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Old 9th February 2010, 11:11 AM   #21
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The ground point is actually a 5mm round pad . Capacitors connect up on one side and the amp grounds connect about 120 degrees away from it.
However I just realised that one channel has 40mm of track common to the transformer 0V line .
I can fix that easily. Will do it and compare the FFT to see what it has done with and without a test signal.
Hmm.... Why was I so careless !
Thanks for bringing this up. I haven't yet checked square waves on the system or taken a frequency response measurement with real headphones connected.
The heatsink temperature is 57 deg C and ambient is 27 deg C with a quiescent current of 135 mA per channel. So at this current one will need a larger heatsink. In an enclosed box the temperature would be higher I guess. Might be a good idea to clamp the transistors to the casing.
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Last edited by ashok; 9th February 2010 at 11:16 AM.
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Old 9th February 2010, 12:01 PM   #22
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"Hmm.... Why was I so careless !"

It's easily done...
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Old 9th February 2010, 12:19 PM   #23
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Yup. That shared section made all the difference. It's dropped to about -76dB wrt to the output at 1 volt. That's close to a 20dB drop !
It's close to the 2nd harmonic level. I must try to get it down a bit more.

After that a few more tests before I get to listen to it again and see if I can notice the improvements. Of course the hum might not be audible now.

At 135mA per channel it isn't exactly battery operable. But at 35mA per channel and high impedance phones it could be used with batteries. Might chew up ordinary cells easily!

After chopping up the traces on this board and setting everything right I might make a couple of more fresh boards.I want to try out the MJE340 as they have much higher (measured) gain than the BD139 I have.

Cheers.
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Old 9th February 2010, 12:34 PM   #24
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When you listen to it and are familiar with the sound try cutting the quiescent current down... you might be surprised how low you can go before it's a problem audibly.
Maybe have it switchable for battery operation... low Iq.
I'm looking at battery power... mine will only really be used as a test amp for evaluating stuff rather than long term listening. Some camera battery packs are pretty cheap nowadays, 12 volt 2200mah etc as they are becoming a bit obsolete with new low power solid state camcorders.
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Old 9th February 2010, 05:01 PM   #25
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Yes you are probably right about reducing Iq to an acceptable level audibly.
I spent a lot of time just now trying to put up the performance curves. But for some reason I couldn't transfer it from the other computer ( which doesn't accept USB memory as of now ). I'll do it later.

I used a Sennheiser HD580 , Philips SHP805 and Philips SHP8900 . I think the 8900 sounds the best of them all and a fraction of the price of the Sennheiser.
I see that many guys recommend the new Philips 5400.

No more audible hum but I can sense the headphone is connected when it is plugged in. That must be due to residual hum though I don't hear it well enough to call it hum.

The LM317 has better performance ( -80 dB hum rejection ) compared to the LM7812 ( -60 dB ). So I'll try this later.
Right now it sounds good. There is enouigh bass to over load the 805's and the older 800's. I don't use any eq. It certainly is louder than I think is safe for continuous listening.

I'm certainly going to make more boards. It's a good gift for those who like listening to phones. Now it will be interesting to see what it does when used as a preamp ! Shall do that sometime soon. A smd input section might be a good idea ! No, I better avoid that. Too many things to be built/completed now.
Cheers.
( I'll post plots tomorrow).
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Old 9th February 2010, 06:20 PM   #26
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Default Here are some measurements

Frequency response with 33 ohm load, distortion against output voltage , Ripple after ground modification,Linear distortion FFT and ripple before ground modification.
Attached Images
File Type: bmp 33OHMFRS.BMP (37.6 KB, 372 views)
File Type: bmp DISTOR-3.BMP (37.6 KB, 211 views)
File Type: bmp RIPPLE-7.BMP (37.6 KB, 147 views)
File Type: bmp 33OHM-LN.BMP (37.6 KB, 145 views)
File Type: bmp 33-OHM-1V.BMP (37.6 KB, 150 views)
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Last edited by ashok; 9th February 2010 at 06:28 PM.
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Old 9th February 2010, 06:49 PM   #27
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It's looking good... it's how it sounds though that matters
Pleased you sorted the grounds out.
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Old 9th February 2010, 07:59 PM   #28
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Hello AM,
it's very interesting, do you have also a pcb layout or a pcb in advance?
I'd like to try this amp.
Thanks
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Guglielmo
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Old 10th February 2010, 03:21 AM   #29
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My current layout is not good enough. I have had to cut traces and add external wires to sort out the earth. Let me finish the modified layout. I'm trying to keep the pcb size down which is already about 80 x 100 mm .
The additional earthing traces require space or I will have to move the parts around and see if that can reduce the tracks. It's and interesting problem but will take some time.

I think it sounds very good. Certainly worth the time having built it. I think it sounds cleaner than the headphone amp I mentioned above. Bass is very nice on the 8900 phones. The other Philips phones overload on the bass notes ! Only the Sennheiser and the Philips 8900 handle the low end without any stress. You could always roll off the bass early by using a smaller input cap for phones that have low end limitations.Tremendous transients and good mids. Maybe I should say 'transparent sound' ? All this with that cheap yellow box 1uF input cap and Samwha electrolytics ! I'm surprised. I will try some Solen's at the input and see what happens.

I'll put up my current board design. Can't say how long it will take to get the modified one ready. I've a lot of other work to get done in the next few weeks.
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Old 10th February 2010, 04:06 AM   #30
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I managed to make a Jpeg file of the board layout. It went through several conversions and the end result looks bad. However it isn't the final one and is just a record of what was done right now.
Cheers.
Attached Images
File Type: jpg JLH-pcb.jpg (52.3 KB, 1784 views)
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