Pass F5 Turbo V2; one more build.

[omishra]

JFETs seem OK.

Good to know this, lucky. See after you biased this amp, it is like tank. But be careful with touching inside while working. We have monster power supply, its capacitor bank discharges after 5-10 sec on moderate output.

there's a bit of transformer hum - very low - audible only if you place your ear on the amp. Not through the speakers. from the amp itself. i have tried changing the powerpoint, and also tried using a stabilizer. little bit of difference.
now need to try mounting the transformer tighter, rotating it, and/or perhaps adding two caps back to back in the AC in.

Any other suggestions?

You mean that DC blocking caps? its okay. One more thing you can try - interchange L and N of mains at transformer primary.
Otherwise it is normal for toroidal, as it depends upon magnetic flux through its core. That's why we use 2.5x to 3x transformer of required load. not less than 300VA per channel. How much is your transformer VA?

 

[surfatwork]

interchange L and N of mains at transformer primary.

tried that - no difference.

How much is your transformer VA?

800VA, so plenty of oversizing. I suspect a bit more of padding at the bottom may do the trick. Will put in a thick rubber disc below - at the moment, there are only some thickish felt pads there. will try the DC blocking caps only last - I wouldnt DC on the stabilizer output, so that doesnt seem likely to be the cause.

 

[surfatwork]

I put in a piece from an old yoga mat and the hum is mostly gone. now just waiting for those MOSFets.

 

[Tmr_suresh]

tried that - no difference.


800VA, so plenty of oversizing. I suspect a bit more of padding at the bottom may do the trick. Will put in a thick rubber disc below - at the moment, there are only some thickish felt pads there. will try the DC blocking caps only last - I wouldnt DC on the stabilizer output, so that doesnt seem likely to be the cause.

just read the post number 30 in my maya amplifier thread.

 

[surfatwork]

That is your problem - Single ground isolator. Use two of them, each one from 0 of filter board to isolate chassis ground. Now you have shorted two grounds at isolator pin. Though transformer is common, both 0 are at different level by few milli volts due to different Vf of diode tolerances.
Still you face the problem then you may need to swap secondary wire pairs of any one channel PS.

My ground loop problem has NOT been resolved :-(
To repeat the issue: With nothing plugged into the inputs, the amp is silent. If i plug in a stereo RCA cable and the other end into a source (e.g. phone), it buzzes like crazy. Plugging in only one RCA works fine.

Got the new MOSFETs and put them in.
Wiring as below, with grounds from the two channels leading to 2 separate ground isolation bridges.



Tried swapping secondary pairs - no difference.
Resistance between two RCA grounds is 20ohm (as I believe it should be because of the 2x10ohm NTCs)
RCA sockets NOT shorted to chassis ground
Speaker terminals NOT shorted to chassis ground

What do you think?

 

[sadik]

Check the RCA sockets, sometimes due to over heat during soldering there are chances of getting the RCA sockets damaged. I had such issue in my built. i changed the socket and the humming was gone.

Sadik

 

[surfatwork]

Check the RCA sockets, sometimes due to over heat during soldering there are chances of getting the RCA sockets damaged. I had such issue in my built. i changed the socket and the humming was gone.

Sadik

thanks.
Tried soldering wires to the input terminals directly (without mounting the sockets on the chassis) and feeding in the signal. No difference - works great with one channel plugged in but the buzzing starts the moment the other channel is plugged in.
Really puzzled as to how the ground loop is being formed.
Audio ground on external source -> Left Input audio ground -> Left channel PS ground on amp board -> Left ch ground isolator -> 10ohm NTC -> Chassis ground -> 10 ohm NTC -> R Ch ground isolator -> R Ch PS ground on amp board -> Right Input Audio ground -> Audio ground on external source. This is the only loop I can see, but it has 20ohm on it. What other loop could be there?

Apologies if this is a dumb question, but I assume that if I set my DMM to AC (AC not DC) and measure the output of the power supply section, it should read zero (or close to zero), right?

edit: please ignore question. going a bit nuts. but for the record, learnt that one cannot use cheap multimeters for measuring AC when there is a DC component.

 

[Tmr_suresh]

buzzing indicated ground loop picks transformer hum .
try this for one input channel connect both wires (signal and ground say left channel) . for the other channel(right) just connect the signal wire and cut the ground wire . now test it and give me a feedback.

Regards,
R.Suresh.

 

[omishra]

Can you try disconnecting one connection to chassis. Choose any one channel and remove wire from its 0 to isolator.

 

[surfatwork]

Hi, so finally got some time to look into my F5T again. To recap, I had a severe ground loop hum when a signal with common ground was connected.
Here's what I found:

1. You cannot use a transformer with 2 Secondaries for a stereo power supply by replicating the rectifiers and filters. It "will" result in hum. You need to either use one +V/0/-V for both channels, else use a transformer with 4 secondaries.
2. The second reason for the loop was my metal back plate on which the input RCAs and speaker terminals are mounted (insulated from the plate). If I connect BOTH the input and the speaker outs directly to the amp terminals, the amp is silent even with a common ground input. But using either the RCAs or the speaker posts mounted on the back plate results in ground loop humming. This baffles me - as I said, all terminals have been properly insulated as far as I can check - there is no continuity between any of the terminals and the back plate.
Would much appreciate if anyone can shed any light on this.

For the moment, I plan to fabricate a plastic or wood backplate, and order twin 300VA transformers for a dual mono setup.

 

[anilva]

I have not been following your thread but what you say regarding one single transformer with two secondaries powering a stereo setup is not right. All my 6 pass amp builds use exactly that and have no problem. Look at the Diyaudio build threads and follow exactly as suggested.

 

[surfatwork]

I have not been following your thread but what you say regarding one single transformer with two secondaries powering a stereo setup is not right. All my 6 pass amp builds use exactly that and have no problem. Look at the Diyaudio build threads and follow exactly as suggested.

Anil, I have spent a load of time researching this - there are several threads on diyaudio about this exact same thing. Even one of our fms (manniraj) has had exactly the same problem. My mistake that I didn't research it beforehand.
If you have had success with this configuration, please do advise how. Thanks.

 

[manniraj]

Anil, I have spent a load of time researching this - there are several threads on diyaudio about this exact same thing. Even one of our fms (manniraj) has had exactly the same problem. My mistake that I didn't research it beforehand.
If you have had success with this configuration, please do advise how. Thanks.

Hi surfatwork

I did not have any issues with my Aleph J power amp build using the Universal PSU v3.0 board. My transformer also was as per the Aleph build thread dual secondaries connected to 2 bridge rectifier modules to a single v3.0 PSU. Absolutely no hum or noise and it was dead silent.
But when I built the Shaan's V4 power amp I used the CRC psu from the diyaudio group buy, I decided to make it partial dual mono means single transformer with dual secondaries connected to the 2 bridge rectifier modules but from the module it was shared to the 2 CRC psu's. This resulted in the hum and the reason which was discussed extensively by some members on the diyaudio forum was that the CRC psu to get rid of hum we need to use 2 independent transformers if you need to power 2 individual psu boards. Me and my friend who build this dual mono CRC psu could not get rid of hum with single transformer. Here is the link to the thread and please read from page#59 onwards
http://www.diyaudio.com/forums/group-buys/301682-crc-power-supply-class-amplifier-61.html

Here is the schematic of the first watt PSU for your reference.

 

[surfatwork]

But when I built the Shaan's V4 power amp I used the CRC psu from the diyaudio group buy, I decided to make it partial dual mono means single transformer with dual secondaries connected to the 2 bridge rectifier modules but from the module it was shared to the 2 CRC psu's. This resulted in the hum

Thank you, manniraj. This is the configuration I also tried, which resulted in hum and that is what I meant. Using the first watt psu schematic works fine for me too.

 

[manniraj]

Thank you, manniraj. This is the configuration I also tried, which resulted in hum and that is what I meant. Using the first watt psu schematic works fine for me too.

To mitigate this hum issue on the CRC psu to use a single transformer with dual secondaries but with 2 psu's Shaan suggested to tap the pcb board layout to make it like a center tapped one and as per him there will not be any hum issue using 2 psu's with the same set of dual secondaries. You can check the same above diyaudio thread where he has given the schematic and I did try and make one of the psu center tapped but did not find the time to do the same on the other psu. But the center tapped CRC psu with V4 was absolutely hum free. You can also give it a try and I think you were referring to the same when you mentioned +v,0-v in the above posts.

 

[anilva]

To mitigate this hum issue on the CRC psu to use a single transformer with dual secondaries but with 2 psu's Shaan suggested to tap the pcb board layout to make it like a center tapped one and as per him there will not be any hum issue using 2 psu's with the same set of dual secondaries. You can check the same above diyaudio thread where he has given the schematic and I did try and make one of the psu center tapped but did not find the time to do the same on the other psu. But the center tapped CRC psu with V4 was absolutely hum free. You can also give it a try and I think you were referring to the same when you mentioned +v,0-v in the above posts.

I had similar problems like your earlier. After lot of trials and reading, it boiled down to capacitor bank and its ripple current capability. The more current draw from your amp, the more is the demand on the capacitor bank. I use 7-12amps ripple current caps from Panasonic or CDE. Trust me the hum will go away. I am using single transformer dual winding PSU with each rail having 8x25000 uF caps on a 4 pair MOSFET a side F5 Turbo V3 without any problem. Don’t get into the 2 transformer business unless the size becomes unmanageable or you want that topology for better channel separation.

 

[manniraj]

I had similar problems like your earlier. After lot of trials and reading, it boiled down to capacitor bank and its ripple current capability. The more current draw from your amp, the more is the demand on the capacitor bank. I use 7-12amps ripple current caps from Panasonic or CDE. Trust me the hum will go away. I am using single transformer dual winding PSU with each rail having 8x25000 uF caps on a 4 pair MOSFET a side F5 Turbo V3 without any problem. Don’t get into the 2 transformer business unless the size becomes unmanageable or you want that topology for better channel separation.

Technically I am not sure if the amps ratings of the caps in the PSU bank make a difference w.r.t the hum issue. But I have used 15000uf 63v cap of Rubycon so total of 4 caps/psu to make it a total of 8 caps for dual psu bank. I believe the caps with 63v were rated higher in current terms as well. But the issue was with the usage of the single transformer of dual secondaries feeding couple of bridge rectifier modules to the pair of CRC PSU boards. As you see from the diyaudio link which I provided one of the respected members 'XRK' even had the same issue and he got rid of the same with the usage of 2 independent transformers each powering a CRC psu board.
By the way I am also finishing my Sony VFET2 amp build with a single transformer of dual secondaries to a pair of Universal v3.0 PSU boards via couple of bridge rectifier modules. I hope that I do not get the same issue of hum.

 

[anilva]

It's all about ripple being filtered by caps. Value of the capacitance is of no consequence in a CRC circuit beyond a point. It is all about Ripple current rating of the cap. For the same capacitance and voltage rating value of a cap, the ripple current rating could vary hugely. The price of a cap is directly proportional to ripple current rating. Check on mouser site if you have doubts.

Rubycon brand is highly questionable as far a genuine items being sold. If you have bought in any local markets or eBay, then they are mostly fake. Buy CDE, Nichicon, Panasonic on Mouser or Digi-Key a capacitor bank with high ripple current rating and you do not deal with hum anymore. Of course I am expecting all the basic wiring practices are followed to avoid hum.

I have done F5, F5T v2, F5Tv3, Aleph J, F6, M2 so far. All of them use a single transformer dual rectifier design of power supplies like Mr. Pass does. No problem. This is my experience and advice. Your call after that.

 

[surfatwork]

@anilva, if you are using the pass first watt schematic, I am sure it won't have any hum. That's one rectifier + 1 cap bank for each polarity, each supplied from one pair of secondary windings. Both channels are fed from the same psus.
The issue comes when one tries to duplicate all of that in an attempt to provide separate psus for each channel, but using a transformer with only 2 pairs of secondary windings. That will inevitably cause a ground loop when connected to a common ground source, and cause hum. Regardless of the capacitors used.

 

[anilva]

@anilva, if you are using the pass first watt schematic, I am sure it won't have any hum. That's one rectifier + 1 cap bank for each polarity, each supplied from one pair of secondary windings. Both channels are fed from the same psus.
The issue comes when one tries to duplicate all of that in an attempt to provide separate psus for each channel, but using a transformer with only 2 pairs of secondary windings. That will inevitably cause a ground loop when connected to a common ground source, and cause hum. Regardless of the capacitors used.

Ok. Got it. You are using a different topology from what Pass has suggested. For separate PSUs for each channel, it is best to have two trafos and not one trafo.