I have a question that might be really dumb, because I don’t know much about electronics.
But I’ve been told that the TL072 and LM6172 ICs are pin-compatible. So, if I have an audio Eurorack module with TL072s that cannot handle video signals, could I not simply replace those chips with LM6172s? I know the power consumption will increase. Any other potential problems or risks?
The rest of the circuit might still not be video friendly.
Also, the higher bandwidth 6172’s are more prone to oscillations and other high frequency phenomenon. - if i recall correctly they might even burn out if improperly decoupled.
Same with the rest of the circuit; your PSU might well handle the increased power draw, but depending on the number of amps replaced the internal power circuitry on the board might be a problem.
3 Likes
Thanks, Transistorcat, I knew it probably wasn’t gonna be that simple 
most audio circuits are decoupled the same as the video ones. just 2x 100nF caps
some audio circuits have high frequency filtering, which might be something you don’t want in video.
So for some circuits it would be an option to try this, you must know the circuit to know if it is possible.
Also , in my experience, video circuits are buffered on the in and outputs, audio circuits not so much, or sometimes.
What you can try is replace the lm6172’s with tl072: this will give you ‘smearing’ effects and edge softness, blurry stuff. Can be interesting! I tried it by accident when I bought fake LM6172’s on Ebay for the TFG
I might even design a circuit around this !
1 Like
+1 to transistorcat and reverselandfill’s answers.
I’ve cooked LM6172 where TL072s were entirely fine! For example, directly connecting the -IN and OUT pins to create a unity gain op-amp - with a LM6172 I end up with the chip oscillating wildly and getting very hot. A resistor is needed instead…
I’ve also noticed that many audio circuits use high value resistors (10K - 100K) in series with the input, whereas the LZX circuits generally prefer low values (499R, 1K). I’m not entirely sure why this is, but it may be relevant.
That said, if you don’t mind a potential bit of melted IC adventure/cost, go ahead and see what happens! Build it up on a breadboard so you can stick a finger on the chip to check if it’s hot, and see if it starts smoking (that’s what I do ).
1 Like
I know this is a bit of an old thread, but I wanted to share that I successfully swapped out the TL072s for their LM6172 counterparts on a couple Nonlinearcircuits DIY modules, namely the Genie and the Ming Rod (GENiE — Nonlinearcircuits & Ming Rod — Nonlinearcircuits). Both had successful results! I had to experiment a little with resistor values on ins and outs to find something that worked with my Vidiot, but generally the op-amps were okay with 1K on the input. The associated feedback resistors had to be changed as well to match the gains the circuits were designed for. There don’t happen to be any straight unity gain buffers in these two designs so there was no need to alter things more than that, except to cludge decoupling caps onto the power pins which seemed to clean things up. A word of warning though is that they don’t function as well for their intended purpose in the audio realm anymore; probably because there’s so much feedback built into the designs and the LMs are so sensitive. I bought the boards to experiment with and I’m happy relegating them to video, but they’d probably need some properly engineered filtering if they were to run the entire gamut. Still, just wanted to share – there’s some pretty bonkers stuff I’ve been able to coax out of these two, lots of weird wavefolding and mixing!
3 Likes
Very cool, I had considered doing this to a GENiE and a Let’s Splosh for ages but it’s been on the backburner. Thanks for trying it out and reporting back.
Do you have any video you can share of them processing oscillators or external video inputs?
Indeed I have some embarrassingly unprofessional phone videos! I’m a bit new in these parts so I will figure out how to upload and share them here. Maybe this will be a good excuse to get an actual Proper Thing with which to capture video so you don’t have to stare at my messy bench.
I imagine the Let’s Splosh would work well with this treatment too – it’s basically a bunch of Andrew’s signature difference rectifiers which are also included on the GENiE. Probably both could be optimised for video with regards to negative voltages and such, but for mostly just swapping chips and cludging caps this works pretty good 
2 Likes
In any electronics design, every connection – even the traces on the PCB – have some amount of resistance and capacitance. Whenever you combine a resistor with a capacitor, you have created a low pass filter, and that limits the highest frequencies that can pass in the circuit.
frequency cutoff = 1 / (2 * PI * resistance * capacitance)
http://sim.okawa-denshi.jp/en/CRlowkeisan.htm
When we limit high frequencies, we’re removing the fine details in a video signal.
So 100K resistor is going to have 1/100th the frequency limiting factor as a 1K resistor. So lower values make the circuit “go faster.”
There are a number of design concerns that become relevant when this happens, if you compare audio synths to video synths:
With 1K resistors, you start to interfere with the impedance of the IO connections (that’s why we recommend buffering all input connections before any low impedance connections.) That means you need more op amps and buffers in the circuit, than a classic audio synth design does.
Another side effect is that lower resistance increases current, and that’s why video circuits tend to require more power than audio circuits. This is also a reason why video circuits most often run on +/-5V analog rails rather than +/-12V or higher.
And the other new design factor becomes noise – a high frequency circuit contains more noise, generates more EMI and cross talk, than a lower frequency one. So you have to take a lot more care in PCB layout, avoiding long parallel traces, being careful to run transmission lines point-to-point in a circuitous route, etc. 4-layer and 6-layer boards help a lot here (a full coverage unbroken ground plane in an inner layer is the best thing you can do.)
Studying video application circuits is a good way to get a feel for general design rules.
8 Likes
I’d love to see some NLC video too! As an easy option, maybe share vids on the LZX Facebook group and link to them from here?
1 Like
Apologies on the carrot dangle, I’ve been trying to look at upload options – I don’t have any social media, but I think I’m going to hijack a friend’s account to document the modules. In the meantime, I swiped a converter from work to make some actual demos with so you don’t have to stare at my messy bench!
1 Like
Oh shoot I hadn’t considered that before, having mostly just dealt with making audio circuits where it’s not really an issue – that really blows my mind! I knew about video equipment expecting to see low resistance at the output but wasn’t sure why all the other resistor values in a given design also needed to be lower. Thanks for laying that out and for sharing your insights!
1 Like