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Post by dragan on Mar 13, 2012 16:36:24 GMT
Hi, first to introduce myself, as this is my first post on this forum. My name is Dragan, I come from Novi Sad, Serbia, and I like headphones i was searching for a cheap head amp to start me with the hobby, and to brush up on my soldering skills, as it's now 8 years since i've held the soldering iron in my hands so, I've found a nice review of BT928 and mods done to it by... You take a guess. decided to buy it, and it arrived in post to me not 2 hours ago. Connected the rear of the amp to power and my iPhone, and the front to my AKGs K77, and... Disappointment. Pure disappointment. First, there's a lot of noise coming from amp just as it idles. A lot! (compared to iPhone or iMac phones out). Second, the sound is so muffled, and when I play any nice blues/jazz/rock song with a quiet intro (Allan Taylor - Colour to the moon - for instance), it's just crackling all the time, no matter what tone pot setting I use. Guitars in the songs are just unbearable, totally distorted, (as any other highs). Only part of the sound that is half acceptable is the vocals. Yes, I am aware of the improvements with tone pot removed, but I'm thinking now, should I even go there, as the standard sound is just horrible, and i suspect that there is something wrong with the amp. Any advices?
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Deleted
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Post by Deleted on Mar 14, 2012 4:17:02 GMT
There isn't a great deal to go wrong in an amplifier using only a dual opamp. I would open it up and do a close examination for possible damage during transit.Check for lifted/ broken PCB tracks around the larger components such as the electrolytic capacitors etc. Alex
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Post by dragan on Mar 14, 2012 9:15:41 GMT
^ thanx on your answer I did that straight away, and altho soldering on the board looks a right mess (even I would do a much better job), I couldn't find anything fishy, apart from the sides of the pcb (top). It looks like something leaked over it, and kind of... lacquered it glossy spill on the edges: I would say it's just from heating the board while soldering, but then again, it shows even in places where nothing is soldered - edges will post a snap of the bottom side later today, thanks for help!
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Deleted
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Post by Deleted on Mar 14, 2012 10:42:44 GMT
That may be overspill from cleaning the copper side with Isopropyl Alcohol after completing soldering.That is not unusual. Alex
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Post by Deleted on Mar 14, 2012 12:11:04 GMT
Hi Dragan
have you tried the amp with another source (something other than your Ipod) ?
just a long shot but it may help to eliminate that first before getting stuck in to the amp.
welcome to RG
take care
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Post by dragan on Mar 14, 2012 13:27:09 GMT
i have tried it with iPhone, iMac and my old portable CD player which all work fine with K77s alone. Once the amp is in the way, it distorts a sound (highs especially) a lot. It sounds as if the highs are losing their connection somehow, don't know how to describe it otherwise.
Tried checking all the plugs and jacks, rotated plugs (RCAs, DC in and headphone plugs) to see if there would be any change - None.
I have also tried plugging the amp in different sockets around the house, just to eliminate interference with other appliances, but no difference.
Also I've let it drive K77s for 5-6 hours, hoping that the amp would burn in, but maybe a tad bit of a difference.
Should I give it more than that?
I am no audiophile, and my hearing is not 'finely tuned by years of listening hi-fi' but even I can hear that the amp is distorting sound, as I said A LOT.
Now, K77s are by no means great headphones, but I have heard them amped with only a cheap 10 plug head amp for auditioning headphones, and I must say that even that noticeably improved sound over my iPhone/iMac out (added quite a healthy dose of bass and kind of a better separation), so I've decided to get some kind of an amp, and later change the headphones to something more serious.
will post pics of workings side of the pcb, so everyone can see.
edit: see and hear for yourself:
amped:
unamped:
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Post by dragan on Mar 16, 2012 9:53:34 GMT
news: yesterday afternoon I called the seller, explained the situation and he agreed to give me a replacement. It arrived about 20 mins ago, and I have just tried it. It is - OK. And by OK, I mean, its not buzzing all the time, just when at full blast at full input volume. Insane volumes. I wouldn't say it's great, but I did not expect great in the first place. I now have a base for further improvements, and will post them here, or I'll start another thread. Which ever mods prefer. tone pot first to go.
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xerxes
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Post by xerxes on Mar 16, 2012 13:08:30 GMT
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Post by dragan on Mar 16, 2012 15:21:10 GMT
^ yep. As I said in the first post: so, I've found a nice review of BT928 and mods done to it by... You take a guess. deff gonna get a dip socket, and the best one I can find, but, there lies my problem: not much "exotic" components are readily available here in Serbia. I can probably source anything, but time and costs of getting any higher quality (for example) op amp would be few times more than the cost of the actual op amp (or any other component). So, I'm gonna do what improvements I can without too much trouble. Found the decent dip socket, 2134PA, and resistors, which almost finishes the story, but still did not find what to replace caps with. Also, any advices could I, and how - change the amp to be battery powered as well? And the last question for now, could this amp (in its modified form), drive AKG K500s ... Even half decent - should I say?
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xerxes
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Post by xerxes on Mar 16, 2012 16:28:37 GMT
Ebay might be your best bet for some components, I often find it works out cheaper than some of the larger suppliers, RS, Farnell etc. that will only sell some items in multiples and charge quite a lot for shipping, although shipping time can be quite long from Ebay: www.ebay.co.uk/itm/10x-8-PIN-GOLD-DIP-IC-SOCKET-PANEL-ADAPTER-SWAPPING-G8S-/390338236624?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item5ae1f8d0d0 - they do list Serbia in the list of available postage countries. What voltage and output power is the existing power supply? I used a couple of 12v SLA batteries in series as a 24v supply for my WNA amp for a while, but to be honest I got fed up with messing about charging them and built a linear PSU for it in the end which was better than the original SMPS that came with it. I'm not familiar with the AKGs; Frans?
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Post by dragan on Mar 16, 2012 17:40:28 GMT
What voltage and output power is the existing power supply? now that you ask, and after checking, I find it quite interesting that actual AC/DC adapter that came with the amp is 15V, 300mA (as said on grottos page), while the DC in on the back of the amp actually says 15V 200mA I assume that "Frans" is a member of the forum, better familiar with AKGs? gone to find my sucker and to warm the soldering iron
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xerxes
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Post by xerxes on Mar 16, 2012 18:06:21 GMT
15v 200mA will be the voltage and current draw, 15V, 300mA will be the maximum it can supply; it's common to over specify the power supply. If it's a linear power supply it may not be that bad; I'm not familiar with the BT929 or the power supply, so I couldn't say. I don't know how familiar Frans is with AKGs, but he is a proper electronics engineer that actually understands electronics, whereas I'm really a cobble things together, "electronics by numbers" sort of guy. As such, I would defer to him about any technical stuff.
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Post by dragan on Mar 16, 2012 18:47:38 GMT
first mods on the BT 928: disassemble: tone pot removed: unnecessary caps removed: back in one piece again and working: also, note the volume pot - it is at 1/4 of the travel so quite a bit left for driving something other than 32 Ohms K77s
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Deleted
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Post by Deleted on Mar 19, 2012 11:07:07 GMT
Hi dragan Nice work a before and after mods sound comparison would be most interesting. take care
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Post by dragan on Mar 22, 2012 18:45:44 GMT
before (with tone pot) lack of bass, muffled sound all around.
after (without tone pot): sounds rather OK. For what it is.
There is a distinct hiss/hum when low impedance HP are connected. (32 Ohms), but it is almost non audible with high impedance ones.
I'm happy for now, but will change things that are mentioned here and in other threads and web pages concerning this amp.
Bottom line would be - quite ok for the price
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Post by Deleted on Mar 22, 2012 20:52:42 GMT
Hi dragan nice work and interesting to have your thoughts keep us posted on further mods take care
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Post by dragan on Mar 28, 2012 17:21:17 GMT
ok, amp is again in pieces. Four 47 Ω resistors are here in the middle of the photo, and I take it that 2 are for each output (1/4 and 1/8). so, what if I change just 2 for 120 Ω, and leave other pair as they are? Just for the sake of comparing the sound?
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Post by dragan on Apr 6, 2012 9:50:47 GMT
also, would 1/4 W resistors be enough? I couldn't get any stronger at my local electronic store. Now when I look at them, they are sooo skinny
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Post by Deleted on Apr 6, 2012 11:17:16 GMT
Excellent Idea
Make the 2 on the left (big socket) 120 Ohm (1/4 watt is MORE than sufficient). Make the other 2 (connected to the small socket, follow the traces on the PCB) 10 Ohm (also 1/4 watt) Opamps can only deliver upto 50mA so there is little power.
Replace both 10uF caps for 100uF caps (max 470uF, 16V min). Replace both 47uF caps for 1,000uF or 2,200uF caps (16V min). Make sure the voltage rating at least equals that of the power supply voltage. The big cap near the DC input can be made larger as well (in value) the voltage rating must be at least 25% above the powersupply voltage.
The on-off 'plop' will be louder b.t.w. The volume pot might 'scratch' the first few seconds after power up.
You can lower the gain by changing the 100k (brown, black, yellow) resistors to 10k and get more travel on the pot.
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Post by dragan on Apr 6, 2012 11:59:09 GMT
Excellent Idea Make the 2 on the left (big socket) 120 Ohm (1/4 watt is MORE than sufficient). not that I doubt it, but still.. these 1/4 watt metal films I have are tiny tiiiiny... and they are actually 150 Ohm (thats nearest to 120 they had in stock)Make the other 2 (connected to the small socket, follow the traces on the PCB) 10 Ohm (also 1/4 watt) Opamps can only deliver upto 50mA so there is little power. Replace both 10uF caps for 100uF caps (max 470uF, 16V min). done: 100uF 35V now inReplace both 47uF caps for 1,000uF or 2,200uF caps (16V min). not done, they did not have 1000uF in anything less than 35V and these look a total overkill on the board (should find 25V)Make sure the voltage rating at least equals that of the power supply voltage. DoneThe big cap near the DC input can be made larger as well (in value) the voltage rating must be at least 25% above the powersupply voltage. done - sort off. Didn't change the value - it's still 1000 uF (what value to use?), but I've changed the voltage rating (20V to 35V)The on-off 'plop' will be louder b.t.w. did not find any big difference by nowThe volume pot might 'scratch' the first few seconds after power up. no, can't hear itYou can lower the gain by changing the 100k (brown, black, yellow) resistors to 10k and get more travel on the pot.
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Deleted
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Post by Deleted on Apr 6, 2012 13:12:38 GMT
I'm not sure how close to 120ohms you need to be (i.e. are you trying to match to 120r headphones?).
If you want 120R without having the actual rated resistor, you can pop a 600R in parallel to the 150R to get the 120ohm you're after.
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Post by dragan on Apr 11, 2012 9:38:16 GMT
changing to 120 Ohm is still pending, I have another thing I'd like to solve now:
a friend gave me his cheap 2.1 speakers, and I am using these now in stead of built in iMac speakers. Altho Neo 2.1 is shite compared to any dedicated speaker/amp it is still a lot better than built in iMac speakers.
What I would like to do is to put BT 928 between iMac and Neo 2.1, so I could control Neo 2.1 volume with BT 928 pot, and to mute Neo 2.1 when I plug headphones in.
Possible? Would BT 928 do this by itself, or do I need to make some changes to it and how? I can't check that atm, as I don't have two RCA to 3.5 mm cables (I'm using one to go from iMac to BT 928 input, but still have to get a second one to go from BT 928 output to Neo 2.1)
Reasons why I'm trying to do this:
I like the sound of Neo 2.1 more than built in iMac speakers I can control the volume of speakers with a pot, rather than with system volume I don't have to move the iMac and change cables every time I want to change from speakers to headphones (it should be a case of just plugging in headphones in BT 928, speakers mute and I can listen to headphones)
Also, I mentioned earlier that there is quite an amount of noise coming from BT 928 when I plug AKG K77s in, but that its a lot lower when using AKG K500. Now, when I plug Neo 2.1 in 3.5 mm out on BT 928 that same noise is heard A LOT.
How can I lower this noise? It is coming either from BT 928 or its wall wart.
I have this recorded, will upload to yt so someone can hear and identify it.
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Deleted
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Post by Deleted on Apr 11, 2012 10:05:43 GMT
a resistor division network (in the plug/cord) should do the trick. The noise is not coming from the wallwart but the opamp and it's high gain setting (high feedback resistance).
Lowering the gain will also lower the noise level. You should lower the gain as far as you can (can get enough power from the HP's with the volume set to max level) That 100k feedback resistance is simply MUCH to high.
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Post by Deleted on Apr 11, 2012 11:19:45 GMT
Dragan,
If you still have the 50k pot you can mount it back in there and the cap that was in series with it you can replace with a 1k resistor. This way the tone control pot now has become a gain control pot !
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Post by dragan on Apr 11, 2012 11:31:43 GMT
interesting Yes, I still have every original component
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