arc xdi 1200.6 slight speaker hiss

[bnae38]

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?

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 .

 

[bnae38]

Here is a pic I found of the guts

 

[Perry Babin]

Did you ask the tech at arc if they could fix the problem if you sent the amp to them or is it common to all of these amplifiers?

 

[bnae38]

He said it was not a common problem, they should be silent.

After scouring the internet searching for people with similar problems, I was able to find a few.

Sending it in is kind of last resort, but may come to that. I'm in contact with him again.

 

[bnae38]

I reached out again anyway, have yet to hear from him. Open to ideas on what to look for?

 

[Perry Babin]

One thing you'll have to do is define what 'silence' is for an amp. His definition and yours may be different.

Is there a point on the gain control where you can get acceptable volume from the amp without hearing the noise from a normal listening position?

 

[bnae38]

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]

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.

 

[Perry Babin]

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.

 

[Perry Babin]

Squeal? Is this a new/different problem?

If ARC is willing to repair it free of charge, I'd send it to them.

 

[bnae38]

I suppose shorting the input out doesn't rule out the input stages.

Times like these I wish i had schematics.. or a good unit to compare to!

 

[bnae38]

I asked for details on the rma process. If it's free, you got it.

Yeah, talked about supply squeal in original post too.

 

[Perry Babin]

The noise from the transformer may be solved by moving the windings slightly. I didn't see that as a problem. I assumed that the amp would have been mounted in the trunk.

 

[bnae38]

Yeah in the trunk. I can hear it with the hatch down sometimes when I'm tuning etc, but not with it closed.

 

[bnae38]

http://www.ti.com/lit/ds/symlink/opa1662.pdf

Any thoughts on these Perry? Just started using them at work. Other than the package being wrong..

 

[Perry Babin]

That looks like a very good op-amp but not all pin for pin compatible op-amps are drop-in replacements. Sometimes you get strange problems. If they don't cause any problems and the preamp section is where your noise is being generated, they should help.

 

[bnae38]

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.

 

[Perry Babin]

I don't know enough about the circuit to advise on what you can do to break the various sections. If you have an audible circuit tracer (you can probably build one), listening to the various points through headphones may be better than using a scope.

 

[bnae38]

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]

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