Alctron SD202 Passive DI "little" problem

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damien

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Hi,
I bought an Alctron SD202 Passive DI to check what I could get for 20$, it was also the perfect time to check the manufacturing quality of Alctron because I was thinking about purchasing their 1073 preamp.

They claim to deliver "warm, vintage-flavored direct sound from your bass, keyboard, or acoustic guitar." with 103dB s/n.


sd202 (2).jpg


I received it and was really disapointed because it was obviously not working.

I opened the box and here is what's inside :

PXL_20210924_182837368.jpg


As you can see the transformer, is broken and the leads cut off from the terminals.

alctron defect 05.jpg

That's a shame.

So I was thinking about removing it from the PCB, extend the enameled leads and solder it back to check what it's capable of.

And I thought that the enclosure, the PCB could be nice to put a "better" transformer on it.


What would you suggest?
1.Do you think I should forget about it?
2. repair it and leave it as it is working?
3. replace the transfo with something "better"?

Any experience on affordable balancing transformer to replace this one in the 10-40$ range? I checked on aliexpress but it's pretty hard to find.
The only available on the market are Jensen, Cinemag and so on for +100$. There's probably a more affordable brands?
What would you recommand?



Thanks a lot for your insight!
 
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abbey road d enfer

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They claim to deliver "warm, vintage-flavored direct sound from your bass, keyboard, or acoustic guitar." with 103dB s/n.
A ridiculous claim since no measurement conditions are produced.
What would you suggest?
1.Do you think I should forget about it?
2. repair it and leave it as it is working?
3. replace the transfo with something "better"?
I would suggest you start with option 2, for the sake of education; you may discover that this xfmr is actually brilliant and inexpensive (which is usually mutually exclusive). If you discover it's crappy, that would be a good warning to other potential buyers.
The latter is more likely, considering the specified input impedance of 15kohms, which may make it just adequate for synths and active instruments, but is utterly inadequate with passive instruments.
You may then proceed to option 3.
Very good DI xfmrs are usually expensive because they require winding and assembly techniques that are labour extensive.
Any experience on affordable balancing transformer to replace this one in the 10-40$ range?
Not really. You may check Sowter, but I wouldn't expect them to be significantly cheaper.
I checked on aliexpress but it's pretty hard to find.

The only available on the market are Jensen, Cinemag and so on for +100$. There's probably a more affordable brands?
I don't think there are any chinese DI xfmrs with the combination of knowledge, experience and manufacturing techniques capable of approaching the quality of the usual suspects.

But I don't know all the xfmrs in the world...
 
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Tubetec

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The Alctron MP73eq had caught my eye too , looks like good value for $299 .
There is a few who have upgraded the transformers to carnhill , swapping the op transformer would be easy , the input wont fit the pin config of the original ,so needs to be hardwired to the board .
Very interested to hear from anyone who has used one of these .
picture below of a unit fitted with carnhills
 

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damien

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@Tubetec Exactly! that MP73eq had good reviews, but when I saw how's my DI was neglected at the factory, I am hesitating a little bit to be honnest :) They are talking about it on gearspace.com Alctron MP73EQ - Gearspace.com


@abbey road d enfer I totally agree with you, I was just curious about what they could provide for such a low price, and I have my answers. I'll try to get it replaced or if refund I'll repair it, provide some measures to the world, and probably replace the transformer with a 12:1 from a reputable company for guitar and bass direct recordings.
The enclosure and the design are good though. the case is study and they put some RF filtering on the output ground path, there's a 10dB pad on the input, ground lift, and the PCB is thick and clean.

I heard about Crimson transformer, but they are hard to find, and also there's this little euro DI transformer from Monacor for 10$ :
It seems they put this very transformer on their StageLine Passive DI box and they have good reviews.
It's 47K to 600Ohms 10:1, so I see it more like a line level output transformer to feed a soundcard or a mixer. Is it going to be adequate to feed a microphone input though?
 
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Tubetec

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Id be inclined to think that DI box left the factory working , its possible it got dropped during shipping/delivery at some point . I agree though it doesnt say much for the quality when only the fragile wires hold the transformer down to the board . You could always use a little hot glue to stick it down for peace of mind.
 

damien

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There's a white plastic bracket that goes through the transformer under the windings and that holds the terminals. This part is actually broken. The amount of force to bend it backwards like this would have been huge. Knowing the packaging is absolutely perfect, I guess it's been forced in the enclosure on the wrong slots/rails and the transformer broke at the base during the assembly at the factory. Maybe I'm wrong though.
I'll rewire the leads to the PCB and hot glue the whole thing and I'll see what this thing can do.

There's 4 leads to the output winding, One to the ground, one to cold, one to hot, but the 4th one cut really close to the winding, I have no idea what it is meant for???
 

abbey road d enfer

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There's 4 leads to the output winding, One to the ground, one to cold, one to hot, but the 4th one cut really close to the winding, I have no idea what it is meant for???
There's several possibilities there, one of the most probable is the secondary being constituted of two halves, but in order to go forward, can you trace the schematic, how are these 4 leads connected to the rest of the circuit?
 

damien

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@abbey road d enfer Here are the pictures of the PCB.

As you can see it has TRS input and "thruput"
R1 = 1,1K
R2 = 1,1K
R3 = 1.1K

R4 = 5,6K (to "thruput" if -10dB pad on)
R5 = 5,6K (to input winding if -10dB pad on)

R7 = 100 Ohms to Hot XLR Output
R8 = 100 Ohms to Cold XLR Output


R9 = 51Ohms
C1 = 103J100 RC filtering to Gnd
alctron SD202 01.jpg

alctron SD202 02.jpg



It's a pretty classic diagram but this hanging lead from the output winding is really strange.

wire.png

What do you think about it?
 
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damien

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@abbey road d enfer this is very very possible. I don't know how other DI transformers are usually wired so I can't say anything about that.
Sorry I can't measure the resistance, the wires are enameled, I get "open loop" on every combination.
 

Whoops

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1.Do you think I should forget about it?

Why would you do that?
You already spent the money , it's an easy fix

2. repair it and leave it as it is working?

Yes just repair it, it's easy to fix, you have to be careful and gentle, do it slowly but easy in the end.

3. replace the transfo with something "better"?

You can replace the transformer for something better, but something better could cost almost the same as a good passive DI in the first place.
I don't know the ratio and specs of that transformer or circuit, but Jensen and Sowter are expensive. Cinemag a little cheaper than Jensen. Edcor and OEP are more affordable, but I don't know if those would be better than that transformer or not.
You also have Carnhill but I don't know if any of their transformers fit the bill.
Overall you have to add on top of the price of a replacement transformer the Shipping Fees and the Customs Fees
 

damien

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@abbey road d enfer Alright! So I removed the transformer from the PCB.
I removed the enamel like @Whoops advices and tinned the wires.

It says 20:1 to start with.

alctron DI transfo 03.jpg

alctron DI transfo 04.jpg

Then I checked the resistance:

input winding shows 105 Ohms

On the secondary here's what I could get :

wires are 1,2,3 and 4 from left to right.

between 1&2 = NUL
between 1&3 = NUL
between 1&4 = continuity

between 2&3 = continuity

between 3&4 = NUL


Wire 1 (the shortest) was probably soledered to the middle terminal (ground)
Wire 2 was also probably soldered to the middle terminal. (ground)
Wire 3 was soldered to the Output Cold terminal
Wire 4 (the longest) was soledered to the Output Hot terminal (100% sure)



Is that making any sense?

Can you please confirm that I should put the wire 1&2 together to the ground from what you see in the closeup of the windings?

Thanks a lot for your help, it's my first real practice on audio transformers ;)
 
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abbey road d enfer

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@abbey road d enfer
Wire 1 (the shortest) was probably soledered to the middle terminal (ground)
Wire 2 was also probably soldered to the middle terminal. (ground)
Wire 3 was soldered to the Output Cold terminal
Wire 4 (the longest) was soledered to the Output Hot terminal (100% sure)



Is that making any sense?
Yes; it seems that the center tap (the connection between 1 & 2) is grounded. It's a little unusual, but not problematic.
Can you please confirm that I should put the wire 1&2 together to the ground from what you see in the closeup of the windings?
You may experiment. First join them together, but leave them unconnected to the center pin. If it works like this, you may want to compare between unconnected and connected. There are pros and cons for each.
 

damien

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I extended the wires, and put some nail polish on them to create isolation.

alctron DI transfo 05.jpg

1&4 = 3 Ohms
2&3 = 3 Ohms

if I join 1 & 2 together I have 6 Ohms between 3&4.

I also checked the PCB and the center terminal is on its own, not grounded. So it seems you were totally right and 1&2 connects the 2 secondary windings together.

is it normal to have 100 Ohms on the primary and 6 Ohms on the secondary for a 20:1 ?

Thanks
 
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abbey road d enfer

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I extended the wires, and put some nail polish on them to create isolation.

View attachment 85092

1&4 = 3 Ohms
2&3 = 3 Ohms

if I join 1 & 2 together I have 6 Ohms between 3&4.
It all seems normal.
I also checked the PCB and the center terminal is on its own, not grounded. So it seems you were totally right and 1&2 connects the 2 secondary windings together.

is it normal to have 100 Ohms on the primary and 6 Ohms on the secondary for a 20:1 ?
The DCR ratio does not really reflect the impedance ratio, but usually, the higher Z winding also has a higher DC resistance.
Seems all kosher.
 

damien

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It all seems normal.

The DCR ratio does not really reflect the impedance ratio, but usually, the higher Z winding also has a higher DC resistance.
Seems all kosher.
Well I soldered everything properly and it's not working.
With the transformer on the PCB I still have 100 Ohms between the input poles, and 6 Ohms between the output ones.
Something is probably wrong in the connections of the 4 wires to the 3 terminals.
I have no signal at all on the output. The input loop is ok, there's something wrong on the transformer side. I'm a bit lost right now.
 
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damien

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Dear me from the past, 1&3 must be wired together.
2 to -
4 to +

I removed the transformer and tried all the combinations.
It seems to work right now on the bench so I will install everything properly and test it tonight.
 

damien

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So it's working as it should! Everything is fine and I put a big blob of hot glue around the transfo to keep it where it should be.

I really wanted to thank you all for your help.

Do you know the best way to measure the frequency response and THD with Room EQ Wizard for exemple while respecting the input & output impedances?
 

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