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Old 6th January 2005, 08:31 PM   #51
Previously: Kuei Yang Wang
 
Join Date: Nov 2002
Konnichiwa,

Quote:
Originally posted by Rod Coleman
If you short across F+/F- the current stays at 1,3A - so the currentsource transistor's dissipation rises to ~10W (for ~9V raw input) - fine for TO220 types mounted on <10 deg. C/W. In the gyrator side nothing happens!
Shortcircuit +F to the negative rail will do nicely.

BTW, improvement to your gyrator....

Add a resistor of 0R47 in series with the emitter, connect a 1K resistor in series bith the base of the small signal transistor, return a 47uF or so capacitor from +F to the base....

Quote:
Originally posted by Rod Coleman
As for whether the gyrator contributes greatly to the improved sound, I suspect that varies with the leakage capacitance (through the filament transformer) back to the "B-" terminal of the rig in question. Big leakage = current path for diode recovery pulses.
Of course, with good layout and schottky diodes, plus CMC filtering this is mostly taken care of anyway....

I am thinking again, sadly immediatly sending complexity through the roof again.... Use an improved gyrator (the one with the added rc noted above) and make one in each line (one PNP Darlington and NPN Darlington. Set the voltage between the base's via a Phototransistor or the like, use a CC in series with a LED and a TL431 as sense element to get the voltage right....

I thinlk I'll stick with LM1085....

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Old 7th January 2005, 10:58 AM   #52
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Default Anothe doodle

Quote:
Originally posted by Kuei Yang Wang
BTW, improvement to your gyrator....

Add a resistor of 0R47 in series with the emitter, connect a 1K resistor in series bith the base of the small signal transistor, return a 47uF or so capacitor from +F to the base....
A bootstrapped second-order gyrator! Ideal if you run out of schottkys...

Quote:
Originally posted by Kuei Yang Wang

I am thinking again, sadly immediatly sending complexity through the roof again.... Use an improved gyrator (the one with the added rc noted above) and make one in each line (one PNP Darlington and NPN Darlington. Set the voltage between the base's via a Phototransistor or the like, use a CC in series with a LED and a TL431 as sense element to get the voltage right....
you're on to something there. If we shift it to a Kirchoff-architectured circuit, some of the simplicity comes back. We could increase the current source to the maximum expected 300B, eg 1,5A and bleed off a variable amount until 5V appears across the filament. How about -

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For the current sources, choose discretes or regulators according to taste.

A circuit that 'flows' like that lays out nicely on 'hacksaw-matrix' copper with hardly any spaghetti wiring.

The great benefit is that voltage control is achieved without any compromise to the high dynamic impedance (looking back into the power supply) from either filament terminal.

For those that like twinkling LEDs, we could put one in series with the base - LED lights only after the filament has warmed up! Actually, the LED would also help align the bleed to zero when 5V is achieved... getting around the 2V saturation of a 431.
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Old 7th January 2005, 11:28 AM   #53
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Default Re: Anothe doodle

Konnichiwa,

Quote:
Originally posted by Rod Coleman
A bootstrapped second-order gyrator! Ideal if you run out of schottkys...
Yup. You can also use it with a nice FET (I use 2SK2610) as small, cheap and lightweight replacement for HT Chokes.... Another trick for that one is to actually tap off the lowered and filtered DC reference from the rail of an earlier stage in the Amp which gets an RC Filtered supply directly from the first reservoir capacitor leaving minimal componentry for the bootstrapping...

Quote:
Originally posted by Rod Coleman
you're on to something there. If we shift it to a Kirchoff-architectured circuit, some of the simplicity comes back. We could increase the current source to the maximum expected 300B, eg 1,5A and bleed off a variable amount until 5V appears across the filament. How about -

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Yes, I thought about that too, the shunt across the heater may be an issue up to a point, plus we reject a lot of extra heat...

Quote:
Originally posted by Rod Coleman
For those that like twinkling LEDs, we could put one in series with the base - LED lights only after the filament has warmed up! Actually, the LED would also help align the bleed to zero when 5V is achieved... getting around the 2V saturation of a 431.
I would probably use some kind of optocoupler and use the LM431 with the LED in the Cathode line and a J-Fet CCS from ahead of the positive gyrator. I'll draw out what I have in mind when I get the time....

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Old 7th January 2005, 11:41 AM   #54
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Default Re: Re: Anothe doodle

Konnichiwa,

Quote:
Originally posted by Kuei Yang Wang
I would probably use some kind of optocoupler and use the LM431 with the LED in the Cathode line and a J-Fet CCS from ahead of the positive gyrator. I'll draw out what I have in mind when I get the time....
I forgot, I recently had to put P-Spice on my work PC, so here a quick drawn out IDEA. A real circuit would add short circuit portection, the darlington transistors obviously could be replaced by LT1085/1033 for NPN/PNP respectively for people like me who like "bullet proof" stuff....

Click the image to open in full size.

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Old 7th January 2005, 12:36 PM   #55
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Default Build Fun

Quote:
Originally posted by Kuei Yang Wang
Konnichiwa,



I forgot, I recently had to put P-Spice on my work PC, so here a quick drawn out IDEA. A real circuit would add short circuit portection, the darlington transistors obviously could be replaced by LT1085/1033 for NPN/PNP respectively for people like me who like "bullet proof" stuff....

One difference between our two circuits (& one would probably need to build to know if it was important) has to do with the dynamic impedance across the filament as frequency increases beyond the control loop's bandwidth.

On one hand, the current source (ie collector facing the filament) goes high-Z as the loop loses control; for the gyrator/voltage source, or a regulator IC in any configuration, (ie.: emitter facing the filament) a low impedance is presented.

It begs the question whether any (low level) shunting of AF or noise HF across the filament affects DHT performance.

Fun to build & find out, I expect!
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Old 7th January 2005, 01:04 PM   #56
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Default Re: Build Fun

Konnichiwa,

Quote:
Originally posted by Rod Coleman
One difference between our two circuits (& one would probably need to build to know if it was important) has to do with the dynamic impedance across the filament as frequency increases beyond the control loop's bandwidth.
Hmm. The loops turnover in both cases will be pretty low, in my case there is a 2nd pole, so it would a 2nd order rather than 1st order handover to the bootstrapped gyrator (which in effect behaves much like a choke). The final limit actually will be in either case the 10K resistor which remains across the filament from the 431 Voltage sensor.


Quote:
Originally posted by Rod Coleman
On one hand, the current source (ie collector facing the filament) goes high-Z as the loop loses control; for the gyrator/voltage source, or a regulator IC in any configuration, (ie.: emitter facing the filament) a low impedance is presented.
It is my understanding from discussions with GR and others that if the cathode delivers different current levels (due to signal) it will cool/heat up more which will shift emission, thus we want low impedance at very low frequencies (say below 0.5Hz) but high impedance in band for the audio signal.

Quote:
Originally posted by Rod Coleman
Fun to build & find out, I expect!
I do to.

But guess what, my next 300B Amp gets a simple Schottky, Cap, Resistor Cap and Common Mode Choke supply, no fancy schmancy 'tronics....

But the one AFTER THAT will get electronic chokes in the +B Supply and the heaters.... ;-)

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Old 7th January 2005, 05:53 PM   #57
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Default Re: Re: Build Fun

Quote:
Originally posted by Kuei Yang Wang
Konnichiwa,




It is my understanding from discussions with GR and others that if the cathode delivers different current levels (due to signal) it will cool/heat up more which will shift emission, thus we want low impedance at very low frequencies (say below 0.5Hz) but high impedance in band for the audio signal.

Makes sense and aligns with Guido's words at the start of the thread. There seems to be a performance gain with current driven heating (high signal impedance drive) that is lost with (low Z) voltage drive (reduced further by the regs output cap).

Quote:


Hmm. The loops turnover in both cases will be pretty low, in my case there is a 2nd pole, so it would a 2nd order rather than 1st order handover to the bootstrapped gyrator (which in effect behaves much like a choke). The final limit actually will be in either case the 10K resistor which remains across the filament from the 431 Voltage sensor.



Looking from the viewpoint of audio signals across the filament (looking backwards if you like), they will pass current from emitters to collectors much easier (Z = 1/gm) than Collector to emitter (Z = Megohms). The low impedance loop is closed through the power supply caps. So I reckon we need to see a collector (or drain) facing one side the filament if we are to expect a high impedance.

Quote:

But guess what, my next 300B Amp gets a simple Schottky, Cap, Resistor Cap and Common Mode Choke supply, no fancy schmancy 'tronics....

But the one AFTER THAT will get electronic chokes in the +B Supply and the heaters.... ;-)

I have to admit to getting poor results with B+ gyrators so far.. but I need to get some decent HV capacitors stocked up to do it right - What do you recommend? Are those LCR audio grade types worth trying?
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Old 7th January 2005, 06:08 PM   #58
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Default Re: Re: Re: Build Fun

Konnichiwa,

Quote:
Originally posted by Rod Coleman
Looking from the viewpoint of audio signals across the filament (looking backwards if you like), they will pass current from emitters to collectors much easier (Z = 1/gm) than Collector to emitter (Z = Megohms). The low impedance loop is closed through the power supply caps. So I reckon we need to see a collector (or drain) facing one side the filament if we are to expect a high impedance.
The question is "how high an impedance"?

A bootstrapped gyrator with a darlington transistor amplifies the 0.5R emitter resistance by it's transconductance (which is huge) up to quite high frequencies. Together with a few smallish value L's that should kill any issues.

But I'll have a think how we can have two collectors facing the Filament.

Quote:
Originally posted by Rod Coleman
I have to admit to getting poor results with B+ gyrators so far..
Hmmm, never had any problems, I use them quite often, both bootstrapped type and normal (Capacitor Follower) type.

Quote:
Originally posted by Rod Coleman
but I need to get some decent HV capacitors stocked up to do it right - What do you recommend? Are those LCR audio grade types worth trying?
No idea. I use high quality MKP's and that's that.

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Old 9th January 2005, 11:39 AM   #59
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Default Bootstrap Gyrator Trial

Quote:
Originally posted by Kuei Yang Wang
BTW, improvement to your gyrator....

Add a resistor of 0R47 in series with the emitter, connect a 1K resistor in series bith the base of the small signal transistor, return a 47uF or so capacitor from +F to the base....
The bootstrapped gyrator looks, on reflection, too good to miss. So I added it to my amp last night (on one side) and played a few LPs through it.

No great surprise to report a positive feeling about the result: a general sense of added freshness is as near as I get get to describing it. But OS-CONs are involved, and mindful of the counsel enshrined in SANYO OS-CON Technical Manual 5.2 (1997) one must "age the tone quality of OS-CONs" specifically "it is recommended to wait at least ten hours before evaluating and analysing the OS-CON's tone quality"

It will be a few days before I can get the professional ears round again, but by then the organic caps will be deeply composted and I'll be able to give a considered report.

I'll need the second opinion (first opinion in weight) to counter my autosuggestibility where design changes aimed at reducing filament noise are concerned. Previous gyrator tweaking (transistor types, Szlikai vs Darlington) was well worth the effort and I'm expectant of every degree of noise reduction to be perceptible... maybe it's time to ransack my book of UHF chip filters, or look at RF power transistors..

Meanwhile, Wang-san, I tip my hat in your direction for a top circuit.

IMPLEMENTATION

9,2V supply, second order gyrator, Darlington of FZT1051A(Zetex) + 2SD2061(ROHM).
First stage 1K (ROHM 1206 chip) 560uF/10V SANYO SVP.
Bootstrap stage: 1K (C film) 47uF/16V OS-CON (Nippon Chemicon 16FF47M) 0,47 Ohm nonmagnetic wirewound (a bit of L in this helps rather than hinders, I suspect).
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Old 11th December 2009, 10:39 AM   #60
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Default 5-yearly update!

I have had requests for more information on the filament circuit across the five years it has been up here.

Mostly, component questions - It seems that all of the transistors listed are no longer obtainable.

And the circuit published changed a little, in order to use a regular darlington configuration for the gyrator section.

So here's a suggested circuit and parts data for anyone wanting to build today. The ST 2FT1480 is a new part, but has the desirable high ft of 100MHz. I would like to specify a nice TO-220 isolated package NPN with 100M+ for ft, and when I find one I'll try it. Meantime the MJE15032 should work OK, but in the current sink section Q3/Q4, be sure the pair don't oscillate. Hold an AM radio close to it if you don't have a scope. Using the MJ for both Q3 and Q4 may fix the problem if it occurs. In any case, keep the wiring for the current sink as SHORT as possible, especially keeping the base resistors close to the transistors.

The bootstrap circuit C1/R5/R2 is optional. It raises the impedance seen at the gyrator's base, and aims to reduce ripple. Easy to try, but build without it at first!

Be sure to use a heatsink for the power transistors!
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