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Old 1st May 2010, 08:31 PM   #411
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Hi,

Anyone could suggest a matching Tube or SS power amp project (to use with 26 pre)? Need bit of power though.

Last edited by coolzero; 1st May 2010 at 08:40 PM.
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Old 1st May 2010, 09:20 PM   #412
Sheldon is offline Sheldon  United States
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Quote:
Originally Posted by E.T. View Post
Would a 26 have enough grunt to drive a 6as7? I'm looking for that "liquid sound" of DHT's. Someone suggested that that sound comes from internally generated microphonics, which seems plausible considering how microphonic small DHT's are.
No, not if you want to drive it anywhere close to full output with a standard source. The 6AS7 has a mu of about 2. That means you need to swing half of your desired 6AS7 output swing. The 26 has a mu of about 8. Even if you use a choke or CCS load, with a standard 2vRMS input you'll get a maximum of about 16vRMS to the grid of the output tube. That's a total swing of about 45V to the output grid. Check the 6AS7 grid curves to see where that puts you.

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Old 2nd May 2010, 04:41 PM   #413
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Any workaround to balance off the impact of having a amp with undesirable input impedance to use with 26 pre, without compromising much of 26's potentials?
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Old 2nd May 2010, 05:22 PM   #414
Sheldon is offline Sheldon  United States
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Originally Posted by coolzero View Post
Any workaround to balance off the impact of having a amp with undesirable input impedance to use with 26 pre, without compromising much of 26's potentials?
Sure. Add a follower. SS devices are excellent for current gain, and you have some experience with them in other parts of your circuit. Just do a simple follower - FET, source resistor, and gate stopper. Use can use either depletion mode or enhancement mode devices, and direct couple it. Use the same B+ as your 26. If you want to add one layer of complexity, use a ccs instead of a source resistor. But for swinging preamp voltages, I doubt you'll hear the difference of the ccs. If you run about 10 -20mA through the follower, it will drive any amp you wish. The follower itself will have very little signature of it's own.

Or you can add a cathode follower, if you want a tube device.

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Old 2nd May 2010, 06:18 PM   #415
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Quote:
Originally Posted by Sheldon View Post
Sure. Add a follower. SS devices are excellent for current gain, and you have some experience with them in other parts of your circuit. Just do a simple follower - FET, source resistor, and gate stopper. Use can use either depletion mode or enhancement mode devices, and direct couple it. Use the same B+ as your 26. If you want to add one layer of complexity, use a ccs instead of a source resistor. But for swinging preamp voltages, I doubt you'll hear the difference of the ccs. If you run about 10 -20mA through the follower, it will drive any amp you wish. The follower itself will have very little signature of it's own.

Or you can add a cathode follower, if you want a tube device.

Sheldon
Thanks Sheldon. Any reference schematic you could provide me start with or link to refer for more details?
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Old 2nd May 2010, 07:03 PM   #416
Sheldon is offline Sheldon  United States
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Originally Posted by coolzero View Post
Thanks Sheldon. Any reference schematic you could provide me start with or link to refer for more details?
If you search source follower in the tube forum you will find lots of stuff. Also search Mosfet Follies on the web for some basic circuits. Tubelab also uses a source follower in his Tubelab SE.

Here's a simple circuit. The plate of the 26 would be attached to the FET gate. The FET is an example of one that will work. You can also use the IZXS parts or the DN2540. You want a part with low crss (reverse transfer capacitance). If you direct couple (no cap between plate and gate), you want to make sure that your you have at least 20V or so across the FET. This is to make sure that your crss is low and constant.

The source follower has very good PSRR so you can tap your HV supply at a point where the voltage is higher but maybe noisier.

If that's not convenient, you can cap couple to the FET, but you will need a voltage divider to the gate to bias it, let's say two 500k resistors between B+ and ground.

It's a good idea to include a safety diode for the gate. We can get to that if you want to go down this route.
Attached Images
File Type: gif Simple source follower.GIF (1.9 KB, 531 views)

Last edited by Sheldon; 2nd May 2010 at 07:06 PM.
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Old 2nd May 2010, 07:27 PM   #417
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Quote:
Originally Posted by Sheldon View Post
If you search source follower in the tube forum you will find lots of stuff. Also search Mosfet Follies on the web for some basic circuits. Tubelab also uses a source follower in his Tubelab SE.

Here's a simple circuit. The plate of the 26 would be attached to the FET gate. The FET is an example of one that will work. You can also use the IZXS parts or the DN2540. You want a part with low crss (reverse transfer capacitance). If you direct couple (no cap between plate and gate), you want to make sure that your you have at least 20V or so across the FET. This is to make sure that your crss is low and constant.

The source follower has very good PSRR so you can tap your HV supply at a point where the voltage is higher but maybe noisier.

If that's not convenient, you can cap couple to the FET, but you will need a voltage divider to the gate to bias it, let's say two 500k resistors between B+ and ground.

It's a good idea to include a safety diode for the gate. We can get to that if you want to go down this route.
Thanks. I found this article as well.

http://www.audioxpress.com/magsdirx/.../moore2355.pdf
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Old 2nd May 2010, 08:21 PM   #418
Sheldon is offline Sheldon  United States
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It's a good article, but much of it doesn't apply here. That describes the requirements for a power amp output stage. The preamp delivers very little current by comparison, and can be quite simple and still deliver very low distortion output.

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Old 3rd May 2010, 04:04 AM   #419
mach1 is offline mach1  Australia
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Coolzero,

You could also try a simple DN2540 / IXCP10M45S cascode ccs on top of the 26 with the output taken from the top of the current set resistor.

Easy and effective with low output impedance.
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Old 3rd May 2010, 06:05 AM   #420
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Originally Posted by mach1 View Post
Coolzero,

You could also try a simple DN2540 / IXCP10M45S cascode ccs on top of the 26 with the output taken from the top of the current set resistor.

Easy and effective with low output impedance.
use CCs the sound is not like .
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