Salas hotrodded blue DCB1 build - Page 84 - diyAudio
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Old 18th March 2011, 02:43 PM   #831
Salas is online now Salas  Greece
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Solve the unknown breadboarding together then because I must be reading other threads than those with the long list of output jamming DCB1s all this time and I can't be of help.
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Old 18th March 2011, 02:44 PM   #832
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Thanks Salas, my problem is not high priority

I'll try to post a pic later.

I've successfully used a similar delay circuit using a 2n7000 for soft start so I am suspecting the 517 as being deficient. The time delay should be about half of 4.7 seconds with the 100uF 47K R

Last edited by necplusultra; 18th March 2011 at 02:46 PM.
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Old 19th March 2011, 02:13 AM   #833
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Quote:
Originally Posted by johnwmclean View Post
Hello Salas, Iím trying to find out as much as I can regarding balancing a blue DCB1 build (using 2 boards). I have not come across any completed builds that have are balanced as yet, information is sparse, I have spend considerable time searching in vain. Iíve posted this layout in the Mesmerize build thread, AndrewT responded kindly regarding the importance of matched impedances for the input and output, thanks Andrew.
Click the image to open in full size.
Is there anything else that would be of importance with such an undertaking would any component changes benefit from balanced operation? Is this layout workable?
Thanks for a wonderful project.
John
In the attached schematic, it show's 3 pin input and output connectors. If those connectors are XLR connectors then they are mis-wired. The XLR connector pin #1 is only connected to the chassis, it is not part of the audio circuit and should not be connected to audio ground or circuit ground.

Do a search on the "pin 1 problem".
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Old 19th March 2011, 03:39 AM   #834
westend is offline westend  United States
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Originally Posted by Salas View Post
Nice construction. Check also the voltages across the fat resistors (10 Ohm?) next to the blue filter caps to be something around 1.5-2.5V, and the DC offset on audio outputs to be in the very low mV. Then you can safely hook it up and let us know how you liked it. After 48 hours play it will mellow out a bit also.
Thank you. Voltages across the resisitors are 1.57 v. and 1.81v.. DC at the outputs is -1 mvdc and 6mvdc, totally acceptable, IMO. I have it hooked to my F5 and signal coming fom a DVD player (don't have a signal generator) and the sound is very good. It makes my old Infinity shop speakers really sing. I just had a "goose bump" moment on the last track .

It is still a surprise to see the Fairchild MOSFET's running at such a low temp, just inches away are another two pairs of Fairchilds on the F5, they are a bit warmer.
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Old 19th March 2011, 09:40 AM   #835
Salas is online now Salas  Greece
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6mV is not the norm, usually the builders report less. It shows your other channel with -1mV has even more successful audio jfet pair matching. See how much of a DC offset is created at F5's output eventually and if it is still low enough. As for running current, since you got a sturdy construction with big trafo and copper bars under the Mosfets and is so cool, its easy you parallel another 6.8R on the 1.57V side 10R and a 8.2R on the 1.81V side 10R so you go circa 400mA each side. That will give it a ''bigger'' sound. The Fairchild Mosfets are still IRFP9240/240 types?
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Old 19th March 2011, 11:19 AM   #836
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Kevin, thank you for looking over my proposed diagram.

I am no expert but as I understand it Rane noteís are correct if you're dealing with equipment with fully differential (non-ground referenced) topology. However the DCBI are ground-referenced modules, so therefore would need to be connected to XLR pin 1, and since that signal is the shield, then it needs to be connected to the chassis as well.

I may have to use a ground loop breaker though, since this will be a single chassis build.

Do you agree?
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Old 19th March 2011, 11:46 AM   #837
AndrewT is offline AndrewT  Scotland
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The Ground Reference is actually the Signal Return in an unbalanced DCB1.
The Signal Return is common to the two channels of the unbalanced DCB1.

If you connect a +IN signal to one channel and a -IN signal to the other channel then the DCB1 behaves as a balanced buffer. It reads the difference at the two INs and it's output is directly related to the two INs. The commoned Signal Return does not need to be connected to the PIN 1 of the XLR. But this is an electronically balanced circuit. The INs are not "floating" as in a real balanced impedance receiver and so at some point the commoned Signal Return must be referenced to an Audio Ground reference point. Rane and Jensen and Self and Jung all show ways to do this without the PIN1 problem.

BTW,
your volume adjustment pots ruin the whole reason for adopting balanced topology. The pots guarantee that the receive impedances will not match and now balanced DCB1s offer no advantage over unbalanced DCB1 in noise/interference rejection.
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Last edited by AndrewT; 19th March 2011 at 11:49 AM.
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Old 20th March 2011, 12:46 AM   #838
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Thanks Kevin and Andrew,

I shall connect pin 1 only to the chassis, I will also connect the signal return to audio ground via a Rane and Jensen and Self and Jung approved method.

Andrew,
In regards to the volume adjustment pots, you’ve stated in the Mesmerize build thread that if the sum of components in the two balance halves were 0.1% or better matched this could be done.

Other than this time consuming solution is there a better alternative for a balanced DCB1 build? I don’t really know if this is achievable without running into these inherent problems, I keep hitting a brick wall...
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Old 20th March 2011, 06:32 AM   #839
westend is offline westend  United States
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Thanks, Salas. I did see a rise through the miscreant channel at the F5 outputs, perhaps to 12mvdc ( I need to record these numbers, d^%%mit). I will pass along your advice about the resistors to the owner. Yes, the copper heat spreaders and the mass of the case keep everything very cool, I was seeing interior case temps of 84 f. and case temps on the IRFP 9240/240 pairs in the neighborhood of 91-94 f..
Quote:
Originally Posted by Salas View Post
6mV is not the norm, usually the builders report less. It shows your other channel with -1mV has even more successful audio jfet pair matching. See how much of a DC offset is created at F5's output eventually and if it is still low enough. As for running current, since you got a sturdy construction with big trafo and copper bars under the Mosfets and is so cool, its easy you parallel another 6.8R on the 1.57V side 10R and a 8.2R on the 1.81V side 10R so you go circa 400mA each side. That will give it a ''bigger'' sound. The Fairchild Mosfets are still IRFP9240/240 types?
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Old 20th March 2011, 11:09 AM   #840
AndrewT is offline AndrewT  Scotland
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wipers on tracks can never give the matching required for balanced impedance connection.

Switched settings could be used to achieve much better matching, but that would take a lot of accurate trimming to achieve the matching at every step of the volume control.
How well will the switched pot retain that <0.1% matching during the life of the switch?
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