# Another mod thread, ks900.6



## bnae38

As I've geared up my bench over the last year, one of the things I always wanted to do was pick an amp I loved and tweak it to taste.


I've spent the last 8.5 years professionally repairing audio equipment, pre-amps, mixers, recorders, etc (basically opamps and controller circuitry). I've repaired 10's of thousands of units, actually looked the other day at the data lol. At this point I'm fairly well rounded on that kind of gear. Power amplifiers aren't something I've worked on professionally.. I've repaired a couple dozen over the years but really don't have much first hand experience tweaking that kind of thing, or where to even begin. So.. for now I'm just playing with the pre-amps .

I fell in love with arc audio a few years back. Solid products, superior support, and simple aesthetics.. Initially i had a ks300.4, but then got the 3way bug like most... I had an xdi1200.6 for the better part of a year. Through that time frame i was battling noise floor problems and more or less blamed the amp. Came to find most of my floor ended up being my choice of dsp (rf) and a few other issues, gain staging etc.. 

After trying the ks900.6, I really fell in love. It was a bit quieter for one. It's relatively huge, but beyond that I just flat out love these amps. The power structure is just what i need for my 3 way stage. They're just solid and sound great (whole ks line - IMO).

There are things i wanted to change though. Now having a dscope, i knew the even channels had a higher noise floor, perceptible or not i wanted to change that. Figured it was the inverting topology to get our zig-zag half bridge (and bridgeable) outputs. So i removed that.. all channels are now symmetric through the signal path. Of course, amp is no longer bridgeable, and ch2, 4 and 6 are now 180deg off vs the output terminals; but i'll mark them accordingly.. 


I frankly just hate mechanical parts in the signal path. Just opportunities to wear and cause intermittent problems/variances in signal. So, all the pots go. All the switches go.. . I hard set gain for soft clip at 4vrms input on all channels. 

I also changed all the opamps in the signal path. Tl072>Opa1642 Tl074>Opa1644 Ne5532>Opa1602. I also changed all electrolytics in the signal path to Elna Silmic II caps. I'm a skeptic, but i hear good things . We'll leave it at that.


The last thing (for now) I decided I wanted was to ac couple the output for tweeter protection. I hacked in 100v 47u bipolar nichicon lytics before the terminals for ch1 and 2. Initial testing shows 3db down point under 1k (into 4ohm load) as expected. Should have little effect on power response at the frequencies i'll use, but should protect tweets in case something goes wrong or i hook something up wrong .

Should also mention, though i dont have pics of this, both ks900.6 amps i have had problems with the output terminal block. The block has screws in it with the heads soldered to the pcb. The screws are essentially machine screws inside the (tinned copper) block. During heavy current flow, i got buzzes when listening (or measurement sweeps at higher volume) and basically intermittent connection because of this on some channels. I ended up buying these rca jacks (center pin is perfect fit for screw) and pulling the center pins out. Then cut them in half and soldered them into the block with a torch and solder. I took the individual terminal pieces out of the plastic block first obviously. Both my amps are dead solid now . Find it hard to believe nobody else has come across this issue...

Fwiw, getting the block off the pcb is a total pain in the ass. I've done it on both my 900.6's now, but you need to take two large tip soldering irons and hold them on the screw to get the solder flowed. I then had my wife back the screws out one by one. On that note, it'll literally almost be impossible to get the new config block off the pcb (not that i'll ever need to) since i would need to reflow all 12 studs at once. Ramble ramble...




So anyway... imgur is not working atm, guess pics will come later. The input structure is interesting on this amp. Ch1 and 2 are unbalanced, the shield is just ground referenced. The other 4 channels are all true balanced inputs.. thought that was odd. 

Channel 1 and 2 come in single ended and are buffered through the first opamp, no gain and no feedback, and then off to the gain stage before the power amp. The connections to the hpf circuitry were removed, and switches hard wired.

Channel 3 and 4 are buffered on both the + and - side before heading to a diff amp, then to the gain amp before heading to the power amp. Again the connections to the hpf circuitry were removed and switches hard wired.

Channel 5 and 6 presented a challenge. Audio comes in the same as channels 3 and 4 (balanced buffer), before heading to differential amp to make them single ended (referenced to ground). At this point we go to the cluster #@!$ that is the mezzanine pcb. On that board there are multiple routing options (center), bass boost, hpf, lpf. Decided pretty quickly that was going byebye. Luckily the hpf opamp from ch1/2 was right next to the main header. Cut some traces loose and a few wires later it's fully functional . Set gain, added feedback comp caps from the mezz pcb, and added some bulk capacitance just because...

Thread kind of sucks without pics, hopefully i can get imgur working today/tomorrow, but anyway.. 

I'm at the point where i could do my final checks and install this badboy, but am kind of taking my sweet time. I'll probably think of something else i want to play with on it .

Feedback welcome.. anything else you guys would add?

I'll follow up with pics.


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## Lycancatt

I highly approve! this is one of two series of amps I've considered when I finally do a nice build and its good to know theres a little play in what can be done with them.


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## bnae38

Ok pics finally loaded. 

Here's pics of the input panel, will probably do some simple numbering on it. Love this epoxy sculpt. Also this "epoxy" spray paint. Hard as a rock..



































Here's main shot with rundown of various opamps before opamp mods, already has caps changed.








1: Ch1/2 gain amp
2: Ch3/4 gain amp
3: Ch1/2 input buffer on high pin side, ch3/4 diff amp on low side.
4: Ch5/6 input buffer
5: Ch3/4 input buffer
6: Ch5/6 diff amp
7: Ch2/4 inverting amp (removed - bypassed)
8: Ch5/6 input selection buffer (for ch5/6 from ch1/2- removed, not needed)
9: Ch1/2 hpf amp (removed). Becomes ch5/6 gain amp with traces cut etc.
10:Ch3/4 hpf amp (removed)
11: Mute circuitry, left alone.
12: Not in signal path, left alone. Nearest I could tell it is part of protection circuitry/measurement.




Here is the gain circuitry for ch1-4.








With pots removed and net shorted (min gain), clip point was about 4.5vrms with the stock 150ohm series resistor. I changed the series resistors to 240ohm (good thin films), and soft clip .1% is at 4.07v in. Gain control is through negative feedback on the opamps.


Here's the gain circuitry for ch5/6. 








Various cuts to get the re-purposed spot loose from the old circuitry, not going to walk through that. I ended up using the same base config for it as ch3/4. 430ohm set the gain to give me the same clipping points on ch5/6 - roughly 4.07vIn at .1%thdn. 

Like i said before, I re-used the feedback comp caps, and added some more rail capacitance. 



Here is the inverter for ch2 and 4, removed and spots for resistors bridged. Was just a simple inverting output, unity gain setup.. piece of cake.








I still have more testing to do on this.. while it seems like this helped with overall distortion matching ch1 to ch2, ch3 to 4; even channels still have a bit more noise floor. Even on ch6... unknown at the moment.. I probed the inputs to the power amps and they're dead matched at that point. Could be a routing/noise problem i suppose. I have not played with bias, and don't know much about that stuff tbh.. Bias pots seem set somewhat randomly, but nothing to lead me to believe even channels are more out of whack.. 



Here's the output hpf, i originally had small caps hacked in before the output filter, but managed to get the new ones in properly at the output.










Overall shot of input side, nothing exciting.. simply hard wired the switches. I removed the little poly caps which were the inputs to the hpf circuitry for ch1-4. Circuit Y-d off to them and was reconnected after the hpf via switch later.










Here's an overall bigger shot of the input half. Can see all 6 silmic input caps to the power amps in this shot too.


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## SQLnovice

Added you to my friends list for future amp repairs, but let's hope I don't need it. Lol.

Sent from my SM-N910P using Tapatalk


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## forty5cal1911

Looks great brother! Some serious work there.

Now if you can just get some seat time with it....


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## bnae38

forty5cal1911 said:


> Looks great brother! Some serious work there.
> 
> Now if you can just get some seat time with it....


Thx bud. Soooon(ish)


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## DeltaB

bnae38 said:


> I also changed all the opamps in the signal path. Tl072>Opa1642 Tl074>Opa1644 Ne5532>Opa1602. I also changed all electrolytics in the signal path to Elna Silmic II caps. I'm a skeptic, but i hear good things . We'll leave it at that.


You'll love the OPA1602 over the 5532's. Their dynamics don't even compare. In my Yamaha products, I utilized the OPA1612, (which requires more current than the OPA1602, but was a better choice saddled with it's ESS DAC) however, the manner in which the OPA1602 handles dynamics, IMHO, makes it a wonderful choice. In the Denon's that I've done for myself and others, utilizing the OPA1602 saddled with the Cirrus Logic DAC's can and will give you goose bumps at moderate levels.


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## bnae38

Played around with bias voltage a bit, not a whole lot of variability on this one. 100ohm pots vs 500 (and slightly different resistors in surrounding circuitry) on the clarion dpx2251 schematics i'm referencing. They share a lot of similarities, but pre's are basically completely different..

Was doing some reading and considering playing around with gain staging on ch5/6. Needed more preamp gain on them to get the same clipping points. About what i expected, i had gains up a bit more on ch5/6 in the past.

Kinda wanted to play with getting distortion down more on those channels... but came to realize once i went there, i'd be there for weeks and i'd likely blow up an output stage or two lol.... Pulling the pcb from chassis isn't my favorite thing in the world...

So spent the last 2 days getting things cleaned up and installed, moved down to one sub and a proper amp rack instead of having the amps on the back of the seats. Regained a bit of trunk space and will have less weight giving the Impreza a saggy butt..


Hopefully will get some listening in tomorrow.


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## bnae38

Tidied up some cabling and sat down for a listen.


Few tweaks to levels by ear, and brought up to volume.

Honestly it's pretty breathtaking.. the precision, dynamics and clarity. Kinda says a lot since I've really tried to stay skeptical/objective about the opamp/silmic changes. Most of the other stuff i did was from a reliability/stability standpoint..

A part of me thinks some of it is and will always be psycho-acoustics (me knowing the changes are there), but hell with it. We're in it for the entertainment...


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## forty5cal1911

bnae38 said:


> Honestly it's pretty breathtaking.. the precision, dynamics and clarity. Kinda says a lot since I've really tried to stay skeptical/objective about the opamp/silmic changes.
> 
> A part of me thinks some of it is and will always be psycho-acoustics (me knowing the changes are there), but hell with it. We're in it for the entertainment...


Couldn't have said it better myself


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## dcfis

Awesome, glad for you man. Rock it!


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## DeltaB

bnae38 said:


> Tidied up some cabling and sat down for a listen.
> 
> 
> Few tweaks to levels by ear, and brought up to volume.
> 
> Honestly it's pretty breathtaking.. the precision, dynamics and clarity. Kinda says a lot since I've really tried to stay skeptical/objective about the opamp/silmic changes. Most of the other stuff i did was from a reliability/stability standpoint..
> 
> A part of me thinks some of it is and will always be psycho-acoustics (me knowing the changes are there), but hell with it. We're in it for the entertainment...


It is not a placebo effect. And while psychoacoustics are actually content in the recording, I knew what you were saying, however, a simple A/B comparison does show even the untrained listener it is not just your imagination. Give the caps about 50 hours of listening time.

I hope you find renewed enjoyment to the content you like hearing...


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## rton20s

Thanks for linking me back to this. I missed it when you originally posted. If I had to guess, I am thinking that A LOT more people have had issues with the output terminal block than most of us know and than Arc has let on. I'm guessing that with the extent that you went to in order to correct the issue for yourself, there is no easy fix. Do you happen to know if there is a similar terminal block and connection used on the other KS amps? I don't think I have heard of the issue on any amp other the the KS900.6.


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## bnae38

I have a lot of ks's and from what i remember all had the terminal itself soldered to the pcb except for the 900.6. Really wished i had taken pics of it in the 900.6..

It's a fairly painful fix even if you know what you're doing.. pretty tedious. But worth it .


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## rton20s

bnae38 said:


> I have a lot of ks's and from what i remember all had the terminal itself soldered to the pcb except for the 900.6. Really wished i had taken pics of it in the 900.6..
> 
> It's a fairly painful fix even if you know what you're doing.. pretty tedious. But worth it .


Good to know. Probably not worth it for me to do preventative maintenance, or upgrades, considering I already have replacements amps on their way. Perhaps it is something the next owner might want to pursue.


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## bnae38

rton20s said:


> Good to know. Probably not worth it for me to do preventative maintenance, or upgrades, considering I already have replacements amps on their way. Perhaps it is something the next owner might want to pursue.


True enough, if it aint broke.. 

It's funny, when i originally found the problem and wasn't able to reflow to get good performance, i actually took the block off and ran wires out... Looked like hell.

Mulled it over for quite awhile, looking at other terminal blocks to try to hack in etc.. Then finally realized the blocks were solid copper pieces only tinned.. Initally thought aluminum (good friggin luck soldering to that) Obvious choice from there was to solder lugs in them and solder those to the pcb.

Is what it is, poor design.. Really dont think they'll design anything like that again .


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## bnae38

Got one of these in to do a fairly simple repair for a user on here. 

We decided to run full load tests and look for the terminal problem i talked about earlier. Sure enough.. ch3 fairly intermittent under load, and ch6 was god awful. With no load on the outputs, i could run thdn sweeps all day and it would look great.


Got some pics this time of this fairly f'ing ugly process and started into it tonight.

Kinda outlined in post 1 or 2 what the process is, unsoldering the screws holding the terminal block and parts involved.

Here's pics from the start.

Cover off and started removing heat sink clamps. Use a narrow screw drive to pry up they wiggle off with a needle nose pliers.










Gently push transistors off isolators so we can slide board out without dragging them out. The bolt near the bottom in 1st pic needs to be loosened, it holds the pcb in and slips into the bottom heatsink.










Here's the terminal block.


















Using two irons, ideally with two large tips (i'm down to 1..), reflow the end of the screw soldering and use a smallish flat-head screwdriver to unscrew. They're actually philips, but a large tip makes reflow difficult. It is 100% a two-person job....

If the solder joint is not in a good re-flowed state, the pad on the pcb will come loose (which sucks).. Every other pad in a zig-zag pattern is ground and tied directly to a surrounding ground plane (how we get our bridge-able outputs, with every other channel inverted in the middle and again at the output - if that makes any sense)











Ok.. got the block off, the nasty looking joint is ground (thankfully) and it had the original screw torqued way way down (obviously somebody else knew about this problem or saw it on that channel..) 











Cleaned up. Note that the non-grounded pads have a narrow gap to the surrounding ground plane, fairly easy to screw up and short to it when soldering back on later.










Pic of the terminal block removed.










Screws (garbage..)











On to making our posts that will be a dead-solid fix.










Break the shells off with needle-nose, then push through before grabbing and pull out the rest of the way. Is a workout.. 

























Break em in half (there is a center notch) and insert in block pieces.. Block disassembly below too.


























I'll get on to soldering with a torch and the rest this weekend. Obviously the ends will be cut/ground down after soldering 

Yeah.. used to shoot pool here lol.


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## bnae38

Here's the original plot of thdn/power, gain ccw ~.1-4.5 vin. 13.6v supply, 4ohm load.

Also worth mentioning i could look at the fft/waveform from ch3 and 6 and wiggle the terminal when running under load and things would freak out.


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## bnae38

Wrapped up the fix and reassembly, did initial tests.

Pic of new lugs soldered in. I went a little crazy with the torch and amount of solder on the first 3 i worked on and had to grind them down..










And soldered in.











Ran some brief unloaded tests and checks with current limited supply after re-assembly, then ran power sweeps again.

Dropped the amp into the same test setup from before and watched it just work.  These are the first 6 power sweeps I ran tonight.

Pic of fixed results and pic of fixed vs initial results.


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## Mark the Bold

Sorry to necro a dead thread, but my KS900.6 is having issues that may be related to this. 

Specifically, when cold (sub 30 degrees) it either doesn't turn on and goes into Code 15 mode ( 5 fast blinks, then pause, then 1 red flash) until it warms up and works normally. Last year I was debating whether to send it into Arc Audio but that would cost $250 so I decided just to suffer through it.

This year, after leaving my car at the airport for three days, it turns on right away but I get a huge pop and a high distortion sound from one channel that sounds like something blew up in there. 

Long story short, could this output terminal block issue be one and the same as my cold weather issue? Or is it something much worse that a new output block will do little? I am debating whether to just buy a new amp, but I really like this amp and its currently installed in a custom amp housing in my trunk that I'd rather not retrofit for another brand of amp if I do not have to. Appreciate the help and the thread.


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## bnae38

Hi Mark,

No, the terminal block issue would only result in random buzzing distortion from some speakers, never excess noise added. Sounds like you have another electrical issue with the amp.

I purchased my other (spare) 900.6 broken and it had problems with the +/-15v rails, series resistor and regulator were blown. Could be related to that and/or probs with the mute circuitry.


I dont have schematics for these... but some of the clarion schematics are usably close, so not usually too hard to figure out what's what.


Ps, happy to have this bumped


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## steelwindmachine

@bnae38 - checking back on this. How has this amp performed for you? Would you offer up your services?


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## bnae38

I sold it, I run a helix V8 now. I do have a couple 900.6's I purchased that I'm going to refurbish at some point though .


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