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Old 27th June 2006, 12:34 PM   #1
ssmith is offline ssmith  France
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Default help finishing DDDac-Charlize project

Hi everyone,
Have just ordered a DDDac, and wanted to couple it with a pot and a Charlize to make an 'integrated usb-dac-amp'.... but have questions (this will be my first real electronics project!)....

1. The pot:
The DDDAC has an output of 1.6vrms and output impedence of 90 ohm. The Charlize has an input impedence of 20kohm. Not sure what the sensitivity/gain is.
I want to order an Alps pot (log and not linear, I believe is best?) -- to put in between the DAC and charlize (no buffer, active pre) but cannot seem to work out which value -- 10k or 50k.

Yeo says "charlize's input impedance is 20kohm. use 50 or 100kohm pot."

But the Goldpoint site says "the attenuator value is chosen to match the input impedance of whatever it will be controlling. (...) you can usually use a level control value which is LESS than the rated input impedance of the gear it will be controlling. So, if your amplifier has a rated input impedance of 100K, instead of using a 100K volume control, you could use a 50K, 25K, or even 10K level control. (Avoid using a level control value which is HIGHER then the rated input impedance)."



2. PSU: am going to try a 12v linear regulated supply (this one) -- as this could run both the DAC and charlize. 5.1A should be plenty. Any thoughts?

thanks in advance!!!

(Have I searched? 1000's of posts and counting....)
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Old 28th June 2006, 04:49 AM   #2
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Default general questions

I thought a few words about volume controls and power supplies would be in order;

The volume control looks like a load for the devices providing the input signal. If these are all solid state low impedance units you would be best served by a 10k control. However if you expect to use any high output impedance tube gear a 100k unit would be a better choice. Use the lowest value you can get away with for lower noise. The input impedance is almost never an issue in this selection as it usually is set quite a bit higher than any control you might use. There will be a resistor to ground following the control and this value can be adjusted. Once again the lowest value you can get away with is best. I wouldn’t recommend much lower than the control value as the input impedance will change too much as you adjust to higher volumes. To get an idea of how much an effect this input impedance can have on noise turn the volume control all the way down and listen to the residual noise then turn it up and listen. Do this with an open input selected. A 10k control will be nearly as silent all the way up as with the control down. If you have a noticeable level of noise with the control down you might have other issues to be addressed. These could range from too much system gain to bad grounding and on to poor or over stressed parts. Any noise at all is too much if you can hear it. Believe me reducing noise will help!

I personally do not like the power supply you have chosen. These type supplies have very poor quality transformers with a lot of line leakage into the output. I think you will find even a very cheap SMPS will be quieter in the actual application. Just keep it physically away from the high gain circuits and use all twisted or twisted and shielded wiring. Also use 2 smaller power supplies to break the ground cross connection and reduce noise.
Roger
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Old 28th June 2006, 09:47 AM   #3
ssmith is offline ssmith  France
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Roger,
Thanks very much for your reply. I'm a total newbie on electronics, and worry about getting grilled on this forum.... although I have learned alot.

On the power supply, I had kind of thought that a linear model would give 'cleaner' power than an SMPS. I had been looking at the 'ripple' specs in the datasheets, and the linear ones are invariably lower ripple. But I'll look at it again....

On the pot, I confess, I don't really undestand 100 percent. You say "Once again the lowest value you can get away with is best" - but I'm really not sure what this value is. The DDDac is fine into a 10k pot, so should I assume this is best?

regards, ssmith
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Old 28th June 2006, 10:09 AM   #4
JohanH is offline JohanH  Netherlands
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Correct me if I´m wrong,

but the dddac has a passive I/V so wouldn´t it be happier with a buffer between its output and the volume pot?

Cheers,
Johan
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Old 28th June 2006, 06:59 PM   #5
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Default Some more answers

The minimum value for a volume control is set by the units that are connected to it supplying the signals. It is the old weakest link in the chain thing where you use only as low of value as the weakest unit can handle without increasing distortion. With most all modern equipment 10k is fine.
I am a little concerned about the passive output on the dddac. Load on this will effect it’s distortion and filtering characteristics. Cable capacitance will also play a negative role. A good active buffer would be best immediately after the dddac output filter. I would highly recommend a unity gain buffer from National, Burr Brown etc. Yes, you could use a tube stage but you would end up with output impedance almost as high as without it but with all the tube colorations and reliability problems to contend with. There is also the issue of very high turn/turn off transients to deal with too. Not a good idea unless you just have to have that “tube sound”.

Yes, on paper the linear supply will appear as quieter but this depends on how it is tested. If your test setup includes your ears it is a different story. The linear supply noise all is within the audible range while the SMPS’s is most all outside of it. The SMPS has far less line frequency noise problems. This line frequency noise even in very small amounts will mask low level information. The perceived result using SMPS is that instruments and voices within a sound stage are more defined and in focus. Also the back half of the sound stage is now there! This can be quite startling the first time you hear the actual acoustic space the performers are in. Adds a little of the “you are there” to it.
Roger
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Old 30th June 2006, 01:29 PM   #6
ssmith is offline ssmith  France
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Roger,
Thx very much -
I've decided to go for a buffer stage -- will try a Tubalizer tube stage to see how that works out (probably not ideal but its not too expensive and ready-made -- so not too much hassle), but when I get a bit of experience I'll look into the other options you mention.

Regarding the PSU - your advice is very interesting. Linear supplies do have FAR lower ripple (on paper), but I honesty never realised that there was line noise in and out of an audible range.... so I'll look for an SMPS and give that a try.

My aim is basically to make an integrated system where the modules run on 12vdc, the idea being that I can run it off the mains and switch to battery too. Looking forward to experimenting...
Thanks enormously,
stefan
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Old 30th June 2006, 08:34 PM   #7
Pano is online now Pano  United States
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I'll second everything sx881663 says. All good advice.

Just thought I'd add that a 50K pot should work just fine for you. They are often easier to find in a high quality stereo log pot.
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