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Old 13th September 2007, 12:03 AM   #1
Don S is offline Don S  United States
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Default EF Output Stage

I have included my output stage for others to look at, spice and comment about. I am looking for stability, current drive, and reliablity! I am also very curious to know the F3 of this circuit! It would help if I knew the input current needed to provide +25 and -25 amps at the output. Also the SOAR. Please feel free to comment.

I am looking at driving a 3.5 ohm load of a dynamic speaker. 2 ohms would be even better!

Thanks, DonS
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Old 13th September 2007, 12:17 AM   #2
AKSA is offline AKSA  Australia
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Looks good Don,

You could string a 220nF cap across the emitters of the drivers for charge suckout, but this will fly very nicely.

If you go for 25A output from each half, this will be around 8A from each device, and at this current assume a gain around 40, which will require 200mA for each OP device, and thus a total of 600mA from the driver.

At 600mA you would have a gain around 80 from the driver, and this needs 7.5mA of drive at the base.

To allow for a reliably functioning VAS and Vbe multiplier, you should run about double this current as VAS bias, so 15mA should do it. Your 100R of inter-emitter resistance should be about right for this configuration, which is also known as the Self Type II EF.

Hugh
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Old 13th September 2007, 01:14 AM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
the 3pair on 75Vdc will drive 8ohm to 60degree phase angle with Tc<=60degC at the limit of the DC SOAR.
However 4ohm 30degree reaches the same DC SOAR limit.
increasing the load phase angle to 60degrees takes the output stage to 163% of the DC SOAR.
I have no data for the transient capability of these transistors.

Reactive 3ohm loading will increase the stress on the output devices even higher.

I have guessed that the drivers have an second breakdown knee at about 40V. These will survive better than the output devices.
Taking the same phase angles and using output current gains of 58/40/35 for loads of 8/4/2 ohms the drivers stay within my estimate of DC soar upto 70degC.
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Old 13th September 2007, 01:40 AM   #4
AKSA is offline AKSA  Australia
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Andrew,

these are extremely tough devices. Three pairs at 75V will be fine down to 3R.

Hugh
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Old 13th September 2007, 05:05 AM   #5
Bonsai is offline Bonsai  Taiwan
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Hugh, they are very tough devices, but Andrew has a point on load phase angle . There have been a few threads on this subject in the foprum and if you want to go to 3 Ohms (some will want to go to 2 . . . ) and cater for tough loads, 3 output pairs on a 75V rail may not be enough.

BTW there was a nice expose on this subject a few months ago by Keith howard in Hi-Fi News (I think it was this publication). I was really surprised at how bad some speaker loads are - worst was the Finial electrostatic. I'd call it an Amp killer!

I wonder if the drive requirements (15mA) from the VAS on this design are'nt a bit high, especially for 4 Ohms which could lead to non-linearity issues.

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Old 13th September 2007, 10:48 AM   #6
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by AKSA
these are extremely tough devices. Three pairs at 75V will be fine down to 3R.
I agree that the MJ21193/4 are tough devices that should survive even PA use.
Are you saying OK on 3r0 or on 3ohm reactive?
High phase angles take these devices well outside the DC SOAR at the device temperatures I quoted.
It's a pity Onsemi has not updated the datasheet giving more information on transient ability. It may be that their reputation is rightly so and could be due to a high factor between DC and 100mS or 10mS peak currents.

If the output devices do turn out to be so robust that they can drive a severe 4ohm load, then the little bit of spare in the drivers will be used up and the drivers become the bottleneck.
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Old 13th September 2007, 12:30 PM   #7
AKSA is offline AKSA  Australia
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Hmmm. OK, Don, build it and see how well it works.

If it doesn't work too well, or easily blows up, use four pairs.

Simple!

Cheers,

Hugh
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Old 13th September 2007, 01:21 PM   #8
jaycee is offline jaycee  United Kingdom
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I personally like the Triple Darlington (as called by Leach) arrangement. Basically, this is just a "pre-driver" stage driving the drivers. The main advantage is it allows the drivers to always have current flow - which reduces crossover distortion.

http://users.ece.gatech.edu/~mleach/lowtim/output.html explains further.
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Old 13th September 2007, 01:34 PM   #9
GK is offline GK  Australia
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Quote:
Originally posted by AndrewT

I agree that the MJ21193/4 are tough devices that should survive even PA use.
Are you saying OK on 3r0 or on 3ohm reactive?
High phase angles take these devices well outside the DC SOAR at the device temperatures I quoted.
It's a pity Onsemi has not updated the datasheet giving more information on transient ability. It may be that their reputation is rightly so and could be due to a high factor between DC and 100mS or 10mS peak currents.

If the output devices do turn out to be so robust that they can drive a severe 4ohm load, then the little bit of spare in the drivers will be used up and the drivers become the bottleneck.

With 75V rails, a woefully inadequate 3 parallel pairs of output devices will never endure any decent amount of continuous PA use into a 3 ohm load without expiring prematurely.
On semi havenít published any transient SOA charts for these devices, but a good rule of thumb for building robust output stages with these devices, which will be tollerant of a fair amount of abuse, is to use as many devices as necessary to enclose the reactive load line inside the non-temperature de-rated 1sec SOA. Assuming a peak output swing of 65V with 75V rails, you need no less than 7 pairs of devices to enclose a 3 ohm / 45-degree reactive load line inside the non-temperature de-rated 1 sec SOA.
The MJL2119* devices are robust parts, but they are not indestructible. Iíve blown enough up to know.
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Old 13th September 2007, 01:35 PM   #10
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by jaycee
I personally like the Triple Darlington (as called by Leach) arrangement. Basically, this is just a "pre-driver" stage driving the drivers. The main advantage is it allows the drivers to always have current flow - which reduces crossover distortion.

http://users.ece.gatech.edu/~mleach/lowtim/output.html explains further.
I don't believe Leach's argument. I think it is flawed.
Adding a pre-driver does not influence the validity of his claim that preventing switch off is the same as ClassA.

I do agree that the pre-driver is particularly useful for low impedance loads. If the driver and output are kept in the high gain range of currents then a To92 small signal device can perform as pre-driver. But if the outputs and/or the drivers are asked to operate in a low gain current range then a medium power device may be necessary.
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