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In Reply to: Re: Torroids as audio posted by Scholl on February 05, 2002 at 04:45:43:
Hi, the ratio is a little worse than 2 : 1 on the input. I paralleled the primary.My preamp is Nuance by Famco. It has a little gain, but not much. The combinatuion I settled for of gain is right for my application. Were I in need of gain I would use a 6sl7 voltage amp and 6sn7 driver, that would be plenty.
Of interest I am achieving in the real world a gain of 43 and 5 out of my 6em7.
This is a far cry from Lynn's stuff, it is deliberately budget conscious, I have not allowed myself any excesses, therefore no PP chokes for lc coupling that I'd otherwise have used, or better still interstage transformers. The intermediate stages are difficult to inductively couple on the cheap.
I would like to actively load the two intermediate stages, but again I am about simplicity and cheapness for the time being while I explore the bounds of possibility within a budget.
There is no need for an HT transformer, but I have isolated the HT with a pair of 240 to 30v transformers placed back to back, and for good measure I balanced the power within them. Because they were surplus torroids I had to wind a common mode choke for the filtering between the two isolating transformers, had they been E.I. I could have avoided the choke. The amp (largely due to this filtered supply) is absolutely silent, you could hear a pin drop.
You have to decide where to stop. I can see progress in employing this input stage and driver topology in about any push pull amp. What isn't flexible is the torroidal output. This is the achiles heel of the whole set up. Purchase a good pair of push pull outputs and this becomes a very versatile arrangement.
Were I building an amp of this nature from scratch with no holds barred I would simply build Lynn's, and leave all this hard development work in his capable hands.
But this cheap amp has earned a full build job. I am off to punch holes tonight.
Regards
Paul Barker
Follow Ups:
I would like to actively load the two intermediate stages, but again I am about simplicity and cheapness for the time being while I explore the bounds of possibility within a budget.
get a split bobbin and wind a matched CT choke reverse wind one chamber to keep a central ground., butt gap the lams and it will better any active stage for loading a differential pair...if you want to go nuts then put an active load in the cathode...
dave
Hi Dave,That is pretty exactly (but without Butt-Gap) how Jonathan Billington makes the Gridchokes I wrote about a while back. Making them a little gapped and thus to handle some unbalanced DC is easy. At a cost of around $ 25 each, what has anyone got to loose?
Ciao T
i'd still keep the butt gap for a grid choke... grid current gets real ugly with ungapped grid chokes particularly on 80% nickel cores.at $25 each they are indeed a bargain, thats just under a pound of nickel and i have to pay $25 bucks a per pound for nickel alone... i guess the pricebreak for quantity must be huge.
i would almost break even buying them at $25 taking out the core, and tossing the rest :-)
dave
Right Dave, I must give it a go.At the moment I have stuff going on in private life that is keeping me distracted from building, so transformer winding has been shunted to the back. Curiosity alone has driven me to explore these torroids, I cannot really afford this time I am spending, but get restless if I'm not exploring new ground to me on valve stuff.
you could rework the grid chokes T mentions....dave
I could indeed.Actualklky I have some nice 90% Nickel C cores wiating for me to get off my but and do something about. So many projects, such a full life, so little time.
Just enjoying the distraction of these torroids. Someone else has done all the hard winding and connecting up. Was easy for them on their torroid winders for the mass market.
How is you rtorroid machine looking? Will it ever be a runner?
I suppose it would be worth making a shuttle to add the turns to the power torroids, we could get a whole lot of functionality out of them.
Regards
Paul Barker
Hi,
I tried input transformer (oh...) to serve as both voltage gain and phasesplitter... It did work wery fine except for that I needed a 8ohm source to drive them (to low primary inductance)... It was a sort of 220/9V transformer that hardly took 220V before it went into current rush and fitted backwards, well... But driving the poweramp from a poweramp did work fine, but seemed a little OTT in my eyes :-9As long as one don't dare one don't get to know... (all others do is hearsay *lol*)
mvh /Pär
...I'm trying toroids too , in a rather unusual SE parafeed design . It's still in it's infancy , all I can really say is that I'm getting some good (sonic) results . This design uses a line scan pentode , strapped as a triode to achieve about 500r Ra , this sits on the usual unbypassed cathode resistor and runs on 200v on the anode at present. Now the catch ! I'm using Mosfet CCS parafeed loading , I don't own or can afford large chokes or in fact proper OPTXs , so rely on a 400v supply , parafed to a 240v-8v toroid . Driver stage standard '6J5' triodes , both cascaded as an attempt to cancel distortion and keep the supply current also constant , for just a few piffly watts . Will probably make an excellent tweeter driver . Further development under way , may even cane the sink at 100mA and use 12E1 , 5894 or paralleled 7236 as output stage . I agree with Paul , the bass does sound a little bit light ....FB
Hey FB, what's the VA of the torroid that you are using?
...it was rated at 8v 5a . There is also another 12v lower current winding that I've left flapping . Ideally I'd like to find a chunky 240v:2 x 6v but they only seem to be available in the smaller ratings....FB
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