Some kind questions on linearity and distortion
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ginetto61
diyAudio Member

Join Date: Sep 2009
Location: Milano
Quote:
 Originally Posted by godfrey You can get a lot of schematics and service manuals for commercial amps on Jan Dupont's website for free.
Thanks !!!
I do not want to sound naive, but i was implying that a more complex design sounds better than a basic design
and this of course justifies the added complexity
I was correct, wasn't I ?
Thanks a lot again !
Kindest regards,
gino

godfrey
diyAudio Member

Join Date: Nov 2009
Location: Cape Town
Quote:
 Originally Posted by ginetto61 How to connect a basic circuit I need gain of 2 and no more than 2-300 ohm of output impedance about 24V for the supply (two 12V batteries in series). For a one npn line stage
Here you go.

Input impedance is 10K, gain = 2, and according to my simulator:
• Frequency response is flat up to about 1MHz.
• Output impedance is about 200 Ohms, depending on the transistor, except at low frequencies, where the output capacitor dominates.
• Distortion for 1V rms input, 2V rms output is about 0.2% with no load, and double that with a 10K load.
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ginetto61
diyAudio Member

Join Date: Sep 2009
Location: Milano
Quote:
 Originally Posted by godfrey Here you go. Input impedance is 10K, gain = 2, and according to my simulator:Frequency response is flat up to about 1MHz. Output impedance is about 200 Ohms, depending on the transistor, except at low frequencies, where the output capacitor dominates. Distortion for 1V rms input, 2V rms output is about 0.2% with no load, and double that with a 10K load.
Thank you very much indeed
This is the approach that i like and I intend to study and follow initially for a line stage
The PS is not difficult. I can find something with building instructions at least up to 30V easily.
I did it with a LM317 based design and it worked nicely.
One last question because i do not want to annoy too much
Starting from this schema and keeping it like this, which could be the moves to get lower distortion ?
I mean, I understand that figures are not everything
But i think that the exercise of fine tune a very simple circuit is both very challenging but also very intriguing.
I do not know, maybe increase the voltage supply ? change bjt ? increase bias current ?
Have you simulated the circuit ? what software do you use ?
I read that some simulation software are quite reliable in predicting a circuit performance
To end this is the most interesting topic for me presently
Thank you so much for your kind and valuable advice.
Kindest regards,
gino

P.S. for me op-amps can also be good, but are very difficult. Too difficult.
Better stick with discretes.

Last edited by ginetto61; 20th March 2013 at 10:54 AM.

godfrey
diyAudio Member

Join Date: Nov 2009
Location: Cape Town
Quote:
 Originally Posted by ginetto61 The PS is not difficult. I can find something with building instructions at least up to 30V easily. I did it with a LM317 based design and it worked nicely.
That was quick - you built it less than an hour after I posted it!
Using the LM317 regulator was a good idea - simple circuits like this really do need a smooth clean power supply.

Quote:
 Originally Posted by ginetto61 Have you simulated the circuit ? what software do you use ? I read that some simulation software are quite reliable in predicting a circuit performance
I simulated it with SIMETRIX SIMplis. Many others on the forum use LTSpice. Both are free and good, giving very similar results. I've tried LTSpice, but found SIMETRIX much simpler and easier to learn and use.

Quote:
 Originally Posted by ginetto61 Starting from this schema and keeping it like this, which could be the moves to get lower distortion ? I mean, I understand that figures are not everything But i think that the exercise of fine tune a very simple circuit is both very challenging but also very intriguing. I do not know, maybe increase the voltage supply ? change bjt ? increase bias current ?
Good questions. Let's look at a few options:
• change bjt
I chose BC547C because it has a very high current gain. All else being equal, higher gain => higher feedback => lower distortion. There may well be other (esp. more modern) transistors that would be better, but I don't know what they are.
Other reasons I chose the BC547C are that it's inexpensive, widely available and I can remember the part number offhand.

• increase the voltage supply
Higher voltage supply is definitely better than lower. One good reason is that, for the same quiescent conditions (i.e. same idling current and collector voltage), it lets you use a higher value for R3. This reduces the load that the transistor has to drive, which reduces distortion. It also increases open loop gain, resulting in higher feedback, which reduces distortion further.

• increase bias current
I'm not sure about that. I actually suspect reducing the bias current may give better results. It's easy to experiment with this: Just increase R3 to reduce the current or vice versa. e.g. doubling R3 will roughly halve the idling current. Reducing it too far will obviously limit the maximum output swing though.

• reducing collector voltage
I originally aimed to have the collector at about 12V (half the supply voltage), and the collector current at about 3mA which is in the BC547's "comfort zone" where it works well. FWIW, the simulator gives 11V and 3.5mA for the component values I showed.

However, I'm starting to think it may be better to set the collector voltage lower, at about 6V. This will limit the output to about 4V rms, but should reduce distortion at lower levels like 1V or 2V rms. For example: first increase R1 to 2K7. This reduces the collector voltage to about 6V, but also increases collector current. Then increase R3 to 6K8 to bring the collector current back down to about 2.5mA. Increasing R1 and R3 reduces the loading at the input and output respectively, both of which should result in less distortion.

I wouldn't expect any of these changes to make a big difference, but there may be a small but noticeable improvement. e.g., IIRC, the last change mentioned (reducing collector voltage) reduces distortion by about half, according to my simulation.

ginetto61
diyAudio Member

Join Date: Sep 2009
Location: Milano
Quote:
 Originally Posted by godfrey That was quick - you built it less than an hour after I posted it! Using the LM317 regulator was a good idea - simple circuits like this really do need a smooth clean power supply
Hello ! no ! i said that i did built the PS with a kit sometimes ago
But of course I can do it again, The build is easy with pcb and instruction
I do not know why the lm317 is not more popular. It is a great regulator i think and kits are available. I like it
Unfortunately it can be used up to 30V max

Quote:
 I simulated it with SIMETRIX SIMplis. Many others on the forum use LTSpice. Both are free and good, giving very similar results. I've tried LTSpice, but found SIMETRIX much simpler and easier to learn and use.
Thank you for the advice ! I will look for it. Great tools these SW.

Quote:
Thank you very much for this crash course in audio electronics !
It is interesting to see that fine tuning of a simple circuit can lead to better electric performance. This could be a way to nice sound also.
The sim SWs are very nice and useful.
The possibility to predict the performance of a circuit without having to build it is very intriguing
Being ignorant I would use them extensively because they seem also quite reliable
Thanks a lot for the kind and valuable help
Best regards,
gino

Last edited by ginetto61; 21st March 2013 at 09:45 PM.

 21st March 2013, 11:32 PM #16 dmills   diyAudio Member   Join Date: Aug 2008 Location: High Wycombe Simulation is useful to a point, and a godsend for some things, but is only ever as good as the models used. Even in simple audio things sometimes layout can make a significant difference, and while possible to simulate, it is hard to model accurately. I would argue that until you gain the experience to know when the simulation is lying (And they all do upon occasion), paper and a pencil is pretty much as good. Get yoursef a copy of "The Art Of Electronics" for a little light beadtime reading (Warning, the binding is well known to be crap), and Doug Selfs "Small signal audio design" contains some stuff that you will seldom see in one place elsewhere. For learning, you will do as well to look at a few simple stages, and sit down with a pad of graph paper, a calculator, ohms law and Ebbers-Moll or such, simulations are good and quick, but to get the real insights you need to do the math yourself, Ebbers-Moll is good for DC conditions, and good enough for audio band, you don't need Gummel-Poon for that. The LM317 is fairly noisy, has poor HF PSRR (As do many integrated regulators), and fairly poor load transient response, it also has quite a high idle current consumption when compared to more modern parts, it is however easy to use and robust, which is sometimes all that is required. Regards, Dan. Last edited by dmills; 21st March 2013 at 11:38 PM.
ginetto61
diyAudio Member

Join Date: Sep 2009
Location: Milano
Quote:
 Originally Posted by dmills Simulation is useful to a point, and a godsend for some things, but is only ever as good as the models used. Even in simple audio things sometimes layout can make a significant difference, and while possible to simulate, it is hard to model accurately. I would argue that until you gain the experience to know when the simulation is lying (And they all do upon occasion), paper and a pencil is pretty much as good.
Hi ! i am sure that there is much more like the layout you are mentioning
it is complex to get things very right.
But how could you get a distortion spectrum with paper and pencil ?
i know that sim software can do this easily and this is quite important i guess
with the SW you have just to change component values and press enter
i like this very much
Once you have got a nice distortion graph the pcb design part begins
I understand that this impacts a lot on the overall outcome

Quote:
 Get yoursef a copy of "The Art Of Electronics" for a little light beadtime reading (Warning, the binding is well known to be crap), and Doug Selfs "Small signal audio design" contains some stuff that you will seldom see in one place elsewhere. For learning, you will do as well to look at a few simple stages, and sit down with a pad of graph paper, a calculator, ohms law and Ebbers-Moll or such, simulations are good and quick, but to get the real insights you need to do the math yourself, Ebbers-Moll is good for DC conditions, and good enough for audio band, you don't need Gummel-Poon for that
I have to tell you that at school i was much better with instruments than on theory
I like the practical part ... measurements in particular
and i am a avid reader of lab reports, even if i do not understand them completely
I trust the instruments so much more than my ear
and i am not good at theory ... quite limited actually

Quote:
 The LM317 is fairly noisy, has poor HF PSRR (As do many integrated regulators), and fairly poor load transient response, it also has quite a high idle current consumption when compared to more modern parts, it is however easy to use and robust, which is sometimes all that is required. Regards, Dan.
is there any monolithic variable regulator with better performance than the LM317 ?
I mentioned it because kits using this part are very common
Thank you very much again for the very helpful advice
Kind regards,
gino

CopperTop
diyAudio Member

Join Date: Feb 2009
Location: UK
Quote:
 Originally Posted by dmills I would argue that until you gain the experience to know when the simulation is lying (And they all do upon occasion), paper and a pencil is pretty much as good.
Doesn't the pencil and paper lie too? Most paper and pencil 'simulations' start with "let's assume it's an ideal transistor and passive components, and the power supply is a perfect voltage source". The slightest nod towards real components renders your simulation an hour's hard graft with the potential for many mistakes. I would say that the potential for the software simulator lying is lower than the average back-of-an-envelope manual calculation.

I also sometimes feel that discussions of amplifier design are somewhat circular, based on a priori 'knowledge' about what an amplifier does, exclusively in the frequency domain. The circuit simulator is quite happy to let us see what's going on in the time domain, which I find very revealing.

DF96
diyAudio Member

Join Date: May 2007
Quote:
 Originally Posted by ginetto61 I like the practical part ... measurements in particular
Without theory you have no framework in which to interpret measurements. A measurement is simply a raw number without interpretation; it is data but contains no information.

ginetto61
diyAudio Member

Join Date: Sep 2009
Location: Milano
Quote:
 Originally Posted by DF96 Without theory you have no framework in which to interpret measurements. A measurement is simply a raw number without interpretation; it is data but contains no information
I am not so convinced .. anyway i am not convinced of something else
As usual i am confused

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