Alctron SD202 Passive DI "little" problem

[Whoops]

I'm happy that you were able to sort it out,
a lot of things can be learned from these small repairs.

 

[abbey road d enfer]

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?

Not the "best", but actually the only one: Insert a resistor of about 60 kohm between the soundcard output and the DI input. Load the DI output with a ca. 2k resistor and connect it to the soundcard input.
Assuming you know how to operate REW, use the Measure instrument. I suggest you use at least 96kHz SR if your card allows, and an upper frequency limit of 48kHz, so you may see evidence of the xfmr's resonant frequency. In terms of level, you should use about -10dBu.

 

[Whoops]

Not the "best", but actually the only one: Insert a resistor of about 60 kohm between the soundcard output and the DI input. Load the DI output with a ca. 2k resistor and connect it to the soundcard input.
Assuming you know how to operate REW, use the Measure instrument. I suggest you use at least 96kHz SR if your card allows, and an upper frequency limit of 48kHz, so you may see evidence of the xfmr's resonant frequency. In terms of level, you should use about -10dBu.

Thank you so much for the explanation Abbey

 

[damien]

@abbey road d enfer Thanks for your very detailed explanation! It makes total sense. I'll test it this weekend and share what I got Thanks

 

[damien]

Hi @abbey road d enfer, I setup my computer and REW as DIY RE explained in this youtube video . I don't have mic pres on my interface, just line Inputs, I use external ones. So do I have to run the test through the external mic preamp with the overall profile as calibration reference or just :
Soundcard Line Out-> 60K Resistor -> DI Line In -> DI Line Out -> 2K Resistor -> Soundcard Line In ?

PS : since the transformer is 20:1 The DI level output is really low to keep the specs "linear and low THD", it's a bit tricky.

 

[abbey road d enfer]

Hi @abbey road d enfer, I setup my computer and REW as DIY RE explained in this youtube video . I don't have mic pres on my interface, just line Inputs, I use external ones. So do I have to run the test through the external mic preamp with the overall profile as calibration reference or just :
Soundcard Line Out-> 60K Resistor -> DI Line In -> DI Line Out -> 2K Resistor -> Soundcard Line In ?

Seems good to me, as far as measuring impedance, which does not need to be accurate.

PS : since the transformer is 20:1 The DI level output is really low to keep the specs "linear and low THD", it's a bit tricky.

Yes, that's life... If you want to measure THD, you need a preamp.

 

[Whoops]

Hi So do I have to run the test through the external mic preamp with the overall profile as calibration reference or just :
Soundcard Line Out-> 60K Resistor -> DI Line In -> DI Line Out -> 2K Resistor -> Soundcard Line In ?

isn’t the DI output Balanced “Mic Level” out and not “Line Out”?

 

[JohnRoberts]

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?

Adding hot melt may be closing the barn door after the horses have left. A significant part of new product design is surviving abuse during shipping. Transformers are often the weakest link due to their relative mass and weak attachments. Attempting to secure them with just their wire leads is often a source of problems.

To improve the production design using a cheap transformer with no bracket, perhaps drilling holes in the PCB and using nylon cable ties. Cheap cable ties can perform poorly in sheer but are generally strong in tension. Perhaps two cable ties in an x configuration to deal with forces from different axis.

For normal use it will probably be OK as is..... as long as you don't drop kick it like shippers do.

JR

 

[RuudNL]

Do you have an idea about the input impedance at the instrument side?
A transformer with a ratio of 20:1 suggests an input impedance of 800 K.ohm with a load impedance of 2 K.ohm, which sounds reasonable. A lot of 'cheap' DI-boxes have a relative low input impedance, which is not good for many sources. (Usually I prefer an input impedance of 1 M.ohm or higher, in order not to load the input too much.)

 

[Whoops]

Do you have an idea about the input impedance at the instrument side?

Check the second post of this thread...

 

[damien]

isn’t the DI output Balanced “Mic Level” out and not “Line Out”?

My mistake! DI In , DI out (not DI line In and DI line Out)

 

[Whoops]

My mistake! DI In , DI out (not DI line In and DI line Out)

Im talking about what you wrote about the DI output and not the input

 

[damien]

Im talking about what you wrote about the DI output and not the input

I'll use the cleaner/neutral mic preamp I have which is the SPL Goldmike without the valve circuit engaged. OpAmps I have in mine are burr brown and they have very low distortion and a really flat response. It should be alright to level up the "mike level output" from the DI (thanks for the hint no resistors needed on the output ) to Line In levels of my soundcard. I'll post the results here . Thanks for your help. Obrigado.

Soundcard Line Out-> 60K Resistor -> DI In / DI Out -> SPL Mic Pre -> Soundcard Line In

 

[RuudNL]

RuudNL said:
Do you have an idea about the input impedance at the instrument side?

Check the second post of this thread...

Thanks! I overlooked it. 15 K would be a bit too low for most non-electronic instruments.

 

[damien]

I read this article Microphone preamp measurements: How to get the signal level right and this is a very good source of information about profiling DI and mic preamps with REW. Ant the thread has really valuable answers from someone from RME about the correct input/output impedence for measures. I threw it here maybe it can help others.

 

[12afael]

I had the exact same problem with a radial passive. I built a Bo Hansen DI in the box.

 

[damien]

This is what I captured of this DI via my SPL 9844 (set as calibration ref) :








So, to come back to the announced specs, 103dB SN : definitely not.
But 0.05% on the 2nd Harmo is close.
The rolloff in the high-end frequencies starts @14KHz and dips smoothly to -12dB @ 20KHz.

To be honest, I found it very decent for 20$. But with the 20:1 ratio and 15K impedence on the input it's not really meant for passive instruments as described in the specs, or you need a clean hi gain mic pre, the one I used is +72dB of gain max.

 

[damien]

@abbey road d enfer and @Whoops Can you please tell me what you think about how this DI performs compared to other higher quality passive DI according to my tests? thanks a lot for your insight

 

[Whoops]

@abbey road d enfer and @Whoops Can you please tell me what you think about how this DI performs compared to other higher quality passive DI according to my tests? thanks a lot for your insight

I have no idea, personally my performance tests come first of all from listening A/B comparisons.
Only then do I measure with REW, and then I also do comparative tests between different units

 

[damien]

I'll A/B it with my pod bass pro xt DI vs AlctronDI+SPL GoldMike.
Thanks!