joethearachnid
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Post by joethearachnid on Apr 30, 2012 19:14:29 GMT
Look what arrived from Australia today! For a while now I've been meaning to get/build a phono preamp but haven't quite managed to persuade myself that I'd listen to enough vinyl if I had one. The only way I could listen to vinyl before now was through the somewhat poor HPO on our integrated amp, which meant no dedicated headphone amplifiers and so no real incentive to listen. However, bmxargh's recent thread managed to persuade me that this was something I needed. There are plenty of cheapo 'boxes' and also Chinese kits using cheap opamps, but many of the kits require a separate power supply or are poor value. For a while I've had my eye on the Oatley K301 phono preamp, which uses 4 tiny JAN 6418 tubes. I've previously built the K272A, which is a headphone amplifier that uses these tubes, but given the fact that these tubes are hugely microphonic, very sensitive to noise and often completely unmatched gain-wise I soon came to my senses and opted for a nice, reliable discrete solid state design using JFETs. ;D There were some very good words said about the K303 on DIYAudio so I decided to go for it. The main deciding factors for this were that I was familiar with Oatley and knew that they made good stuff, the kit came with a PSU and, most importantly, it was cheap ($30AU). It arrived today after being ordered early last week, which is not bad going considering that it dispatched on Thursday. The kit contains very basic cheapo caps, carbon film resistors and a tiny transformer. I decided to build the kit stock because I'd decided that this was going to be a purely objective build and parts didn't matter (this means I was too lazy and cheap to go out and buy replacements). The build was mostly a very easy paint-by-numbers affair with plenty of space between components and large pads. The board is pretty good quality and nice to work with. Only confusion factors were locations of the 1000uF caps since there are 3 types with different voltage ratings and no indication where they all go, as well as the fact that a number of components had been changed in the diagram because a different JFET was being used than from when the board was originally made. The included RCA sockets are a bit crap but I only installed them for testing purposes. The board size is 138x102mm overall, with 86x102 for the amp board and 52x102 for the PSU board. I want to write that here because there's nothing on the internet saying how big the PCB is and my email to Oatley on the matter went unanswered. Hopefully this helps a lost soul looking to buy an enclosure... Built pictures: How does it sound? Well, I'm not really sure since I don't have anything to compare it to. It seems to make music when connected to a turntable, and the music it's making seems adequately loud and undistorted when a decent condition LP is playing. At the moment there's some 50Hz hum audible just above normal listening levels, but that should go away once I case it up. One question I have is about an LED - the PSU output is 52.9v and 9.3mA and I have seen others put an LED in parallel with the output to make and indicator light. What value of resistor (if any) would I need in series with this to power a 20mA green LED to a reasonable (subtle-ish) brightness? Overall it's a very quick, cheap and easy little kit. I can't say how it compares to other phono preamps but I'm quite happy with it so far. I will case it up tomorrow if my delivery arrives. -JoetheArachnid
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Deleted
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Post by Deleted on Apr 30, 2012 19:32:55 GMT
56k resistor maybe even 100k for subtle LED
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Post by bmxargh on Apr 30, 2012 19:54:35 GMT
Cool mate, fair play to you. A nice bit of commitment and something to show for it........The only reason I'm not tempted is cause I need to sort my deck ;D
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joethearachnid
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Old head on young shoulders.
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Post by joethearachnid on Apr 30, 2012 20:10:25 GMT
Thanks Frans. I'll mess around and experiment between those values.
-JoetheArachnid
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XTRProf
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Post by XTRProf on May 1, 2012 0:56:31 GMT
Cool mate, fair play to you. A nice bit of commitment and something to show for it........The only reason I'm not tempted is cause I need to sort my deck ;D Oh, you will be after the deck rotation episode!
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XTRProf
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Post by XTRProf on May 1, 2012 1:07:11 GMT
For a while I've had my eye on the Oatley K301 phono preamp, which uses 4 tiny JAN 6418 tubes. I've previously built the K272A, which is a headphone amplifier that uses these tubes, but given the fact that these tubes are hugely microphonic, very sensitive to noise and often completely unmatched gain-wise Oh, me too but then I didn't have much time and forgotten all about it until now you mentioned it again. How is the sound of the Oatley HP amp with those Jan tubes. Maybe I will go and have a shot at the Oatley Phono tube headamp since it's relatively cheap.
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joethearachnid
Been here a while!
Old head on young shoulders.
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Post by joethearachnid on May 1, 2012 11:57:05 GMT
The K272A (the Oatley headamp) sounds pretty good... when it wants to. It's very tubey and full of pleasant distortions and 'musicality' - not the most accurate amp you'll hear, but for about £25 total spent on it you can't really ask for much more. The problems come in on the operating side of things. The amp has to run on batteries to maintain a quiet enough power supply, and although others have claimed that one 9v will last for ever, I found that I was changing batteries once every few days (the two AA batteries will last forever, mind). The other problems are microphony and interference - when I was building my amp originally I didn't know to connect an enclosure to ground, so I got loud hum through everything until I eventually did. Metal enclosures are a must. The microphonics are such that switching on the amp, pulling out or putting in a headphone jack and sometimes just playing music can cause a high pitched ringing that often doesn't stop until you restart the amp. For tube imbalance, probably now I'd go to the effort of finding two at least vaguely matched tubes through a method Oatley described, but at the time I didn't really think it would matter and so one of my tubes tends to go very noisy when the battery is running down. I'm actually planning a small re-service of that amp in the near future and may put another tube in instead. When I bought the amp you could get 10 JAN 6418s for EUR10, now they're going for EUR10 EACH in some places. Crazy times. Broadly the amp sounds great when it's working, but it's a lot of effort to keep running smoothly and I really didn't want to deal with that in the phono preamp. -JoetheArachnid
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XTRProf
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Post by XTRProf on May 1, 2012 22:28:30 GMT
Btw, since you are changing 9V battery every 2 or 3 days, might as well get rechargeable 9V batteries. Any good basic mains driven PS SHOULD not give you noise high enough to be problematic. The noise problem you mentioned could be a problem from elsewhere. Hissing, EMI, RFI or what you have now?
Actually, you can get the microphony down quite a lot by putting in O rings or grommets or those teflon tube dampers to damp the tubes. For the last, you can also use those tap teflon sealing tape. Also ensure that the tubes don't move in their positions by contact glueing them to any rock stable surface. The ringing should not continue on indefinitely until rebooting the amp. If that's the case, it's already oscillation. Prof Frans, right?
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joethearachnid
Been here a while!
Old head on young shoulders.
Posts: 380
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Post by joethearachnid on May 1, 2012 23:16:15 GMT
I do use rechargeable 9v batteries, yes. And I have looked into PSUs but have never really got around to building one. Right now when well-powered the only noise is hiss from the amplification circuit itself.
On damping: the tubes have two grommets on them each. I've tried other things like wrapping them in rubber bands, putting foam on the lid and covering them in white tack in addition to the grommets. It still doesn't stop the tubes ringing... I've considered 'potting' them with hot glue but don't really want to do that in case I need to change a tube.
-JoetheArachnid
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XTRProf
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Pssst ! Got any spare capacitors ?
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Post by XTRProf on May 2, 2012 1:23:38 GMT
Right now when well-powered the only noise is hiss from the amplification circuit itself. Hmm, hiss ........... Is it is very bad? Can be tube hiss or too high a gain from the HA for the HP you are using. Usually HPs of 32 or lower ohms will hiss quite badly when driven by a high power (loud) amp. Have you change the output resistor to higher values until the hiss is gone? I've considered 'potting' them with hot glue but don't really want to do that in case I need to change a tube. Oh, don't do that. Pot by using easier removable Silicon will be better.
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joethearachnid
Been here a while!
Old head on young shoulders.
Posts: 380
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Post by joethearachnid on May 5, 2012 22:09:25 GMT
Cased up yesterday: Not the greatest casework ever, but I'm sure there's worse. For all my trouble to shield the PSU from the rest of the amp I still get a (much louder) mains hum in the left channel, though only with the turntable connected. As far as I could tell when connected to the receiver and turned up to full, the mains hum was equal through both channels but the hiss was far louder on the right. (this is only with the turntable, not with any other components.) I don't have any other turntables to test with at the moment and I can't see any flaws with the amp, so for now I'm assuming that the TT is to blame, unless someone can correct me. The LED brightness is perfect (used three 22kohm resistors in series for 66k) but due to the large PSU capacitance it takes a while for the LED to go off. I guess the workaround for this would have been to use a DPST switch instead of a SPST one and wire the LED through it. Listened to some Dire Straits and it sounded great. Some sibilance but can't be sure if that's due to the amp or the condition of the LP. -JoetheArachnid
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XTRProf
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Pssst ! Got any spare capacitors ?
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Post by XTRProf on May 6, 2012 5:37:20 GMT
GI Joe,
Use screened wires for your inputs to reduce hum. You can also use a bleeding resistor if you feel uncomfortable with the LED lighting up too long after off. What value I'm not sure but definitley in the high range. Frans and Alex might be able to offer that value.
Also, is your case earthed since there is mains voltage in that casing? If the shield around the PSU is steel, that will be better to screen off EMI from the traffo as we must have ferrite to reduce magnetic waves.
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joethearachnid
Been here a while!
Old head on young shoulders.
Posts: 380
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Post by joethearachnid on May 6, 2012 19:40:02 GMT
Chong, All the signal wires are shielded copper. The shielding for the PSU is some lead sheet I happened to have to hand. The chassis is not connected to safety ground (since I was trying to avoid hum from ground loops ) but all mains connections are very short and heavily insulated. I've tried connecting the amp to safety ground but it makes no difference noise-wise. Today I went over all my soldering on the left channel (noisy) side of the board and reflowed any joints that looked even vaguely suspect, but to no avail. I did discover that the amp can keep running for around twenty seconds just on the PSU capacitance, but the left channel is active for much longer than the right channel. There are similar effects when the amp is switched on, with the left channel coming on very quickly but the right needing twenty seconds before producing any sound. I've checked the values of resistors directly in the DC voltage path and they all seem identical. Given the nature of this PSU a bleeder resistor seems like it might be a good idea... There's a PDF here with the circuit diagram if anybody more knowledgeable wants to look through it: secure.oatleyelectronics.com/files/k303notes.pdf-JoetheArachnid
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XTRProf
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Post by XTRProf on May 7, 2012 2:51:44 GMT
All the signal wires are shielded copper. Check! The shielding for the PSU is some lead sheet I happened to have to hand. The chassis is not connected to safety ground (since I was trying to avoid hum from ground loops ) but all mains connections are very short and heavily insulated. I've tried connecting the amp to safety ground but it makes no difference noise-wise. Pb or Al or Cu are not good for magnetic field shielding. Carbon Steel or Mu Metal and Permalloy (the last 2 will be best) as they are with conductive ferrite. How to have ground loop if the amp ground is not connected to the safety earth? Anyway, I'm talking from a safety perspective when earthing the chassis as there is mains voltage in what you have just in case of any leak from any unthinkable. I did discover that the amp can keep running for around twenty seconds just on the PSU capacitance, but the left channel is active for much longer than the right channel. There are similar effects when the amp is switched on, with the left channel coming on very quickly but the right needing twenty seconds before producing any sound. I've checked the values of resistors directly in the DC voltage path and they all seem identical. That is a bit strange. 20s difference between L and R channels before active? Have you physically measured all the resistor values before putting in? Not purely depending on resistor colour code. There can be some faulty parts around even if they are new. The usual under guarantee parts problem. Frans and Alex can help you in that schematic read of which points to check for equal voltages for both L and R channel to debug.
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joethearachnid
Been here a while!
Old head on young shoulders.
Posts: 380
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Post by joethearachnid on May 13, 2012 18:53:20 GMT
Update: Through Frans' guidance I established that one of my '2SK264' jFETs was bad. I tried heating it and cooling it as per instructions but had no change on the voltages across the relevant resistors. The problem with replacing it was that I couldn't find any 2SK264s anywhere on the internet, and it wasn't until I spotted a 'related searches' thing on eBay that I thought to check and found that actually the FETs were 2SK246s and not 2SK264s as listed (twice!) in the manual. Annoyingly after ordering a few off eBay I realised that the jFETs I had left over from my Panda were actually K246s (though the slightly lower IDSS GR versions as opposed to the higher BL versions) so I slotted two of those in and have evenly booting channels with equal (and far quieter) hum on both channels. I think that's about as good as I'll get. I also created a fast-off for my indicator LED with Frans' help by using a 15k resistor and a 39v zener in antiseries. Syd was kind enough to send me a few zeners from his dwindling stockpile. The LED takes a while to come on (though I don't mind this since the amp takes about the same time to warm up) but switches off fairly quickly. Broadly, I got it to work eventually. -JoetheArachnid
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joethearachnid
Been here a while!
Old head on young shoulders.
Posts: 380
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Post by joethearachnid on Jun 15, 2012 18:37:32 GMT
Small update in case anybody's out there:
I was still getting some hum through the circuit on both channels (somewhat audible at regular listening volumes) despite replacing the bad jFET experimenting with transformer shielding, so I figured the noise must be because the garbage from the transformer on the PSU board was getting channeled into the amp ground no matter what I did. Isolating the case from amp ground wasn't an option as this just resulted in vastly more noise.
I decided that the only option was to put the two parts in separate cases, but rather than buying two smaller cases and discarding/not using the current one, I instead decided to cut the box in to two parts, fabricate two new end panels and then link the boxes with an umbilical cord. Overall it's worked out pretty well - the noise is much lower overall now, and is only audible at what would usually be deafening levels. The amp is still somewhat sensitive to nearby electromagnetic noise, but with a phono preamp I think that's rather unavoidable and again, that noise is generally not audible. It's all been good experience in terms of dealing with sensitive circuits.
Now I just need to get some more vinyl...
-JoetheArachnid
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