Shorted input means the input pole is to be shorted to ground. If using RCA plugs, a simple plug with a wire connecting the two terminals is good and a good piece of equipment to keep in your junk box.
You should always do the adjustment with the input shorted and ensure your source has zero DC with a directly coupled amplifier like this. Once you adjust it if you see even the slightest DC (or induced currents due to improper interconnects, ground loops, etc) on the input it will simply multiply by the gain of your amp and show up at the output.
You can work out how best to attach the NTC to the FETs. In the ideal case it needs to be able to track the outputs very very closely, I realise that it is tough with two active devices on each channel but it's possible. For example, you can mount it on a bar that joins the two devices and has thermal contact with the screws holding the devices down. This will give you close enough tracking.
Though I have not yet built my version, the PCB I am using keeps the thermistor bang between the two outputs, which are anyway close to each other. Worst case I will do without the thermistor. My problem was never output transistor anyway, but the source and feedback resistors which got uncomfortably warm when the bias was pumped up either by me or by summer.
You should always do the adjustment with the input shorted and ensure your source has zero DC with a directly coupled amplifier like this. Once you adjust it if you see even the slightest DC (or induced currents due to improper interconnects, ground loops, etc) on the input it will simply multiply by the gain of your amp and show up at the output.
You can work out how best to attach the NTC to the FETs. In the ideal case it needs to be able to track the outputs very very closely, I realise that it is tough with two active devices on each channel but it's possible. For example, you can mount it on a bar that joins the two devices and has thermal contact with the screws holding the devices down. This will give you close enough tracking.
Though I have not yet built my version, the PCB I am using keeps the thermistor bang between the two outputs, which are anyway close to each other. Worst case I will do without the thermistor. My problem was never output transistor anyway, but the source and feedback resistors which got uncomfortably warm when the bias was pumped up either by me or by summer.
Shorted input means open input RCA's center conductor shorted to input ground using wire.
Please look at below pic -
View attachment 330523
Here you can see how I made thermistor in place. Thermistor lead are bent to form a loop around MOSFET without shorting it. Note that it makes spring like force to keep it closer to MOSFET body. I already told you about this. There is white thermal paste in between touching bodies to expel any air. This should give good thermal contact and decrease drift.
Last edited:
om,
I just checked after writing my previous post. one of the ntcs is touching the washer. other one is couple of mm off of it. As suggested by you I will use the thermal adhesive to create contact between the ntc and the washer.
Thanks
Pandu
I just checked after writing my previous post. one of the ntcs is touching the washer. other one is couple of mm off of it. As suggested by you I will use the thermal adhesive to create contact between the ntc and the washer.
Thanks
Pandu
bought down the bias to 300mv. I would want to keep the amp inside the cabinet.
DC offset is now below 20 mv.
Bias and dc offset look like changing everyday. Even with small adjustment of ntc positioning. Will check it again tommorrow.
DC offset is now below 20 mv.
Bias and dc offset look like changing everyday. Even with small adjustment of ntc positioning. Will check it again tommorrow.
Pandu
How are you measuring temperatures?
If you are using your hands, note that 55 feels really, really hot the first few times, if you are not used to touching hot amplifiers. It is 18 degrees above your skin temperature. I do remember the hand-over-heatsink test, but some may be more sensitive than others.
If your dissipation is maximum 20 watts per device at 55-60 degrees on the heatsink, there is too little heatsink.
How are you measuring temperatures?
If you are using your hands, note that 55 feels really, really hot the first few times, if you are not used to touching hot amplifiers. It is 18 degrees above your skin temperature. I do remember the hand-over-heatsink test, but some may be more sensitive than others.
If your dissipation is maximum 20 watts per device at 55-60 degrees on the heatsink, there is too little heatsink.
I have heard Class A amps get very hot. This is my first class A amp.
I am able to pass the 10 sec test. I am playing safe here. I can probably go little higher on the bias.
I have B&K 7270 amp with me. That is the reference. It gets too hot. It is a class A/B 7 channel amp. My heat sinks are N9 from
Aluminium heatsink, Heat sink in bangalore, Heatsink stockist, Heatsink dealer, Heatsink supplier, Aluminium LED Heatsinks, Aluminium Reflector Sheets, Aluminium Caul board Sheet 2.5mm, Aluminium Plywood sheets, Aluminium Alloy Rods,plates in 2014, 7. It is 205 mm wide. I use two of them each side with height of the cabinet being 150 mm.
Also checked Diodes and feedback resistors. The diodes are getting may be 40 to 50 centigrade. Feedback resistors little less.
Source resistors are warm.
I am able to pass the 10 sec test. I am playing safe here. I can probably go little higher on the bias.
I have B&K 7270 amp with me. That is the reference. It gets too hot. It is a class A/B 7 channel amp. My heat sinks are N9 from
Aluminium heatsink, Heat sink in bangalore, Heatsink stockist, Heatsink dealer, Heatsink supplier, Aluminium LED Heatsinks, Aluminium Reflector Sheets, Aluminium Caul board Sheet 2.5mm, Aluminium Plywood sheets, Aluminium Alloy Rods,plates in 2014, 7. It is 205 mm wide. I use two of them each side with height of the cabinet being 150 mm.
Also checked Diodes and feedback resistors. The diodes are getting may be 40 to 50 centigrade. Feedback resistors little less.
Source resistors are warm.
Last edited:
I finished off the power supply container today and the amp [V2] plays music. Hopefully I will get chance for a proper listen tomorrow.
The supply consists of a 350VA transformer with dual 25-0-25 secondaries,dual schottky rectifiers and +- 68,000uf per channel with a further +- 47,000uf Rifas per/ch in the amp case.
Looking forward to a good listen.
The supply consists of a 350VA transformer with dual 25-0-25 secondaries,dual schottky rectifiers and +- 68,000uf per channel with a further +- 47,000uf Rifas per/ch in the amp case.
Looking forward to a good listen.
Congrats
You might need two 350Va. Another member just went dual mono and was a bit surprised at the difference.
You might need two 350Va. Another member just went dual mono and was a bit surprised at the difference.
Did not he have 0.4f per channel? That could help either ;-).
I am totally agree with you, Buzz, I think Marra's PSU is too small for F5Tv2.
Got any pics, PKI. Oh yeah. Parts are all in. I have to disppear. Let the sorting begin for F5TGB2.
It is dual mono Buzz; everything is per channel.Listening to it at the moment through a pair of 4ins drvers and I'll try it through my PMC's this afternoon.It sounds quite good through the little drivers
Plus/minus 115,000uf fed by 350VA tranny and parraled rectifiers per channel.
Plus/minus 115,000uf fed by 350VA tranny and parraled rectifiers per channel.
Last edited:
Perhaps PKI would care to comment on in his stock amp and my P3 adjusted amp. It is worthwhile, IMO. Ideeally, you have a good soundcard to help, but mine was adjusted by ear. I did not go for lowest distortion.
What kind of signal did you provide as input to amp and what was the expectation from output?
In my build I never used that P3 trimpot, just excluded while populating the board. But if it really helps then I will try to experiment with it.
How you set that using ear? What is expected from output.