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PostPosted: 24 Sep 2011, 14:51 
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Hi, No problem. It has an output impedance of just under 3K and will match anything over 10K well.

Good Listening
Bruce

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PostPosted: 27 Sep 2011, 20:23 
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gofar99 wrote:
Hi, No problem. It has an output impedance of just under 3K and will match anything over 10K well.

Good Listening
Bruce


Very good.

I've done quite a bit of reading on grounding schemes for audio builds (star, buss, dual star etc...) and I've never seen such a wide range of opinions except maybe in politics.

I would be overly grateful if someone could post a "preferred" grounding scheme for the Forewatt specifically with wooden chassis and metal transformer cover/sheild that would include easy to understand specific instructions for input/output jacks and all signal and power supply grounding. I understand that there's more than one way to do this but I'd rather perform the grounding layout in a way that matches what has been proven to work with this preamp and not experiment on my first build.

I've read the notes in the other thread which details that you should keep the tube heater power supply ground floating which makes sense but the rest is very hard (for me) to digest. Again, would be great to see specific details and not general statements (for us tube audio noobs).

Forgot to include that the power input will be 120V hot, neutral, ground.

Allen


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PostPosted: 27 Sep 2011, 21:03 
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Hi Allen, A can of worms for sure. :? I will see if I can describe how I do it in most projects. I generally use a type of buss grounding system. The start of the buss is at the ground terminals of the inputs and traverses the signal circuit generally from front to back ending with the output grounds. I have also used a buss like that connects the input and output grounds directly and then goes out to the remainder of the circuit. I normally use 12 gauge solid copper wire for the buss but have used silver wire on occasion. The power supply (negative side in my tube designs) all connect in a buss arrangement as well and then there is a single heavy gauge connection between the power and signal busses. Often at the point near where the input buss starts. If there is a three wire AC mains I will make a single connection to the ground. It will usually be at the same spot as the the power and signal grounds tie together and is isolated from both by a 120-150 ohm resistor with a parallel type X2 cap of between 0.1 and 0.22uf. If a metal chassis is used, or there are metal shields that are exposed to contact I attach them to the AC ground. This is for two reasons, first our electrical code requires all exposed metal parts to be either grounded or double insulated (hard to do). Second then the metal shields or cases can act as an EMI shield. A subset of this is if there is a completely internal shield. This can go either way to the AC ground or signal ground. It is hard to generalize here, try it both ways and use the one that works best. The method I use can be called a combination buss-star system as the busses are brought together like a star.

Now for why... Grounding has a number of functions and I won't go into all of them here. But three seem to me most important. You need to consider how power, signal and noise are handled by them. Anytime you send a current through a wire some noise is generated. The lower the impedance of the wire, the lower the noise voltage generated is going to be. The lower the current also the lower the noise. So what you want to accomplish is have minimal current (like power) flowing though grounds that connect signal components. Second you want to avoid alternate paths through the ground circuits. This is called a ground loop and will nearly always introduce hum and noise. The third function I already mentioned is personal safety. Any metal parts of an electronic device can get energized if the proper combination of faults occur. Perhaps not very likely, but being injured or killed is not something most of us choose to do. So if there is an internal fault, you want to make sure any voltages generated are conducted to the AC mains ground and not though your body.

Your other implied question on isolating the heater circuit has a different purpose. When you utilize a "totem pole" type configuration like the SRPP I used, the middle cathode is at about half the B+ voltage applied to the top anode. This voltage can exceed the heater to cathode rating for the tube. It can happen in steady state or during warm up as both sections might not warm up equally quickly. A common way to avoid this is to not ground the heater circuit and apply a reference voltage to it that is about 1/3 the value of the B+ on the upper anode. Then neither tube section will be in danger. There are some limitations to this method. Primarily it places a limit on the B+ that can be used. If you go to high (say over 350 volts) there is no good and safe value that will protect the tube under all situations. Fortunately in the designs I use there is little value in going over 300 v and often (like in the Forewatt) 200-225 is a good value to accomplish what is desired. All this said, many spec sheets on tubes give H-C ratings of up to 200 volts. I used to believe that until I smoked a few. Now I use the much more conservative ratings for the "worst" members of any given tube type which is generally 100 volts. There is another benefit to this method, usually the noise level is reduced slightly as it provides a bleed off path for wandering electrons emitted from the cathode that don't end up on the anodes.

I know the above is not a complete discussion of the issue, but I hope it helps a bit. Sorry for the length of the response.

Good listening
Bruce

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PostPosted: 28 Sep 2011, 21:35 
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I appreciate the informative reply. I'm a little closer to understanding but have some more things to clear up if I may.

In the project post there are a couple of statements that I'm confused on:

The signal and B+ grounds are connected to the chassis through a type X2 rated capacitor and a resistor in parallel with the capacitor.

I fully understand the schematic showing the cap and parallel resistor (X2 cap and resistor) where the common/negative power supply rail ends up.
But, I had the understanding that "signal" was input and output including the ground portions of the jacks. If this is not the case please explain.

I'm also having a hard time understanding what all is included in B+ grounds. I had assumed it was the common side of the B+ power supply circuit only.

The ground portions of the jacks should go to the "floating" signal ground in the circuit

I was not aware of a floating ground or common except for the heater power supply circuit.

Thanks for your patience,

Allen


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PostPosted: 28 Sep 2011, 22:26 
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Hi Allen, you are on the right track. The signal ground floats off the chassis, but is where anything in the signal path that needs a ground connects. Inputs, outputs tube cathodes and grids (via resistors) and some things if used like tone controls. The power ground is everything that is in the power supply including all the filters that need to go to ground. Both grounds tie together usually near the input at a single point. From there the type X2 capacitor and its parallel resistor go to the chassis ground. Often this will be at the power entry if you use an IEC (three wire connector like on a PC). The IEC connector often includes some EMI filtering and perhaps a fuse and many have a metal case with a ground tab. This is where the X2 and resistor should connect. You then have a single point where the chassis connects to the ground and single path from the active circuitry to it. I hope this helps, if not ask again.

Good listening
Bruce

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PostPosted: 29 Sep 2011, 18:57 
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Bruce,

I'm thinking I'm probably not using the right terms in my questioning which is pretty common when a newcomer to a subject starts inquiring.

I think the area I'm having a hard time getting my hands around is the "floating", "not floating" thing. I have understood that floating meant, in the case of grounding, to not connect to earth or chassis in any way but, if I'm reading correctly, you are suggesting that signal and power supply common (eventually) come together at the single point at the mains earth ground via the RC filter. If this is the case, please describe what you mean by floating.

Allen


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PostPosted: 03 Oct 2011, 07:33 
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I know that it may be too long, but for some theory you can read http://www.raleighaudio.com/Audio%20Component%20Grounding%20and%20Interconnection.pdf. I do not fully agree with everything here, and some explanations may be confusing, but I like the general idea.
The term "float" means - some isolation from something. For example, float heating in many SRPP means applying some voltage in relation to ground. It doesn't mean - you do not connect the heating to the ground in ANY way, just not connecting DIRECTLY, which allows to add some potential. Certainly "clear floating" should be fully isolated, but it is not the case.


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PostPosted: 03 Oct 2011, 19:47 
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I did a quick scan of Davenports paper and it should answer a lot of my questions. Thanks for the link! It makes sense what you're saying about clear floating grounds versus some isolation. I'm sure that's what Bruce was implying when he said "floats off the chassis" (after re-reading a couple of times).

If I may ask, what's your impression of your ForeWatt's sound after more listening time?

Allen


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PostPosted: 04 Oct 2011, 02:05 
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allenb wrote:
If I may ask, what's your impression of your ForeWatt's sound after more listening time?

At the time I'm using the same in principle (SRPP), but different in some countings tube preamp. Despite my initial feelings about "abusing" :) the layout initially designed to have rather opposite outcome (current vs voltage) I like it more and more. I'm going to build an integrated full (stereo) amplifier combined the ForeWatt and OddBlock principles, taking good from synergetic effect and using some convenients like remote control, control bus, automatic balancing, safety circuits...


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PostPosted: 05 Oct 2011, 17:08 
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BTW.... I am working on a remote control Forewatt design. I have two already, but both depend on outside sourced sub assemblies that have proven to be less reliable than I like. The current design should be diy friendly and easy to implement. Also there is no reason not to combine the Forewatt and Oddwatts. It works fine. The biggest issue is to be sure that all stages are well decoupled to insure stability. I would not include the input stage in the NFB loop as it is clean and stable as is. :soapbox: As you all probably figured out, I am not a fond lover of NFB and like to keep it simple and as minimal as needed to insure stability, not to clean up crud in the amp (I consider using NFB to do that poor design).

Depending on how you implement the Forewatt part, most notably the available B+ I have some updated cathode resistor values for you. At 230V on the upper anode use 560R for both cathode resistors, at 250 B+ use 910R, at 300 B+ use 1200R. These values give the lowest distortion with JJ ECC802S tubes. If you still have the 215 B+ leave the 470R resistors in place. If you go to the higher B+ levels, be sure to adjust the "bias" on the heater cathode tap. It needs to fall in the 75-80V range for best operation. All these values work on the stand alone Forewatts as well.

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