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 » Tue Feb 16, 2016 2:02 pm

Paul -

Thanks for trying it out!

The only schematic is this one: http://www.ka-electronics.com/images/jp ... _Large.JPG
I never drew one featuring both channels because I'm lazy.

I'm not sure what the maximum short-circuit current is since it has no current-limiting.
With 33R build-outs and a big enough heatsink it will drive a dead short indefinitely.
The Vishay MOX resistors in the BOM are 1W but are in a 1/4 W size. This is the Mouser PN for the 33R: 594-5073NW33R00J
The MOX are used in the supply, emitter and build-out resistors. They are also "defined interruption." (fused.)

For headphones I would build it with 10-33R.
I use 33R for the MDR-7506 and like it. I've also tried 0R and can't say which one is right but there's something different in the low end and I prefer having the current limiting.

A 47R or 49R build-out is the better option when the DCAO is used as a line amp/cable driver.
Driving 'phones there's just a lot of insertion loss with 47R and the board is gain-challenged at -10 levels owing to the fact that it began life as a unity gain line amp.
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Re: Dual Class-A Line and Headphone Output Board Documents

Post by Gold » Tue Feb 16, 2016 2:23 pm

Thanks Wayne. I had most of the parts here already. I had the Vishay fusable R's in 10Ω and 1Ω. I ordered the other values. I also ordered some of the 1W Holco's in some of the same values. I didn't know if the fuseable R's had compromises to make them flameproof. I only used flameproof resistors on rails. I've never put them in the audio path. If nothing else I can brag about the brand name.

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

Post by Gold » Wed Feb 17, 2016 8:33 am

mediatechnology wrote: The only schematic is this one: http://www.ka-electronics.com/images/jp ... _Large.JPG
I never drew one featuring both channels because I'm lazy.
I was talking more about the tracking circuit for use with the 1646. I don't plan on using a 1646 but I don't think I've ever knowingly looked at something like that.

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

Post by mediatechnology » Wed Feb 17, 2016 10:14 am

I was talking more about the tracking circuit for use with the 1646. I don't plan on using a 1646 but I don't think I've ever knowingly looked at something like that.
I wound up never using that and just stuck with the DRV134.
It's an interesting little circuit.
I took it out in the last board spin.
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Re: Dual Class-A Line and Headphone Output Board Documents

Post by Gold » Wed Feb 17, 2016 10:43 pm

mediatechnology wrote: For headphones I would build it with 10-33R.
I use 33R for the MDR-7506 and like it. I've also tried 0R and can't say which one is right but there's something different in the low end and I prefer having the current limiting.
I'm having a little trouble understanding what governs the choice of buildout resistor value for headphones. 33R is basically an impedance match for the MDR600's. You don't recommend going higher in value. The 650's are 300Ω. It seems to me the MDR600's are an impedance "mismatch". Maybe the mismatch is evening out a level difference as the MDR600's must be more sensitive than the 650's? I'm mostly getting a "do not compute".

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

Post by mediatechnology » Thu Feb 18, 2016 7:01 am

ricardo is the expert on headphone build-out.

I put the build-out in primarily for short-circuit protection.
I don't think the reason is to impedance match as much as it is to control the LF resonance of the phones' drivers with a low damping factor.

A lot of people argue for 0R build-out - John Siau of Benchmark has posted here on that subject.
That approach argues for a high damping factor similar to power amp outputs driving speakers.

Tascam has a HP amp with variable build-out.
If you read about it elsewhere the arguments become pretty passionate.

My personal thinking is there is no "right" value - the only right value is one large enough to minimize short-circuit current.
If you were to build it with 300R build-out then plug in the 30 Ohm 'phones the level drop will be huge and there won't be enough voltage gain anywhere to get decent level.

With the 30R phones the difference in the low-end was very subtle.

I found this earlier thread: viewtopic.php?f=6&t=497
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Re: Dual Class-A Line and Headphone Output Board Documents

Post by Gold » Thu Feb 18, 2016 8:42 pm

mediatechnology wrote: I found this earlier thread: viewtopic.php?f=6&t=497
Ah, good thread. Like I said, I'm not a big fan of headphones. That's how I missed it.

Edit: That's a fantastic thread. Thanks Ricardo. It occurs to me that headphones would be a valuable addition to both the shop and the studio. More so the shop.

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

Post by BarneyL » Sat Feb 20, 2016 6:43 am

Wayne, thank you for being so polite in your replies. A few rookie mistakes but thanks for the explanations. Will update pics when I finish the front panel and other set of transformers. Do you think there will be any benefit running direct in bypasing the 1246? Was going to try that with the unbalanced output of the Massive Passive into the DCAO.

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

Post by mediatechnology » Sat Feb 20, 2016 9:21 am

You can use it unbalanced without the 1246 if you want.
Use the link you installed for out to in for the optional HPA level control.

You will have 6 dB more gain without the 1246.
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Re: Dual Class-A Line and Headphone Output Board Documents

Post by Gold » Wed Feb 24, 2016 10:52 pm

I finally got this up and running. I'd like to put it in a 1RU case. I was hoping the rotary switches would work because the only other pots I have that will fit in 1RU are PEC's. The accuracy sucks so I can't use them. The rotary switches unfortunately won't work. There isn't enough range. I have four of the Alps blue velvet I got in a trade. I guess I'll move to a 2RU case. I only have 1 P&G RF-15 so I'll keep that for a rainy day. I want one DCAO in a project box for the shop. That's no problem to fit a large pot. The TKD 2500 series look really nice and like they would fit in 1 RU but they are pricey. I'm trying to stick to the parts bin as much as possible.

I didn't see instructions in the thread for how to translate the mV test point readings into mA. I guess it's ohms law but I'm not getting it.

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