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Old 10th January 2012, 02:25 PM   #1
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Default OTL for 40-80ohm speakers

I have a fresh plan making a little different otl, for driving speakers having multiple 8ohm series connected speakers.
At first, it sounds like quite easy load for otl.

Ok, first of all I want 50watts/channel for 80 ohm impedance.
There should be option for half impedance, like 40 ohms.
4 x EL509 / channel should do this job easily?

powersupply/power transformer will be 2 x 110VAC. 400VA/ch, should be enough..

what do you think about this idea?
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Old 10th January 2012, 06:41 PM   #2
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50W/ch into four (presumably series connected full range) drivers = a sh*tload of db, even from moderately efficient drivers... Are you SURE you need that much drive?
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Old 10th January 2012, 06:56 PM   #3
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50w is quite a lot for an OTL !
Just do some math:
A EL509 as cathode follower has an R-out of 1/(0.018ma/V) = 55Ohms. As circlotron it's a bit lower and as futterman its half the cathode follower R-out. So even without further global feedback this isn't bad.

Power:
P=(I^2)*R, for P=50W and R=80Ohms, this yields: ~0.8A-rms and ~1.1A-peak and for 40Ohms: ~1.1A-rms and ~1.6A-peak.

EL509 max. Ik is 500mA. So you can calc. for yourself how much tubes you need for that much power you want.

I have never built an OTL amp, so I can't tell you about some good designs. But I have some names in my head. Just google for: "Rosenblit OTL", "Sandman OTL", "Pickie Makovski OTL", "Murray OTL"
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Last edited by the_manta; 10th January 2012 at 06:59 PM.
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Old 10th January 2012, 07:05 PM   #4
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there will be 12 x 4" full range drivers (smaller model has only 6 drivers).

But yes, 50W is good to have in reserve.

So as I did calc that amount of tubes required, I got 4 tubes / ch. 2 for negative and 2 for positive side.
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Old 11th January 2012, 03:09 PM   #5
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I just looked into Bruce Rosenblit's book and on page 74 he says: "The maximum current that a 509 can pass is just over one amp."
This disagrees with the datasheet value of 500mA and seems to be the solution why bruce gets 80W into 8Ohms with "only" 8 tubes.

According to this, 4 tubes per channel should be enough. But as I said, have neither used EL509 for audio nor built an OTL
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Old 11th January 2012, 03:45 PM   #6
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I say go for it!
Here is some more info about OTL and calculations with EL509:
Ultra-Linear OTL Power Amplifiers
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Old 11th January 2012, 04:29 PM   #7
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Thank you for info, as this is my very first OTL. I have made tens of traditional tube amps tho..
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Old 11th January 2012, 06:38 PM   #8
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You should consider the schematic on post 25 on this thread:
What tubes for a tube amp?

I think the circuit is very elegant in its approach and the output tubes does not cost that much.
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Old 11th January 2012, 06:43 PM   #9
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If you put a lot of drivers in series then they will get litttle or no electrical damping, unless they happen to have identical bass resonances. You will need to provide mechanical damping instead.
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Old 11th January 2012, 08:37 PM   #10
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Well, there has been OTL amps driving 800ohm speakers.. and those had very clean sound!

DF96, what do you mean about mechanical damping?
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