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diyAudio Member

Join Date: May 2007
No need whatsoever for a input-capacitor if using the topology T.Loesch showed me.
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 25th June 2010, 09:41 PM #532 diyAudio Member   Join Date: Sep 2003 Location: London Hi, This looks a bit foreign to my simple eyes. I can see -9v bias, but where does this go to ground? In the 100K pot that would precede this? Where is the input resistor, grid to ground? Also, with 150v on the anode, 9v bias would give you over 7mA which is a bit strong isn't it? Can you explain the circuit for my help! Andy Last edited by andyjevans; 25th June 2010 at 09:45 PM.
diyAudio Member

Join Date: May 2007
Quote:
 Originally Posted by andyjevans Hi, This looks a bit foreign to my simple eyes. I can see -9v bias, but where does this go to ground? In the 100K pot that would precede this? Where is the input resistor, grid to ground? Also, with 150v on the anode, 9v bias would give you over 7mA which is a bit strong isn't it? Can you explain the circuit for my help! Andy
Hey Andy,

This was only done to show the principle of how it could be done. You are free to use whatever Ia or Ua you like. The only problem is that we canīt choose battery voltage so easy as the Lithiums are 3V.

As there is no grid current we can skip the cap.

Didnīt add the gridleak resistor as I thought it was so obvious it should be there. But it could also be the 100k pot you mentioned.

Below is how a 3A5 could be done. But I would go for a 4:1 on that one.

Just remembered that T recommended Silvermica for the 10n.
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 3A5pre.png (172.6 KB, 815 views)

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Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
Quote:
 Originally Posted by CFT Hi Kevin, Parafeed is still lingering in my mind... Was wondering if you have tried this topology in your 26 preamp? Someone said sound coming from traditional series connection is more natural, while parafeed brings in more clarity but more towards ss kind of sound.... Thanks for sharing!
I haven't tried it in my own, I have heard it in other applications, and an adaptation of my design. I'm really on the fence about the parafeed trend particularly in the case of line level applications, IMHO all of the disadvantages combined of chokes and transformers without the cardinal virtue of not running the BH curve of the output magnetics through the 0 crossing. (To some degree the use of exotic core materials can help a lot here.) In addition you have introduced the need for an additional coupling capacitor.. I've heard parafeed circuits that sounded good, but in general I prefer a really high quality SE transformer with a hefty primary dc current through it. Some small parafeed transformers made of extremely exotic core materials I have heard sounded great..(And were not IMHO good enough to justify their obscene price difference with a good conventional less exotic SE constructed with a similar level of care.) I am really not sure about the vaunted clarity advantage of parafeed per se, certainly haven't heard it frankly, but I think if you are not careful you can end up with significant odd order distortion at the 0 crossing of the BH curve just like in a good push pull transformer.

This is all very personal opinion in fairness and someone else's experiences and preferences can and should be different, and legitimately so.. So what works for me may not be the case for you and vice versa..
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Last edited by kevinkr; 25th June 2010 at 11:13 PM.

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Quote:
 Originally Posted by revintage No need whatsoever for a input-capacitor if using the topology T.Loesch showed me.
Certainly not original to Thorsten, I and others in my immediate audio circle experimented with this as long as 10yrs ago and I found the sound not to my liking, by the time it was the battery bypass cap was of comparable size and quality to what I would have stuck in there in the first place with conventional grid bias, and the problem gets worse the higher the bias voltage required. The sound quality seems to be very dependent on the electrical characteristics/internal construction of the battery chosen, and with the 26 an effective bias depending on plate voltage (6mA) varies from -8 to - 10V and multiple cells in series seem to compound the problem.

I experimented with fixed bias using NiMH and NiCad batteries in the cathode circuit and found this sonically inferior to LED bias in most if not all cases. (I use LED bias, but no batteries currently in sight.) The Sanyo Cadnica batteries seem to have the lowest internal impedance of the batteries I tried and sounded pretty good compared to anything else I tried, but a simple LED trumped them too.. IMO..

Again my comments are preference/experience driven, I think at some point everyone should just do these sorts experiments and decide for themselves. A really good battery (varta?/Sanyo) should make a significant difference in outcome. IF nothing else the learning experience is FUN!

FWIW: I'm currently listening to Morris Day on Vinyl on a singularly unromantic and revealing SME 20/2 so what I do I know.. (I love it)

Edit: Someone might want to try LED bias with the 26 in fact - should not be too hard to arrange..
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Last edited by kevinkr; 25th June 2010 at 11:32 PM.

 25th June 2010, 11:41 PM #536 diyAudio Member   Join Date: Sep 2003 Location: London Hi Kevin, I'm not sure what your preference is here - LED bias in the cathode or battery bias on the grid. I tried LED bias and found it rather hard and unforgiving, though very clear and bass was particularly good. Didn't have those smooth liquid mids as I implemented it. I haven't tried grid bias yet. You say grid bias is better than filament bias, but how does it compare with LEDs sonically, as you hear it? Andy
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Quote:
 Originally Posted by andyjevans Hi Kevin, I'm not sure what your preference is here - LED bias in the cathode or battery bias on the grid. I tried LED bias and found it rather hard and unforgiving, though very clear and bass was particularly good. Didn't have those smooth liquid mids as I implemented it. I haven't tried grid bias yet. You say grid bias is better than filament bias? Andy

Sorry, I seem to have muddied the waters a bit.. Yes, I am strongly in favor of absolutely conventional (fixed) grid bias with the 26 and all other DHTs, and use it extensively in my own designs. IMLE, yes grid bias is superior to filament based bias and I strongly encourage you to implement.. Easiest is a a pair of 9V batteries - one for each channel will last many years..
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diyAudio Member

Join Date: May 2007
Quote:
 Certainly not original to Thorsten
Did anyone say so?

About battery bias in any form the best Iīve heard to this day is filament bias. Matter of taste...............

Last edited by revintage; 25th June 2010 at 11:51 PM.

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Quote:
 Originally Posted by revintage Did anyone say so? Matter of taste...............
No, and Absolutely..
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diyAudio Member

Join Date: Sep 2009
Location: UAE
Quote:
 Originally Posted by andyjevans Hi coolzero, Hmmm. Interesting. It's true I haven't tried grid bias. What capacitor are you using on the input - make and value? And are you using a battery - if so what kind? You got me interested. Andy
Andy,

I tried Mundorf M cap supreme, Russian PIO K75 and Auricap. Mundorf is mile ahead Auricaps comparing the sound. Well worth the money. PIO's are almost similar to Mundorf but marginally Mundorf wins the place because it’s overall performance. But PIO’s are much cheaper solution. Probably you can try Mundorf Silver oils for better results. I am using 0.15u for input cap (check the schematic I posted before). My initial choice was alkaline 9V batteries for bias supply which I recently replaced with a 15V SMPS. This gives me a room to adjust bias using a pot to experiment with different op points without changing the circuit. This goes well with SALAS HV PSU which has an adjustable B+. I havent noticed much of a difference (out of 2 or 3 I tried) with different brands of batteries for bias, but I thnk any decent 9V alkaline like Eveready Energizer would do the Job.
As Kevin mentioned, I tried all these with his kind assistance as well as many other’s valuable advices came through this forum. Thanks to all, efforts well worth after all. For sure, 26 is one certain way of having million dollar sound for a few hundred bucks.
Try the grid bias and see. You will understand.

Last edited by coolzero; 26th June 2010 at 12:41 AM.

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