Kenwood KA-94 : power trafo specs ?

[ricothetroll]

Hi,
I have this amplifier that's quite in a bad shape (lots of bad contacts everywhere). My plan is to get rid of everything in it excepted the enclosure and power trafo and turn it into a diy amplifier.
I need to know the specs of the trafo in order to decide which DIY project is possible with it. It has 5 secondary wires on top that seem to fuel the main PCB with the output section and 3 wires that fuel another PSU board. The last one looks like a center tapped 15V but the other one is still mysterious, as the DMM say they are linked together...
Would anyone have any info about it ?
Thanx in advance !
Best regards.
Eric

[ricothetroll]

Hi, I was finally able to find the schematic here :
KENWOOD KA-94 SCH Service Manual free download, schematics, eeprom, repair info for electronics
With it I could find out that it's a CT secondary with a 52VAC and a 39VAC tap, that explained why I saw all 5 points connected.
What power amp project could I build with it ? I thought of ESP's 3A but I'm afraid that the voltage of my trafo is way too high... I could drop it with a regulator but that would be a huge waste of power, and the available current after the regulator might not be sufficient...
104VAC is also quite low for a decent tube project...
Best regards.
Eric

[Mooly]

If the transformer has 240 volt tappings and you are in a 110 v location....

[jaycee]

Too high for a P3A - but looks like a wellbuilt amp. Perhaps look at ostripper's designs, such as the "Honey Badger" amp. It should work well using the 39VAC taps to produce 55VDC supplies.

You could probably reuse the Kenwood's power transistors too. The outputs and drivers in that thing look good. Some pictures would help.

[ricothetroll]

Hi ! Thanx a lot for your answers !

Originally Posted by Mooly

If the transformer has 240 volt tappings and you are in a 110 v location....

I live in Belgium actually !
Where did you see that info ?

Too high for a P3A - but looks like a wellbuilt amp. Perhaps look at ostripper's designs, such as the "Honey Badger" amp. It should work well using the 39VAC taps to produce 55VDC supplies.

You could probably reuse the Kenwood's power transistors too. The outputs and drivers in that thing look good. Some pictures would help.

Thanx for your advice ! I'll take a look at this project !
I found this picture on the web :
http://domainedelaconque.free.fr/ebay/33280.JPG

You're right I could keep the o/p transistors, but I think I'm gonna get rid of the PCB because I keep losing one side of the amp and it can sometimes be cured by gently pushing/hitting the back of the PCB with a drumstick. I tried to resolder a lot of things but without success. Sometimes the side of the amp reanimates by turning the volume fastly up, but those days that trick doesn't work anymore...

If anyone had a clue about what could be wrong in the amp I'd be glad to hear it, just for my personnal knowledges because I'm actually enthusiastic about building a DIY amp !

I actually have another question : I'm planning to feed the amp with an RME Multiface soundcard and through a Mackie HUI, that works at professional +4dBu level. Will I still need a preamp ? I'm planning to implement a symetrical input, as the rest of my chain is.

I found another project on ESP's site, the MOSFET power amp :
Project 101 - High Power, High Fidelity MOSFET power amplifier
That one could run at +/- 56V too. How do you think this one compares to the "Honey Badger" ? What's the point of using MOSFETS instead of bipolar output transistors ? I guess "soft clipping" is not someting you want to hear about in HIFI applications

This is my first HIFI amp project, for now I mostly built studio stuff (mic pre, compressors...) and guitar stuff (loads of stompboxes and a tube amp coming soon), so please forgive my noob questions !

Best regards.

Eric

[Mooly]

Using any mains transformer on a lower primary voltage reduces the secondary voltage by the same ratio.

As to your fault. It could be nothing more than tarnished speaker relay contacts. The problem dissappearing as you increase the volume is a classic symptom.

Edit... whats the point of MOSFET's ? Many many reasons why designers like them. Rugged, cheap, easy to drive, extended hf response compared to bipolar. Lateral FET's are my preferred choice.

[jaycee]

Not really qualified to speak for certain on MOSFETs but laterals are a lot more expensive and harder to get than good audio power transistors I think. Rod's circuits are usually very good. Are you looking to buy a premade PCB or are you happy to make your own?

Preamp, probably not - just a volume pot, though I would still put a unity gain buffer in front of it so that the preceding stage doesn't see a changing impedance due to the volume control.

Hm, the picture has one pair of output transistors in a TO-3P package per channel. I guess they are using power supply sag to make sure their safe operating area isnt exceeded. I'd recommend two pairs of TO247/TO264 package devices per channel at least if you build your own amp in there.

[ricothetroll]

Originally Posted by Mooly

Using any mains transformer on a lower primary voltage reduces the secondary voltage by the same ratio.

In Belgium we're actually running on 230V

As to your fault. It could be nothing more than tarnished speaker relay contacts. The problem dissappearing as you increase the volume is a classic symptom.

Hmm you're right I might try to temporarly short the relay to see if that solves the problem ! Then I'd keep the amp as it is and maybe build another amp then later for fun
The relay is here to make sure no DC offset is present on the output, right ? I'll check the output signal with a scope/DMM before conecting a speaker.

Hm, the picture has one pair of output transistors in a TO-3P package per channel. I guess they are using power supply sag to make sure their safe operating area isnt exceeded. I'd recommend two pairs of TO247/TO264 package devices per channel at least if you build your own amp in there.

Do you mean that the output transistors are oversized compared to the PSU/heatsinks ?

Best regards.

[Mooly]

If the relay comes apart (a good many have a top that just pops off) then just pull some paper or card soaked in WD40 through the contacts to clean them.

Although the relay protects from DC offset etc it can also be used as a delay to prevent switch on and off noises that most amps make.

[ricothetroll]

Hi,
I changed the relay and now... it works perfectly ! I just resoldered a few cold joints here and there on the cinch input PCB, cleaned the pots and now the amp is working like new !
Finally this amp won't be the home for a new DIY amp but at least it gave me clues for an eventual project of that kind.
Thanx a lot for your advices and tricks ! If I had suspected the relay earlier I'd have repaired that amp years ago !
Best regards.
Eric