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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 19th July 2019, 06:09 PM   #1121
tcpip is offline tcpip  India
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Originally Posted by tomchr View Post
Which volume of Linear Audio?
#5 I think.

Quote:
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.

There may have been something inherent in Self's design that allowed such a setup to work, but I would not recommend it for any of my amps.
Tom
Thanks. Yes, I believe Self's differential input circuit was unusual. Not sure how. And yes, I see what you mean about leaving that pin floating when the RCA socket is used. Thanks.
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Old 6th August 2019, 01:49 PM   #1122
tcpip is offline tcpip  India
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I'm intending to drive my Modulus686 with 24VAC secondaries of my transformer.
  • What will be the final DC rail voltage after the power supply? What is the typical drop expected after rectification? I'm using IC bridges, not individual fancy diodes.
  • What is the peak AC output voltage I can expect the amp to deliver into 8 Ohms (for brief stretches of, say, 30 seconds) without clipping, in my case?
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Old 6th August 2019, 01:56 PM   #1123
Kay Pirinha is offline Kay Pirinha  Germany
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The answer to both questions depends on the properties of your mains transformer, your rectifier and your filter capacitors.
Best regards!
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Old 6th August 2019, 02:02 PM   #1124
tcpip is offline tcpip  India
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Originally Posted by Kay Pirinha View Post
The answer to both questions depends on the properties of your mains transformer, your rectifier and your filter capacitors.
What is the answer to Question 2, if the answer to Question 1 is "31V"?
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Old 6th August 2019, 03:20 PM   #1125
asuslover is offline asuslover  Romania
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Modulus-686: 380W (4Ω); 220W (8Ω) Balanced Composite Power Amp with extremely low THD
Quote:
Originally Posted by Kay Pirinha View Post
The answer to both questions depends on the properties of your mains transformer, your rectifier and your filter capacitors.
Best regards!
Of course !
just because u have 31v after rectifiers that does not mean anything. Type of transformer and transformer va is very important too.
More va equals less voltage drop under load.
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Old 6th August 2019, 03:34 PM   #1126
tcpip is offline tcpip  India
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Originally Posted by asuslover View Post
Of course !

just because u have 31v after rectifiers that does not mean anything. Type of transformer and transformer va is very important too.

More va equals less voltage drop under load.
Assume I have a higher xfmr VA rating than what Tomchr recommends.
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Old 6th August 2019, 03:51 PM   #1127
sledwards12375 is offline sledwards12375  United States
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Any 686 builders out there looking to use switching supplies for their build? I have a lightly used pair of Meanwell 400-27 supplies available. PM me for details.

Steve
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Old 6th August 2019, 04:17 PM   #1128
tomchr is offline tomchr  Canada
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Quote:
Originally Posted by sledwards12375 View Post
Any 686 builders out there looking to use switching supplies for their build? I have a lightly used pair of Meanwell 400-27 supplies available. PM me for details.
Those are nice supplies and work very well in the Modulus-686. My MOD686 amps are all powered by the RPS-400-27-C (caged version of the RPS-400-27). You'll get ~130 W into 8 Ω and 200 W into 4 Ω with that.

Tom
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Old 6th August 2019, 04:28 PM   #1129
tomchr is offline tomchr  Canada
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Quote:
Originally Posted by Kay Pirinha View Post
The answer to both questions depends on the properties of your mains transformer, your rectifier and your filter capacitors.
Exactly!

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Originally Posted by tcpip View Post
Assume I have a higher xfmr VA rating than what Tomchr recommends.
That doesn't give enough information to answer your question. We'll need to know:
  • Mains voltage
  • Transformer turns ratio
  • Transformer parasitics (at least DCR of primary and secondary windings)
  • The characteristics of the transformer core
  • Characteristics of rectifier diodes
  • Parasitics of power supply PCB and wiring
  • Capacitance (and ESR) of reservoir caps
  • Etc.
Throw all that into a circuit simulator and you'll probably get an answer that's reasonably close. Reality will still be slightly off from the simulation. Then throw in that your mains voltage may vary by 10% in many parts of the world and all bets are off.

That said, the back-of-envelope answer to your question is "around 32-33 V most likely". It'll droop a volt or two if you run the amp at clipping levels with an 8 Ω load. That should be enough for around 170-180 W into 8 Ω.

If you would like greater precision in the numbers, use a switching supply, such as a pair of Mean Well RPS-400-27-C.

Tom
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Old 6th August 2019, 04:43 PM   #1130
tcpip is offline tcpip  India
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Originally Posted by tomchr View Post
If you would like greater precision in the numbers..
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.

Thanks.
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