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Old 4th October 2011, 05:33 AM   #1231
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Hi Ale,

Don't worry too much about that noise. When the filaments are hot you can still have parasitic currents, even if the HT is off. These can create this effect. When the filament is off too, it should be silent.

Best regards

Thomas
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Old 4th October 2011, 05:35 AM   #1232
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@ikoflexer: That hum is probably being picked up by the plate chokes. Try to rotate them by 90 degree and see if that changes the hum. Or shield them

Best regards

Thomas
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Old 4th October 2011, 09:25 AM   #1233
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Recently I did some HT supply test. I have a test bed in order to test (quick and dirty) PSU and preamp configurations. I compared 3 different transformers, 3 different rectifiers and caps of several sorts and values. The lineamp in this test bed is a CCS loaded 6N6P. That’s what my new pre started with until I got converted to DHT. The test were performed with the Tantlabs E-choke which is giving approx. 65dB (2000x) of ripple rejection. So: transformer-rectifier-capacitor-E-choke-capacitor
Capacitors: Because the E-choke ripple rejection is so high I could test with a wide variety of capacitor values. I found that it sounded better and better by lowering the total amount of capacitance before and after the E-choke and ended up with only 20uF before the E-choke, while the circuit draws a total of 50mA! So that’s about 0.5uF of pre-filter capacitance per mA current draw. When the capacitance is lowered further PSU hum starts to creep in. Quality of the capacitors is very critical and none of the electrolytics could satisfy. Even BG-NX 68uF-350V in anti-parallel configuration. For example 2x10uF Obbligato polypropylene copper tube sounded a lot better than one standard quality Chateauroux 18uF polypropylene. Further bypassing with smaller values of HQ capacitors is recommended. I use capacitors that would be fine for signal coupling purposes (a lot of Russian stuff for financial reasons) and found they had about the same specific sonic properties when used as PSU bypass cap. So you can cook the way you like it. I bypass with several caps of lower and lower values from uF range to pF range.

Next time: Rectifiers.

Peter
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Old 4th October 2011, 10:29 AM   #1234
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question: can i replace a resistor with a CCS in a CLCLRC filter before a transformer loaded 26 ?
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Old 4th October 2011, 12:38 PM   #1235
iko is offline iko  Canada
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Quote:
Originally Posted by Vinylsavor View Post
@ikoflexer: That hum is probably being picked up by the plate chokes. Try to rotate them by 90 degree and see if that changes the hum. Or shield them

Best regards

Thomas
Thanks Thomas, that's the same advice Kevin gave me, and I tried it. Took one of the chokes and put on it longer wires. Rotating the choke did change the size of the hum ripple, but couldn't get it small enough not to be heard. Shield the choke also didn't fix the problem.
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Old 4th October 2011, 01:53 PM   #1236
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Noticed that when HT was off had a distinct hum noise which is gone when I apply the HT. Any ideas why this could be? I have the output floating to the amp i.e. the LL1660 secondary is not grounded... >>

I've experienced exactly the same thing! But when switched on I don't get hum problems.

I'm not using my DHT stuff right now because I need more power for my new Mark Audio alpair 10 speakers, which I like a lot - great unit. I'm looking at a Pass First Watt build, or Le Monstre or equivalent. My goal is to drive this with DHTs. The F5 has gain in it, but the F4 is unity gain.

any thoughts on this?

Andy
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Old 4th October 2011, 02:53 PM   #1237
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Hi andy , i had the hiriga "le monstre" 8 watt for a little time in my house ( with AVG uno nano) and it sounds not so good face to my Diy 6n6P/AD1 4 watt amplifier.
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Old 4th October 2011, 03:13 PM   #1238
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Quote:
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question: can i replace a resistor with a CCS in a CLCLRC filter before a transformer loaded 26 ?

Yes you can, audio current loop is now between the last capacitor and the "cathode" of the DHT. I in fact do this in one of my power amplifier designs, make sure to make that last capacitor large enough that you do not get an in band resonance between the cap and primary inductance which will produce bass boost - which may also be useful in some instances.

Doing this will result in constant current operation, and very good isolation between the supply and your load. I use fixed bias in this configuration.
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Old 4th October 2011, 03:27 PM   #1239
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thanks kevin , is there a rule for the size of this last capacitor? i 'v got 50ufMKP..


rohan
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Old 4th October 2011, 03:44 PM   #1240
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thanks kevin , is there a rule for the size of this last capacitor? i 'v got 50ufMKP..


rohan
Depends on what the inductance and rp are, but I'll wager that 50uF should be sufficient with the rp of the 26 and the primary inductance of that transformer. To be absolutely certainly simulating it in spice would be helpful. You can use filament bias or fixed bias, cathode bias is possible as well, just be sure to use a large enough value that the supply cap is the dominant LF pole in the system.. (Several hundred uF)
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