line level attenuator calculation

[Edwardb]

Hi,

I have a minidsp as active crossover with some high sensitive drivers.
The problem I have is that there is a lot of noise from the tweeters and from the mids.
Now I heard that adding some attenuator will solve the noise problem.
based on experience from others seems that I need -6 db from the minidsp to the amp.

my questions are: what kind of attenuator should I use.
I have seen some on the internet, but they are expensive.....
so thinking of building my own.
Now the question is what attenuator design should I use?
next is then what resistors values would then be ok to use?

I have to say that this is not my expertise... but want to learn... 🙂

Thanks in advance for helping me out!

 

[AndrewT]

Are you using balanced impedance links between the units?
Or unbalanced?

 

[Edwardb]

Hi Andrew,

I'm using unbalanced links.

 

[kinku]

I might be looking for something similar . We both started the thread around same time!!
Do you use analog input to Minidsp and Is it an HD version?

 

[audiofan]

Would you draw a block diagram of your set up ..

 

[AndrewT]

Hi Andrew,

I'm using unbalanced links.

That makes attenuation much easier and much cheaper.

Would you draw a block diagram of your set up ..

Yes, we need inforamtion to work with.

If you know input and output impedances, then mark that on your diagrams.
If you know speaker driver sensitivities add that as well.

 

[Edwardb]

So, atttached is a drawing from the current setup.
The drivers are 4ohm drivers.
The Rotel is driving the tweeters and is a RA-01 40w
The TDA7498 is driving the mids.

I don't know the impedance details from the amps.
the details for the minidsp are here: https://www.minidsp.com/images/documents/Product Brief-MiniDSP.pdf

Hope this gives you a better understanding of the setup.

 

[audiofan]

To have 6dB attenuation you need 2 resistors of the same value connected as a voltage divider ... any value will give 6 dB attenuation.

But you must not overload the output of the digital unit ( 560 ohms according to manufacturer ) and have an output impedance lower than 10 % of the input impedance of the 7498 amplifier. ( 48 K ohms ... value I found on internet )

So a pair of 3300 ohms resistor would fit .... giving an output impedance of 1650 ohms to feed the amplifier and an input impedance of 6600 ohms as load on the DSP ..

according to me it is all you need... many other value may be used and will be OK.

 

[Edwardb]

Hi Audiofan, Thanks for you reaction.
I will give it a try with 2x 3300.

I have one question. How did you calculate the resistor values? .. not to overload the minidsp, but have not lower then 10% of input impedance for the amps... That's something I really want to understand.

 

[AndrewT]

What load can the minidisp drive?
Does that include an allowance for capacitances?

 

[Edwardb]

the output impedance of the minidsp is 560ohms

 

[rehepapp]

Look here:
T-Glied und Pi-Glied - Daempfung Pegeldaempfung Spannungsdaempfung Pad Pegel Leistungsanpassung T Pi pad Spannungsanpassung - sengpielaudio Sengpiel Berlin
Daempfungsberechnung T- und Pi-Glied - Pegeldaempfung Spannungsdaempfung Daempfung berechnen Pad Daempfungsglied pi und T und H dB Dezibel - sengpielaudio Sengpiel Berlin
Voltage divider - loaded and open-circuit dB calculator damping volts potentiometer circuit impedance damping pad decibel dB voltage attenuator impedance bridging matching - sengpielaudio Sengpiel Berlin
Voltage divider - unloaded or open-circuit voltage divider dB calculator damping voltage volts potentiometer circuit impedance damping pad decibel dB voltage attenuator impedance bridging matching - sengpielaudio Sengpiel Berlin

 

[AndrewT]

What load can the minidisp drive?
Does that include an allowance for capacitances?

the output impedance of the minidsp is 560ohms

That's output impedance. What load can the minidisp drive. Does the minidisp have sufficient current capability to drive some capacitance in parallel to the rated load?

 

[Edwardb]

Here is the documentation of the minidsp: https://www.minidsp.com/images/documents/Product%20Brief-MiniDSP.pdf

 

[Edwardb]

To have 6dB attenuation you need 2 resistors of the same value connected as a voltage divider ... any value will give 6 dB attenuation.

But you must not overload the output of the digital unit ( 560 ohms according to manufacturer ) and have an output impedance lower than 10 % of the input impedance of the 7498 amplifier. ( 48 K ohms ... value I found on internet )

So a pair of 3300 ohms resistor would fit .... giving an output impedance of 1650 ohms to feed the amplifier and an input impedance of 6600 ohms as load on the DSP ..

according to me it is all you need... many other value may be used and will be OK.

What I do not understand is how you have calculated the output impedance of 1650 ohm.

 

[Davey]

The standard miniDSP unit already has a limited output of 0.9VRMS maximum. If you attenuate that output to reduce noise, you'll also be reducing the available voltage to 0.45VRMS. Will that be adequate to achieve the SPL's you desire from your Rotel amplifiers?

When calculating output resistance of a voltage divider you return the input side of the series resistor to ground and compute the parallel resistance.
It's actually 1779 ohms in this case since the series 3300 ohm resistor also has another 560 ohms added from the miniDSP unit.

Dave.

 

[Edwardb]

that's another good question. How do I know that?
Also I still have the question how you calculate the output impedance.

 

[audiofan]

Calculation for output impedance ..

 

[Edwardb]

Hi audiofan, thanks for the clarification. It is now more clear to me.
What do you think, Would a voltagedrop to 0.45v enough to drive the amp?

 

[turk 182]

why doesn't he adjust the gain in software to get a 6 db reduction it's not like he's running into distortion is he? dropping the gain should lower the noise floor in the mid and tweeter