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Old 7th December 2012, 04:38 PM   #2251
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I just found an interesting web site on impedance testing of different kinds of power supplies:

Testing Power Supplies

It doesn't seem to be a problem shunting VR tubes with bigger capacitors as long as it is a correctly sized resistor in series with the capacitor. Haven't tried it yet but I'll guess I have to give my share to science and give it a go (any warnings are very welcome).

Last edited by Vbenonisen; 7th December 2012 at 04:56 PM.
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Old 29th December 2012, 11:31 AM   #2252
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Hello again. I'd like to raise the issue of the anode load for the 26. I've been using 2x Hammond 156C chokes in series, but this is prone to hum. As an alternative I'm going to try and optimise a Lundahl LL1667/10mA. In it's raw state this doesn't sound as good as the Hammond combination, but it should have less hum. I tried adding a 4.7k resistor between the Lundahl and the anode and this seemed to clean up the sound. The Lundahl datasheet lists 270H inductance for the 15mA version, so I'm thinking the 10mA version should be over 300H. That's the same as the Hammond combination. Nevertheless, adding a resistor seemed to help.

Can anyone comment on this? Am I correct in adding the resistor between anode choke and anode, not anode choke and B+? Any comments about the size of the resistor? Has anybody played around with choke+resistor combinations?
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Old 29th December 2012, 11:34 AM   #2253
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Hi Andy,
Hope you had a great Christmas!
Adding a resistor between the primary and the anode will increase the effective anode resistance and therefore impacts the low frequency response. The reduction of the hum you perceive is in essence a degradation of the bass response...
Ale
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Old 29th December 2012, 11:41 AM   #2254
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Hello Ale! I see you are in Saturday Morning Hi-Fi mode..... thanks for coming in here.

I think we can't rule out the physical construction of the choke as a source of hum - the Hammonds seem to pick up hum more than the Lundahls. Maybe there is more than one factor in play here? Have you tried a choke+resistor combination any time?
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Old 29th December 2012, 11:57 AM   #2255
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I'm certainly working this morning. I've been building DHT power supplies for the last three days (4-65a and 46 in filament bias)

I haven't tried that combination am afraid. The LL2745 I'm currently using in the 26 preamp are very quiet. They are not shielded in my case as I'm still running with the breadboard (see picture attached). Of course if you place your hand very close to the OT you will notice the induced hum. Otherwise is not noticeable. If you want to use the chokes then I will suggest to place them inside their own little metal boxes?

From a sound point of view, would be interesting to hear your feedback as to whether the addition of the resistor will improve sound rather than reducing hum (and therefore also impacting the bass response of the pre-amp)...

Ale
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Last edited by mogliaa; 29th December 2012 at 11:58 AM. Reason: Forgot the attachment!
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Old 29th December 2012, 01:39 PM   #2256
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Hi!

putting a resistor in series with the choke will not impact the frequency response. In fact it would even improve it. A few kOhms should not make a big difference though. I have found placement of anode chokes to be critical. Also be careful how you orient the two chokes of each channel. They can interact with each other and produce warps in the frequency response.

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Old 29th December 2012, 01:40 PM   #2257
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I was thinking something like this -
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Old 29th December 2012, 01:41 PM   #2258
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Hi!

Quote:
Originally Posted by mogliaa View Post
Of course if you place your hand very close to the OT you will notice the induced hum.
I'm sure its not the transformer that picks up the hum, but probably the 26s which are prone to pick up of external fields. I have used these transformers in phonostages and they don't pick up anything. That's with signals at least 20dB below line level

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Old 29th December 2012, 03:26 PM   #2259
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Quote:
Originally Posted by Vinylsavor View Post
Hi!

putting a resistor in series with the choke will not impact the frequency response. In fact it would even improve it. A few kOhms should not make a big difference though. I have found placement of anode chokes to be critical. Also be careful how you orient the two chokes of each channel. They can interact with each other and produce warps in the frequency response.

Thomas
Thomas,
Technically, I think it does. Albeit it will only be a minor impact and as you say it may be unnoticed or perhaps add some good results. If we don't take into account the load, then the pole is impacted as you have the inductor in series with the added resistance. I did a quick simulation to show this.
Ale
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Old 29th December 2012, 03:27 PM   #2260
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Quote:
Originally Posted by Vinylsavor View Post
Hi!



I'm sure its not the transformer that picks up the hum, but probably the 26s which are prone to pick up of external fields. I have used these transformers in phonostages and they don't pick up anything. That's with signals at least 20dB below line level

Thomas
You may be right here as both are very close. When I tested that I found that the hum increased with the hand close to the 26 as expected, but then moving it over the transformer it had same effect. Can't contradict what you're saying as in my case both are very close.
Ale
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