Avalon - U5 DI Preamp

Phil Smith · Post by **Phil Smith** » 23 Jul 2015, 20:31

Just quick snap shots of current progress..
Almost all traces on the main board, all top layer traces on tone and boost boards and my vision (

) of boost pcb bottom layer (the boost board is pretty simple)
The chip on the tone board (RN1) is just resistor network MDP16-03 10?G with 8 resistors (pin 1-9, 2-10, 3-11 ... 8-16). According to its datasheet ( http://www.vishay.com/docs/31511/31511.pdf ) "G" stands for 2% tolerance and 10? is resistance - but on the photos third digit/symbol is "4" as far I can see, but in the datasheet in should be R, K, M or 0.. BTW, for example, 104 makes sense - 10 to the power of 4 = 10 kOhm. Somebody please check me on this one out.

Regards,
Phil

okgb · Post by **okgb** » 23 Aug 2015, 19:43

Bumpdate any progress ?
I've never used one but the people who I know that own them , Like them
[ and it seems to be more than " I paid allot so it's gotta be great " ]

2weiund2wanzig · Post by **2weiund2wanzig** » 12 Sep 2015, 08:02

Are photos of the tone board bottom still needed? I could try to take one, just have to ask a friend if i am allowed to open his U5

Bottom is the side pointing to the back of the U5?

Phil Smith · Post by **Phil Smith** » 27 Jul 2016, 05:58

Hey everyone!
Sorry for delays.. what a busy year!..

Unfortunately, current progress is still the same, cause there's all I can draw with these pics of the guts..

2weiund2wanzig wrote:Are photos of the tone board bottom still needed? I could try to take one, just have to ask a friend if i am allowed to open his U5 Bottom is the side pointing to the back of the U5?

Yes, indeed!
I hope your friend still has one))
I'll send you a PM.
Actually there are several "unseen" sectors:
1) Obviously, a bottom (soldering) side of the tone board;
2) Nice photo of the motherboard's component side / soldering side of the front panel area (switches, LEDs, etc).

Cheers!

ilcaccillo · Post by **ilcaccillo** » 13 Sep 2016, 03:50

I've used this DI quite a few times, I was never really impressed by it's sound.
It's just too much circuitry for the goal it wants to achive, being a DI.

It sounds definitely worse than High quality DI's that cost less like the Aguilar Tube DI, Radial with Jensen transformer or the plain and simple EMO passive DI Box.

Arnostuck · Post by **Arnostuck** » 12 Apr 2018, 00:08

Hi,
Here's the list of the components seen (except resistors) on the pcb ASSY 5600-9520 produced in 2005 :

Capacitors :

-C1,2,3,4 : 2200uF 63v Nichicon CE 85° (LQ/M) LS=22mm
-C6,8,9,11,13,14,15,18,29,30,31,38,39,40,43,44,45,46 : 100uF 50v BC036 T8 LS=9mm
-C7,12 : 10uF 63v BC036 T8 LS=5mm
-C50,C51 : 10uF 63v BBC036 T4 LS=5mm (under pcb: C50 between R93 and Q24 // C51 between R90 and Q24)
-C52 : yellow tentale ? MEC 68J NPO

Can someone please help me to confirm the brand/model of the grey caps C43,44,45 100uF 50v ?

: U5 capacitors

At first sight, I'd say Vishay Cap BC036 from 2005 :
-Rated RMS ripple current at 100 Hz, 85 °C = 250mA
-Max. leakage current after 2 min at 50v = 33uA
-Max. impedance at 10 kHz and 20 °C = 0,7 ohm
-Useful life: 3000 h at +85 °C

I will perform a full recap, what would you advice as replacement : long life, low ESR, high ripple current ?
Panasonic FM ? The same Vishay but newer ?

Diodes :

-D1,2,3,4 : 1N4004
-D5,6,7,8,9,10,11,12,13,14,15,16,17,18 : 1N4148
-ZD1,3 : unreadable
-ZD2,4 : unreadable
-ZD5,6 : unreadable

OPAMPs & Tansistors :

-U1,2 : National LF412CN (code JR56RD)
-Q1 : O.N. TIP31CG + large heat sink
-Q2,3,9,13,17,21,24 : C546B
-Q4 : O.N. TIP32C + large heat sink
-Q5,6,7,8,12,16,20,25 : C556B
-Q10,14,18,22 : ST BD139
-Q11,15,19,23 : ST BD140

Special resistors :

-VR1 : trim Bourns W203 545C (Bourns part 3296)
-RN1 : Dale Resistors Network 104G MDP1603 (Dale part 0418)
-R100 : added under pcb 475R 1% 1/4w

Enjoy
Arno

Arnostuck · Post by **Arnostuck** » 12 Apr 2018, 16:16

Hi, small add-on about diodes and Opamp (ground is the reference point for measuring voltages) :

ZD1 and ZD3 are small size, same as 1N4148 but first branding is 1N7..., the next letters are on the dark side of the pcb.
ZD2,4,5,6 are 1N4744A and looks like Vishay diodes.

-ZD1 : 1N7.?.. (anode= gnd cathode=+6,22v)
-ZD3 : 1N7.?.. (anode= -6,30v cathode=gnd)
-ZD2 : 1N4744A 15v (anode=gnd cathode=+14,4v)
-ZD4 : 1N4744A 15v (anode=-14,88v cathode=gnd)
-ZD5 : 1N4744A 15v (anode=+17,3v cathode=+32,3v)
-ZD6 : 1N4744A 15v (anode=-32,3v cathode=-17,5v)

OPAMPs & Tansistors :

-U1 : National LF412CN (code JR56RD) alim +15v/-15v // polar. pin1=+7,5v pin7=+7,5v
-U2 : National LF412CN (code JR56RD) alim +15v/-15v // polar. pin1=-9,5v pin7=-8,8v

Enjoy

Alco34 · Post by **Alco34** » 04 May 2018, 18:50

Hello,

Beautiful topic !!!
The Avalon U5 is a best seller DI for Bass, very expansive, and there are no schematics on internet.
This topic is the only one on the subject of the inside box.

To undestand why the U5 is a good product, i decided to make reverse engineering on this unit, but I haven't any U5.
I use all the informations on this topic (very nice pictures of PCB and list of components).

I began to establish a schematic to try a DIY U5 DI and the principle is following:
The U5 uses 4 amplifier stages:
1st stage is the head amplifier with gain and HF limitation.
The passive tone shaping folows 1st stage.
2nd stage is to bufferize tone control and deliver headphone signal
3rd and 4th stages are the symetrical outputs (one for each polarity output with cross coupled feedback gain like DRV134 chip).
The input signal of 3rd and 4th stages is the headphone signal.
The microphone level is made from line output by resistor pad.

All the circuits on the right side is for power supply. The heatsinks are on power supply regulation made by discrete components.

Each amplifier stage is designed a like a power amplifier with a class A output stage, a voltage amplifier, a singleton input stage and a DC servo control by AOP (LF421) to perform DC coupling by removing any DC capacitors.

But there are 2 capacitors at the input of 1st and 2nd amplifier stage: this is not a real DC coupling amplifier (DC coupling only on symetrical drivers).

To finish the job,I need some help : anyone have the values of resistors? the values are sensitive and I can't find this from pictures because the color code is not easy for me (color vision mismatch).

Once validated by Spice simulation, I plain to post the scheme of the U5 to validate the design and help to explain the functions.

The Spice simulation will complete the PCB work, that is incomplete.

Regards

K.cohnen · Post by **K.cohnen** » 04 Jun 2020, 07:53

Hi everybody,
what is the latest status on this project? Is there a schematic to share? My U5 needs repair and a schematic would really help to find the failure.

Regards
Karl

peterc · Post by **peterc** » 08 Jun 2020, 08:17

Bump as well, would love to see this happen.
Thanks!

K.cohnen · Post by **K.cohnen** » 14 Jun 2020, 18:08

Maybe I can solve the open question in regards to ZD1/3. Since I can clearly read 53A on my ZD1, it must be a 1N753A. The datasheet says 6.2V, so it matches with the measurements.

K.cohnen · Post by **K.cohnen** » 15 Jun 2020, 17:09

This is the schematics of the very first part after the black transformer. I know it's not a lot but a first start. I am currently working on the next section where the +/-32V and other lower voltages are generated. Maybe someone can verify the +/-50V in another unit.

K.cohnen · Post by **K.cohnen** » 17 Jun 2020, 16:55

This is my approach on the 32V, 15V and 6.2V part.

Avalon U5 32V.pdf: (109.53 KiB) Downloaded 525 times

Component information (I have not taken the resistors out to verify the values):
R5, R10: 4.75k
R6, R11: 1k
R7, R12: 3.3k
R8, R13: 3.3k
R9, R15: 1.5k
R14: 18.2k
Q1: TIP31CG
Q4: TIP32CG
Q2,3: BC546B
Q5,6: BC556B
ZD1,3: 1N753A

Looking forward to comments / remarks.

K.cohnen · Post by **K.cohnen** » 17 Jun 2020, 20:20

One general note on the U5:
It seems to be recommended to replace all blue Vishay 100uf 50V 85°C electrolytic capacitors by a 63V equivalent with longer lifetime because they have the tendency to fail. The datasheet of Vishay states for endurance 2000h@85°C (useful life at 105°C = 750h). I have replaced all of them with Wuerth ones: 100uf 63V 105°C and an endurance of 7000h@105°C. I could have taken Panasonic ones that even have an endurance of 10,000h, but the distance between the pins is only 3.5mm. The Vishay and Wuerth capacitors have both 5mm. Nevertheless the Wuerth caps have a slighly bigger diameter, but still fit.

K.cohnen · Post by **K.cohnen** » 19 Jun 2020, 15:51

One final word from my side:
With the schematics from the power supply part I have been able to identify a broken transistor as the remaining problem. So in total I ended up with 4 broken Vishay caps and one broken transistor. Since my unit is working again I don't see a need to continue with creating further schematics. But I would like to take the opportunity to thank everybody who participated in this project! I would never got so far without the information on this post.

Madmax · Post by **Madmax** » 17 Jul 2021, 20:23

Hello,
I'm quite new here but not to the U5 nor to electronics

My U5 has suddenly quit working: the lights come on and show the correct positions of the switches, the unit has power, but no sound comes out. It must be one of the first gain stages that is broken, because not even the "signal"-LED lights up when playing. It must have broken when not in use (otherwise I would have noticed it. It's sitting in my rack, not being used a lot but ON most of the time...

Has anybody got an idea of which part this could be? Or has enybody traced the first few steps of the signal path? I'm vaguely thinking of that little single transistor with the tiny heatsink on it...

Any info would be greatly appreciated, thanks in advance!

Max

K.cohnen · Post by **K.cohnen** » 18 Jul 2021, 18:36

Hi Madmax,
I was told that in most cases the capacitors fail. So you could have a look if one of the blue caps look suspicious.

But I would also recommend to check the voltages mentioned in the drawing of my post from 17 Jun 2020. Could still be a power supply problem as it was in my case.

Cheers
Karl

Madmax · Post by **Madmax** » 03 Oct 2021, 20:06

Hi there,
thanks for the input! Haven't had much time to do things over the summer, now I'll give it another go...

How did you find the broken caps? Did you measure them in the circuit (which is often inaccurate depending on the rest of the circuit round a cap...) or solder them out anyway and check then?

I can sort of see the signal path running from bottom right through Q7 along to the left through Q8-11, then up towards the back of the machine... I'm losing the signal somewhere around C24, further along (in the "power amp"-section there's no signal at all, but the supply voltages seem to be OK (+-32V / +- 15 V / 6,2V)

Am I right about the general signal path?

Still haven't found a schematic, probably there just isn't one online (that would be new one on me with "vintage" equipment

)

Best regards, Max

K.cohnen · Post by **K.cohnen** » 03 Oct 2021, 21:15

Hi Max,
I swapped all caps proactively without even looking which one failed. I have a little measurement device that allows me to test them afterwards. They are still very cheap and replacing them is generally a good idea. But sometimes you can see that the top of the cap is blown up upfront.

If the power supply is not your problem you can also look at below website for a simplified schematic. Maybe that helps. Since my problem was gone after the power supply fix I did not study the schematics any further.
https://t-sap.com/index.php?option=com_ ... &Itemid=41

K.cohnen · Post by **K.cohnen** » 03 Oct 2021, 21:23

Last recommendation: You can put a test tone on the input. I wound recommend e.g. 1kHz sine. Then use an oscilloscope to see at which transistor the signal is still present and after which transistor is disappears. You can create the test tone on a computer DAW. And I bought a cheap 50 EUR oscilloscope from China which works fine.

Avalon - U5 DI Preamp [schematic]