ThermalTrak based Amplifier finished

[Arius]

Hello,

I'm happy to say that I've completed one channel of my OnSemi ThermalTrak amplifier. It is essentially a modified Leach clone (many thanks to Jens).

The amplifier is completely modular with onboard PSU and protection. Just plug in the transformer, signal input and speaker outputs. It measures 13.3" X 2.95" - the goal was to make it a compact module to enable me to build a 5-channel amplifier.

Picture:
(sorry, both the camera and my camera skills suck)

[Arius]

Here is a picture of the bare PCB...

[Arius]

I don't have access to a distortion analyzer so with a HP 33120A Function Generator and an Agilent Infiniium scope, I took some measurements.

Some scope waveforms....

Yellow = Input signal
Green = Output signal across dummy load
Purple = PSU +ve rail

[Arius]

Another scope waveform:

[Arius]

Schematics attached....

[Arius]

I must add that I haven't been able to fully characterize electrically the Vbias circuit based on the ThermalTrak diodes. It works but so far, I can't tell if it is doing a better job than a plain old VBE amplifier transistor.

I have access to a thermal chamber for testing electronic products but I'm not sure if heating up the entire module and measuring Vbias proves anything. It's the junction temp vs Vbias that really interests me.

Other than that, the module's was completed yesterday and it sounds very good. Can't say too much as it is hard to compare a mono module vs my stereo hifi setup. Work is going slow on the 2nd module...

[BlackMTX]

Nice PCB layout,compact...

[clem_o]

Hi Arius,

Why not just measure idling current before and after giving the amplifier a 'good exercise'? If the idling current dominates over the rest of the amplifier, that should be a pretty good test. Most amplifiers drift all over the place, specially from startup when everythng's cold...

Cheers!!

[acenovelty]

Hello Arius,
That's a really fine lookin' layout. Maybe you would be willing to tell more about the circuit and construction details. Like the purpose of a relay on the board and other small details.
How does the bias adjustment compare to a Leach board and so on.
Inquiring minds want to know.

Regards

[Arius]

Hello Ace,

Thanks for your inquiry. Um, I did post the schematics here so all the details are there.

The relay is for DC fault, turn on/off, overtemp protection. Bias is adjusted just like a regular Leach or any other regular Emitter-Follower class b amp.

There's really nothing special about this amp other than the ThermalTrak's diodes reacting faster to temp changes. Like Clem suggested, I think I'll use my test setup and 300W dummy load to run the module through various levels of input signal and see how the Vbias tracks temperature changes. It won't be accurate as I'll be measuring Vin vs Vbias instead of Tjunction.

BTW, my toroidal transformer is Antek Inc's 700VA type. It has pretty good load regulation. Rail voltage dropped from 53V to 50V at full load. With 50V rails, I get Vout = 44V before clipping so that's 120W into 8R. Heatsink is a Conrad MF25-75.

The PCB is double sided with ground planes. Noise/hum is inaudible even with the input floating. The purists probably won't like to see the PSU circuit onboard, but my goal is for a compact, modular amp to enable 5 channels - minus the wiring nightmare. To remove one module from the chassis, there are only 3 connectors to unplug (4 if one chooses to bring the status led to the front panel) and 2 screws.

If anyone wants any detail, just ask away. Just keep in mind I'm not an amplifier expert. Just know a bit here and there. I'll start work on the AC soft-start/soft poweron board next.