Wharfedale Pro Kinetic 18BA active subwoofer troubles

[irod]

Hello to the Community!

I have a Wharfedale Pro Kinetic 18BA active subwoofer in repair. It was totally burnt (PSU and the D-class amp), but I've ressurected it) Now it works in general, but doesn't produce an expected power of 300W RMS @ 8Ohm, and if you be so kind to help me, I will appreciate it greatly.

PSU produces +-75..80V (depends on AC voltage in my wall socket), I load the amp with a huge 10 Ohm resistor, use my oscilloscope and see a very ugly distortion when the output signal voltage is > 55V (sinus peak). It's NOT a D-class native clipping, cause the feeding voltage is much higher (about 80V). I've examined the preamp and discovered that the maximum sinus (in peak) it can produce is 4V (8V peak to peak). When I try to turn the gain pot to get more, the "limit" LED lights and the sinus is malformed. When passed to D-class amp this malformed signal causes the heavy distortion at sinus peaks and VERY strong output snabber heating (up to smoke

Then I examined the D-class amp PCB to find out the nominals in the irs2092s feedback circuit - in order to calculate (theoretically) the total D-class amp gain. The fb circuit schematic in attach, and I guarantee it's right.

I've got the gain coef = (22k+39k)/(1k+3.3k) = 14.2.
So, if I have 4V (in peak) on the amp's input, I will get 4*14,2 = 56,7V! Very close to my observations.
It's ok, but in this case the maximum power I can get from the sub is 56.7*56.7/(2*8) = 200W RMS only, much less than 300W from the specs...

The complete D-class amp schematic is also attached (2092.pdf, diagram at the bottom of the page). It's 99% close to my PCB, except the power voltage (I have 80V) and some nominals, but in general it's ok.

I supposed the problem is in the unbalanced jacks I use to send signal to the preamp, but when I attach a balanced XLR to it I get just the same limitation - 4V peak (2.85V RMS) on preamp output before clipping. Supposed the sub needs both R and L inputs for efficient work - no, no effect, just the same limit.

From the one hand, +-4V 2.85V RMS looks adequate, it's about +6dB... and my mixer has just the same output level before clipping.
But from the other hand, where is the 300W RMS subwoofer output power, declared in 18BA specs?)))) and what's the point using +-80V PSU if we never get more than +-57V on the load?)

So I'm standing on a fork)
The first way is to increase D-class gain ratio to 18-20 (changing 22k resistor to 39k, for example).
The second - to increase preamp output level. Not so easy, because I have no docs for it, PCB only)
The third - place the sub into production as is)) but I'm afraid the studio guys will think it's too quiet, put the gain to the maximum (no paying attention to the red clipping LED) and burn it out again very soon...

I've asked for Wharfedale Pro support but didn't get any intelligible response yet))

Will appreciate any help
Looking forward for your answer
Sorry for my English)
Regards
Yuri

 

[Whoops]

Those class D amps and switching PSU's are so hard to fix and troubleshoot that is normally much better just to buy replacement (New) boards from the manufacturer.

Ask Wharfedale how much is to order and buy replacement boards

 

[irod]

Class D & switching PSUs have very similar schematics and operating principles) I really have more problems with the preamp and it's 10+ opamps))
I've found a limiter circuit on the preamp PCB, looks like this:

And I'm 99% sure the problem is in a dried C76 cap. And maybe I should adjust the limiter threhold setting the R9/R10 divider values.
I will try it and write here about the results.

 

[Khron]

That may be a limiter, but the output is actually IC6-A's pin 1. No signal will be coming out of that JFET's gate...

 

[irod]

No signal will be coming out of that JFET's gate...

It's hard to dispute it) Yes, the circuit looks non-standard and weird a bit, but it works))
I've reproduced it in emulator (falstad.com, file in attach) and it really limits.

Maybe the point is that the inverted signal comes from IC7-B pin7 and is mixes with the straight signal through the FET's ds junction.
Or maybe some profi will explain us how it works)))

 

[javier]

hola amigos soy nuevo en el grupo quisiera saber si alguien tendera el esquemático de la fuente conmutada de este amplificador Wharfedale Pro Kinetic 18BA que me lo pueda facilitar se lo agradecería mucho ya que tengo unos componentes quemado y no se su valor muchas gracias.

 

[Khron]

hola amigos soy nuevo en el grupo quisiera saber si alguien tendera el esquemático de la fuente conmutada de este amplificador Wharfedale Pro Kinetic 18BA que me lo pueda facilitar se lo agradecería mucho ya que tengo unos componentes quemado y no se su valor muchas gracias.

I believe the official language of the forum is English. In case the web browser you use has some auto-translate feature, be aware that it only works one way (translating everyone else's messages into Spanish; it does NOT translate your messages back into English).

 

[cyrano]

From Google Translate:

Hello friends, I am new to the group, I would like to know if someone could provide the schematic of the switching power supply of this Wharfedale Pro Kinetic 18BA amplifier that could provide it to me, I would greatly appreciate it since I have some burned components and I do not know their value, thank you very much.

 

[Whoops]

hola amigos soy nuevo en el grupo quisiera saber si alguien tendera el esquemático de la fuente conmutada de este amplificador Wharfedale Pro Kinetic 18BA que me lo pueda facilitar se lo agradecería mucho ya que tengo unos componentes quemado y no se su valor muchas gracias.

Official language of the forum is English

 

[ccaudle]

in this case the maximum power I can get from the sub is 56.7*56.7/(2*8) = 200W RMS only

The specifications for the device indicate 20dBu maximum input, which would be 7.75V RMS, or just under 11V peak. Perhaps you just need a device which can generate a higher level signal.
2.85V RMS is only about 11dBu, which is low for a device intended for professional use (typically the maximum output should be at least 18dBu for pro interface standards on balanced connectors).

 

[Hubbub]

Just an idea... perhaps the 300W RMS rating does not apply for steady-state sinewave use? The signal limiter has a time delay for it to activate. Possibly a tone burst shorter than that time delay would allow the output to reach the full 300W rating. Might be worth a try?

 

[ccaudle]

I really have more problems with the preamp

Is there any opportunity to bypass the preamp while testing and verify that if you can get higher amplitude out of the preamp the power amp section works OK?
Or perhaps remove R10 and see if that will disable the limiter circuit to confirm your suspicion that the limiter circuit is specifically the problem.

 

[Hubbub]

Simply adding a short across R9 will also disable the limiter, might be easier than lifting R10. Has basically the same effect plus keeps the loading of R10 on the preamp for a more demanding test.