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Old 17th November 2011, 10:32 PM   #11
agdr is offline agdr  United States
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Default O2 headphone output relay - part 2

Next the relay contacts are soldered to those output 1R resistors and P2 holes as mentioned. The first 3 photos shows the connections.

Then the power wires for the control circuit have to be soldered onto the power managment circuit mosfet outputs, as the next photo shows. The wires are run down through one of the holes RocketScientist has provided in the PCB, by the 470uF filter caps, and across the bottom to the mosfets. I'm connecting here so the relay current comes right off the O2 mosfets and is not pulled through board PCB traces.

Next some insulating electrical tape is applied to the back of the control board, as the next 2 photos show, and the board fit into its final resting place.

Some tape then goes over the top of the whole thing, as the next photo shows. Finally.... it all slides into the B2-080 chasis in the final photo.

Happy DIYing! And congratulations again to RocketScientist on the O2. It is a great sounding amp - and a lot of fun for DIYers like me to mess around with.
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Last edited by agdr; 17th November 2011 at 10:36 PM.
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Old 25th November 2011, 12:38 AM   #12
agdr is offline agdr  United States
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Default O2 amp input attenuation mod for even less gain at 1x

This modification increases the value of the R3 and R7 input resistors on the O2 to 4.99K. The change attenuates the incoming signal voltage by 1/3 since the two resistors form voltage dividers with R20 and R14. This mod is most useful for cases where even the 1x gain position is not low enough. More specifically for getting more rotation out of the pot in the 1x position with sources that have very high output signal levels.

I have a DAC and a laptop that both output fairly high levels. Not enough to cause clipping with the O2, but enough so that "loud enough" with my particular headphones occurs at only 30% rotation on the pot with the DAC and 50% with the laptop. Ideally I would like more pot rotation. I know upfront that I won't be using this particular O2 with anything else that would need more gain (more pot rotation). After this mod the laptop is now at about 80%-90% rotation for "loud enough" even for low recording levels and the DAC is up to 50%-60%.

A downside of this mod is not much pot rotation left in the low gain setting for future devices. If more gain is needed in the future with different input sources then pushing the gain switch to the higher setting would be required. Essentially this mod is custom tailoring the pot rotation to specific input sources/devices by "using up" the excess pot rotation. Another downside is increased noise generation by the higher value input resistor, as RocketScientist explains in that part of the O2 tech section writeup.

A side benefit of this mod is 50% higher input impedance at 15k vs. 10k originally.

This mod is easy. R3 and R7 are just replaced with 4.99K resistors

SFR16S0004991FR500 Vishay/BC Components Metal Film Resistors - Through Hole

The original RF input filter had a corner frequency of 2.6mHz

1 / [(2)(PI)(274R)(200pF)] = 2.6mHz

so C11 and C12 also have to be reduced or the gain roll off would start to enter the audio band. Rather than maintain Fc = 2.6mHz I'm using 68pF of the same series as the original cap (MLCC C0G) to give a corner frequency a bit lower at 470kHz.

K680J15C0GF5TL2 Vishay/BC Components Multilayer Ceramic Capacitors (MLCC) - Leaded

I'm impressed with the results! Just what I was trying to achieve. I also tried 10K resistors for R3 and R7 to form a 50/50 voltage divider, but found that using the laptop they didn't provide enough gain for the very lowest-level recordings I could find. The pot hit the end of travel and I still needed it louder. The 4.99k resistors on the other hand still had enough pot travel for slightly "too loud" even with the soft sources.

Last edited by agdr; 25th November 2011 at 12:52 AM.
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Old 25th November 2011, 03:43 PM   #13
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Default awesomeness

This is excellent. Thanks for sharing. Please continue to do so.
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Old 27th November 2011, 09:13 AM   #14
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These are great looking mods, I've done or will do pretty much all of them.

Thanks for the explanations as well, it's a great learning experience, I suppose.
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Old 27th November 2011, 09:55 AM   #15
Turbon is offline Turbon  Sweden
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Yes, keep up the good work agdr and thank you for sharing!

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Old 2nd December 2011, 08:04 PM   #16
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Originally Posted by agdr View Post
1. rectifiers changed to 2A 100V ultrafast silicon rectifiers powered by the 16VAC 1000mA Triad unit in the BOM (WAU16-1000). The mouser link for the diodes is
I put a 0.1 uF/250V ceramic across each of these two stock diodes.
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Old 2nd December 2011, 10:58 PM   #17
agdr is offline agdr  United States
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Thanks for the comments! My favorite mod so far is the output relay mod, if for no other reason than I'm still amazed I was able to get relays and control circuit to fit in the tiny space available.

dirkwright - that is fine. 0.01uF ceramics would even work.

Some additional info and typo corrections I've been meaning to post:

attenuation resistor mod-

In my equation to calculate the corner frequency of the existing circuit those input capacitors C11 and C12 are 220pF of course, not 200pF. The result is correct though of Fc=2.6mHz. Just a typo.

The net result of the attuation mod will be fractional gain, of course, in the low 1x setting. (2/3 * 1x) = 0.67x. The antenuator resistor will affect the high gain setting the same way. So if your gain switch resistors are set up for 2.5x high gain, the new net result with the attenuator will be (2/3 * 2.5x) = 1.67x, which is fine since the whole assumption is the input signal levels are too high to begin with to allow full travel of the pot. To preserve a net total gain of 2.5x in the high gain switch position the O2 high-gain resistors would need to be 549R (3.7x basic gain), (2/3 * 3.7x) = 2.5x. Mouser unfortunately is out of those until the first of January:

270-549-RC Xicon Metal Film Resistors

Output relay mod-

The silicon test lead wire I used there is 24 gauge, not 26 gauge. Which explains why I kept cutting off one of the 7 strands in the wire every time I stripped one of those wires! Don't know why I started thinking it was 26 gauge. So from that TestPath link I posted it is this stuff

Cal Test CT2956 5 10 > Silicone Test Lead Wire, 7 Strands, 2A, 24AWG, 0.055 OD, Green, 33 ft CT2956 5 10 Cal Test Test Equipment

One unexpected effect showed up that I've decided is a feature, not a bug. When listening just on batteries the relays will engage and cut off the headphones for about 1/2 second when the AC adapter is plugged in. This happens because the rails suddenly go from battery voltage, 16Vdc or so, up to AC voltage of 24V. That means the timing capacitor in the control circuit goes from being fully charged to needing to charge up 8 more volts again. The cut-off time is only 1/2 second since the capacitor is mostly charged to start, of course. But since the whole design goal is to disconnect the headphones when startup events occur, I consider this effect right in line with the what the circuit should be doing.

Another feature of the design using that tiny 2.2uF capacitor for the timing cap is that it doesn't hold much charge. Which means that no matter how fast you push the O2 on/off switch back and forth you get the full 2 seconds of relay "off" time. In other words just any amount of off time at all for the O2 is enough to dump the charge on that 2.2uF and force it to recharge, giving the 2 second relay delay. That was the theory anyway when I designed it. Sure enough the actual results match.

On the control circuit, instead of using a 27Vdc zener across the relay coils for suppression, a smaller 20Vdc-or-so zener can be using in series with a reversed biased 1N4002. With the 27V option the rails can go up to 27V momentarily when the relay cuts off. The other option is probably the best in retrospect since the rails would go no higher than 20Vdc. The relay turn-off time table in that link I posted actually assumes the latter, a zener just under rail voltage in series with a reverse biased rectifier diode. Either way though is better than just a reversed biased rectifier with no zener.

And finally in my description of the control circuit that 10V zener is performing a logical "AND" fuction, of course, and not an "OR" as I wrote.

Last edited by agdr; 2nd December 2011 at 11:01 PM.
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Old 8th December 2011, 05:05 AM   #18
regal is offline regal  United States
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Thanks for the idea to add a resistor between the filtering caps.

Easiest way I think it to cut the trace between caps (make sure with DMM) then lay a 2W 10ohm wirewound 4527 SMD resistor between the cap terminals.

Also really like the tip about doubling the cap and halving R12/13 to minimize offset.
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Old 8th December 2011, 11:33 AM   #19
agdr is offline agdr  United States
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Originally Posted by regal View Post
Thanks for the idea to add a resistor between the filtering caps.

Easiest way I think it to cut the trace between caps (make sure with DMM) then lay a 2W 10ohm wirewound 4527 SMD resistor between the cap terminals.

Also really like the tip about doubling the cap and halving R12/13 to minimize offset.
Using SMD resistors is a good idea! I hadn't thought of that. There are actually 3 cuts between the 470uF caps for that CRC filter as I have in those first 2 posts. Two on the bottom and one on the top. He has a trace on the top in parallel with the one on the bottom on one rail. Yes definitely check the cuts with a DMM. I did that on all my cuts and forgot to mention it in any of the posts above. It would be easy to miss a wire "whisker" on a cut, even with a magnifying glass.

Last edited by agdr; 8th December 2011 at 11:42 AM.
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Old 8th December 2011, 08:05 PM   #20
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I like the relay mod! What's up next: stepped attenuator ?
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