Crackling noise from bridge rectifiers (100Hz) is heared

[fotios]

Hi to all

I am under a big enervation from day before yesterday.
I had completed my amplifier from Thursday, and i made with success the tests of auxiliary protection, indication, and EMI suppression circuits. These are:
1) Mains input thermal breaker, Tyco 5A/250Vac--> Pass
2) Mains switch push on type DPDT, ALPS 4A/250Vac. Accross each pair of switch contacts, are soldered two 100nF-X2 caps to suppress spikes and thumps during on/off.--> Pass
3) For inrush current limitting of the 500VA toroid xformer, i use a Rhopoint "surge gard" type NTC which is shorted after 0,5 sec from a 30A rated Matsushita SPNO relay--> Pass
4) For output protection (either speaker or amp. output from short circuit) is used a 30A rated Matsushita SPNO relay, one per channel.
a) Turn on delay predicted at 3 sec--> Pass
b) DC level for output disconnection predicted at 8Vdc--> Pass
c) Thermal protection managed by a LM35DT mounted on main heatsink predicted at 80^oC--> Pass
d) Current limitters (except the incorporated VI limitter into each channel pcb) of output transistors seperate for PNP and NPN rows, predicted at 2,5A per transistor--> Pass
e) VAS signal versus output signal comparator for tracking differences (i.e. output heavy distortion) predicted at 1,5Vrms--> Pass
Why all of that? Because this amp. supplied with +/-60Vdc (individual per each channel, there is NOT contact between the gnd nodes of the two channels) from a 500VA xformer and 28000ìF smoothing caps per channel, can produce a massive power of 190Wrms/8Ù per channel. It is not a toy... because to its dynamics can be as well a killer for expensive loudspeakers in cases of distortion or malfunction. I respect the speakers of people. Moreover, the protection circuit is fully buffered from the audio path and it is fully transparent.
And now the bad news... The 100Hz noise produced by the bridge rectifiers, it pass through the audio path of amplifier in its output. I have tried everything in the bridges... 10nF Murata caps accross each terminal... only one accross the xformer secondary AC terminals... caps of different value, ceramic, MKT, MKP... but nothing.
It should be pointed, that i use a big 15" PA speaker of Peavey (350Wrms, 103dBSPL-1W) to be sure that i hear everything possible noise. The buzz of 100Hz is heared from this speaker enough clear.
Some indicating values taken with the scope (AC coupled) respect to gnd are:
a) On the supply rails of amp there is presented the 100Hz ripple with a shape of inverted sawtooth and with a value of 168mVpp.
b) After the VAS stage, is reduced significantly and finally presented in output not as sawtooth, but as a line with debounches of small period (at the same frequency of 100Hz) which off the amplitude is 7-8mVpp. The output offset is only 2-3 mV (added the noise). Although the values presented are very small, the annoying noise generated from the bridge rectifiers (i know, i know... it is also the current throw back to windings of xformer etc..etc) it is audible.
In the amp board, i use one 100ìF elec. and one 100nF MKP per rail for decoupling. I tried also to add more or less capacitance for this, but nothing again. I changed the 1000ìF cap between feedback and gnd with a 100ìF... the same.
The toroid that i use it is custom maden for me. It is fully dipped in a metal case with rosin. Its case it is firmly grounded on the mains earth.
I started to think maybe the problem it is the very large OL gain of amp in low frequencies. Maybe the CCS (individual for LTP and VAS) does not operate in 100% of their possibility.
The project it is an all bjt builded AB class amplifier with 50mA Iqs per output device. It used single LTP mirrored in input, VAS a la D.Self, and tripple darlington EF output stage.
Any advice for the poor (according to my hearing) PSR?

Thanks
Fotios

[keantoken]

Hi fotios. Do you have a schematic?

- keantoken

[fotios]

Originally posted by keantoken
Hi fotios. Do you have a schematic?

- keantoken

Hi keane

Please, no... not in common view...
I started this thread after enough thought, because usually i resolve such type issues by alone in a couple of days (i am used in 48 hours continuous work without sleep) but now i am some despaired because the time is against me. This project it is a vital item for my economics, because i am in debts from it and the customer is in waiting for delivering the amplifier. I make this job for living... i ask your comprehension but i don't want - for the above mentioned reasons - to turned the thread in an endless discussion about this and that. Please, respect my difficult place. Let me finish the job, and after we have enough time for post and discussion about the circuitry.
I have a hope, that an expertised person here, he had the same problem like me and thus maybe has the ready solution, or the right suggestion.
Anyway, if you or anyone else want to see the schematic, drop me a link to send you the pdf or you can see it in the forum of lineup. And forget SPICE etc. These problems resolved only in workbench with tools and instruments.

Regs
Fotios

[fotios]

I find the notepad where i had noted some measurements with my gen and my scope:

1) Open Loop gain: for source used a sinus wave of 20mVpp. These are the values obtained in output under 8Ù dummy.
100Hz > 120Vpp (clip, supply rails exceeding) > 75,5dB
1KHz > 120Vpp (clip, supply rails exceeding) > 75,5dB
10KHz = 17,6Vpp = 58,9dB
20KHz = 10,4Vpp = 54,32dB

2)Closed Loop gain: source used a sinus wave of 2,6Vpp. Output under 8Ù dummy:
20Hz = 115Vpp = 32,9dB
100Hz = 115Vpp = 32,9dB
1KHz = 115Vpp = 32,9dB
10KHz = 115Vpp = 32,9dB
20KHz = 115Vpp = 32,9dB
50KHz = 111Vpp = 32,6dB
100KHz = 103Vpp = 31,9dB
150KHz = 90Vpp = 30,8dB (this is about the -3dB point)

Fotios

[fotios]

I removed all the decoupling caps from the amplifier boards - electrolytics and MKP - and the 100Hz noise from the bridges, has been removed completelly from the audio signal!


Regs
Fotios

[dudaindc]

That is weird... it would be interesting putting the caps back (one by one) just to check when noise returns.
I know you are pressed for time - it was just an idea.
Well done Fotios!

[fotios]

Originally posted by dudaindc
That is weird... it would be interesting putting the caps back (one by one) just to check when noise returns.
I know you are pressed for time - it was just an idea.
Well done Fotios!

Thanks dudaindc

To be sincere, at this moment i have shut down everything in my workbench, and i celebrate the disappearence of this devil that i had over my head for 2 days, by drinking vodka.
As i said in each amp board there was one 100ìF elec and one 100nF MKP per rail for decoupling. From the successive trial and error efforts, the only that i remember it is that when i removed only the elec. caps, the problem has remained. I have not tried to keep removed only the MKP and to place again the elec. in their place. I promise to will try tomorrow this test.
Also me i am surprised from this unexpected incident!

Regs
Fotios

[keantoken]

Hey, look at this.

Would this have something to do with it?

http://www.diyaudio.com/forums/attac...amp=1228549975

On a PCB it is important to make star ground so that no noise is injected into the ground paths as a result of the huge sinking currents from the rectifier. The ground point should be exactly in the middle between the two caps.

- keantoken

[roender]

Originally posted by keantoken
Hey, look at this.

Would this have something to do with it?

http://www.diyaudio.com/forums/attac...amp=1228549975

On a PCB it is important to make star ground so that no noise is injected into the ground paths as a result of the huge sinking currents from the rectifier. The ground point should be exactly in the middle between the two caps.

- keantoken

I'm 100% sure that this is the problem. The decoupling PCB caps must have an separated return path to main starground, and common ground connection point must be right in the middle of them

[AndrewT]

Originally posted by fotios
I removed all the decoupling caps from the amplifier boards - electrolytics and MKP - and the 100Hz noise from the bridges, has been removed completelly from the audio signal!


Regs
Fotios

Originally posted by roender


I'm 100% sure that this is the problem. The decoupling PCB caps must have an separated return path to main starground, and common ground connection point must be right in the middle of them

you must keep the dirty ground from the decoupling caps separate from the clean signal ground.
In a monoblock this can be done on the PCB.
In a multichannel amp it requires separate signal ground and power ground returns to a central audio ground ( or two audio grounds if you choose).