A new proud owner of an AU-999 :) Subwoofer question.

I cant recall if someone here has put all the mods in the same place - there was one that came out just recently about matching the hfe of the input transistors on the driver boards and replacing the stock carbon resistors with metal film. That was the last one I did.

There was also one before that that involved replacing a couple of ceramic caps on the driver boards with polystyrene caps.

One thing to look for - if you are removing the driver boards be incredibly careful with extracting the round metal heatsink-mounted transistors from its spring-loaded metal socket. replacements of these can be a bugger to locate.

Before I did the mods on this amp I thought it sounded completely arse. Lifeless, flat, no stereo image and most importantly - NO BASS!

 

@ Slimecity I think you are referring to some driver board mods I did a write up on like 2 years ago. Those were mods already practiced by Leestereo, Trnsfrmr and others, so I tested them out, combined them and listened in between, then organized them into a step by step "how to". I did the driver board mods after Kevin's and definitely bumped up the amp a notch, even if some of them were subtle.

After Kevs pre-amp mod, I'd put the bass EQ mod 2nd since it brings out the fat side the 999 otherwise lacks, now greatly helped by the pre-amp mod.

Eternal kudos Kev

...... the 3rd best mod imo is balancing the differential pair. If they are already "close" you won't hear much of a difference, but since you have 10% 45 year old resistors in those critical positions not to mention the condition of the transistor pair, you will likely hear some imidiate and noticeable improvement in detail across the board.

In subsequent designs you'll see Sansui paying much more attention to this part, either by actual trimpots, tighter tolerance resistors or current mirrors. So while not a design flaw pr se, you can argue that the diff pair scheme in the 999 is quite crude with room for improvement. Now, this is something you can measure beforehand to see if it's worth the time. You want the currents through the pair to be in the vicinity of 1-2% of one another.

Like Kev says, strike the ztx795's and use KSA992's - less capacitance and easier to match as they come with very little hfe spread from the manufacturer (always a reassuring sign)

 

Okay, I ordered these KSA992's instead of the ZTX795A's.

Is this the write up you were referring to, stereofun?

I actually have a 300B SET that I put together, and thinking that I might end up using the AU-999 as a pre-amp part of the time and use it's own power amp the rest of the time I don't want to heat the house up for some background music

Just soaking up as much knowledge as I can

 

Yeah - that's the one

 

Yup - you got it Stereofun - sorry my memory is terrible.

i felt that this mod made a pretty noticeable difference in my unit too

 

OP, you may find this AU_999 thread relevant as is shows frequency response before and after filter removal and discussion of impact of the various mods (just one person's (my) point-of-view). There you will also see a bit of what Stereofun says about the KSAs and matching. Agree with Stereofuns comments and also agree with Kev on the KSA vs ZTX there.

 

Hey Smurfer, thanks for the comment!

That's a great write-up.

So, you still prefer the AU-777A over the hot-rodded AU-999, eh?

I'm wondering about that NFB mod... yet another mod to add? lol

 

It's really just a matter of taste after mods. After mods the 999 sounds like an 888... very very food but a different flavour to the 777 earlier traditionally late 60s warmer tone. They 999 does some other things better. Just own as many sansui as possible and enjoy the feast.

The feedback mod was huge for me. So far i havent heard feedback from anyone else trying it.

 

Essentially you just change those resistors and thus the ratio... you ended up at 47/2.2 = 21.4 which sounded the most akin to the 888, right?

So, you ended up with a lower feedback value, right? Does this mean that the volume pot at the same setting will sound louder afterwards? It'd actually be a perk for me to have to turn the knob more than stock due to my new speakers being very sensitive -- the other way around might not be quite as ideal (but the mod is still worth it if it tightens up the bass).

 

Also, I decided to listen to the phono section this evening. It definitely doesn't seem as "lively" as my Pro-ject Tube Box S. It seems a little... "reserved" or maybe "veiled" on the high end, and boring on the low-end..? It does pick up plenty of detail in the mid-range, tho. I do recall seeing someone mention that their vinyl playback improved drastically with the improvement of the amp itself thru the modifications.

Does the phono signal just go through the selected pickup resistance (30k, 50k, 100k), through a small filter circuit and straight into the preamp board? Just wondering if the phono improvements would scale proportionately with the all of the amp mods due to that being nearly 100% of its path. I'd be a happy camper if the mods resulted in vinyl playback sounding very "natural".