Hello
I am using a BA283AM as a makeup amp after a balanced passive EQ (Eckmiller W86). I have hooked up my circuit as follows :
- input → 1:1 Sowter isolation transformer → Passive EQ (34dB loss)
- passive EQ → 1:1 Cinemag isolation transformer → hot to (L), cold to ground (J) on the BA283AM
- BA283AM → VTB1148 : (M) to pin 1, (B) to pin 3
- Output Xfrmr : pin 5 to hot, pin 8 to cold.
Ground : input pin 1, output pin 1, SMPS V-, balanced cables shields and pin (J) on the BA283AM
VTB1148 pin 2 & 4 linked, pin 6 & 7 linked, resistor and cap between 5 & 8
24VDC from a Meanwell SMPS to (M)
My problem is, I have a PSU noise in the audio whereas normally the Meanwell SMPS are very quiet but for now on my circuit is not earthed, and more disturbing : I seem to lack some headroom ? I have set up the bias with my oscilloscope...
I am not familiar with the BA283AM so maybe the lack of headroom is normal when running 34dBs of gain ?
The EQ is capable of pretty broad boost in 6dBs increment for the low end, and when boosting by 6dB I can clearly hear distortion. The level is dropped by 34dBs after it, so I wonder what is clipping... ?
My best bet yet is some sort of impedance mismatch. What's the ideal impedance the BA283AM wants to see at its input ?