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PostPosted: 16 Nov 2018, 15:11 
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Joined: 26 Dec 2016, 03:46
Posts: 180
Location: Bayarea
I just did this for the fun of it, I took pictures of the output at 300KHz.

This is 300KHz 65Vpp full power. You can see it's SLEW RATE limited. Frequency starting to roll off.
Attachment:
OPS6 300KHz 72Vpp 4ohm.JPG



This is 300KHz but only about 40Vpp. The sine wave is still looking decent. This is 50W output. So it's still respectable looking.
Attachment:
OPS6 300KHz 39Vpp 4ohm.JPG


EF has no gain, so you get the full BW of the transistors. You use 3EF, you make the input impedance of the output stage very high. Now you can use small transistors to drive the EF output stage. You can easily get small transistors with fT over 100MHz. You put gain on the small transistor, even you design to have gain of 100, your BW is still at theoretical of 100MHz/100 = 1MHz.

Designing class A is the easiest, you bypass all the difficulty and challenges. But you only limited to low power amps. It is a big big deal to design a 50W class A amp, each channel produce about 120W of heat sitting there. Also, more importantly, 50W class A of WHAT LOAD? It is double as hard to design a 50W class A amp to drive 4ohm speaker than 8ohm. Rule of thumb is a 50W class A for 8ohm is 25W class A for 4ohm. A lot of the high end speakers are 4ohm. More importantly, the crossover inside the speaker can have impedance bottom out at 2ohm or less. So your amp can go in and out of class A. Then it's trouble big time. Krell has a KSA250 250W/ch class A amp. It sit there an burn 1300W without input signal. It's hot like a stove. Unless you live in really cold countries, you need air conditioning to run that amp.

There's a lot of knowledge to go into designing class AB, how to optimize crossover distortion. Class A amp don't have to worry about this. But once it goes off class A and into class B, then all the distortion appears if it is not designed for it.


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PostPosted: 17 Nov 2018, 16:36 
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Joined: 23 Feb 2017, 02:02
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I'm feel like your right about most of what you said and I don't have anything to say, also nice results


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PostPosted: 22 Nov 2018, 03:14 
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Joined: 26 Dec 2016, 03:46
Posts: 180
Location: Bayarea
I strongly suggest you to read the power amp books by Bob Cordell and Doug Self. You can download both free on the internet, you just have to look for them.

I particularly like Bob Cordell, he was an analog IC designer like me, we designed opamps. His book covers different power amp designs. The circuits are really like the internal of the IC opamps. He explain the internal stage by stage, how to compromise. He did NOT invent the circuit, these are very standard design in IC opamps that are very well researched.

If you really want to understand IC opamp design ( same as power amps), I used an IC design book by Grey and Mayer that explain in more detail. This was the bible when I was designing IC. Cordell did a good job extending this into power amp design. If you look at my schematic posted, I even have page number labeled referencing to Cordell's book. Again, those are neither mine or Cordell's idea, it's just standard IC opamp design that has been around for a long time.

I am still researching into high end power amps, I can tell you the circuit is one thing, BUT it's not everything. There are so much more in this power amp design. More I get deep into it, the less I look at the circuit. Circuit idea is dime a dozen, you look at the extreme high end amps ( I mean really high end amps that are over $10,000USD or more), their circuit might look very simple, I can tell you it's NOT that simple.

Another suggestion I have. You need to be careful with the long wires and vector boards. The moment you break into high frequency response, the more it is sensitive to oscillation and wire inductance and crosstalk. This is a whole different ballgame than the circuit schematic. I saw pictures of one of your amp in a real chassis, you have long wires running from power transistors to the board, it will be a bottle neck when you get into high speed EF output stage.

You spoke as if you are so sure about what you are doing, so I did not comment any further, and you refused to open up about your design. I assume you know what you are doing. I design my circuit as if it's RF circuits and it matters. I do everything on PCB, it's not that expensive fabricating from China, learn how do high speed layout, optimize the component placement to minimize the critical signal trace.............I have been doing high speed and microwave pcb layout for a long time, battle is won or lost on the layout. You look at my layout posted here, it looked very simple, not that many traces running for such a huge circuit ( 3 pages). I put a lot of effort doing the layout packing this onto a 2 layer pcb. I have ground plane, power planes and even the output plane. I can tell you, the pcb layout of the Hiraga you posted, I took one look, I started laughing. Whoever did the layout had NO IDEA about circuit layout.

My suggestion to you, worry less on the circuit design and idea, we already succeeded in getting 0.002% THD, running over 300KHz BW. You don't need to reinventing the wheel. Study other people's design. You understand other's design, then you can find ways to improve them and call it your own. I keep a whole library of schematic and service manual of hundreds of power amps and studied through them. Look into high speed design and layout. Get a distortion analyzer like the one I have. Only those can tell you how you are doing.


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PostPosted: 23 Nov 2018, 01:31 
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Joined: 23 Feb 2017, 02:02
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Hey thanks for the suggestions I did already gone through most of the books you've listed before you told me about it.

Never really had a plorbem or much with vector boards with lots of builds audio transitors are not RF too.

With regards to innovative schematics you can't really appreacate how much better and diffrent it sounds unless you listen to it yourself.
I've pretty much gone through all or most schematics guys have done other than output stages. I did go through other dudes output stages but I never built any.


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PostPosted: 23 Nov 2018, 21:12 
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Joined: 23 Feb 2017, 02:02
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With proper pcb and good layout sureley gain some small sound quality but as mentioned before it takes more time to build and research something and more money with proper pcb.

The main difficutly really I'm having with protoype pcb is that poor solder joints with moderate quality cheapo chinese solder, with large schematics and sensitive op amps with mega high gain things go wrong with poor solder joints that look fine but actually is not.
Dosen't happen very often but its main plorbem I have this year.

I never use flux or clean my soldering head as often but now I have to with more components and high gain op amps to prvent plorbems.....
The pre amp 4 stage +-12v and valves, about a month ago. Had suffered from a long term worsening dry joint and seldom loss of sound and short protection kicks in had to replace all transitors and rewire poor wires porplery to fix it........


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PostPosted: 06 Dec 2018, 04:27 
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Joined: 26 Dec 2016, 03:46
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Location: Bayarea
I am waiting for the pcb of my 4th amp coming back. At the mean time, I am designing a hybrid power amp for the fun of it. Now I see why you need 4 stages. I designed 3 stages frontend, loopgain is too low. The 12AX7 has gain only about 24 per stage, it's low. The first stage just break out even with the closed loop gain of the amp ( 20), I have only loopgain of the rest of the two stage which is only about 450 or so from simulation. It's way too low. I looked at other tubes like 6GC7 and 6FQ7, even 12AT7, there gain is even lower than 12AX7 as the plate resistance is too low, amplification factor is only about 20!!

I can get so much more gain in the two stages SS design. Even adding the 4th stage, loopgain is still only about 75dB. But according to simulation, my frequency response is over 300KHz. I don't believe it.

My SS amp circuit has a provision to use other frontends by just plugging into a connector, so all I have to do is layout a small frontend board. Make sure it works before putting everything onto one pcb.

I bet when all said and done, it's going to be similar to yours, two stages of IC opamp, then driving two stages of tubes. Keep the tubes at larger signal to get the characteristic of the tube sound. Should be an easy design.


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PostPosted: 08 Dec 2018, 14:07 
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Joined: 26 Dec 2016, 03:46
Posts: 180
Location: Bayarea
This is the simulation of THD with LTSpice on my 4 stages hybrid design

Attachment:
20KHz 24Vp.JPG

Attachment:
10KHz 24Vp.JPG

Attachment:
5KHz 24Vp.JPG

Attachment:
1KHz 24Vp.JPG


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PostPosted: 09 Dec 2018, 21:00 
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Joined: 23 Feb 2017, 02:02
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Well the graphs loooks nice with low distortion.
Since you said a open loop gain of 75dB
I belive your using local feedback to stablize the op amp to get a finite gain then closing the loop at the end with all stages feedback.

My 4 stage hybrid op amp or 4 stage op amp acts as a single standalone op amp with very large gain gauranteed minimum of 163dB at low frequencies and theorotical gain of 200dB which it should have.

I designed a 5 stage op amp but unfrotunatlley it worked without a output stage but with ops it oscillated badly.
I plan to do more in 4 stages with hybrids then going to look back to 5 when I finished upgrading all of my amps to latest hybrid 4 stage schematic


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PostPosted: 10 Dec 2018, 00:14 
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Joined: 26 Dec 2016, 03:46
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Location: Bayarea
It is not a number game, it's about the THD. If each stage has very low THD, even a 40dB loopgain can be better than 200dB of stages with high THD.

Besides, it's not the low frequency loop gain that matters. You have the limit of what is the frequency where the loop gain = 0dB. If it is 300KHz, your loop gain at 20KHz is going to be no more than like 30dB or so even if you play game with the poles and zeros. More loop gain at low frequency is just pushing the first pole lower and lower and start rolling off sooner. With local feedback, you push the first poll to much higher frequency. You don't gain anything having high loop gain at low frequency.

It is almost more important to make the amp faster than to talk about high loop gain. That's the reason all my amps are over 300KHz BW. I never even measure the real BW of my amps, as I always have a low pass filter of 350KHz at the front end to block the AM radio frequencies from entering into the amp. One day, I would remove the filter and measure the true frequency response of my amps.


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PostPosted: 10 Dec 2018, 04:03 
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Joined: 23 Feb 2017, 02:02
Posts: 243
WIth high gain this means the front driving stages have voltage swing of approximatley zero, with votlage swing of zero the distortion is approximatley zero. This mean op amps openloop linearity are determined by the linearity of the last votlage gain stage.

You want very high open loop gain to kill all distortion from not only the gain stages but also the distortion from output stage.
Having a linear gain system with low ol gain dosen't help because the output stage lacks nfb and output is now most dominant factor of distortion.

My amps have extreme low distortion where if you have a input hf filter you can actually hear the negltible reactive curruents and voltage drops due to hf filtering on input, reducing sound quality


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