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tassosk		#1 Posted : Thursday, May 21, 2009 9:55:23 AM(UTC)
Rank: Member Groups: Member Joined: 1/12/2009(UTC) Posts: 20 Location: Greece Thanks: 1 times		Hi Russ,Brian, While waiting for the new auction of the Buffalo32, I’ playing around with my old OPUS. When I turn on the OPUS I measure at the output between + and GND a peak of 1.3V that fades gradually to 1mV (it takes about 15 sec). Is this normal? I'm asking because this Voltage seems to magnetize the transformer core that I use for balanced to SE conversion. I’ve tried to use OTTO between the DAC and the transformers, in order to isolate the tranys during these 15 sec but the problem still persists. Any ideas? Best Regards, Tassos
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Brian Donegan		#2 Posted : Thursday, May 21, 2009 10:07:21 AM(UTC)
Rank: Administration Groups: Administration, Customer Joined: 10/24/2006(UTC) Posts: 2,868 Location: Massachusetts, USA Thanks: 2 times Was thanked: 141 time(s) in 134 post(s)		Do you have the output capacitors installed (22uF SilmikII)?
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tassosk		#3 Posted : Thursday, May 21, 2009 10:46:26 AM(UTC)
Rank: Member Groups: Member Joined: 1/12/2009(UTC) Posts: 20 Location: Greece Thanks: 1 times		Yes!
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Russ White		#4 Posted : Thursday, May 21, 2009 11:03:14 AM(UTC)
Rank: Administration Groups: Administration, Customer Joined: 10/24/2006(UTC) Posts: 3,979 Location: Nashville, TN Thanks: 25 times Was thanked: 89 time(s) in 83 post(s)		Two things: First the transformer should probably not be connected from +out to GND, but rather from +out to -out. Second. There should not be any offset at all on the output of the Opus if the caps are in place unless they are damaged or shorted. Cheers! Russ Edited by user Thursday, May 21, 2009 11:04:32 AM(UTC)  | Reason: Not specified
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tassosk		#5 Posted : Thursday, May 21, 2009 11:09:45 AM(UTC)
Rank: Member Groups: Member Joined: 1/12/2009(UTC) Posts: 20 Location: Greece Thanks: 1 times		The connection of the transformers is as Lundhall suggests for the purpose, so I' try to replace the caps. I have 4 spare 47uF 50V SilmicII and 4 non polar 33uF 16V BG. May I try them as a permanent replacement or i have to use 22uF only? Cheers, Tassos
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Brian Donegan		#6 Posted : Thursday, May 21, 2009 11:13:36 AM(UTC)
Rank: Administration Groups: Administration, Customer Joined: 10/24/2006(UTC) Posts: 2,868 Location: Massachusetts, USA Thanks: 2 times Was thanked: 141 time(s) in 134 post(s)		If you are connecting them between +OUT and GND, they are not doing Balanced to SE conversion, as you are tapping an SE signal that way. I missed this in your first post. While there is DC offset between +OUT and GND (that the caps are supposed to be blocking), there should be no DC between +OUT and -OUT (the differential signals). If Lundhall is suggesting you shoudl be using +OUT and GND, they are mistaken.
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Russ White		#7 Posted : Thursday, May 21, 2009 11:13:42 AM(UTC)
Rank: Administration Groups: Administration, Customer Joined: 10/24/2006(UTC) Posts: 3,979 Location: Nashville, TN Thanks: 25 times Was thanked: 89 time(s) in 83 post(s)		The value is not terribly important, the optimal value will depend on the impedance of the next stage. We chose 22uf because it would work for a wide range.
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Russ White		#8 Posted : Thursday, May 21, 2009 11:31:47 AM(UTC)
Rank: Administration Groups: Administration, Customer Joined: 10/24/2006(UTC) Posts: 3,979 Location: Nashville, TN Thanks: 25 times Was thanked: 89 time(s) in 83 post(s)		Brian is spot on. If you only using one end of the Opus output your not doing balanced to SE conversion. If you take both OUT+ and OUT- then you actually don't need any output capacitors, as there will be 0VDC common mode voltage. If your using only one end, then the signal is already single ended and your wasting a lot of the potential of the DAC. Cheers! Russ
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tassosk		#9 Posted : Thursday, May 21, 2009 11:33:32 AM(UTC)
Rank: Member Groups: Member Joined: 1/12/2009(UTC) Posts: 20 Location: Greece Thanks: 1 times		Sorry, I did not describe the connections clearly. Actually I use + and – for each channel in order to perform the BAL to SE conversion. I mentioned the + to GND DC offset but the same applies to – and GND in both channels. I measure without the transformers(1527XL). Although I have this initial DC offset (as I said previously the DC goes from 1.3V to 1mV in about 15secs and then remains stable to 1mV without signal) with no transformers it sounds OK. Best Regards, Tassos
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Russ White		#10 Posted : Thursday, May 21, 2009 11:38:20 AM(UTC)
Rank: Administration Groups: Administration, Customer Joined: 10/24/2006(UTC) Posts: 3,979 Location: Nashville, TN Thanks: 25 times Was thanked: 89 time(s) in 83 post(s)		For BAL/SE purposes you don't need the output caps at all, and DC to GND will be irrelevant, as the transformer primaries should not be connected to GND in any way.
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Brian Donegan		#11 Posted : Thursday, May 21, 2009 11:38:54 AM(UTC)
Rank: Administration Groups: Administration, Customer Joined: 10/24/2006(UTC) Posts: 2,868 Location: Massachusetts, USA Thanks: 2 times Was thanked: 141 time(s) in 134 post(s)		Do you measure any DC between +OUT and -OUT?
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tassosk		#12 Posted : Thursday, May 21, 2009 11:52:02 AM(UTC)
Rank: Member Groups: Member Joined: 1/12/2009(UTC) Posts: 20 Location: Greece Thanks: 1 times		I measure 50mv between + and - that goes to almost zero after some time.
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Russ White		#13 Posted : Thursday, May 21, 2009 11:55:43 AM(UTC)
Rank: Administration Groups: Administration, Customer Joined: 10/24/2006(UTC) Posts: 3,979 Location: Nashville, TN Thanks: 25 times Was thanked: 89 time(s) in 83 post(s)		That sounds like differences between the output caps. Try shorting the output caps(or measuring at the input side of them) and it should start out at 0v between OUT+/- and stay there.
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tassosk		#14 Posted : Thursday, May 21, 2009 12:00:34 PM(UTC)
Rank: Member Groups: Member Joined: 1/12/2009(UTC) Posts: 20 Location: Greece Thanks: 1 times		0 Volts
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Russ White		#15 Posted : Thursday, May 21, 2009 12:02:37 PM(UTC)
Rank: Administration Groups: Administration, Customer Joined: 10/24/2006(UTC) Posts: 3,979 Location: Nashville, TN Thanks: 25 times Was thanked: 89 time(s) in 83 post(s)		There is your answer. :)
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tassosk		#16 Posted : Thursday, May 21, 2009 12:04:09 PM(UTC)
Rank: Member Groups: Member Joined: 1/12/2009(UTC) Posts: 20 Location: Greece Thanks: 1 times		The connection of the transformers is the following: In+ (from DAC) 1 Agnd 2 + 4 In- (from DAC) 3 Out+ 5 Out- 7 Connect 6+8 of this schematic: http://www.lundahl.se/pdfs/datash/1527_7xl.pdf Should i try 2+4 of the primaries floating?
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Russ White		#17 Posted : Thursday, May 21, 2009 12:11:27 PM(UTC)
Rank: Administration Groups: Administration, Customer Joined: 10/24/2006(UTC) Posts: 3,979 Location: Nashville, TN Thanks: 25 times Was thanked: 89 time(s) in 83 post(s)		I would try something like 680R in series with +OUT and -OUT connected to 1 and 3. Then simply connect 2 to 4. Also connect 6 to 8. Connect 7 to GND and 5 is your output. I would Experiment with the series resistors but the value I gave should be safe to try. The DAC has a minimum load impedance. You don't want to short it out. Cheers! Russ
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tassosk		#18 Posted : Thursday, May 21, 2009 12:15:06 PM(UTC)
Rank: Member Groups: Member Joined: 1/12/2009(UTC) Posts: 20 Location: Greece Thanks: 1 times		Without the caps?
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tassosk		#19 Posted : Thursday, May 21, 2009 12:18:40 PM(UTC)
Rank: Member Groups: Member Joined: 1/12/2009(UTC) Posts: 20 Location: Greece Thanks: 1 times		I also have 4K between 5 and 7 as Lundhall suggest. Will this give the input impedance that the DAC needs? The imput impedance of the amplifier is 10K.
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Brian Donegan		#20 Posted : Thursday, May 21, 2009 12:22:42 PM(UTC)
Rank: Administration Groups: Administration, Customer Joined: 10/24/2006(UTC) Posts: 2,868 Location: Massachusetts, USA Thanks: 2 times Was thanked: 141 time(s) in 134 post(s)		tassosk wrote: Without the caps? Yes, short or remove the caps.
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