Building an ultimate low power class A amplifier – my way

[Vunce]

Yes, this will be the first class A build with a regulated psu for me. Sort of a learning curve here 😉. I did approach this as building a ‘fancier’ CRC.

Most of my amps run Topless!🤣

 

[gary s]

Cleaning the flux residue will make anyones soldering look better😁.

Quite true Vunce, and they look even better when the back is sprayed with pcb lacquer.

Most of my amps run Topless!🤣

Does that mean Vunce, that you don't quite finish a lot of your amps when it comes to the housing?

 

[Vunce]

Well, sort of…
A proper chassis is by far the most costly part of an amp project. It’s not possible for me to buy a chassis for every build. My plank amps are actually quite nice, but I do have a few in completed chassis’s.
Even when those amps are in rotation I still pop the lid, the insides are much more interesting to look at. Plus, it’s very easy for a quick tweak as I pass by😁

 

[gary s]

That is much the same approach that I use Vunce. I will build an amp loose and then adjust, test and listen to it, then decide if it is worth an "expensive" chassis.

 

[andyr]

Great!
Winter projects are lining up.
I believe I have the parts to build one pair of R21 modules also.

Hi V ... did you get around to building this?

Andy

 

[gary s]

See post #93, with pics, I think Vunce is doing the reg psu modules for this amp at the moment as below.

https://www.diyaudio.com/community/...pply-r21-ps-module.376003/page-6#post-7530252

 

[HAYK]

Distortion
At several watt levels, this amplifier has only very low H2 + H3 and all higher order harmonics are some 30 dB down. Low distortion is maintained up to the clipping level.

What sound card do you use to have -180db floor level?

 

[Vunce]

Hi Andy,

The power supply boards are built and confirmed to work as they should, amp boards also built but not powered yet. Next step is getting the chassis prepared, drilling and tapping....UGH😏. Still waiting for power transformer.

 

[gary s]

Drilling and tapping - that is the fun part Vunce. You mention the power transformer, what sec volts and VA did you settle on for using the prasi CRC - R21 regs?

 

[Vunce]

I ordered a custom Toroidy.
Audio grade 300VA
Pri: 120V
Sec: 4 x 15V5

After I built the psu w/R21 modules and tested with a 250mA dummy load, the decision to increase the trafo secondaries from 14V to 15V5 was made. Since this build is voltage regulated, I'd rather have a few volts to spare and deal with potential increased heat than be under powered.

 

[gary s]

OK, thanks for that info, way to go with a custom build with 4 secondaries.

Are you getting it direct from Toroidy in Poland or do they have a US agent?

 

[tombo56]

the decision to increase the trafo secondaries from 14V to 15V5 was made

You forgot that first 2.5 seconds regulators forward full input voltage. 14.5 V transformers were perfect. With 15.5 V secondaries and 300 VA audio grade Toroidy, you will get 21 V at start. OPA828 is specified for +/- 20 V operation, so short +/- 21 V supply is not a problem.

However, there is no reason to have 3.5 V drop on regulators. They provide full performance with only 0.3 V headroom. Only if mains voltage variation in your area is larger than -10%, then it would make sense to use 15.5 V secondaries.

 

[tombo56]

What sound card do you use to have -180db floor level?

It is Cosmos ADCiso which has floor about -165 dB. Additional 15 dB is added to any sound card using REW option “coherent averaging”. For correct distortion numbers with coherent averaging, you have to use either latest beta build or use “show phase of harmonics” along with coherent averaging for older REW versions.

 

[gary s]

If anything like here, the mains supply voltage is always higher. Our nominal mains single phase AC supply is 240VAC, but here at our house it is running constantly at 248VAC give or take a volt.

 

[tombo56]

Great power conditions. In your case 2 x 14 V is perfect for build with voltage regulators.

 

[tombo56]

gary s had good proposal in another thread.
Use higher resitor values in opamp supply and use 19 – 20 V for amplifier. It would provide several extra clean W at output.
That way 15V5 secondaries would be fine.

 

[Vunce]

Squeezing as much power out of Bliss was my motivation for the transformer secondary increase.
To keep the opamp happy the supply resistors will be increased.

 

[poseidonsvoice]

@tombo56

Thank you for the design, I’ve been lurking on the sidelines…

Can the builder pluck R7, R8 and use a tightly regulated +/- 15V supply for the opamp while using a well filtered (higher voltage bipolar supply, i.e. +/- 25 to +/- 35V) for the driver and output stage? As long as heatsink calculations are considered due to higher dissipation by the ECW Laterals, this should be possible, correct?

Best,
Anand

 

[tombo56]

Yes, it would be fine regarding any supply voltage and max. allowed opamp supply. But, it would not allow higher output power than with 19 – 20 V. Output stage is diamond buffer which is input follower. OPA828 max. output voltage is its supply -1 V. We can have about 11 Vrms or 15-16 W/8Ω.

Additional rail voltage also reduces distortion at max power as ECW there need some 2-3 V Vgs for required output currents.

 

[poseidonsvoice]

Thanks tombo! So I will limit voltage rails to +/-24V for the driver/output stage since there is no benefit in output power due to the design topology and max swing the OPA828 can provide. I’m sure if I want, a regulated +/- 24V for the driver/output stage will also be of benefit since rail sagging won’t occur at max output power either.

Now I understand why you designed it this way!

Best,
Anand.