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Old 27th August 2009, 06:22 PM   #481
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Location: France
Even with a 12V 4A power supply ? ( monitor power supply)
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Old 27th August 2009, 06:52 PM   #482
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Even with a 12V 4A power supply ? ( monitor power supply)

I have a "CLAC" when I turn on the amp, do you have a solution ?

thanks
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Old 28th August 2009, 10:06 AM   #483
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Can I remove the diode after the self ?

thanks
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Old 28th August 2009, 03:11 PM   #484
DrLex is offline DrLex  Belgium
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Join Date: Aug 2009
Quote:
Originally Posted by jkeny View Post
I'm expecting a Lepai TA2020 in about a week & was invesitigating mods. Are there further mods in addition to Dr lex's ones? Can anyone give a link to the dc offset correction mod schematic? I have some 10uh toroid inductors which I intend to try - is this the correct value?
10uH is the correct value, if the toroids can handle 3A then they're right.

Here's the DC offset correction for one channel: http://www.diyaudio.com/forums/showp...&postcount=465
For the other channel, use pins 12, 13 instead of 9, 10.

Either bypassing the tone control section or correcting it is also highly recommended. As djnemesi and col already figured out, the tone control is a 'James' Baxandall variation. From simulations it appears the component values that are installed as well as the ones printed on the PCB, are both horrible. With both controls set to neutral, the frequency response is nowhere near flat, it has a huge bump in the midtones. And to top it off, the components are cheap and inaccurate.
The first attached image shows the responses for bass/treble controls set to 0, 2.5, 5 (flat), 7.5 and 10 for the values that are installed. The curves correspond reasonably to what I measured when taking some extra loss of bass into account. The second image shows curves for the printed values, not much better.

I would suggest the following replacement values (component numbers as on the webpage). This will give the much more balanced characteristics from the third image. As soon as I have the right components, I'm definitely going to do this fix:
R1 (installed 10k, marked 15k): 20k
R3 (installed 1k5, marked 1k): 2k2
R4 (installed 1k, marked 1k5): 10k
C1 (installed 10n, marked 472): 5.6n
C2 (installed 47n, marked 393): 56n
C3 (installed and marked 10n/103 near bass pot): 1n
C4 (installed and marked 10n/103 near treble pot): 10n

There is an additional resistor not in the original James schematic at the treble potmeter (installed somewhere around 300Ω, marked 100). What this one does, is for some reason preventing that the treble control can be set all the way to 0% or 100%. It can be safely replaced by wire links or by any other small value (the marked 100Ω is OK).

Use metal film resistors and your preferred type of film capacitors. Even for C4 which stays the same value you should remove the existing components and replace them by something of higher quality. You can also replace the potmeters although for my needs they are acceptable.
Attached Images
File Type: png James-lepai-installed.png (4.9 KB, 1181 views)
File Type: png James-lepai-printed.png (5.3 KB, 1114 views)
File Type: png James-Lex3.png (4.5 KB, 1080 views)
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Old 29th August 2009, 04:15 AM   #485
msj965 is offline msj965  United States
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Hello
Is there any specific watts for the resistor? what is the best one to buy.
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Old 29th August 2009, 05:11 AM   #486
col is offline col  Australia
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DrLex, well done thats awesome!

I will have another go at the tone controls and implement the DC offset circuit when I get the chance.

I also want to replace the volume control with a stepped attenuator that I got off Ebay, It is the same size so should be a drop in replacement.

http://cgi.ebay.com.au/DACT-Type-21-...d=p3911.c0.m14


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Old 31st August 2009, 03:31 AM   #487
msj965 is offline msj965  United States
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Default Hello Drlex:)

just a small Q's regarding the R & C that you put. As a newbie on elctronics do I need to change 2 pairs of the value that you mention?example R1=20k.there is one another beside of it, do i need to change that too? and One more thing on the input resistor between 2.2uf pin 9,10 do it really need to change it to 20k as marked on the pcb is 8.25 something?
Thanks,
Ann




Quote:
Originally Posted by DrLex View Post
10uH is the correct value, if the toroids can handle 3A then they're right.

Here's the DC offset correction for one channel: http://www.diyaudio.com/forums/showp...&postcount=465
For the other channel, use pins 12, 13 instead of 9, 10.

Either bypassing the tone control section or correcting it is also highly recommended. As djnemesi and col already figured out, the tone control is a 'James' Baxandall variation. From simulations it appears the component values that are installed as well as the ones printed on the PCB, are both horrible. With both controls set to neutral, the frequency response is nowhere near flat, it has a huge bump in the midtones. And to top it off, the components are cheap and inaccurate.
The first attached image shows the responses for bass/treble controls set to 0, 2.5, 5 (flat), 7.5 and 10 for the values that are installed. The curves correspond reasonably to what I measured when taking some extra loss of bass into account. The second image shows curves for the printed values, not much better.

I would suggest the following replacement values (component numbers as on the webpage). This will give the much more balanced characteristics from the third image. As soon as I have the right components, I'm definitely going to do this fix:
R1 (installed 10k, marked 15k): 20k
R3 (installed 1k5, marked 1k): 2k2
R4 (installed 1k, marked 1k5): 10k
C1 (installed 10n, marked 472): 5.6n
C2 (installed 47n, marked 393): 56n
C3 (installed and marked 10n/103 near bass pot): 1n
C4 (installed and marked 10n/103 near treble pot): 10n

There is an additional resistor not in the original James schematic at the treble potmeter (installed somewhere around 300Ω, marked 100). What this one does, is for some reason preventing that the treble control can be set all the way to 0% or 100%. It can be safely replaced by wire links or by any other small value (the marked 100Ω is OK).

Use metal film resistors and your preferred type of film capacitors. Even for C4 which stays the same value you should remove the existing components and replace them by something of higher quality. You can also replace the potmeters although for my needs they are acceptable.

Last edited by msj965; 31st August 2009 at 03:38 AM.
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Old 31st August 2009, 08:41 AM   #488
DrLex is offline DrLex  Belgium
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Join Date: Aug 2009
Quote:
Originally Posted by msj965 View Post
just a small Q's regarding the R & C that you put. As a newbie on elctronics do I need to change 2 pairs of the value that you mention?example R1=20k.there is one another beside of it, do i need to change that too? and One more thing on the input resistor between 2.2uf pin 9,10 do it really need to change it to 20k as marked on the pcb is 8.25 something?
Thanks,
Ann
Yes, of course there are two pairs for each value, after all it's stereo. I have borrowed one of col's photos to indicate the positions of all components. I haven't checked which exactly belong to the left and right channel, so the 'a' and 'b' components don't necessarily correspond. R7 is the extra resistor at the treble control. As you can see, the 8.25 resistor is not part of the tone circuit.
Attached Images
File Type: jpg lepai-james.jpg (56.1 KB, 1073 views)
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Old 31st August 2009, 12:09 PM   #489
msj965 is offline msj965  United States
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Join Date: Feb 2009
Default Thanks Drlex:)

Drlex,
Can we eliminate the pop noise on this amp?

Quote:
Originally Posted by DrLex View Post
Yes, of course there are two pairs for each value, after all it's stereo. I have borrowed one of col's photos to indicate the positions of all components. I haven't checked which exactly belong to the left and right channel, so the 'a' and 'b' components don't necessarily correspond. R7 is the extra resistor at the treble control. As you can see, the 8.25 resistor is not part of the tone circuit.

Last edited by msj965; 31st August 2009 at 12:22 PM.
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Old 31st August 2009, 01:40 PM   #490
jkeny is offline jkeny  Ireland
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Join Date: Feb 2007
Location: Dublin
Thanx Dr Lex,
Very comprehensive reply

Finally I'll search for the schematic of the Lepai so I can
- bypass the Tone controls
- wire up the mute circuit for pop control
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