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Old 15th September 2013, 05:47 AM   #11
ChrisA is offline ChrisA  United States
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Originally Posted by kingneb View Post
Now to address some of the hum and hiss issues. I tore apart the grounds and signal cables for better wiring.....
The firt thing I'd do to address the hum problem is TIGHTLY twist all the AC wires. The mains wires and the heater wires and the transformer leaders. All of these should not show any air gap. Some people will go as far as 8 turns per inch but 4 turns per inch might be fine. The current in each wire cancels the field made by the other wire. If the wires are touching the field is nearly zero at 8 wires diameters distant.

No not coil up and zip tie "extra" AC wire. It needs to be twisted with it's partner and cut short and routed as close to the chassis as can be

Many times DC heaters can be more of a problem then AC. If there is no regulator the recifier noise can be hard to get rid of. A simple lm317 can remove that noise.

Here you can see a spring reverb unit I built. notice the green and yellow heater wire. It is solid #18 wire I twist using a drill motor. I'll make up 10 feet of it at a time. It is about 8 turns per inch. Also all other AC is twisted by the stranded kind just does not hold a tight twist
https://www.dropbox.com/s/nes72pnq0w...gle%20view.jpg

Here is a small 5W guitar amp. The layout is not perfect but I tried to mostly copy the 1950's vintage Fender. All the transformer leads are twisted and as short and direct as possible. All AC wires cross others at 90 degrees angles to minimize contact. Also more of the same green/yellow heater wire I make with the drill and a vice.
https://www.dropbox.com/s/vhj9ykofaodw936/guts.jpg

What seems to matter the most is the last 1/2 inch of wire before the tube sockets. Make sure there are no loops in the AC. Also if the heater wire is stuck to the chassis, flat and the signal wire to the grid comes in like a dive bomber then the two are at 90 degree angle and have minimal coupling

Shields do not work with AC mains and heater wire, the field is magnetic not electric

Route heater wire to output tubes first, then to input tubes. So there is less current in the wires going to the input tubes. Hum does the most harm in the input tubes because it gets amplified more. So if heaters are in parallel the output tube is first in the chain.

Use tube shields. They protect the tubes and hopefully shield out hum and noise.

Finally if you still have hum. Cut the heater leads and hook up some AA batteries. They will power the heaters long enough for a test. Is the hum gone? if so the heaters were the route. If the hum is still there your heater system is good and hum must be in the high voltage system.

Here is Merlin's grounding page. This is best modern practice.
http://www.valvewizard.co.uk/Grounding.pdf

Last edited by ChrisA; 15th September 2013 at 06:02 AM.
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Old 15th September 2013, 01:48 PM   #12
BRSHiFi is offline BRSHiFi  United States
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The heaters are all regulated DC (7812 regulator) and elevated to 120 volts. If the heaters are regulated DC, why would hum and wire twisting be an issue?

I also think the zip tied AC Wires are far enough from the small signals to avoid hum inducement, or is this not the issue?
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Old 15th September 2013, 03:28 PM   #13
SY is offline SY  United States
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First, have you confirmed that the circuit is not oscillating?

Second, the ground end of your feedback loop is physically at the first stage?
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Old 15th September 2013, 03:36 PM   #14
BRSHiFi is offline BRSHiFi  United States
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I do not see any odd waveforms on the scope with or without a signal, so it does not look like it is oscillating.

The feedback loop is grounded at the first stage.
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Old 15th September 2013, 06:06 PM   #15
BRSHiFi is offline BRSHiFi  United States
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My plan is this.

1. Have only a single very short wire run from the main filter cap, transformer center tap, and LM317 ground to a lug. This is the main star point.

2. Revise the regulator board to remove the potential dividers, which will be built point-to-point. Merlin advised there should no long interstage ground wires from the stage ground to the filter cap ground. My existing design violates this rule. Building the potential divider near each amplification stage should correct this fault.

3. Create separate stars for each of the three stages for each channel. I made the unit so both channels share a star.

4. My design uses feedback, so Merlin advised grounding the output jack to the first stage. I did not do this before. I ran it directly to the main star point.

5. Ground each RCA jack in the input bank to the first stage ground. I did this.

6. Run the safety ground through a hum blocking network to the main star point. I did not do this.

Now, here is where Merlin is not so clear. Where should I connect the chassis to the circuit ground? For a single channel unit, he says at the input jack. For multichannel, the point where the channels meet. I am doing stereo, two separate channels. This is not covered.
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Old 15th September 2013, 06:54 PM   #16
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by kingneb View Post
The heaters are all regulated DC (7812 regulator) and elevated to 120 volts. If the heaters are regulated DC, why would hum and wire twisting be an issue?

I also think the zip tied AC Wires are far enough from the small signals to avoid hum inducement, or is this not the issue?
OK regulated DC is not gong to contribute to hum, still I'd twist them so they don't pick up "whatever" and act as a conduit for it.

AC mains wires are never "far enough away" Certainly if they are inside the same chassis as the signal wire they are very close. Even the wire in the walls of the house cause hum in a high gain amplifier. This is why people use balanced microphone cables, common mode noise is pervasive in the environment. Some people who build preamps for recording (gains is up to abot 65dB) will place the power supply on the floor and still use mu-metal shields on the input transformers. HiFi preamps are not so sensitive but the goal is "no audible hum" is hard to get to unless you do everything you can think of.
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Old 15th September 2013, 07:07 PM   #17
BRSHiFi is offline BRSHiFi  United States
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The easy way is fully balanced audio. I have yet to make a completely balanced system as the standard for consumer hi fi does not use this.

It would be nice if all consumer equipment, especially turntables, took advantage of balanced topology. I guess they did not for cost reasons.
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Old 15th September 2013, 07:42 PM   #18
ChrisA is offline ChrisA  United States
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Originally Posted by kingneb View Post
The easy way is fully balanced audio. I have yet to make a completely balanced system as the standard for consumer hi fi does not use this.

It would be nice if all consumer equipment, especially turntables, took advantage of balanced topology. I guess they did not for cost reasons.
Yes. back in the Tube era the added cost for balanced audio is just about double. You need expensive transformers on both ends of every cable and twice as many tubes and more power and more heat.

This stuff was expensive. The old Hammond organs used in larger size churches were balanced. At the time, one of these larger systems could sell for more than the median price of a house in the US. Quality commercial audio used to be very expensive. But the pro-audio gear has been using balanced connections for at least the last 60 years.

I've always thought at a DIYer should build something he can't buy. Why not take the leads from a phono cartridge and treat them as balanced and keep the concept going all the way through to the power amps? But insted we tend to copy old consumer-level designs.

My next project will be a microphone preamp with in-built optical compressor/limiter. It will have balanced XLR input and outputs and I'd like to maintain a balanced topology inside. I'm only working with breadboard experiment now. I'm looking at the LA-2 compressor as a model
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Old 21st September 2013, 04:45 PM   #19
BRSHiFi is offline BRSHiFi  United States
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I worked hard at properly re-wiring the unit.

There is still some issues with the volume pot grounding still. There is some faint buzz that disappears when the chassis is touched with the volume up. It is a stereo volume pot. The case of the pot measures 5 ohms between the pot case and the chassis. How should I ground this stereo pot?

Also, I have some radio station interference along with the buzz when AM 1400 is broadcasting during the day. They increase their transmitter power in the morning. Without tube shields, more microphonic tubes pick up AM 1400 clearly audibly when your hand is placed in close proximity. The first stage is the most sensitive to this problem. Is this normal? Can it be remedied?
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