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Modulus-686: 380W (4Ω); 220W (8Ω) Balanced Composite Power Amp with extremely low THD
Modulus-686: 380W (4Ω); 220W (8Ω) Balanced Composite Power Amp with extremely low THD
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Old 6th August 2019, 04:48 PM   #1131
tomchr is online now tomchr  Canada
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Quote:
Originally Posted by tcpip View Post
Thanks a lot for the first useful response. I needed a back-of-envelope answer. I guess I should have specified "I want a figure +/-1V, and with reasonable assumptions" when I asked.
My answer won't be accurate to 1 V. Think about it... Mains variation alone would cause 3.3 V variation.

If you want 1 V precision, use a regulated supply.

Tom
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Old 6th August 2019, 04:57 PM   #1132
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Originally Posted by tomchr View Post
My answer won't be accurate to 1 V. Think about it... Mains variation alone would cause 3.3 V variation.
Somehow, I have succeeded in confusing all of you about the context of my question. I don't need a very precise answer. I just wanted to know a "nominal" answer. I'm not trying to do anything with edge cases or corner cases. Sorry about this. You're of course correct that I won't get even a 10% accurate answer if I map the nominal to the real world. (I live in a country where the real mains voltage is often 30% lower than the nominal figure in many parts of the country. Voltage stabilizers are a household device in every household, and every store selling a refrigerator or AC will sell you a compatible voltage regulator too.)

But I just needed a nominal figure, with "reasonable" assumptions on xfmr, diodes, etc.
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Old 6th August 2019, 05:04 PM   #1133
tomchr is online now tomchr  Canada
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Well, in your case then: There will be some voltage present on the output of the power supply. Maybe. If it feels like it...

Under nominal conditions, expect around 32-33 V with a 24 VAC transformer.

Tom
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Old 6th August 2019, 05:57 PM   #1134
BrianL is offline BrianL  United States
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Modulus-686: 380W (4&#937;); 220W (8&#937;) Balanced Composite Power Amp with extremely low THD
As a rough rule of thumb I multiply the transformer's secondary voltage by 1.5 to account for high line voltage, higher no-load voltage on the transformer secondary, and because it's an easy number to remember. If that number pushes any limits, then it has to be considered more thoroughly.
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Old 12th August 2019, 04:37 PM   #1135
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Yeah, and under nominal conditions, under load, the DC voltage is more like 1.3-1.35x the AC output of the transformer. It's closer to 1.4x at idle.

Tom
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Old 13th August 2019, 02:50 PM   #1136
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I don't like the idea of leaving XLR Pin 3 floating. That means the inverting input of the differential receiver is floating.
I prefer to connect XLR Pin 3 to the RCA shell (along with XLR Pin 1). Unfortunately, it isn't possible to have both RCA and XLR connectors hooked up without using a switch to select between the two, though.
I found this in Self's Audio Power Amplifier Design Handbook:
Click the image to open in full size.
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Old 15th August 2019, 07:15 PM   #1137
tomchr is online now tomchr  Canada
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Oh, that's a neat trick! Basically, you take a standard differential input and connect the single-ended input through a 100 Ω resistor to the non-inverting input of the opamp. When the XLR is unplugged, the opamp works as a buffer. Nice!

The obvious drawback is that the circuit won't work with both the XLR and RCA sources plugged in. It has to be one or the other.

Tom
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Old 15th August 2019, 11:32 PM   #1138
tcpip is offline tcpip  India
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The obvious drawback is that the circuit won't work with both the XLR and RCA sources plugged in. It has to be one or the other.
Yes, it was never intended to work with both sockets connected to cables.

And the input circuit of the Self preamp in Linear Audio doesn't even have these resistors, IIRC.
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Old 16th August 2019, 07:49 AM   #1139
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Modulus-686: 380W (4&#937;); 220W (8&#937;) Balanced Composite Power Amp with extremely low THD
We had a "similar" discussion some months back in the THAT thread, about the Witlock's JENSEN AN-003 note. That note shows an unbalanced to balanced interface that uses only a pair of resistors and a cap, no transformers involved.

I think the consensus was that
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THAT Driver differential output > Balanced output > pseudo-differential cable
and
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It's not the best way of doing things, but better than not doing it.
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Old 16th August 2019, 10:58 AM   #1140
tcpip is offline tcpip  India
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We had a "similar" discussion some months back in the THAT thread, about the Witlock's JENSEN AN-003 note. That note shows an unbalanced to balanced interface that uses only a pair of resistors and a cap, no transformers involved.



I think the consensus was that and
I'm confused here... We were discussing inputs and your response mentions output.
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