MiniDSP -> Passive preamp/potentiometer. Impedance matching? - diyAudio
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Old 23rd August 2011, 11:43 PM   #1
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Default MiniDSP -> Passive preamp/potentiometer. Impedance matching?

Hi!

I'm currently planning my first real DIY audio project.
The first stage to building this is to design a MiniDSP based "speaker processor" wich will basically play the role as DAC with s/pdif input, active crossover and preamplifier with analog passive attenuation between the MiniDSP analogue outputs and the 4 channels of amplification for the pair of 2-way speakers.

Like such, from start to finish in the signal chain:

1. Digital signal (s/pdif)
2. MiniDIGI
3. MiniDSP 2x4 (4 analogue outputs)
4. Vishay P11S logarithmic potentiometers (passive preamp) for 4 channel attenuation (i.e., a total of four stacked P11S on a single shaft connected to a single volume knob)
5. 4 poweramps for 4 channels of amplification for a pair of 2-way speakers

Now, my concerns are with impedance matching, as I want each dedicated amplifier to have full control over the driver it's connected to, in terms of impedance and damping factor.

What do you think would be the safest impedance on the actual potentiometer? I believe the output impedance of the MiniDSP is 0.5k Ohms (560 Ohms to be specific).

Perhaps a 5k Ohms? Are more precisely, a total of four in a stacked configuration (the Vishay P11 series can be easily modified to be stacked), like this one (in english): https://www.elfaelektronikk.no/elfa3...9-85&toc=20164

Open the PDF datasheet to see the stacked configuration

Thanks!
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Old 23rd August 2011, 11:56 PM   #2
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You can attach a pot meter to the MiniDSP motherboard, which allows you to set volume. 10K lin if I remember correctly. Very easy. You can then connect MiniDSP directly to poweramp.

Vac
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Old 24th August 2011, 03:46 AM   #3
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I am new to the MiniDSP and am thinking of doing something similar....my understanding of this...and probably something you may benefit from,,,is the use of the PEQ active XO plugin will handle all of the XO operations including output to each driver. If you use the PEQ plugin I don't think you would need an analogue control to each driver....just the one main volume control as mentions in the last post. Why use an analogue "pot" in this signal path when the PEQ will do this digitally? Or am I incorrect in my understanding of the way the system works. The PEQ handles the XO function as well as the attenuation of the signal to each driver in the 2way arrangement.....or did I misunderstand the way it works?

My idea was to use my own DIY DAC End 2 and my Parasound Jfet preamp driving into the MiniDSP/PEQ......output to each of the four drivers going into a Parasound HCA800II for the tweeters and a Parasound HCA1000 for the mid woofers. Of course the Parasound amps have a nice arrangement of pots, one for each channel, that allows you to set the volume output of each channel, or they can be easily bypassed which is what my intentions were.

Please let me know if my plans are flawed....

Jeff
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Old 24th August 2011, 08:24 AM   #4
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Thanks for your replies
The reason I want to avoid volume control in the digital domain is because it's a lossy process, how overrated the problem may be, I want to maintain the dynamic range from the DAC of the MiniDSP. And by feeding an un-attenuated, loud signal to the analogue volume control, maximum SNR is maintained. Even if it has "more headroom" by doing this in 24 bits, the SNR ratio still narrows when going down.

This might become noticable if I decide to go the high-efficiency pro-driver route, not sure.
Granted I'll probably start with digital volume and then "upgrade"
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Old 24th August 2011, 09:08 AM   #5
tnargs is offline tnargs  Australia
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Quote:
Originally Posted by lofgrenmeister View Post
The reason I want to avoid volume control in the digital domain is because it's a lossy process, ... I want to maintain the dynamic range from the DAC of the MiniDSP. And by feeding an un-attenuated, loud signal to the analogue volume control, maximum SNR is maintained. Even if it has "more headroom" by doing this in 24 bits, the SNR ratio still narrows when going down.
No, you lose headroom just as surely with an analog volume control.
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Old 24th August 2011, 09:46 AM   #6
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Yes but atleast the digital SNR is maintained, there'll obviously be some analogue degradation of the signal but atleast I'm not "killing bits" :-) But if this is at all noticable under normal conditions I don't know, perhaps i'm just paranoid
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Old 24th August 2011, 05:30 PM   #7
qusp is offline qusp  Australia
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no..the SNR is NOT maintained, you are already acting in the digital domain to attenuate signals for your XO, you are not even adding a further process, doing it in the digital domain with the one pot is a far better solution and cheaper too, with perfect channel matching. by adding noise (and you will surely be adding more noise with your 4 pots), you ARE losing bits and unnecessarily adding a FURTHER lossy process. this feeling and folklore that seems to ignore analogue attenuation as a source of information loss has got to wake up sometime, its by far the lossier of the 2 given enough bit depth in the digital process and certainly less convenient. no need for worry about impedance matching that way, the mini DSP will surely drive your power amps inputs.

Last edited by qusp; 24th August 2011 at 05:52 PM.
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Old 24th August 2011, 05:50 PM   #8
qusp is offline qusp  Australia
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sorry i just reread my post, the caps are meant for emphasis, not yelling
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Old 25th August 2011, 02:12 PM   #9
tnargs is offline tnargs  Australia
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Thanks qusp, you added the detail I was too lazy to explain

So much anti-digital religion in the audio world, it has become self perpetuating.

It is a big loss for audiophiles to buy into the pro-analog attitudes.
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Old 25th August 2011, 03:37 PM   #10
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Quote:
Originally Posted by lofgrenmeister View Post
Thanks for your replies
The reason I want to avoid volume control in the digital domain is because it's a lossy process, how overrated the problem may be, I want to maintain the dynamic range from the DAC of the MiniDSP. And by feeding an un-attenuated, loud signal to the analogue volume control, maximum SNR is maintained. Even if it has "more headroom" by doing this in 24 bits, the SNR ratio still narrows when going down.

This might become noticable if I decide to go the high-efficiency pro-driver route, not sure.
Granted I'll probably start with digital volume and then "upgrade"
You don't loose bits by using the volume control built into the MiniDSP AFAIK. I have used it and would not take other approach.

vac.
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