6AH4 Preamp..A great option for line stage - diyAudio
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Old 10th July 2014, 08:28 PM   #1
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Thumbs up 6AH4 Preamp..A great option for line stage

Started researching a good line stage to match these 1625 power amps. After a lot of reading, stumbled on the low mu approach. The more I read, the more it made sense. So, found this design by Thomas Meyer, which I found intriguing. Was initially going with a 12B4, but I love the sound from octal tubes, so went with Mr. Meyer's design.

The parts cost forced me to re-consider this project. However, after a talk with my friends at Heyboer, found out they could make the iron for a reasonable price.

Since I wanted this to come out great, enlisted my buddy to build it. He is much better at this type of work than I am. My skills are OK, but nothing like his. Since he was intrigued with the design, he happily accepted the job.

The only real change made was using a Khomozo stepped attenuator connect to the input vs. the transformer volume control on the output. Used a slightly beefier power transformer, DC'ed all the tubes, and went with 6DE4 damper tubes.

This preamp sounds better than anything I've ever heard. It's shocking just how good it sounds.
Attached Images
File Type: jpg 110 watt 1625.3.jpg (291.4 KB, 539 views)
File Type: jpg 6AH4_sch.jpg (256.3 KB, 550 views)
File Type: jpg 6AH4.JPG (909.0 KB, 550 views)
File Type: jpg bottom_1.jpg (467.4 KB, 488 views)
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Old 10th July 2014, 10:43 PM   #2
farhun is offline farhun  United States
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Are all the chokes and line out transformers Heyboer made?
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Old 10th July 2014, 10:52 PM   #3
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Quote:
Originally Posted by farhun View Post
Are all the chokes and line out transformers Heyboer made?
Absolutely. The only choke not made by Heyboer are the ones on the 1625 power amps. All the iron on the preamp, and the power and output iron were made by Heyboer. Outstanding quality.
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Old 10th July 2014, 11:56 PM   #4
farhun is offline farhun  United States
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I'm sure about their quality, I have a small pair of trannies made by Heyboer. Congratulations on both, they look very nice and clean. Good work!
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Old 11th July 2014, 10:57 AM   #5
JLH is offline JLH  United States
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I like the 6AH4 too. Built a small 1W power amp using the 6AH4 in a DRD configuration with 6J5 driver tube. Sounds much better than what it deserves. The 6CK4 is the 6AH4's bigger brother. In addition, the 6BX7 is very close to two 6AH4's in one octal bottle.
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Old 11th July 2014, 12:58 PM   #6
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Quote:
Originally Posted by JLH View Post
I like the 6AH4 too. Built a small 1W power amp using the 6AH4 in a DRD configuration with 6J5 driver tube. Sounds much better than what it deserves. The 6CK4 is the 6AH4's bigger brother. In addition, the 6BX7 is very close to two 6AH4's in one octal bottle.
The Stax Headphone Amp boffin over on the headphone website pushes the 6CK4 as a replacement for the 6CG7/6FQ7 in their amps. Too much rework to achieve for that project.

I went with this approach because I liked the design. Thomas Meyer is one smart guy, and it's pretty easy to make it sound right. All the iron in the power supply and the output makes it one very low noise and clean sounding preamp. I would put this up against any commercial product.
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Old 11th July 2014, 01:50 PM   #7
JLH is offline JLH  United States
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The 6CK4 is not as linear as the 6AH4 or triodes in the 6BX7. I would never consider the 6CK4 as a replacement for the 6CG7/6FQ7 - they are nothing alike.
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Old 11th July 2014, 02:01 PM   #8
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Originally Posted by JLH View Post
The 6CK4 is not as linear as the 6AH4 or triodes in the 6BX7. I would never consider the 6CK4 as a replacement for the 6CG7/6FQ7 - they are nothing alike.
Makes we wonder why he would push the 6CK4 for the Stax. I'm happy with the 6CG7s in the Stax head amp, and have no mind to modify it.

I really do like the 6AH4 as a preamp tube.
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Old 11th July 2014, 02:19 PM   #9
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Hi!

Great that the 6AH4 works out so well for you. For those interested here is a link to my post about this circuit:

VinylSavor: Making of a 6AH4 line stage: circuit

And the assembly of the preamp:

VinylSavor: Making of a 6AH4 Line Stage: Assembly

And here an implementation of that circuit with separate PSU chassis:

http://vinylsavor.blogspot.de/2012/0...t-10-6ah4.html

Best regards

Thomas
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Last edited by Vinylsavor; 11th July 2014 at 02:37 PM.
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Old 11th July 2014, 02:48 PM   #10
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Quote:
Originally Posted by Vinylsavor View Post
Hi!

Great that the 6AH4 works out so well for you. For those interested here is a link to my post about this circuit:

VinylSavor: Making of a 6AH4 line stage: circuit

And the assembly of the preamp:

VinylSavor: Making of a 6AH4 Line Stage: Assembly

And here an implementation of that circuit with separate PSU chassis:

VinylSavor: The Modular Preamplifier, Part 10: 6AH4 Linestage

Best regards

Thomas

Thank You, Thomas, for your work and support with tube audio. Lots of great information on your blog.

Here is a picture of the power amps I'm using with the 6AH4 preamp. The combination sounds outstanding. The 6AH4 preamp makes the power amps sound a lot better than they did before. Amazing upgrade.
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