Transformer Hum

[dkotsop]

I am new to the forum and I know this is an old topic but haven't really been able to find the answer.
I'm building a power supply for an aleph 2 amp consisting of a 1000VA - 34V transformer, 4 - 100,000uF capacitors rated at 50V, 2 - IXYS VBE55-06N07 DIODE BRIDGE FAST DIODE ECO-PAC1, a Hammond 159ZL 2.5mH inductor. I have nothing mounted or enclosed - just at the testing stage to make sure everything does what is suppose to do. I have also made a soft start circuit which works fine. I am using C-L-C filtering for DC output and I have a dummy load consisting of 4 - 100W light bulbs connected in parallel which draws roughly 3A of current when connected across the +45 and -45V rail voltages. Checked the power supply in stages.
1. Connected load across the output of the transformer connected in series 68VAC and the current drawn was roughly 2.9A. Transformer is dead quiet - no hum. (Note: the output reads higher at about 76VAC since my mains are 123VAC to 126VAC - higher than the 117VAC input spec and I'm not at full load)
2. Connected load across rectified waveform - again no hum transformer is dead quiet.
3. Installed 2 100,000uF filtering capacitors after rectifier. Connected load across and the transformer hums excessively. As I lower the current (unscrewing light-bulbs the hum obviously reduces - slight hum even with only 1 light working but tolerable)
4. Tried connecting complete circuit (reading about +/-47VDC on each rail)with inductors and 2nd bank of caps - same excessive hum.

I don't know if the transformer or my caps are defective? Everything seems to be working properly except for the hum.

I experienced the same problem with the 2nd transformer (for my 2nd channel) and tried switching out the caps. Tried connecting ground plate of capacitors to earth ground and also tried leaving it isolated from ground - no difference.

I don't see how I can have a ground loop problem as the circuit is simple. I have also read that DC in the supply could cause hum but I don't understand why I don't get the hum when connected to the transformer directly of after the bridge rectifier?

Is it possible that these caps are incompatible with the transformer?
They are Nippon Chemi-Con U32D
SPECS 50V 100,000uF, ripple current 31.43A, ESR 6.8Mohms, tolerance +/-20%, 2000hrs@105C

I have enclosed a diagram of the power supply and would appreciate any help or advice.
Thanks

 

[Zen Mod]

PSUD2

sim , then you'll know what's happening

read help , to be able to squeeze maximum from sim

 

[rtirion]

You could try L-C instead of C-L-C and see if the result (hum) is the same.

100000uF right after the diodes is a lot. Read very large charge pulses.
They could result in excesive hum.

 

[dkotsop]

Thanks for the reply. You were right about the large capacitance right after the diodes! Tried the LC configuration and the transformer quieted down significantly. Still a little hum present but in an enclosure - it would be barely audible. Simulating both circuits, with the C-L-C configuration I get 1.2mV ripple with this load. With the L-C configuration I do get significantly more ripple (about 100mV) and my rail voltage went down from 48V to 40V. Can I go with a smaller capacitor before the inductor. I think the design from KK-PCB calls for 2 - 47,000uF caps, then a 2.2mH inductor followed by another 2 - 47,000uF capacitors. Am I better off using 2 smaller caps in front of the inductor (totaling 94,000uF or just 1 smaller cap (say 22,000uF)? Tried simulating the small cap in front and I get about 7mV ripple. (hopefully not much hum - although there is no way to simulate this)

 

[dkotsop]

Just tried various amounts of capacitance in front of inductors. I tried 50,000uF and 33,000uF - they both still generate the same amount of hum. The only thing that seems to work is the L-C filtering to reduce this transformer hum although not really happy with the ripple rejection of this setup. Any other thoughts would be appreciated.

I thought of getting the transformer encapsulated.

 

[Andersonix]

To rule out poorly wound transformers causing the hum, you could build a DC blocker. If you still have lots of hum after blocking any DC, then I think it's 'bad' transformers.

This DC blocker works. Follow special instructions for 120VAC...
http://sound.westhost.com/articles/xfmr-dc.htm#outro

 

[rtirion]

The IXYS are Fast Recovery diodes, which could cause some hum as well.
You could try to slow the diodes down a bit by adding 4 100nF capacitors for each bridge rectifier.
1 cap per diode.

 

[dkotsop]

I will try the dc blocker and slowing down the diodes tonight and see if either results in a reduced hum. I did simulate the dc blocker and it does work nicely. Thanks for your suggestions rtirion and andersnix. I'll post again and let you know how things work out (fingers crossed).

 

[rtirion]

DC Blocker works... when there is DC on the AC Line or a slight difference in a dual PRIMARIES xformer. Still it doesn't hurt to try.

1 - Use other (simple) bridges (not FRED's)
2 - 0,05 Ohm or 0,1Ohm between bridge plus and cap plus to limit charge pulse of a first cap. Whatch power rating of resistor i.e 0,1 Ohm /25 Watt or something like that. This R gets hot
3 - Considering a slightly alternative filter i.e R - 0.22C - L - C - C where C = 100000uF

 

[dkotsop]

I just tried the DC blocker and afterwards a 0.1ohm resistor after the rectifier before the first capacitor and either one was successful. After playing around with the simulator I'm beginning to believe that it's not the fast recovery diodes but the limitation of the transformer. I experienced no buzzing if the load was placed immediately after the transformer, or after the bridge rectifier, or if I get rid of C1 and just have an L - C filter. In all those cases, the transformer output current never exceeded a peak of 9A. If I place a capacitor (even a smaller one), the current coming out of the transformer reaches a peak of just over 21A due to the rapid changing voltage of C1 as it charges and discharges (this current comes in 2ms short bursts). I think the humming/buzzing is a result of these high current peaks (and due to the structure of the transformer).
Thanks for all your input.
At this point, I would welcome any other thoughts or advice.

 

[Fusor]

Inrush thermistor on primary fixed transformer hum for me with similar circumstances.

 

[6L6]

Have you tried standard (slow) diodes? The fast diodes are making your charging pulses more intense because the caps have longer to discharge between switching cycles...

 

[hm4nine]

I had a similar problem with my 1000VA trannies for my poweramps. I used HexFRED diodes and a CRC-filtering (2x23.500uf-0R11-117.500uF). First I used a standard tranny from a well known German company (it was noisy without any load), then from a Polnish specialist (it was dead quite until caps with 4A-load was connected), a British tranny behaved similar, and at least I had success with a custom made tranny from a small Germany company. But its core was much bigger then usual.
Tranny noise is a well know problem, not only for DIY-people, but also for amp-building companies.
For quietest results personally I will never use a big standard tranny any more.....

 

[hm4nine]

Inrush thermistor on primary fixed transformer hum for me with similar circumstances.

This I tried, too. But noise was down only a few moments until thermistors became hot and a low impedance.

 

[dkotsop]

Well - I just tried using a standard bridge rectifier instead of the HexFREDs. Transformer is still noisy. I didn't try the thermistors since I was under the impression that they limit the inrush current at startup (higher resistance at low temp) and once they heat up, the resistance drops allowing the current to flow continuously (I'm using a soft start circuit instead). I think I will contact the company where I purchased the transformers - maybe they can help.
Thank-you all your input. It's much appreciated!
(If there is something I am overlooking - please let me know)

 

[dkotsop]

Good news! It was the transformers. I picked up a couple of different transformers that were barely completed (to take home and try). These things were so quiet I had to put my ear next to them to hear if they were on.
Thanks again to every ones help and suggestions on this forum.

 

[6L6]

I've had a very similar issue - the short story is that replacing the transformer cleared it up just like your problem.

 

[OllBoll]

I had issues with transformer hum too, probably from DC in the mains. Now I use an SMPS so no hum anymore =)

 

[JZatopa]

I am having this problem with my f5 amp. I replaced my 300va Antek transformer with a 600va Antek transformer, thinking there was something wrong with my transformer. I still have a physical hum from the transformer. I have tried some of the things on here but still have a hum. Anything not mentioned in this thread that hasn't been mentioned?

 

[dazed2]

Just curious as to what transformers they were?

Also I'm really surprised you can charge 400,000uF at 47VDC without a slow charge circuit!
I have 470,000uF in line with a 3A draw with a softstart and there was no way I could start it without blowing 10A fuses! I had to put a slow charge circuit.


Hmm thinking about your test situation, it is possible that those lightbulbs won't simulate a real amplifier load. Light bulbs are purely resistive but an amplifier is not a pure resistive load per say. It will draw 3A if you bias it that way right from start.
I may be wrong on this so if anyone can verify this I'm all ears.