G'day Chaps,

A fresh project to keep me busy during these strange times.

I found myself in a phone conversation with the boss the other day. He was expressing an urgent desire for a half decent amplifier. Low cost and fast.

After much head scratching I hatched a plan......

It had to be parts that were cheap...

It had to be parts I have on hand due to the frickin lockdown.....

It also had to be parts I don't intend to use in something else important.

So:

For the preamp section I would reuse the guts of the Backup Preamp Mark I. The old Jaycar PSU and preamp board complete with fake log pot.

For the power amp section I would use some Ebay LM1875 kits I purchased for all of $3 earch iirc.

Maybe some upgrades from the parts bin over the dubious items in the kit but using the chips that come with the kits.

For the main PSU I would whip up a simple CRC supply on some vero board out of the parts bin.

For the power transformer I would use the power transformer out of the Sony HCD-H405 midi system that featured in another thread where its outer chassis was repurposed for sheet metal.

The power transformer was currently unused and virtually free. I assumed it would have all of the appropriate voltages for what I wanted to do. I also decided I could reuse the heat sink from the Sony also.

After much digging I managed to find a service manual for the amplifier. Nowhere could I find any mention of the voltages coming out of the transformer. But I could at least find a circuit diagram for the windings so they could be safely measured.

 

After measuring the voltages I had a small dilemma. The secondaries were 35-0-35, 15-9-0-9-15 and 0-3.3. All what I was hoping except for the high current secondaries....

A genuine LM1875 can theoretically handle +/- 30V but not the +/-50V that was going to come out of any PSU attached to 35-0-35 secondaries. The chips that come with these kits are certainly not a genuine LM1875. If I was lucky they would be a UTC TDA2040 which can only handle +/- 20V max, at worst who knows....

I spent a fair while trying to figure out how to get rid of enough voltage to maybe use some genuine LM1875 chips I have in stock.

I fiddled with a CRC

I flirted with a CRCRC

maybe a even CRCRCRC

Sheer impracticality aside. The cap cost was mounting and I simply don't have enough high power resisters of appropriate value on hand during lockdown.

What to do?

A good nights sleep and I had the answer!

I had been planning to test out my LM1875 Big Cap boards in the Gainclone Boombox which would leave with with a set of Jaycar LM3876 boards surplus.

A genuine LM3876 can safely handle a lot more volts!

I fiddled around some more with vero board and a CRC supply trying to get the voltage into something like reasonable limits, when I decided if I am going to waste power dissipating voltage I may as well do so during regulation.

The sigma22 can work off a centre tap unlike my LM338 boards which require dual secondaries. It also has double the heat sink for high current demands. Higher performance to boot!

I purchased a Sigma22 kit off Ebay a year or 2 back but I never got around to using the thing. Some of the parts supplied were dubious looking and I had concerns about the correct functioning of the non-genuine board.

www.ebay.com/itm/Sigma22-Linear-Regulated-Power-Supply-PSU-kit-for-Preamp-headphone-amp/322436678247?_trkparms=aid%3D111001%26algo%3DREC.SEED%26ao%3D1%26asc%3D20160908105057%26meid%3D194e31500e964903ba4641d7e222e27c%26pid%3D100675%26rk%3D1%26rkt%3D15%26mehot%3Dnone%26sd%3D322436678247%26itm%3D322436678247%26pmt%3D0%26noa%3D1%26pg%3D2380057&_trksid=p2380057.c100675.m4236&_trkparms=pageci%3Ae110f47a-8121-11ea-8cba-82ec941e5128%7Cparentrq%3A8b424ac41710a4b7b739d716ffdc2a98%7Ciid%3A1&autorefresh=true

Since I have my own boards for this schematic brewing based off of the original design, I figured I could use this kit with a few bits replaced here and there without missing the parts later on.

I ended up using all of the parts from the kit. Output was set to +/- 36V

I replaced the trimput with a fixed 5k resistor for 36v operation.

I replaced the supplied .47R 2w cement type resistors with some 3w Metal oxide types for the ballast resistors.

I replaced all of the supplied 220pF caps as they appeared to be film types whereas ceramic types are specified in the original.

I had to use a pair of my good RN55 10k resistors due to losing some of the supplied generic types under the workbench.

The kit never seems to have come with filter caps so I used a pair of Nippon Chemicon 63v 10000uF caps I got cheap on trademe some time back

I do seem to have lost an extra 1.1v compared to what was intended however this is better rather than worse as +/-35 seems to be ideal for an LM3876.

The rails are rock steady at idle, no movement on my DMM. I am very happy with the symmetry, both on 34.9v.

One side note is that the LED seems to be running rather bright by my estimation. It is dropping 3v which is not what I intended, I had hoped to set the dropping resistors correctly so that the LED was dropping 2.2v @ 20mA.

My guess is that the LED is drawing 18 or 19mA instead of 20mA which drops the voltage dropped by each dropping resistor. Each is a 2.2k and a 10k in parallel to create a 1803R 4w resistor as I had no 1.8k of decent wattage in stock.

Is this likely to be an issue?

I don't know the model of the LED but it appears to be a standard green LED.

Nice!
Where is the Led in the schematic? I’d run it at far less current to make it as bright as needed. Most modern leds are bright enough on 2mA.
Or is it being used as a voltage reference in place of the zener diode?

AMR-iFi R&D

Hi Colin,

Thanks for the feedback.

The 12v zener diode is the reference for the error amplifier.

The LED sits across the + and - rails at the output of the PSU.

+/-35v at the output so I had to drop 33.9V across each dropping resistor to make what I assumed would be a 2.2v drop over the LED.

Should I be upping the resistance a bit?

G'day Folks,

A bit more progress after some setbacks on the chassis work.

I took a bunch of materials into work on yesterday morning to use the drill press for some nice perpendicular holes (unlike my best efforts with a cordless drill).

I drilled a few extra holes in the base plate for mounting components.

I also drilled out the coarse threaded holes on the flanges out to 3mm for tapping with m4 thread. These will be the attachment points for the wooden enclosure.

I drilled out the top section with 10mm holes on a 30mm grid. Mainly for ventilation but I think it will look quite nice at the end.

The cheeks or side panels is where things went awry....

My first error was to not mirror the two sides properly. I only had cheap C/D grade 18mm ply on hand which means one face is ok at best and the other side is scabby and riddled with filler. I simply failed to account for this at all. =

Thus when I went to test fit the enclosure I discovered that one side had the scabby side out with all of my markings etc on display.....

My second error was 1 hole on each cheek being 5mm out in the horizontal plane. I am not certain whether this was a calculation error, a measurement error, warping due to my lousy panel beating or perhaps because I measured before panel beating....

My third error came to do with counter sinking holes so that the m4 bolts I have on hand can be used. I figured it would be an easy job to do at home with the hand drill so I didn't bother to do this on the drill press.....

When it came to countersinking time, I discovered that with an 8mm twist drill bit and a cordless drill, that it is quite difficult to do a good job of countersinking. One hole was completely drilled out to 8mm when the drillbit took a nice deep bite....

So I had to carefully use what plywood I had left to make a new pair of cheeks.

This time I was more careful with my cuts as there were a little bit wandery last time. So I pulled out my rip cutting guide rail and took the extra 5 minutes to setup the slide compound mitre saw for the right angles.

This time things came out a bit tidier.

I was very careful to mirror the 2 cheeks properly. Even being careful I almost slipped up twice.

Today I went back in to work for another go with the drill press.

I made sure to take the time at work to do the countersinking neatly and without any drilled out holes.

The 2 holes that were 5mm out last time lined up perfectly. I took a bit of a risk here but not re-measuring I simply accounted for the 5mm.

Viola!

Front

Front 3/4

Inside

I still need to chisel out a small rectangle of the left side on the front panel for the power switch.

I haven't settled on the back panel layout yet.

I will need to drill holes for a pair of rca sockets for input and some speaker binding post for output.

I will also need to chisel out a socket for the IEC socket.

I am reusing the primary side fuse that comes mounted with the power transformer as this saves me a small bit of bother.

I always struggle with wiring nicely around fuses however I am yet to find a IEC socket with both fuse and line filter that doesn't cost far too much.

I've been beavering away quietly on this for about 2 weeks now so let me introduce some more of the challenges.

Space is rather tight so I have had to think had about how to use 3d space effectively to keep things as far away from each other as possible.

Many hours have been spent both scratching head and rubbing chin to figure out:

- Where things should live

- How said things should be mounted

- How to mount the mounts

Due to a chronic shortage of wood screws under 25mm length I have had to be very careful about where I use wood screws for internal mounting.

So here is my solution regarding the mounting of the power amp boards around the heat sink. I chose to use the original heatsink for the chassis minus the fan which I hope to avoid using. I am also mounting the heat sink in almost the original location. I moved it about 20mm towards the right side of the chassis as the stock location was very close to the power transformer .

The original amp stage in here was a massive chip the likes of which I didn't know existed. A Sanyo STK4211. Supposedly 2x 70w

Mounting 2 LM3876 boards on a heat sink designed for 1 massive IC required mounting them vertically as well as grinding off some lugs on the mating surface of the heat sink (I guess these were locating lugs for ease of mass assembly?).

I used a spare piece of vero board and a small right angle bracket to make a vertical centre mount for the 2 amp boards.

The next challenge was mounting the relay speaker protection board. 

This one probably gave me the most trouble as there is very little space between the outputs of the amp boards and the back panel.

I considered mounting it to the side but this drastically complicated the wiring from the input.

Firstly because it put the board right where the wiring should go. But more importantly because the relay board requires an AC connection and I don't want AC wiring any where near my input wiring.

I considered mounting it to the back panel but space is very tight and this probably would have effected heat sink function. It also complicated and lengthened the wiring to the speaker jacks.

I considered various schemes of mounting the board in the vertical plane around the amp boards but it always got in the way of wiring.

I considered various schemes to use the space at the front above where the preamp board will live but once again it got in the way of wiring and I don't want the AC of the relay board near my preamp section.

In the end I figured out that I had created a nice flat surface across the edges of the 2 amp boards and the centre mount. I could use this as a base for another bracket in the horizontal plane across the top of the boards and over to the right hand side.

This meant I could keep the AC wiring as far away from the signal stages as possible.

It allowed me to run the input wiring under the bracket keep it all far away.

It also tidied up some issues with the wiring around the amp boards.

So here we go:

Another small right angle bracket added to stabilise the relay board bracket

Notice the small right angle bracket on the back side to stabilise the bracket against the side of the enclosure. While writing this post I have come up with a better solution involving a small rectangle of wood underneath the bracket to stabilise it against the enclosure that I will try at the next test fitting.

I find that chassis work & chassis planning is highly time consuming - but most important for good signal paths & for future servicing access!

DarkLantern blog - darklanternforowen.wordpress.com

G'day Owen,

Yeah, I agree.

I would rather spend the time on it now and do it properly than deal with the downstream rubbish of poor planning.
Ask me how I know about that one...

G'day Guys,

Another headache in this build has been the issue of where to place the preamp PSU.

I spent a while with the thinking that I could mount it in the space above the preamp board, maybe near the top of the enclosure to maximise distance....

Then I remembered the darn volume pot. This will be dead centre in the space above the preamp section.

I decided this wasn't going to work as this would put the PSU too close to the sensitive input wiring and also create a bit of a wiring headache.

So this had to go in the last remaining unoccupied space in the enclosure: above the main PSU board.

This then meant I had to come up with some kind of bracket. Careful attention would also have to be made to ensure I leave enough space for the power switch wiring which would be in the vicinity.

I managed to re purpose this guy from the original Sony. It only required a bit of bending and drilling to make a mount for the preamp PSU board that screws directly into the wood above the main PSU.

The PSU hands underneath the bracket like so

Here's a top down shot showing how it fits.

My fears of the bracket being a bit flexy and wobbly proved true so I had to add the m3 standoff from the right hand side of the enclosure to stabilise the bracket in an extra dimension.

Always good to keep the AC PSU bits as far away as poss from the low signal parts.

DarkLantern blog - darklanternforowen.wordpress.com

Afternoon guys,

Work has been continuing.

I have now completed all of the hole drilling, socket chiselling etc in the metal base plate and the wooden enclosure.

Power entry, I/O, power switching and volume control have all be accounted for as well as mounting everything and some extra holes and slots drilled in a haphazard fashion around the main PSU board for extra ventilation.

Barring any surprises, that's it.

I left the back panel till last as I wanted to figure out how I was going to solve the speaker protection location before committing to a back panel layout.

Having finalised that back panel, I've started with what amounts to final test wiring. I can now do all those little things to make sure the thing works properly.
I will assemble the amp completely for testing and then I will take it all back apart to finish, prep and stain all the woodwork before final reassembly.

I might even do something about the scabiness of the base plate. Maybe....

I decided that the simplest way to mount the I/O jacks into the plywood chassis was to use some simple small metal plates harvested from the old Sony chassis.

I mounted in the Ins and Outs separately as it may simplify things later down the road. Not the prettiest solution, but perfectly serviceable. Its at the back so hopefully nobody is gazing back here.

One particularly fun bit was the issue of the IEC socket as there was not a lot of clearance with the power transformer.
I had to put it as hard against the chassis as possible to maximise distance from the xformer.

The input wiring is now complete

A single set of inputs makes things nice and easy. No switching to worry about.

The volume pot was mounted upside down to simplify wiring and access. I like to keep signal wiring symmetrical in length between channels hence the little bit of laxity.

The trusty jaycar preamp board also known as the Backup Preamp Mark I. This was the most upgraded version with an NE5532 before I moved to the dual stage LM49720 design for the mark II.

The next step is to finish the amplifier stage.

G'day guys,

More progress.

The mains side wiring is all completed.

I ended up having to solder the wires to the back of the power switch. I would have preferred to use spade connectors like usual but this would have pushed the wires out too far towards to PSU heat sinks. By using soldered connections I could ensure vital clearance.

Here is how the wiring worked out underneath the power supplies.

Add the main power supply

And then add the preamp power supply

Finalised safety earth.

Finalised I/O

As you can see, space is very tight. Cramming everything necessary inside and then wiring it all up in such a way that minimises potential interference has been quite a challenge.

Tomorrow morning I will give everything one last poke over with the DMM to check everything is wired properly and then I will fire it all up for testing.

After that I get to do take everything apart again for finishing and staining on the woodwork.

G'day Guys,

I hit a snag today.

I did some final tests that everything was connected properly, hooked up the mains via and RCD and powered up.

No smoke. Supply voltages read correctly. Relays triggered correctly.

Power down and added some test speakers.

Power up and as soon as the relays triggered the left channel began jack hammering.

I've not seen such behaviour before but the left speaker is thumping downright violently.

I've cycled up and down several times to isolate the source.

I first bypassed the relay protection board considering this to be the most likely source. Not here

I then isolated the amp stage from the preamp stage. Not here either.

The issue only effects the left channel which leads me to think it won't be a PSU issue as this would surely effect both channels.

I am not ruling out a grounding issue but I think this is unlikely due to the issue only effecting one channel.

I've since disassembled the amp so that I can easily test things one bit at a time.

I did not test the boards before assembly as they were last working perfectly in the Boombox several weeks ago.

I suspect something has been damaged on the left hand amp board during all the handling involved in construction.

Does anybody know what that jack hammering is? Oscillation?

Any suggestions as to the likely location in the circuit?

For reference here is the schematic.

Psu schematic as well just in case.

Hi Sadface, did you manage to get it sorted?
I’d disconnect the pre from the poweramp and see if it still oscillates. Motorboating sounds like oscillation. Check the zobel network (2.7 ohm and 0.1uf), and the power supply caps (220u and 0.1u).
Oscilloscopes are very handy to fault find!

AMR-iFi R&D

Hi Colin,

I haven't sorted the scope yet. It's on the horizon.....

I let the project sit for a couple of days but I attacked it again yesterday.
I started by tidying up some wiring and fixing a grounding error: I had taken the return paths from the speakers back to the chassis earth star instead of the 0v output of the power supply.

Today I measured the amp boards outside of the enclosure one at a time. At the output: DC was 0.5mV and 0.3mV. Ac was 4mV and 3.6mV.
Grasping the bare end of the + input wire between my fingers I get a jump to about 200mv ac in both channels which seems about right.

Time allowing, I will reassemble everything again tomorrow and then see what I can pick up at the speaker jacks.

G'day Guys,

Some good news.

While everything was apart I took the chance to lower the current on the power indicator LED so its now at 3mA and not so overly bright.
I also took the time to redress some leads here and there that I hadn't gotten too before.
Finally everything was reassembled back into the enclosure.

Everything fired up and measured well so I connected some speakers and hey presto, the amp is no longer jack hammering the speakers.
So I have learnt a bit more about how not to do grounding. Unfortunately, not enough as there is a fairly noticeable 50Hz hum.

Two steps forward and one step backwards....

A USB scope is on its way from China so if this drags out for long enough I will have some proper tools at hand....

In the mean time I might take "working" unit in to see the boss and see if we might be able to fabricate a larger and nicer looking enclosure now that the lockdown is sort of over. He's fairly hand with sheet metal and a welder so maybe just maybe....

Good detective work, well done.

Is your PSU/signal Ground 'lifted above chassis/mains Earth?

DarkLantern blog - darklanternforowen.wordpress.com

G'day Owen.

No I haven't lifted the ground at all.

Here is a wiring diagram.

A few notes:

The rca sockets are grounded through the volume pot and preamp.

There is no ground connection from the preamp board to the power amp boards. Hot wire only.

I have not returned the -ve speaker binding post back to the amp boards but back to the 0v at the output of the psu.

Only the hot wire passes through the speaker protection board.

I am considering 2 approaches to the hum right now.

a) A rework of the wiring.

The ground return of the protection board and the psu wiring for the amp boards run about an inch away from the big EI power tranny. This seems like a very likely source of 50hz hum.

b) Lifting the grounds, possibly a loop breaker.

In the mean time. Maybe somebody can spot a flaw in my wiring scheme.

It looks like the preamp-amp signal bds are Gnded out through the output protection bd only?

Conventionally, the signal section (preamp-amp) is Gnded near the signal input end (& in close proximity to Chassis Gnd-Earth pt) - not the amp output end.
(Why? My understanding is that this keeps internal Gnd 'loop' shorter/smaller, to minimise 'antenna area & hum pickup.)

In your case, the chassis is wooden, but the Gnd (star) pt would normally still be near the input sockets.

PS. The PTX has a screen? Maybe Gnd the TX frame at least.

DarkLantern blog - darklanternforowen.wordpress.com

Mains earth should go straight to the chassis first for safety, then the input socket rca grounds (via a lifting resistor if needed). An alternative to this is to connect the mains earth to the casing, then to the power supply ground, and have some small capacitors tying the two rca socket grounds to the chassis right next to the sockets.
The power supply ground for the preamp needs to be separate from the poweramp power supply ground. If they’re joined on the power supply pc board, separate the two so that the two transformer grounds are separate. Connect the preamp ground along with the signal to the poweramp boards. Speaker grounds go to the center of the two poweramp power supply caps, right next to (and at the same point as) the same ground reference for the poweramp board.
There are variations to this though, designing grounding can be tedious!

AMR-iFi R&D

G'day Guys,

Many thanks for the feedback.

I tried to make things as clear as possible in the wiring diagram.

Input signal ground -> volume pot - > preamp board
The preamp board ground -> preamp PSU -> Chassis Earth
Power amp board ground -> power amp psu - > chassis earth
protection board ground - > chassis earth

I originally had a separate grounding wire from the chassis earth start to the power transformer however I removed this to see if it effected the hum which it did not.
On the original Sony donor amp it was a class 2 (2 pin power plug) device with no chassis earth so I decided the extra earth lead was unnecessary. I have continuity between the transformer and the chassis earth via the chassis anyway.

Sure, but it was tested to class 2 safety standards, a diy build isn’t. I‘d still use the safety earth, especially for an amp you’re making for someone else. Safety is good!
With the grounding as it stands you’ll get poor grounding for the signal between the pre and power boards. The poweramp input stage negative feedback resistor needs to be at exactly the same potential as the ground reference for the preamp (it’s feedback resistor and rca socket grounds) and you can do that by wiring them together without the influence of other polluting ground sources such as the power supply earths.

AMR-iFi R&D

May 14, 2020 19:19:09 GMT 12 colinf said:

Sure, but it was tested to class 2 safety standards, a diy build isn’t. I‘d still use the safety earth, especially for an amp you’re making for someone else. Safety is good!
With the grounding as it stands you’ll get poor grounding for the signal between the pre and power boards. The poweramp input stage negative feedback resistor needs to be at exactly the same potential as the ground reference for the preamp (it’s feedback resistor and rca socket grounds) and you can do that by wiring them together without the influence of other polluting ground sources such as the power supply earths.

Thanks for clarifying regarding the transformer grounding. Safety first.

My original thought was that best practice would be to earth the shielding of the power transformer. Somehow, I talked myself out of the idea once it started humming. I will correct this.

I am a bit confused as to what you are suggesting regarding the grounding between the preamp and power amps.

Lets see if I can clarify.

The preamp psu is on a separate pcb to the power amp psu.

Both psu pcbs have their own separate connection back to the chassis earth.

There is no connection between the two psus other than the termination of their earths at the Earth star.

If I have understood you correctly, you are suggesting that I run a ground connection between the preamp and power amp boards.

I understand why the ground references between the stages need to be same. However would this not result in a large ground loop running in a circle from the earth star > preamp psu > preamp board > power amp board > power amp psu > earth star.

This was my thinking for omitting the ground connection between the stages and only running the hot wire.

Many Thanks

Sadface

Yes, a ground loop would result. That’s why you need to disconnect the two psu grounds from the chassis ground. Make the respective psu board grounds go straight to their respective audio boards. Run only one earth wire to the chassis ground from the preamp ground.
If anything connected to the input rcas is earthed as well you could also get a ground loop. So the single ground wire connecting to the chassis earth inside your amp could use a 100 ohm resistor to break the potential ground loop. Make it a wire wound power one, from experience they can receive a static jolt that can burn out a small resistor under certain conditions.
😀

AMR-iFi R&D

For this 'Ground break', I've been using lately a 10R w/w 5W in parallel with 0.1uF Class X cap.

DarkLantern blog - darklanternforowen.wordpress.com

G'day Colin,

Many thanks for your patience, I understand now.

Regarding the relay speaker protection board.
The board has a single ground plane across it's psu, controller and both channels inputs and outputs.

I think I would be correct to have only the hot wires coming out, however should I be running both the hot and ground wires from the amp boards to the protection board such that the protection board is grounded through one or both of the amp boards?

Run the speaker terminal grounds to the psu board at the same point as the ground reference for the amp boards. The protection board ground can go there too. The output from the amp boards can be connected through the protection relay as normal. If you have separate power supply boards for each poweramp channel, connect the protection board ground to one of them.
Grounding is fun! You need to be a signal integrity detective (SID).

AMR-iFi R&D

G'day Guys,

One step forwards, one step backwards.....

I rewired everything again this evening as per the suggestions.

Power amp PSU disconnected from chassis ground,

Preamp psu discconnected from chassis ground.

Added ground wires from the preamp to the power amp boards.

Single chassis ground now comes from the preamp board.

The protection is now grounded to the power amp PSU.

Power up

All the leds came on

Relays turned on after the regular delay.

Power off and I can smell burnt silicon.

I will leave everything over night before I start taking things apart. However my nose is drawn to the protection board.

My sneaking suspicion is that I might have damaged the power transformer as I get continuity between the 35v taps and the 15v taps.

I am pretty certain I did not have continuity between these before.

Also, a variac would be good in addition to the oscilloscope, so you can turn it on at low voltage so you can spot a problem before it kills something at normal voltage.
Do the 2 power transformer windings have centre taps, and are they connected to ground? If so that’s where the continuity would be coming from between the two windings.

AMR-iFi R&D