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Old 2nd June 2008, 10:47 PM   #1
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Default Very nice sounding chipamp-driven working class A amp

If you have read the thread "hum to noise problem in class A chipamp" that i started to ask about the problems i found while developing that amp, you will thing that it isn't a good thing to build. Well.. the problems are gone .

Now it sounds very good. It's sweet and the sounds emerge from nothing and fade to nothing in a freakin' way, No 700 KHz on the output, no 3.5 MHz... Very quiet, with separated instruments and a very "black" background.

It's also the first amplifier i've heard whose bass is almost inaudible at low volumes and gets presence when increasing the volume. If you are familiar with psychoacoustics this would seem the thing to expect, but in all class B amps that i've heard the highs became prominent and agressive as the amplifier is pushed.

The solution (and revision to the schematic) to the oscillation problem was adding a cap between the inverting and noninverting inputs of the amp. It also thanks good cables and low impedance sources (it was really noisy when driven with a 5 meter unscreened power wire).

The electronics are cheap (excluding supply and heatsinks), you can get a stereo one for less than $15 (< $5 for the LM4562 (digi-key) , < $5 for the transistors and < $5 for the passives.

The TIP142 are choosen because they are very cheap and easy to find, of course adding a better transistor could give better sound and stability*.

At the moment the amp is stable without any compensating cap while not clipping, but it has a "tick" clipped region, meaning that the driver does not like to have its output clipped.

I would suggest to power it from 18.5V rails and to feed the LM4562 with 17V regulated to try to get clipping at 11 Vrms. This should give 15W of output power which seems enough if you have high efficiency loudspeakers or a family (With 89 db loudspeakers people living with me do complain).

The "key" advantadges of this amp are, apart from good sound, the ability to "tune" it by changing the few parts involve:

(1) Output transistors: These have to withstand high power dissipation and have a sufficient beta at the operating range not to place a tremendous stress on the driver opamp (2.7A center-point value with 20 mA drive current means that the opamp should be giving a maximum admisible of 10 mA at 2.7A, so a beta of 270 is the bare minimum. I don't know if there is any single transistor that meets these needs, but if exists then it would probably be a better choice than power darlingtons for speed reasons, this is a compromise between what sounds "worse", stressing the opamp or using slow darlingtons.

I don't recommend to use power mosfets because the RC filter formed by the gate stopper resistor and the gate capacitance will place a + 90 deg phase shift about 2 MHz, in the middle of the opamp banwidth (5.5 MHz with a gain of 10). This will make a power oscillator for sure and the only solution will be compensating the amp.

(2). Opamps: I go for the LM4562 because is universally accepted as sounding good. The Burr-Brown and Analog Devices ones are also fine and may suit your taste. there is a wide variety of high-fidelity opamps, all of them very good. Distortion in input stages and offset voltages depend on transistor matching and this is best done on-die, so i wouldn't be surprised if somebody reports that amp to sound better than very expensive gear.


(3) Passives: The good bias current and offset performance of these opamps makes no need for the feedback cap, the one that passes the most signal in all designs and the one that can be blamed the most for "capacitor distortion". I've found that the input cap to make slight differences, there is nothing wrong with choosing the one you like the most but it's not a good idea to obscess with it. I don't expect resistors to make absolutely any difference in the sound.

Dissipation is 50.4 peak and idle for the upper transistor and 25.2 idle, 50.4 peak for the bottom ones. This means you should use big heatsinks.

To avoid "out of current" clipping (which does not sound good) loudspeakers should not go lower than 6.5 Ohms at any range. You can make it able to go lower if you add an extra constant current sink at the bottom, having to care only about not exceeding Q7 SOAR. 4 Ohms loudspeakers are completely unusable here. I have doubts about paralleling the upper transistors since any difference in temperature would make a tremendous difference in current sharing. I would love to read advise about it, maybe a "base stopper" resistor? If paralleling is possible then maybe 4 Ohms loudspeakers would be very good as they would allow for 30W of output power with the opamp output swing.

Temperature compensation: The amp does not need temperature compensation of any kind and there is no way it can go into thermal runaway. Bias current goes lower as the programming transistors heat, but this is not much as long as they are far from the hot transistors (about 50 ma).

The design sounds very good. The only objection would be that it is not the fastest amplifier i've heard, but anyways it sounds very, very good, it has a sort of respect and politeness with the music that it's almost impossible to find in class B amps, while it is as clear as opamps can be.

I would love to read from anybody who builds it, mine does work. if you have any problems ask here. If somebody wants to develop a board is of course invited to do and all credit goes to him (orcad makes me want to dissapear). The sound of the amp is worth the effort*.

*I belive that the schematic is so simple that it has to have been posted before. All credit about how it sounds goes to national semiconductor.

I would add that i've found there is not much innovation on the forum, almost all threads refer to people having problems soldering a bunch of parts to a board. Nothing bad about helping people get good sound, but i would also love to read from people trying different things.
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File Type: pdf 5w to 15w chipamp.pdf (14.7 KB, 1574 views)
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Old 3rd June 2008, 01:54 PM   #2
eketehe is offline eketehe  Indonesia
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Sounds great..
I'll learn the schematic, n check if parts are available,hope will have the sensational sounds.

Brgds.
Eka
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Old 3rd June 2008, 02:16 PM   #3
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Quote:
Originally posted by eketeheSounds great..
I'll learn the schematic, n check if parts are available,hope will have the sensational sounds.

Brgds.
Eka
Nice to hear that you have interest on it. The opamp on the schematic has nothing to do with the opamps i reccomend, i put it here because it had the same pinout than all hi-fi dual opamps so i could try to develop a board. The output transistors are a "baseline choice", they are as common as the 2N3055 and you should not have any problems finding it. If you have a preference for other darlingtons or want to make a discrete darlington with couple of transistors and a couple of resistors you can of course do as long as they withstand the dissipation (better if 3A 40V is below the secondary breakdown region, otherwise you may get an amplifier that explodes when it is powered unloaded.). (*)

I would also suggest to place the opamp in a socket. Then you will have an easy time changing it and you can first try to run the amp with a very cheap one (TL072 will do) and then upgrade to the one you like the most.

(*) Forgot to say that the upper transistor dissipation goes 100.8W peak, 50.4 idle and average in the "No load" condition. The conclusion is that the amp should run unloaded only for offset checking (should be arround 10-40 mV for most opamps)!
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Old 3rd June 2008, 02:32 PM   #4
gpapag is offline gpapag  Greece
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Hi ionomolo

Nice that you provided the schematic.

I have 3 questions:

1. You draw the op. amp. power supply lines as +18V and -12V. Is it correct?

2. There are two constant current sinks in the schematic. Do you think that these are enough to drive lower than 6 Ohm speakers?

3. The dissipation you are quoting for the "bottom" transistors is for one CCS or for the two CCSs?

Regards
George
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Old 3rd June 2008, 03:06 PM   #5
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Quote:
Originally posted by gpapag
1. You draw the op. amp. power supply lines as +18V and -12V. Is it correct?
No. The -12V is a mistake, i'm feeding it +/- 12V from scientific power supplies because i'm not listening loud (Yeah, krell's way, dynamic adjust of class A bias, too bad it has to be done by hand).

Quote:
Originally posted by gpapag
2. There are two constant current sinks in the schematic. Do you think that these are enough to drive lower than 6 Ohm speakers?
It depends on how loud you plan to play it. if you want 10Vrms (10^2/6 = 16W) you need a current of 14.1/Zmin where Zmin is not 6 Ohms but rather the minimum impedance of your loudspeakers (btw the only manufacturer who openly quotes that is B&W, i'm not saying that others hide it but that it is not as openly exposed). Each sink takes something between 1.4 and 1.32 A depending on the temperature of the programming resistor. If it takes less then it means that the amp is overheating. The two current sinks in the design will give the minimum current needed for 16W in 6 Ohms. If loudspeakers dip a bit then it will clip.

Quote:
Originally posted by gpapag
3. The dissipation you are quoting for the "bottom" transistors is for one CCS or for the two CCSs?
The dissipation is for each transistor.

EDIT: Powering the ompamp from lower rails as suggested 17V vs 18.5 is not only to get the maximum power from it but also to prevent "terrible clipping".
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Old 3rd June 2008, 03:39 PM   #6
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I have fulminated terrible clipping, improoved a lot the noise performance and removed traces of unstability in square waves by adding a 100nF cap between ccs's base and V-.

EDIT: removed previous edit
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Old 3rd June 2008, 07:02 PM   #7
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I post revision 2 with some errors corrected. The caps C7 and C8 are imprescindible. It also corrects some minor mistakes in the original version.

The cap C12 makes the amplifier more stable but it might not be requiered. The idea is that decouples placed near the active parts should be small since these decouples are to kill high frequency garbage. If you place a big cap there it will load with sharp pulses due to rectifier spikes and will probably inductively couple a buzz and make the clean ground less clean. I have no buzz in my design so i let it there.
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Old 4th June 2008, 11:11 PM   #8
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Default help with opamp and transistor choice

Well, i found that adding the C7/C8 caps improoved the amplifier performace while trying random mods to make thething more stable, but credit to this comes to AndrewT who suggested me to add these much before i started to try that mods. Sorry for not having mentioned him early.

Since i have built that amp i get hooked to music much more than i used to do with my previous reference (LME49810 + MOS), but today i decided to do some A/B comparison and found that the LME49810 had some sort of special transparency that the Class-A one didn't have. The LME+MOS one is a fast cold amp, while this is a warm polite one, but there is something i like in having a female singer shouting at my ears.

Any idea on which opamp/output transistor could give a faster sound? I've been looking at spice simulations and it seems that the pole created by the gate stopper resistor and the cgs could stil allow the amplifier to be stable. Should i go mosfet or there are fast nice bjt output transistors there?

Could the burr-brown OPA627/637 be a good choice?
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Old 6th June 2008, 04:46 PM   #9
eketehe is offline eketehe  Indonesia
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Hi Ionomolo,
The idea to build this class A is coming since i already have pair of LM4562 in hand and wait for a good idea,

I just find all parts, and being noticed from a friend that class A amp is not for beginners.

well... how hard it will be?
what kind of PSU i will need for stereo ?
if TIP 142 is too late for LM4562, is LM833 or TL072 will suitable?

i am a newbie

need your honesty sir... will i able to do this?

its ok with all parts i have buy, i can make a BJT with its pair TIP147

Brgds.
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Old 6th June 2008, 07:46 PM   #10
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I was going to pm you to warn about some of the issues regarding class-A amps as i've noticed that your aren't among the oldest-registered members.

Well, the true begginers where those who built the first amps, they had nobody to ask! And they did build class A amps because they are the conceptually-easier.

About the opamp, i suggest to try with the TL072 and then replace it with the LM4562 once you know it won't explode. The output transistors aren't the best you can find, but they will do the job.

There are only four things you must know before buliding it:

The transformer must be able to supply continuous > 3 A for mono and > 6 A for stereo.

The heatsinks should be very big. Running the amp without heatsinks will immediately destroy output transistors and loudspeakers.

You cannot feed it higher than rated voltage. I suggest you to start with anything arround 12-14 Vdc and a single ccs (you will notice that there are two identical sub-circuits connected to the speaker output. Start building it with a single one). This will allow for 6-8W of output power, will require a much less expensive supply (1.5/2A will do) and will let you know if you want to invest in the big heatsinks and transformers requiered for maximum output.

I suggest you to post or send me by pm a picture of the transformers and heatsinks you plan to use.

The amp does not have an incredible amount of parts so it should be an easy build (it took 30 minutes for me to do a mono one with all parts over the bench).

The amp is not very powerful so you need to use efficient speakers and not listen to outrageous levels.

At the moment i'm trying to design a faster output stage (either bjt ccs-loaded sziklai or mosfet ccs-loaded sziklai) but this will be a hell to get stable, the "slowness" of the output transistors plays for your part if you are new to this since it will not requiere the extreme care with layout that faster parts need.

I belive you will like it because it's a very "politically correct" amp. Nothing prominent, warm, a bit of "tubey midrange" and low distortion, which means it's not adding so much of its own to make anything not liking its character hate it.

The picture attached is from the first prototype i did (6W powered from +/- 12V with a single ccs.). I belive you shouldn't have any problems building it.

Somebody has done anything releated to ccs-loaded sziklai (replacing the usual 100R with a ccs arround a couple of mA's) *?

*This simulates to give a sub 0.01 % 1KHz THD open-loop close to the clipping voltage due to the immense amount of local feedback it has.
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