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  #1  
Old 24th August 2019, 11:40 PM
sharkey22 sharkey22 is offline
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Default Zobel network on HD3S

Hello all.

I have noticed that the HD3S headphone amplifier has a Zobel network consisting of a 10 Ohm resistor in series with a 0.1uF capacitor across the output.
This is not present in my older WAD HD83. The rest of the circuit appears to be identical, the obvious difference being the addition of the DIP switches for more convenient impedance switching in the newer design.
I am curious why the Zobel network was added to the newer HD3S and whether there is anything to be gained by adding it to my HD83.

Thanks.
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Old 26th August 2019, 10:20 AM
Richard Richard is offline
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Default Re: Zobel network on HD3S

Hi Sharky,

As I understand it (John or others feel free to correct!) it's a series RC network but doesn't really function as a Zobel in that it doesn't maintain the headphone's impedance (in a similar way to one used in a speaker crossover for instance).

The headphone's impedance may rise with frequency due to the inductance of the voice coil. What this network does is to give a path to ground for HF in the MHz range, rather than run the risk of HF oscillations being passed back through the feedback loop which might cause audible problems lower down the range.

It might be easier to think of the 2 components separately. The 10R sets the lowest impedance of this path. The 0.1uF cap would have a reactance of 160R at 10KHz so almost no effect. At 100KHz it would be 16R and at 1MHz 1.6R. So it will always be at least 10R, and offer that path of 11.6R at 1Mhz.

I've not heard of any problems with HD83 but fitting this network will cause none either and is probably good practice as HF oscillations can sometimes be present unknown without specific checks using a scope etc.
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Old 28th August 2019, 02:30 PM
sharkey22 sharkey22 is offline
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Default Re: Zobel network on HD3S

Thank you Richard.

I have added the components to the phones socket as suggested. No issues.

Am I correct in saying that these components also ensure that there is always a load on the transformer even if no phones are connected?

Thanks. Ian.
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Old 28th August 2019, 03:23 PM
Richard Richard is offline
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Default Re: Zobel network on HD3S

Hi Ian, if the 10R were across the output by itself it would be a load at audio frequencies, but with the 0.1uF cap in series it means it's only a load at supersonic frequencies (as only higher frequencies can pass the cap).
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Old 28th August 2019, 07:27 PM
John Caswell John Caswell is offline
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Default Re: Zobel network on HD3S

I couldn't improve on Richard's comments if I tried - excellent description.
You don't really need a dummy load on the HD83 but if you have standard switched jack socket it is easy to arrange.

John
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  #6  
Old 28th August 2019, 09:06 PM
sharkey22 sharkey22 is offline
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Default Re: Zobel network on HD3S

Thanks for the comments. Very comprehensive and helpful.

I am not familiar with electronics theory and although I have owned valve amps for decades I only recently learned that no load on output transformers is potentially harmful to them. Fortunately I have not killed any of my valve power amps although I frequently swapped speakers without shutting down the amps in the past.

Thanks again for the HD83 advice.

Regards Ian.
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  #7  
Old 28th August 2019, 11:51 PM
John Caswell John Caswell is offline
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Default Re: Zobel network on HD3S

Ian "Sharkey" wrote-
"I only recently learned that no load on output transformers is potentially harmful to them."
Not strictly true, damage may occur if you swap speaker leads or have o/c secondary whilst the amplifier is being driven. I have see some wonderful lighting displays
Obviously if in doubt power down the amp, but no harm is likely occur if you have signal level at zero when you swap speaker leads.

John
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  #8  
Old 29th August 2019, 01:31 AM
colin.hepburn colin.hepburn is offline
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Default Re: Zobel network on HD3S

Quote:
Originally Posted by John Caswell View Post
Ian "Sharkey" wrote-
"I only recently learned that no load on output transformers is potentially harmful to them."
Not strictly true, damage may occur if you swap speaker leads or have o/c secondary whilst the amplifier is being driven. I have see some wonderful lighting displays
Obviously if in doubt power down the amp, but no harm is likely occur if you have signal level at zero when you swap speaker leads.

John
Hi John/All
Yes I was under the same opinion on this that the TX could be damaged in this way .on push pull TXs if driven or no single applied thanks for clearing that up john. Does the same apply to the single ended TX as it would be drawing current still with no single applied?
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