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12AU7 / IRF612 Headphone Amp - Update
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Author:  roggom [ 05 Sep 2008, 02:25 ]
Post subject:  Re: 12AU7/IRF612 update

I installed a dpdt power switch to the original design and have a filament loop LM317T 2.2 Ohm (3w) so that one way will have 12 volts to the filiments and the other 6.3. I rolled a ECC 88/6dj8 from west Germany and it sounded pretty good, a lot more gain. This would be better suited for line level vs. headphone level inputs.
Next was the 6N6P, again great sound, but this is a little current hog and my voltage regulator was at its limit. I think this one better suited for an independent filament power supply, as the normal filament current is spec'd at 900mA

6H1n: no way, I got a couple of these guys on ebay as a "so called" ecc88 replacement, They run at 600mA and it ran into clipping at 9 oclock. It needs a min of 24 volts to think about waking up.

All in all the ECC 82 12au7 sounds the best in the application but I like being able to roll tubes with a flip of a switch.

Author:  Gio [ 05 Sep 2008, 07:24 ]
Post subject:  Re: 12AU7/IRF612 update

I've seen the 6DJ8 used at lower voltages often. Do you think a line level 6DJ8 would compare to a cmoy? Obviously power draw is very different.

Author:  roggom [ 05 Sep 2008, 09:46 ]
Post subject:  Re: 12AU7/IRF612 update

Yes it has the same sound characteristics with regard to gain, I think a 47k pot in place of the plate resistor will help to set the plate voltage. With the 47k R and a 12AU7 it is 5.8 this is good for a decent swing voltage; which should be half the PS. The ECC88/6dj8 has more gain but with the stock resistor only 3.8 V of swing. So it would be best to bring the R down and the Plate voltage up a little , or bump the PS voltage up to 15 or 18vdc.

Author:  roggom [ 05 Sep 2008, 23:18 ]
Post subject:  Re: 12AU7/IRF612 update

Just wanted to add that there is more response in lower frequencies and virtually no noise.

Author:  Gio [ 06 Sep 2008, 09:39 ]
Post subject:  Re: 12AU7/IRF612 update

It that with the tube in comparison to an opamp?

Author:  roggom [ 06 Sep 2008, 11:16 ]
Post subject:  Re: 12AU7/IRF612 update

Yes that is with the 6dj8 tube va and mosfet output stage. It has much gain but makes up with a lower bass line and not as much high freq noise as an opamp by itself. I was surprised at the performace from the 6N6P and would be up for using it, but would have to build a seperate 6v 1.5A power supply for it alone or beef up to a TO-3 package LM317. The heaters are 900mA but it will heat up consideralbly.

Author:  roggom [ 28 Oct 2008, 21:14 ]
Post subject:  Re: 12AU7/IRF612 update

I was able to build and smoke test the HV version of this amp. I am very pleased as I went very simple and inexpensive on the upgrades.

I used three radio shack transformers: (2) 6.3-0-6.3 1.2A and (1) 6-0-6 450mA for the filaments.
The larger trannies are hooked up back-to-back to give me around 160VDC after rectification and pi filtering. I tapped into the 14vac between the transformers to get 18vdc after rectification and filtering for the mosfet follower output stage. The 12AU7 is directly heated with the smaller transformer. I tried other higher Mu tubes with the center tap to get 6.3 but they were noisy. The 12AU7 is dead quiet into my Sennheiser HD497's. I have a case for the PS just need to case the amp up.

For the amp I had to separate the two stages with a .1uF 400v orange drop ( I salvaged some from a high voltage power supply) and then added a 100k pot and 66k resistor bridge. I only had the 100k pot, a smaller one would have been better. Under load my drain voltage was around 14vdc, so I set the source to around 6.75vdc.

I am listening to some Diana Krall right now, her voice is just...well you know.

Darn it, this high voltage trend is going to get expensive :).

Author:  roggom [ 28 Oct 2008, 21:17 ]
Post subject:  Re: 12AU7/IRF612 update

Schematics

Author:  Gio [ 28 Oct 2008, 23:57 ]
Post subject:  Re: 12AU7/IRF612 update

Hi Rogers, these tubes do sound much better at high voltage. Interesting use of the back to back transformers. To keep costs down, I have had good luck picking up power transformers from old vacuum tube radios at thrift shops as well as radio and hamfests. After a coat of paint they look pretty good. :mrgreen:

Cheers

Author:  roggom [ 01 Nov 2008, 15:41 ]
Post subject:  Re: 12AU7/IRF612 update

Unit has been working for a couple of days and I rolled various tubes (12AU7 & ECC82), I like the sound of the clear top RCA 12AU7A the best, I have also added a 100uF AC bypass (Ck) across the 2k (Rk). and changed the inter stage coupler from a 100nF 400 to a 1uF 250v for some better bass response. My curve is much better in simulation and actual output. I am getting around 20db of gain across the board. Its in a temporary case until I find another chassis, but I may whip up a little cage and some paint (not ready for final pic)

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