 1.7 to 1.8V Alex
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Post by marzukia on Sept 28, 2009 12:58:05 GMT
I had another look over the board and schematic and I realized that I've been getting the collectors and bases mixed up! Even though you've told me several times to check it! The circuit diagram confused me a bit. So I went and moved some things around and put in even more hookup and now I think the readings looks a bit better? I've just gotta figure out what's wrong with the negative side readings for Q3 emitter and Q2 collector. (At this stage everything's been moved around so much I wouldn't be surprised if something's been damaged so I should probably get a fresh set of transistors) Q1 base +7.1V -6.9V Q1 collector +7.1V - 7.0V Q2 base +7.1V -7.0V Q2 collector +11.9V -15.0V Q1/Q2 emitter +6.7V -6.5V Q3 base +1.7V -9.1V Q3 emitter +1.2V -8.5V Q5 collector +2.0V Q4 emitter +0.6V -0.4V
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Post by Deleted on Sept 28, 2009 13:18:52 GMT
marzukia Concentrate on one section at a time. The BC550C/BC560C are separated from the other end by a 10uF capacitor , so get that area working correctly first. Remember that a working transistor has approximately .6V difference in base and emitter readings. Q1 does not have this.Q1 base and collector readings are virtually identical too. That is incorrect. What kind of meter are you using for these readings ? Alex
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Post by marzukia on Sept 29, 2009 12:39:41 GMT
I've been using two cheapie meters from Jaycar and DSE, I get the same readings with both of them though. I'm going to sit down with the board when I can and go over the wiring again... (Thanks so much for walking through this with me BTW, I really appreciate it as I don't have a clue what I'm doing most of the time) This is the layout that I'm supposed to be using for this board, I just copied it off the photos in the group buy thread (because I couldn't locate the PCB artwork) and scribbled it on the protoboard. If it's not too much trouble could you skim over it and tell me if I screwed something up?
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Post by Deleted on Sept 29, 2009 21:03:29 GMT
Marzukia I have sent you an email with previous artwork attached. Alex
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Post by marzukia on Sept 30, 2009 6:21:11 GMT
Alex, thanks a lot for the pics! I'll check it with my layout and fix up stuff.
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Post by Deleted on Sept 30, 2009 6:23:36 GMT
Thank Chris for the photos, not me !
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Post by marzukia on Sept 30, 2009 7:54:44 GMT
I guess I should've asked for the artwork when I first started and save myself all this grief huh, instead of trying to wing it. ;D I've fixed up my old layout using those artworks and I think I should have all the wiring right this time? EDIT: I didn't see those photos until just now, I'll be sure to shoot him an email with thanks (:
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Post by marzukia on Oct 8, 2009 6:26:47 GMT
I'm still waiting for fresh transistors to arrive so I went ahead and wired up everything sans JLH. I fried some earbuds that I used to test the amp, I seem to be getting a lot of DC in the left output (about 13V). Right and ground are fine. Any ideas on what to check?
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Post by Deleted on Oct 8, 2009 6:35:59 GMT
I'm still waiting for fresh transistors to arrive so I went ahead and wired up everything sans JLH. I fried some earbuds that I used to test the amp, I seem to be getting a lot of DC in the left output (about 13V). Right and ground are fine. Any ideas on what to check? Marzukia Are all the LEDs alight ? Check especially that you have all transistors in their correct locations, and that there are no solder bridges between tracks on the PCB. Check this in good lighting using a magnifying glass . It is always helpful to have one working channel though, as this permits comparison voltage readings. Alex
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Post by marzukia on Oct 8, 2009 10:10:50 GMT
Alex, while checking the traces I found one link that I'd forgotten to solder in. Sorry! ;D Everything's reading fine now. Sounds pretty good too. I'm wondering though, how loud is this thing supposed to go with a standard build? I've got Grados and I have to crank the pot on the SCHA a little over double than I have to on my META42 (desktop and standard gain). Maybe I've just got the gain on the META42 too high all this time? And another question I've had in my mind for a while: should I be unlugging my headphones when I turn this amp on and off (does it have a DC thump?)? Thanks again!
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Post by Deleted on Oct 8, 2009 10:50:48 GMT
Marzukia Glad to hear you have it working. Best results are always obtained with a minimum of attenuation. All that matters is that you are able to achieve maximum comfortable listening volume with your usual source. If you can't get that, then the "Tweaks" thread shows how to increase gain. The circuit as it stands has a gain of 2. If you get maximum useable volume at a low volume setting, you are throwing away performance in areas such as distortion and signal to noise ratio.
With the JLH in line there is normally no switch on thump, as the supply voltage rises relatively slowly.As for switch off, I am unsure as I normally take my headphones off, then turn it and the DAC off. I would be surprised if there is any marked turn off thump. Alex
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Post by marzukia on Oct 8, 2009 11:35:10 GMT
Thanks for clarifying Alex. I do get comfortable volume levels at around 5-6 on the dial so it's perfect. Now I just hope I can finish the JLH with no more issues.
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Post by marzukia on Oct 15, 2009 6:55:45 GMT
I got the transistors and finished off the JLH. Except a minor mishap involving reversed wires and exploding 100uf caps (hence the brown fuzz in the pics ;D) everything's working fine and I get pretty good readings. Some pics here; it's not much of a looker inside but I think it looks pretty tidy from the outside and I'm happy with it. Thanks again for all the help with this build, Alex!
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