"Another Poor Man's" Fairchild 660/670

[rotheu]

The wave again starts at the peak, so the first cycle is only 75% even though for some reason it looks extended, but we can ignore those 25%. With 1uF cap the second half of the first cycle is already attenuated by at list 63%. That means that the attack time with 1uF cap is ~half cycle of 11025 Hz. That's 0.045mS! Too good to be true. That's  2X faster than Fairchild  Huh Shocked

Lolo, it looks like it's half of that! Am I wrong somewhere?
It's extremely fast but very smooth, no artifacts. Only very first peak is distorted and it's just smooth attenuation and symmetric sine wave.

I'm almost about to do what Analag did (remove the schematic) ;D

 

[lolo-m]

I consider ( but I'm maybe wrong ) that attack time is the time taken by the comp to stabilize. And I think it takes around 2 cycles to be at max compression.
I don't know if I'm wrong, your way to read attack makes sense too .  ???

 

[rotheu]

I consider ( but I'm maybe wrong ) that attack time is the time taken by the comp to stabilize. And I think it takes around 2 cycles to be at max compression.
I don't know if I'm wrong, your way to read attack makes sense too .  ???

From what I know, definition of the attack time is: time that it takes to charge the capacitor by 63% (meaning 63% of attenuation?) and release time is the opposite (time it takes for capacitor to discharge by 63%). I might be wrong, but ether way it's really really good!
Still can not understand how is it possible ???

 

[analag]

I'm almost about to do what Analag did (remove the schematic) ;D

Yeah I remember how that was...change the output tubes though. You will be amazed. I don't know what it takes to "approve" a schematic, but I know what I've learned from doing this design twice. The low frequency distortion can be eliminated too, but that's another subject.

 

[rotheu]

I'm almost about to do what Analag did (remove the schematic) ;D

Yeah I remember how that was...change the output tubes though. You will be amazed. I don't know what it takes to "approve" a schematic, but I know what I've learned from doing this design twice. The low frequency distortion can be eliminated too, but that's another subject.

I get low frequency distortion only with very fast attack and release times, position 1 with 1uF cap, that's 0.1mS attack and 50mS release and at list 15dB of GR. Isn't that normal? Also what output tubes would you recomend, assuming that the transformer is Edcor XSM rated at 2.5W?
Can you, please, express your views on how it get's so fast with much weaker SC. I can't comprehend that. Thank you

 

[simonsez]

Yeahhh....this is definitely  my next project!! I love vari-thing, my wife is vari too... ;D

 

[lolo-m]

I consider ( but I'm maybe wrong ) that attack time is the time taken by the comp to stabilize. And I think it takes around 2 cycles to be at max compression.
I don't know if I'm wrong, your way to read attack makes sense too .  ???

From what I know, definition of the attack time is: time that it takes to charge the capacitor by 63% (meaning 63% of attenuation?) and release time is the opposite (time it takes for capacitor to discharge by 63%). I might be wrong, but ether way it's really really good!
Still can not understand how is it possible ???

Thanks ! If someone could bump on this, it will help poeple to know what's the real way to determine attack and release times...
And again, I'm really impressed by your comp !

I'm almost about to do what Analag did (remove the schematic) ;D

Yeah I remember how that was...change the output tubes though. You will be amazed. I don't know what it takes to "approve" a schematic, but I know what I've learned from doing this design twice.

I'm not disapproving any schematic (Rotheu or Analag's). I'm just trying to learn. Discution is the only way... If I wasn't impressed by your work (both) I wouldn't bump here !!!
I bought 4 PM660 boards, isn't it an approval ?

 

[[silent:arts]]

Thanks ! If someone could bump on this, it will help poeple to know what's the real way to determine attack and release times...

http://www.groupdiy.com/index.php?topic=10215.msg120923#msg120923

 

[lolo-m]

Thanks, Volker !

 

[flaheu]

yes, thanks for this link, so instructive !

Love the joke with the mathematician and engineer 

 

[rotheu]

Thank you very much! That clarifies a lot of questions.

I totally agree with PRR and others, saying that numbers in mS don't mean anything and that you just have to tweak the pots until you match the A/R times to your program material. No question about that. The only problem is, in case of Fairchild, I think having resistance in series with CV (attack potentiometer) will change the character of the compression and the way it behaves. There got to be a reason it was designed this way, using different capacitance values to change the A/R times. That's why I went the same way with my timing network. Unfortunately, this way limits you to certain defined values in mS, which you have to choose to your taste. Timing in my network can be customized to individual requirements. I chose those, because they cover pretty wide range and I never use release times longer than 0.5 Sec. With this network you can have 36 different combinations, any attack time with any release time. Also, with switchable controls it's much more accurate in case of stereo linking.

 

[veermaster]

Rotheu,
thanks a lot for your timing network sheme, that´s just what I was looking for!
emre

 

[rotheu]

Rotheu,
thanks a lot for your timing network sheme, that´s just what I was looking for!
emre

You are very welcome! Just keep in mind that it's designed for Fairchild type side chain amp with low impedance drive. Don't even try to put it after 6AL5 !!! ;D

 

[lolo-m]

Can you, please, express your views on how it get's so fast with much weaker SC. I can't comprehend that. Thank you

I wonder if the huge gain of two 12AX7 triodes doesn't overdrive the 12BH7 so much that the current in the transformer is a saw tooth... I mean so much pair harmonics makes a jump from no current to max current and then decreases slowly... This could explain the ultra fast charge of the TC cap...
Analag ?

 

[rotheu]

Can you, please, express your views on how it get's so fast with much weaker SC. I can't comprehend that. Thank you

I wonder if the huge gain of two 12AX7 triodes doesn't overdrive the 12BH7 so much that the current in the transformer is a saw tooth... I mean so much pair harmonics makes a jump from no current to max current and then decreases slowly... This could explain the ultra fast charge of the TC cap...
Analag ?

I think the first 12AX7 is mostly operating as detector and it doesn't have a lot of gain. The rest (second 12AX7 and 12BH7) are running in one of my 175 based compressors with 10% more feedback and 6BC8 in front of them as actual signal amp. It's definitely colored amp with around 30-35dB of gain (overall, including 6BC8), but so far it didn't sound to me like saw tooth wave generator ;D But if I'm wrong and what you described is the case with the side chain amp, do you think it's a problem as long as it performs as it is now?

 

[Larrchild]

Nice work Rotheu!
You have all the good parts of a Fairchild built.
6BC8's have a "speedbump" in the transfer function at about -12v that 6BA6's, which look like a near-straight line out to 80v, do not, but your sidechain needs are smaller.
If you move to 6BA6's, you'll have to increase sidechain torque, but otherwise, enjoy!
Bravo.

Add some negative feedback to the sidechain 2nd 12AX7 cathodes. It'll get even faster.
BTW, did you ever try a 12AU7 there? Might be a better fit.

 

[rotheu]

Nice work Rotheu!
You have all the good parts of a Fairchild built.
6BC8's have a "speedbump" in the transfer function at about -12v that 6BA6's, which look like a near-straight line out to 80v, do not, but your sidechain needs are smaller.
If you move to 6BA6's, you'll have to increase sidechain torque, but otherwise, enjoy!
Bravo.

Add some negative feedback to the sidechain 2nd 12AX7 cathodes. It'll get even faster.
BTW, did you ever try a 12AU7 there? Might be a better fit.

Hi, Larrchild, thank you very much!

I think I'm gonna stick with 6BC8's. I like what they do.
I can easily slam it by 20dB of GR, that's more than I will ever need.
Also, like you said, the sidechain needs are much smaller, which makes it possible to use cheap
transformers and have maybe not exactly the same but very similar results.

6BC8's sound very different in Fairchild type compressor, compared to something like UA175,
which makes me think that in case of compressors the sound is not that much
in the tubes and transformers, but mostly in topology and RC network. I might be wrong though...

That's interesting that you advised me to add some feedback to the 2nd 12AX7,  because I actually
reduced it, probably thinking in terms of voltage gain. Now when I think about it... Is it the
output impedance, which will be lower with more negative feedback, meaning the sidechain will
have more current drive?

I will try 12AU7 instead of 2nd 12AX7. Do you think that will require some component
changes?

Analag advised me to change the output tubes (12BH7). I'm thinking of ECC99 , but my concern is
the output transformer rated at 2.5W RMS. ECC99 can have 5W output. Do you think it's a problem
in case of sidechain to overdrive the transformer by that much? What tube would you recommend?

What do you think about my idea for the timing network? It's a big load on the
sidechain, especially with long attack and short release settings, but so far everything
worked nice. Love those long attack/short release things...

Again, thanks a lot, Larrchild!

 

[emrr]

Remember, transformer wattage rating is at lowest specified audio freq.  At least it's supposed to be.    Do you expect to pass 2.5W of 20Hz?  Or something higher, on average? 

 

[rotheu]

Remember, transformer wattage rating is at lowest specified audio freq.  At least it's supposed to be.    Do you expect to pass 2.5W of 20Hz?  Or something higher, on average? 

Hi, Doug

That's right. Does it mean that, if you drive it with more power, the transformer will pass higher lowest frequency?
If that's the case, I should be fine with 60-70Hz, anything lower than that will cause distortion and weak bottom end, especially with
faster A/R settings.
But in that case, what happens with lower frequencies, are they gonna cause some kind of a problem or they will be just filtered
out completely?

 

[nielsk]

Isn't power log to frequency?