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Old 29th January 2011, 12:43 PM   #3611
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I would also try lowering the resistors to 100ohms or less.
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Old 29th January 2011, 12:54 PM   #3612
sendler is offline sendler  United States
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Default Phase

All of your comments about needing filters and having weak bass sound like you are wired out of phase. The board has a confusing layout as the signals at the first coupling caps are -++- with both positive legs for the left and right channels, next to each other in the middle. If it isn't phase then perhaps you have something bypassed that is shorting the 2.5v dc component of the dac chip output to ground through a resistor. Where are you tapping the direct output of the chips?
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Quote:
Originally Posted by sinski View Post
In the last days i compared the gigadac + Jensens JT-11-EMCF with some very good commercial dacs.

There is one thing immediatelly noticeable - weak bass performance and weak dynamics of the gigadac compared to other dacs.

Even gigadac with Jensens compared to gigadac+opamp has a big problem in this area.

I tried different preamps and different dacs and i think that it is a general problem.

Could it be, that a dac chip through transformers has just not enough "power" to drive preamps and that is the reason for the bad bass performance ?

any chance to make it better ? i already changed the Rs on primaries for 180 ohm, it is better than 500, but the bass is just very week
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Last edited by sendler; 29th January 2011 at 12:59 PM.
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Old 29th January 2011, 03:49 PM   #3613
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i am taking the outputs directly from a dac pcb, like on this picture:

Click the image to open in full size.

PIN 19 - Output Channel B-
PIN 20 - Output Channel B+

PIN 23 - Output Channel A+
PIN 24 - Output Channel A-

Because of the 50Hz hum i connected the minus of the rca output on the case to the minus on pcb. RCA outputs are isolated from the case.
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Old 29th January 2011, 05:02 PM   #3614
sendler is offline sendler  United States
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Default dc

Quote:
Originally Posted by sinski View Post
i am taking the outputs directly from a dac pcb, like on this picture:
Is the dc between each pin and ground the same?
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Old 29th January 2011, 05:27 PM   #3615
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You have AC grounded your output signal. Your hum is coming from incorrect orientation of the trafos. Mave them as far away from the power trafo as possible and reorient them. Sometimes just a few degrees eliminates any hum.
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Old 29th January 2011, 07:17 PM   #3616
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The output transformers are nearly 40cm from the power transformer and preamp is also 40cm apart, there is no other gear close to the dac.

Without earthing there is a strong 50Hz hum.

This is my test setup:

Click the image to open in full size.

Click the image to open in full size.

This an example how Neko Audio is placing the same transfomers (i bough mine from Neko Audio on ebay):

Click the image to open in full size.

So, i assume, that my transformer are placed correctly.

Maybe the earthing is the problem, but without connecting the minus from rca output to the minus on the pcb (blue and red cable on the right, where the usb power cap was) there was a very strong 50hz hum. This hum is not present in my system, when i was connecting other dacs, SquezeboxClasic and Touch or CD-players. My system is ABSOLUTELY quiet. Very, very little noise, no hum

This is the application manual from Jensen, brown on input is +, and red is -, on the output orange is + and yellow is - , i hope i am right.

http://www.jensen-transformers.com/datashts/11emcf.pdf

This is application example how to connect a cirrus dac to output transformer:

http://www.jensen-transformers.com/as/as093.pdf

It is a different transformer, but i assume that output and input filter values are correct for my transformer.

I am a beginner, i have very little idea about electronics, more experience than knowledge, but i have already build few much more complex things which worked perfect.

This bass thing is really strange, i don't know where the bug is.

Maybe somebody can help
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Old 29th January 2011, 07:52 PM   #3617
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You don't see any windings grounded on their schematic, do you. You are shunting the output signal to ground. The output is plus and minus, not plus and ground. The shell of the RCA is referenced to ground at your preamp, it should not be grounded to the dac circuit.

Please remove those grounds and turn your output trafos in a different direction, or possibly turn your power trafo a different direction.
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Old 29th January 2011, 08:56 PM   #3618
sendler is offline sendler  United States
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Default ground

Quote:
Originally Posted by sinski View Post
Because of the 50Hz hum i connected the minus of the rca output on the case to the minus on pcb. RCA outputs are isolated from the case.
Tying one leg of the output connector to the gound of the board is not the problem although you can try moving them to the pin #21 at the chip and reinstall the missing cap. I was also unable to run my transformers with the ground floating due to hum and had to do the same thing. This is the normal way to convert balanced to single ended. Sometimes you can get away with a floating ground and sometimes you cannot. It looks like your wiring is correct through the transformers. Are you sure the big cap you removed is just for the usb? You can try removing the filter and listen without it to find out if maybe it is wired wrong and causing the problem. Measure the dc voltage between each of the dac outputs and ground. They should all be about the same around 2.5v. Any differences can indicate a short or a bad chip.
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Old 29th January 2011, 09:29 PM   #3619
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Yep, the cap Sinski removed is definitely for the USB 5V supply
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Old 29th January 2011, 11:21 PM   #3620
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OK, first i found the source of the hum.

Usually the system need one connection to 230V ground. I checked the system - my sb touch ps do not have it, dac, preamp and poweramp also not. 230V ground was always connected to the case.

So i connected the 230V ground with the pcb ground of the dac (- pin of the usb cap) and the hum dissappeared, than i removed the connection between the minus of the rca output and pcb ground, it was no more necessary.

Than i measured frequency response 20Hz-20Khz -10dB and 0dB, it is a perfect stright line. I was thinking that the problem is solved, but than i did again the connection between rca minus and pcb ground and measured the frequency response - it was absolutely identical.

I am thinking that my problem with bass and dynamic is a "dynamical" problem. When a longer bass line is appearing, than the gigaworks dac is loosing the breath, small dac chip has not enough power to keep it playing dynamic music. A dac where opamps ps is stabilzed can deliver the necessary current in a changing dynamic music. Hmm, we will see, i lowered the primary resistors to 180 from 230 and the dac became louder.

Is there any way to measure the behaviour of the dac under dynamic load ?

Last edited by sinski; 29th January 2011 at 11:26 PM.
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