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Old 19th March 2008, 11:30 AM   #1
Dxvideo is offline Dxvideo  Turkey
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Default Complete pre/power amp project.

Hi all,

I am working on a digital preamp+power amp project. Everything is nearly finished. However I'd like to have your opinions about the project;
Click the image to open in full size.
The psu will be;
Seperated analog (+/-15v with 7815/7915 and +/-5v with 7805/7905) and digital power supplies for preamp and unregulated +/- 30v psu for LM3875.
Preamp and power amp will be seperated enclosures.
Power amp will be in dual mono design (with dual transformers)

Anyway,
Do you think will LM3875 still need a buffer?
Or do you have any advices?

Thanks in advance..

PS: R5 would be 20K... Sorry for mistake..
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Old 19th March 2008, 01:52 PM   #2
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Default op-amps in unbalanced audio

Hello! I'm afraid I'm not at the level where I can comment on your circuit as I'm in teh middle of designing my first bass pream & D.I atm, but I'm having grave difficulty simulating it in multisim so I was actually wondering if you would be so kind as to offer me some advice?? ..on which op-amps to use in unbalanced audio and how to power them using a single 9V battery??

cheers!

Jenks
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Old 19th March 2008, 02:03 PM   #3
Dxvideo is offline Dxvideo  Turkey
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Hi Jenks,

You may want to use a hi quality opamp like me. Generally all opamps can work with dual 5v rails. And you can use TLE2426 Rail Splitter IC to have a dual 4,5v from one 9v battery..
The opamp advice may be;
- LM4562 (dual)
- OPA2134 (dual)
- OPA627 (single)
- LT1115 (single)
All these opamps can work with +/-4,5v easly..

I hope these will help you..

And what about my question? (to the others)
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Old 19th March 2008, 02:12 PM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
R1 should be before C1. Otherwise you are relying on the source impedance for your RF filter.
The Unity Gain feedback link is shown shorted. Check the datasheet for recommendation on whether a slew rate limiting resistor should be added to the link.
The RF filter for the power amp is omitted, use small C parallel to R4
C3 is too small. It almost coincides with the high pass filter at the input to the power amp. Increase it at least 40%.
I'd be wary of C4, particularly this very high 47pF value.
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Old 19th March 2008, 02:42 PM   #5
Dxvideo is offline Dxvideo  Turkey
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Dear Andrew,
R1 will be putted before C1,
But I couldnt understand "slew rate limiting resistor". An unity gain stage needs a NFB resistor or what you mean?
Concerning C3: if I remove it from NFB line an put it to parallel of R4 then its ok for the frequency compensation?
I will put a 100u instead of 47u for C3, I will use a standard nonpolar electrolytics for that position, do you advice a black gate for this. Does it make difference?

Thanks a lot..
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Old 19th March 2008, 03:04 PM   #6
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by Dxvideo
But I couldnt understand "slew rate limiting resistor". An unity gain stage needs a NFB resistor or what you mean?
some opamps require a resistor in the feedback link when used at unity gain. Check your datasheet.
Quote:
Originally posted by Dxvideo

Concerning C3: if I remove it from NFB line an put it to parallel of R4 then its ok for the frequency compensation?
no, add C5 // R4 to create an RF filter at the input of the power. The chipamps are usually internally compensated for adequate stability if the gain is greater than a value quoted in the chipamp datasheet.
Quote:
Originally posted by Dxvideo

I will put a 100u instead of 47u for C3, I will use a standard nonpolar electrolytics for that position, do you advice a black gate for this. Does it make difference?
I have never spent the cost on expensive Black Gates. I use a 1uF PP //cheap electrolytic
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Old 19th March 2008, 03:09 PM   #7
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Ok thanks.
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Old 19th March 2008, 04:09 PM   #8
Dxvideo is offline Dxvideo  Turkey
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But...
Even after that modifications, will LM3875 need a buffer?
I asked that because in the past I have some detail lacking problems with unbuffered gainclones..
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Old 20th March 2008, 08:29 AM   #9
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Connecting AGND and DGND directly might produce some undesired sounds, better option would be connect both grounds with a 100R resistor or a 10mh coil.
LM3886/3875 doesnt need a 47p cap on negative feeback loop, i d prefer removing and replacing 1k negative feedback resistor with a 20K, also zobel network needs to be recalculated.
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Old 20th March 2008, 01:19 PM   #10
AndrewT is offline AndrewT  Scotland
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Hi Dx,
no one seems to want to answer your buffer question.
I suspect that is due to not understanding what the digital circuit is presenting at it's output.
The need for a buffer will depend totally on the source impedance over the frequency range.
Try asking in the digital section what the output impedance of the digital circuit is and what load it can drive.
With this information we analogue folk can then give you advice.
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