Started to design solid state amp + calculations

Hello,

After building and error searching for circuits coming out of elektor and other sources for the last 10years, I wanted to design an amp myself. I know, you guys have helped others a lot of times. I hope you all can help me know. I didn't study electronics but electromechanics, so.

1) I want to use the power supply I've laying around. 2x500VA/ +-75VDC, 20000µF
2) it should be a Class A/B amp with the 2SK1530 and 2SJ201 devices as number one.
3) see the XLS-file.

http://be.msnusers.com/BenJanssen/Documenten/project1.xls

As you all can see:
Is this all correct? See the links in the file, that's where I get the knowledge from.
So, I need to switch three ouput devices in parrallel to get about 250Wrms/8Ohm and 150Wrms in 4Ohm to be safe according to the SOAR curves.
I know I could use the MJ15003/4 devices, they have some kind of broken SOAR diagram, or the BUZ900/905DP devices (250W). But I want to use MOSFET's and the BUZ types are hard to find and rather expensive to match.

I have calculated the Power dissipation in the output devices according a phase shift off 45° (the worst case).
Am I to carefull here?

Any other suggestions maybe? Please tell so?
Is this all correct?
Can anybody tell me how to calculate the power dissipation in the VAS section when I use a cascode arrangement with BC550/560 (40mA bias) and MJE340/350 devices?
How can I calculate the gain in the VAS? Or are there maybe some basic rules for the gain in the differential input, the VAS and the ouput devices?

Thanks in advance
Ben
 
I've changed the layout of the xls-file.
You can see that with 4 output devices the max output can be :
*250Wrms/8ohm
*200Wrms/4ohm
*100Wrms/2ohm
to stay within the SOA of the mosfet.

Is this correct or am I to carefull here?? The SOA curve is drawn with a case temp of about 50°C (120W).

Also, van anybody tell me please how to calculate the power dissipation in a cascode arrangement in the VAS with a MJE340/350 and BC550C/560C. Such as used in the Crescendo ME amplifiers.

Thanks
Ben
 

Attachments

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Nelson Pass

The one and only
Paid Member
2001-03-29 12:38 am
Bensen said:
Is this correct or am I to carefull here?? The SOA curve is drawn with a case temp of about 50°C (120W).

Also, van anybody tell me please how to calculate the power dissipation in a cascode arrangement in the VAS with a MJE340/350 and BC550C/560C. Such as used in the Crescendo ME amplifiers.

While your math appears correct offhand, you will want to
exercise caution using Mosfets or any other semiconductor at
anywhere near their ratings. Personally, I try not to exceed
about 20% of the dissipation rating (maybe 40% peak) in a
production piece. I have reliably taken that up to 33% without
breaking parts at home. Above that, you start losing them.

The dissipation of a cascode is the same as that of a single
device used as a VAS, but with the heat shared by the gain
and cascode transistor. It's the standing current times the
CE (or DS) voltage of each device.
 
Sorry can't read your attachments as I don't have anything that can open an XLS file (no spreadsheet/excel).


Generally you will need 4x amp power into 8ohms in device capacity, as a minimum. If you're substantially raising heatsink temp and/or going for considerably more into 4 ohms you will need maybe 50% more device capacity. A soft supply and low Vce sat (output stage losses) will keep this down. This should allow for 8ohm of any phase angle or 4 ohm of 45degrees while still quick and easy to drive.


Nelson goes for OVERKILL but it's more demanding to drive and so on..... more for deeper pockets. With wide range music there is an inbuilt derating of needs in the crest factor of maybe 5-10 times so NORMAL use is forgiving, and there are many commercial products with less.


I use a 250W/ch 8 ohm amp prototype at home with 6 pairs 2SK405/J115 i.e 1000W of devices/channel.

As for your Vas, the MJE's would do all the dissipation while the BCs have only a few volts across them so take the supply voltage(one side to zero) minus a couple for the BC and multiply by the stage current.

ps I have a new 150W/8ohm amp with 30mW standing driver dissipation, 50mW max so I'll use a BC546/56 comp pair - a derating of over 12 times from their 625mW free air 25C rating.
And it's running Class A in normal operation. Hang the expense.

How bout that Nelson?
 
Damm, I toucht I was maybe to carefull. But its the other way.

amplifierguru, as you said, using only 33% of de max dissipation power is only for the guys with deep pockets. This should be than about 8 mosfets in parrallel.

But using 6mosfet's in parallel is also it bit to much for my pockets.
How about the ETI466 and the crescendo (1982) with only 2mosfets (2 x 125W). They should give according to the specs about 200Wrms and 140Wrms in 8ohm.
And my crescendo runs perfectly during the last 7 years many times in maximum power, even with clipping.
How could this one survive?

VAS:
70V rails-0.1V VCEsat bc546= 69.9V * 0.04A bias = 2.8W
This is no problem for one MJE device.

Thanks guys
 
amplifierguru said:

Generally you will need 4x amp power into 8ohms in device capacity, as a minimum.

I use a 250W/ch 8 ohm amp prototype at home with 6 pairs 2SK405/J115 i.e 1000W of devices/channel.

Amplifierguru,

If you would do the following:
150W/8Ohm 55Vdc 4devices(600W) --> your rule
250W/8Owm 70Vdc 4devices

You would see that in both graphs the SOA curve of the device is at a equaly destance from the power dissipation curve.

So, wy would the second one not be as safe as the first one?
Wy would there else be something like a SOA curve?

Greetz
Ben
 
Hi Benson,

How about that, a B.Sc in maths and physics and I can't even manage (6x100W)x2 =1200W! (I stated 1000W in earlier post)

And yes Benson it was simply convenient to add on one extra MOSFET per supply at the board design stage for a 20% overbuild on my rule of thumb, and considering I had plenty.

5pairs 100W devices (K405/J115) =1000W device power
6 pairs 100W " " =1200W " "

A slight premium - as I said 4 times was a (sensible balance) minimum.

Cheers,
Greg
 

kASD

Member
2004-12-25 12:24 pm
N/A
The Overkill Phenomena

The Overkill Phenomena, always saves you when the amp is driving a highly reactive load in the live program. If for some reason it fails to drive then it will eventually end up in smoke and the whole audience will blast you with anything they have their access to and you will have no time even to run, and then the sound contractor which faces these type of events would simply smash the amp to the face of his creator and the creator would never ..............:D :D ;)

In Pro-world overkill has something to do with the responsibility of standing straight and tall in front of hardcore abusive conditions. No matter what type of situation arises overkill has his bulletproof shield to save the amp.

We use overkill in output Mosfets, just 16 X 300W 41 A Mosfets for just 1200W provides an ample room ...........

Sir Nelson Pass's Class-A amps dissipiate massive heat, but they are safe because of the Mr.Overkill in output devices and massive heatsinking.

By the way Mr.Greg what is the maximum wattage of amp , built by you yet...... I mean to say wattage of single channel not multi channel........just curious to find out the max wattage design from you


regards,

Kanwar