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Old 25th September 2012, 03:18 PM   #541
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A big vote for split bobbin, and a lack of enthusiasm for shielded torroids. Even with the shield, a torroid will not provide the equivalent high insulation resistance or low capacitance that a split bobbin will. And you always have the challenge of lead exit with the torroid - no matter what you do you have to get those leads past the shield, which will crowd into the other winding if you have evenly distributed windings (which is really one of the few purposes to torroids, low magnetic strays).

I digress. There is an additional check I would recommend, even when using split bobbin. Each primary lead has some capacitance to core/ground, whether it be large or small. There will usually be one of those leads which has a slightly higher capacitance to ground. It is desired to have the lead with the highest capacitance connected to neutral, to reduce mains induced leakage current to ground. Fortunately, the test is quite easy.

Connect core to secondary. Core to be ungrounded for the test. Energize the primary with line voltage. Measure AC volts with a decent high impedance meter (no VOM's or wiggys) from core to earth ground/3rd prong. Switch primary lead polarity on xfmr, and repeat test. One of those measurements will almost always be greater. The primary lead polarity with the greatest measured voltage is the one whereby you have applied the hot to the highest capacitance lead. Therefore, you have identified the highest capacitance lead - mark this lead as the one you want to connect to neutral, not hot.
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Old 25th September 2012, 07:15 PM   #542
euro21 is offline euro21  Hungary
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Quote:
Originally Posted by Rod Coleman View Post
effective capacitance from primary to secondary.
Some rapid measurements:
1.) custom wound 2x15V -2xSM75- : 130pF, 160pF
2.) Indel 2x17V toroid: 440pF, 370pF
3.) MerkelBach split bobbin 15V-0-15V: 90pF !!
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Old 28th September 2012, 08:25 PM   #543
Bigun is offline Bigun  Canada
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Default DIY Schematic

Hi Rod,

I'm toying with the idea of making my own pcb for a filament heater in order to meet certain requirements for parts, size etc. It would surely be easier to use yours and I might still go that route - it's early days...

But if I want to DIY this thing, is there a schematic that you have released for public use - i.e. which post should I go look at for a useable circuit even though it won't have all the benefits of your latest but proprietary circuit ?
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Old 29th September 2012, 04:49 AM   #544
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I am a little surprised by how the R1,R2 resistor types can change the sound in Rod's kit. The resistors is vitreous wirewound type, 0.68ohm in R1/R2 position. they are connected in parallel to give around 0.33ohm in my GM70 kit version.

I paralleled 8 x 2.7ohm (2W, carbon film) resistors and installed into the R1/R2 position. there is an immediate discernible shift in the sound. Can u imagine this? a current sensing resistor for a filament reg supply, and the type of resistors make a difference!

The carbon film gives more lush to the vocals. However, i still prefer what is provided in Rod's kit. Carbon film softened the bass lines, and the upper frequencies sounded a little fuzzy. anyone here tried carbon film in r1/r2 and like them?
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Old 29th September 2012, 04:55 AM   #545
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someone here has replaced the tiny cap C3 with physically larger ones. due to the space contraint on the pcb, some members here soldered wire leads out from the PCB and then soldered the larger caps to the two wire leads. This effectively lengthen the the cap leads, and it has no real support ; the cap is 'dangling' in the air. I have a feeling that this may not be a desirable change since the cap need to be small with shortest lead possible, base on the fact that it is a compensation cap.
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Old 29th September 2012, 09:13 AM   #546
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I'm seriously wondering to what extent tweaks in Rod's boards are audible, and to what extent users are hearing what they think should be happening, like carbon = warmer etc. Believe me, I'm open to either interpretation. In terms of audibility I believe I hear quite audible differences in the cathode resistor in filament bias - I tried numerous ones and settled on some discontinued Dale wirewounds I got on ebay. I disliked thick film ones in particular, and I thought I preferred the vitreous enamel ones (e.g. the green Welwyn ones but also other varieties) to the white ceramic and alu clad ones. But having said that, I might not have heard any difference if I'd listened blind. My expected rank order would have been to prefer vitreous to alu clad and ceramic, so was I hearing what I expected to hear? Against that, I expected the thick film to sound the best and I was surprised that they didn't, so it seems that my ears have the ability to "think" for themselves and hear what's actually happening.

I can't report on components in Rod's boards because I haven't yet done any critical listening. I know the Danes have and reported hearing differences in the caps and set resistors. It's a bit more complicated with me since I still use Rod's original circuit, though I also have his new boards as well. At some point I'll have to see if I can hear any differences.

But I think we're often in the situation where we have to A-B different parts and make a choice. And I wouldn't like to say for sure now much we actually hear differences and how much we think we hear differences based on what we expect to hear. I'm open to either interpretation or a mixture of both. The one thing I'm sure of is that it's very easy to exaggerate the scale of changes and say "much better" etc. I'm pretty convinced that changes like this are a lot smaller than we think they are.
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Old 29th September 2012, 09:26 AM   #547
Desmo is offline Desmo  Denmark
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I quote myself on the ''upgrade of R1 and C3 issue''.

I have upgraded with these parts. Here in Denmark we are (at least) two Tram II owners, and when I bought these parts I bought them for both myself and Bjarne (''Beardman'' here on diyaudio). I installed the parts, listened and heard a difference that was quite easy to hear. I did not tell Bjarne what I heard, so it was interesting to compare our experiences after upgrading R1 and C3... Bjarne heard exactly the same difference / improvements.

That is good enough for me

However, I do agree that in hifi we often tend to put the differences under the microscope, and subjectively small differences are passed on as substantial. For instance many of my audio-buddies tell how big difference in sound their very, very expensive power cords make. Personally I hear just very small (if any) differences on power cords, so that is not where I spend my money on my system...


Quote:
Originally Posted by Desmo View Post
I have now updated C3 and R1. For C3 I have choosen Amtrans AMCH mylar caps... Good sound, decent price, not that big so they can actually fit, finally they are spec'ed for high temperatures. For R1 I have choosen 12W Mills wirewound, they sound great and have a big surface area to dissipate the heat from so they don't run so hot.

When not using R2 there is actually decent space for the caps, so the installation can be quite clean as seen here I agree with the previous posters: Upgrading these gives a clear sonic improvement..!

Click the image to open in full size.
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Last edited by Desmo; 29th September 2012 at 09:29 AM.
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Old 29th September 2012, 09:26 AM   #548
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I'm going for 12 watt Mills just to be safe,
and because I like them.
If they sound better, that's just a bonus!

Still thinking about what to use for the 220n cap.
Wima have some small 5mm grid caps that should fit right in.
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Old 29th September 2012, 09:31 AM   #549
Desmo is offline Desmo  Denmark
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Yes, the Mills are very good for this purpose. Very transparent, not colored as some of the carbon types... Regarding the caps, then look for caps that both sound good and are spec'ed for high temperatures
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Last edited by Desmo; 29th September 2012 at 09:33 AM.
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Old 29th September 2012, 10:20 AM   #550
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It's convenient to use polycarbonate caps - they're small. I don't see them about much but they used to be plentiful. Remember those small green and silver ones with the legs welded on either side? You can see them in this link.

Polycarbonate Capacitors
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