# d3a gyrator loaded parafeed design



## adamus

Hi all.
   
  Thought i would post this here.
   
  There has been some discussion on diyaudio about the sonic merits of a mosfet gyrator Vs a CCS anode load for triodes (or in this case a triode wired pentode - the D3a)
   
  I have already built a CCS loaded d3a amp so now i am building a gyrator version to try. The salas HV shunt is used for B+, leds for bias.
   
  circuit here
   

   
  Power supply is shown below. the only change is that i am using 30r instead of the 56 shown, this gives 60ma total, 40ma for the load and 20ma for the shunt (it needs a margin)
   

   
   
  I have just about finished the perf board prototypes. The HV shunt is done and tested so we are nearly there.
   
  IF anyone spots any issues with the circuit, please shout. Rather than go round in circles, there has already been a discussion about the relative merits of parafeed Vs single end OPT, so lets not go through that topic here. 
   
  Cost is mostly dependant on the output transformers, whilst testing i am using some super cheap edcors ($20) but if this shows promise then electraprints will be purchased.
  My guess is all parts, excluding the case can be bought for <$200


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## jdkJake

Forgive my inexperience, does V3 represent a battery in this instance?


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## adamus

it could, or you could use an led, or a bypassed resistor.
   
  the gyrator is actually a constant voltage device at dc, so set the plate voltage and then use the bias as you normally would to set operating conditions. personally i will be using a single red led and bias the tubes at about 20ma. I may have to use an ir led to get a low enough plate voltage. 
   
  interestingly, the gyrator does not have the PSRR of a ccs, so the salas shunt (or any other reg) is essential really for a headamp. if you wanted to do it more cheaply, it would only take another mosfet acting as a vreg to improve the circuits PSRR a lot, and thaat would allow you to have cheap components in the PS, just a CRC would probably be ample.
   
  error in schematic: the cap should be connected between the 2k2 and the 2m2 resistor, not to the gate directly as oscillations are likely.


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## digger945

Interesting. What kind of steel would you be considering for the Electraprints?
   
  Have you ever considered using an active current source/sink(I am referring to an adapted Aleph CS)?
   
   
  Lots of good reports on parafeed I've been reading lately.


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## adamus

10k : 300r / 32r M6 from jack or the mu metals from sowter. I had a pair from jack that went into an amp for a mate and the sound that thing made was jaw dropping. The edcors do a job, but they are audibly compromised for sure once you have listened to the electraprints (i still prefer them to a big electrolytic
   
  I'll go and have a poke in the aleph thread - thanks for the pointer. 
   
  I have also just finished a C3g(s) amp, its a bit rough looking but i will post some pics later. the c3g is more polite than the d3a (it doesnt sing to itself without the use of stoppers more pins than i care for!) and the finished result is superb.... but I think the d3a just edges it for detail.


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## FrankCooter

Thanks for sharing your schematic. Unfortunately, you're probably sailing  over the heads of most of the people here (me included). A little general information about the use of a gyrator as an anode load would be appreciated. I know that a gyrator is roughly an electronic choke without the ability to store energy.The trade-offs between a plate choke and a CCS are well known. CCS generally wins in all areas except for swing and voltage drop. I assume a gyrator would compare similarly to a CCS against a plate choke, but how do you expect the gyrator to compare to the CCS?  
   
  Like your shunt regulator. Any idea if it could be scaled  up to a 600v range?


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## dsavitsk

Quote: 





> Originally Posted by *FrankCooter* /img/forum/go_quote.gif
> 
> I assume a gyrator would compare similarly to a CCS against a plate choke, but how do you expect the gyrator to compare to the CCS?
> 
> Like your shunt regulator. Any idea if it could be scaled  up to a 600v range?


 
   
  I've not built one, so I am as curious as you to hear what Adam has to say. But, I would expect it to be similar to a CCS in terms of performance.  As I understand it, basically, a gyrator allows you to set plate voltage while the tube finds its own current, while a CCS allows you to set current while the tube finds it's voltage. I'd think the gyrator might be advantageous for DC coupling because of that.
   
  The shunt should scale, providing you can find parts that can handle the voltage. Only change I might think about would be a better CCS feeding it, though all things considered it should be fine.


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## adamus

Quote: 





digger945 said:


> Have you ever considered using an active current source/sink(I am referring to an adapted Aleph CS)?


 
   
  Thinking about this, if you effectively CT the source resistor on the lower dn2540 (just half the value and use 2 resistors) then take the output from that point, so that not give us the anti triode / aleph config (or something close).
   
  Frank :
   
  Thanks for the reply.
   
  The idea is basically a version of wavebourn (diyaudio member - a very clever man!) SVCS or stabilised voltage current source.
   

   
  It sims like a plate choke (very much like) but obviously has the limitations of CCS in that it wont 'store' energy so B+ needs to be high enough for required swing. 
   
  To date i dont know the sonic benefits until its fired up. Others that use them say it sounds quite different to a CCS and imparts more of the triode signature. You'll also find views that a simple ring of two ccs is better technically. Proof will be in  the listening.
   
  Re the salas shunt reg, scaling up to 600v may be a problem. I know people have taken it over 400v with high v silicons ..... but unless you can find fets that can take the voltage i am not sure.


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## nikongod

I havnt read up on gyrators too much, so take this a good guess:
   
  Would replacing R5 in the OP schematic with a CCS improve PSR? It looks like the most obvious path for PS noise to enter the circuit compared to a more conventional Depletion mode CCS.


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## adamus

yes you could, not much current needed so a little lnd150 or maybe even a crd could be used, or to cut down on parts, forget the cascode.
   
  As i am suing the salas HV shunt reg, i am not to concerned about the PSRR.


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## regal

Keep us posted,  not sure what you mean as to where to place the misdrawn cap.  Also how do you "dial" this gyrator in (trimpot across R1)?   Sounds like the typical hook up to a 9V battery won't work with this,  I guess what is the setup procedure ?
   
   
  Frank, the MJE350 limits the Salas reg to around 300V,  you can go a bit over 400V with an [size=10pt]MPSA94 ,   the thing also requires a huge heatsink  if you want it to last a tube burning out.   For example the amp I'm working on which is 20 mA per channel I have an 8x3x2 heatsink,  but the payoff is extremly low B+ output impedance.[/size]


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## adamus

regal, the cap as drawn is between the the anode of the d3a and the gate of the dn2540. it should really be to the left of the gate stopper (inductance of the longish cap lead will make the dn2540 oscillate if the wrong side of the stopper.
   
  re 'dialing in'. This is a constant voltage device so i will just go for the plate voltage i want, and as you would normally just use cathode bias to set the operating point (at the fixed voltage). Very approx the plate voltage is that created by the voltage divider on the left minus vgs i think. in the opening post, the resistors R5 and R6 are infact a 500k pot. the 2M2 is two 1M and a 200k in series (the 2m2 i had were measuring all over the place  - maybe a limitation of my dmm?)
   
  revised schematic


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## regal

Thanks I think the constant voltage may make a lot of sense for parafeed hybrids,  the dc servos usually junk up the hybrid stage but without voltage fluctuations on the parafeed cap might be able to get away with a nice diamond buffer sans dc servo.


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## adamus

here is a c3g version...with what looks like a neat way of biasing with very very ac impedance (although 2.5v is the only value, but thinking about it, find a device with a suitable vgs and stick in there....)


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## regal

Tl431 would basically be putting an opamp in the signal path so it isn't a fashionable thing to do but let us know how it sounds vs Led's.


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## adamus

True but I am not one for fashion. The datasheet suggests a very low dynamic impedance and what is there looks more linear than an led. As you say, proof is in the listening.


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## regal

Also might want try one of these http://www.fairchildsemi.com/ds/2N/2N7000.pdf  with the gate tied to the drain to the cathode would give about a similiar bias.


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## adamus

update: I have it on a perf board ready to test
   

   
  It'll find a test home in my tweakers amp which is currently a c3g parafeed. By the way, the c3g ccs loaded parafeed is currently biased by a tl431.... i think i prefer it to leds!!! the bass seems better... which could always be a small change in operating point.
   
  test home


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## regal

Adamus is the tl431  adjustble under the cathode,  could you set it for 10V's or higher for tubes that require a lot of bias?


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## adamus

its adjustable upto 36v (max rating) and achived using a couple of resistors - the datasheet explains better than i can. Watch the power rating though, it soon falls off with increased temp. Another thing to add is that i have a slightly increased noise floor with tle431, not noticable with senns but is with grados. I am looking for an osciallation but cant spot one, but my scope is a bit rubbish. The c3gs and d3a LOVE to oscillate, i doubt this would be a problem on non 'extreme' valves.
   
  another method that would probably yield better reults is using a pnp in emitter follower.. you get the stable vref from the leds and the low dynamic impedance of the pnp - you can also scale power requirements with pnp type. cathode voltage = vref + Vce(which is about 0.7v)


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## regal

Quote: 





adamus said:


>


 


 Brilliant.   Thankyou very much adamus


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