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Old 20th March 2013, 07:21 PM   #1
BRSHiFi is offline BRSHiFi  United States
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Default Preamp Grounding and Hum

Hi all,

I am building a simple preamp circuit on a breadboard. This is mainly to optimize the circuit conveniently.

All the circuit is is a 12BH7 differential stage with a current source tying each cathode together. The output is picked off of the phase+ portion through a cap straight to the output jack. The power supply is just a pi filter with a resistor and two caps (22uf each).

I am using a DC filament supply with a 7812 coming off of a separate transformer winding.

The preamp grounds are:

- CT of B+ winding.
- Main filter cap.
- Filter cap after resistor.
- Input RCA jack.
- Input resistor (no volume added yet).
- LM334 ground and current set resistor.
- Output RCA jack.

The filament supply grounds:

- CT of filament winding.
- Filter cap.
- Ground of 7812.
- One end of filament.

I have been messing around by putting the grounds in different places but I always have hum and buzz. What is the best way to ground a small signal system to stop the humming? I have long mastered this exercise on power amplifiers but preamps seem to be a different story. I know the circuit is on a breadboard but even with that it still should be possible to make it work.
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Old 20th March 2013, 08:06 PM   #2
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Hi kingneb, a couple of thoughts.

First, does the amplifier hum all by itself with nothing connected to input(s) and headphones connected to the output?

Second, does the system hum when you connect the preamp to your power amp (if you do), still with nothing connected to the input(s)?

It's possible you have a ground loop when the preamp is connected to another component of your system - if so, the solution will be different than if the hum is present in the preamp all by itself.

Also, can you post a schematic of your design?

Best luck.

~ Sam
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Old 20th March 2013, 08:44 PM   #3
BRSHiFi is offline BRSHiFi  United States
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Join Date: Aug 2008
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My headphones are 32 ohms. Nevertheless I hooked them up and a faint buzz came through when I hot switched the input. The residual hum level with my headphones cannot really be determined as the audio is too soft due to the low impedance.

It hums (buzzes more than hums) when a power amp is connected, with an input or without an input, and even shorted. The level changes with a short, no source, or a source connected.

I took out the power supply and substituted a an old 1960 Heathkit 6L6 regulated bench supply. Still hums/buzzes.

The schematic is attached.
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Old 20th March 2013, 08:49 PM   #4
DF96 is offline DF96  England
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Exactly how are the grounds connected? Buzz is most likely to come from the standard mistake of injecting PSU charging pulses into the audio ground, by connecting everything to a star point.
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Old 20th March 2013, 09:20 PM   #5
BRSHiFi is offline BRSHiFi  United States
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On the breadboard there are horizontal strips along the bottom. All grounds run to one of the strips.
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Old 20th March 2013, 09:50 PM   #6
DF96 is offline DF96  England
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OK. Not easy to be sure from the picture, but you seem to have avoided the usual problem of poor CT and reservoir cap grounding.

You might have some induction. Try reducing the size of circuit loops. I realise this is not always easy with breadboards. However, you could put the rectifier diodes closer together so you have a smaller loop from the transformer secondary.
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Old 20th March 2013, 09:55 PM   #7
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Hi kingneb - OK, for some reason I though you were taking the output off the cathode, my bad. Yes, the headphones will load the plate and the audio will be quiet.

Still, the experiment seems to indicate that the hum is in your pre-amp if you can hear any hum (even at a low level) with the preamp input disconnected and output just going to the headphones.

The 47k/22uF B+ filter should do a pretty good job of power supply filtering, so I wouldn't suspect the B+ filter for now.

One thing I would do is to float the filament supply from your common ground. The power transformer might be injecting B+ rectifier pulses into the filaments by connecting the 12.6VCT to your amplifier common ground.

Since the filaments are not part of the signal path in your circuit, you can just float the whole filament circuit from your common ground.

Nice breadboard, BTW - I love those prototype boards, they certainly make for easy circuit changes!

Best luck.

~ Sam

EDIT: Looked at your photo again, it looks like you may have already separated the filament ground from the amplifier common... Gotta get me some new glasses

Still thinking about the problem. I also see where you've disconnected the B+ rectifier from your protoboard and it looks like the alligator clips are going off to your external bench supply (?)

2nd EDIT: The longish-leads from your 7812 regulator might need some extra bypassing. Something between 1uF-10uF right at teh 7812 output before the twisted-pair filament wires. The longish-leads can lead to less effective regulation in the 7812, might cause some noise. Just a thought.

3rd EDIT: You might also try using just a fixed resistor cathode bias on the LTP. See if the LM334Z is contributing to the problem.
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Last edited by rfengineer2013; 20th March 2013 at 10:23 PM.
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Old 20th March 2013, 10:21 PM   #8
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two 22uf caps and a resistor isn't much of a power supply. Have you tried adding some capacitance and see if that reduces hum? PSUD can be used to check theoretical ripple.
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Old 20th March 2013, 11:05 PM   #9
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Kingneb, are you sure you have >12VDC at the input to the 7812? You need 14.8VDC minimum at the LM7812 input to give you 12VDC at the output.

The transformer wires (green with green/yellow CT) look like a 6.3VAC CT winding - which probably won't give you enough DC volts at the regulator input to produce a 12VDC regulated output.

But sometimes color codes aren't the best way to determine transformer voltage...
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Old 20th March 2013, 11:56 PM   #10
BRSHiFi is offline BRSHiFi  United States
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Just be clear, I am at present using the Heathkit bench power supply for B+. I restored it a few years ago, all new parts in it. As I said before, it has no affect on the buzz.

I did try to float the heaters on the filament supply. This makes the buzz extremely horrible.

The winding on the transformer is 24 volts so 12 volts regulation is not the issue.

Also, there is a non-zero potential between my preamp input ground and my PC case. I got a pretty noticeable tickle from this potential difference. Also, this non-zero potential causes AM 1400 to be picked up to varying degrees.

Different grounding experiments change AM 1400's audibility but it becomes clearly audible when an input is added that grounds to my PC, whether that be the sound card or my USB function generator.
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