Kenwood KA-405

[Karl vd Berg]

Hi all,

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



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?





Many thanks!

[Ian Finch]

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.

[sakis]

Ian many of those amplifiers called the phono stage as equalizer board ..It is correct but "in a way"

as about the mic mixing in most of these amplifiers i remove it completely since involves mixing of the music signal with something else

Nice amp if repaired properly can be really quick still power will be low ...

Kind regards
Sakis

[Karl vd Berg]

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?



The back for connections:



Thanks, guys! Really appreciate your help!

[Mooly]

The tone controls are almost certainly in the line input route and probably use standard opamps of the day. Might be worth looking there.

[sakis]

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

[Karl vd Berg]

Originally Posted by Mooly

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...



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



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]

{See below!}

[Karl vd Berg]

Originally Posted by sakis

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

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!)

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

[sakis]

Looks good ...