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Old 7th October 2004, 05:05 PM   #1
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Default An unusual hum problem. Need help.

Hi there,
I have just got working my latest Gainclone. Its still just on its board and all hanging out. Its been a long painful drag to get it to this stage, lots of blown fuses,transistors and one blown LM3875. Still it was all down to my own stupidity with Zeners.
The configeration of this my ultimate Gainclone is -Dual mono back to each transformer, ultra fast soft recovery diodes, Capacitance multiplier smoothing on top of 4,000uf for each rail, buffered by OPA627 with descrete regulation, I have also tried something I read about a while ago - that is placing 4.7uf caps on the output of the transformer secondarys, this is said to extend the charging cycle of the diodes to minimise spikes- seems to work. Finally I have 47uf of electros, bypassed with 4uf of tants, bypassed by 0.1uf of ceramics on the chips.
Oh and of course it has the T-network using 10k 100ohm 10K. 10K from none inverting to ground. Offsets of about 68mV and 45mV.
The rails seem to have about 3volts of droop with a 120hz cycle on them, the cycle is about 2volts peak to peak, with an additional overall drop of 1volt when going hard. There also seems to be a bit of line noise at 1uS, but this is everywhere, so it might be the scope.
Can't tell you how it sounds at the moment, just glad to get the ******* working. seems OK on my test speaker- quiet and powerful.

A day later :


Just packaged up the new amp. Had it playing on my main system. Even though its very early days yet it sounds very promising. I am hearing details which I never heard before. Canít say that the bass is noticably improved by the regulation. Has a slightly unreal quality, but I have a feeling that this will improve as the chips burn in. Very smooth presentation- much more so than the Valve buffer, which I just switched back in for comparison. There are so many differences between the two amp that I wouldnít like to say that itís the buffers that causing the difference. For one thing the LPF on the valve buffer is using 1n2 and the Opamp buffer is using 1n1.
The only main problem I have is a bit of a hum . Itís a bit strange because it only appears when both channels are plugged in. Take out one phono plug and all the hum disappears. It seems to be something to do with the buffer stage. I have non-isolated phonos on the case, I have screened cables from the phonos to the buffer, with the screens terminated at the phono sockets and isolated at the buffer. I also have a single earth wire from the phonos to the main star ground ( this is the only case earth at the moment). On the output of the buffer I have screened cables going to a delay relay. The screens are terminated at the relay and isolated from the buffer. From the relay there are screened cables to the LM3875, the screens are tied to the screens from the buffer, and isolated from the LM3875. The relay screens are sent by a single wire to the star ground. So the screens have no continuity through the buffer, and only at the main star ground. The two channel screens are tied together at each point and ran to the star ground with shared earth wires.
I have one star earth point for the whole amp - I use small wire from signal grounds and large wire from power grounds. This methodology has created silent amps previously.

The problem I have is that I have to do quite a bit dismanteling to modify the amp, as the amp board slides in and I have to take out the separate power supply and back panel in order to get at it, this is no easy job. So has anyone got any pointers to save me having to do this more than once.


I have a feeling that the problem is either down to the phono sockets been in contact with the metal case, or that there is an earth loop between the pre and power amps. I just can't figure why the problem only manifests with both phonos plugged in and not with just one in either channel. This is why I only spotted it when the amp was boxed up as when I bench tested it I only tested one channel at a time.

Any help would be gratefully appreciated.

Shoog
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Old 7th October 2004, 05:53 PM   #2
Thoru is offline Thoru  Netherlands
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Hi,

I'm not an EMC expert, but I always learned that such problems are usually outside the device.. That means that the cables (which act likes antennas) are the problem. Especially because you mention that the problem only occurs with both plugs attached, I would think the problem is external. Do you use proper coaxial shielded wire? (no pigtails etc). Are the sockets coaxially connected to the casing? Is the casing earthed?
Ground loops are generally a good thing, difficult to explain, but as long as the common mode current can't couple into the differential mode circuit there isn't a problem. By using coaxial shielding (at both ends!) the common mode current can't induce a voltage in the differential mode circuit.
You can compare this with plumbing... If you fill your amp with imaginary water and the water can't get out in anyway (there are no 'leaks') then any hum can't get in.
There is a lot of literature about this stuff...
Anyway, I can't give you a solution, I only want to say: look at the externals of the amp, likely there are the cause...

Remco Poelstra
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Old 7th October 2004, 06:28 PM   #3
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Shoog, you made a chassis star ground and then you used non-isolated RCAs?
You need to isolate them.
Also, if you post a pic of the amp it would be easier.
When you have humm connecting both channels and no humm with only one, there's an indication that the channels are too far away from each other and/or there's a long way between each channel and the star ground.
You should run a thick and short cable joining the two channel's power grounds and from a mid point of that cable connect the wire to the star ground.
But, of course, this is useless if you have non-isolated RCAs.
You need to solve these two problems.
Tell us your results.
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Old 7th October 2004, 07:44 PM   #4
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I will try isolating the sockets as my first option. Some of the earth cable runs might also be a problem of been to long. Putting the sockets isolated will somewhat spoil the esthetics which is a shame.

I will report back. Might be a day or two.

Thanks

Shoog
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Old 8th October 2004, 03:30 AM   #5
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hm similar problem here
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Old 8th October 2004, 10:26 AM   #6
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Default same again!!

similar problem here with an stk4142 II, absolutely quiet with no input, little bit with one and lots when both are conected. is there no way to block this, eg caps? or is it down to re-wiring the input from scratch. i cant stand the noise, get the amp out the kitchen!!!!!!! c ya later, steve.. ..
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Old 8th October 2004, 06:29 PM   #7
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From those who have a similar problem can I ask are your phono sockets isolated from the case.
From this info maybe we can see if we are on the right road.
I am just isolating my sockets as we speak so I will have a report back shortly.

Shoog
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Old 8th October 2004, 07:02 PM   #8
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Quote:
Originally posted by Shoog
I am just isolating my sockets as we speak so I will have a report back shortly.
The grounding arrangement between the two channels, as I said in a previous post, is very important too.
Anyway, I still wonder, why don't you guys test everything before?
The box is always the last thing I do.
When I mount everything on the box the amp has already been playing for days.
You could avoid a lot of trouble by having a method.
Specially if you later find out that you made the wrong wholes on the wrong places on the case.
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Old 8th October 2004, 09:35 PM   #9
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As I said before, I had tested the amp out of its case, but only one channel at a time. This was why the problem didn't show up. I have learn't for next time.
In a house with children it is not safe to have an amp hanging out of its box whilst testing.
hot

The good news is that isolating the sockets from the case solved the problem and now all is sweetness.

This amp is noticably cleaner sounding than my valve buffered amp. I thank Carlos for his inspiration. I will tweek the valve buffered amp a bit to give it a fairer comparison.

Will keep you posted.

Shoog
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Old 8th October 2004, 10:04 PM   #10
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Shoog: how is your transformer ground connected to the chassis?

Resistor and cap right? Happen to have a picture of your amp?



Now that I think about it, it would have been much easier to build outside of the case first... but would have been a little harder to test.

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