Power amp under development

Hi,
1pair of 190W devices are struggling on a +-50Vdc supply the 8ohm 60degree phase angle load is already outside the DC SOAR @ Tc=25degC.
increase Tc=50degC and the loadline is getting close to the 10mS SOAR.
Now increase the rails to +-60Vdc and the 8ohm phase angle has to be reduced to 18degrees to hit the DC SOAR and at 50degree phase angle the loadline coincides with the 10mS SOAR (peak dissipation=294Wpk)

I don't recommend 1pair for real speakers and supply rails over +-48Vdc.
 
AndrewT said:
Hi,
1pair of 190W devices are struggling on a +-50Vdc supply the 8ohm 60degree phase angle load is already outside the DC SOAR @ Tc=25degC.
increase Tc=50degC and the loadline is getting close to the 10mS SOAR.
Now increase the rails to +-60Vdc and the 8ohm phase angle has to be reduced to 18degrees to hit the DC SOAR and at 50degree phase angle the loadline coincides with the 10mS SOAR (peak dissipation=294Wpk)

I don't recommend 1pair for real speakers and supply rails over +-48Vdc.


Andrew
Point taken. That clears up any indecision that I might have had!! I'll stick to the two pair plan.
Hari
 
AndrewT said:

two pair will drive 6ohm OK.
and 4ohm for domestic duty.


Going by Quasi's power selection table on his website, for a DC voltage rail of +/- 60 driving an 8 ohm speaker, number of FETs required per module is only 2. I assume that 2 per module would mean just one pair and not one pair per rail. Maybe I've got it wrong. Could you comment on that?

Hari
 
Hi,
Quasi and I differ in how close we run the output stage loadline to the DC and/or 100mS SOAR after temperature derating.
Quasi uses 100mS whereas I use DC SOAR, but Quasi clearly states this amp is not for PA duty.
These differences in design philosophy can a do make a big difference to operating conditions.
 
jethari said:



Going by Quasi's power selection table on his website, for a DC voltage rail of +/- 60 driving an 8 ohm speaker, number of FETs required per module is only 2. I assume that 2 per module would mean just one pair and not one pair per rail. Maybe I've got it wrong. Could you comment on that?

Hari

Hari,

Quasi's Power Selection Table lists total number of output MOSFETs required, not pairs required. So you are correct when the table lists 2 MOSFETs required that means one pair. Another clue is the table only lists number of MOSFETs required in numbers evenly divided by 2.

As an aside the devices you have suggested I would treat as same as an IRFP450 for the purposes of the Power Selection Table. Andrew does use a different set of criteria for what a device power handling is capable of for a given load and Tc. Using calculations I developed a couple years ago, before Andrew made me aware of the Benson spreadsheet, with a spreadsheet of my own creation found the IRFP450 was best limited to 112.5W per device at Tc 70C. In that case the load was 8 ohms one pair would be ok with about 55W using the load/2 rule of thumb to allow for phase margin. I like some overbuilding. The Benson spreadsheet Andrew uses is more accurate in terms of SOA calculation details, but it does allow one to see the effect a load can have on guiding one on device suitability and selection.

If you look at the SOA graph of the device datasheet you will find that will quickly help you figure out initialy if a device may be suitable for your requirements. You will be amazed how the SOA may not list DC or even 100ms SOA limits on many devices and that some devices DC SOA is the same as devices that look better from a Id or Vdss initially, but fall short big time on the SOA for what you thought the device might be able to handle. Look at the Pd and derating of the device as well as the SOA curve as part of your quick initial screening and calculate Pd derated for 60C or 70C first to help you screen devices initially that may be of interest to you.

The calculations in my spreadsheet look at a number of device specifications and perform deductive calculations as well from the raw specifications, but are nowhere as complete and exact as the Benson spreadsheet.

Andrew knows much more about EE that I will ever know. I just know enough to be dangerous ;)


Regards,

John L. Males
Willowdale, Ontario
Canada
18 July 2007 08:12
Official Quasi Thread Researcher
 
Hi OQTR,
I don't like the way you implied I cannot be dangerous.
I make just as many mistakes as the next man/woman.

Do you use Benson's?

But to the real point of this post.
In the UK EE means electrical engineer/ing.
eg.
MIEE = member of the institution of electrical engineers.
IEE = institution of electrical engineers.

That includes the subgroup of electronics as well as many others.

What does EE mean in other countries?
 
AndrewT said:
Hi OQTR,
I don't like the way you implied I cannot be dangerous.
I make just as many mistakes as the next man/woman.

Do you use Benson's?

But to the real point of this post.
In the UK EE means electrical engineer/ing.
eg.
MIEE = member of the institution of electrical engineers.
IEE = institution of electrical engineers.

That includes the subgroup of electronics as well as many others.

What does EE mean in other countries?

Hi Andrew,

I was not implying you cannot be dangerous at all. I just said with my little knowledge I was dangerous.

Mistakes is how we all learn. I will not get into the magnitude or type of mistakes that are valuable vs very dangerous. My reference to dangerous for me was just the usual a little knowledge is dangerous.

I have not had the time to figure out how to use the Benson spreadsheet nor input the values from the any of the devices my spreadsheet calculated for, which as about 40.

I had used EE in the context of "electrical engineer/ing" as that seemed to be a common term used on this forum.


Regards,

John L. Males
Willowdale, Ontario
18 July 2007 08:44
Official Quasi Thread Researcher
 
AndrewT said:
Hi,
Quasi and I differ in how close we run the output stage loadline to the DC and/or 100mS SOAR after temperature derating.
Quasi uses 100mS whereas I use DC SOAR, but Quasi clearly states this amp is not for PA duty.
These differences in design philosophy can a do make a big difference to operating conditions.

I'm looking at strictly domestic use for the amp. It was just a thought whether a one pair setup would survive the conditions that I'm looking at. That's what is driving me to look around for other similar devices. I'm confining my search to devices are freely available in India.

Hari
 
Hari,

Two points:

1) I noticed in your last PCB you have a link from R27 to a trace on the outside of the V- wide rail trace that then appears to go to T10 and nothing else in between. Why not make a link across the V- rail between R26 and R30 to the trace you have to T10 and then delete the trace from the relocated link all the way around. It would make the path from R26/R27 to T10 much shorter. You could then move the V- rail trace on the side of the PCB over to allow for more room for the C13 - wider trace I mentioned previously.

2) Regarding your question of alternates for the output MOSFETs, can you source either the APT4025BN or APT4020BN with any greater ease to yourself and at reasonable cost? I lucked out a couple years ago with a store that just happened to carry some. I bought them all, about 25 I think. Then a year later the store got in more, about 18 I think and bought them. I did inquire if the store could get more, but they never followed up. The APT4025BN/APT4020BNs have great power handling ability and very good Ciss. Failing that there are a few other choice APT devices, one I cannot rememember the part number for that could handle a 4 ohm load at 75W as I recall. I was never looking for the APT4025BN I now have, but I looked the datasheet up when they appeared in the store and discovered what a gem they would be. Sadly I need at least 56 and ideally 80 so I can build all woofer, mid and tweeter amplifiers I want to build for an active crossover based system. As a side note I am still uncelar if a single pair of output devices is allows better speaker damping or if having a few parallel is. There are less losses with a single pair and the option of not needing the source resistors, thought for a bit of protection it seems like a good idea to still use the source resistors with a single MOSFET pair. I believe the IRFP250N would also be a good choice for you if you can source at reasonable cost.


Regards,

John L. Males
Willowdale, Ontario
Canada
18 July 2007 19:29
Official Quasi Thread Researcher
 
jethari said:


I'm looking at strictly domestic use for the amp. It was just a thought whether a one pair setup would survive the conditions that I'm looking at. That's what is driving me to look around for other similar devices. I'm confining my search to devices are freely available in India.

Hari

Hari,

Are you driving a 2 or three way speaker system and would it be passive or active crossover? If you are driving a passive crossover based speaker system then I would suggest you have am amplifier of 60-100W RMS and of the MOSFETS with a Pd of 190W you will likely need at least 2 pairs of such rated devices. Passive crossovers are well known to add load complexity to an amplifier, but if you are driving a 2 way speaker system it will be less comples.

Have to run, very loud thunder crack here in case power goes off.


Regards,

John L. Males
Willowdale, Ontario
Canada
18 July 2007 19:36
Official Quasi Thread Researcher
 
keypunch said:


Hari,

Are you driving a 2 or three way speaker system and would it be passive or active crossover? If you are driving a passive crossover based speaker system then I would suggest you have am amplifier of 60-100W RMS and of the MOSFETS with a Pd of 190W you will likely need at least 2 pairs of such rated devices. Passive crossovers are well known to add load complexity to an amplifier, but if you are driving a 2 way speaker system it will be less comples.



John
I'm using a three way passive crossover speaker system of equivalent impedance of 8 ohms.
Take a look at the STW26NM50 at http://www.rapidonline.com/netalogue/specs/47-0442.pdf. This looks more suitable for a single pair version (sorry for being so dogged about the single pair :smash: )

Hari
 
Hi Hari,

in case you really want to design this particular amplifier in order to be used for a particular pair of speakers, it might be a good idea to know the actual impedance of those particular speakers.

If the speakers' impedance response doesn't allow for a single pair of output devices due to dips well below the nominal value, this would be an important design criterion...

OTOH, if the impedance response is very flat and unproblematic, we might find some suitable output devices (for use in a single pair version) in a jiffy.

Do you have the possibility to measure the speakers?

Cheers,
Sebastian.
 
Hi,

You could do a simulation with programs like UniBox , WinISD or Basta!, just to name a few...

Unfortunately you would have to know each individual speaker's impedance response and the exact L, C and R of all your crossover parts.

But measuring isn't difficult. Rod Elliot gives a brief introduction on measuring loudspeaker parameters, starting with impedance measurement. You would just need a power amplifier, a resistor, a sound card and a measurement program.

I recommend Speaker Workshop or LIMP. Reading the tutorial will reveal if this is for you. ;)

All you actually want to know as an indication w.r.t. your choice of amplifier output devices is the minimum impedance and the average impedance at lowish freqencies.


you can get a clue by measuring the resistance (not impedance) at the speaker terminals.

This would not be more than a clue, unfortunately, as the crossover in a 3way speaker can have significant influence on the overall impedance response...
The main problem here is that impedance can be significantly lower than dc resistance at some frequencies.

Cheers,
Sebastian.