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#26 pre amp

Hi

Were you taking the o/p from the the gyrator o/p or from the mu follower o/p ? The mu follower o/p
Impedance should be less than 40R. I have driven my hd800,s direct off the mu follower o/p succesfully with my 2p29l . However it measures and sounds better with a 4:1 step down in parafeed.

By the way I think 6.8uf is far too small to drive a 300R load more like 100uf.
 
Circuit from #4780

Somehow never noticed this before: the regulator output is decoupled from the load which allows for a reasonably large cap to be used.

My main gripe with vacuum regulator tubes has always been the severe subjective lack of bass and this circuit probably solves this. I would go for an even larger cap.

What do people who have built it think of the bass?
 
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Joined 2009
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Hi

Were you taking the o/p from the the gyrator o/p or from the mu follower o/p ? The mu follower o/p
Impedance should be less than 40R. I have driven my hd800,s direct off the mu follower o/p succesfully with my 2p29l . However it measures and sounds better with a 4:1 step down in parafeed.

By the way I think 6.8uf is far too small to drive a 300R load more like 100uf.

Headphone Sensitivity / Power Requirements Compared | Headphone Reviews and Discussion - Head-Fi.org

Power required to reach 120db:
HD600 - 199 mW - 26 ma - 7.74v rms

#26 at working operating point: 5-6mA

... just physics...

I guess the worse problem is the output impedance because sounds enough high to listen properly. I believe it's the main reason but I will try lytic cap 220uF or 330uF, what polarity must be connected to the headphones?
 
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Joined 2009
Paid Member
Hi

Were you taking the o/p from the the gyrator o/p or from the mu follower o/p ? The mu follower o/p
Impedance should be less than 40R. I have driven my hd800,s direct off the mu follower o/p succesfully with my 2p29l . However it measures and sounds better with a 4:1 step down in parafeed.

By the way I think 6.8uf is far too small to drive a 300R load more like 100uf.

Have you measured the output impedance?
 
I measured the frequency response into a 340 ohm load with a variety of coupling caps. The result tallies with Ale,s calculation of o/p impedance of his mu follower.

When I tried the direct o/p I used a 100uf/500V Kemet DC Link polypropylene not an electrolytic. If I did I would connect the positive to the follower o/p. This value of cap gave a -3db point of 5hz with the roll off beginning at 30hz. I chose this as my hd800 (and my hd600,s) have a bit of a rolled off bass and I didnt want to to make it worse with an earlier bass roll off.

I quite liked the result with the direct drive but with the parafeed 4:1 step down I can use a 2.2uf cap
(Mundorf) which is probably of better quality and I think I prefer it. It measures better distortion wise anyway.

Of course Im using a 2P29L at 18ma which is probably better for direct drive. The 26 might work better with parafeed. I can wind my own tx,s but suitable tx,s shouldnt be that expensive.
 
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Joined 2004
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Bela I have it: 5687 triode, opt 15k/600R loaded 15mA each triode sounds wonderful but not DHT.

The fact that you have tube equipment, which contains ..kOhm:600R transformer does not mean that it is suitable to drive headphone.

I attached the 801a, Noguchi 24k:600 (FM-24k-600) preamp schematic, which is DHT, has enough low output impedance, but NOT designed (I was the perpetrator :p) as headphone amp!

High Ri tubes (as 801 and #26 too) very badly tolerate variable (tend to lower than calculated) load impedance.

Instead of its choose lower Ri tubes for driving headphones.
 

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Hi All,

For those of you who have successfully been able to run their 26 preamp with exposed tubes:
Do you have any tips you can share?

I can get the pre to be quiet if I completely shield the tubes. But the globe (balloon) versions are so nice looking. It would be nice to have them on display.

Best regards,
John
 
John suspended platform is the way to go.

Hi Merlin el Mago,

Thanks for the tip. I currently have them mounted on rubber isolation stand-offs like your picture. (I am actually working on trying to improve this a little by making a mass-loaded platform that will be suspended.)

I don't think my trouble is microphony. It seems to be related to electrical/magnetic fields.

If I put a temporary cowling of heavy aluminum foil over the preamp and ground it, it is tolerably quiet. If I take it off, the 60hz hum is extreme-- I would estimate ~60 db.

It might be grounding related. This is my first foray into filament bias.

Regards,
John
 
Merlin

I dont get to visit this site often hence late reply.

I did ask whether you were taking the o/p for the headphone from the gyrator o/p or the mu follower o/p. I assumed (wrongly?) that you were using Ale,s hybrid mu follower boards.

These have two possible o/ps. One is a gyrator o/p which is high impedance ie the valves Ra. This o/p is too high to drive a headphone direct. It can be used with a step down transformer or a follower. The mu follower o/p has a low impedance, this is what I use. With this o/p the freq response tells me what the o/p impedance is hence my reply.

If you are using just a gyrator then sorry if I misunderstood you.
 
What filament/heater regulators are you using?.


I am using Rod Coleman's Filament regs.

I have the raw HT and LT power supplies housed together, but in a separate box from the preamplifier--situated about 1 meter away.

The Filament regs are in the preamp box about 22cm from the tubes. I also have the Salas SSHV2 mounted in the preamp box. I am sharing B+ with both channels.

I read a comment on Ale Moglia's site where he briefly mentioned separating the HT and LT raw supplies. Is this necessary for quiet operation?

Thanks again for your input!