6111/6n16b - linestage noise problem - diyAudio
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Old 8th April 2012, 05:01 AM   #1
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Default 6111/6n16b - linestage noise problem

Just completed a build of the attached linestage.

The rca inputs are dc coupled (1M ground resistor) prior to being wired to a 100k stepped attenuator, which is then dc coupled (1M ground resistor) prior to going to the input of the pcb.

All the rca jacks and the stepped attenuator are grounded to the pcb, the rca jacks are also grounded to the chassis (which is in turn grounded to the transformer).

However, there is a humming/buzzing noise even at minimal volume (it is also there regardless of whether I wire in the stepped attenuator or not).

Is anyone able to see a design flaw in this circuit that would lead to such noise?

Note that I used R5/R9 as 2k (schematic shows 28k).
Attached Files
File Type: pdf 6111 schematic.pdf (11.7 KB, 317 views)

Last edited by lordearl; 8th April 2012 at 05:03 AM.
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Old 8th April 2012, 07:36 AM   #2
dgta is offline dgta  United States
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Schematic doesn't show your "1M ground resistors" and attenuators so I can't tell what that's all about.
R2 and R3 should be on the other side of the 100k if they are intended as stoppers.
Not clear how you ran your circuit and chassis grounds. But the input jacks should be grounded only to the circuit, not the chassis. Sounds like a ground loop issue.
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Old 8th April 2012, 08:49 AM   #3
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Thanks for the assistance. I've taken out the grounding which links the sleeves of the input and output jacks, but the noise persists. You are right, it certainly sounds exactly like a grounding problem.

Grounding structure is as follows:

(1) RCA input jacks, sleeve is running to the stepped attenuator
(2) Stepped attenuator ground is running to the pcb GND input
(3) GND output on PCB goes to the RCA output jack sleeves
(4) RCA output jack sleeves also connected to the chassis ground, which is the same point as the transformer earth

Is this clear enough, or should I post a picture?
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Old 8th April 2012, 02:57 PM   #4
dgta is offline dgta  United States
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Hard to diagnose without seeing the physical elements but let's try.

1. Remove any and all connections between circuit ground and chassis ground.
2. By "transformer earth" I hope you mean the metal frame of the transformer is connected to the chassis. No transformer windings should be connected to chassis ground. If your power supply is not on the same pcb, it too should not be connected to chassis ground.
3. I assume it's built on a metal chassis. Make sure chassis is connected to mains ground ("earth").
4. Make sure input and output jacks are connected to the pcb with shielded cable. If the pots are not on the pcb, make sure they are also connected with shielded cable.
5. Short out the input jacks with shorting plugs, At this point you should have no noise. If you still do, it's likely not a grounding issue. Measure the noise frequency and go from there.
6. If the noise stopped, connect circuit ground to chassis at one point only either directly or via parallel RC circuit. You may have to experiment.

Google and read about grounding techniques. For example:
http://www.geofex.com/article_folder...nd/stargnd.htm
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Old 8th April 2012, 04:10 PM   #5
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Is that really all of the power supply filtering used? If so, you'll need more.

All good fortune,
Chris
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Old 8th April 2012, 04:43 PM   #6
dgta is offline dgta  United States
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It just occurred to me that this is a 6111 tube. I don't know what B+ you're running, but if it's less than 63V as suggested by the PS cap, you probably have a non-optimal design. The data sheet suggests much higher B+ and much higher anode loading.
http://frank.pocnet.net/sheets/127/6/6111.pdf

Depending on the B+ voltage and tube current, Chris may be right about more filtering.
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Old 8th April 2012, 09:29 PM   #7
DF96 is online now DF96  England
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With such small anode resistors you will have virtually no PSRR - you are listening to your PSU ripple, which is rather crude with no smoothing (just a reservoir cap). Buzz is almost inevitable. You need more smoothing, and check that the PSU ground arrangements are correct so you are not also listening to charging pulses in a ground link.
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Old 8th April 2012, 09:31 PM   #8
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Re the B+, the pcb calls for 35VAC = 50vdc, I assumed the tube was biased correctly in the circuit? Quite easy to increase the voltage, I ordered a few spare pcbs for testing so can alter the value of the capacitors, too. I've increased the filtering to 3300uF.

I'll try to remove the connection between earth and chassis ground and see if this improves the noise. The chassis is connected to mains earth.
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Old 8th April 2012, 09:41 PM   #9
DF96 is online now DF96  England
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Under normal bias conditions the 6111 has an anode impedance of 4k. Your bias may differ, but it probably won't be hugely different. 28k anode resistor and 4k anode impedance means PSRR is about 1/8=-18dB. You have 2k instead of 28k, so your PSRR will be 2/3~-3dB. Throwing away 15dB-worth of PSRR will create buzz in any weak design. This is a weak design because it has a crude PSU. Don't increase the reservoir, instead add an RC smoothing section. You need to stop and think about why it is failing, not just slap on sticking plasters.
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Old 8th April 2012, 10:02 PM   #10
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Quote:
Originally Posted by DF96 View Post
Under normal bias conditions the 6111 has an anode impedance of 4k. Your bias may differ, but it probably won't be hugely different. 28k anode resistor and 4k anode impedance means PSRR is about 1/8=-18dB. You have 2k instead of 28k, so your PSRR will be 2/3~-3dB. Throwing away 15dB-worth of PSRR will create buzz in any weak design.
OK thanks - I ordered a couple of kits and each of them came with 2k for R5/R9, so I assumed it was an improvement on the original circuit, which showed 28k.

I'll take out the 2k for R5/R9, what is the optimal value for these resistors? While we are in the desoldering phase, are there any other improvements to be made?
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