Dual Class-A Line and Headphone Output Board Documents

Construction information for DIY projects, including the MS Mid Side Matrix, Elliptic Equalizer, Mastering Console, Phono Transfer System, Insert Switcher and the Dual Class-A Amplifier. You can post your baby pictures here.
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mediatechnology
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Re: Dual Class-A Line and Headphone Output Board Documents

Post by mediatechnology » Wed Mar 30, 2016 3:08 pm

Paul -

I've had a customer with this issue and have seen this myself.
In both cases the pot was mis-wired because the silkscreen created confusion at the Phoenix connector.
"I" and "O" refer to the circuit inputs and outputs, not the pot's In and Out.

Check that the "O" Phoenix terminal is to the end terminal of the pot. (Terminal 1 on the TKD.)
The "I" Phoenix terminal is to the wiper. (Terminal 2.)
I think ground on the TKD is marked "C."

With a 50K pot you might want to add a coupling cap due to bias current.
The value is a little high for the load impedance of the 1646 and may bow the taper but that's not your immediate problem.
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Gold
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Re: Dual Class-A Line and Headphone Output Board Documents

Post by Gold » Wed Mar 30, 2016 3:16 pm

mediatechnology wrote: "I" and "O" refer to the circuit inputs and outputs, not the pot's In and Out.
Ahh, that sounds right. I misunderstood the labelling. I should have double checked the schematic. I probably did just that for the other build and figured "I got this" for this one. Nope. Thanks.

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Re: Dual Class-A Line and Headphone Output Board Documents

Post by Gold » Wed Mar 30, 2016 7:42 pm

Yep, that was it.

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Re: Dual Class-A Line and Headphone Output Board Documents

Post by mediatechnology » Wed Mar 30, 2016 7:48 pm

Been there myself: viewtopic.php?f=6&t=745
If you want to skip to the punchline its the last post. :oops:
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Re: Dual Class-A Line and Headphone Output Board Documents

Post by Gold » Wed Mar 30, 2016 8:22 pm

mediatechnology wrote:
With a 50K pot you might want to add a coupling cap due to bias current.
The value is a little high for the load impedance of the 1646 and may bow the taper but that's not your immediate problem.
I must admit I don't really know how to look at the DRV134 for input impedance because the usage is non standard. With a 10k pot the maximum source impedance would be 2k5Ω. Even that seems high as a source impedance for a 1646 (DRV134). A 50k pot would present a maximum source impedance if 12k5Ω.

How would I calculate the value of a coupling cap?

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Re: Dual Class-A Line and Headphone Output Board Documents

Post by mediatechnology » Wed Mar 30, 2016 9:03 pm

I have to look at the input impedance of a DRV134 in that configuration.
When I looked at that it was for a 1646 at around 4K minimum.
I think the DRV134 is actually higher.

My guess is 47 uF would be a good value based on the 1646.
But if the bias current DC isn't audible I wouldn't add the coupling cap.

FWIW: A good RC calculator can be found here: http://www.muzique.com/schem/filter.htm

The 1646/134 do require a low source impedance when they're used in the "conventional" balanced output configuration.
In this topology the source impedance doesn't degrade performance like it would in a balanced output.
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