Ross Compressor Circuit Analysis -- OTA-Based Pedal (Guitar Amplification and Effects)

[@_-Skeptic-_]

You know, I found your channel when I wanted to learn more about OTA.

[@willmcbride4435]

Neat. A clear and interesting analysis. Makes one want to build it. Thanks, Prof!

[@Lantertronics]

You are welcome! :)

[@nethoncho]

Awesome video. Thanks for the link to the Ross pedals and for sharing this.

[@joerectifier]

Excellent explanation….when you were explaining the “detector” and the discontinuity and turning transistors on and off based on how hard we are playing, I once again thanked God for parallel compression and the pedals with a Blend function. It just too invasive without the original signal - that’s probably subjective but it makes things tidy in my world

[@guitarmon5765]

Aaron is simplly great, still miss his class even graduated several years

[@ChromaticReflection]

Aaron, nice circuit walkthrough. The BJT detector and OTA feedback to make a compressor is a pretty cool circuit. Thanks for the awesome material.

[@Lantertronics]

Glad you enjoyed it!

[@edsavage6214]

Aweome lecture! Keep it up Aaron! It would be amazing, and also quite challenging to see the Mutron Octave Divider explained :)

[@RexxSchneider]

So, Vb = 940K * Ic/β + Vbe + 10K * Ie. Assuming β >> 1, we have Ic = Ie, and we know Vb = 3V and Vbe = 0.6V. That gives 2.4V = Ic * (940K/β + 10K). If β is about 200, then Ic = 160μA and so the dynamic base resistance is 26mV/160μA = 160Ω. The input impedance looking into the base of Q1 turns out to be around 160Ω x 200 = 32K, making the total ac input impedance of the circuit 40K at frequencies where the 10nF input capacitor has an impedance much less than 40K (the -3dB frequency is about 400Hz - that's considerable bass roll-off). [Edit:] A further analysis would be helpful, looking at the fequency response to see how much bass is attenuated, and to describe why that may be desirable in a compressor.

[@ScottyBrockway]

The ROSS compressor (and most of their other pedals) were straight up stolen from MXR. They even got sued initially for using the same trade dress. I would like to see analysis of the original circuits, particularly the phase 100 and phase 90, phasers that use both LDR (phase 100) and FETs as variable resistors (Phase 45/90). The Blue Box is also an interesting circuit using CMOS logic flip flops to do octave dividing and a comparator circuit for the fuzz.

[@Lantertronics]

HAH! I didn't know that! I could have titled this video MXR Dyna-Comp analysis. I just googled some schematics and found it's the same topology, right down to the way the input buffer is biased.

[@deepsea313]

@Lantertronics I second the request for analysis of the MXR Phase 90/100 phase pedals! I just bought one!

[@Swodie_Jeetin]

Vintage Ross phaser is positive ground topology with p channel transistors, mxr used conventional negative ground with n channel transistors. Not all Ross pedals are "exact" copies.

[@TDanNix]

just watched a video by JHS pedals, it was on the Fuzz O Tron kit that is a copy of the Fuzzrite pedal and it uses this same arrangement of the grounded emitter transistors as the output signal

[@BryanDenham]

@Lantertronics @Aaron Lanterman, i would like to say thankyou for your videos , i would Love a video break down of the (catalinbread antichthon fuzz tremolo). it seems to be extremely simple with a few components yet it does some things like the Tremolo sound from adjusting your guitar volume , a OD sound or a kind of Fuzz sound . i have this on a bread board it works great and would love to understand more about this circuit, how does the Ge transistor in Q1 even turn on i dont see how its getting power and that Tremolo effect is really cool to me i just want to understand whats going on with this circuit . Thank you for your time.

[@Resevil]

I’m off to pick up my JHS/Ross compressor from the post office and just just noticed this video!

[@Lantertronics]

Excellent! JHS makes great stuff.

[@Aleph_Null_Audio]

Maybe stick with the compressor theme and analyze the orange squeezer next?

[@oasntet]

I'm trying to get an intutive understanding of OTAs the way I've developed for op amps, but it's not quite coming to me. The main issue I have is the _why_ - in this circuit, why go through the trouble of using an OTA? What's the advantage? Is it simply down to being able to control gain externally? Because I'm pretty sure I could work out a variable gain circuit using a couple ordinary opamps...

[@Lantertronics]

You use an OTA exactly because you can control the gain. If you can come up with a way to make a voltage controlled amplifier with only op amps that would be a massive advancement in electronics!

[@Lantertronics]

Most of your op amp intuition probably relates to negative feedback forcing the input terminals to have the same potential, which isn’t in play with OTAs in general.

[@oasntet]

@@Lantertronics the first few VCAs I looked into building don't use OTAs, they use a differential pair of bjts and two op amps...

[@tomthepom98]

@@oasntet Sounds like you built a discrete OTA with some voltage controlled current sources around it

[@Lantertronics]

@@oasntet Ah, you didn't mention adding external BJTs. The differential pair is half of an OTA. The core of an OTA is a differential pair with a current mirror stuck to it. So sure you could build a compressor with a matched BJT pair and an op amp. It's controlling the bias current through the pari that gives you the variable gain. If fact you'll often find OTA structures inside an op amp, with a fixed bias current source. Take a look at the TL072 schematic -- you have a JFET differential pair (with pFETs so things are upside down relative to the way they're usually introduced in textbooks) with a 3 transistor NPN current mirror. Rene Schmitz has some VCA circuits with differential pairs and op amps on his website.

[@carsonyuh]

Volteej