Next build...01A preamp with center channel

analog addict

analog addict

Glory or Death!
Starting to look with designs for a new preamp with a real center channel. I don't have a tube designated for the mixer, but am leaning towards a 6SN7. HV PS is just roughed in so far, but will feature a VR tube regulating at 150V. The basic pre amp doesn't get much simpler than this. Thoughts?

01A preamp by Analog Addict, on Flickr
 
In my opinion, the 01A has wimpy electrical characteristics for topology you depict. I'm not sure how well it is going to work as an outboard preamp driving a power amp through interconnect cables, but it depends on specifically what it will be driving. The 01A has rp of about 10K, and gm of only 0.8 mA per volt. By comparison, a 6SN7 has rp of 7.7K and gm of 2.6 mA per volt (3x as much transconductance). If the cathode/filament is fully bypassed, output impedance will be about 7K, which is high by any standard of an outboard preamp driving interconnect cables. Depending on how much output voltage swing you need, you could be creeping into slew limited territory with the 0.47 uF output coupling cap. How fast can the tube change the voltage across the output coupling cap? Under slew limited conditions, the preamp could sound sluggish and anemic. To minimize this, you would typically want to do one or more of the following: a) use a higher transconductance tube, b) idle the tube at a higher quiescent current, c) reduce required output voltage swing, d) make the output coupling cap smaller.

Additionally, the other typical cautions are in order here: drive power amps with at least 10x greater input impedance than the output impedance of the preamp, and don't drive high capacitive loads.

On the other hand, if you wanted to augment the basic preamp design by adding a direct coupled output buffer, it would eliminate all of these concerns in my mind. There might be other approaches to consider that might lessen the negative effects of some of these concerns, such as utilizing an active plate load.
 
Finally getting back to this after an employment change...

So, in a nutshell, you're suggesting a cathode follower to allow for long cables and/or use with a SS amp?
Well, I've always believed in making my signal path as simple as possible, and not adding any elements not explicitly needed. I have noticed that a previous pre amp build does have some issues with very long interconnects, but the intended use is only with tube amps and only with interconnects of 3' or less.

Here's the current schematic...

Untitled by Allan Shang, on Flickr

and the DC filament supply....

Untitled by Allan Shang, on Flickr

I've been over the schematic with Dennis Had, and he thinks it will work fine, although he's not a fan of the 01A since he thinks it's typically pretty microphonic.
Guess we'll find out.

Also, I decided to go all in on this build, so Tony, Dennis' set up man for the Inspire gear not only drew out, drilled, and pre - populated the chassis, he also talked me into a revolutionary concept for a home built tube amp, a remote control for the volume pot. The two black boxes snuggled up to the left underside of the chassis are the additional electronics to run the volume pot, and the little key fob is the remote. What a concept! Dennis was kind enough to throw the chassis in with a bunch of his at the powder coater, so it's also professionally powder coated.

by Allan Shang, on Flickr

Untitled by Allan Shang, on Flickr

Untitled by Allan Shang, on Flickr

The little dongle on the back is the antenna for the remote.

Untitled by Allan Shang, on Flickr

The underside ready for soldering!!!

Untitled by Allan Shang, on Flickr

I've got a lot on my plate right now, so it may be a few weeks before I can get much done on this. I suppose I should take a few minutes and make sure I have all the necessary parts on hand...
 
kward said:
In my opinion, the 01A has wimpy electrical characteristics for topology you depict. I'm not sure how well it is going to work as an outboard preamp driving a power amp through interconnect cables, but it depends on specifically what it will be driving. The 01A has rp of about 10K, and gm of only 0.8 mA per volt. By comparison, a 6SN7 has rp of 7.7K and gm of 2.6 mA per volt (3x as much transconductance). If the cathode/filament is fully bypassed, output impedance will be about 7K, which is high by any standard of an outboard preamp driving interconnect cables. Depending on how much output voltage swing you need, you could be creeping into slew limited territory with the 0.47 uF output coupling cap. How fast can the tube change the voltage across the output coupling cap? Under slew limited conditions, the preamp could sound sluggish and anemic. To minimize this, you would typically want to do one or more of the following: a) use a higher transconductance tube, b) idle the tube at a higher quiescent current, c) reduce required output voltage swing, d) make the output coupling cap smaller.

Additionally, the other typical cautions are in order here: drive power amps with at least 10x greater input impedance than the output impedance of the preamp, and don't drive high capacitive loads.

On the other hand, if you wanted to augment the basic preamp design by adding a direct coupled output buffer, it would eliminate all of these concerns in my mind. There might be other approaches to consider that might lessen the negative effects of some of these concerns, such as utilizing an active plate load.
Click to expand...

The 01A tube preamp has some of the sweetest characteristics of all direct heated filament tubes, especially if driving a SE direct heated filament power amp. If you haven't built it and listened to it, judgement should be held. As with the 26 heater supply should be wel filtered, and it will have microphonics that can be tamed with dampeners attached to socket mount
analog addict said:
Finally getting back to this after an employment change...

So, in a nutshell, you're suggesting a cathode follower to allow for long cables and/or use with a SS amp?
Well, I've always believed in making my signal path as simple as possible, and not adding any elements not explicitly needed. I have noticed that a previous pre amp build does have some issues with very long interconnects, but the intended use is only with tube amps and only with interconnects of 3' or less.

Here's the current schematic...

Untitled by Allan Shang, on Flickr

and the DC filament supply....

Untitled by Allan Shang, on Flickr

I've been over the schematic with Dennis Had, and he thinks it will work fine, although he's not a fan of the 01A since he thinks it's typically pretty microphonic.
Guess we'll find out.

Also, I decided to go all in on this build, so Tony, Dennis' set up man for the Inspire gear not only drew out, drilled, and pre - populated the chassis, he also talked me into a revolutionary concept for a home built tube amp, a remote control for the volume pot. The two black boxes snuggled up to the left underside of the chassis are the additional electronics to run the volume pot, and the little key fob is the remote. What a concept! Dennis was kind enough to throw the chassis in with a bunch of his at the powder coater, so it's also professionally powder coated.

by Allan Shang, on Flickr

Untitled by Allan Shang, on Flickr

Untitled by Allan Shang, on Flickr

The little dongle on the back is the antenna for the remote.

Untitled by Allan Shang, on Flickr

The underside ready for soldering!!!

Untitled by Allan Shang, on Flickr

I've got a lot on my plate right now, so it may be a few weeks before I can get much done on this. I suppose I should take a few minutes and make sure I have all the necessary parts on hand...
Click to expand...


The 01A tube pre-amplifier has some of the sweetest characteristics of all direct heated filament tubes, especially if driving a SE direct heated filament power amp. For all naysayers, if you haven't built it and listened to it, judgement should be held. As with the 26 heater supply should be well filtered, and it will have micro-phonics that can be tamed with dampeners attached to socket mount. (Treat the tube like a large diaphragm microphone in a suspension.) Have fun, don't give up, and you will be presently surprised with the delicate finesse this tube will give to the music.
 
I've been using this 01a pre for 2 years now, no microphony troubles. Sounds great with my 45 amp. My pre uses Rod Coleman regulators that starves the filaments @15 volts, black box to the right is the power supply.DSC_0362.JPG DSC_0368.JPG DSC_0365.JPG
 
Guess I'll have to try mine again. I used Coleman Regs, Bartola gyrator loads and Bartola followers. Never could get it quiet and it was rather lifeless in both bass and treble. I was going to rebuild it for either 4p1l or 2p29 but a move stopped all audio projects for 8 months. Still don't have my workshop assembled.
 
Impedance mismatches will cause it to sound lifeless, especially in the bottom region. Anode follower config with that tube especially just doesn't have any snot to drive things, so that might be the issue. Never seen an 01a as a cathode follower, but I suppose it must be possible if inconvenient thanks to the direct heated nature of things. A 6J5 can be had in a shouldered bottle, isn't direct heated, and makes a fine follower (its half a 6SN7) though. Could probably also use a 27, its another period triode but indirect heat.

or if you don't mind a little sand in the mix, an op-amp can be configured as a zero gain buffer to drive absolutely anything you could want.
 
Shame about that, would have been cool to run that with my Rauland amps. Did we get those hooked up before you headed out ?
 
Had a little bit of time, so I started on getting organized and actually did get some minor assembly done. I noticed that Tony only gave me 3 sets of Jacks along with the center channel, so I had to do a little drilling. I hate this part because I can't drill to save my life. So, it looks a little crooked, and I shortened the distance between jacks as not to crowd the remote antenna. The three sets of RCAs nearest the side will be the inputs and the two and a half sets closest to the middle will be the outputs. I also pretty much finished the AC supply and started on the ground bus...

Untitled by Allan Shang, on Flickr

Untitled by Allan Shang, on Flickr

Untitled by Allan Shang, on Flickr

Untitled by Allan Shang, on Flickr
 
More progress...

Preforming high voltage lead

Untitled by Allan Shang, on Flickr


High voltage PS finished with noise suppression cap on the VR tube, 150V supply to 6SN7 plates, and high voltage side of one 01A finished

Untitled by Allan Shang, on Flickr

Both 01A high voltage sides finished. Using medical quality vintage Vit Q caps on all the outputs. Started the signal side with wrapped 1K resistors into the 01A grid to aid in noise shielding. Decided to kill two birds with one stone/try something new and used the ground for the volume pot as the coaxial wrap around the input resistors. Yes I know the shielding is grounded on both sides, but this ground has got to be there anyway, so why not make use of it? Also ran the output from the switch to the pot, using my scheme of pure silver 20 gauge wire wrapped/shielded by hand. Worked well on the last preamp, so guess it's now a "signature design element" ;-)

Untitled by Allan Shang, on Flickr
 
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