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Old 28th August 2011, 03:46 PM   #11
AndrewT is offline AndrewT  Scotland
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I do not recommend any of the more complicated chipamp topologies until after you have learned how to properly build a simple AC coupled chipamp.
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Old 28th August 2011, 04:08 PM   #12
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I took a closer look at your diagram and noticed you show only one set of power supply decoupling caps for both amp circuits. For best performance and reliability, you must decouple each of the chips power pins with its own dedicated set of caps, no sharing between multiple chips, and put the decoupling caps as close as physically possible to each chips' power pins. It's also best to put independent output zobels on each chip output pin, not after the current sharing resistors. In other words, you should build two completly independent amp circuits that are gain matched for minimum output offset voltage, then connected together with current sharing resistors.

Mike
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Old 29th August 2011, 12:18 AM   #13
lnh is offline lnh  United States
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Andrew:
I am not building this amp first. I am starting with the preamp/headphone amp first to actually see what I'm getting into...I am doing no modifications to that one whatsoever, though, so I don't really have any major questions about it. In fact, the direction this first schematic is taking, I may build the original circuit this one is based off of, and simply add on to it.


Michael:
I can't say thank you enough for helping me with this. I am learning a lot so far...I'm also reading constantly, including Bob Cordell's book which I just got.
The direction this seems to be taking seems to be similar to strapping amps as would be done in car audio amplifiers. If you're unfamiliar, basically it is two amps that are hooked together using one of the amp's crossover controls as the master, with one input and one output total. The amps must be capable of being strapped in the first place though.

Updated schematic with new output zobel networks and some added notes.
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Old 9th September 2011, 08:43 PM   #14
lnh is offline lnh  United States
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Well, I've started on the project officially. The PSU I have now will only supply a couple amps with +-12v rails, so I will be replacing it soon, but it will work just fine for the first phase.

I've ordered all the parts to make the headphone/preamp and volume control. Got some plated perfboard, terminals, connectors, fuse holders, etc. The order should be at my house on Mon or Tue hopefully. All that I have left to get is a couple heatsinks and associated hardware and some insulated standoffs for the board. Also going to pick up some dip sockets for the LM and TL chips so I don't have to solder directly on them.

All resistors ordered are Vishay/Dale 1% with the exception of a pair of 4.7ohm ones which were out of stock so I got Panasonic ones. All caps are Nichicon low leakage/esr 10% ones.

Will post more as things move along, and wish my luck in my first endeavor.
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Old 10th September 2011, 09:09 AM   #15
AndrewT is offline AndrewT  Scotland
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2Aac is just about adequate for a mono chipamp.
That size will even power and stereo chipamp with only a little loss of performance.

Allow a VA rating ~one times to two times the maximum total output power.
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Old 10th September 2011, 07:43 PM   #16
gootee is offline gootee  United States
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For your summing amplifier circuit, there are some example schematics and very simple equations in application notes AN-20 and AN-31, which can be found and downloaded at national.com . Note that those appnotes don't show things like decoupling caps, in their schematics. You should use at least a 0.1 uF X7R ceramic in parallel with a 10 uF electrolytic on each IC power pin (to ground).

This has some great information, in general: (see especially section 7, for now, on filtering for RF): ADI - Analog Dialogue | Op Amp Applications Handbook

Make sure your input signal ground references run separately from the other grounds, all the way back to the main psu caps' gnd. Search and read about (and then implement) "star grounding".

Keep all enclosed loop areas as small as possible, so they aren't antennas (for hum and RF). i.e. twist tightly together (and shield if possible), or run traces very close together for, all conductor pairs, including transformer winding lead pairs, dc power pairs, signal/gnd pairs, output pairs, etc.

I would also add a low pass RF filter to every chip input, and find out how to filter RF at speaker outputs and power supply inputs of each chip. Might as well do things right, if you're going to do them at all. At least leave PCB pads or space to add the components later, e.g. a hand-wound air core 5 uH coil in parallel with a 10 Ohm resistor in series with each speaker output. Otherwise RF can get in through the speaker leads, make its way backward through the feedback loop, and get to the chip inputs, after which it would get rectified by pn junctions, which might be obvious or, more likely, just change some internal operating points, with "who knows what" effects that would be as likely as not to degrade the sound quality.

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Tom Gootee
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Old 13th September 2011, 09:40 AM   #17
lnh is offline lnh  United States
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Quote:
Originally Posted by AndrewT View Post
2Aac is just about adequate for a mono chipamp.
That size will even power and stereo chipamp with only a little loss of performance.

Allow a VA rating ~one times to two times the maximum total output power.
The final transformer is 400VA for the +-28VAC secondaries, and 60VA for the +-15VAC secondaries (4 secondaries total). The eventual power output that I'm looking to use is ~200W for the three channels on the 28V side, plus the little bit that the various other boards will use on the 15V side. I figured this should cover any power needed without a problem, and leave a little room for upgrades and efficiency reasons. Also, it is a shielded transformer, so that will be nice too.

Quote:
Originally Posted by gootee View Post
For your summing amplifier circuit, there are some example schematics and very simple equations in application notes AN-20 and AN-31, which can be found and downloaded at national.com . Note that those appnotes don't show things like decoupling caps, in their schematics. You should use at least a 0.1 uF X7R ceramic in parallel with a 10 uF electrolytic on each IC power pin (to ground).

This has some great information, in general: (see especially section 7, for now, on filtering for RF): ADI - Analog Dialogue | Op Amp Applications Handbook

Make sure your input signal ground references run separately from the other grounds, all the way back to the main psu caps' gnd. Search and read about (and then implement) "star grounding".

Keep all enclosed loop areas as small as possible, so they aren't antennas (for hum and RF). i.e. twist tightly together (and shield if possible), or run traces very close together for, all conductor pairs, including transformer winding lead pairs, dc power pairs, signal/gnd pairs, output pairs, etc.

I would also add a low pass RF filter to every chip input, and find out how to filter RF at speaker outputs and power supply inputs of each chip. Might as well do things right, if you're going to do them at all. At least leave PCB pads or space to add the components later, e.g. a hand-wound air core 5 uH coil in parallel with a 10 Ohm resistor in series with each speaker output. Otherwise RF can get in through the speaker leads, make its way backward through the feedback loop, and get to the chip inputs, after which it would get rectified by pn junctions, which might be obvious or, more likely, just change some internal operating points, with "who knows what" effects that would be as likely as not to degrade the sound quality.

Regards,

Tom Gootee
Thank you very much for your input Tom. I already have noise issues from the computer output that are very obvious through my headphones...not so much when using my current speakers, so RF noise is a major concern of mine. I'm reading that chapter now, and will be reading the App Notes most likely tomorrow.



At this point I have all of the components to start on-hand except the heatsinks...I'm just going to use a chunk of aluminum or copper scrap for this for now. I also need to pick up some thermal compound, but that's no big deal either. This week I'm going to figure out layout and go from there. Since I'm switching to a much larger power supply and transformer for the next step, I'm going to avoid the headache and extra cost of paralleling the 1875's and just use 3886's or similar. Far easier construction and much easier to find information on. Learned a ton though through this process which is what it's all about, right
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Old 14th September 2011, 06:48 PM   #18
gfiandy is offline gfiandy  United Kingdom
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In answer to the last point in your first post how to combine the input from the two RCA inputs. A virtual earth summing amplifier is probably the best way.

see:-
Op-amps - The Summing Amplifier (Voltage Adder)

Regards,
Andrew
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Old 17th September 2011, 08:39 AM   #19
lnh is offline lnh  United States
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Quote:
Originally Posted by gfiandy View Post
In answer to the last point in your first post how to combine the input from the two RCA inputs. A virtual earth summing amplifier is probably the best way.

see:-
Op-amps - The Summing Amplifier (Voltage Adder)

Regards,
Andrew
Well that's disturbingly simple lol. Thanks!





I've got most of the first board done...have to flip around a transistor (wasn't paying attention), and then drill mounting holes for everything and I should be ready for some listening. Most likely tomorrow evening or Sunday.
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Old 17th September 2011, 02:42 PM   #20
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Keep in mind that the summing amp is also a signal inverter, so you may have to acount for that function as to how it could cause signal polarity mis-match with the other circuits in the system. An inverting unity gain amp in front of the summing amp would do the trick, or you may be abe to invert signals elsewhere in the system.

Mike
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