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Old 5th January 2013, 02:23 PM   #2281
Magz is offline Magz  United States
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You could also try biasing through an input transformer, thus eliminating the coupling cap without putting a resistor in the cathode. This has the added advantage of a very low resistance to ground for the grid, like a grid choke would provide. It sounds much better to me than grid bias with a cap. I talked about it back in post #2228 of this thread:

#26 pre amp

One thing I learned is that the output impedance of your source is important to get right in this arrangement in order to get the best squarewave performance. I found that a total of 1000ohms worked best with the input transformer set up as I have it (LL1676, 2||2:1||1, 2nF+850ohm snubber on secondary) so I added 800ohms to the primary, since my source has a 200ohm Zout. Here's a link to the scope shots:

Sanity Check - Input Transformer Overshoot
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Old 5th January 2013, 02:57 PM   #2282
H.Michi is offline H.Michi  Japan
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Rod, I am sorry, afore-mentioned residual noise level 0.31mV was incorrect!
It was 0.031mV.

Result of your check list:

1) 0.5mV~8mV(at the peak)
2) 0.5mV~5mV(peak)
3) 0.12mV Filament to Firament:0.03mV
4) Capacitance from primary to secondary:?
Filament transformer constraction:E-I core
Install an isolating transformer
4) Fitted 2200 mF
5) Not yet fitted

After installed 2200 mF and isolating transformer, the residual noise is improved to 0.025mV.
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File Type: jpg IMG_4919.jpg (164.1 KB, 410 views)
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Old 5th January 2013, 05:08 PM   #2283
работник
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Hiro, The measurements look very good to me.

Possibly, the common-mode noise voltage can be reduced a little more. It is difficult to improve on 25uV - but maybe this voltage is caused by some leakage current from primary to secondary of the filament transformer. The isolating transformer is very helpful, but perhaps a common-mode filter can be considered.

The drawing shows some ideas for common-mode filtering. You already have the isolating transformer, and excellent differential-mode filtering, so the common-mode choke (Murata) + capacitor is another option.

Murata choke:

52106C | Common Mode Chokes | Magnetics | Products | Murata Power Solutions
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Old 6th January 2013, 11:40 PM   #2284
12B4A is offline 12B4A  United States
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I finished building a test circuit this weekend of a 26 parafeed headphone amp for my Sony 7506's (60 ohm). To my relief it is dead quiet. It's a take off of kevinkr's design using Rod's filament board, tomchr's maida reg, and K&K ccs (maybe I should put a diyAudio emblem on the front). It was a good way of weeding out the duds and sputzers in my 12 year old collection and it would be nice to have a completely unnecessary headphone amplifier using 80 year old active devices.

The problem I'm faced with is the output tranny is not optimal and is heading to another project anyway (MQ EXO-45). I tried using a 120V:7V 5VA toroid for the parafeed trafo and it doesn't work at all in this situation. I'm now looking for a suitable headphone/signal level parafeed (no gap) transformer to use and Lundahl is too much for what I want to put in to the project. I was thinking this Edcor might be what I'm looking for but I'm not sure. Anyone know if it is suitable?
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Old 7th January 2013, 06:48 AM   #2285
H.Michi is offline H.Michi  Japan
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Quote:
Originally Posted by Rod Coleman View Post
Hiro, The measurements look very good to me.

Possibly, the common-mode noise voltage can be reduced a little more. It is difficult to improve on 25uV - but maybe this voltage is caused by some leakage current from primary to secondary of the filament transformer. The isolating transformer is very helpful, but perhaps a common-mode filter can be considered.

The drawing shows some ideas for common-mode filtering. You already have the isolating transformer, and excellent differential-mode filtering, so the common-mode choke (Murata) + capacitor is another option.

Murata choke:

52106C | Common Mode Chokes | Magnetics | Products | Murata Power Solutions
Rod,
Thank you for your advice.
I tried fitted comon-mode choke in my stock , but the result was getting worse.
It is difficult to reduce the noise from this level.

As you mentioned, the isolating transformer is quite effective.
Best regards
Hiro
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File Type: jpg PSU top.jpg (297.9 KB, 333 views)
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Old 7th January 2013, 03:11 PM   #2286
tcqanh is offline tcqanh  Viet Nam
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Quote:
Originally Posted by H.Michi View Post
tcqanh,

I`d like 26pre rather than 71a.
Although 71a was a capacitor cuppling with normal biasing.
I will try 71a with fillament bias later.

Regards
Hiro
I did build RCA 71A pre amp with plate choke and fixed bias. Its sound is really attractive for Single end fan like me. I have also RCA 26 pair. Looking the plate shape, they are nearly the same. I cannot distinguish them from others. So I think their sound are much similar.

Hopping you will try 71A with filament bias in the same circuit in someday.

Last edited by tcqanh; 7th January 2013 at 03:16 PM.
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Old 8th January 2013, 09:03 AM   #2287
tyo94 is offline tyo94  Indonesia
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Quote:
Originally Posted by tcqanh View Post
I did build RCA 71A pre amp with plate choke and fixed bias. Its sound is really attractive for Single end fan like me. I have also RCA 26 pair. Looking the plate shape, they are nearly the same. I cannot distinguish them from others. So I think their sound are much similar.

Hopping you will try 71A with filament bias in the same circuit in someday.
Have you compared the sound impression between 71A and 26?
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Old 8th January 2013, 01:42 PM   #2288
H.Michi is offline H.Michi  Japan
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Teqanh and tyo94,

First of all, my 26 pre has been remodeled from 71a.
I will try again 71a with filament bias in someday.

71a and 26 looks to be similar at glace, but 71a is originaly designed for power tube use meanwhile 26 is for voltage amplifier use. I feel 71a was not so exciting as center of the sound shifted to lower range, while 26 is well balanced in all sound ranges, higher in clarity and transparency.
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Old 8th January 2013, 03:10 PM   #2289
tyo94 is offline tyo94  Indonesia
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Hi Hiro-san,

I am sorry if there's misunderstanding caused by my question. Actually my question is addressed to tcqanh. I have curiosity about differences about 71A and 26 tubes, if he do comparison between both tubes yet. Anyway, thanks to you for explaining that.

Actually, there's no reason that for tube cannot be used for preamp. Thomas Mayer's project based on 10Y tube for his preamp. I had my friend also using power tube (RS241) for preamp. I don't know about the sound impression of 10Y as preamp, but RS241 based preamp feels so impressive (at least for me). The sound stage, clarity, transparency and detail are superb.

Anyway, is there any info for good DHT tube beside 26 which can be used for good DHT preamp?
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Old 8th January 2013, 03:14 PM   #2290
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4P1L One more 4P1L SE
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