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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 18th August 2019, 03:13 AM   #1141
tomchr is offline tomchr  Canada
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Quote:
Originally Posted by tcpip View Post
Yes, it was never intended to work with both sockets connected to cables.
Yeah, but somebody will do it. Especially in a preamp or headphone amp. The use case for a power amp is much simpler. Neat trick regardless.

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And the input circuit of the Self preamp in Linear Audio doesn't even have these resistors, IIRC.
Yeah. The 100 Ω shown in the power amp book isn't technically needed.

Tom
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Old 18th August 2019, 03:55 AM   #1142
tcpip is offline tcpip  India
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Originally Posted by tomchr View Post

Yeah. The 100 Ω shown in the power amp book isn't technically needed.
What I'm wondering about is: are these resistors (let's just say the pair of high value resistors for the XLR input) needed before the relays of the input selector board, or can they be omitted from there and the preamp (or other differential input circuit) be given the responsibility to do whatever is needed?
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Old 18th August 2019, 04:27 PM   #1143
tomchr is offline tomchr  Canada
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If you're taking Doug Self's circuit from fig. 20.5 in the power amp book, I'd probably do the switching after the two 10 kΩ input resistors, but you could just as well switch just after the input connectors. Do check the chapter on signal switching in his Small Signal Audio book if you want the switching accomplished cleanly.

I don't like the highish impedances of Self's input. 10 kΩ adds a lot of noise. So I tend to use an instrumentation amp input. It costs an extra (dual) opamp, but gives you very high input impedance. This makes life easier for the RF input filter and allows me to set the input impedance to whatever I want. With the IA input, I'd just switch the two input signals between XLR and RCA. Just keep in mind that you need to control the common-mode voltage if you sense the RCA differentially.

Tom
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