DIY Audio Projects Forum
 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

DIY Audio Projects Forum

Welcome to the DIY Audio Projects Message Forum. Use these forums to discuss Hi-Fi audio and to share your DIY Audio Projects. Registration is free and required to post messages and view the file attachments. Registration will only take a minute and registered users do not see any advertisements. After you have completed the online registration process, check your email (including spam/junk folder) for the verification email to activate your account. New members are under moderation - so your posts will not be visible until approved by a moderator. See the Read Me 1st, Forum RULES and Forum FAQ to get started on the forum.

It is currently 26 May 2018, 18:15

All times are UTC - 6 hours [ DST ]




Post new topic Reply to topic  [ 17 posts ]  Go to page Previous  1, 2
Author Message
PostPosted: 10 May 2018, 06:20 
Offline

Joined: 23 Feb 2017, 02:02
Posts: 130
Well if the op amp(gain stage) is designed well then it should be tollerant to capacitive loading from the output stage internal capcitances. Poorly designed op amp then I'd agree that it may be a plorbem with highly capcitive output stage.

I'm using half fake Chinese tip41c as my head amps output stage(takes only 32W max), also tried tip3055 2955 fake transitors which both case I get same frequency response and slew rate.
My amps gain stage not excatly tuned for high frequency but with 90 degree phase margin slew rate is arround 15v/us.

If I was to convert to power amp All I would be doing is resetting resistor values to suit higher voltages or use parallel transitors for my op amp stage to stand higher voltages without reducing bias current.


Top
 Profile  
 
PostPosted: 10 May 2018, 13:24 
Offline

Joined: 26 Dec 2016, 03:46
Posts: 38
Location: Bayarea
What do you mean by opamp gain stage? You use IC opamp or you mean type opamp design in discrete form.

No, power electronics is nothing at all this simple. I thought is simple too before. It is not simple like you described by just scaling up. It described in Cordell's book about input capacitance of transistor proportion to current. When power goes up, capacitor loading the front end change drastically. You get into conditional stability problem. Amp don't oscillate that easy in small signal, only in large signal. You see oscillation riding on part of the sine wave!!!

Power electronics is really a separate topic. I used to think how hard can it be in audio frequency after years of designing RF, ultra low noise high speed analog, analog IC design and all other analog circuits. I was surprised by how power electronics different. That's where simulation models fail big. They cannot account the change of characteristics accurately.

Look up the thermal derating of TO220, when temperature is up, you don't get 20W or anything close. That's the reason people use many pairs of big transistors that rated 250W each and they still burn transistors if not careful.

I actually thought of using many pairs of TO220 as output stage, it did not pan out at all. The reason I thought about it is I want fast transistors of over 100MHz to get better stability. Long story short, I need like over 20 pairs or more. Can you imagine the work to drill holes and mount the transistors with isolation pads. Let.s just say, I spent the last night mounting my 9 pairs on the pcb and heatsink. Just one amp, hours. I spent one night drilling and tapping hole on the heatsink. Imagine triple the amount of transistors.

TIP42 is too old, too slow. It's not easy to find good transistor. I use 2SC4883 and 2SA1930 TO220 for pre-drivers, over 100MHz, high beta 150V. They are all discontinued, I bulb buy 100 each as I use 2 pairs only for each amp. The faster the transistors, the easier to tame the stability.

It is not that simple, that's why I take on power amp as a challenge.

You have the book by Bob Cordell? It's a very good book. I talked to him a lot on the forum before. We both were analog IC design engineer before ( power amp is like a big discrete opamp inside). We both have a lot of insight in this subject. He talked a lot about these subject in the book.


Top
 Profile  
 
PostPosted: 10 May 2018, 13:39 
Offline

Joined: 26 Dec 2016, 03:46
Posts: 38
Location: Bayarea
I go on a separate post as it gets too long.

We only talked about the theoretical amp design so far. There is the real world. When you run like 10A signal, a wire is not a short circuit, the grounding and power distribution becomes a big problem. The ground becomes a design by itself, so is the power distribution. You need to think like the current, where does in go. Designing the power supply and distribution is a separate subject. I have to say, there are still things I cannot explain in the power amp. I thought I've seen enough electronics. But when you are pushing to 0.001%THD, it's like RF, nV level of noise, High voltage etc. There comes a point of time it's hard to explain.

Imagine the simple bridge rectifier gave me problem, the turn off is too slow and it affect THD reading. I have to resort to pay $25 each of the bridge rectifier to lower the THD and make the wires less sensitive to where it rest. I can explain it now, but that alone took me like 2 or 3 days. It's like voodoo that I can change the THD reading by waving my hands on top.

When you get down to 0.001%THD, everything matters. To get 0.1dB flatness to 20KHz, you need an amp that response way into hundreds of KHz. To prevent large signal distortion, you need like slew rate of 30V/uS.


Top
 Profile  
 
PostPosted: 10 May 2018, 15:15 
Offline

Joined: 23 Feb 2017, 02:02
Posts: 130
I meant op amp type design in discrete form
I have not looked at the book by bob cordell.

Further more all I had talked about is what happens in my real life testing, as said before I do not belive in circuit simulators nor use them.

I had emphasied head amp able to drive 8 r headphones to be same as power amp, becase mainly what made most of my head amps have cross over like distortion was from a large load like 8 or 4 r.
I think this is because of the current drawn out of the op amp when voltage swings it reduces gain and causes it to mailfunction.

Fruther more when there are multiple output devices normally the predriver handles the capcitance at ease.
This is becasue its low output impedance and has high current sink source capaciblity, this allows like 10 pairs internal capacitance to charge up quickly, therefore 10 and 1 pair should hair fairly simliar response.

I've not tried more than 3 pairs yet on scope, but I would be making a class a amp with 8 pairs of output when I have time I already have the case.

I like everything Japanese but not the transistors
I normally not chose high frequency and low capacitance capacitors because japanese transitors 2sa 2sc 2sb 2sd has poor sound quality compared to just a geniune tip3055, pre drivers are also worse than a tip41c.
I like mj14003 14002 and tip36c and 35c +3055. I like mpsa06 or mpsa42 for small dudes.
MJ14003 has arround a BE BC voltage at 0.45v measured on meter BE voltage is also like 0.02 0.03 volts higher than BC voltage, this kind of high power transitor really had improved my lisetning expreience.

Small signals to-92 I find fairly simliar to non japanese ones however I still prerfer to stay away.
Just every time I test japansese transistors they give me poor sound quality. Its hard and mid range sounds distorted like mp3.
I've got like 10pairs or so of diffrent japanse transitors from geniune broken down yamaha and luxman machine, amplifier board was fortunate enough to not be dead. Just sound quality not up to standards.

Normally cross over like distortion and higher order harmonics are at higher frequencies, its not auditable theoriotically. I gave one of my audiphilie mates a amp with oscillations, he couldn't tell the diffrence.
From contiuous and precise listening though, high frequencie distorions are actally slightly heard, the oscillations or cross over like distortion are at high freq, the high frequency noise some how is able to get distortion into the 20-20khz range.

I do have some of douglous self amp books, only found it slighlty usefull when I was still a noob. I don't find it usefull anymore though because I am getting into pro range.
I think the main plorbem with self is that he trust and uses the circuit simulators too much. Where my real build has different from sim a few times or even not work I stayed away from sims.
Virtually everything is talked with reference to circuit simulators, it needs more parcticall and more verbal therotical explnantions on whats going on.


Top
 Profile  
 
PostPosted: 10 May 2018, 18:21 
Offline

Joined: 26 Dec 2016, 03:46
Posts: 38
Location: Bayarea
I personally don't think transistors make any difference to the sound. I choose transistor for stability. I don't like book by Doug Self at all.

I was on DIYaudio, I was talking with Cordell, Self joined in and got into big argument between the two. Cordell and I both design analog bi-polar IC, we see eye to eye in a lot of things. Power amp is just a big discrete IC. I don't agree with Cordell 100%, they are all human, nobody is perfect. I just look at his explanation and think about it. I did verify a lot of his writing with the IC design book by Grey and Mayer, and from my own experience of designing opamp, I agree a lot with Cordell. I think Self is a little full of himself.

I think for small signal like preamps, simulation should be quite accurate as devices are working in linear mode. LTspice is quite good and is free. Just download from Linear Technology site. It just not accurate with large signal that device becomes non linear.

Ha ha, when you add more drivers and predrivers, that's when trouble starts.

When you said you use 20 transistors with the tube for preamp, are you trying to shape the sound, frequency response. That's the reason that got me curious. Because normally, all the circuit I saw in preamp are just straight pass, no tone circuit, no shaping the response. I just don't see how you need all the transistors to do a straight pass.

I found all the tone control, presence, loudness control contaminate the sound. I always put them on bypass. total passive preamp with just a volume pot gives the most fidelity. I am at the process of finding room in the power amp to put in the motorize volume and remote control to put the volume pot close to the power amp so the short coax won't load down the volume pot to get the best frequency response.

Yes, crossover harmonics are higher order, but remember it down mix and get right back into the audio band.


Top
 Profile  
 
PostPosted: 10 May 2018, 23:51 
Offline

Joined: 23 Feb 2017, 02:02
Posts: 130
For my pre amp zero eq control, just direct input and pot on pre amp output.
20or so transitors are to get high gain charteristics and get lots of feedback so distortion=0, this allows me to test weither or not valve sound is distortion to the highest accuracy
Conclusion valve sound is not distortion but a phenomina when electornics flow through a air gap


Top
 Profile  
 
PostPosted: 11 May 2018, 05:39 
Offline

Joined: 23 Feb 2017, 02:02
Posts: 130
I personally not like eq either, thats why all my stuff is zero eq.
I feel like eq leads to distortion


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 17 posts ]  Go to page Previous  1, 2

All times are UTC - 6 hours [ DST ]


Who is online

Users browsing this forum: No registered users and 2 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Jump to:  
DIY Tube Projects :: DIY Tube Amp Kits :: DIY Speaker Projects :: DIY Solid State Projects :: DIY IC / Op-amp Projects :: DIY Phono Projects :: DIY Cable Projects :: Hi-Fi Audio Schematics
© diyAudioProjects.com - Terms of Service - Privacy Policy