DIY preamp
Thought I would share this.
See pics attached that show the various measurements of distortion and square wave performance.
A few points first though:
1. The preamp is a solid state design with a current mirror loaded LTP input stage, single ended vas and single ended output stage.
2. Global feedback is only about 13dB @ 1kHz, falling to zero dB at about 300kHz. Mostly local feedback is used to reduce distortion.
3. Output stage bias is nominally 100mA (high) but varies (slightly) to maintain a constant heatsink temperature of 40C.
4. Linear Systems bipolar (input and mirror) transistors are both dual package matched devices - for lowest distortion, thermal tracking and low offset voltage.
5. A combination of discrete series and shunt regulation used throughout, and the linear supply is also choke regulated - and located in a separate enclosure.
Distortion fft plots
Distortion testing was done at 1vrms, but is not amplitude dependent.
The test frequency is 6khz rather than 1khz - to better stress the device under test.
The first chart shows the loopback test which is the performance of the sound card itself.
The second chart shows distortion of the preamp into a 47k impedance
The third chart shows distortion of the preamp driving a very low 100 ohm load.
Square wave testing
This is used to test stability and bandwidth
1st pic @ 1kHz
2nd pic @ 10kHz
3rd pic @ 20khz
4th pic @ 50 kHz
5th pic @ 100 kHz
6th pic @ 200kHz
|