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Old 11th March 2014, 10:18 PM   #1
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Default Considerations about headph. OCL

I have headph. amplifier for my 300 ohm Sen580, acc. this schematic:
A Single-Ended OTL Amplifier for Dynamic Headphones | HeadWize
It's cool, but I don't like output caps.
I think about some OTL-OCL like this schem:
diytube.com • View topic - OTL amplifier
using 1 output tube, one input triode with small or middle mu /I don't need full driving 6AS7/ and interstage PP transformer. Is it possible to find PP interstage with K = 2,3 for full driving 6AS7?
With 1 6AS7 Rz = Ri/2+mu, or 270/4 = 67 ohms /I sow as I remember, maybe in Atma-sphere/.
Or interstage PP renders my idea in all that?
Please, You can critic my consideration.
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Projects: OTL-OCL 6AS7 Gen. Electric, SEs 2A3 RCA, 300B JJ, 6S4S, 4P1L, EL11 Telefunken, 6AS7 RCA, 6S33S, 6S41S, 6S19P, E84L Siemens, E236L Telefunken, headphone ampl. OTL Loftin White 6AS7 RCA,... PPs 6005 Gen. Electric, EL34 Mullard, 6L6....

Last edited by azazello; 11th March 2014 at 10:21 PM.
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Old 13th March 2014, 12:07 PM   #2
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You should try a simulation in spice first before building it. Just looking at the OTL amp, I would say it's risky. There is positive voltage on both the + and -. Headphones aren't wired that way. Also, this design depends on the balance of the output tubes, which is doubtful over a long time. I prefer to design things that won't hurt me or other equipment if something goes bad.
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Old 13th March 2014, 06:11 PM   #3
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Quote:
Originally Posted by dirkwright View Post
You should try a simulation in spice first before building it. Just looking at the OTL amp, I would say it's risky. There is positive voltage on both the + and -. Headphones aren't wired that way. Also, this design depends on the balance of the output tubes, which is doubtful over a long time. I prefer to design things that won't hurt me or other equipment if something goes bad.
Thanks!
I have experience with my DIY OTL 6AS7 and I know problems, that can occur.
I listen it more than year and I haven't imbalances. My tubes are matched and I adjusted currents after few weeks. My OTL gave me an idea for OCL headph. amp.
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Projects: OTL-OCL 6AS7 Gen. Electric, SEs 2A3 RCA, 300B JJ, 6S4S, 4P1L, EL11 Telefunken, 6AS7 RCA, 6S33S, 6S41S, 6S19P, E84L Siemens, E236L Telefunken, headphone ampl. OTL Loftin White 6AS7 RCA,... PPs 6005 Gen. Electric, EL34 Mullard, 6L6....

Last edited by azazello; 13th March 2014 at 06:13 PM.
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Old 13th March 2014, 06:43 PM   #4
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Hi,

Since you're well aware of the possible risks with a regular loudspeaker you're quite possibly also aware that the risks are even much higher with a 300R headphone?

Up to you but I'd rather put in a large coupling cap or have a custom OPT wound.

The only time I'd use an OTL amp with headphones is when the headphones are of the ESL type. Other than that it's just an accident waiting to happen IMO.

That said, the design of the OTL amp seems sound enough for use with regular speakers.

Ciao,
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Old 13th March 2014, 06:50 PM   #5
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I haven't experience with headphones and Your opinion is "+" for me, thanks!
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Projects: OTL-OCL 6AS7 Gen. Electric, SEs 2A3 RCA, 300B JJ, 6S4S, 4P1L, EL11 Telefunken, 6AS7 RCA, 6S33S, 6S41S, 6S19P, E84L Siemens, E236L Telefunken, headphone ampl. OTL Loftin White 6AS7 RCA,... PPs 6005 Gen. Electric, EL34 Mullard, 6L6....
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Old 13th March 2014, 07:04 PM   #6
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hello ,

me build proto to good link :

Bauanleitung für einen OTL Headphones Amp

and me build , build to 3 day so easy music
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Old 13th March 2014, 07:17 PM   #7
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Hi,

O.K. But I don't quite understand why you'd want to use an interstage xformer (as a splitter, I assume?) but do away with the OPT which after all can be a very simple device to build?

I looked at the OTL from Tubeamp (same guy that used to post here too?) and you could lose the input cap if all your gear has a coupling cap at the output. After that you'll only end up with just the pair of small coupling caps from the phase splitter to the input grids of the output tube.
PSU is one winding and FWB per B+ and separate bias.
So, the worst thing that can happen is that if one of the two rails fails you end up with the other B+ on the headphones which would kill it in an instant.
Fast blow fuses and/or crowbar protection seem in order.
Since the bias is common to all the output tubes it may be a good idea to either select them (these are not twin triodes) or use individual bias trimming for each triode.
Or, and I have no idea if these are available in Bulgaria, use single triodes such as EC360 (RFT) or A2293 (1/2 6AS7G).
With a mu of only 2 for the 6080 family the curves are pretty close to vertical line in an OTL so runaway is just around the corner.
In praxis this turns out to be less problematic but still.

Ciao,
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