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Old 29th November 2012, 01:26 AM   #2231
Magz is offline Magz  United States
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Those Lundahls I'm using are amorphous Cobalt core with no air gap, so they'd be very susceptible to core saturation, but I haven't seen it. The DC grid current is very tiny, unless the tubes are very gassy.

I asked Kevin Carter of K&K Audio (Lundahl dealer) about it before I did it, and he said that normal tubes without high leakage will work fine. My results have convinced me that's true.

In any case, the transformer wouldn't be damaged by the DC, the core would saturate, which can be remedied by passing an AC signal at the saturation level...demagnetizing, in other words.
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Old 2nd December 2012, 12:40 AM   #2232
Magz is offline Magz  United States
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In case anyone is interested, I measured the AC impedance of a Duracell Alkaline 9V and an Ultralife Lithium 9V, only one example of each, both new. Both measured 9.4V DC. I measured using my Escort LCR meter with a 1000uF, 25V Panasonic FM cap on the + terminal to block DC. The results at 100, 120, 1000, 10000 Hz for each:

Duracell Alkaline: 12.2, 11.2, 2.8, 1.6 ohms

Ultralife Lithium: 4.3, 4.0, 2.4, 1.2 ohms

I was curious how they compared as I'm using battery grid bias in series with the grid. I prefer to use the lithium because of its longer shelf life, but I wanted to check the AC performance first.

Listening-wise, I don't think I can tell a difference.
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Old 2nd December 2012, 06:51 AM   #2233
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In terms of core saturation, how would one know that a core had been saturated and needed demagnetising?
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Old 2nd December 2012, 09:34 AM   #2234
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Quote:
Originally Posted by andyjevans View Post
In terms of core saturation, how would one know that a core had been saturated and needed demagnetising?
Demagnetizing?
Core saturation isn't remanence (in most cases).
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Old 2nd December 2012, 10:33 AM   #2235
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Andy search on G "phono cartridge demagnetization" some info...
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Old 2nd December 2012, 12:21 PM   #2236
Magz is offline Magz  United States
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Quote:
Originally Posted by andyjevans View Post
In terms of core saturation, how would one know that a core had been saturated and needed demagnetising?
Your bass response would drop off noticeably.

I've been back and forth with Kevin C. and Per Lundahl about this several times over the last few years in reference to a couple different projects. It seems the LL1674 and LL1676 are susceptible to developing some core magnetization if they are subject to too much DC current, which will cause the bass response to drop off. Per has implied that simply measuring the DCR with a multimeter is enough to do this.

The cure according to Kevin is: "High level (just below saturation) low frequency sine waves do the job the quickest. You can do that in circuit if you have a source of sine waves and a 'scope to take the primary up to saturation and back it down a bit."

Saturation is said to be ~16VAC rms or so at 20Hz on these. The datasheet shows 22VAC rms at the secondary at 30Hz giving 1% distortion.

All that said, I have now put about 20 hours or so on the LL1676 input transformers with the battery below the secondary, and the bass response is still thunderous. Magnetization from grid current doesn't seem to be an issue, at least with the 26 globes I'm using.

Last edited by Magz; 2nd December 2012 at 12:24 PM.
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Old 2nd December 2012, 05:25 PM   #2237
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Default Good power supply - Black art?

I've worked some time on my 26 preamp, but have some trouble figuring out the optimal power supply for the job. As of now I have 20H-300uF-5H-100uF, and finally for each channel 5H-20uF-0D3 regulator tube.

This works and sound fine, but I was wondering if there is some kind of theory behind making a REALLY REALLY good power supply. I do not have a scope but uses Duncan Amps PSU designer. Is there something that signifies a good PSU that I should look for when designing or is it just a question about ripple, taste and black art?

Regards,
Vidar
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Old 2nd December 2012, 06:44 PM   #2238
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The 26 preamp draws a fairly small idle current, and the peak signal current is also small.

There are many valid choices to make, but here's some considerations:

- choke input filter is recommended (best regulation, lower EM radiation)
- Large L, small C in a LCLC or LCLCLC (etc) is perfectly valid, and allows the use of audio-MKP capacitors (or better), and no electrolytics.
- next most important:high quality trasformer. choose EI, not toroid , and make sure it is split bobbin (Hammond call this 'dual bobbin').

If you can't get a good trafo, please use an isolating trafo (eg 230:110V or 230:230v) and treat its secondary as a kind of electrostatic screen (connect one side to ground, connect cap 1uF 440VAC MKP across secondary).

- experiment wit gas tubes and shunt regulators, and find something you like - or stay with passive!

- try some tubular ferrites on the mains lead.

- RC snubber across the secondary: try 47nF 1500V in series with 100 ohm carbon composition.
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Old 2nd December 2012, 07:13 PM   #2239
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What do the snubbers on the secondary do, and how big resistor in wattage?
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Old 2nd December 2012, 09:09 PM   #2240
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When the rectifiers switch-OFF, the leakage inductance of the trafo can yield a voltage spike. Series RC circuit damps the pulse (C-only may cause ringing).

0,5W carbon composition is fine - the power dissipated is very small.
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