HK 670 integrated amp sim, mods etc

It's the loop gain (not open loop), so it's the gain available for feedback. It's low in this amp, as it is in many of the old simple and good sounding amps. Normal in a lot of modern amps is +50dB, and that usually gives lower THD numbers.
Most of the stages here are used for current gain instead of voltage gain, and that gives it low feedback even if it has many transistors. This is why I liked the design.
I have found I liked the sound of amps with less global feedback (like JLH -69), ranging from maybe 20-30dB, I find the sound 'smoother', less forward/more depth, less fatiguing, with less global feedback. However, this usually comes with sloppy bass (bad control of the woofer), so with the nested feedback there can be a compromise between the two. The output impedance of the amp will vary with frequency, so you will se a slight variation in frequency response with load, but it's 0.1dB or so..
This also happens in the treble with a lot of modern amps, where loop gain decreases and distortion goes up, not to mention FR of class D amps without PFFB!
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Excess gain I think they also call it. Its a nice design. With some servicing and analysis for mods you get a great amp on the cheap. They toss away many older good amps nowadays. HK has a tendency to many bad caps when older, maybe the voltage specs used were tighter, as the brands were reputable. Unusually almost all caps had to be replaced in this highly regarded HK deck for instance.
Yes, that generally matches my impressions too, and that is generally how I like amps to sound in mids and treble, not too detailed and in my face. It might sound impressive first, but I get listening fatigue from it. I want a more relaxed/laid back sound for long listening sessions. I also agree with raising the volume to hear more of the details, this is what I generally tend to do with low loop gain amps, but my ears don't complain even if I listen a bit louder. (Neighbors or better half might though.. :) )
I'm very picky with bass, I like dynamics and control, not just some ill defined low end rumble. I listen to a lot of rock and metal, and the drums and bass has to be tight and precise. This is why this 'trick' suits me well. The .68n cap is on the smaller side, giving more punch in the upper bass, I might prefer a slightly higher value, (I went from 1n to 0.68n) but I think that depends on the speakers too.
It's all a matter of taste. For the same reason I prefer sealed speaker boxes over BR.

I can use the HK as a universal amp for testing any speaker, it will not have any issues with low impedance loads. The Alpha Nirvana class A is a step up in SQ, but is built specifically for 8ohm speakers.
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Interestingly, transparent amps utilizing feed forward error correction for extremely low THD+N, also fed from regulated robust & low noise SMPS PSU are chiefly noticeable for bass control, slam, and upper bass body with across the spectrum wealth of creamy detail, not etched. The way you would like it. At least as I subjectively perceive them. THX AAA tech as showcased in Benchmark AHB2 power amp is a prime example to listen to if you ever get a chance.
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I can use the HK as a universal amp for testing any speaker, it will not have any issues with low impedance loads. The Alpha Nirvana class A is a step up in SQ, but is built specifically for 8ohm speakers.
The HK has the full facilities commercial integrated amp's complex signal routing to overcome too. So far so good. Congratulations!
No benchmarks here, only bench supplies :) I'm not sure if listening in a store with unknown speakers and unknown acoustics would actually tell me anything about the sound of the amp. I would have to connect it at home I think. I did listen to a really expensive setup in a terrible listening room once (store), and it sounded horrible, but I think it was just the room.

Relating to PS I have noticed huge PS caps helping to improve bass on several amps. I tried to sim that, and could see something that might explain it.. Output impedance of low loop gain amps seems to increase when impedance of the PS caps increase. With a lot of loop gain, this was not an issue, but it might explain why big caps and low PS impedance seems to improve bass (subjectively) on simple amps like the JLH etc.
I could see the a little bit of same when I tried to sim this HK, but with the increased loop gain in bass, it was no longer visible.
I'm gonna answer here rather than in the other thread. So, about this "bass and capacitors" thing... First with a DC power supply where capacitors don't matter. Signal frequency is 60Hz. I'm feeding 2A peak AC current into the output, so the output voltage is due to the amp's nonlinear output impedance. This makes distortion a lot easier to see.


Then, activating the "transformer" and the rectifiers, at 50Hz. I'm using 30 signal periods, so we can see a complete beat of 50Hz signal versus 60Hz mains. Green is with 10.000µF supply caps, red with 1000µF which is clearly not enough. The harmonics plot in the bottom left looks a lot nastier!


So that supply cap makes non-harmonic distortion... I don't know if that's the cause of the bass feeling less solid, who knows.

Feeding the drivers from a cleaner supply makes no difference.
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Ah, now I see you wrote here too :) I already answered how I did my sims in your output stage thread: increased impedance DC-supplies to the PS caps, and injecting current (1A) to the output, and plotting the AC voltage of the output vs frequency. I also tried changing the loop gain of the amp, and it has a lot of influence on the output impedance. My guess is that it would influence the distortion too, with less loop gain there is less gain available to correct the supply ripple, so more ripple products get through to the output of the amp.
If you are referring to some instability in the sim, I mentioned before that the models for the transistors are not correct.
I did measure distortion before and after, and also basic checks of stability on a scope after the mod, and it looked good. I just used the sim to alter the compensation to have unity gain at the same frequency as before.
@ Rallyfinnen Hello I am new to this forum and read your modification for the HK670 did it make any big improvement and would you consider doing this for me for a decent price as I have this integrated amp myself since it was new, live outside Nora Örebro län, just curious ;-) Also is this amp ok for 4 ohm speakers like Infinity RSIIIb?
I have read something that some of those older Infinity RS speakers are a difficult load, so I can't make promises. However mine has been fine with every speaker I tried. If you are using it with those speakers now and find it a bit sloppy in the bass, the modification will definitely be an improvement and make the bass tighter.
About me modifying it, I could do it, but sending a heavy amp like that back and forth is a bit risky (I'm located abt 1hour north of Gothenburg). I would recommend finding somebody nearby who could do it. It's not very complicated, and I think the info is all here in the thread, and I could answer questions if needed. A hifi repair shop or DIY amp builder could do it for you.

If you're not able find somebody in your area, you can contact me via PM and we can discuss the details.