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Old 14th August 2003, 09:07 PM   #11
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Ah, so that's what you meant when you said "isolated" earlier. How do I electrically isolate the tab from the (common) heatsink but still maintain good thermal contact? Are there plastic (or similar) washers that are designed to do this? Maybe I should just buy lots of cheap heatsinks. Or cut up scrap metal into little pieces.

I also saw Schottkys in the more "normal" diode-looking packages - those wouldn't need heatsinks to perform as spec'd, and would take up less space too. So, is there a specific reason you recommended I get TO220-type packages?

And thanks everyone for your patience, I know these are really basic questions. But that's what happens when you have a little bit of theory under your belt but zero practical experience

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Old 14th August 2003, 09:07 PM   #12
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Hi,

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However, now that I think about it, if I got B+ up to the 400V range, I could try the Loftin-White schematic you showed me, with the 6SL7 SRPP direct coupled to the 2A3. That would get rid of one cap in the signal path.
I like that Loftin-White quite a lot. If you think you can build one with what you have, why not?

Quote:
And speaking of caps - given the fact that my speakers show a pretty significant IMD increase below about 60Hz or so, I think it makes sense to keep an input cap in the amp to high-pass everything at around 90Hz, right? The negative impact of that cap should be less than the improved clarity in the speaker's output. I have a polyester or polystyrene in there now (don't remember what I picked up at the surplus store), I'll probably play with other types too.
If you're certain the IMD comes from the speakers....?
Still, limiting the bandwidth to 90Hz and upwards will certainly rob the music of its fundament.
Whatever you decide, using a polyester cap at the input of the amp defeats the purpose of building a Loftin-White in the first place.

If you must use a cap, go for the best you can, you can also tailor the cap at the output of the preamp if you use one.


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I'll need to decide on whether to get the 3K one or the 5K one.
Using the best OPTs you can get will never hurt and at 80$/EA they sound like a steal to me.
If its' 2A3s you're using then 3K should be your choice.

Hope this helps,

P.S. Brett, take good care of yourself.
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Old 14th August 2003, 09:19 PM   #13
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Hi,

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I also saw Schottkys in the more "normal" diode-looking packages - those wouldn't need heatsinks to perform as spec'd, and would take up less space too. So, is there a specific reason you recommended I get TO220-type packages?
Single Schottkys would be my choice.

Did the TO220 come recommended by anyone?
I think the single one are much easier to work with and I hear people saying the sound better as well.

If you must use heatsinks for any reason, I'd recommend using one per application iso sharing a single one.

With the tabbed dual Shottkys only the cathodes that are electrically connected to one another can share a common heatsink.

Beware that these heatsinks will carry dangerous amounts a current and voltages here so no poking fingers, please.

Cheers,
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Old 14th August 2003, 09:26 PM   #14
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If you're certain the IMD comes from the speakers....?
That's what the manufacturer said, and a review/test in a magazine showed it too. I think my speakers are happier if they're not playing any bass.

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Still, limiting the bandwidth to 90Hz and upwards will certainly rob the music of its fundament.
Well, I do have a subwoofer. That gets a full-range signal from my linestage's other pair of outputs, and right now I have some strange filter network in front of it (don't remember what slope/XO points I have in there any more) that seems to blend reasonably well with the mains, within the limits of room modes and so on.

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Whatever you decide, using a polyester cap at the input of the amp defeats the purpose of building a Loftin-White in the first place.
That's what I was thinking too. So I need the best cap I can get, and between using it at the input vs. using it between driver and output stages, I'm not sure which one will do less damage. The coupling cap will have much mode DC across it, so maybe that's a reason to prefer having the cap at the input? There's another option here, reduce the value of the coupling cap so I do the filtering there. I was advised against doing that though. I can see how it would muck with things if I had global feedback, but I don't. So, the third option might be to use a smaller driver-output coupling cap.

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If you must use a cap, go for the best you can, you can also tailor the cap at the output of the preamp if you use one.
No caps at the output of the linestage. If I wanted to use a cap, which would be the 'best' place to put it - linestage output, amp input, or inter-stage coupling? I'll see how much teflon caps will cost me on eBay... 100K input R means I'll need around 0.022uF for 72Hz. Grid leak is 470K, so if I put the cap there, I can use an even smaller value. If not teflon, what else would you recommend? Polypropelene?
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Old 14th August 2003, 09:28 PM   #15
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Beware that these heatsinks will carry dangerous amounts a current and voltages here so no poking fingers, please.
Damn, didn't even think about that. Thanks.

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Did the TO220 come recommended by anyone?
I think Brett mentioned them earlier in the thread. I might have misunderstood him, maybe he was talking about the regulators.
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Old 14th August 2003, 09:35 PM   #16
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Default Hot under the collar

Schottky diodes are intended for switched mode power supplies, which don't use bridge rectifiers, that's why you buy them as single packages, or, at best, dual.

There are various heatsink insulating kits available, ranging from traditional mica to various plastic types. You need a low thermal resistance (degrees C per W). If you use mica, you must smear both sides of the mica with a thin coating of silicone heatsink grease to fill any voids that would otherwise insulate. Similarly, thinly smear the device and the area of the heat sink to which the device will be mounted.

Inevitably, insulating kits have thermal resistance and cause the device to be hotter than if it was bonded directly to a heatsink. You might think this is a bad thing, but it is not necessarily the case. If you use individual heatsinks, where will their heat go? If they are inside a chassis, they cannot lose heat, so their thermal resistance rises dramatically. Even a Schottky diode will heat significantly at 2.5A. A proper, external, finned heatsink with devices on insulating kits is a better bet than individual heatsinks, no insulating kits, and poor airflow.
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Old 14th August 2003, 09:47 PM   #17
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Hi,

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So I need the best cap I can get, and between using it at the input vs. using it between driver and output stages, I'm not sure which one will do less damage. The coupling cap will have much mode DC across it, so maybe that's a reason to prefer having the cap at the input?
Coupling caps work best when biased with sufficient DC voltage.
Putting a cap at the input of the amp will have it no DC bias on it.
Not so good.

By reducing the value of the cap between the driver stage and output valves you can filter just the same.
Remeber this filter is made out of an RC network so always look at the R value following it when doing the maths...smaller caps quite often sound better than bigger ones.

NFB? Is this a SE amp or a PP?
Applying global FB with SE amps is almost impossible to do and with a PP the global FB loop won't care, it only feeds back what it was being fed from the output.

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No caps at the output of the linestage. If I wanted to use a cap, which would be the 'best' place to put it - linestage output, amp input, or inter-stage coupling? I'll see how much teflon caps will cost me on eBay... 100K input R means I'll need around 0.022uF for 72Hz. Grid leak is 470K, so if I put the cap there, I can use an even smaller value. If not teflon, what else would you recommend? Polypropelene?
No caps at the linestage? Is it a passive? If not how is DC blocked?

Where to put it is ultimately up to you.
Anyway you turn it some part of the gear is going to en up with it's bandwidth limited.
Normally, I'd put it at the preamps' otput where you could install a second set of outputs, say one full range, the other with a bass cut-off suiting your needs.
That would do the least damage to rendering your gear "my system only"

Cheers,
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Old 14th August 2003, 09:49 PM   #18
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EC8010, I see your point, but given my construction skills (or lack thereof), mounting these on an external heatsink will be a pretty complicated proposition

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Coupling caps work best when biased with sufficient DC voltage. Putting a cap at the input of the amp will have it no DC bias on it. Not so good.
OK, so my assumption was backwards. That's never happened before

The amp is SE, no NFB.

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No caps at the linestage? Is it a passive? If not how is DC blocked?
It's a passive autoformer. DC blocking... I'm not sure if an autoformer will provide DC blocking, since it's the same wire that connects input and output (this would not be the case with a transformer). There's a cap at the output of my phono stage. I believe there is one at the output of my CD player too. To be honest, I hadn't thought about DC blocking at all.

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That would do the least damaged to rendering your gear "my system only"
That's not really a concern. My stuff is ugly enough that I doubt anyone else will want it Seriously, I can back these changes out if/when I sell something. Though, I see the logic in what you're saying. Especially since I already have 2 outputs on my linestage (and only 1 input for now).

FWIW, pictures: Felt like trying something different (warning: images)
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Old 14th August 2003, 10:24 PM   #19
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Hi,

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It's a passive autoformer. DC blocking... I'm not sure if an autoformer will provide DC blocking, since it's the same wire that connects input and output (this would not be the case with a transformer).
An autoxformer would be a xformer with a 1:1 ratio.
In this case its sole purpose would be to block DC.For that to happen you do need separate primary and secundary windings.
So input and output wiring must be separate.

Unless the DC voltage at the input is either: neglegibly small ( which I doubt) or carries only a very small amount of current ( more likely) it won't be working well causing core saturation leading to hysterysis distortion.

So I can only assume some measure was taken to avoid DC at its input a la parafeed or the xformer was gapped for SE service.
This is what bottleheads like us call a WOT.

I'll take a jump into the link you provided...I can't possibly remember all that goes on here.

Cheers,
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Old 14th August 2003, 10:36 PM   #20
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An autoxformer would be a xformer with a 1:1 ratio.
Not in my linestage - it has multiple taps for the output, even though everything's one long wire - the taps are wires that "come out" of the windings, then "go back in" to continue making more turns. So, that's how it works as a volume control - depending on which tap is selected, I get a different number of turns on the output side (the input signal goes across the full number of turns).

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So I can only assume some measure was taken to avoid DC at its input a la parafeed or the xformer was gapped for SE service.
Could be the latter, because there are E-I laminations in there that alternate in their interleaving. There's only one autoformer per channel, no AFAIK there's no parafeeding going on. I could ask Dave Slagle about this.

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This is what bottleheads like us call a WOT.
What's a WOT?

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I'll take a jump into the link you provided...I can't possible remember all that goes on here.
And I don't expect anyone to either
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