Modding and Audiophilism

i asked previously ,but wasnt registered..

thank you

Hi

I had plans to make the HK990 work without DSP board itself ( for people that use just analog inputs and don’t need digital inputs ). Currently work related stuff is chewing up my free time so i didn’t develop anything for it....

Additional things were removed in the Frankenstein 2

- BL1 replaced this time as it started to develop the failure

- R171 / R172 removed

- Q21 / Q22 removed

- DW6 / DW7 removed

- C24 / C63 removed

- C162 / C23 replaced for Nichicon Muse KZ 100uF / 25V

- Added additional TPS7A4700 / TPS7A3301 instead of all the above removed components to generate the needed 7V for the volume control chip ( HK990 has a similiar thing but with LM317 style regulators and the same volume chip )

After a long while of "why am i not satisfied how my HK990 sounds" i decided to dig a little deeper into this.... Turns out HK990 is NOT suitable for high efficiency speakers as it's not linear in sub 1W output. ( i can prove this easily with calibrated mic measurements of each amplifier ) After i've seen the response of stock HK980 that my parents use compared to my lightly modded HK990 i decided to get another HK980 and finally do all of the treatments that i ever wanted to do. Below you will find a log of what was changed ( for now ) if you want to do something like this yourself. The change is very drastic coming from stock HK980... even my first Frankenstein HK980 or lightly modded HK990 can't come close to this in terms of sound.

The sound output now is truly something else... highly detailed... dynamics are typical HK sound but much more pronounced.. very nice soundstage ( you can clearly hear what is happening far away from the mic if you are listening to classical music or some of the beatles / acdc tracks ). All in all about what i would expect from stock HK990 but sadly my dream amplifier dissapointed me... I wouldn't reccomend this to someone that's new to this as there are some challenges space wise and i don't want you to damage something just because you have seen some random guy on the internet :)

Incase someone decides to do this the 4x Nichicon KS holder is here.

There is also updated version of phase plug for Fostex Fe206En ( courtesy of hundreds of measurements with the amps ) and you can find it here. This design might change as i'm still experimenting.

The changes are as follows:

Main board - IC20 (KA7815) / IC21 (KA7915) replaced for  TPS7A4700 and TPS7A3301. the "drop in" replacements i found here years ago: https://www.ldovr.com/category-s/117.htm* - IC11 / IC12 (OP275) replaced for 2x OPA1612 - C57 / C58 / C64 / C65 replaced for Nichicon Muse ES bipolar capacitors 100uF 16V  ( on my board C53 / C54 are bridged... if they exist on your board replace with the same Muse ES capacitors ) - C55 / C61 replaced for Nichicon Muse KZ 100uF 25V ( couldn't get the 16V like the original on the board ) - C120 / C121 replaced for Nichicon Fine Gold FG 2200uF 25V later replaced for Nichicon Muse KZ 2200uF 25V since it's smaller ( originally it's 1000uF 25V but i wanted to see if there is any difference ) - C116 / C117 replaced for Nichicon KA 1000uF 25V ( originally it's 220uF 25V again... testing for differences.  Also KA was chosen because there was no space on top of the board and this one is quite compact ) - BL2 replaced for HFA08TB60 ( mainly because of soft recovery characteristics ) - BL1 can be replaced with any rectifier bridge just in case you get random overheat errors ( wouldn't be the first HK980 i've seen where 2W06M rectifier fails randomly )

Amplifier board(s) - BR1 replaced for HFA25TB60 ( previous amplifier was done with HFA08TB60 which is 8A rectifier diode but it's bigger brother is now in to test the differences ) - C2 replaced for Nichicon Muse ES bipolar 470uF 25V - C3 / C4 replaced for Nichicon Muse KZ 47uF 100V - C5 replaced for Nichicon Muse KZ 470uF / 25V - C7 replaced for Nichicon Muese KZ 10uF / 25V - C13 / C14 replaced for Mundorf Supreme 0,22uF - C17 / C18 replaced for 4x Nichicon KS 12000uF 63V ( compacter version of KG type I ) - C20 replaced for Nichicon Muse KZ 100uF / 25V - PETG 3d printed capacitor holder with bottom cover for isolation purposes

After long hesitation i ordered new caps for the baffle step correction circuit that’s connected to my FE206En horns.

I ordered the usual suspects:  Mundorf Supreme without realizing they will be a LOT bigger than the previous ones. That’s why now i have leaning tower of Mundorf :)

Sound as usual improved quite a lot with more details... even less typical fostex midrange shout ( yay ) .. more bass .. more more more of everything :D

Hi

At the moment the “mods” are not really that deep.. it’s more or less replacing the old / heatsoaked components for new fresh ones. I have few plans on modding the 990 quite a lot but due to current world situation i have a lot business trips all around the world.. meaning no regular posts on this blog as i don’t have time to do anything.

For the fans you have two options:

- replace the original fans inside the amp with Noctua NF-A9 FLX and grab some random 5V usb charger with USB cable you don’t need. You can use the charger directly from the outlet which is fine but not ideal as the fans will run all the time even if the amplifier is off... The better option would be to wire it in parallel with the toroidal transformers. This means the fans will turn on when the amplifier turns on.  For the usb cable pinout... match black and red from the usb cable with the Noctua fans, cut / isolate white and green cable because you don’t need them.  With this option you will get approximately 48-50c constant temperature and very cool insides which is great for the components.

- replace the original fans inside the amp with Noctua NF-A9 PWM and temperature controller like this for example https://www.aliexpress.com/item/32973508411.html. With this you will need separate 12V power supply and a little bit of patience to set it all up. This is by far the most effective option as the board will ramp up the fans when needed. I would aim for about 50c temperature because if you choose anything less they will run always with full speed trying too cool the amplifier down.

In both ways i highly reccomend REPLACING the fans inside not just putting the fans on top of the amplifier as it will be also more beneficial for all the other boards / components inside.

If you get this error on the screen and no sound on analog input it seems that your problem might be the +15 / -15 rail. More explicitly one of the regulators ( LM317 / LM337 might be bad ).

The time has come... the first experimental mods on one of the HK990s.

First very visible mod are the dual Noctua NF-A9 FLX because the original fans were so noisy ( probably bearings ) i could hear them if listening on a quieter setting. They are also wired to permanent 5V power instead of temperature control on the fuse board which seems to be enough to keep the amplifiers happy at about 50c.

Whole +15 / -15 V section got the upgrades with the usual suspects:

HFA08TB60 replaced the original rectifier

Nichicon KA / FG for replacing the capacitors

TPS7A4700 and TPS7A3301 instead of LM317 / LM337

Nichicon ES on signal path

Sound has changed a bit but not in a way i was predicting....

You knew this was coming right ? 

The only thing that was bothering me were the rectifier diodes for the opamp section and analogue part of the DAC.

While it was already opened i couldn’t resist swapping them for HFA08TB60 soft recovery diodes. And damn.... IT’S ALIVE MUHAHAHAHHAHAHA

Obligatory “what’s inside” photo.

As stated in previous posts this is Topping D70 with dual AK4497 DACs and quite nice power supply section with TPS7A4700 / TPS7A3301 which are my favourite. So nice that you can buy a DAC and don’t need to do anything to it to sound better. Or do you ? =)

First day of the new year and i need to start it right :P

The first ever mod for the HK990 comes in form of changing the opamps and capacitors on the XLR board.  The original opamps were changed to LM4562 ( i got 2 spares for the HK980 mod ) along with Nichicon Muse ES for the power supply stuff.

Seems like it responds very nicely to better stuff just like HK980. This in turn means more modding posts for you :)

As a last bit of news...

The absolute last mod on the HK980 was replacement of the original opamps to LM4562 which opened up the amplifier massively.  I almost dare to say it sounds better than my newly acquired HK990 but it lacks control that HK990 can put out.

That’s right... I got a second HK990 so now i have 2 of the same amplifiers again. One working and one broken still. You know the drill. Fun times ahead :)

Small sneak peek of what is coming ... 

Blurry picture from my mother :)

I bought my dream amplifier in broken state off ebay... for those of you wondering it’s Harman Kardon HK990

The description said right channel is working normally, left on the other hand is cold and no sound.  Just in case i got my hands on the toroidal transformer for the amp module.

A bit of an upgrade to the low end of the spectrum.

I decided to try something a bit more expensive and with lower Fs.

So as of yesterday i am running Alpine SWR1542D with 23Hz Fs instead of ESX SX1240 with 28Hz Fs.

This change might not sound that drastic to you but if you look at it from another side its rougly 6dB difference... Meaning i need 4x less voltage on the coils to achieve the same frequency response. ( keep in mind that i have DSP set up to produce flatness all the way to 18Hz )

Not that something is wrong with the original choice of ESX subwoofer but i wanted to see what an upgrade it is if you switch to something better made.

The result is very nice and detailed bass.. Much much better dynamics and now the subwoofer seems to match the main Fostex fullrange speakers a lot more.

Out of curiosity i started to experiment on the power supply side for my SMSL M8 DAC.

The chosen components are pretty standard for me: 

- HFA08TB60 for the rectifier

- Nichicon Fine Gold / Muse for smoothing

- TPS7A4700 regulator for required 9V  ( i like this regulator since it has adjustable output and very low noise )

- Random transformer i found in my parts bin

The outcome is very surprising... i went in with doubt, that’s why i put the header on the board to easily remove the regulator and use it somewhere else. Turns out it will stay there :)

Sound opened up like i would never imagine. Stereo imaging widened... a lot bigger soundstage and very very balanced sound  ( before it was a bit shouty in midrange and highs probably because of original switching power supply )

If anyone reading this has SMSL M8... WHAT ARE YOU WAITING FOR ?!?!?! :D

I replaced opamp power supply capacitors to 105°C Nichicon KA series capacitors today along with remaining original 2W06M rectifier for some generic one ( hidden under the pcb again).

The replacement of the 2W06M was preventative maintenance as this amplifier exhibited the same clicking sounds as the broken one i bought with "OVERHEAT" issue.... Seems like this is the common thing to go bad on these amplifiers. The amplifier will shut down thinking it's overheated either instantly or after a while.