newbie - Kenwood 3020SE mains noise?

[jimh0612]

Hi I'm new to this game but here goes.
Have Kenwood 3020SE amp, sounds good but lot of noise, like mains hum, even with no source connected.
Have read various advice on earthing and am very confused.
Why only two core mains lead ie no earth?
(Original) two core mains lead cheap and nasty, so gonna change this, should I use shielded 3 core, connecting the internal yellow/green (earth) wire to the case for safety?
Read that the shield should be left disconnected at the amp end but connected to the earth pin of the mains plug.
Spent ages reading loads of threads and confused, like I say.

[halo0925]

Are you getting the noise/hum all the time signal in or not?
If you have humm constantly being an OEM amp I would think you have a bad diode in the power supply,capacitor, or regulator.

[jimh0612]

Thanks will check. Will also report on mains cable upgrade when done.

[pinkmouse]

I used to have one of those, they were nice amps.

I suspect it may not be an earthing problem, (unless someone has been fiddling around inside), rather the PSU caps aging and letting mains hum through. The amp is old enough, especially if it has been well used, to make a complete recap worthwhile.

[rja]

I have one of these amplifiers - they do indeed sound lovely!

I can't really offer any help, as I've been unable to find a service manual for the KA-3020SE. But I have recapped the power amplifier section - when it wasn't screwed down to the chassis(initial testing after swapping caps), the amount of noise passed down to the speakers was truly horrible. My advice would be to pull the case, and try tapping around the screws and the grounding point near the relays - the black wire that comes to the front of the chassis.

Failing that, it'll be time to start looking at replacing capacitors. I changed everything apart from the 10,000uF Elna PSU caps. Using a mixture of Rubycon ZL and ZA caps, and Panasonic FC caps - because the ZL and ZA range isn't terribly great for some values. I also bypassed a lot of them with 0.1uf polypropylene - the whole lot came to under £10, and is highly recommended.

Failing that, convert it into a Gainclone? You've got a nice PSU inside, can scavenge the 6A rectifier - and you've then got 35v+/-. A lovely passive pre-amp when the source direct feature is enabled, and a huge heatsink. Should make for a cheap experiment! I thought about doing this when the amplifier sounded totally awful after recapping - but after 20-30 minutes use, it sounded lovely again, so never investigated further.

--Rich

[jimh0612]

Hi Rich

Interesting ideas. To be honest the amp hasn't had much use due to young family/long work hours over the years so in theory everything should be in good condition.
Embarrassingly I have found the solution to my problem - I moved my old Marantz CD50SE away from the amp and hey presto! the hum disappeared......
So I bought an old CD63KI and modded it....bought a matching PM66Ki and modded it.... and now I realise that when I felt my amp was overbright and a little harsh on the headphones (where I do most of my listening late at night) I realise that the sound is probably due to the opamp in the headphone section
So I dont know whether to mod the 3020SE or go down the gainclone route like you say.
Any ideas how much improvement I could get?

[rja]

No idea. But I run a pair of Sennheiser HD480II-13R headphones from my 3020SE, and think it sounds lovely through them. It doesn't use any opamps, it takes feed directly from the speaker channels. Presumably, it'll just be taking it from the two NEC ICs used for the class A section of the amplifier, at low volumes.

It's a nice sounding amplifier, in my opinion. Should be quite easy to gut and turn into a DIY amplifier, if you really fancy - the preamp is entirely passive when source direct is enabled. Will just be a case of following the traces from the volume pot to the ribbon cable - I think it was 2 pins in the middle. Should be a doddle to do - but I don't know how the gainclones cope with driving a headphone load?

Or, if you wanted to be really flash, build it into a big headphone amp . Build a cmoy or Pimeta inside the 3020...

--Rich

[Ipanema]

Does anyone have the schematic of this popular amp? It would be interesting to know what make this amp sounds so good.

I'm also looking for the schematic Pioneer A400. Appreciate if someone could post the schematic as well. Thanks.

[Leolabs]

Check for dry solder joints too.

[rja]

Originally posted by Ipanema
Does anyone have the schematic of this popular amp? It would be interesting to know what make this amp sounds so good.

I don't, but from what I can figure out from looking over the amplifier, it works something like this:-

Passive pre-amp when tone-defeat enabled - literally from the RCA sockets, through a resistor, to the front-panel PCB selector switches, to the volume pot, to the power amplifier.. When tone controls enabled, it takes the same route, but also goes via the balance, bass and treble pots, and a nasty opamp. Can't remember what it is - probably an NJC4580.

Obviously, there is additional circuits for phono inputs. These are located on the input selector board. Two more NJC4580 opamps are used on these inputs.

For the power amp, I believe it works in two ways. There 2 small NEC class A chip-amps(1 per channel), but I can't remember the part number. This is used at lower power output, and is probably the only part of the amplifier I have heard! I'm sure I looked through the datasheet for the IC, and noted it had 50w output. I don't think this is the case in this amplifier, as there is a separate class-B section for when higher power is called for. There are two bias pots, which I assume are for controlling when the switchover takes place on each channel?

The SE had a beefier PSU, but there are no markings to suggest what its capabilities are. I seem to recall either 30v+/- or 35v+/- was available after rectification, and 60v beforehand? Anyway, from the regulator, there are two 10,000uF 42v Elna capacitors, followed by two 1000uF Elna capacitors. In fact, I tend to think that all electrolytics were Elna, but only 2 were from their audio ranges. I guess the nice sound just comes from a rather simple design, half-decent components, and glowing reviews...

All this may not be totally accurate, as it's from memory - and I don't want to start stripping my amplifier down at 4am .

Two sets of speaker outputs are provided, switched by 2 relays. I always thought this was a nice touch at the £200 price-point when new. I didn't have a lot to do with this amplifier when new, but I did work part-time in an audio-visual shop whilst at college. The KA-3020SE was very-much in the same league as the Marantz PM-44SE and the Technics SU-A600/900 IMO, if lacking a little outright power in comparison.

Some pictures of the original KA-3020 can be found here. Whilst somewhat different, it is largely the same

The 3020SE, PM-44SE, SU-A600, 900, and the Pioneer A400 were all the 'killer' amps of the late 1990's. From memory, all sounded very similar - each was extremely capable with all genres of music. I doubt any of them would disappoint most users - some will prefer the tonal characteristics of one, whilst others would prefer a different model. The differences were small IMO - you'd need a rather good pair of speakers to notice anything significant.

The Kenny is the cheapest to get now - mine was £21 + post on eBay. A400 and the Technics amps tend to fetch £60 to over £100. I didn't put much effort into finding a PM-44SE, as there seemed to be quite a few reports of faults.

ETA - I've had no training in electronics, so I might be talking complete tosh. The extent of my capabilities are a logical mind, some experience of soldering through my jobs - I repaired mobile phones for a bit, and spent several years as an IT engineer. Don't take anything I've said as gospel - I have no service manuals for the amp. But it should give a fair idea of how things might work, in the land of Rich...

--Rich