Gain - Have I Got this Right?

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Ray P
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#1 Gain - Have I Got this Right?

Post by Ray P »

I've got plans to build a MoFo Power Buffer and I've got many of the required parts collected already. I've been thinking about how much voltage gain I will need in front of the MoFo in the context of my speakers, which are BLHs using 15ohm Lowther drivers.

My speakers are nicely efficient and I've been using them hooked up to flea-power OTL amps outputting a Watt or two, which is plenty enough to create high volumes in my listening room. The amplifiers I'm using, being valve based, are limited by their current capability (compared with solid state devices) and using 15ohm drivers gives the amps a slightly easier time; as Mr. Ohm tells us;

To deliver 1W to an 8ohm speaker requires 2.82V at 0.35A
To deliver 1W to a 15ohm speaker requires 3.87V at 0.25A

So, if we assume 1W is sufficient in my 15ohm speaker system context and also assume that my source component has an output of 2V, then I need 3.87V at the input to the MoFo so my source components output needs a voltage gain of 3.87/2 = approx x1.95. OK so far?

At this point it occured to me that with a MoFo using 15ohm drive units may actually be a disadvantage because it is a current gain device and to deliver 1W with 8ohm drive units would require less voltage gain - 2.82/2 = appprox x1.41 - that's pretty close to being able to drive the MoFo straight from the source component (something I think Steve S actually did).

What am I missing?
steve s
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#2 Re: Gain - Have I Got this Right?

Post by steve s »

My take is your' ideal' 16ohm speaker mainly helps the damping factor, I would say the lowthers are close to completely self damped anyway.. so I'd say you have very little difference sound wise, from a damping/ listening point of view, so my advice would be to try it.
Loudspeakers impedance varies anyway.
I'd advise a gain of 5 in your driver valve or pre as a minimum. But I'm contemplating driving my second mofo with a medium gain triode (x30 ish) just to see how it sounds compared to bring driven by a small output valve.
I've not used mine for a while, been happy with my 1 watt p650 amp which I quite like the sound of too.
But the bass is slightly tighter and more detailed on the mofo, whilst the music has more air and sweetness on the valve amp, but theres really not much in it.
I dont know how that would relate to a blh, as all mine where not great at bass detail or realism compared to a dedicated bass driver.
My ultimate plan is to drive the mofo with a bigger high gain output valve, but I'm waiting on a large pair of interstage transformers being wound first, which I would guess will be next year when I get them.
The tube manual is quite like a telephone book. The number of it perfect. It is useful to make it possible to speak with a girl. But we can't see her beautiful face from the telephone number
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Nick
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#3 Re: Gain - Have I Got this Right?

Post by Nick »

What am I missing?
Nothing, I think you have it about right. Same reason for 4 and 2 ohm speakers in car audio. More power for limited voltage rails.
Little known fact, coherent thought can destructively interfere with itself leaving no thought at all, that’s why I prefer incoherent thought.
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Ray P
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#4 Re: Gain - Have I Got this Right?

Post by Ray P »

Thanks both.

The reason I ask is based on an assumption that you don't want any more voltage gain than you actually need (otherwise you just have to burn it off) and with that consideration I'm thinking of trying a valve-based delta-sigma dac valve-based output stage that has diferent gain options based on which valves are utilised - I am contemplating a minimalist project of equipping the delta-sigma output stage with a MoFo Power Buffer.
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#5 Re: Gain - Have I Got this Right?

Post by steve s »

Sounds a good idea to me ray..
The tube manual is quite like a telephone book. The number of it perfect. It is useful to make it possible to speak with a girl. But we can't see her beautiful face from the telephone number
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