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Can I use 1.1 instead of .47

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kevinkr

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Low frequency cut off too low. Long coupling time constant in conjunction with supply time constants can result in LF instability, aka "motorboating" because of the way it sounds.

That said I can't imagine that in a GM70 design the supplies would be shared across power and driver stages given the very high voltage required for the GM70.. (I run mine at 1kV) This makes the concern about feedback through the power supply moot as long as there is nothing common to both. i.e. Separate windings on a common power transformer is ok, otherwise it could be an issue.

Depending on the driver stage topology the large capacitor may result in very long recovery times if the GM70 is driven into grid current. I would recommend using a lower value capacitance.
 

kevinkr

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Parallel pair is 2.2uF, series pair would give 0.55uF - IMO depends on what the rest of the circuit is to determine whether the time constant is reasonable or not. (I see you edited after I started to write my response. lol)

Not safe to assume that he means he is using a pair per amplifier, I would usually interpret this as one cap per amplifier.

I'm not a fan of series caps except in power supplies where higher voltage ratings are sometimes required and appropriately rated caps are not available on the market.

In addition two series caps will have twice the ESL and ESR of a single capacitor and likely a lot more than one cap of the appropriate value regardless of what it is compared to.
 
djn said:
In this particular application does a really good cap make a difference or is a run of the mill cap work?
Assuming this is just an audio coupling cap then no, "a really good cap" makes no difference. At least, no improvement. "A really good cap" may sometimes be too large (and so introduce too much stray capacitance) or too unreliable (such as almost anything with a paper dielectric).
 
I'm partial to both the Clarity Cap ESA which is reasonably priced and the TRT Infinicap. Neither will break the bank, but are significantly more costly than most members here are going to recommend. Worst case if budget is a concern and you believe I am a crackpot then go for a Sprague orange drop from the 715P series - I doubt you really need a 0.47uF, but that is dependent on what you are driving resistance wise in the grid circuit. Can you post a schematic of your design?
 
djn said:
What brand to you recommend D?
I don't know if that question was intended for me, but if it was I don't recommend brands because I don't attach importance to brands. A brand is merely a name on the outside, and nowadays it tells you almost nothing about what matters on the inside. A century ago brands mattered, because Blogg's widgets were made in Blogg's factory by Blogg's workers so when you saw a widget labelled 'Blogg' you knew it was either genuine or fake.
 
Good point D. I've worked with a number of foreign manufacturers in the past for Ford and we held them to our standards. So in that industry it does matter. Maybe not so much for small electronics.

Thanks Kevin. I'll look into those.

Azazello, I've been working on and off for 5 years on this amp and finally have the time, money, and motivation to finish it, so I decided not to change up the design now. I'm going to use stock caps to couple then after listening for a while, I'll look into interstages.
 
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