HK 670 integrated amp sim, mods etc

Sure! You just need to find C4, R4 & R5 on each board, that is where the modification is done.
-Jumper the LTP emitter resistors R4&5 (or replace with 1ohm resistors) change the compensation cap C4 to 5,6p or 6,8p. I prefer MICA, but NPO is ok too.
-Then you put a resistor and cap (I used poly cap) in series (C11/R35) and put them parallel with C4. This is the nested feedback. The values are a bit 'to taste' as I describe in post 5, 4 and 26. I used 0.68n & 860k. You could probably run some wires to the outside of the amp to play around with these values and listen to the result. The values do not effect the stability of the amp, just the amount and frequency response of the global feedback in the amp.

As mentioned earlier, I also adjusted bias and removed the muting circuit, but bias can be adjusted per manual to 30mV, and the muting circuit is not a significant change, I just wanted to do it :)

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Ok we will see otherwise I will contact you for this, as for bi-amping like some folks suggest on other forums what would you say about that?
I have never dealt with such things before and the more power the better the bass response etc.
I remember now that my Speaker 1 button/function suddenly stopped functioning 2 years ago so I use the speaker 2 since then, is there an easy fix for this?
 
Bi-amping can be good if you have a 'sweet-sounding' amp you want to use for mid/treble, and another amp that has good control for bass. High power is not always the same as good bass control IMO, high damping factor/low output impedance is more important for bass I think. Then there is also the matter of volume matching the amps, having a separate pre-amp so you can use the same volume control etc..
This mod should give you what you need in one amp.

I have no clue why one speaker output is dead. Could be button, relay or something else..
 
As long as this HK670 have the high damping factor and can manage low impedance I'd better stick to this instead of all the hassle w. bi-amping + the speakers RSIIIb are not originally made for this without internal mods/wiring, I never play music on very loud levels anyway***
 
I think it will be fine. You get higher damping factor in the bass with the mod, mid and treble is about the same as standard (depending on R35) as the original amp.
You could say the R35 resistor determines the sweetness vs detail in the mid/highs. Higher value = more detail, lower value=sweeter sound. The C11 capacitor value determines at what frequency the damping factor starts to increase, lower value, higher frequency. You should end up with values not too far from mine I think.
 
@ Rallyfinnen again, I might consider buying a Vincent SA-31 preamplifier with a Cambridge Azure 651w poweramp that delivers 100w @ 8 ohm and 150 w @ 4 ohm (for decent money) or is it no big difference vs the HK670 specwise would you say, except maybe some "obvious" refinements in soundstage + resolution?
 
I think you know more about the Cambridge amp than me :)
For me DIY is part of the fun, and to me it's more satisfying to improve something than to just buy something and use it. If you like the sound of the HK, and you have confidence to modify it, I think you should try modifying it.
 
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I'm looking at doing this on a HK670, so just wanted to confirm in summary:

Change R6/R7 from 180 ohm to 10 ohm, 1% 1/4W
Change C6 from 3 pF to 5.6 pF, 5% 50V
Add series 860k 1% 1/4W and 680 pF 5% 50V in parallel with C6.

The diagram and part numbers is a little different from what you had in your original post, so I'm going off the attached diagram.

Also - is Ceramic 50V ok for the capacitors? Using Mica and 500V is a lot more expensive.
 

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From my supplier:

@ 680PF the Mica's are $10 ea or a minimum order of 10 units for Foil. There's a 680PF axial general purpose film one I could try?

other Mica's are only available @4.7PF, 5PF or 6.8PF...what value would be best? I'm using an 820k resistor as they don't have 860k either.