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Old 20th June 2009, 09:00 PM   #1
PJPro is offline PJPro  England
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Default First Lm3886

I am currently building my first Chip Amp using the chipamp.com dual mono kit and documenting the build here.

I have just bought a couple of transformers from RS Components. I went for these . Hopefully, they're OK for my needs.

What I would like to do is use a regulated power supply. Can I simply place a separate board between the chipamp.com power supply board
and the amp board? If I can, what would the circuit need to contain?

Many thanks for any advice/recommendations provided.
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Old 20th June 2009, 09:48 PM   #2
PJPro is offline PJPro  England
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Clearly, I understand that I need to provide a regulator chip (LM338 or similar)
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Old 20th June 2009, 10:33 PM   #3
Minion is offline Minion  Canada
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No , not allways , you don"t really need a regulator , that transformer is a 2x25vac which will give you about 2x35vDC which is perfect for 8 ohm speakers .... Regulation would be needed for 4 ohm speakers ....

If you were to use a regulator you could run leads from the PSU board to a regulator board and then to the amp board .... Yer gonna need some heatsinks on those regulators ....

Regulators aren"t really that common in Power amps , the large power transformers usually have adequate regulation for a Poweramp not to sag to much under a load , especially if you are useing a 160Va transformer for each channel as a LM3886 will suck only about 80-100Va ... Pluss with regulation you are causeing extra heat and dropping volts which could be used for more power .... some poeple swear by regulation in power amps but I never noticed any sonic differances between the two ....

Cheers
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Old 21st June 2009, 12:00 PM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
the 25+25Vac transformer is compatible with the National 3886 chips, if you use 8ohm speakers.

For a first amplifier project build a simple transformer, rectifier and smoothing capacitor Power Supply (PSU). Test it with a light bulb tester.
Connect the completed chipamp PCB, test again with the bulb tester.
Check output offset (mVdc) and output mVac with the input RCA shorted.
Connect your source and check the output voltages again.
Measure the output and switch off. Leave to drain the smoothing caps and measure again as you switch on.

Now connect a dummy load. Repeat your testing.

Remove the bulb tester and plug direct to the mains supply.
Check output voltages.
Switch off and finally connect your speakers. Listen.

If all is OK, complete a second channel.
Then decide whether you want to add the complication of a PSU regulator.

Three items to note from the chipamp.com site.
1.) the input has no DC blocking capacitor (=high pass filter). Be very careful to check that your sources cannot send a DC offset to this circuit.
2.) the input has no RF filter (=low pass filter). This is needed to attenuate high frequency non audio interference.
3.) the documentation refers to a snubberised PSU NOT and regulated supply.
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Old 21st June 2009, 12:36 PM   #5
PJPro is offline PJPro  England
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Quote:
some poeple swear by regulation in power amps but I never noticed any sonic differances between the two ....
Well, I've been reading the decimal dungeon pages which definately claim an improvement....especially with the discrete regulator circuit.

The reason I'd like to use a regulator (and I understand the issue with temperature) is because I haven't selected my speakers yet. I went for 25v to cover 8 ohm speakers.

But should I go for, say, 4 or 6 ohm speakers, I'd like to be able to "tune" the power output to match the requirement of the speakers. Moreover, should I change my speakers in the future, I'd rather tweak resistor values than buy new transformers.

Or is there an easier way to achieve this flexibility without unduly impacting sound quality?
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Old 21st June 2009, 03:25 PM   #6
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by PJPro
Or is there an easier way to achieve this flexibility without unduly impacting sound quality?
build discrete.
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Old 21st June 2009, 05:51 PM   #7
PJPro is offline PJPro  England
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Quote:
Originally posted by AndrewT
Hi,
the 25+25Vac transformer is compatible with the National 3886 chips, if you use 8ohm speakers.

For a first amplifier project build a simple transformer, rectifier and smoothing capacitor Power Supply (PSU).
I have built a couple of snubberised PSUs already using the kits from chipamp.com.

Quote:
Originally posted by AndrewT

Test it with a light bulb tester.
Connect the completed chipamp PCB, test again with the bulb tester.
Check output offset (mVdc) and output mVac with the input RCA shorted.
Connect your source and check the output voltages again.
Measure the output and switch off. Leave to drain the smoothing caps and measure again as you switch on.

Now connect a dummy load. Repeat your testing.

Remove the bulb tester and plug direct to the mains supply.
Check output voltages.
Switch off and finally connect your speakers. Listen.

If all is OK, complete a second channel.
OK. Thanks for the advice.

Quote:
Originally posted by AndrewT

Then decide whether you want to add the complication of a PSU regulator.
Please see my other post above. It was blocked awaiting moderation. Essentially, I would like to introduce some flexibility into the amp with regard to speaker selection. I believe that regulation of the power supply was a simple way of achieving this (for 4 & 6 ohm speakers). Is there a better way?

Quote:
Originally posted by AndrewT

Three items to note from the chipamp.com site.
1.) the input has no DC blocking capacitor (=high pass filter). Be very careful to check that your sources cannot send a DC offset to this circuit.
2.) the input has no RF filter (=low pass filter). This is needed to attenuate high frequency non audio interference.
What can I do about 1 and 2? How can I add high and low pass filters into the mix? And why don't these enhancements feature in the design of the chipamp.com board? Is sound quality impacted?

Quote:
Originally posted by AndrewT

3.) the documentation refers to a snubberised PSU NOT and regulated supply.
Yeap, the chipamp.com PSUs are snubberised. I wondered if there was a way to insert an additional board between the chipamp.com PSU PCB and the chipamp.com amp PCB and what this additional board would require.
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Old 22nd June 2009, 08:57 PM   #8
PJPro is offline PJPro  England
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OK. Perhaps I'll post my own idea in the hope that it'll stimulate a bit of debate.

The schematic for the snubberised chipamp.com PSU looks like this.

Click the image to open in full size.

This is the only schematic for the PSU on that site, although it appears to leave out the snubberising caps. When comparing this to a regulated design like the one below (from Decibel Dungeon), the similarities before the regulator are evident.

Click the image to open in full size.

So, am I able to do something like this? Please note, I have left off the snubberising caps.

Click the image to open in full size.

I have simply taken one of the application hints for the LM338 from its datasheet and slapped it onto the end of the snubberised chipamp.com PSU. By tweaking the values of R5, R7 and the Trimpot, one is able to adjust the level of regulation and, therefore, the voltage supplied by the PSU. Incidentally, this schematic looks remarkably like the one described by Tangent for the TREAD.

Tangent TREAD Schematic

So, am I a mile off or does this look feasible? Any comments welcomed.
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Old 23rd June 2009, 07:06 PM   #9
PJPro is offline PJPro  England
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OK. I can't be a mile off as I would have expected to get a kicking by now.
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Old 23rd June 2009, 07:29 PM   #10
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I suspect it would work ok. My thoughts are:
a) is the 5A limit of the LM338 enough for the chip and voltage you are using? The LM3886 can peak at a little more than 6A.
b) the snubbering is probably not necessary. Voltage regulators offer some smoothing of their own.
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