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Old 19th December 2011, 05:01 PM   #1
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Default New Project - Hybrid Amplifier High Quality

Hi all.
I want to make an amplifier with high power like solid stage and also wanted it to sound natural like tube valves.

Remove components the output transformer to structure machine not too bulky.

ClassA output is quality but it is very hot,large heatsink,power supply many caps,...and expensive => I do not like.

I choose classAB,high quality + easy + inexpensive

My project here.

I have designed circuit, not yet layout.
Everybody see have to change anything? if it's OK, I will layout.

Comments please.

Thank everybody.
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Old 19th December 2011, 05:55 PM   #2
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Tube

- Pentode set at 0.5mA
- Triode set at 3mA
- Gain # 50

Solid

- CCS set at 10mA
- Output stage set 100mA

Opam is DC servo. It is ensure that in output stage always 0V DC.
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Old 19th December 2011, 06:02 PM   #3
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Output impedance of the triode about 4700R.

Input impedance of the solid stage very large .
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Last edited by hien.t.nguyen; 19th December 2011 at 06:05 PM.
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Old 19th December 2011, 07:38 PM   #4
AKSA is offline AKSA  Australia
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I like the topology and I'm quite confident it will sound very good.

Nice effort!

I would lower gain on the pentode a little to around 30dB overall; Av = 50 is a little high.

I would make the CCS more stable by using two voltage reference diodes and increasing R1 and R23 so current is around 12mA overall. Servo is good, well set up.

You should have source resistors on M3/M4, all around I would use 0.15R or 0.22R.

Instead of using D2/D3/Z1/Z2, I would simply use a single 4.7V zener across the two gates. This will confer adequate protection and is simpler.

You could replace the two pos and neg supplies on the tube with a single 250V positive B+ supply, and reference the cathodes and input grid to ground. This would be quieter.

Nice circuit, congratulations!

Hugh
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Old 19th December 2011, 07:57 PM   #5
pergo is offline pergo  Italy
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nice design! I am a fan of hybrids i've built several ones.

I agree with AKSA. For single HV supply, you can remove input cap.
I don't use it with this topology, even in my car-audio designs (lot of vibration) without any problem during the years

Make Av= 20-30. I often use 20 (but i use Mu-follower preamp)


You can lower the Mosfet's gate stopper. I use successfully 100ohm for N & P channel, without oscillation.

100mA output's idle current is great for Laterals
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Old 19th December 2011, 08:01 PM   #6
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I would take another op amp for the DC-reg. The OPA134 is not realy good for this job.
A low offset typ like OP177 ist the better choise.
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Old 20th December 2011, 02:28 AM   #7
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Thank everybody

After receipt the comments of guys, I have these changes.
1. Add two diodes for CCS
2. Replace the OPA134 become AD706J
3. Reduce the gain for the first stage, only about 30 times

I remain symmetrical power supply for the first stage, because it's convenient for the rectifier (I'll use the double voltage circuit from AC main power supply)

Regards
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Last edited by hien.t.nguyen; 20th December 2011 at 02:31 AM.
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Old 20th December 2011, 03:23 AM   #8
kevinkr is offline kevinkr  United States
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The symmetrical supply may be convenient, but it certainly is not optimum from several standpoints.. First it requires an additional coupling cap, and the negative supply is a convenient point to inject additional noise into the driver circuitry, in addition there is another set of caps in the audio path tying the negative rail to the audio ground. Just remember to reference the filament supply to something close to the negative rail. I suspect that rather than the nice controlled warm up behavior you would get with the single supply you are going to get a very large negative transient at the output at power on and this will necessitate a muting relay that might not otherwise be necessary.

One other comment involves the linearity of the driver circuit, with bias of just 340mV the maximum linear swing at the plate is going to be pretty limited - perhaps significantly less than 20Vpp. The bias needs to be significantly greater than the input voltage required to achieve your output power goal - which also means you may need to select a different tube or run the pentode section (triode connection is not helping here) at higher current. (Tubes used in amps with significant OLG with lots of global feedback applied only amplify the difference between the input and feedback signal (error signal) - this is not the case when running without global feedback - hence their signal handling is quite limited, and you need to choose something more suitable - for example something like the 6SN7 or 6H30P would probably do a much better job here.)
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Old 20th December 2011, 03:41 AM   #9
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Quote:
Originally Posted by kevinkr View Post
The symmetrical supply may be convenient, but it certainly is not optimum from several standpoints.. First it requires an additional coupling cap, and the negative supply is a convenient point to inject additional noise into the driver circuitry, in addition there is another set of caps in the audio path tying the negative rail to the audio ground. Just remember to reference the filament supply to something close to the negative rail. I suspect that rather than the nice controlled warm up behavior you would get with the single supply you are going to get a very large negative transient at the output at power on and this will necessitate a muting relay that might not otherwise be necessary.
You right,
I will reconsider this problem,I can changes

Please more comments,everything else.

I until not yet layout.
I am waiting everyone finished comments,I will layout and share PCB for all

Thank all.
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Old 20th December 2011, 04:12 AM   #10
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Quote:
Originally Posted by kevinkr View Post
...

One other comment involves the linearity of the driver circuit, with bias of just 340mV the maximum linear swing at the plate is going to be pretty limited - perhaps significantly less than 20Vpp. The bias needs to be significantly greater than the input voltage required to achieve your output power goal - which also means you may need to select a different tube or run the pentode section (triode connection is not helping here) at higher current. (Tubes used in amps with significant OLG with lots of global feedback applied only amplify the difference between the input and feedback signal (error signal) - this is not the case when running without global feedback - hence their signal handling is quite limited, and you need to choose something more suitable - for example something like the 6SN7 or 6H30P would probably do a much better job here.)
Beginning I chose tube 6AN8 but now it is difficult find,not multiple. I chose tube similar is 6U8A by popular,natural sound similar to 6AN8/7199 /,... I've heard.

I liked the sound of rustic, natural, not too sharp, ...should 6AN8 is choice suitable in my opinion.

The problem output impedance in first stage,I was overcome by cathode output,Z output only 4760R.

Input impedance solid stage very large,in high frequency lateral mofet needed more current to win parasitic capacitors inside the mosfet,and 3mA is enough to do it.

The problem of non feedback global,I want to so,If have many feedback the sound not natural,heard the same as solid amplifier

Of course the cost is output high impedance,low damping factor,...I parallel two lateral mosfets because it.

Regards,
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Last edited by hien.t.nguyen; 20th December 2011 at 04:16 AM.
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