Despite the chaos, I should be nice to have a chance to set-up workshop again afresh, I expect.

DarkLantern blog - darklanternforowen.wordpress.com

G'day Guys,

A bit of ill new before the progress report.

Some low-life has helped themselves to my tools in my workshop in the basement.

They buggers took most of my cordless tools, batteries, chargers, quick-clamps and goodness knows what else.

The build is back in progress however.

I soldered up the relay board. I do love the clear housings on the relays. I look forward to watching the mechanism during testing.

Wiring hasn't started yet but the finishing work on the front panel has. Today I liberally applied some danish oil to the front panel.

I used the keep it wet method. Every time the stain got dry I slathered on some more to keep it wet. I applied goodness knows how many coats over 6 hours this afternoon.

Something like 10 applications at a guess. The early results are promising.

Here's a photo pre-wood work for a colour reference.

Wood a;ways looks appealing - what timber?

DarkLantern blog - darklanternforowen.wordpress.com

28mm Brown wood ply, I got a decent sized off cut from plywood city in Penrose.

Looks good. What’s the advantage of a keep-it-wet method?

AMR-iFi R&D

Thanks Colin,

From my experience the keep it wet method allows one to get more stain in with one coat.
I find that once a coat is allowed to dry, further coats don't really seem to penetrate.

So by applying as much as possible in the one coat, one can get deeper penetration and a richer colour.

G'day Guys,

Wood working is finished. I've applied 4 coats of varnish. I had some mishaps, I still struggle to do varnishing well but this time is better than my last couple of attempts.

Pre-varnish, just a light sand with a 220grit sanding pad.

1 coat of varnish

2 coats of varnish.

And here is a test fit of the front panel on the chassis post varnish

The wiring is now progressing along steadily. Nothing fancy on this build in the way of wire, just some generic multi-strand pvc light duty hookup wire form Jaycar.

I considered using magnet wire but it seems problematic to twist into pairs and mark easily so I'm sticking with the cheap stuff for this build.

Nice.
I tend to use satin or matt polyurethane - it hides a lot of flaws
I'm starting to use water-based varnish, not as hard wearing, but mostly you don't need the most robust protective coatings.

DarkLantern blog - darklanternforowen.wordpress.com

G'day Guys,

Wiring is progressig nicely. Here is my wiring scheme draft.

I'm thinking that since I have separate preamp boards for each channel I am best to ground the amp at the output of the preamp PSU.

Here is the wiring diagram for the speaker protection which I think I have wired faithfully. I have left out the AC connections in the wiring diagram.

Any mistakes, suggestions or critiques?

Looks ok. Does each poweramp board have a ground lift resistor on the input/negative feedback connection? Also add small ceramic caps 0.1uf or so from the input RCA socket grounds to the metal chassis next to them. That routes most RF picked up by the interconnect shields (which are really antennas) straight to the casing before it gets to the grounding inside.

AMR-iFi R&D

Hi Colin,

Thanks for the feedback. I will chuck the ceramic caps on the RCA socket grounds to the chassis.

Here is the schematic for the power amp boards.

I don't see anything that looks like a ground lift resistor, here is the schematic.

G'day Guys,

Progress continues on the wiring scheme.

The power amp section is mostly wired up now although I need some 4.8mm spade plugs for the binding posts.

I've started on the preamp section now.

I am taking my time, making sure the runs are no longer than needed, keeping things tidy, keeping things away from each other etc.

That must be satisfying putting the various boards you know so well together.

AMR-iFi R&D

G'day Guys,

A wee update. The amp is almost done. I have yet to solder up the rca sockets at the input or wire up the volume control but the rest is all done.

However......I decided to change tacks on this build.

I decided I wasn't happy running the chips so close to the voltage limits.

I purchased the power transformer from an Onkyo TX-SR606 home theatre amp off trademe.

These are a 550w unit with taps for 21-0-21 @ something like 6A so its higher current with a more appropriate voltage.

This amp will be doing duty driving some 4 ohm speakers, so the extra current will be nice and the lower voltage should mean less chance of clipping, hitting the protection circuit or simply overheating.

Its going to require a bit of reworking.

I suspect I will need to redo the transformer mounting and I will also need to resize the dropping resistor in the speaker protection section as I will need to drop a lot less voltage at circa 28v compared to 41v as before.

Previous me was nice and used 35v filter caps in the preamp power supply so the extra voltage on the 19-0-19 taps just gives me more headroom for regulation albeit with a bit of extra heat.

As I would have suspected, I need to drill some new mount holes for the transformer.

I am wary of drilling holes with any of the electronics present in case a metal chip or some swarf creates a short somewhere so it looks like complete disassembly is in order.

I am thinking it is also a good idea to ground the transformer itself to the chassis earth for safety's sake.

My thinking would be to scratch some of the enamel off the tx frame around one of the mounting bolts and using a couple of star washers and a ground lug under one of the bolts to ensure a good contact.

I need to change the dropping resistor in the speaker protection from the current 375R to 91R since.

Dropping from 28v psu to 24v for the 2x 12vdc relays in series gives 4v to drop.

4v @ 0.044A

R=E/I = 4/0.044 = 90.9R.

P=EI = 4x0.044 = 0.176W

I happen to have some 91R 1w resistors in stock. 1W should provide sufficient headroom for 0.2W continuous so I think we're all good.

Tomorrow I will sling some solder and get my drill going. It might be that this finally makes some noise tomorrow afternoon given there is not actually much to do.

I’d totally concur about Earthing the PTX frame. (I’m sure colinf too, has advised this elsewhere.)

To save some disassembly, maybe mask off the rest of the amp while you drill the chassis - just tape & paper maybe & thoroughly vacuum up the swarf.
Or mask off the top side & drill from the other side. Pilot holes first if necessary.

DarkLantern blog - darklanternforowen.wordpress.com

Agree, if the amp is driving 4 ohms the power supply voltage shouldn’t be higher than +/-28v. The datasheet for the LM3886 states 68w at +/- 28v for 4 ohms. A 500va transformer might look a bit overkill for a chip amp but it didn’t stop Krell and others using oversized transformers and power supply caps on their amps!
Definitely ground the power transformer, the amp chassis and the casing of the mains power switch if it’s metal. Also note that mains internal wiring has standard colours. Brown for active, blue for neutral and yellow-green for earth. The insulation thickness needs to be 0.7mm or greater.

AMR-iFi R&D

Nov 14, 2020 19:56:34 GMT 12 colinf said:

 Brown for active, blue for neutral and yellow-green for earth.

I remember a mnemonic that went something like

Brown is the colour of the dirt you'll be buried in if you touch it. Blue is the colour of the sky you'll still see if you touch it.

A bit crude, and I don't remember where it came from.

I've survived the odd HT DC shock and also a few  or 1-2 mains AC shocks.

Not pleasant, but in the moment what can you do?

One was in bare feet on wet grass reaching up to touch a faulty caravan 230V exterior plug/socket, in my mid-late twenties.

What a thrill! ;=}))

Not recommended !

Nov 14, 2020 19:56:34 GMT 12 colinf said:

Agree, if the amp is driving 4 ohms the power supply voltage shouldn’t be higher than +/-28v. The datasheet for the LM3886 states 68w at +/- 28v for 4 ohms. A 500va transformer might look a bit overkill for a chip amp but it didn’t stop Krell and others using oversized transformers and power supply caps on their amps!
Definitely ground the power transformer, the amp chassis and the casing of the mains power switch if it’s metal. Also note that mains internal wiring has standard colours. Brown for active, blue for neutral and yellow-green for earth. The insulation thickness needs to be 0.7mm or greater.

IEC colours if I recall correctly?

I've been sticking to this convention for a few years now. I typically use the 10A/250VAC stuff from Jaycar. According to the specs the insulation should be 0.825mm.

I am a big fan of oversized power supplies so I am pretty happy to go to a larger transformer.

I am always disappointed with the size of transformers in commercial offerings. They seem to be sized to provide barely sufficient current relative to the specs whilst minimizing cost. 

Same with capacitance, I am yet to open a commercial amp with more than 10000uF per rail in the main power supply, often less.

The way I see it, by over-engineering the PSU I am decreasing the chances of failure due to over heating. I am thinking the PSU should be 'stiffer', less prone to sag and therefore less likely to clip.

Now that I know about these Onkyo transformers I will be keeping an eye out for my onkyo amps to salvage. It seems like the HDMI boards on these amps are prone to failure so these amps seem to come up for salvage a bit.

I have another one that is already earmarked for my next project. $20 for a salvage amp that yields a 500VA transformer is a good deal IMO. I paid $50 for the second one but its still cheap compared to buying such a PTX new.

I also find the other secondaries quite convenient for an integrated amp as there is taps for preamps and maybe a dac stage if one wanted.

Yep, IEC colours.
I’ve received the odd shock as well over the years. That horrible tenseness and heat that takes over your arm muscles. The Voltac is on the table, I can turn it off pronto if I need without touching its metal casing with my free hand. I developed a system for avoiding shocks over the years. If I’m about to switch something high voltage on I make sure I know where I’m potentially grounded (I generally wear croc shoes around the house anyway), what I’m touching at the same time with my body and use only one arm for testing with a multimeter. I’ll fix the black lead on first so I need to move only the red lead at a time. If it’s over say 600v (like the 211 amp at 1000v) I generally wear a light glove or a sandwich bag on the hand holding the meter probe.
I like bigger power transformers as well but there comes a point at which the physical hum from a big transformer becomes intrusive for me. One of my favourite things is to switch transformers for other equipment off if they’re not needed and let your ears relax. It might be only quiet hum but for me it can re-focus my mind on hum instead of the peacefulness without it. I’m beginning to like switchmode power supplies as they are silent physically. We just need to filter out all that switching noise before it hits the audio circuit!

AMR-iFi R&D

Nov 15, 2020 20:38:40 GMT 12 colinf said:

I like bigger power transformers as well but there comes a point at which the physical hum from a big transformer becomes intrusive for me. One of my favourite things is to switch transformers for other equipment off if they’re not needed and let your ears relax. It might be only quiet hum but for me it can re-focus my mind on hum instead of the peacefulness without it. I’m beginning to like switchmode power supplies as they are silent physically. We just need to filter out all that switching noise before it hits the audio circuit!

Is it steel transformer cores vibrating coupling to a steel chassis, whether bolted down, or raised up on say rubber grommets, or is something else the cause, as discussed here?

Without permission, in pursuit of knowledge, a couple of scans from an appendix in a borrowed library book:

Just for comment.

Too complicated to go in to why the scan pic resolutions seem so different right now.

Cheers!

G'day Boys,

The amp went through the first phase of testing tonight.

The power tranny is giving me 21.2v - 0 -21.2v unloaded on the main secondary. 19.1-0-19.1 unloaded on the preamp secondary.

The main power supply is giving +/-28.5vdc unloaded.

The speaker protection is working properly.

A couple of seconds delay on power up.

Instant disconnection on power off.

Protection is triggering when dc is applied to the inputs of the protection from the main supply rails via a 100k resistor.

Now I can put the preamp psu in for testing, wire everything back up and see if it doesn't hum when I apply some speakers.

All things going to plan I will give it a round of high volume testing on the weekend while I do the weekend chores.

DarkLantern blog - darklanternforowen.wordpress.com

All running ok?

AMR-iFi R&D

IT'S ALIVE!!!!

After many months, all initial testing has now been passed.

DC offset is 2.4mV left channel and 1.6 mV right channel.

The speaker protection is working nicely, no clips pops or thumps getting through to the speakers on power off.

So far I have only plugged it into the speakers from the Boom Box for testing as there are not too important (more on that soon). With the volume turned up to the maximum I can get some hissing from the speakers with no signal present but nothing audible otherwise.

Thankfully no hum or buzz of any sort, just music coming through the test speakers.

All of my struggles with the Corona Amp a few months back seem to have paid dividends as this has worked properly first time.

Tomorrow I will put this into the lounge room system and see what the sound is like with spotify through a PS4 as the source and the venerable Miller and Kriesel Volkswoofer and S1A combo.

Front View

Top View

Left Side

Right Side

Rear View

Overall Guts shot

Amp Boards

Preamp Boards (The Mark III preamp boards recycled from my backup preamp project)

Power Supply input side

Speaker Protection and Outputs

Extension shaft through the Heatsink

The only difficulties I've had recently was the binding posts I used for the outputs were not of amazing quality. Purchased off trademe and not recommended.
I will probably replace these with something higher quality.

I also had a bit of difficulty soldering the 0.1uF ceramic caps from the input rca ground lugs to the chassis. I'm not sure if it is the large thermal mass, the mild steel or both but I struggled to get solder to flow onto the chassis.

Yes, thermal mass I'd say - detach the (chassis) lug next time perhaps.
(And/or use an iron with temperature boost for these situations.)

All that work, drilling, troubleshooting, messing with varnishing, worthwhile.

IEC mains filter sockets - I have usually used just plain (unfiltered) IEC mains sockets - but recently have encountered a learning experience with the reduction of Mains HF & RF noise, which has resulted in much better sound. More on this in a later thread maybe....

DarkLantern blog - darklanternforowen.wordpress.com

Well done!
Use solder lugs to mount the 0.1u caps to the chassis. Too much time soldering the steel with a small iron may destroy the cap and any plastic insulation washers on the rca sockets nearby, also stress the iron element if it’s a temperature controlled iron. To heat the chassis you need a large iron and lots of flux, but that might also bubble the paint on the back. Solder lugs would be best!

AMR-iFi R&D

The solder lugs would have been a good idea. I even have some in stock....

I was also considering simply using some star washers with a bolt and nut.

It took it into work today and ran it through a PAT, phase, earth bond, insulation resistance and leakage tests all passed.

This evening I did some testing at a decent volume on the lounge room system. Works perfectly and sounds great!

Nice and accurate as expected from solid state. Not too bright but plenty of detail.

I really am happy with how the brown wood ply came out. I'm glad I have some more for future builds, its a lovely colour.

Now I can finally start on the next build. Rebuilding my dual mono 6SN7 preamp.