Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: FranciscoPerez on January 19, 2017, 06:07:35 am
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Hi,
It's been discussed before here, but can't seem to find the corresponding threads. :(
Do some of you guys use film caps in your PSU instead of electrolytics? They last longer, have low ESR/ESL ratings, high ripple current capability, and are currently quite "cheap".
What would be the cons, if any, of using film caps in the psu section?
I was wondering if I should buy some and use them in my next build...
A quick search at mouser showed this, among others:
http://www.mouser.es/ProductDetail/Vishay/MKP1848C61050JK2/?qs=%2fha2pyFaduiaB6KwCqVWXvK4v1A%252bfX6Ay68Bl8ETHMpcYvIs6Tt%252b9w%3d%3d (http://www.mouser.es/ProductDetail/Vishay/MKP1848C61050JK2/?qs=%2fha2pyFaduiaB6KwCqVWXvK4v1A%252bfX6Ay68Bl8ETHMpcYvIs6Tt%252b9w%3d%3d)
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What would be the cons, if any, of using film caps in the psu section? ...
Cons:
They're HUGE compared to an electrolytic with the same ratings.
They cost more than an electrolytic of the same ratings.
That's it.
You can see examples of the size issue in this amp (http://el34world.com/Forum/index.php?topic=14263.msg134514#msg134514) and this amp (http://el34world.com/Forum/index.php?topic=21418.msg227394#msg227394).
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APPLICATIONS
• Renewable energies inverters
• UPS
• Battery chargers
• Motor drives
I clipped the quote from the datasheet, appears PSU is a good match. never used them though
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Here's a site with a build based on Motor run caps in the PS. I found that interesting and according to the video and dialog at the site, it's supposed to be very quiet.
http://diy-fever.com/amps/mesa-mark-iic-preamp/ (http://diy-fever.com/amps/mesa-mark-iic-preamp/)
silverfox.
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Thank you guys,
Yeah size might be an issue... especially in higher capacitances.
They seem to offer up to 6 caps in the same envelope too, never saw this before !
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Will have a look silverfox, thanks!
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Google Carr amplifiers, they use those big solen fast polypropylene caps.
In one factory tour video he says they sound warm.
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They seem to offer up to 6 caps in the same envelope too, never saw this before !
Are you talking about motor capacitors ?
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At an Ham Fair I got 5 x Iskra 20uF 900V DC @ 85° capacitors (new) for 3€ each and ... they are big, 30mm x 41mm x 44mm
they looks like this
(http://thumbs.ebaystatic.com/images/g/pBcAAOSwqBJXXFwo/s-l225.jpg)
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EDIT: For those that are on this side of the pond, may be this is a good sale
http://www.ebay.at/itm/132008997730?clk_rvr_id=1155826668929&rmvSB=true (http://www.ebay.at/itm/132008997730?clk_rvr_id=1155826668929&rmvSB=true)
Franco
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One enduring myth in regards to PSU caps is that "they don't matter because they are not part of the audio circuit", until you realize that PSU caps are an AC path to ground.
For example, in the typical triode stage, there will be a 100k plate resistor. On the plate side of said resistor is your output (via coupling cap so DC doesn't pass through). On the other side, there is the VDC supply AND a big fat cap to ground.
Flip this around, look at it, and what do you see? The plate resistor and PSU cap are in fact in parallel with the coupling cap and following volume pot (or tone stack), at least when it comes to AC (aka audio). Therefore, the reactance and ESR of the cap will affect the gain structure.
That said, while it might matter in Hi-Fi applications, it hardly makes a difference for guitar amps which are much more frequency limited anyway. So in the end, cost and size are the deciding factors and people are content with electros. Some even prefer the sound with electros even, because lack of fidelity is sort of the selling point of a guitar amp, to a point.
But this is all subjective and you might prefer the sound, or you might not even hear a difference.
Getting the "sound" of film PSU caps can be approximated by bypassing the big electro caps with a smaller film cap (like put a .1uF cap across the 20uF+ normal cap). This will take care of ESR/ESL and render the power supply effectively transparent. Won't give you the other advantages of a film cap though.
I tried it once on my first preamp stage node, didn't hear a difference.
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One enduring myth in regards to PSU caps is that "they don't matter because they are not part of the audio circuit", until you realize that PSU caps are an AC path to ground. ...
Flip this around, look at it, and what do you see? The plate resistor and PSU cap are in fact in parallel with the coupling cap and following volume pot ...
I see the filter cap as being in series when you look at the path of current for a tube stage.
Electron current is emitted from the hot cathode, flows out the plate and through the plate resistor, through the filter cap to ground, up through the cathode bypass cap (if present; through cathode resistor if not), and into the cathode completing the loop. This path is relevant to alternating current.
Direct current cannot flow through the filter or bypass caps, so it goes through the cathode resistor (establishing bias voltage) and through the decoupling resistor in the power supply (providing a convenient way to set the supply node voltage).
... Flip this around, look at it, and what do you see? The plate resistor and PSU cap are in fact in parallel with the coupling cap and following volume pot (or tone stack), at least when it comes to AC (aka audio). Therefore, the reactance and ESR of the cap will affect the gain structure. ...
Show me an example of this as a measurement of a physical circuit.
Retreating to the theoretical for a moment, a "perfect" filter cap would look substantially like 0Ω for all a.c. and like ∞Ω for d.c. If the parallel view held, we have 0Ω in parallel with the plate load, coupling cap, grid reference resistor/volume pot, which gives us 0Ω total resistance. So that seems to not hold for impacting "gain structure".
If the ∞Ω case for d.c. was instead the issue, now the imperfection of leakage (and maybe ESR, indirectly) seems to figure in as it could reduce supply voltage if we had an extreme case. Of course, the "extreme case" we're likely to see which would have a noticeable impact here is the cap failing as a short-circuit.
The reactance of the cap (~90Ω for a 22µF cap at 80Hz) matters most in determining how effectively circulating a.c. will be kept at that audio stage & filter cap, and isolated from disrupting other stages further upstream. And since that cap reactance is working against 1k-20kΩ of decoupling resistor, ESR, etc would have to be HUGE to have a discernible impact.
... Getting the "sound" of film PSU caps can be approximated by bypassing the big electro caps with a smaller film cap (like put a .1uF cap across the 20uF+ normal cap). This will take care of ESR/ESL and render the power supply effectively transparent. ...
Electrolytics can have a rising impedance due to imperfections, which the small bypass caps seek to neutralize. However, this extra impedance isn't typically significant until you're up to the RF range, which means at worst you're looking at fighting oscillation gremlins or distortion products downward-modulated into the audio range.
Hi-fi folks usually point to noise or "harshness" as the main problems created by power supply electrolytics. I used Solens in the first scratch-built amp I made, partly due to some vague suggestions it "would sound better."
My only real reason for using them since has been "they never need replacing" and is the only benefit I'm sure I've gotten from them. It's also the only reason I'd offer others for why they might consider using film caps in the power supply.
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Are you talking about motor capacitors ?
I was refering to the Vishay caps I linked, they have 6 and 12 pin version of their caps containing 3 and 6 caps inside the same envelope.
Great discussion guys, thank you! I'm glad I started this thread :icon_biggrin:
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Looking at the photo is Silverfox's link (Reply #3), those look to be plastic housed motor start caps, though they're supposed to be motor run caps. Motor start caps need to perform for only a fraction of a second, so they're made cheaply. They'll fail if run for longer than a short pulse. My understanding is that motor start caps are housed in plastic; oil-filled motor run caps are housed in metal. Can someone confirm this?
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> motor run caps are housed in metal. Can someone confirm this?
No.
Further, many motor duties formerly handled by oil caps are now done with electrolytics. The technology improved so much that bipolar e-caps can give "useful" life in motor-RUN duty.