Filters , poles, interaction, resonance

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IslandPink
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Re: Filters , poles, interaction, resonance

Post by IslandPink » Mon Dec 04, 2017 7:33 pm

Now, I had never seen this - there are a lot of great things on Bjorn's site :
http://kolbrek.hoyttalerdesign.no/index ... r-supplies
Not read it all yet, either !
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izzy wizzy
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Re: Filters , poles, interaction, resonance

Post by izzy wizzy » Mon Dec 04, 2017 8:18 pm

Very interesting. The one that catches my eye is the compensated single VR tube; stable Z and phase over wide frequency spectrum. Food for thought.

cheers,

Stephen

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Re: Filters , poles, interaction, resonance

Post by IslandPink » Mon Dec 04, 2017 8:27 pm

That's what I use on the phono amp - 2-off OD3 in series. I haven't tested the compensation across them, I got approx. values from Gary Pimm, and it's around 0.1uF and 390R .
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Re: Filters , poles, interaction, resonance

Post by izzy wizzy » Tue May 15, 2018 5:55 pm

I tried Bjorn's idea of approx 10u and 100R across a 0d3 to even out the response in my line stage. WOW! I did this months ago and haven't gone back for a comparison again.

By comparison, I would have said previously it sounded like the impedance curve in terms of expression but after it was far more even across the range. If that makes any sense ... so hard to explain. Much like hi Z EQ networks sound like the RIAA curve while low Z ones sound more even. Same kind of idea. It was nice to be able to thank Bjorn directly at Munich for his idea ... which he hasn't tried himself BTW :)

cheers,

Stephen

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Re: Filters , poles, interaction, resonance

Post by IslandPink » Tue May 15, 2018 6:22 pm

Nice .. I must try that myself then - so you went 100R and 10u across each OD3 - or do you only need 150V , not 300 ?
One must think the type of cap is important at 10uF - what did you use ?
I might just try going to something bigger than 0.1u but not quite so big , first , for a pair of OD3's.
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Re: Filters , poles, interaction, resonance

Post by izzy wizzy » Tue May 15, 2018 7:52 pm

I only have one 0D3. If you look at the impedance curve, at low freq he measured the 0D3 at about 100R and with 10u and 100R (he used 91R and 9.1u), the tc is about 1kHz where the 0D3 starts to rise rapidly so from then on, the 100R starts to get to work in parallel and evens out the impedance of the shunt reg. With two in series, you might have to double the values R and halve the C to achieve similar. I didn't try anything else or take it out again which is something I should do for completeness.

OK now it gets embarrasing. I think I used whatever came to hand out of the cap box. IIRC, it's a WIMA MKC4 of about 35 year vintage which is a metalised polycarbonate but it's face down in the bottom of the box and it's very tight in there so can't say for sure.

cheers,

Stephen

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Re: Filters , poles, interaction, resonance

Post by izzy wizzy » Sat May 19, 2018 1:27 pm

Thought I'd better go revisit this 10u 100R bypass thing as quite a bit has changed. First I took it out and couldn't really tell. So I put a switch on it and can switch it live. Can't really tell any difference :oops:

This happened also on this line stage with the power supply cap. Standard BHC electro vs BG WKz. There was a difference but couldn't really decide. I wonder if this has something to do with it being differential? I dunno. Stupid hobby.

cheers,

Stephen

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Re: Filters , poles, interaction, resonance

Post by IslandPink » Mon May 21, 2018 11:42 am

He he !
Well, I think it being differential will make the PS choices a lot less significant - it certainly seemed like that when I did the Aurora PP 300B amps. In the case of the phono you're not getting that close to full swing on the devices either, so the AC demand on the PS will be very small.
Must look at your last phono now - hadn't realised it was PP.
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Re: Filters , poles, interaction, resonance

Post by izzy wizzy » Mon May 21, 2018 1:46 pm

Thinking back, I think the psu wasn't set up currents wise as well as it is now and maybe that's why the bypass made a difference; significant difference IIRC. It might also have been put in when I had the CCS in the tail without the tx on the input. Now it's so much better balanced, as you say, maybe it makes it less sensitive to the PSU.

And following on from that, I went back into the phono these last few weeks to see if a few tweaks might improve things. Had some better transformers to get the PSU performing more how I intended with good results. This allowed more current through the stages to good effect; all compromises I knew about and plthought not big, certainly now better. Tried dampers instead of the mercury and was very disappointed to find themercury so much better. Weird! I want to try mercury in the line stage and am hoping it makes no difference.

Bigger CC grid/screen stoppers on the pentode reduced noise so it's now completely inaudible with headphones off the line stage. We hear so many say pentodes are noisy and so shouldn't be used for phono. Real world would seem to indicate otherwise. Still got it a bit more microphonic than I would like.

I'm fiddling with the EQ to get better value caps in there which has been beneficial. It's quite easy to hear even though I'm only looking at 0.2dB deviation in the sim beforehand. We shouldn't be able to hear that but it's obvious.

It's got a PP output stage coz I had a PP tx and also I wanted to try DC coupling without massive decoupling caps. Seems to work but then I have no comparison.

cheers,

Stephen

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