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Old 29th July 2008, 07:36 AM   #1
jeppix is offline jeppix  Italy
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Default Hybrid Car Amp... let's try

Hello to everybody!!!!!

I'd like to make a Hybrid car Amp.... but I'm not so good in schematics

These are the options I'd like to have:
- Tube preamp
- Final stage with transistors or ICs
- Easy PSU

I think tube preamp and transistors/ICs in final stage is the best solution so you don't need output transformers.

Easy PSU is important for me because I have some trouble in wiring transformer so if the circuit use battery voltage is better.

What about using subminiature tube (for example 6021W) for the preamp?

For the final stage I think using STMicro TDA ICs, but every ideas are welcome.....

Let's try..... and sorry for my awful english!!!!
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Old 29th July 2008, 09:58 AM   #2
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You might want to build it as 2 units.

1st stage: tube buffer. Cathode follower maybe?

2nd stage: SS amp.

John C.
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Old 30th July 2008, 06:38 PM   #3
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I've actually successfully done that (built a tube preamp and separate giant solid-state amplifier) but with no interstage processing to remove the tube tonality.

I will be posting them soon on DIYAUDIO.com. I'm trying to finish the cooling fan controller for the amplifier this week.

Just as a side note, it sucks, but you may have to "suck it up" and deal with the difficulty of power supply design/troubleshooting for any "real" tube preamp.

Otherwise, the old Planet Audio hybrid amps are still popping up on eBay. I was planning on buying one just to duplicate the PCB & parts list, along with examine the tube layout, but they're still quite popular!! (Had one years ago, wish I kept it!)

Edit: For a starting point (tube preamp and PSU do a search for "Silicon Chip 12AX7 tube preamp" and you'll see what's involved. Their PSU is not so hard to build, but I found an error in the regulation part when I built it. Fixed it and removed the transformer noise. Egad!
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Old 30th July 2008, 08:39 PM   #4
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Default Just asking...

I am not that familar with tube amps; but won't the road vibration cause distortion in the signal path?
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Old 30th July 2008, 09:13 PM   #5
jeppix is offline jeppix  Italy
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Hello to everybody!

Making some "googling" I found this:

PAIA TubeHead
A 12V tube preamp, there is a kit but also the schematic.

But I also found this:
LVPS Power converter
and the schematic
It is a converter from 12V car battery to 170V (in the same site there is also the version for 250V and 300V)
It seems quite easy to build but before using it there are to monitor some points with a scope... that I don't have a scope!!!

If this power supply is ok there is no more the problem of 12V because you can make a "normal" (high voltage) preamp.

For the final stage, with this voltage, you have 2 options:
1. a tube power amp
2. a transistors/ICs power amp (scaling down the voltage).

I prefer the second choice for 3 reasons:
Firstly if you make a tube power amp you also need output trasformers. Secondly, as says benchtester, I think vibration can cause distortion.
Thirdly having an all tubes system is very big and I don't have a "Stretch Limousine"

What do you think?
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Old 30th July 2008, 09:45 PM   #6
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It depends on the tubes used and the design, etc. But there are several awesome (and really expensive) commercial tube amps & preamps for the car. They have no problems with vibration/microphonics.

I tested my preamp first for microphonic effects over my normal driving route and had no issues.

Jeepix, my suggestion (only a suggestion, there are people a lot more experienced than me here!) is to look at the 12AX7 Silicon Chip design above in my post. It is fairly simple, and my guess is that you could get the SMPS working with no oscilloscope, though not recommended.

Otherwise, if you want to take the easy route, you can do like one guy on DIYAUDIO.com did and get a good 12VDC-120V AC inverter then use a simple linear power supply to power either an amp or preamp.

I'm thinking about doing that for a short time as getting a custom SMPS working can be difficult or time consuming.

Yes, one of the sacrifices you have to make for a custom amplifier, especially a tube amplifier, is the size! But that's engineering-almost everything's a tradeoff.
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Old 30th July 2008, 09:47 PM   #7
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The tube preamp only needs 42v. That's easily produced by a few extra windings or a doubler off of the switching power supply you'll build to power the output stage.
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Old 31st July 2008, 06:56 AM   #8
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Location: GP (Gangstas Paradise)
Can be done, would not recommend the valve part, unless you don't mind changing valves every 3-4 months. But maybe you can get some NOS valves that where used in car radio's, bit hard to find, some of them where made to handle extra vibrations... If it where for Hi-Fi, I'd have recommended 12BH7, Quietest valve I've ever heard in a preamp...
It's one of the best feelings in the world when you build something without it going up in smoke............
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Old 31st July 2008, 08:35 AM   #9
jeppix is offline jeppix  Italy
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thanks MartyM for the advice.

Is it this the preamp? 12AX7 Tube Preamplifier kit

It is quite easy to build... not much components. I will design the PCB this night.

Now before deciding the power supply I must decide what to use as Final Amp.....

I don't know what final amp use: tube, transistor, IC?

Transistor is problaby easy but I have a lot of voltage for the preamp that I have to scale down (I don't know how ) but I think with some regulator and big heatsink. And also big heatsink for the transistor, I think....

IC and what a gainclone? With this I problaby loose all the tube benefits...

Tubes are very beautiful. I already have the right voltage, and for a complete amp I could also made a tube crossover

I'm really in trouble

What I build??? Surely the tube preamp....

Every advice is welcome.... so when I decide for the final stage I also think about the power supply....

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Old 31st July 2008, 02:24 PM   #10
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Hello! Yes that is the tube preamp I'm referring too.

Well I can tell that perhaps you might consider the National Semiconductor LMxxxx line of Overture power amplifier chips. I used the LM3886 for testing the tube preamp.

Those have design examples that work and they sound nice. I could hear the tonality of the tubes as long as I wasn't using a preamp that did audio processing or etc.

I'm not sure how other amplifier designs or ICs would work with tubes
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