smokingmachine said:
Smokingmachine,
The short answer is two as long as the load impediance is 8 ohms. I based this on the following data from the respected datasheets (some calculated values per the the datasheets):
Vds Id/100C Pd Pd@70C
===========================================
IRFP450 500 8.7 180 112.5
IRFP140 100 22.0 180 138.0
IRFP240 200 12.0 150 96.0
The above noted Pd/70C assumes sufficient heatsinking to ensure Tc does not rise above 70 C and is the maximum power the noted devices can handle. This means if your rated resistive only load impediance is 4 ohms, then power at 8 ohms resistive is Pd@70C / 2.
To answer a previous question of why Id for some devices is higher, but the Pd is lower is because the Id rating is the maximum Id specification which is not at the device maximum Vds rating. The higher the Pd rating and lower the derating vs the rated Id. This is why is may seem a higher Id rating does not translate into a higher device Pd rating, or at times even a lower device Pd rating.
In my opinion the only device that will allow a safe 200W for 8 Ohms and a speaker phase angle of 45 degrees (using the rule of thumb calculation: rated impedance / 2) would be a pair of APT4025BNs atg Tc 70 C or similar or better APT device.
Regards,
John L. Males
Willowdale, Ontario
Canada
25 May 2007 22:56
25 May 2007 23:18 Typo Correction. jlm
Official Quasi Thread Researcher
Dear Sirs,
I'm finally on the route to finish my NMOS200 amp. That was my first PCB to do, and I'm quite happy with the results so far.
Mr. Quasi, thank you for the project!
I'm finally on the route to finish my NMOS200 amp. That was my first PCB to do, and I'm quite happy with the results so far.
Mr. Quasi, thank you for the project!
An externally hosted image should be here but it was not working when we last tested it.
what kind of capacitors are the best for nmos500/350?(ceramic, tantal,...)and what their voltage has to be?
can i use 200 ohm power resistors(5w) for soft start, becouse i can not get 220ohm?
the coil on the output can be air coil or not?how much current will pass trough the coil?
for 2x400 rms at 4 ohm what main capacitors for the power supply will do the job?will 20000uf/100V be good for positive voltage rail and the same for negaive voltage rail for both channels?
i will use 1,2kw transformer(hope it will be enough) and a greatz that can handle 35 amps and 400V-better more than less
thx
can i use 200 ohm power resistors(5w) for soft start, becouse i can not get 220ohm?
the coil on the output can be air coil or not?how much current will pass trough the coil?
for 2x400 rms at 4 ohm what main capacitors for the power supply will do the job?will 20000uf/100V be good for positive voltage rail and the same for negaive voltage rail for both channels?
i will use 1,2kw transformer(hope it will be enough) and a greatz that can handle 35 amps and 400V-better more than less
thx
KSE340/350 in place of MJE340/350
Well, after months of infrequent revisits to my implementation of the Quasi N-channel amp, I was able to get the amp to work correctly.
Let me describe the problem I faced: First, the PCB was my own design, more like a conventional design with all the power Mosfets aligned to one side of the board. I used 3 x IRFP450 per half, all other parts were standard and as per schematic, the only exceptions being that the MJE340/350 parts were KSE340/350 plastic insulated types. Voltage was +-63 volts DC. I used a different circuit for both soft start and DC protection. Leach's clip indicator was also made integral on the amp board. (Input transistors are very much 2SC1845s.)
The problem was that the music had a crackling noise/distortion with every beat even at very low levels. I tried Iq from 100 mA to 300 mA but the phenomenon did not go. Initially this phenomenon occured only when playing both channels. I thought this might be a ground issue and rewired the amp after which the distortion was heard even while playing one channel only.
However, Iq and offset could be set very precisely and there was absolutely no drift.
I tried other voltages viz., +-35vDC, +-42vDC, +-56vDC apart from +-63vDC. I did not notice the problem with +-35 and +-42 volts. Hence, I changed R6 to 22K and R12/R15 to 68E, in place of 18K and 47E respectively. I checked only one channel with +-56 volts DC this morning and the music was clean. Sounded pretty good too.
Only could not bring down offset below 63mV. I need to check whether R6 or R12/R15 has effected the change.
My question is whether there is any difference between MJE and KSE (plastic insulated version) types?
Has anyone else run Quasi's amp with high rail voltages and standard parts without any problems?
What could be possible reasons for the phenomenon I encountered?
This is only a preliminary report and I will give more details hopefully tomorrow, but this is something I thought I'd share for others' insights and comments.
Thanks,
Well, after months of infrequent revisits to my implementation of the Quasi N-channel amp, I was able to get the amp to work correctly.
Let me describe the problem I faced: First, the PCB was my own design, more like a conventional design with all the power Mosfets aligned to one side of the board. I used 3 x IRFP450 per half, all other parts were standard and as per schematic, the only exceptions being that the MJE340/350 parts were KSE340/350 plastic insulated types. Voltage was +-63 volts DC. I used a different circuit for both soft start and DC protection. Leach's clip indicator was also made integral on the amp board. (Input transistors are very much 2SC1845s.)
The problem was that the music had a crackling noise/distortion with every beat even at very low levels. I tried Iq from 100 mA to 300 mA but the phenomenon did not go. Initially this phenomenon occured only when playing both channels. I thought this might be a ground issue and rewired the amp after which the distortion was heard even while playing one channel only.
However, Iq and offset could be set very precisely and there was absolutely no drift.
I tried other voltages viz., +-35vDC, +-42vDC, +-56vDC apart from +-63vDC. I did not notice the problem with +-35 and +-42 volts. Hence, I changed R6 to 22K and R12/R15 to 68E, in place of 18K and 47E respectively. I checked only one channel with +-56 volts DC this morning and the music was clean. Sounded pretty good too.
Only could not bring down offset below 63mV. I need to check whether R6 or R12/R15 has effected the change.
My question is whether there is any difference between MJE and KSE (plastic insulated version) types?
Has anyone else run Quasi's amp with high rail voltages and standard parts without any problems?
What could be possible reasons for the phenomenon I encountered?
This is only a preliminary report and I will give more details hopefully tomorrow, but this is something I thought I'd share for others' insights and comments.
Thanks,
smokingmachine said:
the total soft start resistance should be about 10r to 30r for 110/120Vac and about 20r to 60r for 220/240Vac.
Three or four 200r in parallel get you into the ballpark for 220/240Vac.
If you build it right then >80Apk at the output. Build it badly and you could be around 20Apk to 30Apk.
For 4ohm loading, I suggest +-40mF per channel.
AndrewT said:
hy Andrew, can you tell me in some other words what to do with softstart resistors?
4x220 ohm parallel gives 55 ohm and 20 watt?
in my case 4x200 ohm parallel gives 50 ohm and 20 watt?is this good or not?
and i have some more question-
the 5watt resistors at nmos fets are 0.33 ohm or 0.47 ohm?
and can i use 220 ohm trimpot for setting the bias(in the scheme is 220 ohm)?
smokingmachine said:
Smokingmachine,
The 5W output MOSFET device source resistors can be either 0R33 or 0R47 for the NMOS350 or NMOS500. The NMOS200 uses a parallel set of 1R0 which is basically the same as 0R47. I would ask you if are you refering to the NMOS350/500 or the NMOS200 for your question?
Quasi has a preference for 0R47 source resistance, but many of the builders have the personal opinion that 0R47 is too high. There has been some discussion about this in the thread in a few places. Initially as the amp went through design itterations and later as a few builders ask the question you asked.
My guidance is if you are not making any attempt to match the MOSFETS then go for the 0R47 for the NMOS350/500 or the parallel 1R0 for the NMOS200. That said, one builder found some interesting issues when he did not match the MOSFETs, but I do not recall off hand what value he used for his source resistors. I am home late again tonight and ill and have been ill for about a week, so I do not have the time to research to find the different areas in this thread where the discussion occurred about the source resistance purpose and dimensioning.
With respect to the 220 trimpot for setting the bias, are you refering to VR2 controlling the bias of T8? If so, I am not sure I understand your question if you can use a 220 ohm trimpot when that is exactly what it is in the schematics and you are refering to as well.
Regards,
John L. Males
Willowdale, Ontario
Canada
Official Quasi Thread Researcher
29 May 2007 22:16
29 May 2007 22:23 Typo correction. jlm
29 May 2007 22:46 Typo correction. jlm
Hi,smokingmachine said:
softstart is there to reduce the current during the first few cycles at start up.
The transformer primary usually has a resistance of just a few ohms.
Any added resistance in series with this will reduce that initial current.
Make the added resistance too big and the soft start resistors will overheat too quickly.
Fo a significant reduction in start up current the values I gave earlier work well.
Your four 200r in parallel give a total resistance 50r. This will work for a very few seconds. But you must use a switch (relay or triac or FET) to short out the resistors very soon after start-up. 200mS to 500mS is usually OK.
KSE340/350 in place of MJE340/350
Yesterday I had posted regarding some of the issues I am facing with Quasi's Mosfet Amp.
Today I tried again and I was able to set offset to zero volts. With rails of +-56 volts, Iq of 110mA and 3 pairs of IRFP450s the sound is not as good as playing the same amp with rail voltages of +-35volts or +-42 volts, but with R6 and R12/R15 in original values as per schematic.
The changes I have made are changing R6 to 22k and R12/R15 to 68E. With original values and rail voltages upwards of +-56 there is audible distortion.
Yesterday I had posted regarding some of the issues I am facing with Quasi's Mosfet Amp.
Today I tried again and I was able to set offset to zero volts. With rails of +-56 volts, Iq of 110mA and 3 pairs of IRFP450s the sound is not as good as playing the same amp with rail voltages of +-35volts or +-42 volts, but with R6 and R12/R15 in original values as per schematic.
The changes I have made are changing R6 to 22k and R12/R15 to 68E. With original values and rail voltages upwards of +-56 there is audible distortion.
Re: KSE340/350 in place of MJE340/350
Samuel,
Do you think the distortion you experienced may be a result of the KSE340s/350s you used instead of the MJE340s/350s?
Does your reference to "R12/R15 to 68E" really mean "R12/R15 to 68R0". You have used the "68E" term in two postings and I am not familar with the "E" use for resistance.
R6 has had some history and in fact the latest values quasi specified for R6 are 18K0 for rails >= 65V and 6K8 for <= 60 V. I am not sure if R12/15 had different values during the design phase of the amp.
If I am feeling ok later today I will dig up some notes and the links for the R6 history which may shed some light on your experience and findings. That said, I think I can say there have been a number of builds using the 18K0/47R0 values at >= 56V rails without the issues you experienced, but, and this is the but, I am almost certain in all of those cases the amps were using MJE340s/MJE350s. If this is true then it suggests a slight difference with the KSE340/350 parts. I for one would be interested in this as I have the KSE parts. I just have not had time to start building, partly due to the challenges in finding some parts and mostly due to various personal crisis over the last 18 months. I am not experienced nor have the knowledge like quasi and all of those that have contributed to the design review and post design refinements to know if your changes would work just as well for the MJE340/MJE350 or if your findings would be is one uses the KSE parts instead of the MJE parts.
That said maybe we need to see if we could find someone with some MJEs to send to you to have you try the adjusted values you have determined work ok with the rail voltages you tried and then the original R6 and R12/15 values to see if you experienced the same distortion problems.
Regards,
John L. Males
Willowdale, Ontario
Canada
30 May 2007 08:32
Official Quasi Thread Researcher
Samuel Jayaraj said:
Samuel,
Do you think the distortion you experienced may be a result of the KSE340s/350s you used instead of the MJE340s/350s?
Does your reference to "R12/R15 to 68E" really mean "R12/R15 to 68R0". You have used the "68E" term in two postings and I am not familar with the "E" use for resistance.
R6 has had some history and in fact the latest values quasi specified for R6 are 18K0 for rails >= 65V and 6K8 for <= 60 V. I am not sure if R12/15 had different values during the design phase of the amp.
If I am feeling ok later today I will dig up some notes and the links for the R6 history which may shed some light on your experience and findings. That said, I think I can say there have been a number of builds using the 18K0/47R0 values at >= 56V rails without the issues you experienced, but, and this is the but, I am almost certain in all of those cases the amps were using MJE340s/MJE350s. If this is true then it suggests a slight difference with the KSE340/350 parts. I for one would be interested in this as I have the KSE parts. I just have not had time to start building, partly due to the challenges in finding some parts and mostly due to various personal crisis over the last 18 months. I am not experienced nor have the knowledge like quasi and all of those that have contributed to the design review and post design refinements to know if your changes would work just as well for the MJE340/MJE350 or if your findings would be is one uses the KSE parts instead of the MJE parts.
That said maybe we need to see if we could find someone with some MJEs to send to you to have you try the adjusted values you have determined work ok with the rail voltages you tried and then the original R6 and R12/15 values to see if you experienced the same distortion problems.
Regards,
John L. Males
Willowdale, Ontario
Canada
30 May 2007 08:32
Official Quasi Thread Researcher
smokingmachine said:
Smokingmachine,
I am not sure if you can use a 200 trimpot or not. My hunch would be yes, but quasi would know if you can and any side effects or if he may even suggest placing a 10R0 in series on each side of the 200 trim pot to make up the 20R difference. Sorry I do not have the EE knowledge to know the absolute answer to your question. This is not a question that has arisen as yet I believe.
Regards,
John L. Males
Willowdale, Ontario
Canada
30 May 2007 08:38
Official Quasi thread Researcher