"Another Poor Man's" Fairchild 660/670
- Thread starter rotheu
- Start date
Help Support GroupDIY Audio Forum:
Just tried increasing the plate voltage for 6BC8s from 100V to 120V (150V is maximum rating for 6BC8) and different values instead of 350 Ohm resistor in the cathode circuitry to see what will change at different currents through the tubes, but liked original configuration the most. End up using 360 Ohm (more common value) and same 100V on the plates. Also, with this values the VU meter shows exact amount of GR in dBs all the way down to 20dB of GR.
I just changed 1uF cap in the timing network with 100nF. That gives you practically instant attack (no transients pass through) and 5mS(!) release on position 1. This thing is completely crazy! No thumping at all! Low end of course gets distorted, but vocals (also distorted but in much nicer way) are just coming out of the speakers! This is just to play around, I don't think it's the way to go, but I'm just surprised how stable this refrigerator looking device is at this crazy speeds! Thank you very much, Mr. Narma! 
Viitalahde
Well-known member
I approve that prototype. Proper work!
I'm going to start with a similar mess later this year. PCC189's in parallel and a gainclone amp driving the sidechain. I want to try out something Rein Narma would've done if he had chip amps in the 50's.
I'm going to start with a similar mess later this year. PCC189's in parallel and a gainclone amp driving the sidechain. I want to try out something Rein Narma would've done if he had chip amps in the 50's.
lolo-m
Well-known member
There's always something to learn... I don't have a 660 schemo and thought 670 had only two 6386 per channel :-[ ;D :-[rotheu said:
MartyMart
Well-known member
Now that's what I call PROPER bread - boarding !!!
Fabulous, what a gorgeous MESS
MM.
Fabulous, what a gorgeous MESS
MM.
Ok, some attack times versus 3 different caps in the timing network 1uF, 2.2uF and 4.4uF. The sine wave is 1KHz, so 1 cycle is 1mS. Sine wave starts at the peak so the first cycle is not full but only 75% or 750uS. It seems like the attack time with 1uF cap is somewhere between 0.1 and 0.2 mS. Thats very close to Fairchild 0.2mS with 2uF cap. I don't quiet understand how is it possible with much weaker control amp.
Edit: The attack time for 1uF cap is 6 samples, at 44.1KHz that's 0.136 mS.
Edit: The attack time for 1uF cap is 6 samples, at 44.1KHz that's 0.136 mS.
lolo-m
Well-known member
Can you make some tests with a 12K sine wave if you're in 48kHz, or a 10500 Hertz if you're in 44K1 ?
I want to compare some results ;D
I want to compare some results ;D
lolo-m said:
Tried to record 10500 Hz sine wave ( I'm on 44.1KHz) on the audio track to do the test, but for some reason it's all distorted??? Doesn't look like sine wave, more like saw wave but it's randomly distorted. It didn't go to the compressor, just internal bussing in protools, exactly the way I did it with 1KHz wave. Any ideas?
![[silent:arts]](https://cdn.imagearchive.com/groupdiy/data/avatars/m/0/172.jpg?1620371656){kind=avatar}
[silent:arts]
Well-known member
wouldn't 11025Hz better compare to 44K1 ?lolo-m said:
[silent:arts]
Well-known member
normal. how many samples from a 11025 Hz sine wave are there at 44.1kHz ?rotheu said:
draw it up, take a ruler and draw a line from one point to the other.
has nothing to do with Pro Tools btw
lolo-m
Well-known member
Yes 11025Hz of course sorry ;D[silent:arts] said:
It will look like a triangle in soundforge too, and will look like a pyramid in cubase or nuendo...
A quarter of the sampling frequency allow you to have a regular "something" wave... You can have normalising the signal a full wave... That's why I asked for those frequencies...
[silent:arts] said:
Ok, understood. Then there is, 11025 Hz
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 ???
lolo-m
Well-known member
My knowledge don't allow me to understand why but you've got around 0,2ms of attack time with a 2uF... Really impressive !!!
