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PostPosted: 06 Sep 2018, 00:14 
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Joined: 23 Feb 2017, 02:02
Posts: 397
I'm probing low imedance output of a head amp, with 1x probe fine pick up oscillations
10x probe can't see high frequency oscillations. The resistor divider and compensation built inside probe filters out hf oscillations and scope canot pick up.
50mhz scope fairly new 2016-2018 model.

I use my own output stage in head amps, I kept my output stages same in all amplifiers cause no new inovations yet in this area.

By measuring 90 degree phase marign, with and without load 8r, the square wave response that shows up on scope is very close to 90 degree phase margin. This means virtually zero overshoot.

I don't have THD meter, however at higher frequencies and 8R load sine wave distortion is visible to eye so I tune like this.

My supply voltage is +-12v. the maximum swing is about 1.4v to rails on both sides, don't remeber excat values but swings very close to rails.
Its also capable of delivering at least 6w class ab into 8r

I have not measured -3db point on amp or maximum frequency response, but sine wave voltage is arround 84% norminal at 124khz and heavy compensation on feed back resistor to reduce sine wave distortion on 4stage op amp. Without load the distortion is not visible and heavy compensation is not required to reduce distortion.
3 and 2 stages are worse than this in max frequency response.

I reduced my slew rate by about 6-8times of that without any compensation added in feedback network that sets gain in my 4 stage op amp.
With 3 and 2 stage I reduced slew rate about 10x.
Optimised to reduce sine wave distortion to minimum while sacrificing frequency response.

From exprience square wave response dosen't effect sound quality, so I am not fussed about limited frequency response and round square waves


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PostPosted: 06 Sep 2018, 01:27 
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Joined: 26 Dec 2016, 03:46
Posts: 194
Location: Bayarea
Your scope probe might be defective. I use 10X probe for 200MHz wide band. The limitation is the ground lead of the probe. For HF, I use a fitting that solder onto the pcb that has very short ground lead. I have been using 10X probe for decades, never have problem you describe. 1X probe has too much capacitance and change the circuit.

If you see distortion on the sine wave, THD is really really bad. You can't see 0.1% distortion with naked eyes at all. If you are into designing amps, spectrum analyzer is a must, just as important as scope. I use this one:
https://quantasylum.com/products/qa401-audio-analyzer
If you don't have a spectrum analyzer, you are shooting blind. You can look for other ones that might be cheaper. You cannot tell distortion without this. All the stages you put in, I would expect your THD should be lower. If you can see distortion, something is wrong. If you see distortion on sinewave, it must be like 1% or worst distortion. . Also, distortion almost double if the load goes down to half. I measure mine at 4ohm which is a lot harder on the amp that spec 8ohm.

In my amp, I tested at 22.5V peak ( 45V peak to peak) when using a +/-32V power supply. With higher voltage, THD number will go up some. But in no time the sine wave will show any distortion until clipping as shown in the picture even at 20KHz.

As I said, we can't talk much more without schematic. Designing low power amp is very different from higher power amp that use over +/-30V rail. I don't think you will see all the problem and difficulties I was talking about large signal with 12V rails. No wonder you don't put a lot of emphasis on the output stage. Try one with at least +/-25V and you see what I mean. It's the output stage ( OPS) that's the challenge. If you look at the front end of some cheap amp vs Krell/Mark Levinson/Bryston type of high end amps, they are almost the same, it's the OPS that separate the good and the bad.

I don't know the characteristic of headphone. I bet there's no crossover network inside. Having a crossover is a totally different animal. It is so so much more taxing on the power amp. Crossover network will present very drastic impedance peak and valley and tax the amplifier and affect stability. That's where the OPS becomes very important. Design one that actually drive real speaker and see.

Square wave is for looking at stability and slew rate. You need slew rate for higher power amp, not just BW.


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PostPosted: 06 Sep 2018, 02:36 
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Joined: 23 Feb 2017, 02:02
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I don't want to risk my schematics being coppied or pattned by others, sorry I can't share.

Its my emitter resistorless output stage that controtubtes to high distortion, biasing of OPS could be further improved to reduce distortion, however with some currently used tricks to OPS, generally the distortion can be minized and not visible at 24khz,
even without any tricks just a very small cross over like distortion at sine wave.
Also in theory a small cross over like distortion at higher frequencies are not auditible because this is well above human hearing range 20khz.

IMO these are two great reasons for me to stay away from fiddling with my emitter resistorless output stage.
I'm not too hyped up about output stages at the moment so I haven't invested any extra time into emitter resistorless OPS research.
At the moment I really like desiging gain stages and putting valve voltage gain inside a +-12v op amp.

My +-24v class a amp with just a standard 2 stage op amp runs fine with distortion not visible at 24khz and pure resistive load
It can do much better with 3,4 stage.
Consumes about 2-2.5amps and +-24v without music and load per channel.
Measuring with DC AMP meter the dc current dosen't change untill the speaker requires above 2.5amps

This cross over like distoriton only happens with near pure resistive loads, with fair inductance 50uH or higher the distortion should be lower than standard output stages. Even my worst emitter resistorless stage dosen't have visible distortion with 100uH or larger inductive 8r loads.


Last edited by ILoveHiFi on 06 Sep 2018, 02:59, edited 1 time in total.

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PostPosted: 06 Sep 2018, 02:50 
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Joined: 23 Feb 2017, 02:02
Posts: 397
This is what happened and I am up to this conclusion at the moment.
With regards to scope, I don't think its deffective.
I used to design some two stage op amps and I didn't have a scope at the time.

After I have a scope.
The square wave looks very good 90 degree phase margin, large slew rate minimum compensation and using fancy 10x probe.
However the square wave is a solid filled square due to hf oscillations when viewed on 1x probe.
keep in mind head amp low impedance output stage, it is not oscillating because of pure capacitive loading.

I'm not too hyped up about distortion anylizer at the moment, since its the listening exprience that matters, if it sounds better then fine.
Some valve amps higher distoriton but sound better.

My +-12v head amps do drive 8r speakers at farily loud levels, sounds great and good for medium volumes.
Driving 8r headphones are harder for my amps than 8r speakers becuase inductance much lower.


Last edited by ILoveHiFi on 06 Sep 2018, 03:04, edited 2 times in total.

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PostPosted: 06 Sep 2018, 03:04 
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Joined: 26 Dec 2016, 03:46
Posts: 194
Location: Bayarea
I think crossover distortion is about the most important thing on sound quality. Crossover distortion happens with low frequency, it is audible. Crossover distortion is constant regardless of the music volume. That is crossover distortion affect the sound at low volume listening the most. That's the reason cheap amps sounds good loud, only high quality amps sounds good at low volume. That's the reason why people go class A. That's where the hard part of higher power amps, balance of heat vs sound quality. I use only 32V rails because of the limitation of the heatsink on the chassis. One of my amp has big chassis that costed me $350 from China, I can only use 43V rails and bias at 1A. It's getting hot.

You don't use emitter resistor at the output stage, that only work if you have a single pair of output transistors. It will not work if you have more no matter how hard you try to match transistors. There is no short cut, more pairs lower crossover distortion which is the key of a high quality amp.

Well, it's not too much to talk as I can't see what you are doing. I just don't see your point of 4 stages with my experience. Particularly you even see distortion, that's really bad. As I said, I use 2 stages and achieve that kind of distortion that you don't see in most commercial amps. The sound of all three of my amps beat the PA-7 stasis amp designed by the famous Nelson Pass of Pass Lab. That's an amp with quality of $3000.

If you feel so strongly about your design, you can try patenting it. I wrote my own patent that I got approved, I think it costed me about $1500. But before you even spend money and time to write up a patent, do a google patent search to make sure there's nothing like yours. You need to define what make yours so special, what advantage you have compare with all the other amps. Buy a book on how to write your own patent. That's what I did and I got the patent.


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PostPosted: 06 Sep 2018, 03:08 
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Joined: 23 Feb 2017, 02:02
Posts: 397
The +-24v amp has 3 pairs of lighly matched tip36c output transitors and runs very hot heat sink 58-63c approx at 16-20c ambient.
With this kind of power consumption and biasing 1 pairs of transitors will not surive.
WIth 60w per transitor the heat rise due to case to junction thermal resistance already yeilds 60c temp rise. If the voltage swing goes maximum then about doubble the transitor will blow.

The cross over like distortion only appears at 16v pk-pk or higher for class a +-24v amp
and starts appearing at about 3v pk-pk with my head amps.

With tricks on OPS 2 to 4 stages op amp the distortion is not visible at 24khz.
Even without tirkcs distortion is not visible if you use any load that is 100uH or higher.

Thanks for the tips on buying a book as tutorial on patent I might do that.


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PostPosted: 06 Sep 2018, 03:23 
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Joined: 26 Dec 2016, 03:46
Posts: 194
Location: Bayarea
KochiyaYamato wrote:
This is what happened and I am up to this conclusion at the moment.
With regards to scope, I don't think its deffective.
I used to design some two stage op amps and I didn't have a scope at the time.

After I have a scope.
The square wave looks very good 90 degree phase margin, large slew rate minimum compensation and using fancy 10x probe.
However the square wave is a solid filled square due to hf oscillations when viewed on 1x probe.
keep in mind head amp low impedance output stage, it is not oscillating because of pure capacitive loading.

I'm not too hyped up about distortion anylizer at the moment, since its the listening exprience that matters, if it sounds better then fine.
Some valve amps higher distoriton but sound better.

My +-12v head amps do drive 8r speakers at farily loud levels, sounds great and good for medium volumes.
Driving 8r headphones are harder for my amps than 8r speakers becuase inductance much lower.

I don't think your scope is defective, I think you 10X probe might be defective.


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PostPosted: 06 Sep 2018, 03:24 
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Joined: 23 Feb 2017, 02:02
Posts: 397
I have 4 pices of 10x probe, all gave same results, two diffrent brands of 10x probe.
Also using universitys 100mhz scope and their 20x probe also had filtered out hf oscillations


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PostPosted: 06 Sep 2018, 03:35 
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Joined: 26 Dec 2016, 03:46
Posts: 194
Location: Bayarea
I can't find my book on patent, I think it's the first book here by David Pressman. But I am not sure, this must be a new edition as I don't recognize the cover. I bought in 2011 to write the patent. I got my patent in 2014.

https://www.amazon.com/s/ref=nb_sb_ss_i_1_13?url=search-alias%3Daps&field-keywords=patent+yourself&sprefix=patent+yourse%2Caps%2C380&crid=1JV8O74J8I3R9

It's not as hard as you think to write it. But be prepare to be challenged by the examiner. Do patent search first on google patent before you commit in writing. Be very clear on what make yours different, what is the advantage of yours that other people don't.

When you get your patent, then we can talk. As I said, I just don't get it. To me, if I can get away with one stage, it would be better. The more stages, the more you lose the quality. The amp I am building is actually a dual monoblock INTEGRATED amp!!! that is two separate amps in one chassis WITH preamp function that I can choose one of 3 AV inputs. I do this because I don't even want opamp or any active electronics in the preamp, just a motorized volume pot driving the power amp. All this extra effort to eliminate one stage in the signal path.

When comes to hifi, less is more.


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PostPosted: 06 Sep 2018, 03:45 
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Joined: 26 Dec 2016, 03:46
Posts: 194
Location: Bayarea
I found my book, it is the exact book by David Pressman

https://www.amazon.com/Patent-Yourself-Step-Step-Filing/dp/1413313825/ref=sr_1_3?ie=UTF8&qid=1536222383&sr=8-3&keywords=patent+yourself

Find a used one for cheap.

Must be old age, I didn't even recognize the book cover.....and I used that for a year studying and write my patent. Worked for me. I got my patent!!!


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