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Old 10th August 2018, 08:09 AM   #361
indra1 is offline indra1  Indonesia
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Quote:
Originally Posted by andyjevans View Post
... Any other suggestions? ...
I would suggest regular schottky junction diode such as the 1N5822 rather than a regular pn junction device for lower noise but more devices are required due to lower forward voltage drop.

Last edited by indra1; 10th August 2018 at 08:11 AM.
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Old 10th August 2018, 10:00 AM   #362
mogliaa is offline mogliaa  United Kingdom
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01A question
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Originally Posted by Rod Coleman View Post

- the forward voltage will be more stable, and the signal resistance will be lower, if the current is increased to at least 250-500mA. The easy way to achieve this would be to filament-bias the diode string. 250mA would bring the signal resistance of the C3D02060A down to around 1Ω.

Filament bias would also increase the forward voltage, so you would need fewer diodes to get the same bias-voltage.
Hi Rod,
That was exactly my thought initially, but I decided to test it and listen to the stage which was revealing. I went back to my SiC tracing and zoomed the low current section. Between 5-6mA area where I'm currently operating at, seems to be quite linear:

Click the image to open in full size.

However, I agree fully that in filament bias will may be even better due to the current level will push down the dynamic resistance below 1ohm for sure.
Cheers
Ale

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Originally Posted by VT52com View Post
Hey Ale, love your site, some really nice things on there that I want to try out Rod's here too, have a bunch of his filament/bias boards here ready to go into amps.

Still on the fence about CCS/Gyrators in the anode circuit, have always preffered inductive loads and no solid state in the signal path. Maybe you'll convince me one day


sorry for the threadjack
Not a problem and good to see you around here. Is a personal decision. I prefer the gyrator load, however chokes (if properly selected) are very good as well. On the flip side if you are looking at low current and high Ra DHTs, the choke isn't the best option and gyrator will work better overall. It sounds best in my opinion on those specific DHTs .
Ale
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Old 10th August 2018, 10:32 AM   #363
Rod Coleman is offline Rod Coleman  United Kingdom
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01A question
Looks good, Ale - and filament bias looks the way ahead. I am sure we will be hearing from those that try it with filament bias soon.

As for other didoes to try, yes the 1N5822 is certainly worth a look. It is low-cost, and given the quantity of diodes Andy was discussing, it is worth a try.

Normal PN diodes are not noisy - in forward conduction mode, which is 100% of the time, in this circuit. Any sample should measure <1ÁV rms over the audio band, with 250mA flowing. They are worth investigating - even the standard-recovery parts - to reduce the number of parts.

Example: the ultra-cheap 1N4001 gives 0.8V forward voltage at 250mA (the 01A filament current) and <0.5Ω at that current, looking at the Fairchild curves. Much smaller than the others, too.
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Old 10th August 2018, 10:51 AM   #364
indra1 is offline indra1  Indonesia
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Originally Posted by mogliaa View Post
... However, I agree fully that in filament bias will may be even better due to the current level will push down the dynamic resistance below 1ohm for sure. ...
Forward voltage drop will also increase with more current, the number of diodes may need to be reduced (lower dynamic resistance bonus) to maintain bias.
Quote:
... On the flip side if you are looking at low current and high Ra DHTs, the choke isn't the best option and gyrator will work better overall....
There is also an option to use a choke as Rmu on the gyrator.
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Old 10th August 2018, 01:19 PM   #365
deafbykhorns is offline deafbykhorns  United States
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Quote:
Originally Posted by andyjevans View Post
We had a 9 preamp shootout today with Ale Moglia and Tony Rees, with one very significant result which I, for one, didn't expect. This was an 01A preamp (gyrator + source follower) that Ale had put together the day before on a hunch with 6 Cree C3D02060F diodes in the cathode. It was not only better than the other 10Y, 01A, EML20A, 6E6P-DR and 2P29L preamps (most in filament bias), it was comprehensively better in all ways. More transparent, better instrumental timbre, cleaner and all round more engaging and a better listen. If this can be reproduced for a variety of DHTs it marks a compete change in my builds from filament bias to SIC bias. I can't wait to get the soldering iron out - I want this sound in my system!

I'm sure Ale will be writing this up in more detail in his blog and with photos, so this is just advanced notice of what's coming!
'm glad you posted this, the 10Y was going to be one of my tube variants to try.
I think I'll stick with the 01A

Here's a snapshot of current/voltage using Ale's model on 6 diodes
Attached Images
File Type: jpg Capture.JPG (50.0 KB, 152 views)

Last edited by deafbykhorns; 10th August 2018 at 01:46 PM.
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Old 10th August 2018, 01:24 PM   #366
andyjevans is offline andyjevans  United Kingdom
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Quote:
Originally Posted by deafbykhorns View Post
'm glad you posted this, the 10Y was going to be one of my tube variants to try. I think I'll stick with the 01A
We only heard SIC diode bias in the 01A. Doesn't mean that there won't be equivalent benefits for all the other DHTs. I'm rebuilding a 2P29L preamp at the moment to see what that gives.
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Old 10th August 2018, 02:03 PM   #367
merlin el mago is offline merlin el mago
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Quote:
Originally Posted by andyjevans View Post
I have a string of 6 x CSD01060A on my 01A and the bias is exactly 5v. That's 0.83v each.
Quote:
Originally Posted by ui2 View Post
So, according the CSD01060A data sheet, I interpret that you run these diodes below 50mA, right?
All the best,
Ulrich
Quote:
Originally Posted by andyjevans View Post
Should be around 4mA for an 01A.

Regarding plate chokes - yes, I like the NP Acoustics 136H 15mA amorphous plate choke. It's the best plate choke I've used. Nothing fancy cosmetically, just sounds good.
There is something that my ignorant person doesn't understand:

1-Filament of 01A current 250mA voltage 5V
2-If used filament bias resistor as per Moglia schematic 4V across 20 ohms = 200mA (starved) that's OK
3-If substitute the filament bias resistor for diode bias we can get the voltage needed but how can reach the 200mA of current for filament current?
4-Or the 200mA are supplied by Rod Coleman reg. and the string of diodes it's only to reach the voltage grid bias + current plate?

TIA
Felipe
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Old 10th August 2018, 02:08 PM   #368
ui2 is offline ui2  Germany
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Dear Felipe,
the answer to your question is #4 (at least regarding things I reported).
All the best,
Ulrich
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Old 10th August 2018, 02:27 PM   #369
merlin el mago is offline merlin el mago
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Thank you Ulrich. So my target for diode bias for 26 preamp is 10V & 5mA.

Last edited by merlin el mago; 10th August 2018 at 02:36 PM.
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Old 10th August 2018, 06:17 PM   #370
andyjevans is offline andyjevans  United Kingdom
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For a 26 preamp you set up the Rod Coleman regs normally, not in filament bias. This may mean changing one of the resistors (was R8 on my older versions of Rod's regs). So you need the usual filament current and voltage from the reg - 1.5v at 1A or a bit less if you're starving it. You then set up the string of diodes to give you the required bias voltage across it, e.g. 10v.

We're also talking about using the diodes in filament bias, which is another way of doing it and may require heatsinks. But that's another story at the moment.
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