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Old 5th March 2012, 10:20 AM   #1711
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I welcome all developments in the "26 preamp plus" field - adding an extra tube for gain and stepdown to line levels ( 600 to 1K), adding Mosfets and so on.

The problem with a 26 and a LL1660 in 4.5:1 is the gain is pretty much wiped out. For those who are depending on the gain of the preamp because they use a 2 stage amp or an amp needing a bit more gain, then the extra circuitry is pretty important.

I see this as a good line of development in reaching a good model for a 26 preamp with gain AND low output impedence. We have a good basic model already which has been tried and tested, so we could put some effort into this extended circuit. I don't know that it needs a new thread - there's most of the information on this thread so I guess we just continue.

Andy
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Old 5th March 2012, 10:22 AM   #1712
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A lot of folks like LEDs, and they are probably OK.

In circuits that can support the cost, then the buried-zener reference chip, like the Burr-Brown REF10 is the lowest noise, highest stability.

the usual band-gap reference chips are much too noisy to use here.

Zener diodes are only moderately stable, and have some sample-to-sample variation. for the circuit you are using they are too noisy, and need at least some RC filtering.

I'd probably start with some cheap LEDs, then see if you prefer the sound if a REF10 is used.
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Old 5th March 2012, 10:53 AM   #1713
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Andy,
I personally found the cascoded MOSFET Gyrator to sound really good (not the CCS) in this setup. And is dirt cheap for anyone to build them using DN2540s
The 4P1L (or even the 71a) has a low anode resistance and its biased at 20mA so works brilliantly with either a choke or a gyrator. When tried DHT with low anode current and higher anode resistance (such as 30sp, cx112a and 01a) they are not optimised in this configuration. My 45 SE amp has a 47k input resistance and if you factor in the driver miller capacitance in addition to the cable's then they can't perform well.
Choice here is either a DHT cathode follower (with all the additional power supply complexity or a source follower as Morgan Jones recommends
We need to experiment and report the results!
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Old 5th March 2012, 10:57 AM   #1714
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Quote:
Originally Posted by Rod Coleman View Post
A lot of folks like LEDs, and they are probably OK.

In circuits that can support the cost, then the buried-zener reference chip, like the Burr-Brown REF10 is the lowest noise, highest stability.

the usual band-gap reference chips are much too noisy to use here.

Zener diodes are only moderately stable, and have some sample-to-sample variation. for the circuit you are using they are too noisy, and need at least some RC filtering.

I'd probably start with some cheap LEDs, then see if you prefer the sound if a REF10 is used.
Rod,
Never used the Ref10. Worth trying them? I'd probably do a first try with LEDs and report results. Need to check which MOSFETs and bipolar have at hand first. I do have some SMD ones which may pick up if appropriate so could end up building a tiny PCB board to fit in the preamp chassis...
Cheers! Ale
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Old 5th March 2012, 11:28 AM   #1715
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Quote:
I personally found the cascoded MOSFET Gyrator to sound really good (not the CCS) in this setup
Does that mean you tried the CCS and did not like it. Or that you simply did not try the CCS?
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Old 5th March 2012, 12:17 PM   #1716
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Quote:
Originally Posted by mogliaa View Post
Andy,
I personally found the cascoded MOSFET Gyrator to sound really good (not the CCS) in this setup. And is dirt cheap for anyone to build them using DN2540s
Could you give us a schematic, or point to where you posted one?

andy
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Old 5th March 2012, 01:25 PM   #1717
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Andy,
The cascoded MOSFET gyrator I used in the 26 is the same as the one I posted below for the 71a, albeit in the latter I used a BSP129 instead of the dn2540. Similar characteristics but is SMD. You should probably use the TO220 DN2540 which is dead easy to use instead. You can set the operating point with the trimpot and won't change despite the valve ages.

http://www.bartola.co.uk/valves/Valv...amplifier.html
Cheers,
Ale
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Old 5th March 2012, 01:32 PM   #1718
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Quote:
Originally Posted by Bas Horneman View Post
Does that mean you tried the CCS and did not like it. Or that you simply did not try the CCS?
Hi BAS,
I tried the following so far:
OT (LL1660): perfect sound if matching with valve. Good bass and superb tone
Cascoded MOSFET gyrator (DN2540 and BSP129): in Mu-follower mode you can get the lowest distortion and best performance as line driver. Great tone and good bass. Dirty cheap and easy to use. Quite like that you can adjust the operating point very easily with a trimpot.
MOSFET CCS (dn2540): not as great as the Mu-follower gyrator when driving the long cables and amp's driver. Bass good but overall tone a tad worse than the gyrator.

Probably a cascoded PNP gyrator will be better in a 26 due to the low current required. I built the MOSFET version for 4P1L, 6P21S, 46, 71a and 10Y which all run at currents greater than 15mA
I should build a cascoded PNP version for the 26 to test the difference...
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Old 5th March 2012, 01:35 PM   #1719
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Thanks for you post.
Quote:
MOSFET CCS (dn2540)
Single or Cascaded?
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Old 5th March 2012, 01:43 PM   #1720
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Cascoded. I tried with good results a single MOSFET gyrator with good results. However the output resistance is far too low compared with the cascoded version
This is my experience, other may prefer other configurations...
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