John Linsley Hood Class A amplifier. Distortion figures and PSRR look quite nice, but the circuit has only been simulated, I have a TDA1545 DAC to finish If I could only keep my mind on a single project. If my calculations are correct, the amp will deliver 250mW into a 32Ohms load in class A.
PS: Sorry for the extra ground symbol.
The firmware for the DAC input selector has been debugged extensively, and now seems ready for real life testing. The source is here: SPDIF input selector source. The firmware has a manual, and an automatic selection mode.
- If the button is pushed for less than about 2 seconds, the selector enters manual mode, and skip to the next input.
- If the button is held for longer that 2 seconds, the firmware scans each input for one second and selects the first input that has data (audio).
The firmware works by sniffing the I2S data line to the DAC, to see if anything is going on there. To prevent the DAC from playing anything, before an input is selected, a relay has been added to the DAC board, to only enable the I2S data line to the DAC, when a signal has been selected.
When the project is finished the compiled firmware will be made available along with the rest of the design files.
I replaced the PSU capacitors for the power amplifier (C508/C509) with some 15000uF/50V parts I had salvaged somewhere else. This may be a little overboard, since the value of the capacitors in the original may very well, serve as protection against overheating the amp. The big idea being, that as the current draw rises, as the amp is turned up, the capacitors can not maintain their charge, thus lowering the supply voltage to power amp (and adding distortion). There is a possibility, that the amp can now overheat at high volumes, due to the larger reserve.
While I had the amp opened, I adjusted the bias (idle current) as well I upped it to 100mA, measuring 100mV across (R455/R456). This means the amp delivers a little more power before leaving class A, and heats up a bit more. I therefore added an extra heat sink, to the one already installed.
The bass of the amp now sound more controlled, and thereby somewhat less excessive, leaving a little more room for the midrange and treble to shine. This is consistent with what others have heard, changing the PSU capacitors. The increased bias current probably does not improve things to the same degree as the capacitors, and I will not recommend this mod unless you are prepared to keep an eye on the amp, checking it is not overheating. But change those capacitors.
All in all this is a really nice amplifier, and due to the simplicity of the circuit very mod-able. I won't regret the money I spend for a nanosecond.
There was an error in the last PCB posted, here comes the correct version.
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