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PostPosted: 15 Oct 2011, 20:19 
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Hi Everyone, Please com padres, accept my apology in advance as well, especially since this is all occurring on a thread about one of my projects. I agree with Suncalc. I too have a considerable experience in electronics getting real close to 50 years of it. I use all kinds of devices routinely as well. I am open minded about the characteristics of all types of components and circuitry. Tubes are only "horrible electronic devices" to folks that don't understand how they work and what they can be used for. I would never make a computer with them, but I am equally unlikely to use complex computer architecture in an audio amplifier. I do differ a bit with Suncalc on the use of SS rectifiers though (sorry old buddy). All my designs use SS ones. I have managed to tame those pesky things and make them do my bidding. That is part of my goals - to use whatever works best to do what I want accomplished. What is best can include, cost, ease of construction or type of desired output. While I tend to be rather open minded and mellow, I figure that for anyone who doesn't like tubes there are plenty of other parts of the forum and indeed other forums where those opinions will fit better. This is after all a forum section devoted to tubes. Sorry Gio, my bad for speaking out, but broad as assertions like in the subject post just rub me raw. :soapbox:

Good listening
Bruce

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PostPosted: 25 Oct 2011, 20:27 
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I'm trying to lay out the parts for least possible noise carryover to signal wiring/components. I'm not sure about best placement of the final B+ filter cap.

The B+ is filtered through a total of 4 stages. Another thing I don't do is use chokes. I have nothing against them; I just don't need them to get the performance I require. The final filter stage to each tube is isolated from the alternate channel and each uses a polyester capacitor. I have found that using poly (polyester or polypropylene) caps in the final stage seems to improve the overall quality of sound. It is possible that an electrolytic with a suitably sized bypass cap would work well also.

Is it ok to place this cap in close proximity to the tube socket as I will be doing with the coupling cap? Or, do I need to keep it grouped with the rest of the power supply caps?

Allen


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PostPosted: 25 Oct 2011, 20:51 
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Hi, Yes you can place it near the socket. I frequently do. Since the gain is fairly low in the Forewatt parts placement in general is not as critical as in something like a phono preamp. I encourage good building practices in all projects though as it will pay off in subsequent projects. The largest source of hum and noise in the Forewatt is as expected the power transformer and AC mains wiring. If you keep them as far away from the inputs and tubes as reasonable you should be fine. It is important to keep signal wires as short as possible and use shielded wire if they are more than say 2 inches long. This is especially true on the input side. Another thing I suggest is to put a metal plate or shield between the power supply area and the audio circuitry area. Also if the tubes are on top of the chassis using tube shields is helpful in some cases as some brands and variations of the 12AU7 tend to pick up ambient noise. The tube shields and metal shield should go to the chassis ground and not the signal ground for best performance. With a careful build you should be able to get at least 90 dbv signal to noise depending on the brand of tube you use. I get excellent results with JJ ECC802S in this circuit.

Good listening
Bruce

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PostPosted: 25 Oct 2011, 22:22 
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Thanks for the quick response!
Another question arises after reading your response. Would a grounded 16 gauge steel (cold roll sheet metal) dividing plate suffice for a barrier between power supply and signal components? I'll keep 2 to 3" separation gap as well.

If I use shielded wire for inputs should I terminate both ends of the shielding to ground? I seem to recall reading opinions suggesting both single ended and dual ended ground terminations. Many seemed to be saying to terminate only one end.

Allen


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PostPosted: 25 Oct 2011, 23:08 
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Hi, The steel would be fine. Do not ground both ends of the shielded cable. What will happen is that you will create a ground loop and introduce hum. There are two concepts that frequently get mixed up. The following is a simplification and does not cover all cases, but for what we do it is 99% the way to go. Between pieces of equipment that do not share a common ground the shield conductor normally acts as both a shield and a conductor of the signal. Inside a piece of equipment you pretty much want all the signal grounds to come to a common point. I generally use a "buss" wire starting right at the input terminals that goes down the center of the circuit area. Either pure silver wire or heavy (14-12 gauge) copper wire to do this but there are other possibilities. (Star, smaller stars feeding into a single main star and so on). I ground that end of the shielded cables to the buss. The other end goes to the circuitry. All the signal grounds in the circuit go to the buss as well. What you are trying to avoid is having two things. (1) no flow of power through any signal ground. There should only be one connection between the signal and power grounds, generally at the start of the buss or at the center of the star if you used one. Additionally the chassis should be not be directly connected to either ground. I use a type X2 capacitor (about 0.1 to 0.2uf) with a parallel resistor of 120-150 ohm to connect the chassis to the buss. I have also seen rectifier/ capacitor / resistor combinations to make this connection. This is for two reasons first many electrical codes require exposed metal parts to be directly connected to an earth ground (with a three wire cord etc). If you don't use the earth ground then you must double insulate all metal parts (hard to do in diy stuff). The other reason is to make sure that the case acts like a shield and no current will flow through it. Going back to the the grounds the second reason for the internal grounding is (2) no multiple paths for signal grounds that go to the same place. What happens (simplified) is that any wire has some resistance. If current flows through it then there will be noise including hum. If the wire happens to be your shield it will introduce some of that noise into the signal and degrade it. There is a lot more to grounding and shielding and somewhere I recently saw a post on it. If someone can help and pass on the link it may be helpful to you. I hope this helps... it is rather simplified and people write whole books on this subject. Most likely I just confused you more. :|

Good listening
Bruce

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PostPosted: 26 Oct 2011, 19:57 
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I'd like to see the post you referred to for additional help, I need all I can get! The info you've posted helps as well.
I appreciate your explanation of terminating the shield ends but I'm not clear on it yet. Please bare with me.

I ground that end of the shielded cables to the buss. The other end goes to the circuitry

So I should attach one end of the shield to the buss or star ground but the other end goes to the circuitry? By this do you mean the opposite end of the shield connects to the common/negative post of an input which would cause it and the negative lead to share as a conductor?

Allen


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PostPosted: 27 Oct 2011, 03:34 
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I'm sorry to cut in on your conversation, but I think Bruce mentioned my link to one of good book about grounding. The post is in current thread, on page 9, 4-th post from the bottom.
According your question. I'll use Bruce's words in my explanation:
" "buss" wire starting right at the input terminals",
"I ground that end of the shielded cables to the buss."
"The other end goes to the circuitry."
"All the signal grounds in the circuit go to the buss as well."
It means you do not have the place in the circuit where you can connect the "other end shield" - all signal grounds already connected to the buss ans supply the electrical connection.


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PostPosted: 27 Oct 2011, 19:46 
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Poty,

Some of my confusion may be due to envisioning a twisted pair with shield instead of a single conductor with shield.

For example, going from the volume pot to an input jack, you would use a single lead with shield from volume pot to center pin of jack. You then need to get from the jack's sleeve (common return) to the ground buss via a second conductor.

Does the shield only tie onto the bus at the starting point of the bus near the jack and the opposite end of the shield stay unconnected?

I had already read the article you posted on page 9 but it unfortunately, my questions involve a more rudimentary level.

Thanks for baring with me,

Allen


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PostPosted: 28 Oct 2011, 20:27 
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Hi, No problem, thanks for the link. :thumbsup: Let me try something that might help. At least I hope so. The shield of a cable should not be grounded at both ends unless it actually is the return path for the signal it is carrying. Thus a cable between two pieces of gear is attached to a ground at both ends. One inside a device a cable that is not the return path should only be grounded at one end. So in a typical piece of gear like a line stage preamp where the signal goes directly to a volume control there are two options. If you ground everything at the inputs, then the shield going to the control should not be grounded at that end. The ground on the control would have a separate connection to the "buss" or "star" ground. If you do not tie the grounds at the input together and don't make that your grounding point, then the cable will carry both the signal and the return path. Then the shield will be attached on both ends and there will be another connection from the control to where ever the "buss" or "star" is. If this is still unclear, ask again, as it is important to a quality build and we will get you through it.

Good listening
Bruce

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PostPosted: 29 Oct 2011, 11:07 
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Now I've got it. This makes perfect sense to me now and sorry it took so long to finally get it.

I can now finalize the circuit layout and start collecting parts.

While I'm here I've got a question regarding the circuit board mounted transformer. I wasn't planning to mount it to a board with circuit traces but to a drilled fiberglass board about 1/16th thick. Is there a preferred method of attachment without soldering the pins to the board? I guess I could use a couple of tie wraps but that would be a little crude.

Allen


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