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Hi all!

I'm Mike and I am new here at diytube.com. I recently built a Darling tube amp. Unfortunately, I couldn't use the wonderful pcb. My enclosure is too small for that. Since I hadn't a small toroidal mains transformer in my scratch box I used a second enclosure for the psu like that:



The Darling sounds beautiful. I have, however, around 15 mV 50 Hz hum (I'm located in Germany) on the outputs when the inputs are left open (no source connected). With a source connected or terminated with a 100 ohms resistor it is dead quiet, and AC on the outputs drops from to 5 mV which is almost inaudible. Proper star grounding or external power supplies didn't change a thing. The best explanation seems to me that the inputs catch some noise. Can you confirm this effect? If not, do you have any ideas? Thanks in advance!

Mike

While a shielded wire at the input could help, the high impedance of the unloaded grid circuit is probably just hum coupled from the heater inside the tube. Once you have the load connected, the grid circuit is not going to couple to the heaters as easily. So nothing is really wrong.

Shannon

The wire at the input is shielded. Hum from the heater was my first idea. But why 50 Hz, while on all DC lines ripple is at 100 Hz? I have to admit that this is more or less an academic discussion, since the Darling is quiet with a source connected to the inputs. I'm just curious whether other Darling builders observed this effect.

Mike

Hi Mike ...

I wouldn't be concerned with the hum with no input connected.
There should *always* be an input connected anyway.
With no input, the RCA jacks are like antennas for hum.

... tom

Oh right, this is the Darling circuit with a voltage doubler for the filaments. And your transformer is in a separate chassis. What does the power cable between the two chassis look like? It must be some ground loop between the two.

Shannon

TomMcNally wrote:With no input, the RCA jacks are like antennas for hum.

Yes, they are! Even my Fluke Multimeter shows 50 Hz when nothing is connected to the inputs.

@Shannon: My power connection is far from ideal. It's a 4-wire connection: 2 wires for the positive line (B+ and heater) and 2 wires for the negative side. That means that signal ground and power supply ground run on the the same wire. However, for test purposes I tried a star grounding by the book and even separated charging cap (-) from filter cap (-). The 50 Hz hum didn't change. Even with the mains transformer disconnected using an external transformer hum was still there.

The only reason for digging a little bit deaper is: If this effect is inherent to this Darling design, everybody should have this. If not, something in my design is the "problem".

I forgot to mention that th circuit that I use is a tiny bit different from the Clementine. It's this Version. I added gridstoppers, so both circuits are very similar.

Besides, I made a very interesting experience with my last Darling. I used an vintage Telefunken mains transformer. The mechanical hum was crazy! Inductive blind current measured 200 mA, which is a LOT. I couldn't find any electrical or mechanical problems. So I decided to mould it in epoxy resin. To my surprise the mechanical hum was totally gone. And even the blind current normalized.

Mike

The high impedance inputs on the amp will be susceptible to picking up stray 50 Hz AC power line and magnetic fields, the use of shorting plugs should eliminate the hum.

Properly shielded cables from the source to the amps should normally be hum-free.

I read that you did not use the PC boards, but for those who do use them, a common mistake is to forget to safety ground the PC board to the chassis, just a single ground lead is needed, this way eliminates any ground loop problems.

/ed B

EWBrown wrote:The high impedance inputs on the amp will be susceptible to picking up stray 50 Hz AC power line and magnetic fields, the use of shorting plugs should eliminate the hum.

So, you all would agree that this is nothing wo worry about? As I said before, the amp is hum free when a source is connected.

Mike

It is nothing to worry about, if it were an internal power supply ripple issue, the hum or buzz would be 100 Hz and not 50.

/ed B

Ok, guys, thank you very much for the input. Besides, here's an inside view of my Darling. I'm pretty sure, I'll make an acrylic glass front panel someday. I like the enclosure very much. But the 1626s are too beautiful to hide!

Mike