• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Looking for a simple tube preamp build

Dave has a FAQ that explains it, but I'm having a hard time following it completely.
https://intactaudio.com/atten_FAQ.html

If I understand it correctly, it looks like if you put the AVC after the '01A, that would put the source impedance at 10k ohms at 0dB attenuation from the AVC. Then as you attenuate, for each -3dB of attenuation the load resistance presented by the AVC doubles (the output impedance is reduced by 1/2). So at -3dB attenuation Zout is 5k ohms. then at -6dB Zout is 2.5k ohms, at -9dB Zout is 1.25k ohms, at -12dB Zout is 625 ohms, and so on. So if your preamp line stage has 8X gain (18dB), and you use it at input to output = unity gain, that's -18dB attenuation, and the '01A 10k Zout is reduced to something like 160 ohms.

If that's what happens, then wow -- I see what you mean. That solves the problem of the '01A Zout being too high to drive powered speakers and Class D amps.

But still, the '01A will be driven full tilt from the source, so if your DAC puts out 200mV average level, and your '01A amplifies 8X, then that's 1.6V average out from the '01A, with peaks perhaps 12dB (4X) above that, or peaks = 6.4V. THD will be quite high at those levels. Shouldn't the AVC go before the '01A?
 
Last edited:
Dave has a FAQ that explains it, but I'm having a hard time following it completely.
https://intactaudio.com/atten_FAQ.html

If I understand it correctly, it looks like if you put the AVC after the '01A, that would put the source impedance at 10k ohms at 0dB attenuation from the AVC. Then as you attenuate, for each -3dB of attenuation the load resistance presented by the AVC doubles (the output impedance is reduced by 1/2). So at -3dB attenuation Zout is 5k ohms. then at -6dB Zout is 2.5k ohms, at -9dB Zout is 1.25k ohms, at -12dB Zout is 625 ohms, and so on. So if your preamp line stage has 8X gain (18dB), and you use it at input to output = unity gain, that's -18dB attenuation, and the '01A 10k Zout is reduced to something like 160 ohms.

If that's what happens, then wow -- I see what you mean. That solves the problem of the '01A Zout being too high to drive powered speakers and Class D amps.

But still, the '01A will be driven full tilt from the source, so if your DAC puts out 200mV average level, and your '01A amplifies 8X, then that's 1.6V average out from the '01A, with peaks perhaps 12dB (4X) above that, or peaks = 6.4V. THD will be quite high at those levels. Shouldn't the AVC go before the '01A?
TVC Goes at the output, I don’t remember what my final results were but real world results might of been lower than that. My TVC was custom so it wouldn’t saturate at those levels with that tube. THD results were decent(on my HP8903) but more importantly, this preamp was the best I’ve heard. I connected it to my 300b amp, GM70 amp and then to my Threshold S/500. I really didn’t use my SS amp that much but with this preamp, I couldn’t tell the difference between SS and tube amps. The preamp currently drives my SS amp until winter rolls around and the GM 70 heats the room. I might be somewhat biased but 20 other people that have heard this system experienced the same thing, they were stunned by the improvement.
 
I wouldn't be surprised if the improvement from the '01A is from a particularly pleasant coloration from some added H2 and a bit of gain adding some sensitivity... and there's nothing wrong with that.

I've found over and over again that when presented with an A vs B comparison where one choice has a bit more voltage sensitivity and pleasant H2 distortion, I will almost always prefer the more sensitive/slightly more distorted choice.

I had a Hagerman Bugle phono preamp (OPA2134 op-amps) which wasn't floating my boat (kind of flat and dull sounding). I added a crude little 6DJ8 anode follower set to 2X gain after the Bugle (before my selector switch and Intact Audio AVC), and I liked the sound better that way. It sounded 'warmer' and more 'full-bodied'. Certainly there was more harmonic distortion. I still liked it better that way.

I tried that same 6DJ8 anode follower after my DAC. I did not like that. The DAC sounded better straight into the selector switch, etc. So with the digital source I did not prefer the added harmonic distortion. It sounded 'muddy'.

Is your TVC a 'transformer volume control' with separate primary and secondary (and thus DC isolation)? Or is it one of Dave's AVC autoformer volume controls? I have an autoformer volume control, a single winding on the core.
 
https://www.audioasylum.com/forums/tubediy/messages/15/159939.html

https://www.audioasylum.com/cgi/t.mpl?f=tubediy&m=158008


View attachment 1072416

I would add a proper 1M ohm grid leak resistor on the 12B4A, instead of letting the pot be the grid leak. What happens if the wiper loses contact with the track? But that's a quibble.

I'd also reduce the value of the pot to 50k, or perhaps 20k, depending on the Zout of my sources. Another minor quibble.
I found these tubes in my pile up but the 9H 250ma choke seems large.
 
And this one?
 

Attachments

  • frank-6sn7-linestages.gif
    frank-6sn7-linestages.gif
    11.3 KB · Views: 94
This is a dumb question but if I don't need a bunch of gain can I use 300Bs or 2A3s or 45s???
Of course you can. You can use pretty nuch any triode or triode-wired tetrode/pentode than has an acceptable plate resistance.

Acceptable is a matter of debate of course, but I personally am comfortable with 2500 ohms or less, and uncomfortable with above 5K. Many would prefer under 1000 ohms, especially if driving a solid-state power amp, which often have low-impedance inputs.
 

jean-paul

diyAudio Moderator Emeritus
2002-09-20 7:20 am
Germany
"Low" impedance like 10 kOhm :)

The whole issue is that almost no one needs a preamp with modern sources (modern like younger than 20 years). About all modern sources have 2V outputs and below 100 Ohm output impedance. Besides that gain is also not needed. That makes inserting a preamp with less good drive capabilities and equally unnecessary gain and extra noise/hum an awkward item, even with many a tube power amplifier. Today many amplifiers and/or good quality volume controls are 10 kOhm for less noise. This makes the tube preamp sweat so all in all the tube preamp is extra, superfluous....for decades already. However it keeps DIYers busy for some reason.

Much simpler is to invest in a quality source selector and volume control in a good power amplifier with normal gain and 1V input sensitivity. Everything will work out OK then with 99% of sources. As many things have been standardized it is strange to fight that by inserting a superfluous device and then enjoy either maximum volume at 9 o clock and/or lack of bass by own choice.

If there's a desperate need to build and insert a tube device then make sure it is compliant with todays line level voltages and impedances. A tube buffer (so no gain) that laughs at 10 kOhm seems way more adequate. A tube buffer with good quality 10 kOhm volume control itself of course. Or have tubes in the input stage of the power amplifier where there is a need for gain.
 
Last edited:
  • Like
Reactions: 1 user

lavane

Member
2013-10-29 3:32 am
Yes. I built one. It's just a simple single stage design though. It works well with the tube amps I've used it on. Dunno about SS. It has no cathode follower, buffer, etc. If built per instructions it is very quiet. I found the 12AU7 had 0lenty of gain, but the 12AY7 sounded best to me though. The instructions call for an optional cathode bypass caps. I used them, but put them on a switch. I use them in circuit at lower volumes, and switch them off at higher volumes. It just sounds better with them out of circuit at higher volumes. It acts kind of like a loudness control, sorta. I also added the ground lift switch, but have yet had to use it.
 

Attachments

  • 20220607_112902.jpg
    20220607_112902.jpg
    461.9 KB · Views: 17
About ten years ago, a bunch of SMAC friends came over and brought different amps with them. I had a tube preamp and a SS preamp to work with.

we had 8 amps to play with. Half were tube amps and half were SS amps.

we would put an amp on a preamp, turn the amp to full volume if it had a volume pot, then played tunes with volume set with a meter.

we did these with all the amps and preamps.

even thought the color of the music was different, in all cases the music was more dynamic than without a preamp.

that is what I am shooting for with building a preamp.
 
  • Like
Reactions: 1 user