Why Green Pre Rotary Switch

Hello, I asked this yesterday on that other forum but missed any responses.

Why is a rotary switch and trim used instead of a pot for the gain settings on the Green Pre?

Hi ChumpitO,

You would need a 150K Ohm negative logarithmic pot for that - and this is hard to find. Very.

Also, in case of leakage in the 4700/6V (C14), a pot would be scratchy very early - a rotary switch takes this much better. Yes, I changed quite a few gain pots in this type of circuit (Amek 2500, Angela, Soundcraft boards to name a few).

Jakob E.

there's a very accelerated nonlinear change in the resistance if you take a look at the sequence of values. Also, the designer produced variants of the design with different step-sizes... such as unity-gain at minimum, then a 12dB step, then a sequence of 6dB steps... -If you wanted, you could have a few "high-res/lo-res" or "close-ratio/wide-ratio" steps in there. There's nothing to stop you even putting in a 16 or 24-position switch with 4dB or 3dB steps...

Also, it allows use of low-noise metal film instead of conductive plastic or (shudder) carbon pots...

Keith

It also allows you to build and use preamps in matched pairs.

You don't have to guess that the gain pots on each pre are set exactly the same. If you matched the resistors for each gain switch, just set each gain pot to the same position and you're good to go.

If you use 1% metal films, you probably don't even need to bother matching them, really.

EDIT: Because I can't always spell...

[quote author="SSLtech"]
Also, it allows use of low-noise metal film instead of conductive plastic or (shudder) carbon pots...

Keith[/quote]

Hmm, I was assuming from what I read before that the trim was also in the signal path to get values in between. Do you know if the schematic @

is the latest one?

Thank you gyraf and SSLtech :grin:

[EDIT] Okay, consul that makes a lot of sense.

Peter recently replied in the (ahem) old forum that he would post his updated V9b layout/scheme as soon as he finished working out some kinks with the level meter.

~ Charlie

The latest circuit is essentially the same as the cct on my site. It has a few extra bits added to it, i.e. a High Pass filter, output trim & a fancy meter.

Regards
Peter

Sorry if this is rehash, but I couldn't come up with answers to these Q's on my own...

Looks to me like the location and orientation of C14 (4700uF) in the schematic is not the same as it is in the layout. In the scheme, the + side of C14 goes to the + side of C5. But, in the layout it's reversed, the + side of C14 goes to the + side of C4. I can see how it might not make any difference which end of the gain control resistors this cap gets put, but doesn't the orientation matter?

Also, what's the deal with the additional, small caps in parallel with C4, C5, C11, C12? These are in the layout, but not shown in the scheme.

thx!
~ Charlie

Hi Charlie

I've tried C14 both ways, no change in sound or suppression of clicks when changing gain.

The idea behind the parallel caps is to smooth out the sound of the electrolytic capacitors. Electrolytics can sound harsh & parallel poly caps can help fix this. It sounds better to me in this cct.

Peter

Hey, thanks for the reply Peter.

So, what value is good for those parallel poly caps... 0.1uF, like the ones on C1 and C2?

btw... havin' any luck w/ the meter?

Thanks, again!
~ Charlie

moosapotamus,
Have you checked out Kev's site?

http://recording.org/users/kev/

His info on the Green Pre page (namely the parts lists) will tell you all that info. But incidentally, yes,:

C1a,C2a,C4a,C5a,C11a,and C12a are 100N (0 .1uF) (104)MKT caps...

Good luck!
Joel :guinness:

Yes, 0.1uF bypass is good.

The Green V12 seems to test OK on the bench, I'll get it into the studio over the next few days to evaluate the performance, get rid of hums, etc.

Peter

OK. Thanks, guys!

Thanks, Joel. I've been through Kev's Dual Green Pre page, but don't recall seeing anything there about the parallel (bypass) caps. You guys answerd my question anyway, tho.

Thanks, again!
~ Charlie

Hi,
I just finished rebuilding my Green Pre into a 4 channel version. So now there are two v14 channels and my two old v1 Green's in a rack-box. A really tight assembly
When trimming the CMRR, I realized that the newer Green's are more silent. It seems the old 4700uF caps (including the cable to the gain switch) pick up a lot of annoying RF-like noise/hum. When I touch the caps, the noise increases a lot. It's like the cap and cable act like an antenna. The same happens with the new channels, but only a fraction of it (no problem here). The old caps are 4700uF/16V, since I couldn't find 6V caps here in Sweden. Could this explain the noise? The problem remains when +48V is turned off. I use star-ground configuration with a ground lift switch for the signal ground.
-Is the there any difference in design between the versions, that could explain this fenomena?
-Could it be that the PSU now is loaded by two extra channels?

Best regards

Lars

What psu are you using and what is the power xfmr rated at?

[quote author="Ptownkid"]What psu are you using and what is the power xfmr rated at?[/quote]

Hi,
This is the PSU I built:
http://www.diyfactory.com/projects/green/green.htm
I tried it with the 6VA transformer as well as a 30 VA transformer. Same problem occurs. I suppose "xfmr " means transformer...

best regards

/Lars

[quote author="sunyata"][quote author="Ptownkid"]What psu are you using and what is the power xfmr rated at?[/quote]

Hi,
This is the PSU I built:
http://www.diyfactory.com/projects/green/green.htm
I tried it with the 6VA transformer as well as a 30 VA transformer. Same problem occurs. I suppose "xfmr " means transformer...

best regards

/Lars[/quote]

OK,
I found the problem. It seems that the 6VA PCB transformer is causing the noise, after all. I went back to my old toroid transformer (30VA). This also eliminated the 50 Hz hum, picked up by the nearest channel. Perfect! I also had to add heatsinks to the 7815 and 7915. They were getting really hot, from the load of four channels.

/lars