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

[Sp33ls]

Hey all,

I decided to pick up an AU-999 for a solid price in order to upgrade my system from my Marantz 2220b that I recapped and serviced myself (it's a sweet sounding little unit, too!).

This AU-999 is in great shape. It had one owner (he bought new), only has a few nicks on the edges, and the inside looks pristine. Even better, there's no noticeable noise from the speakers when I crank the volume all the way up, and the tone sounds great (it does sound a bit more bright on the top end than my Marantz, but that might be because of the subsonic filter -- I'll look into that later).

My first question is... what's the best tactic to connect to an active [Rythmic] subwoofer that only accepts RCA and XLR inputs (and doesn't have outputs)?

I have an external EQ that I've been playing with too. The tape 1 loop works just fine. But, obviously that's just a line out and doesn't have volume control for the sub.

So, my first question is... why does my amp still have sound output from 'System A' when I pull the pre/main jumpers? Is the selector switch bypassing the jumpers?

When I tried to use a Y-splitter out of pre-out and run that to the subwoofer, and the other back into Main in, I just get a subtle hum out of the subwoofer (as previously mentioned, speakers output regardless).

I'm looking to make the recommended mods for the unit anyways, so I'm okay with doing a DIY mod for creating an ouput for the sub.

I'm very excited to see this amp's potential!
(P.S. does anyone know where I can get a red light bulb indicator for the Tuner source indication??)

Thanks for the help!

 

[kevzep]

So, my first question is... why does my amp still have sound output from 'System A' when I pull the pre/main jumpers? Is the selector switch bypassing the jumpers?

Because that is the way the AU999 is wired, if you look at the speaker selector, it says "Speaker C Separated", so only in this position will the pre be separated from the main.

You could just use the pre out, and as long as you are on Speaker A, B, you are not required to have an input return from the Subwoofer.

Do the mods, they are very easy to do, i highly recommend doing the pre-amp gain mod as minimum...

The amplifier in stock form is very average...

 

[Sp33ls]

Hey Kev,

Thanks for the quick reply!

That's what I was hoping to do, but the weird part is that when I'm on "System A", the sub doesn't output. But, when I switch the System C, the sub outputs just fine. It's like an either/or.. is that normal?

Also, I am looking at the filter caps that people have selected to substitute in there.. they seem to range from 4700uF to 12000 uF. What's the rule of thumb on selecting a proper value? I'm assuming having a high ripple current is desirable for the caps in this position?

 

[Sp33ls]

Are Chem-cons the stock capacitors? The guy I bought it from had it for only 6 months (he is friends with original owner). He said all he did to it was deoxit and cleaned it up a bit, and isn't aware of any prior service.

Anything stick out to anyone? Is it all original or has it be re-capped ?

 

[Hyperion]

Looks original to me, normally the main filter capacitors are the last things to change as part of any refresh. The modification(s) Kev mentioned are excellent and really should be done to unlock the potential of this amplifier.

 

[Sp33ls]

Okay, great. I'm planning on doing all of the mods, in order to maximize the potential of the amp.
Here's the parts list:



One question I do have is... once the modifications are complete, does this amp (sonically) hold its own against the likes of modern units that are in the $2k-$3k range? And against old Mac stuff from its era? I know it's subjective, but I'm just curious. I'll be driving efficient speakers with it (97dB), so power isn't an issue.

 

[Hyperion]

Judging by the comments I read, the modifications put the AU-999 into a whole different league that's for sure. Especially Kev or anyone who has performed the mods, (having heard one 'before' and 'after'), can give you a better impression of just how good these mods are. There are many AU-999 owners who are forever in Kev's debt for the work he put in and the transformation in SQ these mods give.

 

[Hipocrates]

It seems or

One question I do have is... once the modifications are complete, does this amp (sonically) hold its own against the likes of modern units that are in the $2k-$3k range? And against old Mac stuff from its era?

Nope.
The mod will improve the unit for sure.

Compared againts MAC1700... I like more the 1700, (not fair to compare againts MX110z/MC30)

Newer 2/3k, Marantz PM15S2, MA6850

Still they are lovely units and speakers matching will have a big roll to play. I like how sounds with KEF R300 and JBL 4312.

 

[Sp33ls]

I appreciate the input! I'm already surprised at how great it sounds with 40+ year old components, so I'm excited to see how it performs after the upgrade.

Yeah, I suppose it's a case of you get what you pay for... I prioritize sound quality per dollar, build quality, and simplistic design. I do have an affinity for tubes (I've built several), but honestly, question if it's worth the high-maintenance. lol. So, I was in search of great sound for a great value, and it sounds like the AU-999 fits the bill nicely.

It's hard to "settle" in this hobby lol.

Nice speaker recommendations, Hipocrates. I'm running B&W 602 S2s right now, and looking forward to trying a pair of Zu Cubes (full-rangers with co-axial tweeter for high range). From what I've read online, it sounds like these units tend to do well with high-efficiency setups..? Fingers-crossed.

 

[Hipocrates]

B&W 602 S2s

Funny that you mention those speakers, those were my first speakers a while ago.

tend to do well with high-efficiency setups..?

To my ears, they sound better with ''laid back'' speakers. Efficiency will come to play if you listen loud and with demanding content.

I love the 999, I used to have 2, trade one for a pair of speakers and I regret it. The little 999's are keepers and I'm pretty sure that you will enjoy them greatly, specially in that condition.

One thing, Their rectifier is a little small, so I tend to upgrade that part on this fellows.10A 400V is what I use.

And don't settle, you don't need to

 

[kevzep]

Okay, great. I'm planning on doing all of the mods, in order to maximize the potential of the amp.
Here's the parts list:

View attachment 1227828

One question I do have is... once the modifications are complete, does this amp (sonically) hold its own against the likes of modern units that are in the $2k-$3k range? And against old Mac stuff from its era? I know it's subjective, but I'm just curious. I'll be driving efficient speakers with it (97dB), so power isn't an issue.

I had a MC2205 and a C32 at the time I did all these mods, and once complete, I thought it sounded better than that McIntosh combo....Obviously not as powerful though.
I dont have anything modern to compare it to, only my Main McIntosh system which clearly is in a different league.....

To note, you must have quite a late production model as there are no subsonic filters in your unit. Normally the caps are Grey Elna's looks like you have that slightly later series in black.

I also would not just refer to the manual for ordering parts as there were so many production changes along they way that weren't documented that you might miss something or order too much.
Look at what you have and make a list from your unit.

 

[Sp33ls]

Interesting. That's a good point. I was basing my list off of this post, using the manual as a reference for any discrepancies:

Would you still recommend the other mods (your pre-amp & bass-eq mod, Increased low end by Trnsfrmr, and Driver Board Mods by Stereofun)?

With newer revisions, would you expect only subtle changes like a different capacitance/resistance in a few areas, or is it common to observe a completely different topology with different transistors, additional components, etc?

I wonder if this is a clue as to why I don't hear anything from pre-out when I'm not on the "separated" channel?

 

[kevzep]

The most important mod to do on this amplifier is the preamp mod, there is a fundamental design error in the way they Bias that stage, and it simply doesn't work properly.
So this is something that needs to be corrected.
All the other mods, changing caps etc, is all a matter of personal taste. Although, getting rid of those green caps is not a bad idea, putting some nice stacked film in there might improve the linearity of its phase and frequency response.

The next most important one is the bass EQ mod, this circuit is very very subtle and if you want the bass to increase when you apply gain, you need to do this mod.

Really the only difference is the absence of the troublesome transistors and the subsonic filter.

I suspect and I have never tried it or looked into it, but I would have thought that when the Pre/Main is separated, it would have still had audio at the pre-out connections, but apparently not, you might have to run a Y lead, and use one part of the Y to connect back the AU999 power-amp, and then the other part of the Y to go to the sub.
See how that goes.

 

[Sp33ls]

Yep, it looks like mine was from 1972.. one of the latter units to be produced.

Adding images as a reference. Not surprisingly, I have the F-1228-1 board, which matches the manual I downloaded. But, I wonder how many differences I'll spot.

Yeah, I was surprised tho that "System A" still works even without the return signal to "Main in". And doing that created a "hum" in the subwoofer as well (no hum when it's on separated, though, works just fine). So, the jumpers appear to be useless..? haha

I'm also curious about the defeat setting... if I were to use an external EQ, would you recommend I just defeat the tone controls and do the adjustments solely from the EQ? And, would that render the pre-amp and tone mods useless? I'm still going to do them regardless, but just curious of where it bypasses at

 

[kevzep]

I'm also curious about the defeat setting... if I were to use an external EQ, would you recommend I just defeat the tone controls and do the adjustments solely from the EQ? And, would that render the pre-amp and tone mods useless? I'm still going to do them regardless, but just curious of where it bypasses at

You need to do the preamp gain mod, as explained earlier, there is a design error, and the pre-amp does not work properly, its got nothing to do with the EQ.

Yeah, I was surprised tho that "System A" still works even without the return signal to "Main in"

Again, I have explained this earlier, it is designed that way, the pre-amp is internally connected to the power-amp EXCEPT when on speaker system C. It is clearly indicated on the front panel. "Speaker System C Separated".

 

[Sp33ls]

Oh, yeah, that was a silly remark on my part. I meant to say that when System C is selected, I didn't hear anything from the power-amp, even with the jumper connected.

But, you know what? In my excitement of testing things out, I ignored the fact that there's a "System C" speaker out like an idiot lol. So, I'm sure it's all good. I've got it unplugged and on the desk ready to make mods once the parts arrive.

I was also initially surprised that my subwoofer was outputting much of anything at all, because I assumed that there would be a subsonic filter in there preventing it's frequency band ( crossover point currently set to about 65 Hz).

I'll just make sure to include that in my final listening report. Thanks, Kev!

 

[kevzep]

All good, and yes you'll need to use the Speaker C outputs.

The curve on the subsonic filter is fairly subtle, from memory its something like 6db/Oct so fairly shallow, so you will get that low end still getting though...

 

[stereofun]

Just checked your pics - I don't see a subsonic filter in there. Not all models had them. So no need to worry about removing. I never did an A/B when I removed my subsonic, but I saw a graphic presentation on it, and the bass gets robbed quite a bit - probably the reason they eventually got rid of it. What is your serial number ? I bet yours is one of the later models.

P.s I see in your cap-list that you have both outputs and drivers to be replaced by Onsemi MJ's - these are great replacements, but I sure wouldn't bother if you already got a complete, original and working set - alternatively you can rebuild it with the new sets and then as the amp is stable and "secure", put the old ones back in (just be quick on the bias as it might move a bit)

 

[kevzep]

I wouldn't bother replacing the outputs at all if they are original and working, I wouldn't even pull them and swap for others, its not necessary.

Leave the drivers (TO-66) on the chassis, just leave them as well..

Post #11 we established the Subsonic filter is not there, the unit is from 1972.

 

[kevzep]

Also use KSA992 instead of the ZTX795A.

Take the MJ21194 and the MJE15033 and 15032 off the list you don't need them