Need help understanding the purpose of resistors in DI box schematics.

[Sam0311]

Sorry I wasn’t sure where put this post, please let me know if you’d like me to move it.

Bit of a disclaimer, whilst I have an interest in electronics I have very little practical experience with it. It’s also been a while since I’ve read up on it so I’m a little bit rusty.

I want to try out an experiment where I try to push a transformer past it’s limits with my Eurorack to see how it sounds. Should be a relatively cheap and simple project but I’m struggling to find all of the information I need.

I was looking at one of Jensen’s schematics for a DI box. On the input side they have a resistor in series with the tip of the jack socket and one to ground if you include the AC coupling option.

Link to schematics

I want to better understand the function of these and how they interact with the rest of the circuit with a particular focus on impedance, and know how to choose resistor values for a transformer of my choice (the ones I’m looking at have a 10k input impedance).

I understand that the resistor in the AC coupling option acts as part of a high pass filter but wasn’t sure if the value of this should change if the value of the other resistor or transformer changes.

Cheers guys

 

[Sam0311]

I’ve been doing some thinking and I feel like something like this may work?

C1 and RV1 for AC coupling, RV1 also acting as input attenuator. 10k output impedance is a bit high for Eurorack but if I understand correctly with a standard 100k input impedance I’d only be losing about 1db of gain which isn’t the end of the world.

 

[My3gger]

When starting with diy i tried similar thing with 2ch active DI. Ch1 had OEP mic input transformer, ch2 another mic input transformer capable of taking a lot more signal than OEP before reaching saturation. While we could easily hear difference in sound such circuit aren't really useful for saturating transformers.
Proper line out driver with output transformer would give better results, maybe consider something like this as it isn't too expensive and you can find lots of technical information: CTX Transformer Console Colour

 

[Sam0311]

When starting with diy i tried similar thing with 2ch active DI. Ch1 had OEP mic input transformer, ch2 another mic input transformer capable of taking a lot more signal than OEP before reaching saturation. While we could easily hear difference in sound such circuit aren't really useful for saturating transformers.
Proper line out driver with output transformer would give better results, maybe consider something like this as it isn't too expensive and you can find lots of technical information: CTX Transformer Console Colour

Thanks for this, funny I was actually looking at OEP transformers. Would be interesting to know more of the specifics of your experiment if you can remember? What transformer were you using? How hot was the signal you were driving it with?

Do you think using a line driver is better because it hits the transformer harder or do you think there’s an amount of colour added by the IC/Transistors?

Also do you know if the resistor to ground in the schematic I posted can in affect override the input impedance of the transformer? It would open up a lot of options.

 

[My3gger]

We tried several sources like de-balanced line level, synth and others. For good explanation of DI boxes try searching this:
"On a Stage there's just XLR female connectors that connect to the Mic Preamps of the Live console"
I learned a lot from Bo Hansen's DI and modifications he made:
http://www.hansenaudio.se/techpage.htmLine drivers capable of driving transformers will hit them harder, discrete versions like 2520 also add some color. Now driving transformers into very audible distortion isn't that easy or sounding nice, you might want to check synth forums for other ideas giving better results.

 

[Sam0311]

Interesting cheers man. I tried out my experiment last night. It didn’t not work which was kind of exciting but you were right, the difference in sound wasn’t as drastic as I would have liked. Next step is to decide how I want to crank the signal going into the transformer.

I’m not too worried about it sounding ‘good’, I’m more doing this out of curiosity.

 

[Matt Syson]

There are so many 'conditionals' and a lot of misunderstanding about the use of transformers that it is near impossible to state what ANY set of conditions will achieve. the 221K resistor mentioned previously is likely just a 'bleed resistor' placed to ensure that any residual DC from the input 'source;' gets drained off. The article also mentions having the output pad and the presence of resistors with the aim of turning the following cable capacitance into a 'simple' low pass filter which will supress the natural resonance (peaking) caused by the transformer inductance and cable capacitance. another BIG unknown is the impedance and drive capability of the signal source. Does the driving ampilfier go into current limit protection when faced with a low impedance which may well happen at low fequencies when the transformer core gets towards saturation, where the impedance of the combined transformer and secondary load will 'tend towards' the resistance of the tansformer primary winding. All op amps will go into a harsh 'current limit' protection mode which is not designerd to 'sound' nice but merely prevent the chip being destroyed by overcurrent. A TL07XX family chip has a fairly soft' transition into protection, but then it isn't really suitable as an output driver chip anyway because they start to distort with anything less than about 3Kohms loading. As any audio circuit approached 'clipping' the negative feedback becomes a bit of a liability (although actually vital) because 'simple' circuits can become unpredictable. A bit like closing your eyes when driving faster than 100 Miles an hour perhaps?!
Any 'GOOD' output circuit design takes into consideration the LIKELY output conditions it can encounter, like cable capacitance for (say) 500 feet of typical cable. Load impedances of (say) 600 Ohms and so on so the designer can then declare his(or her) unit to be working to how they imagine and created it. 600 Ohm loading can 'embarass' a fair number of output circuits, particularly on cheap units either because the transistor/IC amplifier circuit or even the value of output capacitors used if they expect the frequency response to be 'flat' down to anything less than 50 or 60 Hz.

 

[merlin]

On the input side they have a resistor in series with the tip of the jack socket and one to ground if you include the AC coupling option.

Link to schematics

I want to better understand the function of these and how they interact with the rest of the circuit

It provides a charge/discharge path for the capacitor. It has no effect on the rest of the circuit provided it is suitably large, >100k should be fine with anything.