Harmon Kardon HD970:inside view

[georgehifi]

I've posted the I/V and discrete output stages here.

Oscillator in HK HD970 CD Player

Cheers George

[georgehifi]

Here is what Lars Viborg of LC AUDIO did to one, below is the pic, and he said it sounds stunning, even dacs at three times the RRP don't come close.
He instaled a LC Zapfilter MkII for the I/V and Output stage board with a Schoktty Shunt Regulated Powersupply board and Toroidal transformer.
And if you want to use it's own transport he put in a XO3 reclocking module and it's own XO Powersupply as well. Major surgery on this one.
I think just the output stage mods are enough and use it as a DAC only with a good transpot like Teac VRDS or alike. But then again a clocked internal transport like this could better the external clocked Teac VRDS?

Cheers George

[georgehifi]

Here is a pic of part of the output stage surgery.

Cheers George

[Extreme_Boky]

Just a suggestion,

The original analog stage uses simple bias solution to avoid a use of sound coupling / DC blocking capacitors. I personally never liked the sound of ZAP filter (too cold, washed-out / sterile). The original IC's are surface mount duals - perfect opportunity to try fast IC's from AD stable, or mellow sounding ones from BB. This will (again, from my experience) most definitely give better results than ZAP filter, for much less money!

Boky

[Leolabs]

Thanks for the suggestions.FET or bipolar OPAMP????

[georgehifi]

Originally posted by Leolabs
Thanks for the suggestions.FET or bipolar OPAMP????

Has to be kept as fet input on both opamps, (as this is a direct coupled circuit from dac output to rca outputs), as bipolar input opamps in general tend to have more dc offset at their outputs an can cause the dc servo circuit to go out of it's nulling range and not null out the dc.
If I were going to do opamp subbing, I would dearly love to use the AD825 smd, but it's a single and I don't know if there is such a thing as a 2 x single smd opamp to dual smd opamp convertor board? Maybe Browndog?
In my experience the AD825 is the gun opamp the OPA627 is good also but doesn't have the slam of the AD825, and then I found that the Zapfilter II has even more slam but can on bad cd's sound a bit cold, but really makes up for it with good recorded material, but it's also more expensive way to go.

Cheers George

[georgehifi]

Your in luck Browndog have them @ $6.95 each, you need 4 of them and 8 AD825 's

http://www.cimarrontechnology.com/in...PROD&ProdID=19

Cheers George

[Extreme_Boky]

Originally posted by Leolabs
Thanks for the suggestions. ET or bipolar OPAMP????

Supply voltage is 24V - so AD8066 (my favourite) should work nicely. This IC requires very clean noise-free supply with proper PCB ground topology.

The second IC is half "DC servo", half diff-to-single-end gain stage (for discreet output) - I would try AD826 here. Alternative: LM4562 (different sound, also quite nice...)

Boky

[BV]

No need(IMO..) for any upgrade.
Measurements for HD970 "as is". Jitter bellow 10ps!!
http://www.diyaudio.com/forums/showt...79#post1186279

[georgehifi]

Originally posted by BV
No need(IMO..) for any upgrade.
Measurements for HD970 "as is". Jitter bellow 10ps!!
http://www.diyaudio.com/forums/showt...79#post1186279

Yes it does, the sound standard is a little warm (OP275) and also lacks a dynamics, it also needs more bass slam, this is compared to two other very good dacs. But it can be heard that it's very extended smooth/clean and distortionless, and low in "jitter" if I can identify what jitter sounds like.

Saying this I believe it can be achieved, with subbing the OP275 (known to be softish) and the OPA2134 for either better opamps, like my personal favorite the AD825, or to do the radical transplant of a Zapfilter MkII. The latter is extremly dynamic (unbelieveably so) and can sound cold on some sterile recordings, but I belive it's just staying true to the source.

Cheers George