Dual Class-A Line and Headphone Output Board Construction Information

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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gstarey66
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Joined: Thu Jul 17, 2014 9:07 am

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

Post by gstarey66 »

Hi,

No, exposed metal on the back side. Glad I hesitated and checked before applying power. Should have insulators tomorrow. Thanks for the help.

Regards,
Greg
gstarey66
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Re: Dual Class-A Line and Headphone Output Board Documents

Post by gstarey66 »

Hello,

Got a little carried away trying to make this amp good enough to keep me happy for a number of years. Sounds very good indeed, limit is my headphones at this point.

Used a MR room proof box that had 1/2" thick aluminum walls, good brass with gold plate connectors, alps blue pot for volume. Power supply was a 35VA transformer, a C - R - C filter (1000uf - 2.0ohms - 1000uf) followed by a LM317/337 regulator with 1000uf on the output. P/S in its own shielded box, hard wired to one end of a hospital grade 16 gauge IEC cord that plugs into the headphone box. Then it gets another 1000uf cap, an RF clamp on filter, then onto the board itself.

On the PCB, input is wires with RF beads (no resisters), 470uf caps instead of the 220uf's. All the IC's have 1uf stacked film panasonic V type caps soldered directly above the chips with leads as short as possible. (Probably overkill with these chips, but works wonders with chips such as the LM6172 op-amps which like to oscillate in the MHZ region.) Output is a wire with an RF bead. Damping factor should be high enough to not be any kind of a limit. ( Interesting paper on that at: http://benchmarkmedia.com/blogs/white-p ... -amplifier )

Heatsinks are pretty much triple the size needed (were free). And so on. Pretty sure everything is sufficient to power planar headphones when I get a change to buy them. Currently biased at 85ma, but will bump that up later.

By using the recycling bin for hardware parts, really didn't spend much at all. Should keep me going for a long time. Hope that all made some sense.

Thanks for the design and boards Wayne. I am very happy with the sound.

Regards,
Greg
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mediatechnology
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Re: Dual Class-A Line and Headphone Output Board Documents

Post by mediatechnology »

Very cool Greg! Glad you like it. Send us some pics when you get the chance.

Sorry that you had the issue of the device not being insulated.
These are the Fairchild BD139s that I've been using. The BD140s are identical.
They are insulated on both the front and rear as well as the mounting hole.
I'm curious enough to order some of the ST parts to see what's going on.

Image
Fairchild BD139s showing insulated case and mounting hole.
gstarey66
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Re: Dual Class-A Line and Headphone Output Board Documents

Post by gstarey66 »

Hi Wayne,

Did try to attach some pictures, but didn't seem to be able to. Don't have a web site to post them on as haven't really needed one for anything else. If there is easy way to attach some pics, let me know and I will do so. Have a good shot of the transistors. If you want to, let me know your email address and I will send them to you and you can delete them after you have looked at them.

Regards,
Greg
BarneyL
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Joined: Fri Aug 22, 2014 8:24 am

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

Post by BarneyL »

Hi Guys, finally getting around to assembling my DCAO and wondered what the minimum thickness for a homemade heatsink is?

I can find 20mm x 3mm x 1metre locally for $5 and figured I could make a 6mm thickness by bolting two bits together with heatsink compound in between.

Planning on using it as an insert in my mastering rig with switchable transformer outputs. I have some neve style 1:2 and planning on a pair of CMOB Cinemag as well. Maybe a no transformer output option too?

Keen to hear it on it's own as well as on the unbalanced output of my Manley Massive Passive, through the 1246 and also unbalanced bypassing 1246.

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

Post by mediatechnology »

The 3mm x 20 mm stock that you have should be thick enough.
I recall Roger using a bar about that height.
I don't see any great harm in having the heatsink couple all six devices.
That way you can pick up some extra length.

Since you're going to be using this as a line output you should be able to use a significantly lower idle current with far less heatsink than my headphone builds.
An Iq of 25-35 mA ought to be plenty driving a 150 or 600 transformer primary or equipment load.
BarneyL
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Joined: Fri Aug 22, 2014 8:24 am

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

Post by BarneyL »

Thank you for the quick replies.

I have Fairchild, 139 & 140 parts luckily.

Will post some pics when it gets further along.
BarneyL
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Re: Dual Class-A Line and Headphone Output Board Documents

Post by BarneyL »

Getting there, passes audio and sounds rather good!!!!!! Have -6dB bypassing the transformers, unity gain using the 1:2 steel transformers. Waiting to hear back from David re purchasing some CMOB's. The lowest I can get the idle current is about 157mV on both channels with the 1K trimpot. Using the GP and Out connected to the switch poles to utilise transformers or bypass transformers, then connected to XLR pin 2(+) and 3(-). Using it with no jumpers, tried push pull and it was a phasey mess?
Everyone who's heard it so far is more than impressed, I think it's going to be a great tool in the mastering rig. Thanks Roger and Wayne.
(also sorry for taking the Pro out of Pro Audio Design Forum....)
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mediatechnology
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Re: Dual Class-A Line and Headphone Output Board Documents

Post by mediatechnology »

Glad you like it! Nice-looking build too.
The lowest I can get the idle current is about 157mV on both channels with the 1K trimpot.
That sounds about right at about 48 mA Iq.
Using the GP and Out connected to the switch poles to utilise transformers or bypass transformers, then connected to XLR pin 2(+) and 3(-). Using it with no jumpers, tried push pull and it was a phasey mess?
After looking at the pic and seeing what you meant when you said no jumpers I might be able to explain why.

If you are using one board and the transformer is connected differentially between the two outputs then only one input can be fed and the jumpers are required.
The jumpers then need to be linked vertically to cross-couple the inputs to provide push-pull drive to the transformer primary.

If jumpers are not used and a transformer is connected differentially and both inputs are driven by Left and Right, then the transformer is going to "hear," and you're going to hear, the Side channel in the recording.
In many recordings and almost all FM broadcasts its a phasey mess.
For you to hear left (or right) only with a push-pull output connection then the jumpers need to be fitted or the two inputs wired out-of-polarity to only one channel.
It then takes one board per channel.
Gold
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Re: Dual Class-A Line and Headphone Output Board Documents

Post by Gold »

I'm finally getting around to building one of these as a headphone amp. I don't really like headphones so it wasn't at the top of the list. I think to fully evaluate the Widener board I will need headphones. Wayne and Roger to the rescue.

I guess I have a few questions. I don't think a full schematic of the board is posted. Is that to avoid confusion or because you don't want it out there? I think I might find that helpful.

What's the theoretical maximum current output, say into a short? I have Sony MDR600. The have a 30Ω impedance. I am planning on using a small build out resistor. I think 1W parts should be safe. Maybe one of those 3.3Ω resistors that are used elsewhere in the build. Maybe I'll probably make a second board with a 47R buildout resistor for Sennheiser 650's. I was looking for 1W resistors at Mouser yesterday. It looks like TE bought the Holco resistor line. I was able to find 1W Holco's in a few values. It looks like most of the line is not stocked but i hit a few values they have.
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