Modding the Harman Kardon 730 phono stage

[r887661]

This is an amazing thread and I applaud your skills and knowledge, masterful.

I have two HK 730, the pretty one has no FM and the ugly one is fully functional. The out come of putting these two in a blender and then halving the bits will result in me being left over with an ugly one with no FM...I want to follow your lead with the phone stage mods. However, the FM does need restoring, Misfitz Audio in Grand Rapids gave it their best shot and could not find a fix....any ideas???

Thank you so much for your time,

Ian

 

[dlucy]

This is an amazing thread and I applaud your skills and knowledge, masterful.

I have two HK 730, the pretty one has no FM and the ugly one is fully functional. The out come of putting these two in a blender and then halving the bits will result in me being left over with an ugly one with no FM...I want to follow your lead with the phone stage mods. However, the FM does need restoring, Misfitz Audio in Grand Rapids gave it their best shot and could not find a fix....any ideas???

Thank you so much for your time,

Ian

I would recommend you start a new thread and let the experts give you options and specific recommendations.

 

[dyche01]

This is an amazing thread and I applaud your skills and knowledge, masterful.

I have two HK 730, the pretty one has no FM and the ugly one is fully functional. The out come of putting these two in a blender and then halving the bits will result in me being left over with an ugly one with no FM...I want to follow your lead with the phone stage mods. However, the FM does need restoring, Misfitz Audio in Grand Rapids gave it their best shot and could not find a fix....any ideas???

Thank you so much for your time,

Ian

I would recommend you start a new thread and let the experts give you options and specific recommendations.

I have never (successfully) messed with an FM tuner. You should follow Doug's advice and open up a new thread. You will get lots of good information.

 

[Pete B]

It does look like the 730 follows the IEC curve which seems to include a rumble
filter mainly for rim drive turntables. The -3dB point Re. standard RIAA looks to
be about 40 Hz if you bump the cap values by 10X then the new -3dB point should
be about 4 Hz. I do like the idea of a subsonic filter to remove record warp components
and I'd like to take a look at an 8 Hz - 3dB point. I'd do it in LTSpice since that's what
I'm used to using.
That filter with unity gain feedback from Q705/706 is a 2nd order Salen and Key type:
https://en.wikipedia.org/wiki/Sallen–Key_topology

There are some similarities to the Tomlinson Holman design in the Advent receiver
and the Apt phono preamps. I simulated the Advent filter here in the second post from
March 18th:
http://www.classicspeakerpages.net/...dvent-model-300-receiver-schematics-rebuild/&

A bit on the Holman papers:
https://www.diyaudio.com/forums/analog-line-level/48419-apt-holman-preamp-sound.html#post541106

 

[dyche01]

It does look like the 730 follows the IEC curve which seems to include a rumble
filter mainly for rim drive turntables. The -3dB point Re. standard RIAA looks to
be about 40 Hz if you bump the cap values by 10X then the new -3dB point should
be about 4 Hz. I do like the idea of a subsonic filter to remove record warp components
and I'd like to take a look at an 8 Hz - 3dB point. I'd do it in LTSpice since that's what
I'm used to using.
That filter with unity gain feedback from Q is a Salen and Key type:
https://en.wikipedia.org/wiki/Sallen–Key_topology

There are some similarities to the Tomlinson Holman design in the Advent receiver
and the Apt phono preamps. I simulated the Advent filter here in the second post from
March 18th:
http://www.classicspeakerpages.net/...dvent-model-300-receiver-schematics-rebuild/&

A bit on the Holman papers:
https://www.diyaudio.com/forums/analog-line-level/48419-apt-holman-preamp-sound.html#post541106

I think it is telling that the HK730 is the ONLY HK receiver to follow the IEC curve. The 630 and 930 both follow the standard RIAA curve. If I recall correctly, the HK730 came out around 1976, the year that IEC proposed their equalization curve. It seems like HK gave it one shot and then abandoned it.

 

[Pete B]

I agree that makes perfect sense.

I've always liked Harman Kardon gear since I scratch built a Citation 12 more or less
copy as a young kid.
I currently own a Citation 11, 17, and just bought an HK730 for a smaller system so I
will be looking into this in more detail.
Edit: Also have Heathkit AA-2500 which is an HK870.
One came up for sale locally at a good price, this thread and the fact that it looks easy to
work on made me go for it. It has not been worked on and works fine! I will recap and
mod it.

 

[oblomov]

Wonder about my A-402 integrated?

Supposed to be basically 730 without the tuner...

 

[dyche01]

I agree that makes perfect sense.

I've always liked Harman Kardon gear since I scratch built a Citation 12 more or less
copy as a young kid.
I currently own a Citation 11, 17, and just bought an HK730 for a smaller system so I
will be looking into this in more detail.
One came up for sale locally at a good price, this thread and the fact that it looks easy to
work on made me go for it. It has not been worked on and works fine! I will recap and
mod it.

I still love love love the HK730. As you say, it is easy to work on and it sounds great.

 

[Pete B]

Interesting that it has a CFP output stage with no current limiting protection other than
fuses. The Citation 12 is a Quasi EF output also with only fuses for protection. Both
rated to drive 4 ohm loads.

 

[Edzo]

Wonder about my A-402 integrated?

Supposed to be basically 730 without the tuner...

I wonder too now.......I like the amp (A-402) absolutely, for listening FM radio (tuner T-403) and good digital audio via Aux. (via Roland Rubix24 ) Lately bought a Telefunken S-800 pick-up, and started playing vinyl again. indeed the sound didn't really please me. I am not sure yet it's the phono-stage, or the record player which is disappointing. Did put a new needle and element in the Telefunken though. Own a 430 Twin Power too, didn't hook it up yet, because this wonderful little receiver resides and sings at my girl's house.
Few days ago I hooked everything temporarily up to my Akai AM-U55, and am short of time now to try. This afternoon had some half hour playing a record over it. Seemed nice, but this overlaps with this other thread of mine:

http://audiokarma.org/forums/index.php?threads/akai-am-u55-seems-to-love-my-speakers.844553/

So not sure yet where is A and where Z ........hope to have some more time soon to figure it all out.

 

[kylerhea]

I hope it's ok to add another entry to this well respected thread.

I have a question regarding the values and type of some of the caps in the signal path:

After some research and actual experience: what would be the optimum value for C715/716?
I have seen that some seem to use 0.1mF at this position.
Does it have to be as high as 0.33 or 0.47 to get a flat frequency response?

My second thought is about the 22mF input coupling caps C701/702: Is their value critical or could I use let's say a 10mF cap instead without losing anything (but rumble)?
Sorry I am not very experienced calculating corner frequencies etc..

Last question concerns the type of cap of C701/702: it is one of the very few blue caps in the original model. Are these special "low noise" types where a UKL is absolutely necessary or could they be replaced with another type of quality cap (such as Nichicon FG, ES, Elna Silmic etc.) without adding any audible noise?

Thanks,

Marc

 

[kylerhea]

Just read the HK402 restoration thread:

Leestereo calculated that the input caps C701/702 at 22mF would give a cut off frequency of 0.08Hz so that they could easily be replaced with a value as low as 1mF.

Why would Harman Kardon use a 22mF there...?

 

[Champco]

Resurrecting a 3-4 year old thread, it may be a while to get your answers but ya never know. It was a good thread thanks for bringing it up.

 

[rudie818]

any more testimonies on how this mod affects the sound? I’d like to try it and get the most out of the HK 730. Thanks

 

[Allison3]

This is a great thread! Does anyone know if HK used the same preamp board in other x30 receivers (330C, 430) and, maaaaaaaaaaaybe the Citation Receiver?

 

[Skywatcher]

This is a great thread! Does anyone know if HK used the same preamp board in other x30 receivers (330C, 430) and, maaaaaaaaaaaybe the Citation Receiver?

Just the 730 as far as I know.

 

[J English]

Amazing work. Wow. My hat's off.

 

[GaryVB]

I tried the mod but not getting any sound. I think it was because I didn’t “bridge the pads” on some of the components I replaced. Does anyone have a better picture of the bottom of the board? I want to test my theory. Then I’ll seek advice when that doesn’t fix it. Ah, I’ll take any advice now please.

 

[Skywatcher]

I tried the mod but not getting any sound. I think it was because I didn’t “bridge the pads” on some of the components I replaced. Does anyone have a better picture of the bottom of the board? I want to test my theory. Then I’ll seek advice when that doesn’t fix it. Ah, I’ll take any advice now please.

I never had to "bridge" any pads, unless there was damage.

 

[dlucy]

I tried the mod but not getting any sound. I think it was because I didn’t “bridge the pads” on some of the components I replaced. Does anyone have a better picture of the bottom of the board? I want to test my theory. Then I’ll seek advice when that doesn’t fix it. Ah, I’ll take any advice now please.

Those black tubes on the foil side are not bridges; rather they are resistors, still part of the circuits, and covered in a black non-conductive tube. They are necessary for proper operation of the board.

Here are my before & after photos.