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Once I finished the prototype I started testing the amp 5.. It immediately turned out that all the electrical signals were OK, and it was possible to control the DC on the output to values below 5mV. I only have my doubt about the mute circuit, as switching from mute to active still gives a "thump" on the speaker terminals.
I did not solder the capacitors supplied that should control oscillations of input and output, and even without these the amp already behaves quite good. I learned from my gainclone experiments that oscillation, even if one cannot hear it, can cause the amplifiers to get very hot. Therefore it is a good idea to experiment with small 100pF and 1000pF capacitors on input and output.
During initial testing, I noticed that the voltage regulator for the 5V rails and the Tripath chip itself become quite hot. So this was reason enough to use heatsinks on those places. The rectifier bridge doesn't become hot at all so we can forget about that one. When building gainclones, I used an aluminum profile for cooling. And given that this amp just like gainclones doesn't really need much cooling under normal operation, I might as well use the same technique.
Although I followed the building instructions quite close, I am not satisfied with the overall gain. Of course the amp is built like a power amp, but still I think the gain is too low for many applications. But after connecting it to other outputs I found out that it can sound rally powerful also. It may have to do with the (inverted ?) nature of the design. Changing the values for R43 and R44 will solve that problem. Jan already included some optional resistor values in the kit, it is just that for mobile devices (iPods, Phones) but also for some CDplayers the "standard" gain of the amp may be on the low side.
After doing the basic electrical test I did a "burn-in" for two days to see if it was completely safe to listen to this amp. As you can see, I used my mission speakers as I like the sound of these relatively cheap speakers a lot and they're my reference for my DIY projects.
The picture above shows some initial setup using an iPod as an input. However the gain is too low for listening on a reasonable volume level.
As you can see, I already drilled an additional hole for another pair of RCA inputs. That is because I was not sure whether or not to transform this amp into an integrated (volume control) or even add a complete pre-stage to the amp. Also, I'm not so happy with the speaker connectors used. That's why I used bananas to connect the cable.
Meanwhile (it's year 2011 now), I'm finally looking into modifying the amp to an integrated amp with volume control and input selection. The modes will be described on the next page..
I like the sound of this amp, although without a little more gain it's practical use around the house is somewhat limited. Friends (with a separate pre-amp) have borrowed For-T for a few weeks and were very positive about the amp.
Of course I would like to find out about the sound quality of this Class T amp compared to a gainclone (power opamp) amplifier. Over the last 2 years I have listened to my clonekits and the For-T for longer periods of time and I like both. For some reason I have not yet done a 1-to-1 comparison (shootout).
When trying to modify the amp, adding a pre-amp and DAC, I have blown up my old For-T. Please continue reading the Fif-T project, where I replaced the broken amp4 with the TK2050 (hence the name Fif-T).
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