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Old 18th August 2013, 05:26 PM   #1811
Junie is offline Junie  Philippines
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Hi Terry, I changed the value of the input cap from 1uf to 5uf (10uf in series) and the gain resistor, Martin and Bonfis covered the issue about gain which is high please read past posts and you can find it.
This amp is worth building you will never be disappointed .

Click the image to open in full size.

Cheers,
Junie
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Old 18th August 2013, 05:26 PM   #1812
bonfis is offline bonfis  United States
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Originally Posted by still4given View Post
Thanks guys. So are your transformers approximately 55-0-55Vac before rectification?

Thanks, Terry
56-0-56 for mine. Only change was to use 100v filter caps. Had made other changes previously as stated in earlier posts but they aren't relevant to the rails voltage issue.
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Old 18th August 2013, 06:19 PM   #1813
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Thanks Guys.

I will do a search of your names in this thread and see if I can find the necessary changes. There is a lot of "color commentary" to sift through otherwise. Hopefully I will be getting my boards pretty soon.

Thanks again. Terry
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Old 30th August 2013, 07:22 AM   #1814
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A question to those to have built a MkIII...

How do you match the output transistors??
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Old 30th August 2013, 07:33 PM   #1815
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Default Not needed to match Ranchu

Try to match the small ones at the input.

We use to buy several transistors and measure the gain..then we select the ones have lesser differences in gain... the gain i am refering too is the one you measure in cheap multimeters that has a socket for this kind of measurement.

PNP and NPN transistors, even being complementary, use to have different gain...usually PNP shows higher gain compared to the NPNs counterpart... at least this happened these last 50 years..i am not sure if they fixed that ... i do think the difference remain considering the ones i have at home.

The consequence of the used of unmatched parts appear mainly in the power amplifier output...there you gonna have some of set milivolts...this is not a real problem.... famous factories accept 25 milivolts as normal and standard.....in the real world, even 200 milivolts ( a huge off set) represents 10 miliwatts into 4 ohms speakers..this does not move speaker and does not produce thump when you connect the speakers and disconnect them from the amplifier.

Power on thump is due to the time needed to charge condensers in the input circuitry... there, when you switch power on, they start to charge and while charging the voltage appear variating in a ramp way..increasing fast...and due to that you see the result as some kind of unstability that pushes your woofer forward or backward...usually this does not harm unless you are using under rated speakers that cannot face this job.

Unmatched transistors, double differential and these kind of amplifiers (leach style and others), because of this difference in gain into NPN and PNP transistors, use to generate harmonics..... and harmonics generated are understood as distortion.... harmonic distortion...some guys loves the result..uncle charlie does not like...i do not use double differential circuits..due to that limitation to find matched NPN to PNP..something factories can do, but we, normal users, have troubles to order really matched counterparts.

Take some of your time to read our forum.... i mean.... the entire forum...since 2004.... several threads teaches us a lot..including this stuff i am repeating to you now...something i have published many times...also many guys did the same..... read the forum will make you an expert, because the good guys in audio are all united in this diyaudio forum..including the big shots...the great Doctors, the ones write books and work in important factories as circuit designers.

That's the reason why Dx amplifiers does not use double differential using complementary transistors...exactly because this difference in gain i avoid the tophology...Dx amplifiers builder does not suffer from this kind of problem.

Meanman is Dx Corporation too... authorized to distribute Dx MKIII pcboards.

regards,

Carlos
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Last edited by destroyer X; 30th August 2013 at 07:47 PM.
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Old 30th August 2013, 10:16 PM   #1816
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Thanks for the explanation Destroyer... I am slowing working my way through the forum posts but looking for the right information in 100s of pages of forum posts is like looking for a needle in a haystack.

I am not complaining because I'm enjoying it all.

Also I have bought some MKIII boards from Meanman in his latest group buy.
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Old 30th August 2013, 10:37 PM   #1817
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Default I have receive 2 complaints about these pcboards

well... 2 complaints is nothing or half of anything...but we have to listen everybody.

They say holes are not correct..there are some holes to insert component leads that are not fine...people have to open the hole diameter.

I would like you to inform this stuff in order to ask Meanman to fix, as he is the one is already producing these pcboards.

Maybe the guy is trying to use non standard parts....so...i am asking YOU... people that is assembling or have assembled to help US to fix the stuff.

regards,

Carlos
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Old 30th August 2013, 11:13 PM   #1818
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Quote:
Originally Posted by Ranchu32 View Post
A question to those to have built a MkIII...

How do you match the output transistors??
ESP has provided a nice article describing how to match transistors:

Matching Power and Driver Transistors
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Old 2nd September 2013, 01:53 PM   #1819
BMW850 is offline BMW850  Netherlands
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I have the 100R resistors soldered (changes from 22R to 100R) R23B R23A and the bias set to 25mv.
I also changes the 180pf to 100pF silver mica capacitors for C10 see schema.

My conclusion is the 180pF silver mica sound better than 100pf.
I continue not post further details about the sound, please let me know what you all find from this change.

Am very curious if anyone else would like to test this out.

Regards,
Rudy
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File Type: jpg 2012-12-21 schema Martin 180pf naar 100pf.jpg (207.7 KB, 179 views)
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Last edited by BMW850; 2nd September 2013 at 01:59 PM.
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Old 2nd September 2013, 02:21 PM   #1820
bonfis is offline bonfis  United States
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Quote:
Originally Posted by BMW850 View Post
I have the 100R resistors soldered (changes from 22R to 100R) R23B R23A and the bias set to 25mv.
I also changes the 180pf to 100pF silver mica capacitors for C10 see schema.

My conclusion is the 180pF silver mica sound better than 100pf.
I continue not post further details about the sound, please let me know what you all find from this change.

Am very curious if anyone else would like to test this out.

Regards,
Rudy
Hi Rudy-
I have tried 180, 120 and 100pf for C10. I can't hear any significant change in the amps sound. Have you listened to two amps with different C10 values at the same time? That's a comparison I haven't made. How would you describe the difference? Theory says the smaller value for C10 should give a higher slew rate but that may not be audible under normal circumstances. I haven't encountered any stability issues with the smaller cap which is the other potential risk of using it.
Steve
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