Aleph J new build testing

[Wang1]

Just carrying out some preliminary bench testing of PCB's while waiting for chassis to arrive.

Power supplies are fine, however when testing amp board i cannot set offset and bias according to the guidance notes.

Adjusting r27 seems to have no effect at all, but by adjusting r7 i can set voltage to required value (anywhere upto 400mV).

Q7 and Q8 get hot while Q5 and Q6 stay relatively cool.

However as i understand r7 is for adjusting offset, but when i measure voltage across signal output/gnd i get full +ve rail voltage ie aprox +26v, and does not change when i twiddle trimmers.

any advice appreciated

 

[Zen Mod]

-ref schematic, post it
- you can't do that without proper heatsinking; except if your goal is to kill those mosfets

 

[Wang1]

i know, i am just doing short test see nothing smokes, i was keeping close eye on temperature of mosfets

 

[Wang1]

ok panic over for the moment, a little more adjustments between r7 and r27 and i have the offset down to 100mV with a bias of 200mV, will leave it like that until installed in chassis.

 

[beeah]

It just so happens that I just buttoned up my fresh Aleph J build today, and am only a couple album sides in with it in my main rig.

Until a few hours ago, I was getting quite frustrated and discouraged over the fact that I could set the bias (that is, once I found a post that convinced me to be less timid with pot adjustment) but trying the zero the offset was like playing whack-a-mole - the tiniest turn of the pot would send the values flying off in the opposite direction without warning.

Today I found a ZM post suggesting that trying to dial in the offset was going to be unsuccessful until the chassis was closed up and the amp allowed to run for a while, such that some thermal stability could take hold.

And what do you know? It worked! I set the bias where I wanted it, attached the remaining panels to the chassis, and let it bake for a spell. I was then able to - quite easily - zero the offset for both channels. Like 0.000, not ballpark.

I’m starting to think this Zen Mod guy might actually know what he’s talking about…

 

[KevinHeem]

...it's all smoke and mirrors, don't believe a word he says.

 

[Zen Mod]

 

[Wang1]

Hello beeah,

thanks, makes complete sense, i will wait to do further testing, too impatient 🙂

 

[Wang1]

Now my chassis turned up, (ordered from Aliexpress), i was a bit dubious ordering from China, first time I used Aliexpress, took a while but arrived in perfect condition and well pleased with it, very good build quality.

Anyway, all installed (except XLR sockets), and now in the process of burning in and testing, all good so far, bias and offset spot on and no drift after coming to full temperature.

Some of the wiring is temperary at the moment, (waiting for some hook wire on order), but my question is concerning earthing, my thoughts were to just put an earth post in the centre of the chassis base somewhere and run all earth wires form the boards and the signals directly here in a star earth config and then connect to mains earth, but would apprectiate any thoughts.

I will post a more detailed build with photos later.

Thanks

 

[beeah]

Great news! Patience and perseverance pay off.

My amp is presently playing music so I can’t double-check at the moment, but IIRC (hey, it’s been a few weeks) the only things connected to my chassis star ground are the PSU board, purple shield wire from Antek power transformer, and a 22 ohm thermistor tying the star ground to the mains ground.

Perhaps I’ve done it wrong, but my amp boards themselves are NOT tied to the star ground. Come to think of it, I should probably ground my negative speaker terminals…

 

[Zen Mod]

Now my chassis turned up, (ordered from Aliexpress), i was a bit dubious ordering from China, first time I used Aliexpress, took a while but arrived in perfect condition and well pleased with it, very good build quality.

Anyway, all installed (except XLR sockets), and now in the process of burning in and testing, all good so far, bias and offset spot on and no drift after coming to full temperature.

Some of the wiring is temperary at the moment, (waiting for some hook wire on order), but my question is concerning earthing, my thoughts were to just put an earth post in the centre of the chassis base somewhere and run all earth wires form the boards and the signals directly here in a star earth config and then connect to mains earth, but would apprectiate any thoughts.

I will post a more detailed build with photos later.

Thanks

is there any thermal goop, considering that you're using mica insulators, for output mosfets?

 

[Wang1]

is there any thermal goop, considering that you're using mica insulators, for output mosfets?

Not yet, temperature of heat sinks stabalised between 45-50 degC , i left it for 2 or 3 hours with the lid on, only thing is as the temerature increases, so the bias decreases slightly then i have to increase bias a little to compenate.

Is this normal?

 

[Wang1]

BTW, this was how i intend earthing

 

[Zen Mod]

each channel needs to have own NTC going to chassis/Safety GND

you need to use thermal goop with Mica insulators, pronto

 

[Wang1]

thanks, will do.

 

[Ben Mah]

Seems like an opportunity lost for dual mono when each channel has its own transformer and power supply and then the audio grounds for both channels are tied together.

 

[Extreme_Boky]

Audio grounds (common returns) have to be routed back to a single point of DC reference - otherwise, there wouldn't be any current flow through input transistors...

However, those paths have to provide as low as possible impedance paths as well (which is easily overlooked)..... so that the noise does indeed couple to the ground while ensuring each return has its own return path. Low ground currents (input stages) may require a slightly different approach, but high return currents (voltage rails -> MOSFETs -> speakers... and then back where it all started) do require a separate return path for the best results. Also, nelson has posted previously... a screw depicting correct grounding... he showed us how to tie everything together at that common point.

 

[Ben Mah]

Some good amplifier wiring advice from diyAudio member Bonsai:

https://www.diyaudio.com/community/attachments/dual-mono-wiring-pdf.937434/

https://hifisonix.com/wordpress/wp-content/uploads/2019/02/Ground-Loops.pdf

https://www.diyaudio.com/community/threads/wiring-up-dual-mono-amplifier.370184/

It has worked for me. My preamp and amplifiers are silent powering my 103dB speakers - no hum at all with ear to speakers.

 

[Wang1]

here is the grounding, note although i have dual power supply tx and boards, the mains power is common IEC.

Chassis grounds direct to earth pin on iec.

Each channel psu board gnd and signal board gnd are connected and led the IEC ground via their own CL60.

When powered up all is stable and temeperature of MOSFETS around 45 to 50 deg c, with 400mV bias, (i changed the insulating pads to silicon).

Just waiting for XLR holes to be drilled in the back plate, then will connect all signal wiring etc, and try it.

 

[Zen Mod]

take care that each Donut is having own separate primary fuse