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Stephen12b  
#1 Posted : Sunday, May 12, 2019 5:37:20 PM(UTC)
Stephen12b

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Location: San Rafael

I finally built my Sympatico amplifiers in response to the heat from my Pass F4. It runs completely cold to the touch at the low volume I play through it. Using 18 V transformers 400 volt amp. Doubled the 15000 caps to keep rails as high as possible about 25 V. Also added a .15uF polypropylene across the 470 uF, not sure if that might be part of the problem.

Wondering if you had any thought on what might be causing an intermittent startup issue where one side or the other seems to stop with -9V offset on both the positive and negative output. If the problem continues the servo pulls things to -7 volts. Shorting the balanced input or simply waiting a few moments sometimes clears the situation. The sound from the speaker is similar to a bug zapper when this is happening.Once stable everything sounds great and seems to hold as long as the amp remains turned on.

The +- 15 volt measure fine, the +_25 volt rails seem fine but I am curious about 25-15 being close to the -9 volt error. Seems like the input might be latching perhaps due to slower start up due to lower rails? What is the drop out voltage needed to maintain the 15 volt regulation? I assume 9 to 10 volts should be adequate.

When working this really is a very good sounding amplifier and I can't believe the lack of any heat.

Thanks in advance for any suggestions to discover and cure what is sometimes happening.

Stephen
Stephen12b  
#2 Posted : Monday, May 13, 2019 1:02:44 AM(UTC)
Stephen12b

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Finally got a scope on it and yes there is high frequency oscillation of almost 5.5 volts with a sinusoidal positive and a flat clipped negative going edge. After a few moments it goes away. Because it happens randomly on one and then the other Sympatico at startup I don't think it is a poor solder connection. They are dual mono sharing one chassis but nothing else. Problem has presented with Buffalo connected or with no connection, the same with the speakers. I tied the two grounds to the chassis but it doesn't make any difference. Oddly I have never observed both channels simultaneously oscillating. For now I disconnect the speakers, turn on the amp and wait until the oscillation stops.

I understand you use a soft start circuit, might that help?

As mentioned I suspect the .15 uF film caps that I added but it is quite difficult to remove them as they are on the bottom of the boards and I thermally glued the boards in place. Brick wall

If you could corroborate my suspicion about the non spec added cap or offer some test it would be greatly appreciated. As long as it stays on it sounds great and performs flawlessly, way better that I expected it would or even could.
Brian Donegan  
#3 Posted : Monday, May 13, 2019 2:41:38 PM(UTC)
Brian Donegan

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Were these pre-assembled Sympatico modules or did you build them up from bare boards?

What value are you using for C1 and C2?

Stephen12b  
#4 Posted : Monday, May 13, 2019 8:18:07 PM(UTC)
Stephen12b

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Location: San Rafael

Hi Brian,
Thanks for responding so quickly. These are assembled units but I broke the rule of building them stock before experimenting to see if they could be tweaked. C1 and C2 are stock. Since I was using 18 volt transformers I paralleled C13 and C14 with additional 18000uF caps using 12 gauge wire about 1" long to keep the rails as high as possible. I also parralleled a ultra low ESR .15uF pp cap across C7 and C8. I suspect that might be the problem. I will try removing them to see if startup stability is improved.
Stephen12b  
#5 Posted : Wednesday, May 15, 2019 3:29:40 AM(UTC)
Stephen12b

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Location: San Rafael

After many words and some blood the 0.15uF caps are out. Unfortunately the start-up instability persists. Sometimes everything starts up just fine, but usually one side or the other breaks into oscillation upon powering up. Next I will remove the extra 18000uF caps. At that point the amplifiers will be stock except powered from +-18 V instead of +-20 to 25 volt. What is the minimum DC rail that will support the 15 Volt shunt regulation? I'm just shy of 25 volts with only 2 mv of the classic downward sawtooth ripple so assume that would work.
Stephen12b  
#6 Posted : Wednesday, May 15, 2019 4:30:39 PM(UTC)
Stephen12b

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Amplifiers are now stock and both are oscillating, Has anyone tried 18V transformers before? Unfortunately I don't have any higher voltage to try.

I hooked up a variable autotransformer set to 140 volts and simply increased the size of the oscillation.

Revisiting your question about C1 and C2 I am measuring .333nF instead of 220pF. Not sure if that is because they are in the circuit and thus measure differently or if i was supplied amps with the wrong value.
Stephen12b  
#7 Posted : Friday, May 17, 2019 6:57:08 PM(UTC)
Stephen12b

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Location: San Rafael

Hi Brian,

You had mentioned that you use a soft start circuit, so after verifying every resister and capacitor and scratching my head and disconnecting one channel, I used the variable autotransformer to slowly bring up the AC to 120 volts and both channels consistently work as expected. Something must be latching up during a sudden power on. Stock I'm measuring 4 mv of sawtooth ripple so I'm thinking of adding a second filter stage with full wave bridge rectifiers to create a "c r c" to allow greater ripple attenuation and to slow down the startup to hopefully avoid the oscillation problems.

Thoughts?

Stephen12b  
#8 Posted : Wednesday, September 18, 2019 6:16:20 PM(UTC)
Stephen12b

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United States
Location: San Rafael

A quick update. The amplifier is stable with different speakers so it must be a unique interaction with the 4 ohm Ramsa pa speakers I was temporarily using. For those curious about the amp, maintaining a balanced connection from the DAC to the Sympatico and on to the speakers is providing some of the best (most detailed without any unnatural emphasis or resonance) sound of any amplifier in my experience. That includes amplifiers costing several thousands of dollars. Definitely worth checking it out.
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