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RC filter component selection
RC filter component selection
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Old 27th October 2017, 02:05 AM   #11
PRR is offline PRR  United States
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Quote:
Originally Posted by robinlawrie View Post
....i cannot find the input impedance listed anywhere
And the schematic is a mess.

It is actually VERY well defined, once you sort it out. There's two inputs, for plain unbalanced or for balanced, both on RCA jacks. Each one is 9.09K.

Yes, 4.7K series resistor will be a 3.6dB loss, very significant.

The other end is: what can your preamp/DAC drive? Some strain on anything under a few K, others will drive 600 Ohms or even 100 Ohms. If you can drive say 470 Ohms, then loss is a tolerable 1dB.
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Old 27th October 2017, 10:02 AM   #12
robinlawrie is offline robinlawrie  United Kingdom
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ok so im fighting fires on multiple fronts here!

i started a project which is balooning uncontrollably beyond my skillset, which extends to reading schematics, soldering and general design/construction.

i had assumed that buying pre-made amp boards and all other major components, it would be fairly plug-and-play, but im rapidly being proved wrong.

ive purchased a vast quantity of hardware now, so there is no backing out!


to extend the remit of this thread, seek help, and stop hijacking the 3255 evm discout thread, i shall explain the whole issue here and hope somebody can help overall!


so, source is a minidsp 2x4 HD, which apparently outputs 0.9v rms at clipping.

amp is the 3255 evm, which apparently has a 9.09k input impedance, and needs a 2v p2p input.

i have two of them.

the amps are set up in 2.1 mode, with (on each side) one channel of the minidsp going to the sub, and one being split in two, with one half having a lowpass on. these two are sent to seperate channels on amp to drive 1.5 way fullrange speakers. lowpass is to ensure treble is from one driver only.

so four channels from minidisp, and six channels of amplification in total.

clear as mud?


so.. multiple issues:

i need to adjust the gain on the amp so it will run well with the lower voltage provided by the minidsp. my bugging on the evm discount thread has narrowed it down to adjusting the four 10k resistors (Rg) on the board. these are microscopic surface mount jobbies.. yay!

however it has been suggested this will have knock-on effects i dont really understand. (rfi rectification)

now the lowpass filters will attenuate the signal further, meaning gain needs increasing even further. trouble is, increasing gain changes input impedance of amp, meaning lowpass filter needs tweaking ( if i understand correctly)



so im basically a bit lost. !
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Old 27th October 2017, 05:39 PM   #13
rayma is offline rayma  United States
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Quote:
Originally Posted by robinlawrie View Post
source is a minidsp 2x4 HD, which apparently outputs 0.9v rms at clipping.
amp is the 3255 evm, which apparently has a 9.09k input impedance, and needs a 2v rms input.
To prevent gain loss, you can use an op amp buffer after the RC filter,
or else an active low pass filter. Both can have the input impedance of your choice.
http://www.electronics-tutorials.ws/filter/fil20.gif
http://www.electronics-tutorials.ws/filter/fil28.gif

If you do need more gain, either of these circuits can provide as much as you want,
up to about six times (+15dB) before the filter amplifier clips from the 0.9Vrms input
(if the filter amplifer uses +15/-15V supplies).

Last edited by rayma; 27th October 2017 at 05:54 PM.
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Old 27th October 2017, 05:47 PM   #14
robinlawrie is offline robinlawrie  United Kingdom
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Sorry i mis-read/wrote. The amp needs 2v rms input.

Would it be preferable to adjust the gain of the op-amps on the amp board rather than add extra amplification beforehand?
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Old 27th October 2017, 05:53 PM   #15
rayma is offline rayma  United States
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Quote:
Originally Posted by robinlawrie View Post
Would it be preferable to adjust the gain of the op-amps on the amp board
rather than add extra amplification beforehand?
It's best not to hack around on the smt board, voiding the warranty, especially if
you have no experience. It may be possible, depending on the details.
Is the complete board schematic available?

Last edited by rayma; 27th October 2017 at 05:55 PM.
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Old 27th October 2017, 06:28 PM   #16
robinlawrie is offline robinlawrie  United Kingdom
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Ahh believe me, the warranty is already very void. Ive modified the boards (removed unnecessary components etc) to fit them in my case. The schematic link is on the first page of this thread.
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Old 27th October 2017, 07:00 PM   #17
robinlawrie is offline robinlawrie  United Kingdom
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And im not toooo worried about replacing smd components, soldering i can do, and my son got a usb microscope for his birthday my issue is choosing component values! Not a clue.
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Old 27th October 2017, 07:04 PM   #18
robinlawrie is offline robinlawrie  United Kingdom
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Quote:
Originally Posted by rayma;5226103..

If you do need more gain, either of these circuits can provide as much as you want,
up to about six times (+15dB) before the filter amplifier clips from the 0.9Vrms input
(if the filter amplifer uses +15/-15V supplies).
Ready Available voltages in the chassis will be 3.3v, 12v, 15v and 51v.
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Old 27th October 2017, 08:25 PM   #19
PRR is offline PRR  United States
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Quote:
Originally Posted by robinlawrie View Post
....source is a minidsp 2x4 HD, which apparently outputs 0.9v rms at clipping. .....
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Old 27th October 2017, 09:19 PM   #20
robinlawrie is offline robinlawrie  United Kingdom
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Well that is good news! Really good news. I was advised of the 0.9v thing in another thread, but possibly i forgot to specify the HD version. The old one was 0.9v.

So now i just need to specify an rc filter that works with the minidsp and the amp with the lowest attenuation possible. Hopefully i wont need to go active.
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