[babs]

I took a little break with my description of my mods because i actually trying  a new Op Amp for the HO outputs
 
More on this, maybe tomorrow....

 

[babs]

OK,  I continue with the HO output amplifiers Mod.
 
(Warning; lot of technical stuff ahead...)
 
The HO drivers are the DX50 Headphones amplifiers
There is two amplifiers, one for each channel (right, left)

 
Pin 1 of these amplifiers are at the top left on that picture (small dot)
 
These amplifiers have the following implementation:
- 9V unipolar power supply
- Single Op Amp, 8 pin Soic case, standard pinout
- Output Bias at 4.5Vdc (I didn't measure the input operating point)
 
In addition, due to the usage of this amplifier, the following characteristics are important:
- High output current, designed to drive low impedance load
- Low input voltage noise
- Medium to high speed (more than 10MHz GBW is a minimum for me)
- Low power to preserve battery
- Low THD
 
As for the differential amplifier discussed earlier,  is not really possible  to put a socket to test lot of OP Amps, and make a choice between sound signatures
 
So I choose for my first try, the sibling of the LME49721 used for the differential amplifier : the TI LME49710MA
Specifications:
- Designed for high quality audio
- Medium speed: GBW = 55MHz
- Low noise: 2.5nV/sqrtHz
- Medium power consumption : 4.8mA
- vanishing small THD
 
The problem with this amplifier (and with the majority of the "Audio" type OP Amp): its not designed to drive low impedance load, it is designed and specified for a 600 Ohms load and greater.
But the majority of the commercials headphones are in the 100 - 300 Ohms range, IEM can go as low as 15 Ohms (as my Senn IE8)
 
Obviously, this not prevent these amplifier to be able to drive lower load than 600 Ohms, but in this case, the behaviour of the output section will be unpredictable; thermal distortion is one of the possibility, or saturation or unstability.
 
The only way to check that is to just try it!
 
So I changed the two amplifiers with the LME49710
 
(As for the previous modification,  the following evaluation is 100% psycho-acoustic stuff...)
 
With the HD650 (300 Ohms), I note an evident increase in dynamic and punch,obviously not to the level of my desktop class A amplifier, but significative and worthwhile.
Soundwise, the new amplifier added to to the already good improvement of the previous differential amplifier Mod, the two models of TI seem to have the same sound signature: the soundstage seem to open up a little and sound seem cleaner.
 
But using such neutral sounding amplifiers lead to a less warmer sound than the stock amplifiers. little clinical on some tracks
 
With the Sennheiser IE8 (15 Ohms), the same  sound remarks applied, but I notice some compression and limiting on heavy music (Muse, King Crimson...)
Even if the IE8 are very sensitive IEMs, the amplifiers seems to struggle with their extremely low impedance.
But I will keep it for the moment...
 
***************************************************
Another worthwhile modification, in the same circuit, is the replacement of the OP Amps decoupling Cap, placed on the top left of the board (see picture)
The original capacitor is a standard 220uF 10V tantalum in a 7343 case
I replace it with 3 x 47uF 10V Oxicap (AVX #NOJC476M010RWJ)
The three capacitors are soldered  in parallel to obtain around 150uF (not my best soldering result...shame)
 
Result: better control over the bass and midbass, more body...recommended
 
***************************************************
I used this HO amplifiers Mod for 4 month with pleasure but with some frustration: I mainly use the DX50 with my IE8 and
the dynamic limitation of the LME amplifiers on my King Crimson tracks trigger to me an irrepressible modification fever!
 
So I search a more powerful Op Amp who will fill the bill
 
And I fund one!:
This an new model of Analog Devices (in the ADA series) the ADA4897-1ARZ
Specifications:
- High speed amplifier GBW = 100MHz
- Ultra Low noise: 1nV/sqrtHz
- Low power consumption : 3mA
- Small THD
- Designed for 100 Ohms load (LME: 600 Ohms)
- 80mA output (LME: 26mA)
 
OK, it is not an "Audio" amplifier; it's an high speed amplifier, and  no specifications goes lower than 100kHz
But the THD at 100kHz is lower than -80dB with a 100 Ohms load, impressive!
These little amplifiers can drive 80mA, much more than the majority of "Audio" Op Amp.
 
OK I will try it!
 
The first impression is very deceptive!: trebble-ish, no bass, complete lack of body...
 
I open up my solder gun...but..., as for any mod, maybe we need some break-in!
 
I let the DX50 put some music on 15 Ohms resistor loads for the night...
 
At morning I put the HD650 on the DX50 for a check and... oh my god!, yes! these amps are the good ones!!
 
The ADA4897 seem to fill all gaps in the previous sound of the DX50:
- Smooth and detailed
- The return of a touch of warmth
- Powerful attack on drums kicks, excellent control of resonance
- No more compression
- 20th Century Schizoid man is now just so beautiful!  (beautiful?...)
 
OK, it is not yet at the level of my desktop class A amplifier, but the differences are not so big, and I can live with that
 
To be continued....

 

[WitzyZed]

a great read! getting giddy as you tweak it perfectly. & as a fan of muse, I'm glad you found a mod to suit their & King crimson's sound. that would drive me crazy!

 

[liddokun]

Just bought a DX50 and I'm so excited about getting it and modding it.  Love modding my equipment! 

 

[palermo]

Hi babs,
Actually, what the stock opamp is used as differential amp? I just finished read your write, but did you mention it?
I think you should try Ad823 for this place. This opamp can operate start from +/-2.5v

 

[babs]

The stock amp used in the DX50 as differential amp is buffed to remove the top marking, so i don't know his model.
But its a low power amp (< 1mA quiescent current), typically, low power amps are slow and noisy, but I can't say more...
 
Yes, the AD823 can be used in this place, but in my opinion, i doubt it can sound better than the LME49721

 

[palermo]

Thx babs. changing the diffrntial opamp may change soundsig significantly to both LO and HO.

 

[egosumlux]

A very interesting read indeed thank you very much this is highly informative

 

[CoiL]

Babs, one question - what FW are You using to evaluate sound and signature changes? If You haven`t tried yet... then Sound Unlocked 1.3.3 @ FLAC-mode / IRR OFF / CPU-Default or Sound Unlocked 1.3.5beta @ FLAC-mode / IRR OFF / CPU-408kHz take this DAP to another lvl soundwise compared to stock or other custom FW`s imho.
I wonder what this FW can achieve along with Your HW upgrades

 Give it a try

 

[mvsainz]

Hey there!
 
First of all, thanks for all the details. I definitely intend on doing a couple of the mods to my DX50 in the near future. But just wonderings... since it gets a little confusing with so many part numbers and different components, would you mind summarizing which are the most worthwhile mods for the general listener (someone with varied musical taste, HP's between 25 and 300ohms, and only wanting to improve on the dx50's already great quality)? Just a list of which mods you are completely happy with, and which were the part number and quantities used. I'd really owe you one if you did! Anyways, in either case I hope I get around to ordering the parts and putting some more time on my rework station

 

[CoiL]

mvsainz, before going "hardcore", did You try Sound Unlocked 1.3.3 FW with different hidden bootup settings @ FLAC-only/lossless mode? It really takes DX50 SQ up a notch
But Your question is x2 from me... also would like a summarize of "best" parts list.

 

[mvsainz]

Truthfully, I've been waiting for the DX50's official firmware to come out. They told me a new version should be coming out and day now, with alleged support (finally!)for m3u playlists. And about the SU FW, I really don't know much about it, and haven't had much luck looking for a decent review online. Searching superficially at least. Any way, I'm glad to see I'm not the only one who would like to see a summary of the mod. Plus, it's just so much satisying to listen to something you've tinkered with yourself. Hell, if at least placebo effect raises the SQ, I'm happy.

 

[ptjw99]

 

Spoiler: Warning: Spoiler!

Now, the tests!:
 
I put on test lot of capacitors, even some capacitors too big to fit on the case!
I will resume my finding for some capacitors with PROS and CONS; each capacitor have a distinct sound signature, each capacitor modify the sound and there is no perfect capacitors
at certain degree, the choice of a capacitor will depend of personal taste, so take my comment with a grain of salt.
All capacitor have a minimum of 100 Hour of break-in
All tests with a Sennheiser HD650
Listed in order of test
 
1) Jovial 220uF 10V (original capacitors)
 PRO: easy sounding, a certain warmth, good bass, good mid (voice), unocuous treble
CON: 'small' sound, restrained soundstage, lack of details and dynamic (veiled)
 
2) Nichicon FW 220uF 10V (#UFW1A221MED1TD)
 PRO: easy sounding, open soundstage, good balance between the bass, mids and treble
 CON: can lack precision and dynamic, some haze over the sound
 
3) Nichicon FG MUSE 100uF 10V (#UFG1A101MEM1TD)
 PRO: natural sounding, open soundstage, good balance between the bass, mids and treble, some warmth
 CON: can lack precision and dynamic, some haze over the sound, limited bass reach  (OK with HD650)
 
4) Nichicon F95 'MUSE' 220uF 6.3V (#F950J227MBAAM1Q2) tantalum SMT capacitor
 PRO: precision and lot of details, good bass
 CON: artificial and hard sound, tiring...
 
5) ELNA RFS Silmic II 47uF 6.3V (#RFS-6V470MF3#5)
 PRO: soundstage ok, good balance between the bass, mids and treble, uncolored, same form factor as original
 CON: lack precision and dynamic, haze over the sound, uninvolving...dull (big deception)  very limited bass reach
 
6) Kemet T520 220uF 10V (#T520D227M010ATE018) low ESR polymer SMT capacitor
 PRO: precision and lot of details, good bass, uncolored
CON: artificial sound, lack balance, uninteresting
 
7) AVX Oxicap 220uF 6.3V (#NOSD227M006R0100) Niobium oxide capacitor
 PRO: precise soundstage, superb bass and low mid, lot of detail, superb control and precision
 CON: can be colored and dark , the treble is a little recessed

 
Conclusion:
I have some difficulty to make a choice between the Nichicon FG and the AVX Oxicap:
- the Nichicon FG is more easy to listen, natural, relaxing; the AVX is more dynamic and involving
- And the AVX seem somewhat colored to me with a little recessed treble
 
But I finally choose the AVX, I can't resist! but its a purely personal choice, it will depend of your sound taste, and your headphone, etc...
 
 
To be continued....

 
I am thinking of replacing the HO capacitors with something more reputable and reliable (to me) like from the Nichicon KW series and I was just wondering what is the correlation between the capacitance values and sound? To my limited understanding, there is a link between the impedance of the headphones used and the lowest frequency passed by the capacitor depending on its capacitance (the formula 1/(2 x pi x F x R)).
 
Is there any reason you replaced the 220uf caps with 100uf caps? Wouldn't that result in the lower frequencies being attenuated more than if you used a higher capacitance value? Or am I missing something out completely?
 
I am using a pair of HD25s with my dx50 and they have an impedance of 70ohms, can/should the capacitance values be changed to something more suitable that allows the lower frequencies to be passed?

 

[babs]

I try the 100uF Nichicon  Muse FG because the 220uF is too big to fit in the case
I try these caps just to check the SQ of these capacitors with my HD650 (300 Ohms)
If your cans have 70 Ohms impedance, you can use 100uF capacitors
 
The main link between capacitance value and SQ is the lower frequency response cutoff:
with the 100uf caps (total=200uF; we have two of them in parallel), the frequency response cutoff will be 2.6Hz (HD650) or 12Hz (HD25)
 
But even if the lower frequency  is OK, bigger caps can sound better than smaller valued caps

 

[CoiL]

  Babs, one question - what FW are You using to evaluate sound and signature changes? If You haven`t tried yet... then Sound Unlocked 1.3.3 @ FLAC-mode / IRR OFF / CPU-Default take this DAP to another lvl soundwise compared to stock or other custom FW`s imho.
I wonder what this FW can achieve along with Your HW upgrades

 Give it a try

babs, can You give feedback to this please?