CF Linestage tweaks

8th Anual CarverFest. September 10th - 24th
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Zoot Horn
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CF Linestage tweaks

Post by Zoot Horn » Sat Sep 27, 2014 8:41 am

Thought I would start a new thread.

My pre has some audible hum,,tried it with two amps,,no source connected, gets a tad louder when volume is increased.I have checked connections,,scrapped more paint from the ground lug/chassis connection.I was thinking about wrapping the ac wires in copper foil? Any suggestions where to look/what to do next?

Edit,, I tried connecting a wire from the grounf lug to the receptale(wall outlet ground) with some sucess.

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Re: CF Linestage tweaks

Post by radioeng2 » Sat Sep 27, 2014 11:07 am

George,

It occurs to me that we need to keep in mind there could be two directions for hum to come from. One is the common ground loop the unbalanced connections give rise to, the other any self created in the Linestage.

With regards to the second...at the fest it showed up on the analyzer that the way we were building, we had more 120hz showing up than is desirable. That's where Bob immediately popped off to Mike that we should double up the ground buss size wire in the power supply. Which we did do on most units. Still the measurable level was there. This is something, when I can get unpacked, I'm going to dig into and see what can be done. The level is low enough to probably not show up on some rigs, but the higher the gain in the following amp or more efficient the speaker, the more possible to hear.

The other way hum shows up is the very common ground loop hum. The unbalanced connection used with RCA's is very prone to this. The CF Linestage does use a grounded outlet. This ideally is the only point going to AC ground in the system. You could experiment in your system lifting grounds to see if you effect or remove hum using a ground lifting adapter.

In the Linestage itself, we did a number of things to minimize and address proper grounding and noise prevention. Among these things was attempting to have all the grounds going to a single point only. Key here is the RCA's all being isolated from chassis. If any don't have the plastic sleeves correct or something allows them to touch the chassis at the rear, then you have a grounding loop.

If in your rig at the fest you didn't notice any hum, the the more likely things to look at is the ground pin on AC in all the components or a loose rear connector that has shifted and is now touching the chassis.

Mark
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Zoot Horn
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Re: CF Linestage tweaks

Post by Zoot Horn » Sat Sep 27, 2014 12:04 pm

Thank you sir,,I'll keep at it

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Re: CF Linestage tweaks

Post by BrianT » Sun Sep 28, 2014 6:41 am

Zoot Horn wrote:Thought I would start a new thread.

My pre has some audible hum,,tried it with two amps,,no source connected, gets a tad louder when volume is increased.I have checked connections,,scrapped more paint from the ground lug/chassis connection.I was thinking about wrapping the ac wires in copper foil? Any suggestions where to look/what to do next?

Edit,, I tried connecting a wire from the grounf lug to the receptale(wall outlet ground) with some sucess.
George

I have had my Linestage on all day Saturday, and are using it this morning. I have no hum.
Do you do the 120v hum mod on the power supply board at Carverfest? (extra ground on the ps board)

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Re: CF Linestage tweaks

Post by Zoot Horn » Sun Sep 28, 2014 7:19 am

not sure Brian,,I have the "double wiring" it that what you are describing?

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Re: CF Linestage tweaks

Post by BrianT » Sun Sep 28, 2014 7:47 am

Zoot Horn wrote:not sure Brian,,I have the "double wiring" it that what you are describing?
Yep
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Re: CF Linestage tweaks

Post by radioeng2 » Wed Oct 08, 2014 2:18 pm

Wonder if Raytheon tubes would sound good? Nice lookin' set of four! BIN of $40+5.
http://www.ebay.com/itm/NOS-NIB-Matched ... 5360wt_728

Can't remember if any of the ones I populated kits with were Raytheon or not.

When folks get the Linestage settled in and used to it's sonics, I really look forward to hearing about tube brand impressions.

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Re: CF Linestage tweaks

Post by frenchguy » Wed Oct 08, 2014 4:14 pm

finally unpacked all the audio gear last weekend, today was looking at my linestage & touched a wire on one of the audio boards & he fell off, so thought I may as well get out soldering stuff & fix that & maybe look at moving my remote volume pot to the front of the chassis so I have manual & remote volume! was not too bad of a job had to unsolder some wires & make longer ones so they would reach the front of the chassis. Melody gave me a hand holding stuff & helping me see what the hell we were trying to do!! =D> after about a hour we got it all back together & the remote still works so we did something right!! :rolleyes: :lol: have not hooked it up yet to try it but it should work, didnt mess with much except the pots! [-o< will give it a try when I get some time in the next few days. had some hum in it when I tried it @ CF, but will have to look into that after I get it hooked up! start night shift tonight so probably wont get much done the next 7 days! ](*,) one bright spot is it is only 11 months till CF15 :D \:D/
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Re: CF Linestage tweaks

Post by angelod307 » Thu Oct 09, 2014 12:46 pm

What is the issue if any to replace the 50k volume pot with a 100k unit like the one that comes in a sunfire tube pre? Both alps, but one has more pins. Eight versus six.
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Re: CF Linestage tweaks

Post by Magnaryder » Thu Oct 09, 2014 2:41 pm

angelod307 wrote:What is the issue if any to replace the 50k volume pot with a 100k unit like the one that comes in a sunfire tube pre? Both alps, but one has more pins. Eight versus six.
Mark will probably be around and tell you the pre was designed for the 50k pot. I brought a beautiful 100k stepped attenuator to the 'Fest and was assured it wouldn't work in Mark's design. But YMMV

ray
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Re: CF Linestage tweaks

Post by radioeng2 » Thu Oct 09, 2014 6:25 pm

There is circuit values on the tube input side that have to do with attenutor value. But it's broad enough that a 100K might well drop in.

But why would you want to? Higher impedance means easier and more susceptible to pick up noise. It also means less current flow and more pure voltage drive which means cables and connectors and mismatched metals become more prominent.

The attenuator we installed, including the R/C ones I think, are the 8 pin models. The forth pin on each channel is for the ability to build a loudness circuit. This is so that as you go lower in volume, the spectrum tilts towards enhancing bass to match the ears characteristics. Whether this matches the gain figure of your amp and the efficiency of your speakers is what makes this very unpredicable. The likelyhood of it really enhancing the bass at the right level is about none.

I don't see a really light input load as a particularly huge goal for me in other words.

Mark

PS...Ray I vaguely remember you mentioning it but I don't off hand recall a reason to not try it if there is a reason to. If it's a nice one, it probably would have a chance to sound better. I did open one of the ones used and it looks nice internally. It uses discrete resisitors of the surface mount style. Under light load conditions, higher wattage resistors have lower noise. So a pot using bigger resistors might have some advantage.
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Re: CF Linestage tweaks

Post by Zoot Horn » Sat Oct 11, 2014 4:23 pm

Hum update,, very little at any volume,, I think I have found some synergy with the Quicksilver amp,, very detailed and vocals to die for,,which I attribute to the Lucas Linestage,, guys ,, we are fortunate to have built and own one!

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Re: CF Linestage tweaks

Post by hewlew1 » Sat Oct 11, 2014 6:27 pm

Then I guess I am twice as fortunate Eh George? Markie may have a kit or two left. :-$
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Re: CF Linestage tweaks

Post by Zoot Horn » Sat Oct 11, 2014 6:36 pm

Thanks Harry,, you read my mind.

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Re: CF Linestage tweaks

Post by frenchguy » Sat Oct 11, 2014 9:25 pm

will have to hook my CF linestage up to see how much hum I have, have not tried it since I moved my volume pot to front of chassis, will hopefully get a chance in the next week or so! ](*,) :)
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