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PostPosted: 26 Jun 2018, 13:46 
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Joined: 25 Jun 2018, 14:02
Posts: 5
Location: Ontario, Canada
Attachment:
diy_4.jpg
Attachment:
diy_3.jpg
Attachment:
diy_2.jpg
Attachment:
diy_1.jpg
Attachment:
Amplifier_diy.jpeg

Hi everyone:

This is Anthony Yip, new member of the forum. I am going to share with you a solid state power amplifier circuit using op-amp and power mosfet. This circuit is my original concept and design, not copied or based on anyone's design.

I have built 6 modules of this amplifier, each 3 modules combined to build a 2.1 system (L & R channels plus woofer driver). I have two such systems in operation.

My amplifier modules are built on prototype circuit boards. They are all working very well and I am very pleased with the performance and sound quality.

I am uploading the schematic diagram as well as several pictures of my diy projects. I can elaborate more on the design and construction of these modules if anyone is interested.

Recently I worked out a design of the pcb board of the amplifier. However, I don't have the chance to actually get them manufactured. I can provide Gerber or dxf files of the design if needed by anyone. You can use my information for diy and personal use. Please do not use my information for commercial purposes.

Happy listening.

Anthony


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PostPosted: 29 Jun 2018, 04:49 
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Joined: 26 Dec 2016, 03:46
Posts: 120
Location: Bayarea
I design power amps also, this is completely different from anything I've seen, I have a big collection of schematics, nothing like this. I spend 15 mins. reading your schematic, I have no idea how this circuit works. I don't follow how the two opamps work. Do you have any write up how the circuit works? Circuit looks quite complicated with multiple signal paths I can see. It's hard to describe as you don't have any reference desinators.

1) output feedback through 10K resistor.
2) Output through 22uF and 18K to the top of the bias spreader.
3) From A2 through 3m inductor and 1K resistor to the two driver transistors.
4) A2 through 1uF and 3.3K to the -ve input of A1.

If you have any theory of operation, it would be helpful to understand it. If not, if I have time and feel like doing it, I'll enter into LTSpice and see what comes of it.

The input impedance is very low, it's about 3.5K or so ( did not calculate, just guessing). This is way to low and some preamp might have problem driving it.

Don't you worry about layout and grounding problem using those type of proto boards? I put all my design onto pcb all with ground and power planes. It's quite cheap to have pcb made in China.


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PostPosted: 29 Jun 2018, 11:29 
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Joined: 25 Jun 2018, 14:02
Posts: 5
Location: Ontario, Canada
Attachment:
Amplifier_diy_1.jpeg

Hi everyone:

Thank you for taking the time to look at my schematic. I am putting together something that I hope will explain the operation of the circuit.

A1 and A2 are two op-amps. A3 is not a op-amp, but an amplifier formed by two driver transistors and two power mosfets. The non-inverting input is the base of the driver transistors, while the inverting input is the emitter of the driver transistors.

The input is fed into both A1 and A2. The outputs of A1 and A2 is combined again to drive A3 to deliver sufficient current to drive the speaker.

Now suppose there is a 1V signal at the input. The output of A2 willl be -3.3V, the output at A1 will be 10V at most but will be affected by the feedback loop (10K/1K).

If we assume that the system is stable and work as designed, then the output at A3 would be 10V. The actual gain of A1 would be =10/3.3/2 = 1.5.

Now you can see that an op-amp with a low gain of 1.5 can drive a power amplifier with an overall gain of 10.

Please note:

A2 is an op-amp with fixed gain of 3.3. If good quality op-amp is used, such a configuration should produce a very stable amplifier.

A1 has a ceiling gain of 10 and is subjected to negative feedback to determine the actual gain. The actual gain is very low and the configuration should again produce a very stable amplifier (subject to A3).

A3 is the most unpredictable amplifier. It's amplification and phase shifts, etc., is affected by the speaker and instantaneous voltages of all driver transistors and mosfets. To minimise the unpreditability of A3, it is prudent to make it's gain as low as possible. However, the output of A1 is limited by the rail-to-rail voltage of +/-15V and the capability of the op-amp. Under a rail-to-rail voltage of +/-15V, most op-amp would deliver a peak output of +/-12V. under peak conditions A2 will also deliver a peak output of +/-12V. The effective input to A3 is therefore +/-24V. The output of A3 need to be +/-30V, therefore the minimum gain of A3 would be 30/24 = 1.25. I set the target gain of A3 at 2.

To stabilise A3, the driver transistors are provided with emitter resistors that provide local feedback. These resistors are made as large as possible, but not too large in order not to adversely affect the voltage drive to the mosfets.

I know this is not enough to fully explain the design of the circuit. I will try to post more explainations later.

Happy listening.

Anthony

P.S. I wonder if I should pm those people who are interested to get further explanations. This post would be quite boring to someone who is not interested.


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PostPosted: 29 Jun 2018, 14:00 
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Joined: 26 Dec 2016, 03:46
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Location: Bayarea
I read through the description. My question is why make it so complicated? You have to have two opamps driving A3.

No offense, my biggest problem from my experience of designing 3 different power amps is taming the amp for stability. It is hard enough to tame a simple straight forward amp, I cannot imagine your amp is easy to tame.

I know you put series inductor//10ohm to isolate the capacitance of the speaker cable from the amp. In my experience, this is NOT a good practice. Under non linear load ( speaker is very non linear), this inductor will increase THD a lot. I did a lot of measurements on this already. I even have a long thread in Audio Karma talking about cable and THD. It started out as some funny effect and turned out to be the resistor I use is non linear. From that, I develope the cable and had my theory. It's long, but it's very relevant.

http://audiokarma.org/forums/index.php?threads/effect-of-speaker-cable-on-thd.764633/

That's the reason NO high end amps put inductor in series. Also, if you scan through my thread, I made my own cable that proofed to compare very well to some expensive cables. It's not my original idea, you can buy it, it's just very expensive. I made it cheap and I have instructions how to make them for $30USD a pair. BUT, those cables are very high capacitance. about 3000pF.

Without the inductor, can your amp stay stable with 3000pF right at the output of the amp ( not through the cable on the other end). It sounds very small, but I can assure you the famous Nakamichi PA-7 I have ( same as the Threshold S300 Stasis that make Nelson Pass famous) sang like a bird with 2000pF right at the output connector. I tested all my amps with 22,000pF and frequency sweep to trigger oscillation.

You measure THD at 20Hz, 1K, 5K, 10K 15K and 20KHz? With 4ohm load?

JMHO



EDIT: Taming the amp is easy, taming the amp and keep the frequency response high and low THD is getting to be an art. Try large signal BW>300KHz, 30V/uS and 0.003%THD at 20KHz with damping factor over 2000.


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PostPosted: 29 Jun 2018, 18:40 
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Joined: 25 Jun 2018, 14:02
Posts: 5
Location: Ontario, Canada
Attachment:
printed circuit board.jpg

Hi everyone:

Before I start I want to express my thanks to Mr. Yungman for his patience to read through and comment on my posts.

Well, here is an image of the pcb design. It is a double layer pcb. The red traces are the top layer and the blue colour represent the bottom layer.

For those of you who have the software to view and print gerber or dxf files, you can pm me and I'll send the gerber or dxf files directly to your emails. If you don't have the software, you can download a free viewer/print software from the internet. Just type "gerber viewer" or "autocad viewer" and search.

I must say that I didn't actually made a physical pcb based on this design. However, the pcb layout was based on my prototype boards of which I made 6 already and they are all working fine.

I know that I haven't fully explained my circuit yet. I'll do that later.

Meanwhile, happy listening.

Anthony


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PostPosted: 30 Jun 2018, 14:59 
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Joined: 26 Dec 2016, 03:46
Posts: 120
Location: Bayarea
Hi

I am not interested in build your amp, I already designed and build 3 amps already and have a few new designs in the pipeline. Also, I am into designing high end amps only. But I would like to talk about the circuit.

I want to hear your side what is the advantage of having this complicated design with two signal paths at the input. In my experience designing electronics all these years, the more straight forward, the better the circuit. If it looks too complicated, it usually is. Attached below is a rough drawing ( without all the details) raster amp that we used in our instrument. It is very simple, we used this as a +/-500V swing amp to drive the raster scanning for our mass spectrometer.

Attachment:
Raster amp.jpg


I copied from other people, modified and used it in our instruments. It should be a reasonable audio amp. The amp really use one opamp A2, A1 is to buffer the input as you need minimum 20K input impedance, or else, some preamp ( particular tube preamp) cannot drive it. You have the same problem in your circuit, your input impedance is only like 3.5K or so. You need to buffer that also.

Signal path is very simple, output of the opamp drive Q1 and Q2 which serves as cascode stage to drive the Q3 and Q4. Similar idea as your amp BUT without driving both the base and emitter of the transistors.

Unless I miss something of your circuit, this is literally half your circuit and pretty much do the same thing. It is going to be a lot easier to tame. We use over 10 of this circuit per instrument, it's very reliable and stable and simple. That's why I asked about the spec of your amp, what is the frequency response ( both large and small signal), THD at at least half power for 20Hz to 20KHz driving 4 or 8ohm.


It is an attractive idea to use opamp in power amp design, quite a few people tried that. the problem is most opamp ( audio quality) can only use +/-22V or below. Having to do level translation and extra power supply make opamp not a good choice for audio power amps. There's a good reason why people don't use opamp in audio power amp. I actually look into opamp also because of it's low THD quality and high gain. But when I expand the circuit out, it quickly became undesirable.

I personally don't like using Drain ( of MOSFET) or Collector (BJT) to drive speakers. Output impedance is too high, only way to lower output impedance is by GNFB, which, is not a good way to achieve low output impedance alone. I much prefer either emitter follower or source follower to drive the speakers. With more pairs of output transistors, open loop output impedance is very low. GNFB is ONLY the final touch.

Also, common source/emitter output stage has much lower frequency response than emitter/source follower. This make the emitter/source follower circuit much easier to tame. As I said before, design is easy, taming is the hard job. 99% of even the high end amp take short cuts by lowering the loop gain to make it easier to tame. But then performance suffers.

JMHO


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PostPosted: 02 Jul 2018, 22:23 
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Joined: 26 Dec 2016, 03:46
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Location: Bayarea
Here is another opamp based power amp design. It uses OPA445 +/-45V opamp, problem is THD is much higher than discrete design.

https://www.by-rutgers.nl/PDFiles/SSA120.pdf

There are so many ideas around, the question is whether it's just a different take or whether the new design is actually better. Just design a new design is not so hard, design a new design that is better is hard in power amps. I have been designing electronics ( not audio) for 30+ years, designed a lot of state of the arts circuit ( at least at the time!!). After I retired, I thought I take up something easier as a hobby and I picked up hifi power amps. I have been working more than just hobby for over 3 years, I find there are a lot more to power amp than just a low audio frequency circuit. There are a lot more intricate things that not even text books can tell you. I won't say it is the hardest compare to RF, medical ultrasound scanners, digital scopes and mass spectrometers I designed, but it's a facet of it's own and it NOT as simple as people think. People think it's no big deal have not design a hifi audio power amp yet. It's NOTHING like small signal and low power audio circuit. Those are kindergarten stuff compare to power amp that deal with large signal.

There are a lot of passionate people in this field that are very talented. I studied hundreds of schematics, quite a few times I discovered some things to improve performance and stability just to notice it's been done. It's subtle but it's there.

95% of the power amps on the market fall into only like two or three categories, most are using discrete front end. Major two are the Blameless design, the other is the Complementary IPS design. There are very good reasons 95% of power amps in the last 50 years use this, they are very well thought out. These are very well described in books by Bob Cordell and Doug Self. The difference is within the concept, people make changes to improve the performance.

I have my own design that is totally different from the two above, I don't want to share my idea. I build it with real pcb and all. From comments, it's still not quite measured up to the two other amps I design using Blameless and Complementary. It was disappointing, but that's facts of life. It did clearly beat the Nakamichi PA-7 by a lot, so it's no dog by any measure. I am still hoping to optimize it to give it a fighting chance.

Who doesn't want to have a design to call it their own. I am no different. But it's not that easy. I am still working on different ideas, hoping some day I can hit a jackpot. Designs including hybrids with tube front and SS output, Error correction for output stage and other ideas. These are also talked about in books.

This is an example of Complementary IPS design by Krell, used in Mark Levinson, Bryston
Attachment:
Krell KSA100.JPG


This is the typical Blameless
Attachment:
Blameless amp.JPG



You can see even at that, both designs have a lot of similarities.


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PostPosted: 22 Jul 2018, 01:04 
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Joined: 23 Feb 2017, 02:02
Posts: 193
Just looking at your block diagram compared with your actuall schematic, the actuall schematic you've drawn up shows one non inverting op amp with pnp common collector buffer driving a unity gain szlkai output stage with a consant current source as its biasing.

In scheamtic the second op amp is inverteing and directly driving the output via a inductor and resistor.

I've suceeded in many innovative amplifier output stages and internal op amp designs

I don't see any inductors presneted in the picture of the amplifier built so I think you've probally drawn something wrong with scheamtics provided in first post


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PostPosted: 26 Jul 2018, 10:52 
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Joined: 25 Jun 2018, 14:02
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Location: Ontario, Canada
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Amplifier_diy_1.jpeg
This is the inductor.


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PostPosted: 26 Jul 2018, 10:54 
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This is the inductor
Attachment:
diy_4_1.jpg

Sorry, I uploaded the wrong file in the preceeding post.


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