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Old 22nd December 2007, 09:56 AM   #1
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Default My Headphone Design

Hi Friends,

I'm a 17 years old tube enthusiast from Brazil, and this is my idea to use two XCL82 tubes to make a OTL design with negative feedback to drive headphones.

In the frist idea is drive only a Sennheiser HD580 headphone, but when I has simulated the pentode of XCL82 in TubeCad the Zout was around 86ohms ( 20mA, 200v Triode Connected, rk = 330ohm, gain is 0.71 ). A little lamp lighting in top of my head with the idea of put negative feedback in the amplifier and reduce the Zout to can drive 32ohm headphone.

Iím thinking to use the XCL82 because I can buy this in Brazil for US$1. This is a very popular tube.

But If I canít drive some 32ohm headphones the idea is make a very stable and very good amp for mine headphone.

The schematic you can see below. Is C1 necessary? Is a good idea change the R1 to a CCS with transistors? I havenít worked with it before.


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Best Regards,
Felipe Navarro
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Old 22nd December 2007, 02:25 PM   #2
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Hi Felipe,

the condenser is blocking DC from the output.
Why do you use positive feedback?

Regards Andreas
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Old 22nd December 2007, 10:39 PM   #3
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Andreas,

The idea is use the negative feedback to make the amp more stable and with a minor Zout. And make the amp able to drive headphones with low impedance (32ohm).

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Felipe
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Old 22nd December 2007, 10:54 PM   #4
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In this case you are applying positive feedback. That will not make the amplifier more stable nor will it decrease the output impedance.

Jan E.
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Old 23rd December 2007, 05:57 AM   #5
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Jane,

Okay. But the're some feedback techniques to make a amp with feedback have a lower output impedance.

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Felipe
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Old 23rd December 2007, 08:29 AM   #6
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Hi Felipe,

for negative feedback pls. connect the feedback to the grid of the input triode.

A better method for degreasing the output impedance is a "ring of three": a CC stage, a CC stage and a CF stage with overall feedback from the cathod of the CF to the cathod of the first CC stage.

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Old 23rd December 2007, 11:47 AM   #7
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AndreasS,

Can you make a draw of it?

Best Regards and a Nice Xmas.
Felipe
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Old 23rd December 2007, 06:35 PM   #8
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Here ya go mod_evil. Feedback should be routed to the grid of the triode. Please ignore the values of components as your shema didnt have any I left them at default. Feedback becomes the ratio of R1 to R2. The input impendance also basically becomes the value of R1 so make sure it is high enough for your source to drive.
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Old 24th December 2007, 04:19 AM   #9
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jerluwoo,

With this topology, do you think I will can drive 32ohm headphones?

I'm thinking to make the amp with OTL but without the feedback. It's a good idea, but if i need to drive 32ohm headphones I can make a buffer with the BUF634T. I haven't Hi-Fi 32ohms Phones.

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Felipe
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Old 25th December 2007, 05:29 AM   #10
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I'm assuming the xcl82 is the same as the ecl82/6bm8, high mu triode/power pentode. Feedback will be required to make the gain of the triode managable, comparable to 12ax7 with slightly lower mu. It may be capable of giving decent volume out of a set of 300ohm phones but definitly will not drive 32ohm. If you really intend on using 32ohm hp's then configuring it as a standard single ended amp with 4k:32ohm output transformer is probably the best way to go. Cathode follwers are funny things. They do not have the drive current that the output impendance and idle current would suggest. It is at the mercy of the plate resistance. As such you'll find most otl designs use a very low Rp triode for the output, usually a few in parallel, but these are still pretty limited on how much current can be delivered to a low impendance load.
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