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Old 20th December 2013, 09:51 PM   #1
Alex M is offline Alex M  United Kingdom
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Default Hum in SVP preamp circuit

I have recently completed Allen Wright's FVP5a/SVP preamp as a "spare" for my RTP3. The circuit works fine, except for some issues with hum in both the phono and line stages. There is a slight buzz as well as a hum that doesn't change in level with volume, which I ascribe to the active line stage, while the hum from the phono stage is quite loud. It vanishes when the mute relay shorts the line output, so the hum is definitely coming from the preamp.

This is my first single-ended valve preamp project, and is the first time I have come up against the dreaded hum. By contrast, any noise in the differential RTP3 is cancelled almost completely in my differential 300B power amps.

I have taken care with the grounding scheme, trying to follow Allen's suggestion, which is to star ground each module separately, and then taking all the grounds from each channel to chassis at one point by the phono grounding posts. The connections to the input and output sockets and the volume control are screened cable, with the screen grounded only at the socket end.

I am wondering whether the line stage noise might be coming from the HT wiring. I use one of Nikos Salas's SSHV2 shunt regs for each channel, but the regulator boards are about 30cm away from the line stage on the opposite side of the chassis. Any noise that gets on the regulated HT rail will be present across the 25K anode resistor of the voltage amplifier valve and so be added to the signal. With the phono stage, with its much higher gain, this problem would be magnified, consistent with what I hear.

Has anyone found they needed a screened cable for the HT in this position? Are there any other suggestions to address this problem?

Thanks,

Alex
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Old 20th December 2013, 10:34 PM   #2
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what´s about the filaments?
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Old 20th December 2013, 11:17 PM   #3
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Does your schematic match the Allen Wright schematic exactly? Can you provide a high detail photo of your build? What is a Nikos Salas SSHV2 shunt reg?

Last edited by scott17; 20th December 2013 at 11:21 PM.
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Old 21st December 2013, 09:12 AM   #4
Alex M is offline Alex M  United Kingdom
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Yes, the circuit is identical to Allen's, except that I used a 5687 for the voltage amplifier and current sink in the output stage (with an 820R cathode resistor for the voltage amplifier to get 6mA in that stage). The filaments are fed by individual LM317 current sources, so AC isn't the issue.

I'll see if I can take a photo or two today.

Th SSHV is here. What I didn't say, which may be relevant to my current problem, is that this regulator has separate sense and supply connections for the regulated HT and ground return. It may be that I need to screen the sense wires.

Alex
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Old 21st December 2013, 04:41 PM   #5
Alex M is offline Alex M  United Kingdom
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Here is a photo of my SVP from the top front. At the left is the line stage, centre the phono stage and right the shunt regulators. The right channel circuits are at the front. You can see the left channel star ground just left of centre rear. As you might guess, the PSU is in an external box.

Click the image to open in full size.

The layout and wiring are a lot messier than my RTP3, mainly because of the nice symmetrical dual mono design of the latter.

Because the HT regulators have a CCS input, and the valve heaters are all current regulated, all the connections from the PSU carry a constant current. That is why I was surprised by the audible hum!

Alex

Last edited by Alex M; 21st December 2013 at 04:45 PM.
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Old 21st December 2013, 05:23 PM   #6
Alex M is offline Alex M  United Kingdom
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Oh, and in case anyone is wondering, the small board behind the volume pot is a JFET tape out buffer, and the one with the two preset pots under the volume control shaft is a delayed mute circuit.

Alex
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Old 21st December 2013, 07:23 PM   #7
pchw is offline pchw  United States
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What if you disconnect the phono section? Do you still have hum?
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Old 21st December 2013, 07:59 PM   #8
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It is unclear from your first post: Are the input and output jacks isolated from the chassis?

Is the heater wiring AC? Is it tightly twisted and routed away from and/or perpendicular to all signal wiring?

Have you tried lifting the circuit ground connection from the chassis? (Not the mains earth)
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Old 21st December 2013, 08:14 PM   #9
SY is offline SY  United States
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Understand that you're working with a circuit with next to zero power supply rejection. So small details of lead dress and grounding are critical. Likewise heater wiring- you have to consider coupling paths, and that includes common mode noise. It might help to draw out specifically all of the grounding paths (power supply, signal, and chassis) for the signal and power supply units. Likewise, it may be helpful to look at the hum waveforms to infer the potential origins.
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Old 21st December 2013, 09:55 PM   #10
Alex M is offline Alex M  United Kingdom
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Quote:
Originally Posted by scott17 View Post
It is unclear from your first post: Are the input and output jacks isolated from the chassis?

Is the heater wiring AC? Is it tightly twisted and routed away from and/or perpendicular to all signal wiring?

Have you tried lifting the circuit ground connection from the chassis? (Not the mains earth)
Chassis connection is to the star earth of each channel at one point. I tried connecting the signal grounds to to chassis via 100R/0.22uF in parallel instead of a direct connection, and found no difference.

As I mentioned earlier, heaters are DC, current regulated.

No I haven't tried lifting the circuit ground from the chassis. I will try that.

Alex
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