The mighty Denon AU-320 S.U.T. modification

GPJ

New Member
Not too long ago I purchased two Denon AU-320 S.U.T.'s. Curiosity always gets the better of me so I decided to take a peak inside as I know these are early 1970 devices. Sure enough, the foam had rotted into dirt on both. Not surprisingly, both were wired slightly differently. One was all stock wiring, the other was redone in a poor way with a big blob of white silicon preventing shorting around the 3 way switch. It looked ugly but the device worked fine.

The signals passing through both devices worked fine with my DL-103 and 103R, but I wanted to clean up the inside, redo the soldering and also the foam. While researching the AU-320 I found an old thread online where a user modified it to eliminate the switches, and turn the top set of RCA's into the 3ohm input and the bottom set into the 40ohm inputs by direct wiring. His photos however, didn't give me any clue about wiring.

I was liking that idea of shorter wire runs and a more direct signal path, however, after literally long hours of search I could not find any information regarding wire color of the transformers. Therefore, I decided the old fashion method of reverse engineering and documenting to find out how this wonderful transformer functioned.

A big part of the puzzle was the 3 way switch and how it routed the signals for each position. Once that was solved, I was able to determine how the wire colors needed to be configured to work exactly how it should but without any switches in the path.

When you modify the AU-320 as I present, you lose all functioning of the switches. The top RCA IN is L/R for 3 ohm function (1:36). The bottom RCA IN is L/R for 40 ohm function (1:10). It becomes an all manual device. One could replace the stock RCA cable, but I find it is a very adequately shielded cable. I did clean up the nearly 50 year old metal connectors on the whole device with Deoxit.

So far, I've only done one device and it works perfectly! It functions the exact same way except without the switching. I could swear it sounds much better, but I'll leave my subjective commenting at that. (except that I do prefer 1:10 with 103R). I do like that I have new soldering and very clean innards.

I did this for nothing but the challenge, and ending up with a better device according to my needs. If you can live without the other functionality, you will have an awesome 1:10 or 1:36 S.U.T.. Prices are still very attractive on eBay. I just bought a Cinemag 3440AH S.U.T. and the newly modded Denon holds it's own. A truly marvelous S.U.T.. Enjoy the attached diagram with the correct color coding! Just wanted to share something and that's it .....

EDIT: CAUTION / DISCLAIMER .... because I literally don't know ....... I assume only one turntable can be plugged in at a time into the AU-320 after this mod. Please use only one set of L/R, RCA inputs when using the device as modified here to prevent potential damage to all related equipment.

AU-320 Mod.jpg
 
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The Denon AU-320 is very under rated I think because of it's age. The thing is, transformers don't wear out. Switch contacts can get dirty and RCA's can get corroded, that's about it.
Most, if not all of these need to be cleaned up with some Deoxit to perform at their best.
Your direct wiring scheme bypasses the switches entirely but as you noted you lose the convenience of the bypass switching and changing the loading on the fly.
The transformers used in the AU-320 have very good specs, bandwidth is 10Hz to 100KHz, down 1db at the extremes. A lot of the newer transformers can't get close to that. It's also triple shielded. I know my AU-320 in my system is dead quiet.
Your AU-320 should sound better than the 3440AH Cinemags as it's a much better transformer.
Your also correct in assuming that only one TT can be plugged in at a time now.
I haven't had my AU-320 apart yet due to some of the screw heads being stripped out. But it's on my to do list to clean up the switches. Do you have any good picks of the switches? Is it easy to get to the contacts?

BillWojo
 
Thanks for the great pictures of the switches. Now I know what to expect.
Inside of the transformer casing is lots of fine wire wrapped around a high nickle/iron core. For the 1:10 step up ratio there may be 100 turns of wire on the core that is hooked up to the cartridge. On the same core is another winding with say 1000 turns of wire, that coil gets hooked up to your preamp. A small voltage on the 100 turns side induces a magnetic field on the core and that magnetic field in turn excites the larger coil with 1000 turns. The voltage is stepped up by a factor of 10.
That coil with 1000 turns actually has a lead out wire on the 1000th turn but the coil continues until it has 3600 turns. This is how they get two different ratios out of the same transformer.
That grey can contains two completely separate transformer assembly's, most likely each is shielded in a can before being put in the big can. The cans use a special metal that can block stray magnetic fields called mu-metal, it's an alloy containing lots of nickle.
Hope that helps you out. For more info just google "how do transformers work", something should pop up.
It's no wonder these SUTS work so well on our Denon carts, they were designed by the same engineers that designed the carts. Very little in the way of compromises had to be made.

BillWojo
 
Dang, for a new member your really making some great post! There is little info on that AU-320 despite the fact that Denon made 10's of thousands of them. Ebay seems to always have a few for sale.
Now, years from now when someone is searching for info on one of these your post will be very helpful.

BillWojo
 
I bought one of these from buyee last year and only one transformer or one channel works.

I've had it apart several times and sprayed deoxit into the switch's several times to no avail, maybe I need to take the switches apart to properly clean them?
I've had numerous people say the t-formers never go bad?
 

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So this is what I found with the AU-320 i bought from a well known auction site. I have two turntables and had got fed up with changing the DIP switches on my phono stage, so this suited me very well for my D-103 and Adikt. But left channel not working. So I opened it up to see the big dodge of goo. Couldn't see any of he wires so I carefully dissected the goo away. It seems to me that neither of the signal leads are attached now. I have looked at the previous pictures and wiring diagrams and I think I just need to re-attache the signalled , to the correct place. Help!
 
Thanks for your help, I am going to have acarewful look at all your pictures and try and work out what is there and what should be there!
 
Last but not least, I decided to make a complete wiring diagram with the correct colors for the stock AU-320.

After two full days, I have had my fill of AU-320 technicalities ..... time to enjoy these great devices. Hope this helps current and future researchers! If the diagram is too small and colors are not showing right, I can email the larger diagram. Seems this forum doesn't like big images.

View attachment 1954759
Would you be kind enough to email me the diagram you have for the stock wiring of the AU-320. I am still trying to work out the exact wiring of my one?
 
Second thoughts, I think I have it now. Both signal lead cores are now disconnected and the violet lead is missing in action!
Thanks for all your help
Now a challenge for my soldering skills.
John
 
Wow, your brave but than again you have nothing to lose. Can you post some more pics of how these are mounted in the can, shielding, etc. Also a coin next to the transformer for scale as I'm sure it's very small.
I wish you the best of luck in your repair.

BillWojo
 
Will do Bill. The can is like a Russian doll. Outer can has rotton foam inside holding concentric inner cans with what appears to be an over complex grounding schedule.
The trans themselves again are wrapped in old Acoustic Research foam surrounds
I have continuity all the way up to the end of the green/orange wires. I'm guessing there is a mucky solder joint , old wire enamel polluting the joint. The owner reported that there was a little hf bleeding through. Beyond this I can't envisage a circumstance where there would be a fail in the coil windings.
If I get a win I'll eliminste the switches and might replace the output cable with my favourite quasi balanced wiring schhedule.
 
I would surmise that the inner cans are Mumetal . Denon have gone to great lengths ro protect the trans. from outside influences. The actual ground continuity wires are a bit clumsy by normal Japanese standards. The layers of the inner cans are spot soldered and the link follows through to the next layer and so on till you get to the black wire. Theres some special Japanese voodoo wrap around the inner can combo. Semi sticky almost like old fashiomed plumbing mastic tape.
Petrol is not budging it.
 
The picture with the sole transformer is the left transformer denoted by the black ground wire.

Left transformer wiring

Orange wire .... signal wire from transformer (to left RCA cable signal)
Green wire ..... wire from transformer to left RCA cable shield

White wire .... signal wire to transformer (3 ohm)
Red wire .... signal wire to transformer (40 ohm)
Yellow wire .... wire from transformer to left RCA cable shield

Black wire ...... to chassis ground lug

I think there is a little more to it than that. One more layer of rhe orange to peel
 
I am simply talking about where the wires attach. (not at the transformer end of it)

Such as, the green and yellow wires are directly soldered to the left RCA shield wire. The red, white, and orange wires are directly soldered to the 3 / /40 switch. The black wire directly soldered to the outer right RCA jack.

How it goes internal to the transformer is by watching you perform surgery ;)


Appo Loggies. If you have a close look at the macro of the green / orange you can clearly see at least two enamelled wires going into the solder joint. Any ideas.
 
If you look at the ground links around rhe switch its bonkers IMO they should have estabished a star ground inside rhe box and kept that away from the signal pair ( as in pro balanced ) until the exit from the box. ( ground lug )
Maybe they had residual hum issues and kept on adding ground pathways. Sure fire way to get loops. Maybe a humbucking coil wound with the OP signal coil :dunno:
Getting to where I need one of those magnifying work lights.
 
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