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 Post subject: Re: RossD's 2A3 SET
PostPosted: 21 Oct 2013, 19:26 
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RossD wrote:
Any luck with the design of the power supply?
Yes. I have a design using your transformer that meets voltage requirements with an allowable 227Ω of available filter resistance. Here is the PS design sheet.
Attachment:
5AR4 Rectifier.jpg

Now for the next question... what kind of a topology do you want to use? There are several trades which can be made. Personally I have been playing with a new filter topology that produces incredible channel separation and a wide deep soundstage. It is based on some standard Triad Magnetics inductors which you can get from places like Allied Electronics for relatively little money. It uses a Triad C-14X as the main filter choke ($15.80 from Allied) and two C-24X inductors (7.98 ea. from Allied) one for each power channel. It will take some space in your chassis (not too much) but the performance can't be beat.

Let me know what you think.


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 Post subject: Re: RossD's 2A3 SET
PostPosted: 22 Oct 2013, 07:13 
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I would like a very quiet power supply in this amp.

Lets go with the multiple chokes/inductors as you see fit.

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 Post subject: Re: RossD's 2A3 SET
PostPosted: 22 Oct 2013, 21:19 
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Ok. Without further ado, here is the schematic for the power supply.
Attachment:
PS Schematic.jpg

Some people may think this a little involved, but I can assure you that you will achieve width and depth of sound stage that rivals the best mono-blocks. Also, this level of filtering virtually eliminates any B+ related hum. This is good because if you want, at some later date you can upgrade the filament supplies to regulated DC for an absolutely hum free design. And the filter chokes are relatively inexpensive ($31.76 total from Allied Electronics).

Let me know what you think.


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 Post subject: Re: RossD's 2A3 SET
PostPosted: 22 Oct 2013, 22:02 
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I think I have my design. Thank so very much for your time and effort. I just hope I dont let your design down with bad building technique or subpar component choices. I will try to keep my build thread here current with the build on my bench. I am not sure if I will get time to start it before christmas or not as this is usually a busy time of year for me.

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 Post subject: Re: RossD's 2A3 SET
PostPosted: 23 Oct 2013, 08:34 
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Matt,
I'm getting the schematics into one drawing partially for ease of use and partial because I then 'think' more about the components and what not. In doing so, I noticed sometimes people float heater windings and sometimes ground them with a couple of 100 ohm resistors. The 6sl7 datasheet states a max heater to cathode of 90v, with the small about of bias and a floating heater, could it potentially be exceeded? Float it or ground it?

Another thing, some schematics seem to imply, or just note, certain capacitors as being snubbed by smaller ones (0.1 uF polies come to mind). Is any of that going on here?

I came up with using a 2 amp slo blo for the fuse. Sound about right?

What about the use of a X2 cap and a parallel resistor at the mains ground to chassis?

Once I get my schematic of your design completed, I'll post it up for everyone, unless you have objections.

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 Post subject: Re: RossD's 2A3 SET
PostPosted: 23 Oct 2013, 17:12 
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I would snub all big electros in the PS with 0.1uf poly of high voltage rating. Also the power tube Ck electro I snub also with a 0.1uf poly. 2A slow blow should work if not 3A will work better.

X2 and 100ohm resistor to ground is a good mix for earthing\ground isolation.

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 Post subject: Re: RossD's 2A3 SET
PostPosted: 24 Oct 2013, 07:24 
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Thanks Mark.

Ok, so which type of capacitor does 'poly' represent? I look at a website like tube depot and there are no less than 5 types of caps that have 'poly' in the title. If I had to guess, you're talking about a Metallized Polypropylene capacitor?

What is your intent with snubbing the power supply electros and snubbing the Ck bypass cap?

I think the hardest part about learning about tube amps, is all of the jargon. Seemingly, for one item, there's the USA nomenclature, UK nomenclature, slang terms, implied terms, personal and regional diction, .... et cetera.

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 Post subject: Re: RossD's 2A3 SET
PostPosted: 24 Oct 2013, 07:53 
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RossD wrote:
Thanks Mark.

Ok, so which type of capacitor does 'poly' represent? I look at a website like tube depot and there are no less than 5 types of caps that have 'poly' in the title. If I had to guess, you're talking about a Metallized Polypropylene capacitor?

What is your intent with snubbing the power supply electros and snubbing the Ck bypass cap?

I think the hardest part about learning about tube amps, is all of the jargon. Seemingly, for one item, there's the USA nomenclature, UK nomenclature, slang terms, implied terms, personal and regional diction, .... et cetera.

Poly is short for polypropylene (any type). I snub electros because they are noisy and Ck caps so the smaller cap will pass high frequency more easily than a bigger electro. Better high frequency response.

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 Post subject: Re: RossD's 2A3 SET
PostPosted: 24 Oct 2013, 19:17 
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RossD wrote:
I noticed sometimes people float heater windings and sometimes ground them with a couple of 100 ohm resistors. The 6sl7 datasheet states a max heater to cathode of 90v, with the small about of bias and a floating heater, could it potentially be exceeded? Float it or ground it?

...

What about the use of a X2 cap and a parallel resistor at the mains ground to chassis?
Everybody get ready, because I'm about to stomp on some toes.

When it comes to grounds, I am of the opinion that most people simply do NOT pay close enough attention. It is really not enough to simply look at a piece of equipment and try to decide on the ground scheme, you really need a "system grounding philosophy" that applies not only to equipment you build, but everything you use. The thing about the fight between the "direct ground" camp and the "parallel cap/resistor camp" is that the actual grounding philosophy is never discussed. The cap and resistor separation between chassis and signal grounds was almost an absolute requirement with two wire ac systems to achieve a safe design. Today it, more often that not, leads to funny ground loops and background noise.

So, what do I mean by a "system grounding philosophy"? This means that the grounds in your system must be managed to both provide safety and to avoid ground loops and currents. This is not as simple as it sounds at first. And it depends on how your equipment is constructed, how it's grounded, and how it's interconnected. It also depends on the type and quality of your power system where the equipment is used. Everyone who has a system should have a ground plan to manage noise and safety. It just makes good sense when connecting multiple disparate pieces of equipment; be they audio equipment, radio gear, or test gear.

Lets look at an example; me. I have a good quality three wire power system with excellent ground path impedance. I have checked ALL my outlets for proper hot/neutral wiring and impedance to ground spike (not a simple task). Given this, I have developed a grounding philosophy for all my equipment. Here it is:

Matt's Audio System "System Grounding Philosophy"
1. All separate pieces of equipment shall use IEC-53 connectors and implement independent 3rd wire safety ground design.
2. Chassis grounds for all pieces of equipment shall be provided by that piece of equipment's safety ground lead. The IEC ground lead shall connect to the main chassis and all exposed metal parts, except for signal/speaker connectors, with a DC impedance not to exceed 2mΩ.
3. System wide signal (and speaker when used) ground reference shall be provided by the system power amplifier.
4. Power amp signal and chassis grounds shall be tied together at only one point, this point shall be within the chassis, and this connection shall have a DC impedance not to exceed 2mΩ.
5. No preamp, equalizer, switch, control, or other equipment shall connect signal ground to chassis safety ground except through a "ground lift switch" which can break ground connection when used in-system.
6. Signal ground reference for equipment other than power amps shall be provided via interconnect shields or separate ground wire connection (e.g. turntables with magnetic cartridges).
7. When multiple signal grounds, via cable shields or ground wires, connect two pieces of equipment, those cables and wires shall be bundled in close proximity to each other to minimize field coupling area.
8. At no time shall signals pass between system components without signal return paths (either shields or conductors in twisted pairs) for each signal connection.
9. Existing two wire commercial components used within the system shall meet these requirements or utilize an isolation transformer to supply AC power.


Now this is my grounding philosophy. I do not mean to suggest that everyone should follow these rules. Everyone should apply rules that are appropriate to their situation. However, I will tell you that my system, regardless of how I configure it, is a silent as death itself with no signal input. And every piece of my equipment has a safety path for fault currents that will not run through anyone touching a chassis or control. This is the way I like it.

So what does this mean to you? The answer is: "it depends". If like me, you have a good quality three wire ground system, you may want to adopt a philosophy like mine. But maybe not. You will notice that this ground philosophy tends to drive you to always use either a power transformer or an isolated switching supply in each piece of equipment (except the power amp, however it's a good idea there as well).

As to your question about the x2 cap and a parallel resistor, the chassis ground should always be tied DIRECTLY to the mains ground for safety reasons. If used, the cap and resistor would go between signal ground and chassis ground. But again, this depends on your overall grounding scheme. As for the 6SL7 heater, I would use two 100Ω resistors to form a center reference to ground.

Now I'll get off of my soap box. :soapbox: This is just one of those aspects of design that I think tends to get over looked. And bad grounding can be a nightmare when trying to figure out from where a hum or buzz is coming.

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 Post subject: Re: RossD's 2A3 SET
PostPosted: 25 Oct 2013, 12:35 
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Thanks Matt, that was insightful.

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