G'day Guys,

I turns out Iied in my last gainclone thread, it was definitely not the last gainclone I will build, especially since I bought 4x LM3886TF to supply the last build......

This one has actually been several years in the planning.

I purchased the power transformers back in 2018.

My other gainclone experiments have largely been practice for this one.

I have long wanted to replace my 'backup' Rotel power amp with something of my own creation.

The brief here is pretty simple: I want a simple, robust proven design. Over-engineered, over-built and gorgeous is what I am aiming at here.

With the arrival of my latest batch of pcbs I can finally get started.

The only item missing right now is some 11k resistors for the feedback network, and some polypropylene caps. These aren't far away so here we go.

The transformers are 120va 2x24v.

Power supplies will be 6x 2200uF per rail for the bulk caps with some smaller bypass caps and a snubber on the output.

I decided against my LM338 regulated supplies for this one in favour of the simplicity and robustness of a bulk capacitor bank.

I will be using the same LM3886 boards I used in my last build. This time with gain set to 12x since my favourite tube preamps all have highish gain.

I will be using the same protection circuit as last time, but different boards optimised for the available space.

The chassis is once again a salvage job. This one is an old Onkyo HT-R530.

I highly recommend grabbing the 500,600 and 700 series Onkyo recievers in faulty and cheap form as they have massive 550-650w transformers with 21v-0-21v secondaries, perfect for gainclones. I picked this one up for something like $20 which is a bloody good deal on a big power tranny alone.

We'll talk about aesthetics later but this one will move away from my usual wooden front panel in favour of something a lovely shade of copper.

So without further ado. Here is a teaser in the form of a layout mockup.

Mains wiring down the centre, power transformers and power supplies at the front, amp stages at the back.

I am pretty happy with this layout, unless anybody can point out any flaws in my scheme here. This is how I will proceed.

G'day Guys,

Things are moving!

This week I managed to get the speaker protection boards and the main psu filter boards complete.

Spot the booboo!

Fixed and completed boards

Speaker Protection boards

I am still waiting on some resistors for the amp boards themselves and some heatsinks for the rectifier boards.

Here is a fresh mockup with the completed boards.

You put the cap the wrong way! Been there done that...

AMR-iFi R&D

G'day Guys,

More parts arrived and thus progress has progressed.

Now that the heatsinks arrived I have completed the rectifier boards.

I've gone with MUR820 rectifiers.

I went with discrete mainly because I want a snubber as well as a suppression cap across the secondary and a pcb is the easiest way to do it in my opinion.

It takes up a bit of space but there is more than enough headroom for current.  I think it is worth it for the sake of robustness. It also looks pretty cool...

With the filter boards

Next up is the amp boards

G'day Guys,

The amp boards are progressing, half the resistors are soldered.

I find I can only get so much work done in an afternoon/evening. Add in some life and progress can feel pretty glacial at times.

I am pretty fussy about prep work so I probably spend 10x the time cleaning pcbs, scraping the oxide layer off leads, brushing the pads with a fibre glass brush etc as I do actually soldering.

I also like to match resistors as close a possible which also adds time. I normally match across channels so for example R3 and R4 are both 1k resistors. I will match so that r3 on both channels is as closely matched as possible, same for r4 etc.

Recently I got a little bit fed up with staring at the chassis and figuring out how to mount everything everything around the existing folds, vent slots, holes and other associated quirks with a salvaged home theatre amp chassis.

The real pain was the lack of a front panel or anything that could easily mount a front panel. I could have made it work eventually but not without wasting weeks and probably pulling some hair out.

So I paid a visit to Smith Sheetmetals in Wairau and had them fabricate a new bottom half of the chassis. It is designed to accommodate the lid of the Onkyo.

So I now have a nice 1.2mm steel chassis with no annoying folds to work around and a solid front to work with.

Here is where I am currently at (minus some work on the amp boards).

I think I will have to mount the amp boards slightly off centre on the heatsink, I don't see this being a huge issue as each heatsink is 0.72 C/W which is pretty over sized for 68w max dissipation on these chips.

By the end of the week I think I will be done on the amp boards, then it is all chassis work from here out.

I have all but abandoned my idea for a bass or copper front as the cost is simply too high unless I can get really lucky with a cheap source of 5-10mm copper or brass plate.....

Back to the old brown-wood ply again I think.

Much cuttting, drilling, filing, sanding, painting, staining, varnishing and polishing to come in the week few weeks.

Mar 18, 2021 21:26:27 GMT 12 sadface said:

I also like to match resistors as close a possible which also adds time. I normally match across channels so for example R3 and R4 are both 1k resistors. I will match so that r3 on both channels is as closely matched as possible, same for r4 etc.

In restoring the Wurlitzer amp I obsessively matched nominal new 5% carbon film resistors, resistor pairs,  to < or ~ 1%.

I took my meter to the store and went through 10-15+ packs to find on value resistors.

It really is a Bell Curve. I found indeed nominal 5% variation on say 100k including that range.

The staff got used to me being on my knees in a carpeted aisle checking & quite quickly sorting open pairs of 5% pairs of resistor packs, trusted or not noticed, to 1%.

I included a few metal film ones, but since originals had been carbon composite and almost all had drifted way out of range, I ended up replacing all of them.

I think matched carbon film and some equally matched metal film are entirely suitable replacements for that early '60's stereo valve amp, but my point was to check.

Yes, matching resistors (and capacitors) per channel, of course! And yes, it takes time ... but I think time well spent.

(I wasn't and am still not fussed about fancy boutique brands, I just wanted accurate values matched across channels and to the original circuit diagram of that era. YMMV;-)

Cheers!

I agree, I'm not fussed with boutique parts either.

I do however, make sure I buy quality name brands: Vishay/dale, Panasonic, nichicon, Epcos/TDK, ALPs etc
I also stick to known suppliers: Mouser, RS, Farnell etc

I got some "LM1875" kits off ebay some time back for some experiments. The chips are certainly not LM1875 chips as they look nothing like genuine chips and under bench marking over at DIYA have proven to have lower voltage and current capabilities than the LM1875 spec sheet.

I might get some things off ebay buy certainly nothing that will be anywhere near the signal path.

Many years back I compared some Mundorf silver/oil PIO caps to some (much cheaper) Solen polypropylene caps as output coupling caps in a tube amp. I heard a clear difference in the bass response with the Mundorfs going much deeper. Except it turned out the Solen caps were smaller in value and therefore were rolling off slightly at the bottom.....

Since then I have become deeply skeptical of much of the fancy stuff.

G'day Guys,

Progress was good over the weekend until it stopped.

I wound the inductors for the Thiel network on the output. 0.7uH as per the data sheet. In this instance, 17 turns of 21awg wire around a 6mm former.

And then I couldn't find my 10R resistors. Inside the bag was some 510R resistors of the same type. I have searched high and low and my stash or Vishay PR02 series 10R resistors is gone....

So now I need to do another parts order.....

G'day Guys,

Parts arrived today. Mouser was a little bit slow on the pick and pack.

But damn Zee Germans are good. DHL picked up from Texas 9am Saturday NZ time. Delivered 9am Tuesday to Wairau.

With the arrival of some fresh 10R resistors the amp boards are finished.

This amp get coupling caps as who knows what might be driving this thing in the future.

The gainclone integrated amp I built last year had LME47920 chips to act as DC blocking but with this being a straight power amp It seems prudent to include them.

Tomorrow I will tidy up the boards, get rid of any flux residue etc.

After that it's mostly drilling and a little bit of cutting before I can start testing. If I get my A into G, it might make some noises by the weekend.

Looks like kitty approves.

AMR-iFi R&D

Mar 19, 2021 20:12:50 GMT 12 sadface said:

I got some "LM1875" kits off ebay some time back for some experiments. The chips are certainly not LM1875 chips as they look nothing like genuine chips and under bench marking over at DIYA have proven to have lower voltage and current capabilities than the LM1875 spec sheet.

I recall Rod Elliott having a few things to write about counterfeits too.

From sound-au.com/

I've got some rare early Sanken amp IC's that I'm sure are original;- it will be a question of what to do with them.

G'day Guys,

Some good progress the last couple of days.

Many holes drilled: The rectifier boards, filter boards and speaker protection boards are all mounted.

Pilot holes for the transformers and chassis feet have been drilled but not out to size just yet.

I am thinking of using some rivnuts to mount the chassis feet as this will save me dealing with nuts under circuit boards later on.

All things going to plan, tomorrow I will deal with the heatsinks and power amp boards.

G'day Guys,

Quite it a bit of progress over Easter. Not as much as I would have liked due to life but good progress nonetheless.

Many holes drilled.

Things went mostly pretty well. Interestingly after I found my cutting oil my hole centres wandered, whereas the holes I drilled dry were all bang on.

Notice the larger holes for the heatsink screws are all off of the centre lines.

There was also some calculation errors, most notably on the power amp mount holes which were 8mm off on both sides (at least I am consistent in my error making)

Here is the right side power amp board with the error holes in full view. A neat 8mm off.

Here is the left side. 8mm off on the horizontal axis (relative to the heatsink) and 1.5mm in the vertical axis due to the heatsink wandering off axis with the cutting oil.

Here is the final mockup of the day. Everything nicely screwed down.

That's as far as I got. Next is to mount the I/O terminals, mains wiring and of course; Some speed holes under the heatsinks for venting.

After that is testing and then its all cosmetic after that.

I also snapped a 1.5mm drill bit.
Blunted an m3 tap and snapped an m4 tap

Clearly I am apprentice grade at machining if I was being generous.

Measure twice, drill once and all that. (I fail that too.)

AMR-iFi R&D

Easy to snap a 1.5mm drill, I've done mine.

DarkLantern blog - darklanternforowen.wordpress.com

Oh I measured thrice.
I simply didn't do well at the drilling part. The rest was calculation error. I'm rather amused that I made the exact same mistake for all 8 holes on the amp boards....

This was all done with a cordless drill except the holes in the heatsinks which I used a drill press for as I needed them perpendicular to the surface since they were tapped.
I found me an old Ryobi drill press for $60 at Cash Converters over Easter. The belt needs replacement but otherwise its in unreasonably good nick considering the price.
I should break a few less taps now that I have a drill press for starting. I always have difficulty starting taps, after the tap is in its not so hard.

This evening I managed to rule up the IEC socket, fuse holder and power switch. For the IEC socket and power switch I will drill out the corners and then use my cordless jigsaw for the rest.

I also drilled out the feet holes and put in some m4 rivet nuts.

Feet mounted, can anybody see my mistake here?

Many many standoffs.

Apr 6, 2021 21:45:28 GMT 12 sadface said:

I found me an old Ryobi drill press for $60 at Cash Converters over Easter. The belt needs replacement but otherwise its in unreasonably good nick considering the price.
I should break a few less taps now that I have a drill press for starting. I always have difficulty starting taps, after the tap is in its not so hard.

Good score!
I've been looking myself, I don't have one now that my son has moved his out!

Using lubricating fluid? Use turpentine if necessary, it helps.

DarkLantern blog - darklanternforowen.wordpress.com

Apr 6, 2021 21:45:28 GMT 12 sadface said:

I should break a few less taps now that I have a drill press for starting. I always have difficulty starting taps, after the tap is in its not so hard.

I bought a set of these a few months ago; I've just used the M3 so far for drilling / tapping into a heatsink.

He used to have imperial ones too.

Metric HSS Tap & Drill Set. 3mm ~ 10mm, One step Drill - Thread and Deburr

www.trademe.co.nz/Browse/Listing.aspx?id=3043766189

I always use a center punch first to stop wandering.

For larger holes, especially in sheet metal, I start with smaller sizes and work up for a cleaner result.

You can get non-wandering drill bits, with a "self-centering split point" but I find a sharp center punch dimple is usually enough.

Much easier to place a small center punch accurately on a line than aim a drill bit at it.

As for presses, I just have one of those things that you mount an electric drill in, pull the spring loaded lever down.

Clearance between the vertical shaft and the drill bit limits the size of work though.

Like this, but a bit fancier - it can clamp the drill at the top as well as at the shaft end.

www.trademe.co.nz/business-farming-industry/industrial/manufacturing-metalwork/drills-accessories/listing-3041758079.htm

Hope that helps!

How do you find those drill taps?

I’ve been eyeing them up for a while but I’m a bit dubious as their efficacy and longevity.

Actually I've only used the M3 once, for one hole.
You do need a speed controlled drill to go slowly once the tap section is reached.
I should have practised on some scrap first to get the hang of it, but just barrelled in...

It looks OK to the naked eye, and an M3 bolt screws in just fine, but looking under magnification I see that there still seems to be a bit of swarf in it, and that I was a bit rough coming to the countersink portion, which is a bit irregular compared to the factory original holes on the right.
Maybe it would have been better to reverse out before then and de-burr / countersink separately.

I don't know how long they'll last but likely enough for my needs especially if only into aluminium, but we'll see.

[Further edit: I see too that I was a bit off the line established by the others, but since they're to mount transistors, lead length will accommodate OK.]

[Even further edit: I am not using the very right-hand (center)  hole; there are two more beside it with the same spacing as the LH two. I just got the height wrong.]

I use the M3 tap regularly. Actually there are 3 in the set, the starting bit, intermediate and bottoming bit. If it’s sheet metal I just use the start bit carefully by hand so I know how much force to put on it, and turn it backwards a little about every turn or 2. If tapping into a heatsink where the bottom of the hole needs to be clean and tapped I use the intermediate bit, then the bottoming one as well.

AMR-iFi R&D

My Dad had some original tap and die sets I remember, maybe from the 50's.
Much darker metal, from memory, possibly more brittle, but I remember the three stages sets.
Thanks.

Apr 6, 2021 21:45:28 GMT 12 sadface said:

I found me an old Ryobi drill press for $60 at Cash Converters over Easter. The belt needs replacement but otherwise its in unreasonably good nick considering the price.

Does it have any speed control? Some have different pulleys and belts...

Is an electronic speed controller possible, maintaining torque even at low rpm, if not?

Like my B&D hand drill?  An external box?

Standard 5 speed drill press. 5 different pulley sizes at each end.

Same as this one but with a lot more surface rust on the foot.

shop.cashconverters.co.nz/Listing/Details/73809237/Rockwell-Drill-Press-13MM350W

There is also this one available at the Cash Converters in Panmure. Different brand but I think most cheap drill presses are much the same. It probably comes out of the same factory as a brand new Ozito from bunnings at $159.

www.bunnings.co.nz/ozito-16mm-500w-drill-press_p0048826

All drill presses I've seen have mechanical belt/pulley manual speed adjustment.
The only 'electronic' one I've actually just has a variable taper pulley.

Being AC mtrs, I guess they'd need Variable Freq Drive.

sadface - that's a nice buy, that Ryobi - 13mm chuck?

DarkLantern blog - darklanternforowen.wordpress.com

Yep, standard 13mm chuck.

The only variable speed drill that I know of the is the Bosch green. Nice kit but not the cheapest.
www.mitre10.co.nz/shop/bosch-diy-bench-drill-710-watt/p/269303

Actually, there is a Full Boar model as well.
www.bunnings.co.nz/full-boar-750w-digital-drill-press-bench-mounted_p0094372

Actually, I've looked into that one - I believe it's an 'industrial' Ozito brand.
That's the one with variable taper pulley for speed change.
Quite grunty motor, 16mm keyless chuck..

DarkLantern blog - darklanternforowen.wordpress.com

G'day Guys,

It was a very good weekend. Along with finishing some projects around the house, I made huge progress on the amp.

The rear panel was completed: IEC socket, fuse holder, speaker jacks, RCA jacks and some screw holes for the lid.

The front panel was finished with a large hole for the power switch. The switch itself will be mounted into the plywood front panel so I wanted wide clearance for the mains wiring.

I also mounted a terminal block on some m4 rivnuts above the power switch to split the mains into the 2 separate power transformers.

And the real big job was to drill 80 vent holes under the heatsinks.

I snapped my 2mm drill bit last night so while I was in Bunnings this morning buying a replacement I took the liberty of buying an Evacut Cobalt HSS drill bit. I've never noticed these before but are seemingly designed for heavy duty into hard metals.

I gotta say, the 2mm cobalt bit served duty like a champ. 80 holes back to back without feeling blunt was pretty impressive in my book. The pilot holes all came out pretty nice, some minor wandering here and there but 95% perfect is pretty good for my engineering skills.

I was planning to go with 14mm vent holes but a calculation error (becoming a theme now) resulted in me sticking with 10mm holes.

I used a $20 craftright step drill bit and I must say, this thing was much faster and cleaner than using a twist drill bit on the sheet metal.

Unfortunately things went awry once it came to deburring. I am yet to acquire a proper deburring tool so I was using a 13mm twist drill bit....

Suffice to say on 3 holes it decided to bite and so 3 of the holes are a bit larger than the rest. This also also required some light panel beating as the metal bent when the drill bit into the hole.

So here we are. I need 3 more m4 rivnuts to secure the lid. Otherwise the chassis work is done for now and it is time for wiring and testing.