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Old 30th March 2010, 09:32 AM   #351
bravi is offline bravi  New Zealand
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Default FET or BJT for sound quality

Hi Rod,

Which one results in better sound quality? Does it differ between preamp and power output tubes?
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Old 30th March 2010, 10:01 AM   #352
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Quote:
Originally Posted by bravi View Post
Hi Rod,

Which one results in better sound quality? Does it differ between preamp and power output tubes?

I haven't done any direct comparisons, sadly. But the different circuits I built did not vary much in sound, so far I can tell. However, the difference between ac, dc (just rectified) and ANY of the circuits is very noticeable.

Best plan is to build according to the parts you can easily get. But if you choose FETs, be sure they are logic-level types with a low Vgs (th) turn-ON voltage - About 1,7V for preference. Allow 2,5V across these types.

If you can get Japanese BJTs on the list above, these may work down to 1,5V across each, but 2 to 2,5V is better for low capacitance.

If you can only get the MJ150xx series BJTs, I would probably build with FETs, since these seem to have poor performance at low Vce.
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Old 30th March 2010, 06:52 PM   #353
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Originally Posted by xecluded View Post
Coolzero,

I don't have technical skill to build the 26 preamp that you've mentioned here but i am thinking about commision it to someone who knows how. What do i need to provide him other than the schematics that you've listed here (is this the final version by the way.) Do you have a parts list required to build this preamp. Also can you give some tips and tricks on how to eliminate the hum that i've heard so much associated with the 26 preamp. I really appreciate any help that you can provide. Thanks.

xecluded
Hi,

Attached schematic shows what exactly I have now.

For CCS between choke and B+ supply, I used K&K audio, compact CCS (K & K Audio - Lundahl Transformers, audio DIY kits and more). Filaments are in series, fed by a SMPS used in DVD players (SMPS fo DVD SMPS - 226 SMPS for DVD CN;GUA products). SMPS works well to reduce the overall hum drastically, which I couldn't control using any other simple filament supply. Plate chokes are from EP (Electra-Print.com High-Fidelity Audio Tranformers). Most of the components I used which I had already. It is not necessary to have the exact same part mentioned. You could easily find alternatives for some of those for a lesser price.

However, to make my pre a pure DIY I am planning to replace B+ PSU with Salas simplistic regulator and Filament supply with Rod's Gyrator/CCS. Hope it will give me a better result. You could easily find the details of those in this thread if you wish to use those as well. This thread includes pretty much everything you need to build a hum free wonderful sounding 26 pre. Good luck.
Attached Images
File Type: jpg 26 my pre.jpg (255.9 KB, 1074 views)
Attached Files
File Type: txt 26 pre part list.txt (189 Bytes, 120 views)

Last edited by coolzero; 30th March 2010 at 07:14 PM.
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Old 30th March 2010, 07:22 PM   #354
Sheldon is offline Sheldon  United States
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I don't understand your B+ supply. You take a perfectly good shunt regulator and follow it with a choke and CCS, then a plate choke. I can't believe that the choke and CCS are necessary for good noise performance. If so, there is something wrong. Normally, the OD tubes are followed by a small (less than 0.1uF) cap. A large cap can cause oscillations. The following choke complicates the picture further. Have you tried it without the choke and CCS, and with a proper small cap?

Sheldon
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Old 30th March 2010, 07:50 PM   #355
Sheldon is offline Sheldon  United States
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Here's an example of a little overkill: The Miada regulator before the current source offers some ripple reduction to the current source. However, the primary reason I put it there was to offer high voltage protection to the current chip, in event of a no load condition (OD tubes disconnected and ps disconnected from amp). This supply powers a phono amp that has a cascode input, with essentially zero PSRR. Note that the zener symbols at the output represent OD2 tubes. There is no filtering after them and the amp is quiet.

Sheldon
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File Type: gif bench riaa power supply.GIF (24.2 KB, 973 views)
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Old 30th March 2010, 07:57 PM   #356
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Quote:
Originally Posted by Sheldon View Post
I don't understand your B+ supply. You take a perfectly good shunt regulator and follow it with a choke and CCS, then a plate choke. I can't believe that the choke and CCS are necessary for good noise performance. If so, there is something wrong. Normally, the OD tubes are followed by a small (less than 0.1uF) cap. A large cap can cause oscillations. The following choke complicates the picture further. Have you tried it without the choke and CCS, and with a proper small cap?

Sheldon
Sheldon, pardon my amateurs know how in this subject. What I did was just a trial and error. For some reason it worked. I would definitely try your suggestion and let you know how it goes. Thanks a lot.
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Old 30th March 2010, 08:04 PM   #357
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Quote:
Originally Posted by Sheldon View Post
Here's an example of a little overkill: The Miada regulator before the current source offers some ripple reduction to the current source. However, the primary reason I put it there was to offer high voltage protection to the current chip, in event of a no load condition (OD tubes disconnected and ps disconnected from amp). This supply powers a phono amp that has a cascode input, with essentially zero PSRR. Note that the zener symbols at the output represent OD2 tubes. There is no filtering after them and the amp is quiet.

Sheldon
Still slowly learning about all these . Your schematic and explanation cleared some of the doubts I had. Thanks again.
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Old 30th March 2010, 08:36 PM   #358
Sheldon is offline Sheldon  United States
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Quote:
Originally Posted by coolzero View Post
Sheldon, pardon my amateurs know how in this subject. What I did was just a trial and error. For some reason it worked. I would definitely try your suggestion and let you know how it goes. Thanks a lot.
No pardons needed. Some trial and error is good. Some engineering beforehand is even better. A current source fed shunt supply is a good combination. That what Salas's supply is. You can do it with what you have. I'd take the current source you are using before the plate choke and put in in place of R4. Increase the current so that you have about 20mA through the VR tubes. Use two OB3 tubes (will give you about 180V. Note R12 from B+ to the connection between the VR tubes on my schema. That's optional, but can help the tubes fire up). Use something less than 0.1uF cap after the VR tubes, or use a voltage divider and two small caps after the VR tubes (see the schematic I posted). Remove L2 and connect the supply to your plate choke. That should be a nice quiet supply.

If you want to try Salas's supply, just remove everything between C4 (I might use something bigger than 1uF there) and the plate choke and plug in the Salas circuit.

Sheldon

Last edited by Sheldon; 30th March 2010 at 08:46 PM.
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Old 31st March 2010, 02:06 AM   #359
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Quote:
Originally Posted by Sheldon View Post
No pardons needed. Some trial and error is good. Some engineering beforehand is even better. A current source fed shunt supply is a good combination. That what Salas's supply is. You can do it with what you have. I'd take the current source you are using before the plate choke and put in in place of R4. Increase the current so that you have about 20mA through the VR tubes. Use two OB3 tubes (will give you about 180V. Note R12 from B+ to the connection between the VR tubes on my schema. That's optional, but can help the tubes fire up). Use something less than 0.1uF cap after the VR tubes, or use a voltage divider and two small caps after the VR tubes (see the schematic I posted). Remove L2 and connect the supply to your plate choke. That should be a nice quiet supply.

If you want to try Salas's supply, just remove everything between C4 (I might use something bigger than 1uF there) and the plate choke and plug in the Salas circuit.

Sheldon
Thanks Sheldon for the advice. I'll try this out soon.

Kanishka
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Old 31st March 2010, 05:29 AM   #360
regal is offline regal  United States
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Quote:
Originally Posted by Sheldon View Post
Here's an example of a little overkill: The Miada regulator before the current source offers some ripple reduction to the current source. However, the primary reason I put it there was to offer high voltage protection to the current chip, in event of a no load condition (OD tubes disconnected and ps disconnected from amp). This supply powers a phono amp that has a cascode input, with essentially zero PSRR. Note that the zener symbols at the output represent OD2 tubes. There is no filtering after them and the amp is quiet.

Sheldon

I have a Broskie PS-2 power supply which is similiar but it has the Lm reg on the output after the 10m45S, so it is a series regulator. I had to go this route because I'm using a SS buffer after the tube which has a DC servo that goes nuts if the B+ voltage drifts even 100 mA which is common for shunt regulation, but this combo looks like it might hold a constant voltage?
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