Compact USB DAC/Integrated LM1875 Amp - Page 2 - diyAudio
Go Back   Home > Forums > Amplifiers > Chip Amps

Chip Amps Amplifiers based on integrated circuits

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 13th March 2008, 03:49 AM   #11
sangram is offline sangram  India
diyAudio Member
 
Join Date: Sep 2002
Location: India
The Alien kits come with an ECS crystal and an associated mylar spacer.

Mouser part no. 520-HCU1200-20DNX
Digikey part no. X172-ND

I don't think these chips are too critical of the type used, but I could be wrong.

Looking forward to a completed project!
  Reply With Quote
Old 13th March 2008, 03:38 PM   #12
dfdye is offline dfdye  United States
diyAudio Member
 
dfdye's Avatar
 
Join Date: Dec 2005
Location: USA
Sangram,

Thanks for forwarding the information along. Coincidentally, I talked to the guy in our department who designs our data acquisition systems, and he essentially recommended the same thing. Always nice to get double verification!

David
  Reply With Quote
Old 15th March 2008, 05:42 AM   #13
diyAudio Member
 
danielwritesbac's Avatar
 
Join Date: Nov 2007
The Allied Electric store brand 36vct (18+18vac) center tap EI core transformer, is very tiny, super strong (rated 1 amper and gladly does 4), and its #227-2060. Its Canadian made, very polite, and its pretty. Of course, that makes sense.
Still, that voltage makes for big heatsinks for LM1875's or about a third larger (whopping big) for an LM1876.

The Parts Express "China Special" transformer has proven itself to be decent at audio amplifiers, its $3 and its a 2 amper 25vct (12.5+12.5vac). This won't play as loud because of the lower voltage, but it will decrease your heatsink size requirements. This low of a voltage may cause unsatisfactory results with LM1876 due to the noises during transients that it can make when undervolted. However, LM1875's will do just fine because LM1875's are "soft clip" amps, unlike the rest of the Overture chips.

These are very small EI core transformers, but "known good" for the application. I believe that the 36vct takes 6800uF per rail while the 25vct can get by on 4400uF per rail--minimums.

For space saving, you can prevent power supply issues without going through all the hoops if you buy 4 (instead of 2) of whatever cap you're using for the NFB protection cap. These are usually things like Mallory SK, Nichicon KZ, Elna Tonerex, etc. . . usually in values around 22uF to 47uF.

Just get an extra pair.
Now, if this additional pair (1 per rail) of smaller "audio type" caps are the first line of defense in your power supply board, right after the diode bridge rectifier, then they will knock off all the racket that big caps can't. After all, even an economical audio style cap, if used for shunt, can wipe out one heck of a lot of racket before it goes anywhere.
Right after those are the larger caps as usual, but now they're running cool because they're not exposed to ripple.

LM1875 and different voltages. . . this is a "biggie" consideration. Well, apparently, it can succeed on many different voltages. Up at the super-high voltages it likes 16 ohm or less load, at 28v it likes big heatsinks, and down at the lower (lowest) voltages, it starts liking 4 ohms and/or really efficient speakers. However, there's a catch, a caveat, a whopper of a problem. The design for low voltage won't produce fidelity at high voltage, and vice versa. So, to avoid a do-over, decide on your voltage versus power versus heatsink size issues first, before tailoring the sound of the amplifier. Well, simply put, decide on the voltage first.

If undervolting (makes amp small because heatsinks are smaller and tank caps are smaller), then consider tailoring the size of the input filter cap to the capability of the speakers. That won't be an electrolytic cap, but Wima are inexpensive and so is most polyester. Some people even prefer MKP. In a situation where you already know what speakers are used with the amp, then just don't have the amp produce bass that couldn't come out of the speakers anyway. That allows a less powerful amp to avoid clipping.

That's all the notes I've got. Have fun!
  Reply With Quote
Old 15th March 2008, 07:47 AM   #14
dfdye is offline dfdye  United States
diyAudio Member
 
dfdye's Avatar
 
Join Date: Dec 2005
Location: USA
Daniel,

Many thanks for the suggestions.

Regarding transformer choices, I'm currently planning on using a PCB mounted, low profile, 24VCT transformer to power the system. As per the schematic, I was planning on using 2X2200uf caps per rail, but after digging through my surplus bin, that will soon change to some esoteric 4700uf axial caps I have (no need to buy new parts, right? ) The rails will be bypassed at the chip as per the schematic (again, allowing for reasonable changes in cap values based on what's in my surplus bin).

The resulting ~+/-18V rails will be more than sufficient for my needs and will, of course, allow use of a modest heat sink.

There have been a few more changes to the overall circuit layout, namely using differential inputs to account for the different USB and amp ground, and changing around components to maximize usage of my "favorite" parts (IE those that I currently own that will work).

I'll post the revised circuit and preliminary board layout when I get it finished (hopefully this weekend).

David

PS Adding the Xformer on board blew the initial size goal, but the new layout seem to be fitting a standard 6" X 6" board nicely.
  Reply With Quote
Old 15th March 2008, 10:20 AM   #15
diyAudio Member
 
danielwritesbac's Avatar
 
Join Date: Nov 2007
Quote:
Originally posted by dfdye
Daniel,

Many thanks for the suggestions.

Regarding transformer choices, I'm currently planning on using a PCB mounted, low profile, 24VCT transformer to power the system. As per the schematic, I was planning on using 2X2200uf caps per rail, but after digging through my surplus bin, that will soon change to some esoteric 4700uf axial caps I have (no need to buy new parts, right? ) The rails will be bypassed at the chip as per the schematic (again, allowing for reasonable changes in cap values based on what's in my surplus bin).

The resulting ~+/-18V rails will be more than sufficient for my needs and will, of course, allow use of a modest heat sink.

There have been a few more changes to the overall circuit layout, namely using differential inputs to account for the different USB and amp ground, and changing around components to maximize usage of my "favorite" parts (IE those that I currently own that will work).

I'll post the revised circuit and preliminary board layout when I get it finished (hopefully this weekend).

David

PS Adding the Xformer on board blew the initial size goal, but the new layout seem to be fitting a standard 6" X 6" board nicely.

Cool! Just remember that, at that voltage, the transients are going to clip sometimes, so don't use LM1876 if a loud clip (spike protection circuit) is a concern. In my opinion, the slight extra room for LM1875 is worth the soft clip feature (absence of the spike circuit).

P.S. You might want to safely drain (put 50 ohm wirewound resistor over it for a few seconds) one of those 4700uF caps and attempt to run a speaker from it as an experiment. Configuration=bass blocker cap, in series with a speaker. Next, add whatever smaller cap (somewhere between 4.7uF and 47uF parallel to the big cap) makes the treble come back to life.
Whatever caps successfully pass an audio signal can be used to shunt. . . without gaps.
This smaller cap goes right on the diodes, and then next comes the big caps, with much less work to do. Well, that's the easy way to get really close, really fast. There's just one problem. Its not necessary to pass a lovely sound during this experiment because that's exactly what will get shunted, so aim for a hard, and very forwards sound. . . . whatever you'd like to shunt (stop). Daniel's bizzare method of power supply cap selection.

EDIT: Most recent method of component selection: Online shopping has become so easy that you can hit the "buy it now" button with a finger, a thumb. . . or a fuzzy white paw. There's eight of those in my house and you wouldn't believe the huge box of random electronic bits that wound up at my house today. The highlight is. . . 57 dual gang stereo potentiometers.
  Reply With Quote
Old 17th March 2008, 07:19 AM   #16
dfdye is offline dfdye  United States
diyAudio Member
 
dfdye's Avatar
 
Join Date: Dec 2005
Location: USA
The PCB rough draft is done, but keep in mind that there probably are a couple of mistakes and a few points of disagreement between the schematic and the PCB. The over-aching goals were to make a one sided PCB to allow easier home prototyping and to use as many parts that I had readily available in my garage. I obviously cheated with the top layer and used that for jumpers (only 5, woohoo!)

The general layout should be pretty self explanatory, and breaks things into three sections: the PSU, UAB/DAC and amplifier. The USB/digital ground is isolated from the central ground via a resistor to keep as much goofiness from the USB out of the amp as possible. The LM1876 is set up with differential inputs to further reject any digital noise. I still don't really like where the speaker outputs are, but I'll worry about that later. I'll probably break up the +/- output blocks to eliminate the long ground run, but I'll see if that matters in the prototype.

Right now, I am ready for bed, so I get to double check the boards later and probably prototype later in the week if I can manage the time. That and a quick order to Digikey and I should be set!

David


PS On a side note, the initial draft used all SMP's, but after realizing that I had most of the "through hole" parts already, I switched out components and the only surface mount parts left are the PCM2707 and the crystal. Using SMP's the layout was substantially more compact, as I am sure you can imagine, and if anyone is interested, I may finish it up, but I figured through hole parts would be more universally used anyway, so I never got around to finishing the SMP version.
Attached Files
File Type: pdf usb amp v0.7.pdf (45.2 KB, 211 views)
  Reply With Quote
Old 17th March 2008, 07:27 AM   #17
dfdye is offline dfdye  United States
diyAudio Member
 
dfdye's Avatar
 
Join Date: Dec 2005
Location: USA
Here is the PCB
Attached Images
File Type: jpg usb amp v0.7.jpg (62.1 KB, 1235 views)
  Reply With Quote
Old 19th March 2008, 10:31 PM   #18
dfdye is offline dfdye  United States
diyAudio Member
 
dfdye's Avatar
 
Join Date: Dec 2005
Location: USA
Several changes before I order boards that I thought I would post.

I'll probably order prototypes tonight or tomorrow, so hopefully this will get fired up before too long.
Attached Images
File Type: jpg v0.9-top.jpg (88.0 KB, 1065 views)
  Reply With Quote
Old 19th March 2008, 10:32 PM   #19
dfdye is offline dfdye  United States
diyAudio Member
 
dfdye's Avatar
 
Join Date: Dec 2005
Location: USA
And the bottom layer. . . .
Attached Images
File Type: jpg v0.9-bottom.jpg (72.8 KB, 1034 views)
  Reply With Quote
Old 22nd March 2008, 04:08 AM   #20
dfdye is offline dfdye  United States
diyAudio Member
 
dfdye's Avatar
 
Join Date: Dec 2005
Location: USA
Not anything too interesting, but I finished off the SMP version of the amp. I think despite the fact that I'll have to buy a few more parts for this board, this is the one I am going with. The grounding is cleaner and the critical elements are further from the transformer with the added "shielding" of the heatsink (not that it will help too much, unless someone has some sinks made of MuMetal )

Also, I added pin headers for USB DAC out and separate inputs for the amp in case things ever need to get switched up somehow. Easier to do now that in the future!

All of the SMP's are 1206, which should be pretty easy to hand assemble. Also, I added in alternate cap pads in case I want to add/change the rail caps. I think that is about it. . .

Anyway, since this seems to have turned into a work-log, enjoy if you are following along, and I'll post more when I do something new!

David
Attached Images
File Type: jpg usb-amp-smp.jpg (82.4 KB, 962 views)
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Integrated LM1875 PCB Nordic Chip Amps 8 8th September 2010 11:18 AM
compact lm1875 amplifier Jamh Chip Amps 9 25th August 2009 12:50 AM
Compact Sub Suggestions lgchristianson Car Audio 4 19th May 2007 11:05 PM
Compact sub DocLorren Subwoofers 11 8th January 2005 12:34 AM
Compact subwoofer Vatos Subwoofers 0 20th March 2003 08:17 AM


New To Site? Need Help?

All times are GMT. The time now is 12:45 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2