Blown channel with bias set to 18mv = 7mv offset at speaker output
Original channel with the bias set is giving me a reading of 20mv
I'll plug it in to see if it sounds ok and then I may match the LTP transistors in the original channel, just to see if it brings the dc offset down.
Do you guys think matching the LTP trannys could bring the dc offset down?
Original channel with the bias set is giving me a reading of 20mv
I'll plug it in to see if it sounds ok and then I may match the LTP transistors in the original channel, just to see if it brings the dc offset down.
Do you guys think matching the LTP trannys could bring the dc offset down?
Even if it does there's no point, other than making neat numbers, in having a DC offset arbitrarily as low as your DMM's resolution. Suppose you had a 5 or 7 digit bench meter - how low an offset would you need to hold for any period, for their display to show all zeros?
As a static figure, it's become an obsession to many audio tinkers but that tiny 20 or 7mV DC is going to vary with temperature and is already completely insignificant to a bass speaker cone which has its own mechanical offset, even new out of the box. You certainly can adjust output offset by rolling transistors but the LTP match wont necessarily be better. Below 50mV offset, this becomes a hiding to nothing as the amplifier manufacturers realized.
Probably not worth bothering then.
There's always a risk that something will go wrong everytime I remove the board. Last night I after setting the bias and putting the lid back on, there was only sound coming out of one channel 🙁 Took me ages to find that there was a fracture around the solder pad of one of the speaker protection fuses. I was very lucky to find it because it was an intermittent fault.
There's always a risk that something will go wrong everytime I remove the board. Last night I after setting the bias and putting the lid back on, there was only sound coming out of one channel 🙁 Took me ages to find that there was a fracture around the solder pad of one of the speaker protection fuses. I was very lucky to find it because it was an intermittent fault.
Yikes!..that's fragile, no doubt due to repeated pulls on the output leads. I've never examined one of these in reality but a small and tightly packed case is not a good servicing proposition for a power amplifier. This appears to have provisions like connectors and fasteners more for ease of assembly than service needs, which I imagine weren't part of the plan.
(Thank you PR Audio)
(Thank you PR Audio)
It may look bad in the photo but compared to some stuff it's an absolute doddle to get the circuit board out.
Took a microsystem apart once and surprised myself when I managed to put it back together.
Took a microsystem apart once and surprised myself when I managed to put it back together.
The output offset is the DC gain times the difference in input voltage at the +IN & -IN pins/pads.
The input current at the -IN pin times the resistance to audio ground sets the -IN pin Voltage
The input current at the +IN pin times tyhe resistance to audio ground sets the +IN pin Voltage
Add on the difference in Vbe to give the total input Vdiff.
The input current at the -IN pin times the resistance to audio ground sets the -IN pin Voltage
The input current at the +IN pin times tyhe resistance to audio ground sets the +IN pin Voltage
Add on the difference in Vbe to give the total input Vdiff.
- Status
- Not open for further replies.