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Old 10th April 2006, 06:56 PM   #1
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Default Circuit and PCB Layout comments

Hi, I am looking for comments on a circuit and PCB Layout for a DAC gain stage. The idea is that you would use a dac with a passive I/V into this which would then provide the amplification. I am currently working on the DAC portion as well, but one could also use a HagDAC (http://www.hagtech.com/hagdac.html) or any number of other projects.

The circuit is a 5842 as a grounded cathode (I think that's the right term.) The plate load is a simple solid state CCS. This should provide a gain of about 43 and an output impedence of about 1.7K which is low enough for a preamp with a 50K pot.

The output coupling cap is connected to pad5 and pad6 and is mounted off the side of the board. The input coupling cap is likely unnecessary with some DACs in which case it could simply be jumpered. Otherwise there is room for a high quality cap there. (I may need an additional resistor to ground before the cap, but I am not really sure?) The heaters must be DC I would think, and this is the part I would especially like comments about as I am not sure the best way to lay them out, or if what I have done is fine.

As a side note, if I have boards made, I think that I will need to do at least 5. If this is the case, I'll likely have 3 or 4 extra. If anyone would want one, send me an email. Price would be whatever they cost to have made, which seems to be in the $15 range.

Oh, the 5842 seems to have 4 grid connections. I used which ever one seemed to be the closest to the input in the layout which is different for each side.

Also, C3 and C4 are not really polarized. Also also you the PS is seperate.

-d

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Old 11th April 2006, 06:48 AM   #2
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Where's the bypass for B+? Seems a little dangerous to count on the current sink to negate it's benefit. Why is C3 so physically large? You probably don't need any more than 0.1uF. Consider removing C1. The lack of it and slight cathode degeneration improve linearity and dynamics. Gain drops a tad, though, and output impedance rises. Boards look fine. Maybe you could cut it in half and use one for each channel? Unless you want the mirror effect.

And what is with this fake cascode current source? I keep seeing it used by everyone here. What's the benefit of such a connection? I don't get it. It's not a true cascode, it does NOT reduce output capacitance. Is it to compensate for Early Voltage? Or something to do with thermal transients from dynamic power loading on output transistor? Can someone please explain it?

jh
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Old 11th April 2006, 11:42 AM   #3
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Using a cascode as a constant current source allows you to sink a healthy anode current but have a high impedance load (roughly hfe1 x hfe2 x Re) with the result that distortion produced by the valve is very low indeed. And 5842 is a very linear valve in the first place. So once you drive a bit of constant current through it, you get a useably low output resistance with low distortion.

However, VHF oscillation is a distinct possibility. The HT needs to be decoupled with a 10n-100n capacitor with short leads/traces to the bottom of the cathode resistor and a surface mount grid-stopper resistor would be best. Most people would rely on the preceding device to decouple any input DC, but you definitely want an output decoupling capacitor.
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Old 11th April 2006, 12:20 PM   #4
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It's a good idea to NOT remove the bypass cap from the cathode resistor. With a CCS as a load, this will actually increase the distortion.
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Old 11th April 2006, 03:32 PM   #5
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Quote:
Originally posted by hagtech
[B]Where's the bypass for B+? Seems a little dangerous to count on the current sink to negate it's benefit.
In my head I was planning to put the final cap from a CLCRC filter near the board, and it was too large to fit on board. I do think I'll add a spot for the smaller decoupling cap suggested below, though.

Quote:
Originally posted by hagtech
[B]Why is C3 so physically large? You probably don't need any more than 0.1uF.[/]
Space for a Jupiter 0.1uF cap.

Quote:
Originally posted by hagtech
Maybe you could cut it in half and use one for each channel? Unless you want the mirror effect.
I think it depends on where I get it made. There are a few deals to get 5 "small" boards for a fixed rate which makes it in my interest to cram as much on each as possible.

Quote:
Originally posted by hagtech
And what is with this fake cascode current source?
In my case, it is shamelessly stolen from a certain hi-fi kit manfacturer. I don't want to say who as people should buy their kit and not get the design from me.


Quote:
Originally posted by EC8010
However, VHF oscillation is a distinct possibility. The HT needs to be decoupled with a 10n-100n capacitor with short leads/traces to the bottom of the cathode resistor and a surface mount grid-stopper resistor would be best.
Is it better to be closer to the cathode, or the top of the CCS, or right in between?

Also, surface mount over carbon? What sort of watt rating does the resistor need, or does it matter, and is it worth adding a spot for a plate stopper as well?

Quote:
Originally posted by EC8010
Most people would rely on the preceding device to decouple any input DC, but you definitely want an output decoupling capacitor.
As the designer of the preceding device, I happen to know that it lacks a coupling cap. However, it seems to me that the raw dac ouput can't possibly have more than a fraction of a V of offset. Maybe it is possible to leave this out all together? I'll have to experiment.

Quote:
Originally posted by SY
It's a good idea to NOT remove the bypass cap from the cathode resistor. With a CCS as a load, this will actually increase the distortion.
I think I'll leave the spot at any rate and maybe experiment both ways.


Thanks for the suggestions. I appreciate it.

-d
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Old 11th April 2006, 03:59 PM   #6
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Quote:
Originally posted by dsavitsk
Is it better to be closer to the cathode, or the top of the CCS, or right in between?

Also, surface mount over carbon? What sort of watt rating does the resistor need, or does it matter, and is it worth adding a spot for a plate stopper as well?
Ideally, you want one end of the decoupling capacitor's short leads to go directly to the top of the current programming resistor in the cascode and the other directly to the bottom of the cathode resistor. If you can't do that, make the traces wide (reduces inductance).

Yes, SM over carbon. Reason being, on a PCB, you can butt the end of the resistor right up to the valve pin with no intervening track. It won't dissipate any power, but you'll probably prefer a larger resistor simply because it's easier to work with.
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Old 11th April 2006, 11:30 PM   #7
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Okay, lots of work.

I made most of the changes suggested except the smd grid stopper. The problem is that there are traces running under the resistor, and using smd will make that portion of the layout a lot harder. I would rather move the resistors closer and mount them under the board if it is really necessary. But, if this is really emphatic, as in it is really necessary, then I will figure out a way to do it.

But, I added caps to decouple the PS. I also added smaller input caps in addition to the big ones.

Here is the changed board

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However, I also made a version using a single 5687. The output impedence is similar, but the gain is a more tame 18 rather than 43.

Interestingly, this would also make a good preamp module, so I think it is more likely that I'll go ahead with this board and save the 5482 for later.

Here is thr 5687 board and partial schematic:

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Old 12th April 2006, 08:58 AM   #8
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Ah, I should have said explicitly. Put the SMD grid stopper under the board with one end touching the valve pin - then there's no problem with positioning.
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Old 12th April 2006, 06:43 PM   #9
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Why would a grid stopper resistor have to be SM type? What is wrong with the classic carbon composition 1/4W or 1/2W leaded type? They work well at high frequencies (bulk type element), can handle power surge, and easily make voltage and layout spacing ratings.

Certainly the reduced inductance is of questionable merit, as the pins and leads within the tube itself far outweigh that of the resistor.

jh
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Old 12th April 2006, 08:02 PM   #10
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Exactly, it's not the resistor's construction (although it might be lower inductance than a carbon composition), it's that the SM resistor can be butted right up to the valve pin - no inductance between the pin and the resistive element.
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