Problem with Autoformer Modules

discussion of magnetic volume control design for both line and speaker levels.

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niclaspa
Posts: 1
Joined: Sat Sep 06, 2014 7:07 pm

Problem with Autoformer Modules

Post by niclaspa »

I bought a couple of autoformer modules for my line stage (schematic attached). Before, I was using a 47k resistive attenuator. I replaced it with the autoformers. The first problem is that it picks up a tiny bit of hum. I guess that I could cure that with some mu-metal shielding.

Worse is that when I turn the switches on the attenuators, there are loud pops and swooshes. Also, sometimes the swooshes appear without turning the autoformers. Some kind of self-oscillation? I have been trying to use two different tube phone stages before the line stage, but with the same result.

I tried to put the autoformers on the output of the line stage. No pops and swooshes, but the first step above silent is pretty loud! Sure, I can reduce the sensitivity with a couple of resistors on the input of the stage, but I am not sure what I have gained compared to the resistive attenuator.

Any advice would be appreciated!
Attachments
Line stage schematic
Line stage schematic
linestage.png (8.56 KiB) Viewed 6866 times
dave slagle
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Post by dave slagle »

Hey,

The hum suggests a bit of shielding is needed. The clicks and pops when switching is either a bad switch or DC on the switch. The swooshing sounds like an oscillation and a 100 ohm resistor at the grid pin of the D3A might help.

I think the real question here is why the d3a for a linestage? If when you place the autoformer at the output your first click (-42db) is too loud, it means you are already attenuation by a ratio of 100:1. This suggests you have way too much gain considering the d3a+4.5:1 stepdown nets you a gain of around 17.

have you tried the autoformers as a passive?

dave
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Willem
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Joined: Tue Nov 03, 2015 9:37 pm

Post by Willem »

The clicks and pops on the attenuator are most likely due to DC buildup on the input grid of the tube due to the lack of a grid leak resistor. If the switch is of the non shorting type, the resulting open grid will result in an impulse being output between each switch position.
docali
Posts: 58
Joined: Thu Oct 16, 2008 7:16 am

Post by docali »

Hello!

The pops are definately related to the D3a operating point shifts. If I remeber well you can read something about this in this forum. I would suggest to NOT use an AVC driving the grid of this tube. Also this tube has in my opinion too less headroom for a line stage.

BR!
Docali
docali
Posts: 58
Joined: Thu Oct 16, 2008 7:16 am

Post by docali »

You should read here about this issues:

[url]http://www.intactaudio.com/forum/viewtopic.php?t=40[/url]
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