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Old 24th October 2012, 05:46 PM   #451
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I was thinking something like the one I've attached. The downside is that you'll need another power supply for it, but it's requirements are pretty small. I'm going to use one of these to convert unbalanced to balanced for that 50W watercooled SOZ that i've been trying to build since the day I first logged into this crazy place
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File Type: jpg P2205556_1.jpg (116.8 KB, 669 views)

Last edited by MEGA_amp; 24th October 2012 at 05:48 PM.
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Old 24th October 2012, 06:49 PM   #452
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Thank you MEGA-amp for noting the missing file. Here it is to go with post#450 above.
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File Type: pdf bridgeACA.pdf (32.1 KB, 129 views)
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Old 24th October 2012, 07:41 PM   #453
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Regardless of the method used to generate, and use the out-of-phase input signals to the 2 ACAs [note post #451 by MEGA-amp], the DIYer will still face the question of how best to connect the outputs of the ACAs. There are three possibilities. I addressed the first two in my earlier post.
  • Each ACA retains its output DC blocking/coupling electrolytic capacitor [3,300 uF].
  • Discard/do not use the electrolytic capacitor of each ACA. But the DIYer must protect both amps and the loudspeaker with fast blow in-line fuses; in case of accidental shorting of the amps' output to power supply ground [e.g the grounded chassis].
  • The third option is a compromise of both preceeding options. Use one DC blocking/coupling electrolytic capacitor and one fast blow fuse only. The simplified diagram is in the attached bridgeACA1.pdf.
  • What do I do with the unused electrolytic capacitor from the second ACA [3,300 uF, paid for!]? Use it in parallel with the other one in its target ACA [ shown dotted]. Double the capaticance can only improve the overall performance of the bridge amp.
But; one must never use any electrolytic capacitor without ensuring that it is properly polarized in its application. So follow this procedure to comply with this rule.
  • Do not connect the loudspeaker.
  • Allow both amps to reach operating temperature.
  • No input signal.
  • Adjust potentiometer P1 in the upper ACA so as to increase the DC voltage at its output node to +10.25 VDC or higher if you choose to 10.5 VDC. The positive lead(s) of the electrolytic capacitor(s) is connected to this output node.
  • Adjust potentiometer P1 in the lower ACA so as to decrease the DC voltage at its output node to + 9.75 VDC or lower if you choose to 9.5 VDC
  • Insert the fast blow fuse [1-2 A] and connect the loudspeaker.
  • Confirm that the electrolytic capacitor is properly polarized. Meaning it has 1/2 Volt or 1 V [your choice] across it.
  • Monitor this voltage on turn-on also from a cold start to ensure that the capacitor is also properly polarized.
  • There is absolutely no relationship between the phase of the input signals to the ACAs and the polarity tweaks across the output blocking capacitor. The procedure is fully applicable to attaching the electrolytic capacitor to the PCB circuit of the lower ACA instead as long as one adheres to the proper polarity across this capacitor.
The best option of the above 3 is of course the one you are most comfortable with.

N.B. The attached file kept detaching spontaneously! Possible software problem.
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Last edited by Antoinel; 24th October 2012 at 07:54 PM.
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Old 24th October 2012, 08:00 PM   #454
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I have been talking about the bridging option with Nelson and came up with the attached.
Still waiting for papa's final nod though.

jan didden
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File Type: pdf ACA bridged.pdf (19.4 KB, 182 views)
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Old 24th October 2012, 08:49 PM   #455
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Quote:
Originally Posted by janneman View Post
I have been talking about the bridging option with Nelson and came up with the attached.
Still waiting for papa's final nod though.

jan didden
Whao! You practiced in your schematic the highest simplicity and efficacy by generating the out-of phase signal to the other half of the bridge from the drain of the 2SK170 front end which is already in the first amp.
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Old 25th October 2012, 12:12 AM   #456
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Quote:
Originally Posted by janneman View Post
I have been talking about the bridging option with Nelson and came up with the attached.
Still waiting for papa's final nod though.

jan didden
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.

@Antoinel, removing the output caps will put DC on the feedback loop. The DC will cancel in the transformer. C3 may have wrong polarity though.
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Old 25th October 2012, 12:50 AM   #457
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Quote:
Originally Posted by Loudthud View Post
.

@Antoinel, removing the output caps will put DC on the feedback loop. The DC will cancel in the transformer. C3 may have wrong polarity though.


Loudtud: 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.
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Old 25th October 2012, 01:37 AM   #458
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Quote:
Originally Posted by Antoinel View Post
Loudthud: I hope that you agree with these two possible options. Please confirm. Thank you.
It sounds right and you are correct to bring up the two caps in series concerns. The other side of that coin is the bias current. A bridge to an 8 ohm load will require more current. A member PM'd me about running the ACA into a 5 ohm load. My suggestion was to add an additional resistor in parallel with R1-R2 and R3-R4 and reduce the supply to about 13V. That kept the power output and distortion about the same. For 19V you should really double the quiescent current.

The output impedance will also increase in a bridge configuration and the distortion spectrum will shift away from 2nd order to 3rd order. If you really like the sweet sound of 2nd order distortion, a 38V rail might be a better option.
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Old 25th October 2012, 01:47 AM   #459
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Quote:
Originally Posted by Loudthud View Post
It sounds right and you are correct to bring up the two caps in series concerns. The other side of that coin is the bias current. A bridge to an 8 ohm load will require more current. A member PM'd me about running the ACA into a 5 ohm load. My suggestion was to add an additional resistor in parallel with R1-R2 and R3-R4 and reduce the supply to about 13V. That kept the power output and distortion about the same. For 19V you should really double the quiescent current.

The output impedance will also increase in a bridge configuration and the distortion spectrum will shift away from 2nd order to 3rd order. If you really like the sweet sound of 2nd order distortion, a 38V rail might be a better option.
Thank you Loudthud for your valuable comments. Bridging two similar amps is not as straightforward as I believed it to be. I hope that DIYers [and I] who are interested in bridging ACA take notice and implement your recommendations.
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Old 25th October 2012, 02:21 AM   #460
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[QUOTE=Loudthud;3214275]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.

QUOTE]



Loudthud: Looking at the schematic of janneman:
  • What is the quality of the signal at the source of 2SK170? Is it high? It is comprised of the pristine input signal, and a clean inverted and attenuated replica of the input signal coming from the output. If the answer to the above questions is yes, then the current flowing through the 1K source resistor of 2SK170 is pristine and of high fidelity. Since the source and the drain current of 2SK170 are identical, it will follow that the voltage drop across the 1 K drain load resistor, and the attendant voltage drop across it are also pristine and of high fidelity for use as the out-phase signal source to the partner of this amp in the bridge.
  • If the answer is no to the above questions, then the out-of phase signal will need to be derived from the the clean output of this amp instead; attenuated in amplitude to suit.
Please clarify. Thank you, and best regards.
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