arc xdi 1200.6 slight speaker hiss

bnae38

Member
2015-08-02 8:21 pm
WI
I've had this amp for a few months now, bought it used. I noticed a hiss coming from my speakers with the volume all the way down. Naturally, I eliminated all but the amp, as well as shorting the rca inputs. On the bench i can produce the same results, noise level is the same even with inputs shorted out. It sounds like normal gain hiss, as if the gain were cranked all the way up, but is there even if the gains are all the way down. Given, it does increase if i turn the gain up (starts to sound louder near half way up).

Numerous times, I let it go, reassembled it and put it back. But now that I have installed a Rockford Fosgate 360.3, it's irritating me again.

Worth noting that the larger toroid near the edge also has a high pitched whine that varies in pitch when i push on it. I scoped the area and all "seems" well. Gate driving does not vary and is a solid square wave. I ordered some high temp rtv silicone and I will try to get it to shut up. The whine is noticeable if i have the back hatchs to the trunk open, but otherwise it is not. Unsure if that is related, but thinking not. Speaker hiss does not vary with the inductor whine.

I've been in contact with Brad @ Arc and he has been very helpful. I froze the area near ch1-4 outputs and noticed a change in the sound of the static. I replaced the two 3.3u caps to no avail. I did not have a lot of hope with that anyway, as the static is present on all 6 outputs.

Scoped various rails, all seem relatively clean. Even went so far as trying probing a .1u across various filter caps to see if i could hear a difference, nope.


As I said, from time to time i shrug it off and tell myself it's normal for a class D, but it seems like it should be better.

Any advice? :dunno:

Also, it's a beautiful and powerful amp besides this issue. I've considered trying to send it back, but would rather sort it out and keep it :).
 
Last edited:

bnae38

Member
2015-08-02 8:21 pm
WI
Here is a pic I found of the guts

17643352622_8c22f62c20_k.jpg
 

bnae38

Member
2015-08-02 8:21 pm
WI
Short answer is no. The background hiss does not increase until about half way up. I am running all channels at about 1/3 gain. The only amp i had laying around to compare it to is an audison lrx2.150 AB. Dead silent.. of course lol.

In a quiet garage and the ignition on with no music playing, i can pretty easily hear it with my head 3 feet from the closest component. I would say it is abnormal. Perhaps half as noisy, I would consider "acceptable".
 

bnae38

Member
2015-08-02 8:21 pm
WI
I'd kind of like to rule the squeal out as just a squeal and not a bigger part of this problem. Hard to believe in coincidence though i see it all the time.....

Scoped large mosfets in the power supply and nothing seems goofy to me.. Gate duty cycle seems solid.

Arc wants me to rma it.
 
I don't generally recommending replacing op-amps because the only difference is generally a lower noise floor (which isn't audible in most situations) but it may help here. If you could break the class D amp from the preamp to see if the noise was eliminated, that may tell you whether replacing the preamp op-amps would help. If you do this, make sure that the replacement op-amps are stable in unity gain circuits. Some are not and will create more problems.
 

bnae38

Member
2015-08-02 8:21 pm
WI
Sounds good, one step at a time.

Next time i have it apart I'll scope the feed to the D section. Not sure why that didn't occur to me... :/.

If i decide to open a net to the irs2093 controller (if i play with ch1-4), would i leave the net open and listen to the output? I can see grounding it as a smart thing or a very stupid thing.... :).

$150 for repair including return shipping. Not bad i guess.. don't really want to be ampless though.
 

bnae38

Member
2015-08-02 8:21 pm
WI
I scoped some more today. Rails are not super clean, the closer I look. Probably about 300mv of crap on them. Could be part of it.

Glued the inductor, no more whine..

Appears they use a freescale sf4 micro to control pwm on 2092s and 2093 parts. Was scoping outputs using fft and could see some points in the audible spectrum, it would move when i pushed on the 2093 area. One big one around 3k. The noise from the output was a hiss in general though, not a tone. So it must be the noise floor in general is up too..

Not sure how ch5/6 (on 2092 controllers) would be affected by the 2093 (ch1-4). Hiss is on all 6 channels; but i ordered a couple 2093 parts. Will be easy enough to change at work with an air pencil. Lovely qfn's..

I scoped the audio inputs to the 2093 and the closer i got the the chip the higher the amplitude of crap i could see on the scope. Net goes through 24k resistors at one point and i saw more noise on the side nearer the 2093. Seems like the noise is downstream.
 

bnae38

Member
2015-08-02 8:21 pm
WI
Fft math on my hantek scope is f'd, it was the pwm frequency even showing up at lower frequencies. So nevermind that.. should have got a big-boy scope.

Also, I removed the front end resistor of one channels to the 2093 and noise output is almost inaudible now on that channel.

Canceled my order of 2093 parts, and looking closer and supply noise again..

Or it could just be design.

Perry, if you were to replace 072 and 074 opamps in an amplifier with something with lower noise, what opamps would you choose? ..not going there yet for the record ;).

Thanks!
Ben