New LM4702 Build

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Hi all -

I'm getting ready to build my first amp build based on the LM4702 chip using pre-assembled modules. At this point I'm not sure of the final design, but the amp may morph into more than just a power amp. I'm thinking of possibly including a pre and DAC modules as well. The whole shebang!

I've started to gather all the parts I need, including a big, hefty toroidal xformer. I'm now starting to look at chassis' and enclosures and hence my question.

The size of the case I'm looking at needs to be about 16 x 12 x 4. I'd like the case to have both top and bottom plates removable for ease of access, drilling and wiring hookup flexibility. I'd also like the bottom plate to be a little thicker than the rest of the enclosure for added sturdiness and strength. At this point I feel I can't be too picky with what I find that comes close, but I suspect aluminum is easier to work with. In the end I do want a case that looks smart, well designed with a bit of eye-candy to boot.

I don't have the equipment to construct my own enclosure so therefore need a third-part solution. Since most of you all are a lot more seasoned with building amps... My question is, where is the best place to start looking for the type of enclosure I'm after? I've seen a few possibilities on the Hammond site and a few on eBay, but still nothing that fits the bill as I envision it.

Can you guys provide some additional links to other sites that offer a variety of chassis' and enclosures for DIY projects. Much thanks.
 
In helping me decide what exact size chassis I need, I cut a piece of cardboard to use as a template for the bottom plate. The attached picture shows a 16" x 12" (cut to scale) piece with a tentative layout of the modules. There's plenty of room using this size, and extra space for additional modules should I decide to expand this project. A 16" x 16" size would give me plenty of room! Par-Metal does have what appears to be a nice, basic case, with no bling, wood sides, or anything else. ;(

I was going to try and balance the weight of the chassis with the module placement, but it's clear to me the toroid will win the day unless I mount it in the middle. That doesn't make the best and most efficient use of the space though. Balance may not be practically possible.

Any thoughts on special consideration to other layout configurations? Are there any general rules-of-thumb for layout?
 

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Here are some aluminum chassis. Chinese supplier, no affiliation use at your own risk.
http://www.vt4c.com/shop/program/main.php?cat_id=1032&group_id=2&hit_cat=

Keep the power caps close to the main amp board.
You can stick the toroid at the front, this seems odd but it keeps the rest of the wiring to the jacks neat and short.

Have fun, don't be too anal, or you will be like me playing tunes from a breadboard a month from now. Although the cardboard would be easy to drill.
 
redjr - since you live in the U.S. take a run through the electronics sections of your local Salvation Army and Goodwill outlets. Some have found good items to gut and use the chassis without a lot of drilling and bending.

Also, remember most cities have small metal shops that will put a couple 90 degree bends on a small sheet from Home Depot for just a few bucks. Check the Yellow Pages.
 
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Here are some aluminum chassis. Chinese supplier, no affiliation use at your own risk.
Audio Catalog

Keep the power caps close to the main amp board.
You can stick the toroid at the front, this seems odd but it keeps the rest of the wiring to the jacks neat and short.

Have fun, don't be too anal, or you will be like me playing tunes from a breadboard a month from now. Although the cardboard would be easy to drill.
Yeah, I have a tendency to be anal! With this template I wanted to get a sense of the size I need based on my components/modules before I buy. I just like to have a plan - even with my DIY projects. It may take longer, but in the end, I'll have fewer surprises along the way, and the result more predictable. ;) I've kinda zeroed in on a Par-Metal enclosure and now just need to decide on the size. Their cases seem very workable - although somewhat bland and industrial looking. I'd really like some integrated wood side panels. Only thing - which is not a show stopper - is there's no removable bottom plate. In the picture, those caps on the PSU module are 10,000mfd each, so I should have enough filtering.

Hopefully, I won't be using cardboard as my bottom plate! :)
 
redjr - since you live in the U.S. take a run through the electronics sections of your local Salvation Army and Goodwill outlets. Some have found good items to gut and use the chassis without a lot of drilling and bending.

Also, remember most cities have small metal shops that will put a couple 90 degree bends on a small sheet from Home Depot for just a few bucks. Check the Yellow Pages.
I thot about finding an old carcus from an amp or something - even looked through my limited supply, but couldn't find anything suitable. And the thot of going to a metal shop did cross my mind. Great suggestion.
 
Hi red

I saw a very nice chassis layout that would work well for Low power DIY amps, but no pictures, sorry...

A thick alu chassis bottom plate that acts as the heat sink, so output devices directly attached. Transformer and pcbs also mounted on bottom plate.

4 sides in wood with nice finish, screwed to the bottom plate, glued at the edges. Wood top, glued to the sides. Servicing from the top.

Cooling was fine, darling ton pair output devices.
 
jim -

Cool. Sounds nice. I looked at some wood chassis, but they seem to get expensive pretty quick too. As I said in my other post, I'm committed to the Par-Metal enclosure now. I bought an assembled AB module that already has the heat-sink integrated with it and done very nicely and compact too. It would just not be feasible to remove it and put it on another (HS). You can see it from the pictures I posted earlier. I'll post a 'before' picture of the case when it gets here.

Even with the Par-Metal case, I'll either be putting the amp module inside, or make a design decision to hang the HS out the back. The latter will require some significant case mods if I go that route. And I don't have any sheet metal cutting, or bending tool either. :( I just won't know until the chassis gets here to see what I'm up against. Should be here next week. While I've been waiting I've been in the process of buying the necessary hardware/electronic parts I'll need for this build and others down the road. I'm just getting back into this hobby after about 30 yrs. so I needed to stock up on a few things. When I sit down and ready to put something together, I like to have most of what I need ready to go. Having said that, I'm almost always running to RatShack or HomeDepot for something! :p
 
Update. Well, I finally got my enclosure from Par-Metal last week. Took a little prodding with them, but they finally came through. Overall I'm happy, but a little disappointed that the internal front panel and rear deck are different heights! :eek: Not by much, but 1/16" makes a big difference when now the heat sink won't fit where I had planned on putting it. Ugh! This is not visually obvious when the case is together, so I plan on living with it. I originally thought I would mount the amp PCB/HS in a vertical orientation over the bottom and top vents for passive cooling. This would have allowed for more room inside too - not that it was needed. :)

Now I have to go to plan 'B'. I've decided to mount the amp module in a horizontal orientation on stand-offs with the HS facing up. This still leaves plenty of room underneath and above the module for hookup and cooling purposes. This seems like an unusual arrangement, but should work fine. Since heat radiates and rises shouldn't be an issue. While tapping one of the 3mm holes for the stand-offs I broke a tap. Those suckers are hard when you go and try to hand file them down. :) Now a little drop of oil in the hole before tapping helps.

I've included a few pictures of the bare case, intended module layout and amp module with stand-offs. Nothing has been secured down yet. Still plenty of drilling to do, waiting the arrival of the IEC connector, etc. Stay tuned...
 

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+-60Vdc should be approaching 200W into 8r0 if you build it right.

+-50Vdc is the norm for a 100W into 8r0 ClassAB Power Amplifier.

According to the seller, my PSU outputs +-60v which can be used (in parallel) to supply both V & Vcc requirements on the amp PCB. I have attached the schematic below.

It appears the +-60v supply the LM4702 module on pins 13-15, and the +-45 Vcc powers the output transistors. Am I understanding that correctly?
 

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Another update

Hi all -

Quick update and some pics. I've finally decided on the enclosure layout of the modules and managed to get everything semi-mounted on their stand-offs. Also made some basic (no load) voltage measurements of the tranny and PSU. All voltages were spot-on as advertised! Continued to wire up what I could. Still waiting on some better binding posts to arrive before starting to drill the rear deck. Also need to get some fuses for the IEC connector. Maybe by the weekend. Attached are a few pictures from today.

My Photobucket site has been updated with more pictures if you care to see more detail of the build.
 
For some reason the pics did not show in my post above. Here they are.
 

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For future reference, remember that with a good carbide blade on a table saw most aluminum heat sinks cut like hardwood. I have used that method to custom fit HS often. It looks like there was plenty of room to get 1/16" + from one side and still have room for mounting hardware. This is in no way a criticism of your approach - just a suggestion of an alternate method to get max air flow around the fins.

Note: All shop safety rules must be applied !!
 
For future reference, remember that with a good carbide blade on a table saw most aluminum heat sinks cut like hardwood. I have used that method to custom fit HS often. It looks like there was plenty of room to get 1/16" + from one side and still have room for mounting hardware. This is in no way a criticism of your approach - just a suggestion of an alternate method to get max air flow around the fins.

Note: All shop safety rules must be applied !!
I probably could have gone that route - had that been my only option. The issue was the PCB and heatsink are so tightly integrated. I chickened out and really didn't want to remove it to cut it down. Had I decided to still mount the HS vertical, I could have, had it been placed right up against the rear-deck. It seemed to fit there, but was still very tight once the cover was on. There are vent slots punched into the top right over where the HS is positioned now. Hopefully this will easily allow the heat to escape up and out. This obviously was not my original design, but has become an 'as-built' situation. Clearly, this is an issue that surfaced as a result of the poor QC of the case manufacturing. No criticism taken. :)
 
One other thing guys I've noticed.... maybe you can help. The leads on the toroidal are 16 AWG and seem to be pretty beefy. I've tried to find some similar gauge, 300v, and weight(thickness), but all the 16 AWG hook-up wire I find is much thinner.

I also noticed that the tranny leads have an extra white internal sheath that I guess adds some extra insulating properties to the wire. This is course adds to the overall thickness. At a minimum, I would like to find some similar size wire to go from my PSU to the amp module - where high current may be likely under some operating condition. I ordered some more wire last night that is 16 AWG, but it's hard to tell from a website, just how thick it is. I thought 16 gauge, is 16 gauge! Can anyone recommend some wire, or at least provide the right AWG markings I need to zero in on, type I should be looking for?

Suggestions, links?
 
I have tried to try the same thing till I realized, if the AC wires feeding the transformer were carrying the full power to the transformer, they certainly could handle the reduced amount. Now I just strip one of the many PC style power cord lying around and use that wire. I cut the transformer leads to a length that is appropriate and transition with a Euro-style screw terminal. Works for me.

I actually cut these too short but still works.

20120327_195453tt.jpg
 
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