Amp Camp Amp - ACA

[jan.didden]

Originally Posted by Loudthud

That won't work as intended. You are picking off a signal from inside the feedback loop where it is distorted to compensate for the non-linearity of Q1, the signal level is reduced by feedback so the level will be wrong and there will be high frequency emphisis caused by Q1's gate capacitance.

Good observation, something I hadn't considered.
Back to the drawing table...

jan

[Nelson Pass]

Originally Posted by janneman

Good observation, something I hadn't considered.

Let's take a look. In an exercise that served me well when I was thinking
about the "SuperSymmetric" design, let's imagine that a small positive
distortion spike occurs at the output of the first amplifier in the balanced
system.

This spike will also be found to be positive at the Gate of the input
transistor due to the feedback loop. This will create a negative spike at
the Drain of that input device, which is the source of the signal driving
the other side of the balanced amplifier. That spike will appear as a
positive spike on its output. Now both sides of the balanced amplifier
have a positive spike.

and Voila!

[jan.didden]

Voila indeed.
It's a balanced bridge having equal distortions at each end of the load so nothing will be heard (of the distortion).

Thanks Nelson.

jan

[Loudthud]

Q1 is not very linear without some source degeneration. Search this thread for some of my earlier posts. Because of the global feedback, the signal at the gate of Q4 is somewhat distorted in an attempt to correct for Q1's non-linearity. The non-linearity increases slightly above about 5kHz because of the gate capacitance of Q1. This further distorts the drain current of Q4.

A simple way to generate the "other" phase would be to replace R11 with a 68.1K resistor and connect the input to the output of the first side. The gain won't be a prefect -1.000 because the open loop gain of the ACA is fairly low. The value will have to be decreased by several precent. I was taught the math sometime in the 70's, at this point I just remember the general concept.

[Zen Mod]

Originally Posted by Nelson Pass

Let's take a look. ......

neat




The Machine that goes Bing - YouTube

[Antoinel]

Originally Posted by Antoinel

Loudthud: You are absolutely correct. Thank you for catching my error. You saved my hide! I failed to pay attention to the origin of the feedback loop. Fortunately, two possible options still remain. I'll post tomorrow the refined diagram of both. Here is the heads-up:

• Keep the output blocking capacitor in both ACA amps. Connect the loudspeaker leads between the existing output ports. Thus, both output capacitors are in the path of the output current flowing through the loudspeaker.
• Keep the output blocking capacitor in both ACA amps. Adjust the ouput DC voltage at each output to 10.0 VDC with the P1 potentiometers as recommended in the article by Mr. Pass. Connect one loudspeaker terminal to the output node of the upper ACA at the positive lead of its output blocking capacitor. Connect the other end of the loudspeaker to the ouput node of the bottom ACA at the positive lead of its ouput blocking capacitor. This arrangement bypasses both ouput capacitors from the path of the output current through the loudspeaker, and simultaneously preserves the original operation of the feedback loop and the front end.

Loudthud: I hope that you agree with these two possible options. Please confirm. Thank you.

As promised, the refined schematic/diagram which is attendant to this post is attached as bridgeACA2.pdf. Here are my general comments:

• I show the passive approach of using a transformer [JENSEN as before] to generate the out of phase input signals to the ACAs. The diagram at the output of the ACAs equally applies to using the approaches posted earlier by Mega-amp and janneman.
• I added detail to the input and output circuit of the upper ACA from the published schematic in the article by Mr. Pass. This detail is not shown for the lower ACA. The purpose of showing this detail is to emphasize that this proposed diagram does not have errors which otherwise will upset the operation of the amp and the transformer [magnetize it].
• The loudspeaker needs to be connected to either output ports [A and A'] or [B and B'].
• The output ports A and A' show that the two output capacitors are in series with each other and with the loudspeaker. Their equivalent capacitance is 1650 uF which maybe detrimental to the low end performance. It is also possible that certain DIYers may object to the potential impact on sound by these capacitors.
• The possible detriment which is associated with ports A and A' disappears when using ports B and B' instead. The capacitors are still present; but the power output signal bypasses them completely. But; one must protect both amps and the loudspeaker with the shown fuses should the output ports B and B' be [inadvertently] shorted to the power supply common/ground

Loudthud raised multiple and specific concerns regarding bridging ACAs; which the DYIer needs to seriously consider in this briging endeavor:

• The principle purpose of bridging is to quadruple output power. But, each ACA [as is] has the ability to put out only 1 Amp and may be unable to generate the needed 2 A [each] for the bridge.
• The intrisic output impedance of each ACA maybe 2 Ohms [for example]. So in the bridge configuration, the loudspeaker is effectively in series with 4 Ohms resistive. This added resistance may rob it from accoustic output; an efficiency issue.
• The concern about harmonic distortion which is disturbed in the bridge, and which is beneficial by design in the solo use of ACA.

[Loudthud]

I'm not sure what Nelson was getting at exactly. Here are a few measurements I took and a scope photo where you can see the distortion at Q4's Drain.

Distortion 1kHz 1 Watt @ 8 ohms
Output 0.79%
Q4 Source 3.46%
Q4 Drain 6.04%

Distortion 1kHz 3 Watt @ 8 ohms
Output 1.63%
Q4 Source 6.55%
Q4 Drain 12.8%
Note that the generator amplitude was 963mV RMS. Signal at Q4 Source is about 140mV RMS.

[Zen Mod]

Nelson is saying that this distortion will be decreased through bridge operation

[jan.didden]

Originally Posted by Antoinel

As promised, the refined schematic/diagram which is attendant to this post is attached as bridgeACA2.pdf. Here are my general comments:

• I show the passive approach of using a transformer [JENSEN as before] to generate the out of phase input signals to the ACAs. The diagram at the output of the ACAs equally applies to using the approaches posted earlier by Mega-amp and janneman.
• I added detail to the input and output circuit of the upper ACA from the published schematic in the article by Mr. Pass. This detail is not shown for the lower ACA. The purpose of showing this detail is to emphasize that this proposed diagram does not have errors which otherwise will upset the operation of the amp and the transformer [magnetize it].
• The loudspeaker needs to be connected to either output ports [A and A'] or [B and B'].
• The output ports A and A' show that the two output capacitors are in series with each other and with the loudspeaker. Their equivalent capacitance is 1650 uF which maybe detrimental to the low end performance. It is also possible that certain DIYers may object to the potential impact on sound by these capacitors.
• The possible detriment which is associated with ports A and A' disappears when using ports B and B' instead. The capacitors are still present; but the power output signal bypasses them completely. But; one must protect both amps and the loudspeaker with the shown fuses should the output ports B and B' be [inadvertently] shorted to the power supply common/ground

Loudthud raised multiple and specific concerns regarding bridging ACAs; which the DYIer needs to seriously consider in this briging endeavor:

• The principle purpose of bridging is to quadruple output power. But, each ACA [as is] has the ability to put out only 1 Amp and may be unable to generate the needed 2 A [each] for the bridge.
• The intrisic output impedance of each ACA maybe 2 Ohms [for example]. So in the bridge configuration, the loudspeaker is effectively in series with 4 Ohms resistive. This added resistance may rob it from accoustic output; an efficiency issue.
• The concern about harmonic distortion which is disturbed in the bridge, and which is beneficial by design in the solo use of ACA.

A few comments: if you carefully balance the bridged output for equal DC (half supply) there is no longer a need for output caps. If you want to play it safe, a single output cap on one side will do.

The bias current should be increased in the bridged mode of course. You don't get anything for free: quadruple the output power will also mean quadruple the (class A) dissipation and that results from a) double the bias current and b) two amplifiers.

@Loudthud: what was the circuit in the '170 drain lead used for your distortion measurements? Did you measure against earth?

jan

[Antoinel]

Originally Posted by Loudthud

I'm not sure what Nelson was getting at exactly. Here are a few measurements I took and a scope photo where you can see the distortion at Q4's Drain.

Distortion 1kHz 1 Watt @ 8 ohms
Output 0.79%
Q4 Source 3.46%
Q4 Drain 6.04%

Distortion 1kHz 3 Watt @ 8 ohms
Output 1.63%
Q4 Source 6.55%
Q4 Drain 12.8%
Note that the generator amplitude was 963mV RMS. Signal at Q4 Source is about 140mV RMS.

Loudthud:It seems to me that loop feedback is contaminating the intrinsic performance of Q4 [in janneman's schematic] which maybe pristine without it. What are the distortion properties at the source and drain of Q4 without loop feedback by comparison? How great is the performance of Q4?