Where to get circuit diagrams for HK6550?


Well-Known Member
Sep 6, 2008
New Delhi, India
Hi Guys
As you are aware I have purchased a HK6550 that I want to mod to use with an external pre. Can someone help me?
Hi Anm,

If you google-search with "HK6550 circuits service manual" or something similar, many sites show up with what you want, but most of them would cost you some money (some are charging of the order of USD 20).

Here is a forum link Audioschema's op Audio Circuit Denmark

The first poster has a website where he has circuits of many equipments. Go to his site and follow the links. You will have to upload by using ftp. He has given the user-id and password for access. He is apparently providing this service free, however, he is requesting for USD 5 donation, which is optional, I guess.

Just try and see what happens.

cranky, you are absolutely correct. All statements.
I show it to Viren and we did figure out the boards are separate, and there was a point where the power board had a cable coming from pre board. It was marked LGR (left/ ground/ right) - so we guessed this is the one. However, wanted the service manual to be doubly sure.
I had the same thing in mind - drill a hole to get a cable out - one end of the cable soldered on to the board, and the other end having RCA plugs, that goes into the OUT of pre.

Cranky, can you download the pdf from the link given by Hemant. The point TPA2 on power PCB is the one I am talking about.

Another quick question - can the pre section of the HK kept connected? Or do I need to solder that one out? Though I think it should not be a problem - but drawing parallels from discussions of two power amps connected to a single speaker, the current can flow back to the other power amp even when it is off and can screw up some equipments. I really want the pre of HK to be still usable, and listen to that too from time to time.


Didn't Viren say he could do this for you?

The power amp section on HK amps is normally on a separate board in the middle of the amp. It should be a very simple matter to locate an entry point for the preamp signal, and drill two holes in the back of the chassis for the RCA jacks or a single one for an entry cable. I recommend the latter, with adequate strain relief. Can you remove the cover and take some pictures? I could help you identify the input points. a schematic would also help, but I don't have it for your amp.
I thought it would considerably increase the resale value :) Except an ugly looking hole on the chasis - I was thinking of drilling the hole on the right side instead of back - as this point was very close to right side, and it wouldn't need to pass over lots of boards and components - a very short and direct path out.

Actually I was thinking of being able to quickly a/b a solid state pre with tube pre just for fun. But if I have to desolder/ resolder I might not be able to switchover too quickly - unless I use a switch. Don't want to put in any more components in the path, but seems that's the only option.

Hello anant,
tried to reply ur pm but ur mailbox is full. Could you please create some space.
the cable from pre section (the one I understand to be THE cable) is soldered on 3 tall pins.

To use a switch, I would need to extend the cable from pre section upto the point where switch can be installed easily. then a cable back to power section. What kind of switch would be good. Would it degrade the sound quality?


TP generally means Test Point. I'm afraid my connection is very basic and downloading it may become an issue right now, but I could try in a day or two.

The cable is usually a single shielded cable, but I don't remember the connector, on the few HK manuals I have it is displayed as a 4-pin cable. Generally powering up the amp and disconnecting the cable, and using a signal generator will quickly point out the input of the amp. The input is normally labelled, so yes, LGR should be the right marking. IIC they are no hard soldered, but I don't know how it is on your amp.

You cannot connect two preamps unless they are in series or switched. This will mean an extra switch or jumpers like the higher end HKs. Series is useless, as it creates even more distortion by chaining the pres together. If you need to, then you'll need a switch on the back to disconnect and reconnect the HK pre. It should work out fine for you, but frankly I would rather you commit to one or the other. The soldering is simple enough, so if you feel compelled to 'go back' you could always do so.

I hope you are aware that this considerably affects the resale value of the HK.
sure will post the pic whenever I open it up next.
I like the idea of switching cables instead of a switch.

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