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Old 2nd November 2014, 10:14 AM   #1
borys is offline borys  Poland
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Default Very Simple Hybrid Ampifier


Inspired by few recent projects I have decided to try a very simple/basic hybrid to stard with.
I like the Hitachi VAS sound so why not to stick a triode input in front of it ?

I have never worked on the vacuum tubes before so I would be very gratefull if someone can guide me to the right dirrection and give me some advice.

I have supplied the amp from 2x30Vac transformer PSU unit, the heater is supplied temporrary from the lab psu. The thd figures arent great but the amp sound suprisingly good, I was really blown away at the first time I hooked it up to the speakers (or maybe I was too excited).

I am going to build another version with another LTP stage, so I would be able to set the triode ''working point'' in more correct way.

The animal:

-input 12AU7 --> biased at 1.7mA total (two triodes), the cathodes are at +1,8V, the anode load resistors are only 3.3k (prapobly it is wrong but with the simple/education version it must be at that point)
-VAS --> Hitachi biased at 9-10mA total, looks that 5mA is enough to drive correctly the pair of renesas mosfets.
-output --> lateral mosfets from Renesas

Special thanks for OS and Valery for helping me!!
Attached Images
File Type: png VSHA simple.png (64.1 KB, 3519 views)
File Type: jpg DSC_3133.jpg (846.6 KB, 3399 views)
File Type: png thd15v.png (22.8 KB, 2656 views)
File Type: png 20kHz.png (41.7 KB, 2451 views)
File Type: png clipping 1k.png (37.4 KB, 2280 views)
File Type: png clipping 10k.png (37.8 KB, 371 views)
File Type: png SR.png (32.1 KB, 333 views)
File Type: png 1k cap load.png (32.9 KB, 303 views)
File Type: png bode simple.png (31.4 KB, 268 views)
File Type: png 20kHz 60V.png (35.5 KB, 308 views)

Last edited by borys; 2nd November 2014 at 10:23 AM.
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Old 2nd November 2014, 10:41 AM   #2
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Thumbs up great

How does it sound in trebles?
regards, Pawel
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Old 2nd November 2014, 03:36 PM   #3
borys is offline borys  Poland
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It sound very well in the whole top octave. It is maybe a bit to bright/sharp (seas 22taf/g with aluminium dome) but this can be tweaked at further development. The amp has its signature, lot of air and plenty background details, mids and vocals are very plesent to listen. Overal I am very happy from the amp as it was only an educational experiment. The HD version will be ready next weak.
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Old 3rd November 2014, 06:47 AM   #4
borys is offline borys  Poland
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I didnt know that heater must be gnd referenced, so i grounded one side of the grid supply and it looks better. The top octave sounds better now.
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File Type: png thd33v.png (33.0 KB, 477 views)
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Old 4th November 2014, 10:53 AM   #5
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If your "HD" version is ready, will you share your PCB design with us? This is an area (tube-FET direct coupled hybrid amps) where I experimented a bit, with very good results, so I am excited to try your version....
How do you plan to solve the startup issue? I guess heater supply first, then delayed supply rails + muting for the output?
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Old 4th November 2014, 11:03 AM   #6
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You may find better results referencing the heater to the positive rail. Also since the VAS is balanced you may be able to side step start delays etc. by simply placing a large value resistor from plate to cathode of each tube.
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Old 4th November 2014, 06:11 PM   #7
borys is offline borys  Poland
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Thanks for advice !!

After I have referenced the DC12.6v heater to ground, the amp is dead quied with no signal. Tell me please around 220k around cathode and plate would be ok ?

Automatic amplifier turn on --> I was going to use triode ccs resistor 3.3k, when vacuum tubes start to consume the bias current the voltage drop across it will reach approx 5.5V and then the speaker relay will turn on, but first I must finish the amp.

The HD version is showing much lower distortion in Spice, wondering how it will look like in real life.
Bellow eagle files of simple version if some one would like to play with it.

I will share the all documentation after I will test and listen to it.
The simple version is singing very nice, I am wondering how about HD version too.


I have forgotten to mention, the eagle board is kind of universal, the amp can be AC or DC coupled, I have used AC coupled version.
Attached Images
File Type: png vsha_HD thd.png (55.5 KB, 468 views)
Attached Files
File Type: zip (115.4 KB, 111 views)

Last edited by borys; 4th November 2014 at 06:32 PM.
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Old 7th November 2014, 06:45 PM   #8
borys is offline borys  Poland
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The HD version is working now. There is hidden diode in the tube socket to make them look nicer :P
The quick home made measurements bellow, the amp is stable, there is no oscillations present when the tube heater and main psu are powered at the same time, only DC is going bit up/down until the NFB is starting to work (heater takes around 5-8s to heat up the tube).
Unfortunatly I have pulse regulated psu for the heater so there are small artefacts present at the output (with good filtered supply all is OK).
Now there is some time for the listening tests of both versions.
Suprisingly the different tube gives a bit different sound signature.
Attached Images
File Type: jpg thd15v.jpg (99.7 KB, 341 views)
File Type: jpg sr.jpg (101.5 KB, 225 views)
File Type: jpg 20khz.jpg (106.6 KB, 172 views)
File Type: jpg 50khz.jpg (104.9 KB, 179 views)
File Type: jpg diodka.jpg (167.1 KB, 798 views)
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Old 7th November 2014, 08:56 PM   #9
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Originally Posted by borys View Post
Suprisingly the different tube gives a bit different sound signature.
Yes, it's written on ice cream box: Happy message in every tube.
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Old 8th November 2014, 05:16 AM   #10
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Hi Peter!

Wow Just noticed your thread
Congratulations, a very cool project!

I'm curious, what is the difference between the HD version and initial one on terms of schematic?

BTW, in my hybrids I have supply for the heaters - 6.3V AC from a small separate transformer (heaters are paralleled) just "floating" - no problem.

If you can't explain it simply, you don't understand it well enough (c) Albert Einstein
See my projects at VIRTUAL ZERO distortion AUDIO
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