Hi all. Got a C Audio SR707 giving some trouble. Problem is DC offset on both channels. One measures 20mV approx and the other measures 40mV approx. Amplifier comes on but does not trigger protect. (I believe that triggers at 1v)
The PNP long tailed pairs have 2sa872 installed and not 1085s as schematic. (Not sure if this is a factory component change ). I tried changing out one of the pairs to a matched Hfe pair (sorry can't remember what transistors I used at the time ) but that made the DC offset even worse at 240mV!... So I put back the 2sa872. Checked current mirrors all fine too. Changed out the 24k resistors on both boards and no change. Not sure where else to look.
The PNP long tailed pairs have 2sa872 installed and not 1085s as schematic. (Not sure if this is a factory component change ). I tried changing out one of the pairs to a matched Hfe pair (sorry can't remember what transistors I used at the time ) but that made the DC offset even worse at 240mV!... So I put back the 2sa872. Checked current mirrors all fine too. Changed out the 24k resistors on both boards and no change. Not sure where else to look.
A commercial amplifier with only 50mV offset is regarded as perfect.
It's not a laboritory DC motor driver and I would say it is well within specification.
You could replace R44 with a 220R preset and that will balance the Hfe values to allow some adjustment but I would say, it's not worth the effort.
It's not a laboritory DC motor driver and I would say it is well within specification.
You could replace R44 with a 220R preset and that will balance the Hfe values to allow some adjustment but I would say, it's not worth the effort.
All sorted. Multi turn pots fitted in place of resistors. Fits well into the space. DC offset sitting at 2mV after warm up.
Also rewired and re-routed line level cables going to gain pots for much lower noise . They were picking up power supply noise from the transformer/main caps/rectifier. Moved them away from that area.
Also adjusted bias measures from gate to gate to 380mV. Runs much cooler. It was originally 500mV and idling too hot.
Also rewired and re-routed line level cables going to gain pots for much lower noise . They were picking up power supply noise from the transformer/main caps/rectifier. Moved them away from that area.
Also adjusted bias measures from gate to gate to 380mV. Runs much cooler. It was originally 500mV and idling too hot.
