Post by sadface on Jun 17, 2020 21:06:53 GMT 12
G'day Chaps,
May I formally introduce my next project. 00940's Open Source LM3386
All things going to plan, this will be my last Gainclone project before I move on to some discrete solid state stuff.
The chassis and the power transformer were salvaged from one of these I got for $20 some time back. Some sheet metal work to enclose the front and probably some black engine enamel will be required.
Then I will clad the thing is some nice pretty wood as is my style. If anybody can suggest a dirt cheap source of nice looking hardwood I would be grateful.
The TX is a nice big toroidal type.
25-0-25 and 15-0-15 secondaries which is perfect. 300w power consumption according to the plaque on the Digitech chassis so I estimate 250VA.
The PSU may be recognised from my JLCpcb thread. 'The Meni'
6x 3300uF for 19800uFper rail.
Unfortunately my transformer Jig is currently at work waiting to explode some capacitors in the carpark so testing will have to wait for now.
Time to start working on the amp boards!
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Post by colinf on Jun 18, 2020 17:32:44 GMT 12
Save them for Guy Fawkes night...
AMR-iFi R&D
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Post by sadface on Jun 18, 2020 19:56:00 GMT 12
It occurred to me today that I forgot to add bleeder resistors to the PSU design It will just have discharge at 3.5mA through the indicator leds....
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Post by sadface on Jun 18, 2020 20:42:22 GMT 12
Probably a silly question.
Am I correct in that if I use my LME49720 preamp boards in front of the LM3886 boards. That I can do away with the coupling caps at the input of the LM3886?
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Post by colinf on Jun 19, 2020 18:38:40 GMT 12
Yes, you can make it direct coupled. Measure the dc at the output, it should be under 100mV. Ideally you want it to be less than 10mV.
AMR-iFi R&D
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Post by sadface on Jun 24, 2020 22:14:58 GMT 12
Work progresses slowly,
I'm really liking the blue and brown on the black boards.
I discovered I left some SMD 1206 sized resistors out of my parts order so Fedx is bringing me more goodies from Mouser. I had to tack on some parts for an up and coming dual mono aikido preamp to get free shipping.
I keep forgetting to order the input headers so another order will be necessary soon
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Post by Owen Y on Jun 25, 2020 9:07:14 GMT 12
Do Mouser offer free shipping like Digikey (over $50 orders)? I am ordering some Pana caps soon & am considering the convenience of RS, for a maybe an extra 10% cost, over ordering from Digikey overseas.
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Post by sadface on Jun 25, 2020 11:00:46 GMT 12
Mouser do free shipping on orders nz $60. Dhl fedx and I think ups are all free. Dhl is the default but I’ve been experimenting with fedx recently.
I’ve not yet tried digikey as mouser generally seem to be slightly cheaper. I prefer element 14 and RS Components as they are cheaper again but they often don’t have everything I want.
Vishal-Dale cmf/rn series resisters are only in certain values from rs but the full range is available at mouser.
I ordered Monday night. Fedx claims I will get the order by 6pm tonight but I think it will be tomorrow morning as the parcel is currently in Aus according to the tracking.
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Post by sadface on Jun 26, 2020 20:45:43 GMT 12
Some more progress.
The shortest electrolytic caps added, spade connectors and most importantly those dastardly SMT resistors on the underside.
Anybody have any suggestions for post solder cleaning?
I scratch off any flux splatters and I wipe with ISO-prop using cotton buds. The cotton buds seem to leave little lint whiskers and the ISO-prop leaves residue.
I've also tried ISO-prop wipes which are nice in that they don't leave any lint etc but they still suffer from the residue and I find them difficult to get into the small spaces.
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Post by Owen Y on Jun 27, 2020 14:05:10 GMT 12
Must admit I've seldom bothered to clean, the few PCBs that I've populated. Maybe less obvious on green boards?
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Post by sadface on Jun 27, 2020 15:19:58 GMT 12
I suspect you are correct.
Blue and green boards don't seem to show things up as bad. The black seems to show up every little stain....
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Post by colinf on Jun 27, 2020 19:19:18 GMT 12
I use RS PCB Cleaning Solvent, cat. 190-8290. Use a small paintbrush and wipe off the excess. It dries slowly, and use it with the window open. Not sure you can get it down under though. 331-1937 flux remover wipes look good if expensive.
AMR-iFi R&D
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Post by Owen Y on Jun 29, 2020 10:50:26 GMT 12
TheArtofRepair dude here recommends applying some (low) heat + iso alcohol, to clean off flux easier & cleaner....
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Post by sadface on Jun 30, 2020 22:21:48 GMT 12
Thanks for that link.
Some good tips in here.
I don't have a desoldering station but I do have a cordless heatgun that isn't too hot.
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Post by Owen Y on Jul 1, 2020 10:42:10 GMT 12
'Cordless' heat gun - that sounds handier than a corded one, like mine.
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Post by sadface on Jul 1, 2020 20:05:23 GMT 12
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Post by sadface on Jul 3, 2020 23:29:02 GMT 12
The last parts arrived today.
All soldered up and ready for testing.
Now I need to do some more drilling and tapping.
Also the fatties arrived with the last parts. Output coupling caps for something in the hopefully not too distant future.
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Post by Owen Y on Jul 4, 2020 10:44:30 GMT 12
Those caps are biggies - are using driving into something 50k ohms or less?
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Post by sadface on Jul 4, 2020 14:06:53 GMT 12
Hi Colin, Yes. They will be driving my Marchand XM9 crossover which has 25k input impedance. With the 1M resistors in parallel with the output of the preamp I get about 24k combined impedance. According to sim.okawa-denshi.jp/en/CRhikeisan.htmA 1uF cap gives about -0.45dB @ 20Hz. -0.29dB @ 25Hz A 2.5uF cap gives about -0.09dB @ 20 Hz. -0.06dB @ 25Hz. I went with the 2.5uF just to make sure the corner frequency is as low as possible and that there is no possibility of bass roll off. I have a fantastic sub woofer and occasionally I like to listen to electronic music that can use properly it so I am quite sensitive to rolled off bass. My favourite song for testing this is Skrillex' remix of La Roux - In for the kill. With a good sub you should feel the whole house rumble.
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Post by sadface on Jul 5, 2020 23:12:48 GMT 12
G'day Guys,
I had some time over the weekend and I managed to get the PSU made for the preamp stage. This one is the same as the one used my backup preamp. This time I used smaller heatsinks as the current draw will be something in the margin of 50mA. I don't estimate more than 20mA per LME49720 and the denoiser power supply uses circa 10mA.
For the preamp I will reuse the mark III LME49720 boards that were just replaced in my backup preamp by the new class A version boards.
Here's all of the pcbs stuffed and soldered.
Now all that remains is the really hard stuff.
I did some layout drafting within the chassis and its going to be a tight fit as per usual. Tomorrow afternoon I should have time to rule up the mounting holes in the power amp heatsink so that I can take it into work and make some nice accurate pilot holes using a drill press. After that its all chassis work.
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Post by sadface on Jul 7, 2020 20:43:01 GMT 12
G'day Guys,
I've been playing around with the chassis stuff and I've come to the conclusion that the Digitech chassis is a bit too small for what I am trying to build here. By the time I put in the Tx and the main PSU half of the internal space is taken up leaving very little room for heatsinks, preamp boards, amp boards and speaker protection.
So now I am going to change course and use a different chassis. I've been keeping an eye on trademe recently for cheap amps to salvage.
I recently picked up a faulty Yamaha HTR-5630RDS for $11.50 Here is the unimpressive guts before salvage. Interestingly the main PSU for the amp stages has a measly 6800uF per rail for supposedly 5x100w. (I had to tear off some of the label to remove the glue) I'm using 19800uF per rail to drive 2x40w and I would have used more if I could fit them on the pcb. I'm sure I will use this pair somewhere at a later date. The rest of the caps on the pcbs were an unknown brand so I couldn't be bothered removing them.
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Post by sadface on Jul 7, 2020 20:51:24 GMT 12
I have a question regarding magnetic and RF shielding inside a metal chassis. I have noticed that most commercial amps don't have a completely enclosed chassis. They generally have a plastic front panel like the Yamaha in the previous post.
Would this have any effect on the shielding capability of the chassis?
Is it necessary to enclose all 6 sides or does the 5 sides design of most commercial amps provide sufficient shielding?
Would there be any benefit to me going through all the bother of fabricating a sheet metal panel to enclose the front or should I just do it the easy way and get a nice piece of wood?
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Post by Owen Y on Jul 7, 2020 21:06:02 GMT 12
Good point about chassis shielding... Many amps have a steel shield internally around the PTX (and all the AC current-carrying bits, rectifiers, etc). At least between any low-signal circuitry (eg input/preamp sections of an integrated amp) that may be in close proximity to the above. The bit that I always scratch my head about is the Power On switch, which often requires AC lines running to the front panel. Putting the switch on the rear, is of course an answer. (With LED on front, if desired.) Or partition off one side for the AC bits?
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Post by sadface on Jul 7, 2020 21:32:09 GMT 12
A perhaps simple option that just occurred to me would be to go with nice simple wood for the front panel and then cover the inside with some kind of foil.
I've seen what appears to be adhesive copper foil before.
Perhaps Aluminium foil would also be perfectly sufficient?
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Post by sadface on Jul 7, 2020 22:13:12 GMT 12
Here's a draft layout inside the new Yamaha Chassis. I am thinking I will run the volume control on a shaft down the right hand side of the amp to keep the wiring runs nice and short. For the sake of my aesthetic sensibilities, I might have to drill a decent sized hole in the heatsink for the shaft to pass through.
Anybody have any critiques or suggestions?
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Post by Owen Y on Jul 8, 2020 11:04:01 GMT 12
Toroids of course, have low radiated EMF. For EMF shielding, steel is best I believe, because of its magnetic permeability.
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Post by sadface on Jul 8, 2020 22:03:03 GMT 12
From what I can tell, by leaving the front of the box unenclosed by metal the EMF shielding is somewhat compromised. Seemingly the manufacturers are not doing this for cost purposes.
Looks like I'm going to want to box in the front of the chassis with some mild steel sheet. Off to the scrappers for me!
There is some funny bends and projections to work around so this is going to require a bit of head scratching.
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Post by sadface on Jul 14, 2020 22:18:10 GMT 12
G'day Guys,
A minor progress update.
I have sorted some solutions for boxing in the front of the Yamaha Chassis. A quick trip to the scrappers yielded me a fluro bulb housing made of 1mm mild steel sheet. This weekend I should have time to trim back the flange back to 20mm and start drilling appropriate mount holes. I might need to use some rivet nuts since its not that thick.
I also got a nice offcut of 28mm brown wood ply from Plywood City in Penrose. My plan is to chisel a 15mm indent or socket in the back side of the plywood to slide over the front section of the chassis that protrudes in front of the lid as seen here.
I want a nice flush fit with the lid of the chassis. This way I can retain all of the existing bracketry.
I also stuffed and soldered the speaker protection board over the weekend. One Rod Elliot project 33 board.
I haven't populated the board that houses the relays yet as I want to measure the voltages of the power supply before sizing the dropping resistor. It should be about 35vdc but I want to make sure....
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Post by sadface on Jul 19, 2020 17:09:21 GMT 12
G'day Guys,
I've made some progress on the chassis.
I managed to box in the front of the chassis with some salvaged 1mm mild steel sheet.
I invested in an angle grinder for the job which is something I've been looking for an excuse to buy for a while now. I am sick of of using a Dremel for metal work.
I've secured the new panel into the front bracket and to the bottom of the chassis using some m3 rivet nuts.
Next weekend I will start preparing the plywood front panel. I am going to have to chisel out a recess or socket in the back side of it to work around the design of the front of the chassis.
In the mean time. I can now properly move forward on the internal layout and chassis work.
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Post by sadface on Jul 20, 2020 19:02:49 GMT 12
G'day Guys,
Here is my latest dilemma: Front panel layout.
Because the chassis only has vent holes in one location. I am boxed into the original location for the heatsink.
I could of course make my own vent holes but that seems more trouble than its worth.
So instead I am trying to figure out how to work around the heatshink being in the middle of where I would like to run a shaft for the volume pot. Here is a 1/2 scale drawing of a few different layout spacings for the front panel. The top is the spacing with the shaft running to the side of the heatsink with the hole centre 10mm to the left giving 7mm clearance with the heatsink. Power switch mounted an equal distance on the left side.
The middle is spacing with the volume pot and power switch mounted 1/6 of the width of the front panel. The bottom is the spacing as 1/6 of the front panel width.
My aesthetic preference would be 1/5 spacing. However this requires the shaft to go straight through the heatsink, as does the 1/6 spacing.
I am contemplating putting a 13mm hole in the heatsink, the trouble comes with dealing with the fins. I think I would need to remove a decent chunk of material from one of the fins to make it work.
Has anybody ever tried such an undertaking?
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