Hello everyone,
Does anyone have a service manual for a Sugden A25, Bias adjustment, offset, etc..
Thank you very much
Does anyone have a service manual for a Sugden A25, Bias adjustment, offset, etc..
Thank you very much
Thank you for your reply, I have already the schematic but can't find the bias setting that I've heard is very high.
I'm not familiar with the amp as such but one thing stands out when looking at images of the internals and that is that the bias will be 'low' for reasons of the heatsink being small along with the small toroidal PSU. In other words it looks exactly like a 25+25 watt Class AB amp would be.
I would say 100ma bias current as that is the nominal sweet spot for that type of circuit and FET's used. No more.
I would say 100ma bias current as that is the nominal sweet spot for that type of circuit and FET's used. No more.
👍
Have a read at the section 'biasing considerations' here.
https://www.ampslab.com/RECOMMMENDED READ/LATERAL MOSFETS/LATERAL MOSFET FOR AMPS.pdf
Have a read at the section 'biasing considerations' here.
https://www.ampslab.com/RECOMMMENDED READ/LATERAL MOSFETS/LATERAL MOSFET FOR AMPS.pdf
I knew I had read something on this somewhere: https://www.canuckaudiomart.com/forum/viewtopic.php?t=52193
Suggests factory recommended setting of 90mA, which obviously tallies with Mooly’s advice.
Suggests factory recommended setting of 90mA, which obviously tallies with Mooly’s advice.
Mick, would you be able to explain how the bias is adjusted on this amp? I have one but am not familiar with the process.
To set the bias current you need to either break the power supply circuit to each channel and insert an ammeter in series, or use a suitable low value resistor and measure the volt drop across tht and calculate the required volt drop using ohms law.
The circuit shows no suitable resistors are used and breaking the circuit, while easy, isn't for the inexperienced.
I see 4 fuses on the board. These could be supply rail fuses and if so then you could remove one per channel and insert a meter in its place or better still (safer for the amp) replace the fuse with a 1 ohm resistor and then measure the volt drop across that resistor and set the bias accordingly.
You would have to look what each fuse does. Are they DC rail fuses (which is what you want) or are they from the transformer to the rectifier (no good) or speaker fuses (no good). If DC rail fuses then they would go to the Drain of one of the FET's in one of the channels. Two fuses per channel, one for plus and one for minus rail. If its like that its easy to do.
If rail fuses you would have to turn the bias to zero on each channel, note the current flowing (using ohms law and the test 1 ohm resistor) and then add 100 ma to it.
If the fuses are shared and one fuse protects the plus rail to both channels (and same for the minus rail) then its still easily doable.
Identify what the fuses do and go from there.
The circuit shows no suitable resistors are used and breaking the circuit, while easy, isn't for the inexperienced.
I see 4 fuses on the board. These could be supply rail fuses and if so then you could remove one per channel and insert a meter in its place or better still (safer for the amp) replace the fuse with a 1 ohm resistor and then measure the volt drop across that resistor and set the bias accordingly.
You would have to look what each fuse does. Are they DC rail fuses (which is what you want) or are they from the transformer to the rectifier (no good) or speaker fuses (no good). If DC rail fuses then they would go to the Drain of one of the FET's in one of the channels. Two fuses per channel, one for plus and one for minus rail. If its like that its easy to do.
If rail fuses you would have to turn the bias to zero on each channel, note the current flowing (using ohms law and the test 1 ohm resistor) and then add 100 ma to it.
If the fuses are shared and one fuse protects the plus rail to both channels (and same for the minus rail) then its still easily doable.
Identify what the fuses do and go from there.