New Lineup IDEA - Power Follower/Output stage - Page 2 - diyAudio
 New Lineup IDEA - Power Follower/Output stage
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diyAudio Member

Join Date: Dec 2005
Location: the north
Quote:
 Originally Posted by Fast Eddie D Kudos. I like this implementation with the local feedback in the output stage. Is this buffer linear enough to operate out of the global feedback loop? It does have local feedback. Because if you build it as a two stage amplifier, you will have less distortion fed back to the op amp. Do you think this is worth a try?
It is very precise with MOSFET output devices. Not much distortion.
My guess it is like THD 0.002%
So it can be used outside feedback loop. I will test to see the figures.

Another thing which Bonsai mentioned in post up here is the DC Offset on output. It can be an issue in real circuit. But I think we can find a way to balance output close to zero.
Another redical solution to DC Offset is to use output cap. This is something we never should be afraid of. Many old amplifiers performed very well usning big output caps.
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 4th November 2012, 02:49 PM #12 diyAudio Member     Join Date: Dec 2005 Location: the north Fast Eddie D. Here are my testings for Power Buffer. I put signal at buffer input. No feedback. I did test with FFT on output. 8 Ohm. 04 Vp = 01 Watt THD 0.00020% 08 Vp = 04 Watt THD 0.00042% 12 Vp = 09 Watt THD 0.00067% 16 Vp = 16 Watt THD 0.00093% We can see the buffer performs very well. Better than 0.001% is very good. So in real circuit the distortion will be very low. My guess is better than THD 0.005%. This is better than most power buffers I know about. __________________ lineup
diyAudio Member

Join Date: May 2010
Location: Skokie Il
Quote:
 Originally Posted by lineup It is very precise with MOSFET output devices. Not much distortion. My guess it is like THD 0.002% So it can be used outside feedback loop. I will test to see the figures.
Good. You do realise the practical advantages to a two stage power amp, right? It can reduce speaker interaction with the amplifier. There are discussions of this in another thread here.

When back emf is fed back into a unity gain feedback network, it is not amplified. When it is fed all of the way back to the input voltage gain stage, it is amplified!

 4th November 2012, 03:20 PM #14 diyAudio Member     Join Date: Dec 2005 Location: the north Yes. I know about this. The speaker has a coil and can transmit pulses into global feedback loop. This is where a circuit like my power buffer can do a duty. One voltage amplifier with local FB. And one buffer to follow. And no back emf to the voltage amplifier. __________________ lineup
diyAudio Member

Join Date: May 2010
Location: Skokie Il
Quote:
 Originally Posted by lineup Yes. I know about this. The speaker has a coil and can transmit pulses into global feedback loop. This is where a circuit like my power buffer can do a duty. One voltage amplifier with local FB. And one buffer to follow. And no back emf to the voltage amplifier.
That's it. My old Nakamicha TA4A reciever works that way. It is very well behaved and very powerful in an "old school" way. It uses a traditional diamond buffer with current sources. It's my favorite solid state amp. It can drive any single pair of speakers "6 ohms nominal" or higher quite well.

 5th November 2012, 09:57 AM #16 diyAudio Member     Join Date: Dec 2005 Location: the north Nakamichi has done a lot of good things. I have own a Nachamichi taperecorder. It was the best __________________ lineup
diyAudio Member

Join Date: Dec 2005
Location: the north
Here is the next part in development of this circuit.

I have added 'real' current sources.
I have added 2 compensation caps. This makes the circuit have good stability. I think it slows down the MOSFETs.

Now is only half the distortion att full output (30 Watt into 8 Ohm).
The input impedance is >11k. THis makes the buffer an easy lood for the frontend.

I now use OPA604 for voltage amp. It has max +/-25 VDC supply.
And so it can put out +/-22 Vpeak.

The diagram shows circuit at full +/-22 Vpeak output.
THD is 0.00067% at 30.25 Watt output.
Where OPA604 contributes 0.00028% and the buffer 0.00039%

These are good figures. It points to we can build a very good amplifier which is not too complicated.
THe MOSFETs run in 217mA idle.
Attached Images
 powerfollower6.jpg (93.1 KB, 675 views)
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lineup

 5th November 2012, 11:51 AM #18 diyAudio Member     Join Date: Dec 2004 Nice idea Lineup, one can construct a pure class-A preamp section also using your topology, as it nicely fits in some of the places of interest.
 5th November 2012, 12:03 PM #19 diyAudio Member     Join Date: Dec 2005 Location: the north Yes. With a discrete Class A preamp we can put out more Watt. Chip opamps are restricted to lower voltage. (Max 30 Watt) I have now adjusted the needed capacitance across GATE-DRAIN for MOSFETs. One cap 1.5nF across seems to be pefect for AC values. The input impedance is set by those two resistors to ground. __________________ lineup Last edited by lineup; 5th November 2012 at 12:08 PM.
diyAudio Member

Join Date: Dec 2005
Location: the north
Here is the AC Analysis with those 2 caps across MOSFETs.
And the current working circuit.
1. OPA604 as voltage amplifier +/-22Vpeak.
2. The Power buffer
Total THD 0.00067% for both parts.
Attached Images
 powerfollower7.jpg (96.0 KB, 621 views) powerfollower8.jpg (108.7 KB, 601 views)
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