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Old 7th September 2008, 01:25 PM   #1
fab is offline fab  Canada
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Default Mix of GB and F5 Amps idea

Hi,

This is the idea I came up while trying to increase the open loop gain of the F5 amp to have a higher more "load invariant" close loop gain (26 dB) . This is really only a mix of a GB-150 (Greg Ball "Amplifierguru") and Pass F5 amp topology. Also, I need a higher power so I plan to use it in class AB mode (about 10W class A and up to about 70W or so in class B) so I save a bit of money on my energy bill and the size of the case which has to pass the WAF...

The C25-C26 caps are bootstrap caps that could be included or not using a switch for audio comparison purpose.

Can you comment on any aspect of this design giving recommendations before I give it a try.... or not...

Thanks
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Old 7th September 2008, 03:56 PM   #2
juma is offline juma  Germany
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I don't know if NTC1 and NTC2 would be sufficient to prevent thermal runaway in output MOSFETs - you plan to run the amp in AB class, it's quite different scenario compared to F5
I'd rather go for Vgs multiplier for ensuring thermal stability.
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Old 7th September 2008, 10:45 PM   #3
fab is offline fab  Canada
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Quote:
Originally posted by juma
I don't know if NTC1 and NTC2 would be sufficient to prevent thermal runaway in output MOSFETs - you plan to run the amp in AB class, it's quite different scenario compared to F5
I'd rather go for Vgs multiplier for ensuring thermal stability.
This a valuable comment. I have never used IRFPxxx devices before. I have used IRF340 before (both in class A and class AB)which are also vertical mosfet devices but different than IRFPxxx devices.

Where would you put the VGS multiplier in this design?
Does someone knows how is done the temperature compensation in the "SKA" amp (GB150)?
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Old 7th September 2008, 11:27 PM   #4
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I don't know for sure but probably manipulating tail current of long tail pairs
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Old 7th September 2008, 11:43 PM   #5
fab is offline fab  Canada
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Quote:
Originally posted by darkfenriz
I don't know for sure but probably manipulating tail current of long tail pairs
You mean in SKA amp because I do not have a long tail pair...

In my schematic, would that be similar to use a NTC like I have shown?
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Old 8th September 2008, 09:14 AM   #6
juma is offline juma  Germany
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AFAIK, SKA uses Lateral MOSFETs (BUZ 9xx) so the temp. comp. is not needed.

You should put Vgs multiplier between the emitters of Q5 and Q6 in you schematic. SEARCH button is your friend
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Old 8th September 2008, 11:03 AM   #7
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They are IRF type FETs, and the Vgs are matched within 10mV. Photo shows a pair of TO-92 devices attached to the heatsink for thermal tracking.

http://www.ska-audio.com/diy/Compone...0x200_150d.jpg
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Old 8th September 2008, 01:24 PM   #8
juma is offline juma  Germany
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Yes, you are right, the picture clearly shows temp. comp. transistors and IRF MOSFETS. I was seduced by old pictures showing BUZ9xx devices.
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Old 8th September 2008, 01:34 PM   #9
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Quote:
Originally posted by djk
pair of TO-92 devices
BC546/556
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Old 8th September 2008, 03:12 PM   #10
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I guess you could use Q5 and Q6 for temperature compensation as they are in mosfets gate-source circuit but you have to experiment with R62/64 and R59/60.
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