radiance
Well-known member
I see the Fairchild time constants uses resistors in series with caps on two positions. Does this give a longer attack time?
[BUILd] PM660 / PM670 SCAmp Booster build and tips help thread
- Thread starter lolo-m
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lolo-m
Well-known member
No, on position 5 and 6 the resistor just slow the release.
With Fairchild's TC network you have to think current. The current coming out of the SCamp charges the network. You'll see and understand that it will fill the smaller resistive element first (the smaller cap) then the most resistive element will make the cap's discharge. On position 5 and 6, there's an additional resistor+cap that will be charged by the current of the SCamp , then by the cap's charge and it have a really really long discharge. Honestly position 5 and 6 are nearly unusable now. They have a super fast attack time and a crasy long release (more than 2 minutes if I remember well). They were made to maintain the level whatever could happen in broadcast (mad DJ, poeple talking too loud or too low, sleeping engineer...) not to saturate the emiter.
I hope I've been clear enough :
.
With Fairchild's TC network you have to think current. The current coming out of the SCamp charges the network. You'll see and understand that it will fill the smaller resistive element first (the smaller cap) then the most resistive element will make the cap's discharge. On position 5 and 6, there's an additional resistor+cap that will be charged by the current of the SCamp , then by the cap's charge and it have a really really long discharge. Honestly position 5 and 6 are nearly unusable now. They have a super fast attack time and a crasy long release (more than 2 minutes if I remember well). They were made to maintain the level whatever could happen in broadcast (mad DJ, poeple talking too loud or too low, sleeping engineer...) not to saturate the emiter.
I hope I've been clear enough :
.
radiance
Well-known member
Thanks Lolo!
One more thing...is there a limit on how small the parallel resistance can be?
In the original Faitchild time constant section the smallest is 68K.
So, the smaller the resistor the faster the release time...why not make it even smaller, say 10K, or even no resistance at all?
I like very fast release times ;-)
One more thing...is there a limit on how small the parallel resistance can be?
In the original Faitchild time constant section the smallest is 68K.
So, the smaller the resistor the faster the release time...why not make it even smaller, say 10K, or even no resistance at all?
I like very fast release times ;-)
lolo-m
Well-known member
I don't exactly know the limit for the parallel resistor but 10K is working for approx 20ms release time. I've got this setting in my TC network : 2,2uF // 10K.
- No resistance (only the cap) will mean no release
- A wire parralleled to the cap will mean no attack and no release = no compression.
Another thing, if you want to slow the attack you've got a few options :
- you can use bigger caps : if you double the caps value you approximatively double the attack time or,
- you can use a little serial resistor added to the 75 ohm I forgot on the SCAB board (Read PIMPING MY OWN PIMP page 2). As it'll make the SCAmp output impedance higher, the caps will charge slowlier and attack time will be longer BUT be carefull with this resistor value as you create a voltage divider with the release resistor so the max compression will be affected.
I don't understand what you mean there but :radiance said:
- No resistance (only the cap) will mean no release
- A wire parralleled to the cap will mean no attack and no release = no compression.
Another thing, if you want to slow the attack you've got a few options :
- you can use bigger caps : if you double the caps value you approximatively double the attack time or,
- you can use a little serial resistor added to the 75 ohm I forgot on the SCAB board (Read PIMPING MY OWN PIMP page 2). As it'll make the SCAmp output impedance higher, the caps will charge slowlier and attack time will be longer BUT be carefull with this resistor value as you create a voltage divider with the release resistor so the max compression will be affected.
hi lolo,
I'm currently building my PM670,
and i would like to put your scamp booster in.
not sure it's the right place,
but i can't find a thread for the boards only,
Was it a one shot run of boards or are they still available somewhere ?
best regards,
Francois
I'm currently building my PM670,
and i would like to put your scamp booster in.
not sure it's the right place,
but i can't find a thread for the boards only,
Was it a one shot run of boards or are they still available somewhere ?
best regards,
Francois
radiance
Well-known member
lolo-m said:
Strange, I had a 470K pot wired parallel to the cap and still got some compression when turned all the way cw which resulted in 0 resitance...well, maybe the resistance was 18R or so due to the pot being not very acurate...but still.
It did sound quite nice...I'll experiment some more with this.
Thanks again for chiming in on this old thread...
Kingston
Well-known member
Are you sure you don't have the 221k resistor of the old time constant network still there?
radiance
Well-known member
Kingston said:
It's still there yes...but that does not matter since the pot I use is parallel to that resistor as well is it?
..Or am I missing something?
Kingston
Well-known member
Oh I see, close to 0 resistance. Don't know what is going on then.
lolo-m
Well-known member
You're right you may have some compression...radiance said:
Looking at the schematics the 0dB point (6BC8 tube bias) is made with a cap parralleled to a resistor. The SCAmp if loaded with a shunt (0 ohm load) is charging the bias cap. This is why you've got some compression.
This is not really safe for the comp so avoid this by using a minimum of 4K7 in the TC network (about 10ms release with a 2uF cap). If you want to use a pot put a minimum resistor (4K7 or best IMO 10K serial to the pot). Every thing will be a lot more predictable that way
.
lolo-m
Well-known member
pacemaker said:
Hi François !
The black market topic vanished for some reasons (I don't know).
First, like I said many times :
Have a working STOCK PM660/670. Test it, listen to it. If what you hear don't please you enough, PM me. There's a few SCAB boards left somewhere...
PS : Le bonjour d'Avignon, tu es où en France ?
Yes. Few left. please contact Lolo-m and I will send it to you .
I have a nicely working stock PM670. I have my scamp board stuffed and ready to go. I've read this thread a few times and am still completley confused about how to hook up the TC network. Verbal descriptions and theorys are going over my head! Reading Chuck D's posts, I have similar questions. Really what I would like to see is a clear list of components for the TC network and a clear diagram on how to hook it up. I have Lorlin 2 pole, 6 position switches.
Help will be much appreciated!
Help will be much appreciated!
OK, I think I finally have a handle on this. I've been studying this switch from page 6:
I was so confused because I didn't understand the values...but now I see that the cap values are paralleled to make up the values as shown in the fairchild TC network. And on the other pole of this switch I just couldn't figure out what the 2k resistors were doing and where did they come from? they are not in the fairchild tc schematic. Then it dawned on me.... that pole with just the 2k resistors is the Analag mode!..is that correct?
I was so confused because I didn't understand the values...but now I see that the cap values are paralleled to make up the values as shown in the fairchild TC network. And on the other pole of this switch I just couldn't figure out what the 2k resistors were doing and where did they come from? they are not in the fairchild tc schematic. Then it dawned on me.... that pole with just the 2k resistors is the Analag mode!..is that correct?
lolo-m
Well-known member
You're right. This switch is to get both modes on only one switch : analag's and fairchild's tc networks are on that switch.Freddy G said:
I don't remember and don't have time to dig but it may need one more switch to toggle between the 2 differents modes...
Here's an novel use for the SCAmp pcb - I'm using it not in my poorman builds, but rather, in a diy franken build.
I had some neumann fader modules which I ripped all the transformers from.
The faders were discrete vca module type driving lowZ faders.
So I have taken the neuman discrete vca modules and am hooking them up to the SCAmp pcb as the sidechain.
Don't see why it won't work!
Am using this in a 3RU build with a pair of Tesla channel strips.
A combo of traffo/fet DI with Tesla early discrete/semi preamp+inductor eq and I'm adding Haufe active transformer main and aux outs.
This will be the last of my franken-strips!
All my parts used up then
I had some neumann fader modules which I ripped all the transformers from.
The faders were discrete vca module type driving lowZ faders.
So I have taken the neuman discrete vca modules and am hooking them up to the SCAmp pcb as the sidechain.
Don't see why it won't work!
Am using this in a 3RU build with a pair of Tesla channel strips.
A combo of traffo/fet DI with Tesla early discrete/semi preamp+inductor eq and I'm adding Haufe active transformer main and aux outs.
This will be the last of my franken-strips!
All my parts used up then
