Capacitive v. Inductive inter-stage coupling - diyAudio
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Old 24th July 2011, 12:19 AM   #1
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Default Capacitive v. Inductive inter-stage coupling

I am building my first tube amp, and I need to make a decision between capacitive coupling and using an LL1660 transformer for inter-stage coupling. I have heard both positive and not so positive about each. I plan on driving a 300b in A1 operation.

Any comments?
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Old 24th July 2011, 12:37 AM   #2
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There are always tradeoffs... I'm still in the process of trying to answer this question myself. Currently working on a GM70 driven by IT coupled triode connected D3A..

IT drivers generally require tubes with relatively low rp if any sort of decent bass performance is desired. In many cases these same tubes work very well with choke loading and capacitive coupling.

Types like the 6SN7 often chosen for driving 300B, a choice I made 12yrs ago, and still live with today are pretty marginal for IT driver application.. (RC coupled, my driver is SRPP based)

So you need to think about what your goals are and what sort of driver complexity you are looking at.

As a personal note I was very suspicious of transformers at one point, today I have SUTs stepping up my SPU, and plate to line transformers in my 26 line stage. My digital hardware is all transformer coupled as will be the next power amp.. And no regrets..

I'd try the IT approach if you have the design chops for it. D3A, C3G, and 7788 are all types to consider and have great linearity in triode driving IT (or anything else for that matter).
"To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine
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Old 24th July 2011, 12:43 AM   #3
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That is good advice, i.e., I think I will simplify first. I can start with capacitive coupling, but keep room in my chassis to also include the IT. Then, when the first design is finished, I can modify the mechanicals, so I can listen to one for a while, then the other. Thanks for the input, it gave me some perspective.
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Old 24th July 2011, 08:46 AM   #4
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If you read up on tests that Lynn Olsen did on Nutshell hifi you'll see you get less harmonics with inductors. Therefore there is proof inductive coupling is better.

However inductors have their own physical and cost issues - so you may consider shifting around your PSU lines so you end up not needing any coupling capacitors. Usually there are only two of them to dodge (3 stage amp) so you need two extra rails (+ve and -ve) - or design a PSU ladder than you can hang the circuit off.

Although PSUs have capacitors too, you can also use followers etc, and also the PSU caps can be nicely bypassed with better caps too - PSRR also helps you here.
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Old 24th July 2011, 10:53 AM   #5
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I've gone through many many versions of a 300b SET, and tried out all sorts of coupling. I eliminated plate resistors because they didn't sound so good as iron and reduced the gain. I eliminated active loads because to me they had a glare and hardness despite their clarity and good bass.

That left me with the choice of interstages or plate chokes. In practice you can't really get 1:1 interstages to work too well with plate resistances over about 6k - you need the inductance. Yes, the LL1660 is one choice. But my input tube of choice is the 26 and I prefer that with a plate choke. In fact I use two Hammond 156C in series so I have 300H inductance. That's beyond interstages.

Coupling the driver to the 300B is great for an interstage - just choose a driver with Rp of 5k or under. Lundahl LL1660, or LL1692A is nice, or I also used and liked Hammond 126C, bifilar wound.

Logic told me to direct couple the 26 to the 46 driver I use, but I didn't like the 46 with a bypassed cathode resistor. I much preferred the 46 with battery bias on the grid (24v from two 23A). So that meant a capacitor coupling the stages. I use a Russian FT-2 or FT-3 teflon. Normally I'd try to avoid capacitor coupling but in this instance it's what gave me the best sound. I found that I liked the sound best when the cathode was grounded, or with the 26 just a 10 ohms resistor in filament bias.

So you have a few options to weigh up which are inter-dependent. Some circuits require a cap, some don't. Some tubes like interstages, others like plate chokes. But if it were me I'd avoid all resistors except for the 300b which I still have in self bias since it's safe and reliable and 300bs are expensive!

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