SRPP with FETS, what output stage? - Page 2

davidallancole · 31st December 2007, 01:08 PM

The load R25 is attached to the ground rail.

Is there some reasson it should be attached to the positive rail?

AndrewT · 31st December 2007, 01:20 PM

The load could be connected to either supply rail.

Why did you choose to connect it to the negative rather than the positive?

I would try both alternatives and decide on sound quality which is better.

I think I know which might be better, but I have only ever built one single ended amp, for headphones, and didn't like the result.

davidallancole · 31st December 2007, 02:33 PM

Hi Andrew,

I choose the ground rail simply because we were always taught in school to connect the load to ground. I will try both and see if it sounds different.

Which one do you think is better? I have seen both ways on different amps. Is there any reason that one may sound better than the other? If the PSRR of an amp is good, I would think the ground rail would be the logical choice if it is wired properly so it is clean. What ever noise or ripple is on the positive line would get sent down the speaker.

Though one should have a clean positive line in this case since the PSRR is very low (don't think there is any rejection).

AndrewT · 1st January 2008, 02:09 PM

Hi David,
I'll not preempt your experiment for fear of biasing the result.
Nor will I explain why one might potentially offer a superior result until after you tell if either sounds different.

davidallancole · 1st January 2008, 03:13 PM

Sounds good Andrew. I should hopefully have something built this week that I can respond to.

davidallancole · 3rd January 2008, 10:10 PM

So I built this up with a darlington output transistor (tip120) with a darlington again used in the ccs. I put a 1k resistor from the positive rail to the point were the gate of the top one is connected to the drain of the bottom one, and then a 1k resistor from that point to ground. The point of this was to help drive that point back up to 1/2 Vc when the darlington transistor was connected as well as it brings the gain of the SRPP down a lot.

It sounds good to me. I can't tell any difference between connecting the output capacitor to the positive or ground rail, but my source is a mickey mouse cd player and my speaker is a 8 inch unit from a cheap guitar amp.

So Andrew, now that I have tried it and haven't heard any difference, do you hear a difference when you do it? I would imagine it is more noticeable on a better system than what I was trying it on. What is your preference for where the capacitor is connected?

AndrewT · 4th January 2008, 11:27 AM

Hi,
I gave up on my single ended about ten years ago and have not returned to that yet.

The speaker connection to the positive rail gives a true constant current draw on the supply as the signal changes. This can help remove some PSU artifacts from the output and can sound a little different, not necessarily better.

When the speaker is connected to the negative the change in current draw on the supply goes from near zero amps to double the quiescent bias current and causes severe ripple in the supply rails when the signal becomes significant. Do not believe the folk that tell you ClasssA is constant current, they know not what they are talking about. Only a very few ClassA topologies achieve constant current with signal change.

How do you achieve silent power up?

davidallancole · 4th January 2008, 04:43 PM

My power supply was outputting only 8 volts to get the 1 amp bias that the unit was set up for. This was a different class a stage that I build to run on +-10 volt rails. So the voltage at the output where the cap goes to the speaker is 3.8 volts when I measured it, thus there was not much voltage change there to make a bump. I didn't hear any thump anyways.

davidallancole · 4th January 2008, 05:39 PM

I checked in some sims about the constant current pull on the power supply you were talking about. Its true and seems like a good way of taking the variable load off of the power supply. With the load going to ground, the supplys current is increasing and decreasing with the music which would be constantly changing the supply ripple, while connecting the load to the positive supply causes the load current on it to be constant, thus causing the supply ripple to be constant. Thank you for telling me that information. I think that is a very neat thing to know about class a amps.

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31st December 2007, 01:08 PM	#11
davidallancole diyAudio Member Join Date: Nov 2005	The load R25 is attached to the ground rail. Is there some reasson it should be attached to the positive rail?

31st December 2007, 01:20 PM	#12
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	The load could be connected to either supply rail. Why did you choose to connect it to the negative rather than the positive? I would try both alternatives and decide on sound quality which is better. I think I know which might be better, but I have only ever built one single ended amp, for headphones, and didn't like the result.

31st December 2007, 02:33 PM	#13
davidallancole diyAudio Member Join Date: Nov 2005	Hi Andrew, I choose the ground rail simply because we were always taught in school to connect the load to ground. I will try both and see if it sounds different. Which one do you think is better? I have seen both ways on different amps. Is there any reason that one may sound better than the other? If the PSRR of an amp is good, I would think the ground rail would be the logical choice if it is wired properly so it is clean. What ever noise or ripple is on the positive line would get sent down the speaker. Though one should have a clean positive line in this case since the PSRR is very low (don't think there is any rejection).

1st January 2008, 02:09 PM	#14
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi David, I'll not preempt your experiment for fear of biasing the result. Nor will I explain why one might potentially offer a superior result until after you tell if either sounds different.

1st January 2008, 03:13 PM	#15
davidallancole diyAudio Member Join Date: Nov 2005	Sounds good Andrew. I should hopefully have something built this week that I can respond to.

3rd January 2008, 10:10 PM	#16
davidallancole diyAudio Member Join Date: Nov 2005	So I built this up with a darlington output transistor (tip120) with a darlington again used in the ccs. I put a 1k resistor from the positive rail to the point were the gate of the top one is connected to the drain of the bottom one, and then a 1k resistor from that point to ground. The point of this was to help drive that point back up to 1/2 Vc when the darlington transistor was connected as well as it brings the gain of the SRPP down a lot. It sounds good to me. I can't tell any difference between connecting the output capacitor to the positive or ground rail, but my source is a mickey mouse cd player and my speaker is a 8 inch unit from a cheap guitar amp. So Andrew, now that I have tried it and haven't heard any difference, do you hear a difference when you do it? I would imagine it is more noticeable on a better system than what I was trying it on. What is your preference for where the capacitor is connected?

4th January 2008, 11:27 AM	#17
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, I gave up on my single ended about ten years ago and have not returned to that yet. The speaker connection to the positive rail gives a true constant current draw on the supply as the signal changes. This can help remove some PSU artifacts from the output and can sound a little different, not necessarily better. When the speaker is connected to the negative the change in current draw on the supply goes from near zero amps to double the quiescent bias current and causes severe ripple in the supply rails when the signal becomes significant. Do not believe the folk that tell you ClasssA is constant current, they know not what they are talking about. Only a very few ClassA topologies achieve constant current with signal change. How do you achieve silent power up?

4th January 2008, 04:43 PM	#18
davidallancole diyAudio Member Join Date: Nov 2005	My power supply was outputting only 8 volts to get the 1 amp bias that the unit was set up for. This was a different class a stage that I build to run on +-10 volt rails. So the voltage at the output where the cap goes to the speaker is 3.8 volts when I measured it, thus there was not much voltage change there to make a bump. I didn't hear any thump anyways.

4th January 2008, 05:39 PM	#19
davidallancole diyAudio Member Join Date: Nov 2005	I checked in some sims about the constant current pull on the power supply you were talking about. Its true and seems like a good way of taking the variable load off of the power supply. With the load going to ground, the supplys current is increasing and decreasing with the music which would be constantly changing the supply ripple, while connecting the load to the positive supply causes the load current on it to be constant, thus causing the supply ripple to be constant. Thank you for telling me that information. I think that is a very neat thing to know about class a amps.