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Old 29th February 2012, 09:40 AM   #1091
AndrewT is offline AndrewT  Scotland
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You're wrong.
The NFB DC blocking cap should never have any significant voltage across it, when the amp is operating properly.

If there is no voltage there is nothing to rectify.

If the amp develops a fault that creates a voltage across the DC blocking cap then the diode limits the cap voltage to <1Vdc. i.e. the cap cannot be damaged when the amplifier output stage blows up.
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Old 29th February 2012, 10:20 AM   #1092
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How much volt should be on otuput? how to regulate the voltage on output?

Regards
Michael
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Old 29th February 2012, 03:37 PM   #1093
BMW850 is offline BMW850  Netherlands
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Quote:
Originally Posted by AndrewT View Post
changing the lower NFB resistor from 390r to 2k2 changes the gain margin and phase margin of the closed loop amplifier.

The amplifier is bound to perform differently, before an after the feedback change.

BMW,
your test and reported conclusion are completely flawed.
Your description of the modification is correct and confirms you are listening to two different amplifiers that by design must perform differently.
Andrew, please tell me how I can do it otherwise, so I change the input sensitivity ď390R to 2.2KĒ to the desired result with my preamp?
I am open for everything even for other suggestions, but please tell me what to do.

You tell me the test and reported conclusion are completely flawed.
That may be, but this amplifier sounds now to a level where I want to go maybe further, even though it is true no more how this amp is designed.

You says, I listen to two different amplifiers, which I can confirm because it sounds different , but as I said this is the level where I want to go.

I have listened a number of years ago to a UCD400 classd amp with a tube entrance, that sounded a lot better than the default UCD400.
I know this has nothing to do with this amp, but the design was also completely different.

Hopefully I become understood correctly, English is not my mother tongue.

Regards,
Rudy
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Old 29th February 2012, 06:43 PM   #1094
AndrewT is offline AndrewT  Scotland
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take your original amplifier and replace the 390r with a 2k2.

now compare the original to the new.
are they the same or different.
measure the original and the new.
are they the same or different.

Now swap the 1% 600mW 2k2 metal film to your fancy and expensive 2k2
measure the two and compare.
are they the same or different.
Listen to the two versions.
are they the same or different.
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Old 1st March 2012, 02:05 PM   #1095
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This resistor creates an interresting lot of fuss... It has lead me to reconsider the value Carlos was suggesting in his schematics.

The suggested 390r value sets the amp to a gain of 121.5x, or near 42dB. This may be good for amplifying directly from a portable device like iPods, but when using a preamp, well, we need to turn down the input pot quite a lot.

My researches on the actual gain of many consumer power-amps shows that the typical gain for an amp of that power varies from 24x to 28x, which is between 27,6dB and 29dB.

Reducing the gain to a level of 28-29dB will raise considerably the signal-to-noise ratio and reduce the electrical noise pickup, which is all good obviously.

Reducing the gain is made by raising the amount of NFB which seems to work in the correct direction also to significantly lower the THD of the unit, AndrewT, is that correct? I still don't have all the knowledge about phase margin though.

So is it all good to increase the 390r resistor value to between 1.7K (28.6x or 29dB gain) and 2.2K (22.4x or 27dB gain) What are the gotchas???

Thanks,
Martin.
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Old 2nd March 2012, 06:02 AM   #1096
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I am making Speaker protection circuit using DX protection circuit. i want to use 12v relay and my power supply is 12v DC.


Do i have to change some values?

http://www.nabucoeletronica.com.br/d...-schematic.pdf

Thanks
Michael
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Old 4th March 2012, 02:26 PM   #1097
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Default Where's uncle Charlie?

Does anyone know if uncle Destroyer DX is well? If anyone knows please give us an update.

Perhaps I missed a post but I did not see any news about him. His last post was early January.

Hopefully he is fine and healthy.

Francois
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Old 4th March 2012, 03:33 PM   #1098
BMW850 is offline BMW850  Netherlands
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Quote:
Originally Posted by canonnica View Post
This resistor creates an interresting lot of fuss... It has lead me to reconsider the value Carlos was suggesting in his schematics.

The suggested 390r value sets the amp to a gain of 121.5x, or near 42dB. This may be good for amplifying directly from a portable device like iPods, but when using a preamp, well, we need to turn down the input pot quite a lot.

My researches on the actual gain of many consumer power-amps shows that the typical gain for an amp of that power varies from 24x to 28x, which is between 27,6dB and 29dB.

Reducing the gain to a level of 28-29dB will raise considerably the signal-to-noise ratio and reduce the electrical noise pickup, which is all good obviously.

Reducing the gain is made by raising the amount of NFB which seems to work in the correct direction also to significantly lower the THD of the unit, AndrewT, is that correct? I still don't have all the knowledge about phase margin though.

So is it all good to increase the 390r resistor value to between 1.7K (28.6x or 29dB gain) and 2.2K (22.4x or 27dB gain) What are the gotchas???

Thanks,
Martin.
Martin here you have a point, this this is exactly where I had problems with my preamp.
I'm not yet started to test the 390R vs 2.2K resistance, hopefully next week.

Carlos has left this forum

Regards,
Rudy
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Old 4th March 2012, 04:04 PM   #1099
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Carlos is OK and happy
Here is his vlog and he is uploading videos almost every day: http://www.youtube.com/user/destroyersoueu?feature=mhee
Watch this he is telling is OK and something about the forum - http://www.youtube.com/watch?v=zMBux...2&feature=plcp

Regards!

Last edited by astankov; 4th March 2012 at 04:12 PM.
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Old 4th March 2012, 11:37 PM   #1100
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Glad to see that Carlos is fine and enjoying life!

I did some adjustments on my amp.

I wanted to get rid of the high-pitched hiss and the electrical hum when the amp is sitting idle.

So I lowered the gain grom 143x to 32x by replacing the 330 ohms resistor by an 1.5K (I went with a 330r instead of 390r initially ... wrong direction one would say )

I also re-routed the power wiring over the main trannys using some custom brackets. Initially, those wires were running just below the signal inputs. (See the pics below)

So how's it now? Very silent and more satisfying. The high-pitched hiss is completely gone as well as the 60HZ buzz...

The sound has changed a bit too, I thought the amp was emphasing too much the high-mids, at the point of being harsh at higher volumes. This tendency is now gone and the frequency response seems more flat and natural.

Pics showing before and after the wiring rerouting:
Attached Images
File Type: jpg IMG_2434.jpg (535.1 KB, 323 views)
File Type: jpg IMG_9027.jpg (412.2 KB, 302 views)
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