This is the little brother of BUF634 — at one time it is OPA634
It have a higher bandwidth 260Mz compared to max buf634 of 180Mhz
Faster at 2500v/us compared to Buf634 2000v/us
But it is not as powerful 100mA output current compared to 250mA
It draws a higher current than BUF634 as well but not much.

The Pin out of OPA633 is unconventional, It is not the same as any other op-amp. And hence we need to bend the pins. By inverting the 4 connected pin it would fit nicely into the D10 without even soldering. Show in the picture are 2 simple modification and 2 with supply caps and double bypass.

How does it sound (compared to BUF634), It is more detailed, more air around the instrument and vocal but and slightly brighter. Pair better with my UM3X. But for my HD595 I prefer the BUF634.

There is a however very slight hiss on opa633.

After

I have completed D4 Modding sometime back, finally I have some time to write this (it so happend I wrote this at about some 25,000ft above sea level). It does seem that most of the modders are busy with the Fi.Q very lonely here.

The changes started off as a Dream of using the D4 at 9V or 12V which proved to sound much better than 5V especially for bigger headphone and definately HD600.  At 12V it drives HD600 comfortabily close to a desktop amp.

The DC to DC converter consists of a filtering circuit before going into the up-converter. A regulator to regulate the voltage to 12V and a enhanced Pie Filter.

When I was taping the 5V from the D4 I noted 2 nos of zero ohm resistor at position L1 and L2 I think the designer at ibasso have in mind 2 little ferrite beads or some small inductor but at a later time decided not to install it. I took out the 2 zero ohm resistor and replace it with 2 small inductors. This would help to reduce ripples at the power lines. There are 2 different power regulator, one for the AMP (L/R and BUF), and another for the DAC + 8616.

The AMP reservoir chosen by iBasso was a 10V 2200uF Nichicon HZ series Capacitor, This is a very good choice by iBasso, Given a choice I would not have change this but the rating of 10V was insufficient and I was force to change it to a 16V part. I chosen a Rubycon ZL series.

The DAC Capacitor needed to be change the 100uf Nichicon Fine gold would not cut it here, I replaced with a Sanyo WG series 1800uf. The reservoir for 8616 was likewise replace with OSCON with WIMA bypass.

When I solder the DC converter to D4, I preferred to remove the battery connector, with that removed I have additional space to put in another reservoir for the AMP. A similar Rubycon ZL. A diode D1 was removed as it was no longer needed. D1 is to prevent a a short if the battery is inserted with the wrong way. Since there would not be a battery here, there isn’t a chance of that happening. By removing that I have an additional 0.4V.

Buffer used here are BUF634. They proven to be a favorite and is notably better for big cans.

L/R Just when I was about to settled with 797ANZ, someone in the forum bite the bullet with 797BNZ, Hopefully he does not posion the owner of the D4, means more work for me.

Additional mod possible is to remove the relay, It is not needed as the owner is mainly using this from powerbook.

Do you think I have void the warranty of this D4 ?

This is the measurement from the Output of the AD8616. The Ad8616 is the opamp buffer after the WM8740 DAC. It belongs to the DAC section of the D4. At the input of the AD8616/output of WM8740 the Signal I pump in using the computer is a Triangle wave. Level set at 100%. From the scope it is a perfect triangle shape. But at the output of the Ad8616 this is what I see.

Sorry for the poor image, I have one hand on the probe and hand on the camera.

So confirm there is clipping.

So how to solve this problem ?

1) You could up the voltage of the AD8616. (Possible to tap from the 5V amp or from WM8740, but could be noisy, need to break traces as well )
2) or reduce the Gain of the AD8616 so that the output voltage is below clipping.

A temporary solution would be to set the volume of your computer to about 80% of max so that the clipping does not occur.

Initial Solution, quick remedy, (Have not spend much time thinking of the best solution). I decided the quickest solution to the problem is to reduce the gain of the AD8616. It original gain of the AD8616 is 2, by adding a resistor from the output of the 8616 to the negative input we could adjust the gain.
The original resistor across is 2.2K ohm.

The Gain resistors are located on the top side of the board, and to solder on that you need to remove the Big Nichicon HZ cap to troublesome, to fix this way. (Ignore the yellow capacitor on  c23 it is an additional bypass to the amp section. which is not related to the mod for the clipping problem. )
So instead of changing the resistor, I found a compensation capacitor beside the AD8616 which is connected in parallel to the feedback resistor. By adding a 2.2K ohm across this I reduce the gain to about 1.5. From the Picture the 2.2K resistor is sitting on top of the capacitor. label C26 and C28.

You could also solder the resistor directly on PIN 6,7 and PIN 1,2 on the AD8616. (If I do that I would be tempted to change the AD8616! ). This would be the recommended solution,  as I have know people who have good soldering skill damage the 1000pF capacitor C26 and C28.

I so happened to have a 2.2K 1% SMD 0805 resistor at home. How lucky.
So after adding the resistor I test it again and the Clipping would not occur.
Now I could feed signal at 100% resolution to the D4 instead of just 80%

It should be placebo, on initial testing I could hear more details from the DAC now. Sweet. Well in theory it is 100% now instead of 80%, so might not be placebo.

Some end notes: This problem occurs because the AD8616 does not have enough voltage and hence is not able to swing to the full potential of the WM8740 at 2V p-p. The AD8616 need at least 4.5V (it is a rail to rail op-amp but still some headroom is required). It is not possible to modify the voltage output of the WM8740. A cleaner solution would be to provide a higher voltage to the AD8616,which seem to have an independent power supply taped from the WM8740. But this requires cutting traces and rewiring, which I would not recommend.

The WM8740 is a 16bits DAC by using it at 80% would means that the resolution of the DAC is reduced. At 80% looses less that 1 bit in resolution but it would defeat the purpose of the dual DAC on the D4. But during normal not critical AB test most would not notice the difference.

This problem only affect the DAC section of the D4 the amp section is not affected.

Wounder why I got them there. I plug 2 of them into my Socket-ed Brown dog and then into the LR of the D10.

Guess what — It worked !!!

How it sound ? — Devoid of highs, Devoid of low, lifeless mid. But hey it worked.

The 741 is THE op-amp that every other op-amp is based upon. The Greatest Grand Daddy of them all. Fit for a museum. For a very long time every engineering book about op-amp feature this. This is the Part which have the most offspring in silicon history, less the transistor.

Note that the 2 op-amp comes from different company, In the pass they shared part number which resulted in numerous compatibility problem. Like what John mention A 5534 from different company at different time sound different. Added with the ease of remark/cloning part numbers, All new op-amp have different part numbers even if they are spec likewise.

ADA4899-1 — It is the top 100 product by EDN for 2006 A great achievement for an OP-amp

This should be the top of the range Low voltage Op-Amp from Analog Devices.
The pin out is different from the standard single op-amp, Like the first exotic there are different mode of operation.

1) It could be disable or power down to save power
2) Normal
3) Low input bias current
For Audio we could try Normal and Low input bias

Look at a chose up picture of Chip, Note that there is a heat spreader at the underside of the the Chip.

How does this sound ? (Note initial impression less than 5 min testing, Normal mode)
Very close to the 2*AD744 in OBCA mode but without the Hissing (silent even at high volume)
The mid are mellow and sweet, There is a sense of creamy smoothness
I was surprise the the high are close to the AD8599 in terms of clarity, Very quick as well. Usually when something is mellow it is not clear. But this has both. Neutral sound.

Picture of the Chip in modified Brown Dog suited for the D10

To the right is 2*ADA4857-1. That is different from the standard single op-amp as well, Note the rework wires on the chips. But It did not make it to my exotic list of Op-amp for the D10.

How does the ADA4857 sound ?
Very sorry because I audition the ADA4899 just before this. Not comparable, but still very strong. It still beats OPA2350 (non class A, I do not have a class A OPA2350 as yet) slightly in the mid and is much clearer in the highs.

Both are audition with AD8656 in the Buffers

After a second round of testing
The ADA4857 is a very detail amp, I heard things I never heard before. I think plenty of people will like this especially people who like the UE, JH IEM rather than the Westone camp. If you like string instrument this is for you. The guitar in Eva Cassidy’s “I know you by by heart” is strikingly life like.

For people whose primary interest is in woodwind instrument and vocal, ADA4899 is better.

Buffers LME49600
This is the upgraded/clone of BUF637

Comes in TO220 style TO263.

This is build to be used mainly as buffers for headphone amps.
Gain is fixed at 1. So it would be very difficult to use for any other application.

This OP-AMP comes in 2 mode, BW at 110Mhz (Normal bandwidth) or 180Mhz (Wide Bandwidth fast mode). Initially I wanted a jumper so that I could easily switch between the 2 modes but due to the lack of space. I made 2 pair, One in each mode.

Doing a AB test with the EL8201, this sounded much more confident and open up the sound stage, The biggest and widest I heard. More body to mid as well. unfortunately it is slightly brighter but quicker as well.

However hard I tried, I have not found a way to fit this in the casing.

EDIT ADDED
I just realize that I have yet to optimized the LME49600 fully for the D10, I only connect one of the output to the D10, if I connect both the output the difference would be like going from the single bypass buffers to the double bypass buffers. The sound impression given above was for Single buffer, Normal mode. For this one OP-AMP there are 4 option to fit it on the D10
1) Single output, Normal
2) double output, Normal
3) Single output, wide bandwidth
4) Double output, wide bandwidth

But judging from the experience of the Single bypass, double bypass testing. the 2 real option should be the both double output ones.

EeeeeeRrrrrrr…… while writing this I realize there are another option. use 4 LME49600 as buffers.
2 for the left side and 2 for the right side. But this would be an overkill, D10 only have a voltage of about 4.5volts. Is there a need or ability to drive that much current with 4.5V. But this is Hi-Fi you never know.

For user who like the sound of AD8656 and Bypass buffers you should try this

Double Bypass buffers.

It sound warmer and there are less sibilance and bass is tighter as well, compared to the bypass buffers that comes with the amplifier.

This one is really easy to make as well just solder a small wire joining pin 2 and 3, and another joining pin 5 and 6 on an empty socket.

Using the single buffers supplied by the factory The output resistance between the L/R amp and the headphone is about 6 ohm. Using the double buffers it is reduce by half or about 3 ohm. Allowing the L/R amp better control of the phone.

This combination sound really good. It is very good for battery life as well.
But is only suitable if you have an easy to drive IEM or headphone.

Blue LED

How does it sound ? It is just bluer, and more Beautiful
Tested with Bluer than Blue by Livingston Taylor, Chesky Records

Some may think that changing to blue LED would require more power and hence less battery life.  But that may not be the case, Taking a textbook LED implementation on 5V a 330Ohm resistor is connected in series with the LED. The turn one voltage for red LED is 1.7V for Yellow is 1.9 for Green is 2.1 and blue 3.1Volts So the effective current for Red LED is 10mA, for yellow it is 9.3mA for Green is 8.7mA and Blue is 5.7mA. Just changing from Yellow LED to blue LED would almost save half the power. Most of which is loss to the resistor. But I am not that power conscious. I reduce the resistor value so that the power consumption is almost the same, The blue LED is too dark if you just change it, without changing the resistor.

This is a 1Khz Square wave using SPDIF. The left and right signal is place close together on a dual trace OSC-Scope

The wave form is mono and I adjusted the amplitiude on the OSC-Scope so that you could see the waveform side by side. The ringing (overshoot and undershoot) is normal.

Using USB and even trying with different computer this is what I get which is NOT normal.

As you could see highlighted by the 2 red arrow the signal is not in phase. It is about 4 degree out of phase, Or about one sample (sampling frequency) off. I could offset the phase in the signal generator so that is how I calculate the difference in Phase.
What this means is that one side of the sound is coming out slightly faster then the other. The Left and Right signal is suppose to be Mono or equivalent to Mono.

I will try to investigate more. Sorry for another troubling find.

Found an Errata from Texas Instrument.
http://focus.ti.com/lit/er/slaz036a/slaz036a.pdf

An extract from the Errata

There is an upgraded part 2906B
And according to the article, publish in Jan 2009 at
http://focus.ti.com/lit/ml/slel056/slel056.pdf

If this is the case, Most of the existing product in the market would be affected as PCM290x is widely used.
EDIT: I wounder if the PCM270x are affected need to investigate

Using a USB to SPDIF is unlikely to solve the problem as the device is most likely using a PCM290x device. Unless it is using the upgraded version.

I need to catch some sleep.

I am really not expecting this.

A am really force to modding this at a very early stage. Ideally I would have like a few more days or maybe week reading and understanding the circuit before any modification. I like to procrastinate and dream about every mod.

The following mod is what I would like to have done at this stage

1) Change the to Output bypass capacitor to MKP, But I do not have the required parts at home.

2) Change 2 of the resistors to Vishay-dale connecting Pin 2 and pin 6 of the LR Amplifier to ground.

The 2 feedback resistors are already using Vishay-Dale and I would like the input resistor to match that as well.

3) Change the 2 Input coupling capacitor to something else.  This would be hard to source as I could only place the cap on this side of the board and I only have about 3 or 4mm thickness to play with.

I end up only being able to have sufficient parts at home for the following mods

A) This 2 capacitors is to help the frequency response of the input DC coupling capacitors. I have wima at home but it just could not fit here.

B and D ) Power supply decoupling. This help reduce the RF interference and is quite effective especially for blocking mobile phone  signal.

C) what I could make do at home with the Output DC blocking capacitor

E) This is to match the other resistors so that we could have a balance output when Gain is set to High.

So far the main benefits I hear from the above mods are better RF interference – this is very obvious

Better clarity at the high end. I would not hear the whistle of Jennifer Warnes’s Lights of Louisianne which is not present before the mod. The mid is more mellow as well.

I am still using the original Op-Amp and have yet to roll, I have gather enough information for Op amp selection.