HK 330C preamp out/amp in mod question

Searches have not revealed the information I seek, hence this thread.

I'd like to mod my 330C to seperate the preamp and amp sections via RCA jacks mounted on the rear of the chassis as is not uncommon on higher end receivers and integrateds.

I'd appreciate guidance from the AK community on this project, I'm not an electronics tech, but I can desolder/solder and follow instructions. I've recapped an amp, and done some cd player mods successfully. I think I can handle this even if it involves opening circuit board traces as long as I can get good, specific instructions.

My thanks to database contributors, we have the HK 330C Service Manual to work from here:
http://akdatabase.com/AKview/displayimage.php?album=18&pos=0

It appears that HK has the job essentially done for you already. Just separate the factory connections from the tone amp board to the power amp board. I suggest adding perhaps 100K to ground at the power amp inputs, just to bleed input coupling cap leakage away. It's possible that you will encounter problems with pre-out and pwr-in levels not being well matched to external components, but a rough calculation suggests that you'll be okay. Power amp input sensitivity should be around 500mV for full output.

I'm sure there are a number of 330c owners here that would like to see a nice picture how to of this modification.

BinaryMike said:

It appears that HK has the job essentially done for you already. Just separate the factory connections from the tone amp board to the power amp board. I suggest adding perhaps 100K to ground at the power amp inputs, just to bleed input coupling cap leakage away. It's possible that you will encounter problems with pre-out and pwr-in levels not being well matched to external components, but a rough calculation suggests that you'll be okay. Power amp input sensitivity should be around 500mV for full output.

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Howdy, Binary. Thanks for the response. I have questions, of course.
1. I see (not too well on my monitor) Lch & Rch in at the amp board. The terminal numbers are hard to read, however. Does Lch=LP406 and Rch=LP410?
2. Signal ground is bonded to chassis, but present input jacks are insulated from chassis and signal ground provided by jumper. Should I be looking for a single (star) connection point for all signal grounds?
3. Suggested 100K resistor from power amp input signal (RCA center pin) to RCA jack shield and jumper from there to the star point (if there is one)?

Mazeppa said:

1. I see (not too well on my monitor) Lch & Rch in at the amp board. The terminal numbers are hard to read, however. Does Lch=LP406 and Rch=LP410?
2. Signal ground is bonded to chassis, but present input jacks are insulated from chassis and signal ground provided by jumper. Should I be looking for a single (star) connection point for all signal grounds?
3. Suggested 100K resistor from power amp input signal (RCA center pin) to RCA jack shield and jumper from there to the star point (if there is one)?

Click to expand...

[1] Looks like LP412 (L) and LP406 (R) to me. Both terminals go directly to the + side of 47uF caps. The preamp outputs appear to be LP508 (L) and LP510 (R).

[2] I would run shielded cables to RCA jacks in the rear panel (floating), and ground the shields on each PCB ground track closest to the hot connections. Add something like .01uF ceramic caps from jack shields to chassis if you encounter RF interference problems.

[3] The bleeders can go anywhere that's convenient for installation.

BinaryMike said:

[1] Looks like LP412 (L) and LP406 (R) to me. Both terminals go directly to the + side of 47uF caps. The preamp outputs appear to be LP508 (L) and LP510 (R).

[2] I would run shielded cables to RCA jacks in the rear panel (floating), and ground the shields on each PCB ground track closest to the hot connections. Add something like .01uF ceramic caps from jack shields to chassis if you encounter RF interference problems.

[3] The bleeders can go anywhere that's convenient for installation.

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1. Finally figured out how to magnify the image and you are spot on, sir.

2. Sounds right, will be done so.

3. Check.

I'm about 60% done with replacement of all electrolytic cap.s in the unit. Preamp/amp jacks installation will be next. I will post results and pic.s when available.

Thanks again, Binary. I hope I have a chance to buy you a drink someday.

Completed a trial hook up of the 330c amp section to a pair of the existing tape out jacks per BinaryMike's suggestions less the filter caps. :thmbsp:

Giving it a listen using a Marantz 1060 pre section as I type, driving a pair of lightly modded Bose 201's. This is my background music at work system.

Ran it for about 6 hrs yesterday, and several hrs today. Sounds better than the 330c as a receiver and much better than the 1060 as an integrated. I'll wait until I try it in my main system before I get into a review w/the pics and all, but I will say this: I doubt a pair of Bose 201's ever sounded this good. I'd better turn it off, I keep stopping to listen. Then I can't keep the grin off of my face.

I'll attempt a what-I-did on my 330c as was requested. See also pic.s and descriptions at: http://s713.photobucket.com/albums/ww135/Mazeppa_photos/HK 330c mod/
I couldn't figure out how to get the pictures in the proper order, hopefully between the text below and the picture descriptions a usable explanation can be obtained.

A BinaryMike suggested, this was really pretty simple. Had to be for me to muddle through it successfully.

The only hiccup I had was getting Common to the RCA jacks properly. When I first checked between LP411 and LP 405 to common (also chassis) I found continuity. So I ran the coax jumpers expecting easy success. Wrong!

Power up was uneventful, but I found a whopping 26VDC offset at the speaker terminals! Power down, start checking, no continuity to common. This was my 1rst mistake. I checked for common at LP411 & LP405 before I removed the preamp/amp jumpers, common was provided from the preamp, no preamp/amp jumper, no common at 411 & 405.

I didn't have to look far to find common on the amp board, very handily located at LP410 & LP404. Jumpered LP411-LP410 and LP405-LP404 w/alligator clip jumpers. Power up ok, DC offset good, hook up in garage system and we're making music. Alright!

Power down, move coax shield from LP411 to LP410 and LP405 to LP404 to get common to RCAs. This was my second mistake, 26VDC offset is back!

Power down, solder jumpers where the alligator clippers were before. Viola! DC offset down, we're making music. Once I tried it in my main system at home, however, the 330c amp sounded kind of dull and flat. Installed the RFI suppression cap.s per BinaryMike and she's sounding as good as ever, better actually as I'm using it with my Emotiva USP-1 preamp. The difference is as much, well, different as being much "better" than the onboard pre.

Motive for this exercise is a notion to use the 330c amp to drive the mid/high drivers of my Quartets in a passive bi-amp scheme.

Please don't judge my workmanship yet, still in experimental stage. I'm still wondering how to get the Pre In jacks to work w/o having ground loop problems. Guess I'll just have to try it and see what happens. When I decide on the final configuration, Amp In/Pre Out, Amp In only, or return to original then I'll square away the wiring and markings.

When you're dressing the ends of shielded cables for soldering, it's important to leave plenty of length on the shield strands. If you don't, then you're likely to melt through the dielectric and create a short to the center conductor when you solder the shield connection. I suggest using a heat sink clip on the shield strands even if the length seems adequate -- some cable dielectrics are quite fragile.

Thanks for the tutorial. I think I'll put that on my list of projects.

BinaryMike said:

When you're dressing the ends of shielded cables for soldering, it's important to leave plenty of length on the shield strands. If you don't, then you're likely to melt through the dielectric and create a short to the center conductor when you solder the shield connection. I suggest using a heat sink clip on the shield strands even if the length seems adequate -- some cable dielectrics are quite fragile.

Click to expand...

Good tip, thanks.

Great little project. Thanks for sharing with us.

I am rehashing this old thread to ask a question?? I wish to do this mod on my unit as well, but I only want the preouts as I am not interested in using the unit for an amp but rather a preamp.... As far as I understand , I can run the other preamp side to RCA's without any need for resistors or a coupling cap??? That aside same as OP's method seems fine??

You could take preamp out from LP508 and LP510 as suggested above. It's generally safe to cut the power amp inputs loose and leave them floating if there's nothing connected to the power amp outputs and there are no internal failures. You're under no compulsion to do this, but preamp load reduction is almost always a good thing.

BinaryMike said:

You could take preamp out from LP508 and LP510 as suggested above. It's generally safe to cut the power amp inputs loose and leave them floating if there's nothing connected to the power amp outputs and there are no internal failures. You're under no compulsion to do this, but preamp load reduction is almost always a good thing.

Click to expand...

Ok, so basically all I really need to do is add some RCA jacks, or borrow one of the tape loops... , connect to RCA's with a coaxial wire from said points from earlier posts?? I don't need to worry about grounding the shields on this side??

Shields on preamp output jacks should not be grounded to the chassis, but to preamp circuit common instead. If you're re-purposing a tape in/out jack, then it's already grounded correctly.

OK! Thanks for the help!