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 » Thu Feb 25, 2016 5:37 am

Ve/Re = Iq (330mV/3R3 = 100 mA)

I go for about 280-285 mV for an Iq of 85 mA on headphone builds.
Set it and then readjust it after about 5 minutes warm-up.
It has a slight negative temperature coefficient.
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Re: Dual Class-A Line and Headphone Output Board Documents

Post by Gold » Sat Mar 12, 2016 9:03 pm

I made two different headphone amps. One for the shop and one for the studio. Below is the one for the shop.

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This is the one for the studio. I ended up ordering the fancy TKD pots. I have to send them back. I ordered 10k parts. They were labeled 10k but were actually 50k. That wouldn't have been a deal killer but the resistance track got funny at the bottom of the throw. There was a smooth log response down to 60k (yes 60k) and in the last few degrees of rotation the resistance went from about 60k to 46kΩ. Weird. The Alps pot in the shop build I've learned is an RK40. These are not made anymore but they make an RK27 which is 27mm square instead of 40mm square. If I had known that I'd have ordered those at $14ea instead of the TKD at $100 ea. The TKD at least feel expensive. The build is working. I'm just waiting for replacement pots.

The switch is to select either the input on the back or the input(s) on the front. The rear input will follow the monitor of the console.

Image


Image

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

Post by mediatechnology » Sat Mar 12, 2016 9:27 pm

Very nice! How does it sound?
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Re: Dual Class-A Line and Headphone Output Board Documents

Post by Gold » Sun Mar 13, 2016 5:32 am

mediatechnology wrote:Very nice! How does it sound?
It sounds good to me. Since I don't do a lot of headphone listening I don't have anything to compare it to. I could bring the shop one home and try it next to my NAD. I wasn't using headphones at the studio before.

I'd like to try a pair as line drivers. I'd have more of a reference to comment on the sound. I may try them as replacement line out cards in the Studer A80VU. Another thing I'd like is a variable gain line amp for unbalanced stuff like iPods and cassette decks. I think I'll probably do that first.

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

Post by JR. » Sun Mar 13, 2016 11:04 am

Is that small one inside a sealed plastic box? If so it might be a good idea to add some air vents on top and bottom to allow convection cooling to exchange the hot inside air with cooler room air. The heat sink just helps transfer the heat from the device to the nearby air, but plastic is a poor thermal conductor so heat will build up inside the box.

If the IU uses a metal bottom chassis it should be fine as is, if not consider some air vents there too.

Looks very professional. Nice job.

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

Post by Gold » Sun Mar 13, 2016 1:10 pm

JR. wrote:Is that small one inside a sealed plastic box? If so it might be a good idea to add some air vents on top and bottom to allow convection cooling to exchange the hot inside air with cooler room air. The heat sink just helps transfer the heat from the device to the nearby air, but plastic is a poor thermal conductor so heat will build up inside the box.
It's in a sealed plastic box. I was going to wait and see how hot it got over time. I haven't had it on all day yet. I'll add some vents if necessary. I'm not sure how I'm going to mount it in the shop. If I cut vents now they will almost certainly be in the wrong place. I used a shielding spray on the plastic box. It was very expensive and highly toxic. I think that's about $25 of coating. Never again. I like the copper tape a lot better.

For the build in the rack case I can add material to transfer heat to the case. I thought I could drill a few 1/2" holes on the heat sinks and insert some round bar stock that attaches to the case and the heat sink.

On this build I used the bonded twisted pairs from Belden Mediatwist for the audio inputs for the first time. I think I will use this in the PTS builds for the phono inputs. It's nice stuff.

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

Post by Gold » Mon Mar 14, 2016 7:49 pm

I left the DCAO in the plastic box on all day today. The box gets a little warm. The heat sink gets hot. I can only touch it for a second or so. I will modify the heat sink. I have another piece of flat bar stock. I'm going to drill a couple of holes through the original heat sink, the box and the new heat sink. I'll use #10 machine screws to attach the inside and outside heat sinks together. I can also use the outside heat sink as a mounting bracket.

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

Post by mediatechnology » Tue Mar 15, 2016 5:09 am

In free air at 85 mA/channel Iq I usually see about 135 degree F on the heatsinks I use.
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Re: Dual Class-A Line and Headphone Output Board Documents

Post by JR. » Tue Mar 15, 2016 11:35 am

While steel is not a great thermal conductor, 2x #10 screws will be a ton better than plastic. Will you sandwich plastic between the two heat sinks? With elevated temperature over time the plastic may deform or flow allowing the screws to get loose. Larger holes with round aluminum spacers could make it a metal to metal connection with improved thermal flow as a bonus, while I am more worried about the mechanical integrity than thermal.. The steel screws should move adequate heat (WAG).

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

Post by Gold » Tue Mar 15, 2016 4:54 pm

I'll connect the inside bar stock to a piece of bar stock outside the box via screws. The screws won't touch the plastic with some clearance. I'll also put nuts where there is exposed thread to increase mass. I may be able to steal a half hour tonight to do it. A picture and all that...

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