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Old 13th March 2009, 10:58 AM   #1
cedus is offline cedus  France
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Default Harman Kardon 730

Hi everyone,

I am more a tube guy (I built, repaired, and even designed a few of them).
Last week I had the opportunity to buy a old but almost like new HK 730.
I decided to test it on my second system with Buschhorn MK2, Philips CD101 and Thorens 125 + 3009 + DL110.
and...
I almost fell in love with them. It's not and by far the best thing I heard but I love the way it presents music.
I even decided to keep it in this system to replace a SE EL84 of my own design.

My question is apart from the usual thing (change the caps, the rectifiers, and perhaps the transistors) is there anything I could do to this amp to make it go even further, change in design better transistors etc.

The beauty of this one is it is a completely dual mono set up (the marketed it "Twin Powered")

I attach the Power Amp schematic for the one who knows. I have no real experience in solid state except reading all the articles at passdiy a few years ago.
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Old 13th March 2009, 12:43 PM   #2
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Default i love to work

with these circuits ....

things i would sure do
------ make sure that the two transistors in the input are perfect matched ( painfull job to do but finally brings results.
------after that a common heatsink to make the work in thermal junction is also good improvemnt

------recap everything without a question and given the oportunity replace all ceramics with something better styroflex or so
------recaping also means that 105 degrees caps are very welcome

------recaping also means that you may increase an ammount of capacitance in caps that are related to psu

------recaping also means that you may bypass the above with good quality 100nf caps

-----recaping also means that input capacitor is to be replaced with something a lot better than a simple electro

----- also after the upgrade of all caps with better quality your aim will be if possible to reduce the value of C409 and 410 from 220pf to something smaller after observing the performance of this cap in square wave >10khz the lowest is the best but something has to be there you cannot remove this totally
will requiere scope and signal generator .

-----next i could remove totally zobel and relocate it in the output with a seperated return to ground
( there is a very good possibilty that the inductor may be totally removed if normal cables of length of about 5 meters each and relativelly easy load is connected to the amp )

((((((removing the inductor in combination of flat type cables didnt work that perfect for me ..... but in my set up a removed inductor with cables that are twisted actually gave the best results from all listening tests ))))))

----and finally after replacing the idle trimmer to a quality multi turn, I would recomend to beef up the idle closer to a safety point
( means: use as much idle is possible but dont tost the amp ) temperature tests to be made and also this is related both with the load but also with the use IE casual listening or full power listening .....

of course all other small thingyies here and there apply like very carefull soldering and inspection , replacing all thermo paste in all heatsinked transitors , inspection of all insulators or replacing with nikasill and totall inspection of the "mechano" like screws connections and so on and on and on

there you go !!!! you got your shelf A WONDERFULL SZIKLAI !!!!!!

it is expected to perform easilly up to 30khz square and sine (sine even more ) and perform with very nice sonics sweet and warm sound that will make you fall in love with it
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SERVICE ΕΝΙΣΧΥΤΩΝ ΚΑΙ ΙΑΠΩΝΙΚΩΝ ΜΗΧΑΝΗΜΑΤΩΝ ΗΧΟΥ www.eastelectronics.gr
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Old 13th March 2009, 01:12 PM   #3
cedus is offline cedus  France
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Thanks a lot for that comprehensive answer.
Everything seems to make sense for me and I will follow that with caution.

One more question. Do you think replacing all the transistors could help? Or is that too much?

Here is the RIAA
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Old 13th March 2009, 01:26 PM   #4
cedus is offline cedus  France
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And here is the preamp.

Could I remove or lower values of some of the signal caps (for exemple there is a 100uF at the output of the preamp and another one at the input of the amp this was done I think because you can use preamp and power amp separately). Can I replace those with nice 4,7uF polyprop.
If not I will use blackgate NX
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Old 13th March 2009, 01:36 PM   #5
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Default well

i can tell you one thing .... been working with variations of this circuit for more than 2 years now .... been trying any transistor that you can imagine ( most of these modern transistor you can find gozillion of circuits in the forum ) but the best i ve ever listen too was the original japanese that i have in stock by accident ....

unless any of your transistor is faulty just let the stay there your money cannot buy anything better i would say

here is a link with some more genneral information

vintage amplifier repair/upgrade manual
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SERVICE ΕΝΙΣΧΥΤΩΝ ΚΑΙ ΙΑΠΩΝΙΚΩΝ ΜΗΧΑΝΗΜΑΤΩΝ ΗΧΟΥ www.eastelectronics.gr
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Old 17th March 2009, 09:02 AM   #6
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how are you doing ....

also i would like to have a picture of your amp ...if possible a close up from the area of where the pcb meets the power transistors

thanks
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