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

[@markhammer643]

Nicely done, and thanks for the nod. About the mods in red... A stock Dynacomp/Ross circuit will have a 150k resistor from V+ to the junction of Q3/Q4. That resistance dictates how quickly the 10uf cap is charged up again. But, as you have indicated, that is going to alter how much current is being fed to pin 5 of the 3080, and ultimately, its gain. Variations in that resistance *below* 150k will speed up how quickly the 10uf cap charges up again. This is often described as "gain recovery" time. However, because reverting back to maximum gain is slowed down by that 150k, the initial transients of rapidly-picked notes can be subdued, due to the circuit not having gone back to full gain. Many manufacturers have substituted a variable resistance in place of the 150k, in order to vary the speed with which maximum gain is recovered. Largely because the net effect of speeding up full gain-recovery can make the initial pick attack of rapidly played notes more audible, it has traditionally been labelled "Attack" on the control panel and schematic. It is NOT, however, the same function as the Attack control on a full-featured studio compressor that had Release, Threshold, and Ratio controls. In those units, the Attack control adjusts how quickly gain is *reduced* in response to transients, and NOT how quickly it is brought back up again. Much to my pleasure, Fender finally labelled the control as "Recovery" (rather than "Attack") on their Bends compressor pedal, and I was pleased to be able to tell Fender designer Stan Cotey in person how much this gratified me. Misnomers aside, the impact of adjustments to gain-recovery time have long been bewildering to players, simply because the impact depends far more on the speed of one's playing than on the setting of the control. Leave enough space between picked notes, and even slower recovery times will not have much apparent effect on what one hears. "That knob doesn't DO anything" is what one will tend to hear in music stores. But launch into much faster playing, and the pick attack will seem to evaporate. Small wonder that the Dynacomp/Ross was traditionally preferred by players wishing to make their Les Pauls seem to sustain held single notes forever (one of the consequences of a slow gain recovery), while country players eager to launch into rapid "chicken-pickin" would often prefer the Dan Armstrong Orange Squeezer, that had a much faster gain recovery. The implementation of variable gain-recovery time has pretty much eliminated the distinction between products on that basis. As an aside, I had worked out the illustrated resistance values with a friend for his well-reviewed Ross-derived compressor, to use with a SPDT on-off-on toggle. The intent was to provide easily replicable fast-medium-slow times. Because effect of gain-recovery depends so much on how fast one plays, I saw no need for anything in between those settings, and that's what he included on the pedals. Some time later, I saw that the toggle was replaced with a potentiometer. When I asked him why he went to the needless trouble and expense of turning it into a pot, he said it was because of consumer demand. Even though it matters little, and most users would likely either turn the control to min or max, it was the "illusion of control" they wanted, and a continuously variable pot gave them that. I might point out that Electro-Harmonix went with the 3-position toggle for their Soul Preacher compressor, and has stuck with that. Customers...bah! Can't pawn them, and can't succeed without them. Happy Chanukah. Being a "Hammer" doesn't necessarily make me a Maccabee, but I'll happily accept the mistaken attribution. 🙂

[@zaum2002]

Thanks for that explanation. I bought a used Dynacomp with this mod and didn’t quite understand the function.

[@Lantertronics]

Thank you for that wealth of information! I am pinning this post for the moment.

[@_-Skeptic-_]

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

[@jonathanke17]

Same here, wow

[@willmcbride4435]

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

[@Lantertronics]

You are welcome! :)

[@gr8pr4mr]

Makes me want to go on your electronics course. 😊

[@MixMeMcGee]

I would love to hear you “talk about that another time” if you can!

[@guitarmon5765]

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

[@jkuebler89]

Mr Lanterman, you are doing amazing work, really appreciate you taking the time to break down these circuits. For me you make it much more digestible. Thank you!

[@Dolan1492]

Professor Lanterman 😘

[@nethoncho]

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

[@fredfabris7187]

Great video! I love circuit analysis of anything guitar!

[@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!

[@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

[@edsavage6214]

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

[@HazeAnderson]

I put together this circuit together from a kit I purchased from Fuzz Dog (called the Super Skuisher) which adds two more pots for Clip and Attack. I was able to obtain a real CA3080 for it. Works VERY well.

[@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.

[@kenzuercher7497]

I have an MXR Dyna-Comp and I'm pretty sure it uses FETs for the gain reduction. I don't care for it much as it could also use a mix control to add the unmodified original signal.

[@LTJR.]

Keeley 4 knob compressor has tone and blend added to what I believe is MXR/Ross comp. I owned and loved an old script model MXR, w/can type transistor. Pretty sure the Ross is very similar, if not a direct knock off, but with more electrolytic capacitors and I can’t remember past that-I built a DIY from one of the sites, and I liked that one also, but not as much as my old one. We had a store called “The Gear Trader” and I was able to play or own or audition all of these things…GOOD TIMES! On one of my boards I had the cheap plastic Dan Electro comp, Which I believe is the same MXR/Ross topology (One or the other) I couldn’t tell the difference between the newer ones. Probably 2000 or so. My buddy came back with about $3000 worth of Chiquita banana boxes filled with the last of MXR parts, somewhere up in Rochester I think…There was a little of everything and a lot of some things… I still have a script logo, blue box, P45&90 and envelope follower, that I put together from parts back then-good times indeed! LT

[@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

[@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.

[@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.

[@DS-bi3fz]

Keeley Compressor is also only a slight mod

[@Aleph_Null_Audio]

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

[@erikvincent5846]

This is really good! Ever look at the Boss OC-2 Octave circuit? That's a fun one.

[@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.

[@alforddavis3427]

Can you discuss the pre/post emphasis noise reduction in this circuit?

[@Lantertronics]

Sorry, which circuit are you referring to? I don't see any particular noise reduction in the Ross Compressor.

[@alforddavis3427]

@@Lantertronics I thought the 0.01uf across the 15k at pin 3 of the OTA was the emphasis and the 0.001uf across the 150k at pin 6 is the de-emphasis. Though they match mathematically, I never thought they matched sonically (and on the oscilloscope) and therefore I used to experiment with reducing the 0.001uf. I assume there was some effect from the rest of the circuit that I never understood.

[@wesmitchem825]

Wow someone was able to get the back off of one of there pedals i couldn't for the life of me

[@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.

[@anotheryoutubed]

The reissue Ross comps by JHS SUCK. They are NOT a 1:1 recreation. I could not get a usable tone out of it, at all.

[@carsonyuh]

Volteej