[lhowing884]

I've read a few EMU 0404 USB mods on the internet and decided to take on this modding project. Some guides were informative, others were not. There still seems to be some unanswered questions relating to what each component does and which ones to replace. I look the liberty to hook up my oscilloscope and poked around in a few places and did the hard work. Here is my end result: (sorry, I only took pics after the mods, so I do not have the before pics to point out which ones are to be replaced)
 
 
DAC decoupling capacitors
From the looks of it and my PCB analysis, the DAC has 4 outputs: Left -,+ and Right -,+. You need to replace the stock 47uf caps (x4) with some good ones of your choice. The other 3 around that area are supply caps (10uf I believe) and are used only for filtering. (Be cautioned: I used 1500 uf Rubycons only to find out DC-DC converter cannot handle the inrush current at turn-on. The unit kept on going in an endless power cycle, sometimes it works, most of the time it doesn't. I played around with the values and 470uf seems to be a good limit on all the supply caps.) Supplying the opamps is -+6V, so there must be some DC-DC conversion going on here. (Noise!) 
 

Four 220uf ELNA Cerafine audio de-coupling caps and rubycon 1600uf filter caps (later replaced with smaller values)
 

The remaining two G-Luxon caps are also used for filtering. They are 220uf, Replace them.
 
The Op Amps
I just chose the 2132 because I am familiar with them. You can almost choose any opamp you want, a majority of them will be a direct replacement with a few exceptions. As far as audio output goes, you only need to replace the headphone amp, (x1) gain stage, (x2) the output/buffer stage. (x2)
 
The signal path goes to the first pair of opamps to the right of the caps. Then it is split between the headphone and output stage. In other words, whether you are modding the headphone or direct line out, this first pair must be replaced. (Why not do all 5 while you're at it?)
 

Burr Brown OPA 2132U opamps. (need five)  
 

Output stage pair on the right and one gain stage opamp is showing on the left. There is one more right under the caps.
 

Lowered the supply caps to 470uf as 1500uf was excessive. I also soldered some silver wire to get some balanced XLR connectors out to my main system.
 
Power Supply
I used a LM7805 linear regulator to replace the wall-wart. You will need to use pass transistor to supply the current since most regulators are rated at 500mA. The one shown is rated at 1A and still could not power on the unit. The MOSFET mounted sideways is the pass transistor. I also had to use a small DC fan running at 7V to keep the heatsink cool. 
 

 

A nice extra supply of filter capacitors is always good. (But don't go too overboard) Use some snubbing capacitors (100nf is a good choice) on the diodes as well.
 
Sound? I will let you be the judge of that. Whats the fun in DIY if you know the results? I will say that putting in the larger caps gave some nice surprises in the low end. 

 
That is it for now. I will edit this as users post comments/questions. Time for bed.

 

[MrTom]

A fantastic write up

Sorry for the semi off topic question, but the snubbing ceramic caps on the rectifier diodes: what values and have you noticed any improvement? I'm thinking of doing the same on my power amp and am getting the information in line before hand

 

[lhowing884]

100nf (.1uf if you like micros) is good. I read in a book someone experimented with values and they found 100nf to be the best. It is also the most common. I never bother to check the waveform before the filter capacitors so I cannot comment. They are supposed to soak up the transients when the diodes switches on and off. I generally see this method employed even on $5k+ equipment and since the caps are very cheap, I do it to all my supplies.

 

[MrTom]

Well I can safely say that I'll be doing this next time the case comes off the amp. If I remember and have time, I'll scope it out...
 
Thank you!!

 

[Jeffhazlett]

Hi,

Loved your post and pictures. I modded my 404 with upgraded caps and dual mounted BB627s. It all sounds great but I knocked out my headphone section. It plays but at very low volumes and distorts like crazy if turned up. I've put in several different opamps I. The headphone position but I get the same results. I don't see any evidence of other components being damaged but I've clearly done something wrong. Any thought would be appreciated. Also on you power supply, I've used a lm7805 with heat sink but no transistor could you give me some detail on how to do that? I've used a 6volt battery with fabulous results, it last about 3 hours at a time.

Anyway thanks in advance for any help,

Jeff

 

[wakibaki]

Jeff

An LM7805 has a dropout voltage of ~2 volts, so you need to supply it with a minimum of 7 volts for it to work properly (regulate). All it's doing is reducing the voltage supply to <= ~4 volts, reducing as the battery voltage drops until the circuit simply stops working at all.

One of the problems with mods like this is that oversize caps are frequently fitted often at some distance from the board on extended leads. This causes a number of problems of varying severity. One is that the total track length between components is increased in an unpredictable manner. This can lead to crosstalk between tracks which didn't exist when the equipment was designed or design tested, leading to oscillation and other problems such as poorer SNR. Another problem is the introduction of series inductance, another possible cause of problems. Perhaps the most obvious and in some ways most serious is the vastly reduced mechanical reliability and increased likelihood of shorting, which can lead to partial failure or total failure of the entire device, which you have learned to your cost. Without the extensive test and diagnostics skills which you obviously do not possess, you will find it virtually impossible to restore the device to working order.

Your best bet is to tolerate the device's reduced functionality and not mess around with it any more. You can, of course buy another one, but learn the lesson of this experience, and do not attempt to modify it on the basis of an ill-considered thread you come across on a forum such as this.

The 0404 is an excellent performing device, designed to a very high standard. It's not a good idea to modify it even if you have extensive construction skills, because it's impossible to ascertain the effect of the modifications without access to sophisticated and expensive testgear.

w

 

[SebastianL]

Does anybody know the voltage rate of the oscillators/clocks? I’m considering exchanging these as I have a pair of spare NDK 3.3V already.