EH - Low Frequency Compressor  [schematic]

Discussion regarding early stompbox technology: 1960-1975 Please keep discussion focused and contribute what info you have...
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mictester
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Post by mictester »

Broadcaster wrote:Just to make this even more confused, here is another schematic for the blackfinger... including the parts around the 3080.

Image

I have long been intrigued by the discrete version of the LFC, but never really understood it, maybe Q2 is FET alike the squeezer? Nah, I just don't get it :oops: .
It's simpler than it looks! The op-amp (741 or if you're going to build one today, use TL 071) is a simple non-inverting amplifier. The gain is set by the ratio of the feedback resistor to the resistor from the inverting input to ground (DC blocked by a capacitor). The 3080 is used as a variable gain amplifier. The gain of the amp is controlled by the current fed to pin 5 from the transistor rectifier circuit. The 3080 bypasses the feedback resistor around the 741, so the gain of the 741 can be changed by increasing the gain of the 3080. The whole shebang works as a controlled-gain amplifier, the gain of which is set by the current developed into the 3080 from the transistor rectifier....

The Dynacomp (and Ross, and lots of others) use the 3080 on its own as a gain control stage. The 3080 is a pretty poor amplifier (it's noisy and has lots of distortion) so EH used this clever feedback-controlled amplifier to mitigate the poor performance of the 3080 and the 741 - as the 3080 gain increases, there's more and more negative feedback around the 741, so distortion is reduced, noise is reduced, and frequency response is improved!

This kind of gain cell is quite effective, and has low noise and distortion, but has a few problems - it can't reduce the gain of the op-amp over a huge range (18 dB is the usual practical limit), it can introduce noise due to the DC level changes in the circuit, and it's more complicated than a simple VCA stage.

It's interesting to note that this type of gain cell was also used in the Morley compressor pedal.
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Post by Broadcaster »

Thanks for the writeup on the 3080 version - so while you're at it, why don't you do the same thing for the discrete version... :wink: . While the rest of that circuit is somewhat off the shelf, why the NPN (Q2) instead of the regular FET (as seen in the OS and others), I've never seen that before (or after, if you prefer). I guess the discrete version came earlier, so there might have been a reason for the change to a 3080 design. I never tried any of these, so I'm just curious.

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Post by mictester »

Broadcaster wrote:Thanks for the writeup on the 3080 version - so while you're at it, why don't you do the same thing for the discrete version... :wink: . While the rest of that circuit is somewhat off the shelf, why the NPN (Q2) instead of the regular FET (as seen in the OS and others), I've never seen that before (or after, if you prefer). I guess the discrete version came earlier, so there might have been a reason for the change to a 3080 design. I never tried any of these, so I'm just curious.
Q2 being an ordinary bipolar transistor has a few advantages - it's cheap (they didn't have to go out and buy extra parts), it has a pretty good range of control, and it introduces some interesting distortion artifacts!

I'll dig out my really simple transistor compressor and put it up here later (when I've changed the pencil scribble in my notebook to something more legible!).
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Post by branleur »

Please! Could anyone post or repost a corrected schematic of EHX LFC! It might be the one IC based or the two IC based, it don't care

With the schematic i can trace a pcb and post it!

Thank a lot

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Post by branleur »

I've drawn the one IC version of EHX LFC (without the CA3080 OTA) with switchercad. The filter section works. But the section driving Q2 didnt't! Some value missing in the schematic posted page 1.

If anyone could help.


here the switchercad file:
LFC 012.rar
thanks
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Post by rhandy gaye »

i haven't been following this thread at all but saw one of these (1972 model ?) in google japan.

a different pcb to the other.

anways.
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Post by JiM »

branleur wrote:But the schematic posted by HereIam2010 is not aviable! :?
You just need to remove the "/" at the end of the URL.
https://img36.imageshack.us/img36/5161/lfc.gif
https://img194.imageshack.us/img194/8397/blackfinger.gif

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Post by branleur »

:shock:

What a hell!! A corrected schem :shock:


Ho man, you know I love you :hug: !


Seriuously


VERY THANKS

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Post by xaxxop »

thanks friends for all the info :applause:

someone is working for the PCB? I could clone the pcb

Leandro.
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Post by branleur »

xaxxop wrote:someone is working for the PCB? I could clone the pcb
Yeah man! But i will use the tonestack coming from the one IC version of the EHX. I've alredy draw the schematic on switchercad, and simulate it. It works :) . The tonestack of the early version of LFC is really interresting. bass frequency are boosted, and there is a bande passe from 600Hz to 15kHz. If i take the time, I schow you.
xaxxop wrote:I could clone the pcb
If you want to have the PCB faster, you schould. I am alway slow :slap:

have a nice day :wink:

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Post by branleur »

here's the schematic i've mix! Do not matter with the voltage divider (R29 and R30) before the input! it's just there to reduce audio'svoltage.
LFC mix 001.JPG
And there the frequency response. Interresting! isn't it?
frequency reponse.JPG
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Post by branleur »

I've drawn a PCB for the EHX LFC. After some disagrement, the LFC seems to work.

I use old low gain transistor (like 2n1711, 2n2905...). Hight gain transistor do not match with the LFC. I use 741 IC. I mods the "sustain" pot from 100k to 1Meg. But the LFC compresse a lot. It can also have some "pump" effect . Something still go wrong :cry: . I did not use 741HC IC, but another 741. I try using TL071, but did not work. Still thinking about the last problem

The filter section working very well :D .

I also use special voltage regulator of my own. It can make +V and -V very symetrical.

The PCB can be placed in hammond 1590C



I will post PCB and schematic, when the LFC works well

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Post by xaxxop »

Great!!!
branleur wrote:
xaxxop wrote:I could clone the pcb
If you want to have the PCB faster, you schould. I am alway slow :slap:
have a nice day :wink:
I am very slow, I have little time. thank you and good luck
Leandro

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Post by branleur »

using low gain transistor seems to be to right way! Stay waiting transistor coming from Banzai :? Banzai crew are very very slow :mrgreen:

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Post by gigelmargel »

branleur wrote:using low gain transistor seems to be to right way! Stay waiting transistor coming from Banzai :? Banzai crew are very very slow :mrgreen:
Glad to here that it works :)
I am very curious to build one...

Thx for your effort!

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Post by branleur »

gigelmargel wrote:Glad to here that it works :)
Not perfectly works! There is too much compression yet, but hope i will find the proper transistor.

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Post by gigelmargel »

branleur wrote:
gigelmargel wrote:Glad to here that it works :)
Not perfectly works! There is too much compression yet, but hope i will find the proper transistor.
I like to have a lot of compression :)

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Post by gigelmargel »

Can you post the PCB, please?

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Post by PierreBPSFSB »

Hello !
Its also my first post ! :)
Here under some info about my BlackFinger i bought in 1980:
Schematic:
Image
Curve i/o Comp:
Image
Layout from "PCB":
Image
ToneBurst response:
Image

** The PCB/Schematic is slighly different from Elecsonic, but quite the same as Boris post and Xaxxop, but (thanks to JiM :wink: ), it is the SAME as Broadcast or Hereiam2010 (that is with the "red" addition of R20,21 & C14 near theCA3080)
- Capa: at that time i was confused, but ALL the 10nF (green point) are actually 100nF, excepted the remaining Tone 2x10nF.
- Diode: silicium 1N914 (or 1N4148)
- Ao1= µA 741, but as Mictester said other like JFet TL071, LF351.. are better for noise.
- PNP= 2N5087 HiGain +LoNoise (or BC 859C)
- NPN= 2N5088 HiGain +LoNoise (or BC 849C)
- Pot: mine was "blank", but Tone should be 50 to100K Linear, Sustain= 50 to 100K Log, Volume= 50 to 100K Log also)
** And it works ! :thumbsup
** The sound is VERY "clear" (like Gilmour, Knopfler), and the "Static" Comp Ratio is "smooth" like Opto Comp (instead of a Limiter, like a DynaComp MXR), but I sold it because surprisely the Compression is (IMO) sounds very hard at the note attack, and even "OverCompressed, with no real sustain at the end of the note... So i understand well the sad "feeling" of Branleur ! (unfortunately, its due to the signal Rectifier topology...).
I think also that it is more OK for Bass & Keyboard.
We can verify on the -not usual- "Dynamic " response, see ToneBurst's start envelop :| (Sorry for the French comments !)
(Now i use since 1982 the DynaComp with its super "plop" at the attack of the note [due to its simple & very clever Rectifier] ... okay the high-medium is C.Santana enhanced ! or the DBX 166XL very transparent & versatile... but in a Rack :cry: )
-------------------------------------------------------

Following the post of Broadcast and Mictester:
- LFC: to return to the LFComp "Q2": for me, its dificult to say if it is a NPN Transistor or N Jfet, because that schematic seems to have value errors, and "strange drawing" about the rectifier, and also the base of Q2 is feeded by summed Current & Negative Voltage... Netherless as you said, yes, a NPN (PNP) could act as variable resistor element with a large attenuation range, but as said the distorsion is quite high, but also the DCcontrol's transistor Base injected current drives to high "offset steps" at each transient.
- BlackFinger: also, following the post, i have seen the Morley (ProComp PCBmod), and it has the same clever use of OTA (attenuation inside the feedback loop, to keep the S/N # constant), but the CA3080 both inputs was "grounded": here the input are signal feeded, so it is more a signal "negative feedback" (with careful phase e- pin2 on µA741) than an output variable conductance.
It is just a remark :wink:

Hope it helps !

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Post by PierreBPSFSB »

Re Hello !
I made a mistake on my first post about the "Layout PCB" picture :oops:
Here under the good one ! :)
Image

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