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#26 pre amp

diyAudio Chief Moderator
Joined 2002
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Salas,

I have tested the SSHV with IRF9610. I prefer this than the original because it gives a fuller sound and better resolution. Also heat is comparatively less. Thanks.

I agree that the resolution is better and its included in the final schematic for long now. The less heat thing should not be though. Probably a Vgs difference must have produced less drop across the setting resistor, thus less current than with your previous PMOS. Nice that you also got some appreciable subjective lift non the less.
 
Does it work without the gyrator?

I don't use the LT108x regs in my amps, but I hear they need more voltage headroom than might be expected, especially at 1A+.

If that is so, you'll need more like a 9V trafo, or to use the transistor CCS, or FET CCS - it's not hard to build if you cut some plain copper board with a hacksaw.
 
I agree that the resolution is better and its included in the final schematic for long now. The less heat thing should not be though. Probably a Vgs difference must have produced less drop across the setting resistor, thus less current than with your previous PMOS. Nice that you also got some appreciable subjective lift non the less.

Agreed. Less heat could be due to the optimization of voltage/current through SSHV.
 
Does it work without the gyrator?

I don't use the LT108x regs in my amps, but I hear they need more voltage headroom than might be expected, especially at 1A+.

If that is so, you'll need more like a 9V trafo, or to use the transistor CCS, or FET CCS - it's not hard to build if you cut some plain copper board with a hacksaw.

Rod,

I haven't tried it without the gyrator. Will check it BTW. Is there any claculation/ratio to figuerout the Vin for this setup? It is bit difficult to find STP55. Hence could you give any alternative whcih is easilly available to use with? Also appreacite if you could post the latest schematic optimized for 26 filament if you have. Just to make sure. :)
 
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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.

Sheldon
 
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.

Sheldon
 
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?
 
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.
 

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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/ax/addenda/media/moore2355.pdf