Chipamp 4780 (2) Beta Build - diyAudio
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Old 31st October 2011, 01:39 PM   #1
JL889 is offline JL889  United States
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Default Chipamp 4780 (2) Beta Build

Post your results for the 4780 (2) beta build here.
BOM for 4780 (2)
LM4780 integrated (blue)
R1 = 221R
R2 = 22.1K (for Stereo, Jumper on back for Parallel)
R3 = 221R (for Stereo, Jumper on back for Parallel)
R4-R6 = 22.1K
R7-R8 = 681R
R9 = 10K
R10,R11 = 2.7R 2W
R12,R13 = 2.2K 2R
R15,R16 = 0.1R 3W (for Parallel only)
R0 = 10k
C1-C2,C7-C8 = 0.1uF
C9-C12 = 10000uF 50V
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Last edited by JL889; 9th November 2011 at 03:27 PM.
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Old 31st October 2011, 02:30 PM   #2
hates ground loops
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Okay, here's my build of the LM4780 stereo board.

The circuit...I have populated the stereo board with parts/values I have had very good experiences with my earlier LM3886 builds. The components' values are very close to the circuits in the datasheet.

The transformer...I'm currently using a (conservatively rated) 180VA EI-type with dual secondaries, and it puts out 22V AC for each secondary. After rectification and filtering I have pretty much exactly +/-28V DC on the supply rails.
The circuit's ground is connected to safety earth via a 10 Ohm 3W resistor to have a ground reference with some damping for eventual groundloops.

The parts...The rectifier diodes are BYV29-500, big caps are Panasonic TSUP 10000uF 50v, small caps are Nichicon Muse 100uF 50V, small bypass caps are 1uF 63V WIMA. The resistors are CMF55 1% MF, except for the 10k muting resistor which is a compact MF. The values are 15k to GND, 221R to the + input, 20k and 1k in the FB path. The snubber resistor is a 2R7 1W MF resistor, and the snubber caps are 100nF film caps.

The amp is DC coupled for now, and both channels' outputs have a DC offset of 60mV, which is perfectly fine.

Soldering...was straight forward, and other than bending the resistors' legs so that they fit, there were no real issues. It's noteworthy that the through-holes of the pcb don't have that bridge/cross type eyelet (don't know the name), so a lot of heat is transferred away from the soldering iron. Thus a fairly strong iron is best, I have a basic 50W Weller and managed to have the solder flow nicely.

What I would change/improve/wish...I'd wish for slightly larger diameter holes for the outputs so I could fit a fatter speaker wire. They are already quite large, but I'd prefer their diameter increased just a bit.
I'd also increase the spacing between the electrolytic bypass caps close to the chip so that also larger diameter caps could be fit nicely. Other than these minor issues the pcb and the layout are outstanding!

The pcb can also be built with two channels in parallel by soldering in a bridge and adding some low-resistance output resistors. The board would thus make a very compact 100W monoblock amplifier.

The amp sounds wonderful btw :-)
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File Type: jpg caps.jpg (293.2 KB, 439 views)
File Type: jpg detail.jpg (307.5 KB, 437 views)
File Type: jpg full.jpg (333.6 KB, 421 views)
File Type: jpg bend.jpg (327.2 KB, 405 views)
File Type: jpg 2011-10-30 21.28.48.jpg (286.5 KB, 115 views)
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Old 26th November 2011, 04:50 PM   #3
GloBug is offline GloBug  Canada
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While I've been waiting for the mailman for the boards to arrive, I've got what should be a decent PT for this project.

It's a 21.5-0-21.5 CT unit pulled from a harmon/kardon receiver. (It also has 13.8vac secondaries) Should be easy enough to get ~28vdc with it.

The back of the receiver said 155w, so I figure it's around 220VA based on an online calculator. Although I don't know the specific mA of the 13.8 secondaries, or weather I need to de-rate the total VA a bit for it.

I've been playing around with Duncan's PSUD a little bit. I thought about using a choke in a pi filter for this project. I have a couple little ones around, although I'm not sure how many Henry's they are or their current ratings. It's an option though, it sure does smooth out the DC like butter in the PSUD.
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Old 26th November 2011, 05:42 PM   #4
hates ground loops
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Here's some good advice that I saved in a txt file some time ago, I believe it is from someone here at diyaudio:

"For a multi-winding transformer, you will be safe if you figure 10% voltage drop under load. Say you have a winding that measures 18 volts RMS open circuit and 10 ohms resistance; 1.8 volts drop will occur with 0.18 amps current. That should be considered maximum for that winding. If any winding is less than 15 ohms, it canít be used as the 120 VAC primary because the inductive current will be too great. Transformers are rated by the volt-ampere capacity (VA). A transformer rated 12.6 volts RMS at two amps has a VA rating of 25.2. Since flux density is the usual limiting factor in transformer design, it follows that a higher VA rating requires more iron; you should be able to weigh a transformer and calculate a VA capability. I weighed some of my transformers that have the VA rating on them and concluded that 20 VA per pound is a conservative number for a 60 Hz transformer."


Concerning the choke...you'll have an idle current of around 130mA per rail, and current spikes up to several Amps. It would have to be a pretty big coil with an air core. These chips have above 100dB PSRR, so I don't really see the need for a choke in the supply.
Also, I don't see how it could be added to the pcb without considerable modifications.
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Old 26th November 2011, 08:42 PM   #5
GloBug is offline GloBug  Canada
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Ya I thought about it after I posted that it likely isn't anywhere near big enough.(Choke)

So I went and weighed this transformer, it's 1.495 kg/3.29 lbs.

I also weighed a known 360VA transformer I have, it weighed 2.975kg/6.55lbs.

So your saying the one I have in the picture is only 65VA and the 360VA one is only 131VA? That is very conservative.

I got to let the first part of your post soak in a bit, I will ohm the transformer shortly.
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Old 30th November 2011, 05:37 PM   #6
GloBug is offline GloBug  Canada
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Quote:
Originally Posted by Rodeodave View Post
Here's some good advice that I saved in a txt file some time ago, I believe it is from someone here at diyaudio:

"For a multi-winding transformer, you will be safe if you figure 10% voltage drop under load. Say you have a winding that measures 18 volts RMS open circuit and 10 ohms resistance; 1.8 volts drop will occur with 0.18 amps current. That should be considered maximum for that winding. If any winding is less than 15 ohms, it canít be used as the 120 VAC primary because the inductive current will be too great. Transformers are rated by the volt-ampere capacity (VA). A transformer rated 12.6 volts RMS at two amps has a VA rating of 25.2. Since flux density is the usual limiting factor in transformer design, it follows that a higher VA rating requires more iron; you should be able to weigh a transformer and calculate a VA capability. I weighed some of my transformers that have the VA rating on them and concluded that 20 VA per pound is a conservative number for a 60 Hz transformer."
OK that makes a little more sense to me now. I'm slow sometimes. Thanks.

I also "sourced" a 26.5 center-tapped transformer.

It should be a good 220VA as well, maybe slightly more, I have not worked it out yet.

So this gives me options of a 4-8 ohm amp or a 8 ohm amp.
Any sonic advantages with going either way? I'm leaning towards lower voltage with 4-8ohm for versatility, however if you figure it sounds a little better with more voltage I can go that way too.
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Old 18th December 2011, 10:39 PM   #7
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Well I'm a bit slower than u guys... I have managed to get my blue board almost populated, thanks to a very nice member of the forum. Just need the 4 10000uf caps and a LM4780 to complete it. My aim is to use the blue board as a bridged output to a bass unit and the red boards are going to be a 4 speaker set up.

I have a Roberts Radio cabinet.. Nice solid walnut... I want to build all the amps into the cabinet and try to fit a remote control for volume and selector. I have just got a home theatre projector and I reckon some movies through a triple 4780 set up would sound pretty good.... A bit of AC/DC will also pop in now n then.

I'm between jobs..."unemployed" at the moment so I cant rush it even if I wanted to, but I will try to get time to keep you posted... I may be out of work, but I never seem to have time free for me
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