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Class D Switching Power Amplifiers and Power D/A conversion

UcD400 Q & A
UcD400 Q & A
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Old 26th August 2005, 09:50 PM   #1301
Yves Smolders is offline Yves Smolders  Belgium
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800VA into 200.000uF?

Fan cooled?

You must be planning on bringing down the house with an army of 38cm subs or something
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Old 27th August 2005, 12:35 AM   #1302
mikemon is offline mikemon  Finland
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yeah... I test it on 2 ohm 680W, and there aint heat at all...
link of the measurement again measurement
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Old 28th August 2005, 02:33 AM   #1303
deandob is offline deandob  Australia
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Default Passive attenuators with UCD

Hi,

I'm planning to build a passive shunt attentuator as the input source for the UCD 400 modules, and was wondering if there would be any problems driving the UCD from a source with higher than normal output impedence?

The shunt attenuator will have a 2K series resistance in each balanced input and a shunt resistance between the balanced inputs that will vary between 0 and 8K, so the input impedence seen by the UCD module will vary considerably. I will be bypassing the coupling capacitor of the UCD module (in fact plan to have no caps in the signal chain, but will use output DC protection), so variation of frequency response with different volume levels should not be a problem. However it would be good to know that this scheme will not add any problems or distortions.

Regards,
Dean
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Old 28th August 2005, 04:32 AM   #1304
deandob is offline deandob  Australia
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One more question, what is the input capacitance of the UCD400? Passive attenuators are sensitive to cable and amplifier input capacitance.

Regards,
Dean
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Old 28th August 2005, 07:54 AM   #1305
sx881663 is offline sx881663  United States
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Default A question of resistance

Dean,
Both +/- inputs of the module are 100k to ground with just a couple of picofarads thrown in. They are not connected to anything else just the opamp inputs. Assuming you will retain the 8620 opamp and the existing circuit there will be no problem at all. This also assumes you will have the volume control in the same box as the amp and use twisted pair or twisted shielded pair cable to couple in the signal. With the rather high source impedance and much higher input impedance it would be a very good idea to not only shield this cable but to carefully route it away from any high current stuff. I would recommend only grounding the shield at the amp end.
Nice to get to answer one of yours for a change.
Roger
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Old 28th August 2005, 08:40 AM   #1306
deandob is offline deandob  Australia
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Thanks Roger.

If it is just straight connect to the opamp noninv and inv inputs then the passive attenuator will work well. Your point about the cable length and shielding is relevant, the attenuator which is a bank of relays with shunt resistors can be mounted near the UCD module. However wont there be a problem without a shield that the EMI from the UCD module or other noise will be introduced into the inputs? With proper layout the CMRR of the opamp will reduce this problem.

What about using low capacitance shielded cable?

The setup I'm planning is:
- Lynx 2B soundcard with CS4396 DAC. No modification to this card is needed as it has an excellent implementation
- Balanced out from the soundcard to the amplifier which has a switch relay, series attenuator resistor (2K Dale for low noise) then 8 x shunt resistors relay switched via PIC between the balanced inputs. To convert this scheme to an unbalanced input simply put another relay to short the -ve balanced lead to earth.
- The attenuator will go straight into the UCD, modified to remove the coupling caps and swap selected caps on board to better types, including the output cap to a Auricap if that checks out with Audience. Change the UCD opamp power supply to one with better regulation.

I'm hoping this will be a "can of woop ***" PC based sound system setup (I hope I got my americanism correct!), ie. extreme UCD....

Regards,
Dean
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Old 28th August 2005, 08:47 AM   #1307
ewildgoose is offline ewildgoose  United Kingdom
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I am doing what you describe with no great issues. Cheap 10K pot with 10K shunt resistors inline with each balanced leg.

Do note a long way back earlier in this thread that Bruno cautions that this is not a great design since the common mode rejection will be signficantly reduced unless there is very good matching of components in each channel

It's working acceptably here in my situation though...
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Old 28th August 2005, 09:17 AM   #1308
deandob is offline deandob  Australia
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With 1% hand picked resistors it should be possible to not significantly reduce the CMRR, which is only appropriate to the series resistors, the shunt between the balanced lines should not matter (as it is common). What I am describing is not separate voltage dividers on each line, but a matched series resistor in each line and a shunt resistor between the two balanced lines.

Regards.
Dean
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Old 28th August 2005, 11:01 AM   #1309
sx881663 is offline sx881663  United States
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Default Resistors,cables and stuff

Chris,
1) Try a higher capacitance cable for the input. This automatically means lower inductance which does have sonic benefits. The higher capacitance will make it harder for the EMI to get in as it sees lower impedance. This also forms a natural R/C filter for RF. The cable will be so short this is not a problem even if the cable capacitance was very high.
2) The best common mode rejection requires absolutely balanced +/- input impedances from DC to several MHz. Balancing the resistors are only part of the problem. You also need to balance all reactive components as well and these will be mostly stray capacitances and inductances so keep leads tightly twisted and the same length.
3) You had asked about the resistors before and I overlooked that there is some benefit to be gained in using .1% parts in the attenuator as well as around the opamp. If this as a lot of expense and work I would say it probably is not worth the effort. The most critical resistors will be the 2k series pass elements and I would recommend the small Caddock MK-132 .1% units available from Michael Percy. (If he only has 1% get 4 and do a best match)
http://www.percyaudio.com
Roger
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Old 28th August 2005, 11:33 AM   #1310
deandob is offline deandob  Australia
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Roger,

I have a lot of shielded cat5 cable left over from wiring my house with audio (balanced line level signal over shielded twisted pair) and a short length of this should be fine for connecting the balanced attenuator to the UCD. The shield should provide enough capacitance and as you say it gets grounded only at the amp. The quality of cat5 wire, accurate twisting which makes it good for ethernet over twisted pair also makes it good for balanced audio.

Regarding the attenuator for balanced lines, I can't think of a passive scheme that would be better in terms of not upsetting the CMRR than the one I am using (exact matched series resistors with a common shunt between the two balanced lines). If someone has a better scheme please let me know.

Regards,
Dean
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