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PostPosted: October 29th, 2021, 5:48 pm 
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Joined: March 12th, 2013, 11:12 am
Posts: 738
Tubes can offer a slower turn-on times, look cooler and are easy to replace. If your PS is well designed, there is no reason tubes rectifiers should sound "soft".

Cogito wrote:
TubeDriver wrote:
Why not do tube recifier? I ended up with a 5Y3 LCLC PS and had to add a separate 12v filament tranny when I went to a 1626 tube.


My understanding is, tube rectifiers sound softer than SS Shottky rectifiers. Once the transients are softened in the preamp, they are lost forever. No?

Quote:
My advice is to put this on a large chassis which will allow you 1) the space to keep things apart when needed and 2) the space to move and/or add things around when needed.


Yes,


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PostPosted: October 29th, 2021, 6:26 pm 
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Joined: January 14th, 2015, 11:15 pm
Posts: 499
Hi Sashi,

Since this is your first scratch build, you are embarking on a journey down the rabbit hole. As you are already discovering, there are many options and it's difficult to choose between them. However, the best way to find out for yourself what works for your ears is to build test mules where you can compare one option with another.

One of the best things I ever did was to build my tube taster so that I could listen to the sound of different tubes. You might find it interesting. The key was using switches so that there was minimal delay between using different tubes.

http://audioratbag.blogspot.com/2012/02/tube-taster-linestage.html

In your case, you could set up dual throw switches so that you compare the various ideas you want to try. For example, you could build modules for the following

power supply filtering - various combos of inductors / caps, electronic
load for the plate - resistor, CCS, gyrator
tube - I know you are set on the 12B4A which is fine, but eventually you may want to try DHTs
cathode - resistor, resistor / cap, diodes of various kinds
output - transformer, capacitor, mosfet driver

So here is some more homework:
https://www.bartola.co.uk/valves/2021/02/06/the-low-gain-dht-pre-amp-library/ for DHT tubes - he also designed a gyrator which is interesting

http://tubelab.com/articles/tubelab-prototyping-system/ - prototyping

By the way, I am a bit puzzled by your need to drive five amplifiers. Could you expand a little about that?

ray


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PostPosted: October 29th, 2021, 8:29 pm 
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Joined: February 28th, 2013, 1:19 pm
Posts: 914
Cogito wrote:

Ok, I understand how CCS impacts the linestage and its advantages. Since I am planning on a regulated B+ and extremely low ripple and noise power supply, CCS is not advantages in those categories.

As of now, I am not convinced CCS is the right topology for my application.
My requirements:
    1. Low Distortion
    2. Low Noise
    3. Excellent Frequency Response
    4. Micro details
    5. Extremely low output impedance
    6. Lots of headroom for transients

My DAC puts out 2.8V for PCM and 1.45V for DSD over RCA. That's 6dB difference. To bring DSD to the same level as PCM with headroom, 9-10dB would be plenty of gain for my system, I think.

Going down my list:
1. Low Distortion - CCS has lower distortion
2. Low Noise - CCS has higher SNR
3. Excellent Frequency Response - CCS has narrower bandwidth due to greater sensitivity to load capacitance
4. Micro details - CCS and Resistive load should be same
5. Extremely low output impedance - CCS impedance is about 1K (Rp). Not sure the impedance of resistive load topology. This is important to me because the preamp needs to feed 5 amps. Transformer coupling instead of CCS or tube buffer stage?
6. Lots of headroom for transients - CCS has limited headroom

May be it is my lack of understanding but CCS doesn't seem to provide the benefits I am looking for.


Shashi,

I am with Ray on the benefits of a CCS. I believe you are incorrect in a number of areas, misreading information in the article that Ray sent you the link for.

1. Low Distortion - correct - CCS loading has lower distortion because it loads the plate of the tube with an equivalent resistor of several megaohms. This results in a horizontal loadline across the tube curves where there is virtually no 2nd order distortion compared with conventional resistor loading, the value of which is limited by the power supply voltage. No other loading linearizes a tube more than CCS loading.

2. Low noise - correct, and with a regulated supply you will achieve vanishing noise levels.

3. Excellent Frequency Response - Incorrect. A CCS WILL have excellent frequency response. Any stage is going to be impacted by MIller capacitance of the following stage, but a CCS is much better in it's ability to drive into difficult loads...such as the input of a Mosfet in a source follower configuration (for driving A2). Look at TubeLab "Power Drive" circuit.

4. Micro detail - I would think the CCS would come out on top due to low noise floor that would otherwise mask low level signal.

5. Extremely Low Output Impedance - Not a problem - The output impedance is using a cathode bypass cap across the cathode resistor, or low equivalent resistance fixed bias arrangement, is the value of the plate load resistor in parallel with the Rp of the tube. Given a 10K plate resistor and an Rp of the 12B4 of around 750 ohms, this results in 10K||750 = 698 ohms. Using a CCS load of 2M ohm, the output impedance is 2M||750 = 750 ohms. This is negligible. Now if you an unbypassed cathode resistor of 500ohms, this multipled by the gain of the tube (say 6) adds to the Rp of the tube, 750+6*500 = 3750 ohms, in parallel with the CCS load of 2M results in a output impedance of 3743 ohms. Not so good. So you really want to use a bypass cap or something like LED or diode fixed bias with a low resistance.

6. Lots of headroom for transients - A CCS loaded 12B4 at 125V/20mA can swing +/-60V. That is an order of magnitude more swing than you need.

Suggest you hit the books some more.

David


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PostPosted: October 29th, 2021, 8:36 pm 
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Joined: February 28th, 2013, 1:19 pm
Posts: 914
Shashi,

I apologize for the snide remark at the end of my post, it was not helpful or kind. You ARE learning, and it will take a few days (at least) of study to understand the circuit and trade-offs. As I pointed out earlier, a CCS load is a complication to building a circuit to understand its operation, but it is a very beneficial compilation. I would definitely use a CCS in my build (and have for a 12B4 linestage), But a resistive plate load (or inductive) can still make a decent sounding preamp. It is often better to start simple and upgrade to understand what each technology brings to the performance.

One thing I left out, the headroom of the CCS is really dependent on the voltage across the CCS, but if you have a decent voltage (50 to 60 V) that is more than sufficient for preamp use.

David


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PostPosted: October 29th, 2021, 9:06 pm 
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Joined: July 17th, 2016, 6:24 am
Posts: 1099
Hey David, Dont worry about it.

I read about CCS from the PDF Ray linked and Merlin Blencowe's book. Merlin had very detailed information with different CCS topologies. He also listed disadvantages of CCS, which is the source of my info.


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PostPosted: October 29th, 2021, 10:03 pm 
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Joined: February 28th, 2013, 1:19 pm
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Shashi,

Read through the Blencowe book discussion on disadvantages of a CCS, and I don't think most of the arguments are really a concern for this application. With a low mu, low Rp tube with a dual Mosfet current source on top, you are going to get outstanding performance. I believe his discussion at the beginning was generic, and included poor performing triode CCS in the mix.

David


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PostPosted: October 30th, 2021, 8:27 am 
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Joined: July 17th, 2016, 6:24 am
Posts: 1099
David McGown wrote:
Shashi,

Read through the Blencowe book discussion on disadvantages of a CCS, and I don't think most of the arguments are really a concern for this application. With a low mu, low Rp tube with a dual Mosfet current source on top, you are going to get outstanding performance. I believe his discussion at the beginning was generic, and included poor performing triode CCS in the mix.

David


Ok. I will try to take modular approach to easily disconnect CCS for testing.


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PostPosted: October 30th, 2021, 12:13 pm 
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Joined: July 24th, 2015, 4:17 pm
Posts: 1701
Location: Parkville, Maryland
My view is that you have to consider that a tube amplifier is a high-impedance device (voltage) and using the solid-state amplifier methodology (low impedance - current) to size a power supply can be counter intuitive.

Big is not necessarily better. Next up is the ripple-current rating of an electrolytic capacitor. You need to make your selection with at least a 20% margin in terms of ripple-current performance otherwise the expected useful life of your power supply is only as good as the capacitor's ability to deal with the ripple current. 105-degree C rated capacitors would be a place to start in your search.

Or -- since you are starting from scratch -- why not take the Conrad-Johnson approach and go all film for your power supply? You can plan around big-ass film caps and not have to worry about service life. Besides it will sound better.

The final thought is that the power transformer has to be able to provide the charging current without stress or it can overheat and even fail.

Food for thought.

_________________
Walt


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PostPosted: October 30th, 2021, 1:43 pm 
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Joined: February 28th, 2013, 1:19 pm
Posts: 914
Shashi,

If sometime you want to hear what a 12B4 linestage sounds like, I just dug out the linestage I built a while ago that ended up having a regulator issue that I finally tracked down this afternoon. I am using Morgan Jones' "Statistical Regulator", which is a current source fed shunt regulator consisting of a series string of 5.6V Zener diodes. Ended up I had 2 bad zeners in the string on one channel, so replaced them and the regulated supplies for both channels work. I did have it jury-rigged to just use one regulator shared between both channels, and it worked OK, but was worried about the current through a single series string of zeners (40mA) so put the preamp away for a while.

BTW, this preamp has CCS loading for each 12B4, with a cathode resistor and ELNA Silmic electrolytic bypass capacitor. Anyway, I have it in my basement near field system and it is sounding very good driving a pair of Pass AmpCamp SE Mosfet Class A amplifiers into my Aria 5R speakers and subwoofer. Definite improvement over a Topping A50s.

BTW, the power supply is a seperate box, connected via an umbilical.

David


Attachments:
12b4preamp.jpg
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12B4 Preamp 3.jpg
12B4 Preamp 3.jpg [ 891.45 KiB | Viewed 7211 times ]
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PostPosted: October 30th, 2021, 3:55 pm 
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Joined: July 17th, 2016, 6:24 am
Posts: 1099
David,
I would love to hear it.
Shashi


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