DHT…. Get rid of the Hum Pot! - diyAudio
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Old 13th May 2009, 06:54 PM   #1
sgerus is offline sgerus  United States
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Default DHT…. Get rid of the Hum Pot!

The other night I was tweaking my 2A3 amp, and came up with a method for very low HUM levels without having a HUM Pot in the single path.

The typical method’s you see for DHT cathode bias are:
1. R & C on the filament transformer center tap
2. R & C from a HUM Pot
3. R & C from the center of two 22R 3W resistors across the filament

A better method I found on a schematic from diy member kmaier is:
4. The cathode bias resistor on the filament transformer center tap and the capacitor on the HUM Pot

The new method I proposing:
5. The cathode bias resistor on the filament transformer center tap and the capacitor on from the center of 2 close (but not always exact) resistors.


Some test results:
1.4mV HUM with method #4
4.2mV with R & C on the filament transformer center tap
3.2mV with R on the filament transformer center tap and the Cap for the center of 2 exact matching 470R’s, .5w, carbon resistors

Why was the HUM so much higher with the two 470R’? Because I need 2 resistors that mimic the HUM Pot

So with the HUM Pot disconnected, I measured it and found it was at 49.4R and 51.5R,
Or 1 side was about 4% larger than the other.

Next I measures my supply of 470R’s and came up with a pair that had about a 4% delta
(450R and 469R in my case)

Using jumper wires I tested the amp with my 450/469 resistors in place of the HUM pot and ended up with the same 1.4mV

Bottom line…. Very low hum without a variable resistor in the single path!
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Old 14th May 2009, 03:30 AM   #2
rtsang is offline rtsang  Hong Kong
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Default Salute to Kmaier

I have applied kmaier's method to my PX4 amp with good results.

I am using AC heat and I found it difficulty to use DC heating as the voltage drop via the rectifiers and resistors would not be enough for a 4V AC to convert to 4V DC and the filament transformer only had 4V windings.

I was able to get around 1.5mV to 2mV of residual noise with PX4 but with PX25 (drawing 2A filament current), I could only able to get around 6mV of noise.

I understand your idea of eliminating the pot and use fix resistors, but I wonder would the drift in the tubes and transformers later make your fix resistors inappropiate ?

I would still like to keep a pot so that I can readjust it later.
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Old 14th May 2009, 11:11 AM   #3
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Interesting idea.

Is there any difference in sound quality that you can make out with all four methods?

andy
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Old 14th May 2009, 11:50 AM   #4
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Hi sgerus,

I think you are just fooling yourself!

With ca 235ohms in series with 470u you just partly bypass the cathode.

So you loose signalvoltage of the same percentage as the humreduction.
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Old 14th May 2009, 01:45 PM   #5
sgerus is offline sgerus  United States
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Andy:
I can’t really tell if the sound is any better, I was only interested in HUM.

Lars:
I will scope check and report back later. I do know that the DC voltage on the cathode bypass cap is exactly the same as with the 100R hum pot.
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Old 14th May 2009, 01:59 PM   #6
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The voltages shouldn´t be different as there is no current flow.

By your arrangement you will get lower gain and higher Zout.

So you actually hasn´t won anything, as the quote between output voltage and hum voltage stays the same.

Still it might sound better.....
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Old 14th May 2009, 04:54 PM   #7
kmaier is offline kmaier  United States
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All,

I did a lot of testing during the prototype design (45s and 2A3s) which eventually resulted in the split balance technique. Sonics are also important in addition to low output noise (hum). Using good quality 45 tubes, I can get hum levels below 200 micro-volts (which is < 0.0002 volts at the 8-ohm tap) which results in a S/N ratio better than 80dB referenced to 1-watt. 80dB is my personal acceptance factor... anything less and I don't use the tube. Anything better and I smile more, have a few pairs that are around 90dB.

As pointed out, the high value resistors simply form a RC network and you lose gain, i.e., you don't have a low AC impedance to ground. The reason you still benefit for reduced hum levels is the RC network is a lower impedance at 60Hz than nothing at all. Still, you have limited cathode (AC) bypass which decreases at lower frequencies.

You're also not able to compensate for the imperfect balance of the cathode versus the grid and plate alignment, it's basically impossible to have the internal assembly perfectly balanced. There's too many variables.... the gauge linearity of the filament wire, the coating on the filament wire (and the consistency of the coating itself), the spiral grid assembly plus the plate assembly and the overall alignment. It might not seem like a lot, but we're talking electrons flowing... they're really small.

I use a pair of low value padding resistors for three reasons: 1- to reduce the resistance that ends up in series with the bypass capacitors (12 ohm resistors results in less than 5 ohms), 2- to widen the adjustment range of the 100-ohm pot (reduces the "knife-edge" null point), and 3- to lessen the (sonic) effect of the element of the pot versus the fixed resistors which should be of similar quality to the rest of the circuit.

Regards, KM
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Old 15th May 2009, 12:38 AM   #8
sgerus is offline sgerus  United States
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Lars was right......

With the 100R hum pot 1 Volt in = 1 Watt into 8R at 1000Hz
With the two 470R's it's up to 1.2V = 1Watt's

SO IT LOOKS LIKE THIS EXERCISE WAS A FLOP!!!

Well, that's how we learn.

Looks like the hum pot's will go back in, this time in // with two 12R resistors for all the reasons KM pointed out.

Thanks Everyone
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Old 19th May 2009, 10:11 AM   #9
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Interesting thread. Can I just make sure I've understood this? The final choice was a hum pot with 12R from each end to the wiper. The filament supply goes to each end of the pot. Where does the wiper go -

- to unbypassed cathode resistor to ground
- to bypassed cathode resistor to ground
- other?

I want to try this in PP so I have two filament supplies, two tubes, but one cathode resistor.

So how exactly do I implement the "best method"?

Many thanks!!

andy
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Old 19th May 2009, 11:16 AM   #10
kmaier is offline kmaier  United States
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Quote:
Originally posted by andyjevans
Interesting thread. Can I just make sure I've understood this? The final choice was a hum pot with 12R from each end to the wiper. The filament supply goes to each end of the pot. Where does the wiper go -

- to unbypassed cathode resistor to ground
- to bypassed cathode resistor to ground
- other?

I want to try this in PP so I have two filament supplies, two tubes, but one cathode resistor.

So how exactly do I implement the "best method"?

Many thanks!!

andy
Andy,

Not sure what you're suggesting is possible... i.e., two filament supplies but one cathode resistor. Are you using directly-heated cathode output tubes in push-pull?

The above, if he really takes my suggestion(s), would have the hum balance pot configured as above but the wiper would only have the cathode bypass cap to ground (AC balance being adjustable) and the cathode bias resistor from the filament center-tap to ground (fixed DC balance via the filament winding).

Can you clarify your output stage configuration and tube types?

Regards, KM
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