Kenwood KA-405

Karl vd Berg

Member
2012-07-16 12:49 am
Hi all,

I'm doing some work on this amp and have a couple of questions:

[IMGDEAD]http://img839.imageshack.us/img839/8605/ka405.jpg[/IMGDEAD]

The picture below shows the pre-amp board. Are these components exclusively for the Phono section? If I don't use any turntable, then I leave it?

[IMGDEAD]http://img200.imageshack.us/img200/1497/p5060559.jpg[/IMGDEAD]

[IMGDEAD]http://img62.imageshack.us/img62/9427/ka405schem.jpg[/IMGDEAD]

Many thanks!
 
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Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
Everything within the dashed lines, including more that is excluded below your pic, is located on that PCB.

It includes the routing of all inputs, selector circuits and the ICs associated with a Mono microphone preamp and stereo MM phono preamp, as the labels indicate, even if the term "equalizer" is misleading. Phono preamps certainly apply RIAA equalization, as part of their primary role in amplification of the typically small, 5mV signal.

Unless you have reason to relace the Phono inputs, leave what you have intact as the amplifier has more value with them in original condition, should you decide to change to another model.

As an alternative approach, the seldom used microphone input could probably be easily replaced with a stereo 3.5 mm socket and wired to the existing stereo "mic mixing" control. You would disconnect the inputs (+ end) of C27,28 from the PCB and connect the inputs to that point. With a suitable stereo 3.5mm jack /3.5mm jack lead, this would allow front panel insertion of even an iPod or whatever portable device you may wish to amplify, complete with separate level control. I add the condition that the input impedance will be high relative to a speaker or headphone load yet this is usually ignored in common use. The present mic.input is mono so unfortunately, is not suitable to be adapted as it stands.
 

Karl vd Berg

Member
2012-07-16 12:49 am
Thanks Ian Finch and Sakis for the inputs. :)

I just would like to replace some components strictly related to the LINES IN (AUX, TUNER and TAPE IN), leaving the PHONO stage out of my mind.

The picture below shows the othe part of that board containing (I believe) the MIC Mixing circuitry. So no LINE IN components whatsoever? :confused:

[IMGDEAD]http://img189.imageshack.us/img189/4444/p5100580.jpg[/IMGDEAD]

The back for connections:

[IMGDEAD]http://img547.imageshack.us/img547/7716/p5100579.jpg[/IMGDEAD]

Thanks, guys! Really appreciate your help! ;)
 
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Karl ...you need to make a small study on the principal of operation of these line of amplifiers first .

All source selector volume balance etch are passive unless tone control is involved and also this will have unity gain .

That means a passive "preamp" will be the cleanest thing you have ever listened but will not have enough gain or guts in very low listening levels .That is generally the passive preamp behavior ... well not really in this case since 100K pot facing a 270K input will be just fine .

Add to this the slightly increased gain of the amp and you are done .

Actually there is no line in parts that can be upgraded ...The all amp though will work a lot better with fresh capacitors all over of better ratings and temp and there is places that you can upgrade Dc blocking caps from electrolytics to something better .

if you are a tone control enthusiast you may also play around the Opamp and try for a better one than a standard 4558 used there ( 5532 is a far better drop in ) play there don't go to any extremes . ( oscillation is waiting for you around the corner )

In the tone control area there is also room for upgrade in the capacitors while you also there don't go for extremes ...

Decoupling and plenty of bypass in the power supply will also make serious changes and that will also offer something more in the secondary power supply area together with some beefing up to any electrolytics there .

Removal of Speakers AB switch and the all wiring will improve crosstalk levels but then you may loose headphone Jack

Some things can be done also in the main power supply like capacitors on the AC side and why not IEC power supply plug with EMI filtering .

IF the picture is enough you may go ahead If not i may be create a complete replacement list with parts and numbers of what i think

Kind regards
Sakis
PS
The above guidance will cost you two plates of nasi bami shipped in Athens ASAP:D
 
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Karl vd Berg

Member
2012-07-16 12:49 am
The tone controls are almost certainly in the line input route and probably use standard opamps of the day. Might be worth looking there.

THANKS!!!!!!! :)

This is a picture from the tone control...

[IMGHTTPDEAD]http://img41.imageshack.us/img41/2566/p5100581.jpg[/IMGHTTPDEAD]

And this is close up of the board with the OPAMP in it...

[IMGHTTPDEAD]http://img13.imageshack.us/img13/1000/p5100582.jpg[/IMGHTTPDEAD]

As you can see it's a 4558DA opamp!

Now, I have right now in my stock-box a brand new OPA2134PA with 8-pin socket. Do you think it's worthy to replace the 4558DA with the OPA2134PA?? I wouldn't to expend further with another Opamp...

Many thanks! :)
 

Karl vd Berg

Member
2012-07-16 12:49 am
Actually there is no line in parts that can be upgraded ...The all amp though will work a lot better with fresh capacitors all over of better ratings and temp and there is places that you can upgrade Dc blocking caps from electrolytics to something better .

if you are a tone control enthusiast you may also play around the Opamp and try for a better one than a standard 4558 used there ( 5532 is a far better drop in ) play there don't go to any extremes . ( oscillation is waiting for you around the corner )

In the tone control area there is also room for upgrade in the capacitors while you also there don't go for extremes ...

Decoupling and plenty of bypass in the power supply will also make serious changes and that will also offer something more in the secondary power supply area together with some beefing up to any electrolytics there .

Removal of Speakers AB switch and the all wiring will improve crosstalk levels but then you may loose headphone Jack

Some things can be done also in the main power supply like capacitors on the AC side and why not IEC power supply plug with EMI filtering .

IF the picture is enough you may go ahead If not i may be create a complete replacement list with parts and numbers of what i think

Kind regards
Sakis
PS
The above guidance will cost you two plates of nasi bami shipped in Athens ASAP:D

Sakis,

That's what I'm doing: recapping the most critical parts of this amp!

I read your post now: VERY INTERESTING!

I'd sacrifice the headphone jack to improve crosstalk levels!

And also do some tweaks in the power supply as well! bellow is my list of new components I can buy in a shop close to home (no boutique caps, though!)

Code:
Power Board

1N4448 - 3pcs

220uF 63V 105°C (Nippon KME) - 3pcs
100uF 200V (Nippon SME) - 2pcs
100uF 100V 105°C (Nippon KME) - 2pcs
100uF 50V 105°C (Nippon KMF) - 2pcs
47uF 50V 105°C (Nippon KME) - 4pcs
22uF 100V 105°C (Nippon KMF) - 2pcs
10uF 100V 105°C (Nippon KMF) - 2pcs
2,2uF 50V 105°C (Nippon KME) - 2pcs

MKT 1uF 63V R5 Raster 5mm Philips - 2pcs
MKP10 (Wima) 100nF 400V - 2pcs
HQ 10nF 63V 1% (Polypropylene Philips KP) - 1pc

-------------------------------------------------------------------

Tone control Board

Meerlagen 100nF 5mm - 2pcs
MKT 220nF 63V Raster 5mm - 2pcs
47uF 100V 105°C (Nippon KME) - 6pcs
10uF 50V 105°C (Nippon SXE) - 4pcs
4,7uF 100V 105°C (Nippon KME) - 1pcs

MKT 1uF 63V R5 Raster 5mm Philips - 1pcs
 
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Karl vd Berg

Member
2012-07-16 12:49 am

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
Ok, I can get the OPA2604AP! :)

I have to put two ceramic caps of 100nF (0.1uF) seried to both rails, right?

As close as possible to pins 4 and 8, if I understood correctly:
http://www.diyaudio.com/forums/construction-tips/179762-how-connect-two-single-opamp-into-dual.html

Hmmm... well yes... and no...

The problem with rail decoupling is "know your enemy". Have you a problem with noise on the rails. Probably not. So what we want is a low impedance supply for the opamp.

I would connect one cap (and tbh probably a small electrolytic such as 10uf) in series with a low value resistor (around 1 to 6.8 ohm) and connect that combination across pins 4 and 8. The resistor actually raises the impedance slightly. but this can actually perform better in practice than just a cap alone which can sometimes provoke or cause unwanted "stability" for want of a better word, problems.

Any caps you add from rail to ground can inject noise from the rail into the ground and if that ground is a signal ground...

I know its splitting hairs and there are no hard and fast rules but that's probably the way I would do it.
 

Karl vd Berg

Member
2012-07-16 12:49 am
Hmmm... well yes... and no...

The problem with rail decoupling is "know your enemy". Have you a problem with noise on the rails. Probably not. So what we want is a low impedance supply for the opamp.

I would connect one cap (and tbh probably a small electrolytic such as 10uf) in series with a low value resistor (around 1 to 6.8 ohm) and connect that combination across pins 4 and 8. The resistor actually raises the impedance slightly. but this can actually perform better in practice than just a cap alone which can sometimes provoke or cause unwanted "stability" for want of a better word, problems.

Any caps you add from rail to ground can inject noise from the rail into the ground and if that ground is a signal ground...

I know its splitting hairs and there are no hard and fast rules but that's probably the way I would do it.
Ok, I'll probably do that, too: add a 3.9 ohm resistor seried a 10uF (25V??) across pins 4 and 8.

Do you know if a 1/2W resistor is enough for the task?
 
do that ...!! then relocate zobel directly on the binding post DO NOT skip it ...

As about OPamps listen to this

While i was repairing one Nakamichi ZXL 1000 ( if i remember correctly ) to keep things quiet ( other people also working in the shop ) i ve done some of the job with headphones I was very surprised to listen music from line in to the headphones where there was plenty of power distortion was low and sound was supreme .

Later on i thought that i should look up the schematic in which i was expecting some small discrete or Class A amplifier that has superb performance actually copy the schematic and make a product out of it and make money

It was only a JRC 4558....Do you want to know why the sound was superb ? CAUSE THESE PEOPLE LEARNED HOW TO OPERATE THE SPECIFIC OP AMP PROPERLY !!!

I suggested that going to extreme will cause more problems than joy astronomical speed and bandwidth OPamps need also proper compensation that probably doesnt exist in the specific circuit ....Oscillation is waiting for you around the corner .

Kind regards
Sakis