First post and straight to the point like post #7 but no answers. First thing too check indeed. Again the common error that device Wazulu by Shakatak with tube ECC666 is good without even looking at GAIN, impedances etc.
We don't even know what brand and type active monitors are used! "Hi I have a red diesel car. Which tires can you recommend?"
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I made a 5687 preamp with the volume control on the output, years ago. You have to remember that if you put the volume control on the output of the preamp, the input is seeing the full signal output from your sources, which is then amplified to a fairly high level, which you then attenuate back to a suitable level to drive your power amplifier(s). This guarantees high levels of distortion.
Let's say your source puts out an average of 200mV of signal, and you will end up using the volume control to return the output from your preamp to 200mV (unity gain). Let's also say you use a 6V6-triode with no feedback, so you get a gain of about 8X.
0.2V(8) = 1.6V
Your 6V6-triode will be putting out 1.6V avg signal at its plate. We then use the potentiometer on the output of the preamp to attenuate that 1.6V back down to 200mV average signal out, which is attenuation of -18dB.
The preamp amplifying device is running full tilt with 1.6V output, with the inevitably higher distortion level that implies, but the output of the preamp is set to unity (200mV in, 200mV out). You've just added the distortion and noise from the amplifying device, with no gain.
What if the volume control is on the input of the preamp?
The gain of the preamp is 8X, and we want 200mV out from the preamp. That means at the input of the preamp we want 0.2/8 volts, which will be 0.025V or 25mV. We use the volume control at the input of the preamp to attentuate the 200mV input signal by 18dB. The difference is that this time the 6V6-triode sees 25mV at its input, which it amplifies to 200mV. At these lower signal levels the THD will be much lower. The penalty will be that all the noise from the 6V6-triode passes unattenuated.
So, your choice with this 6V6-triode preamp is:
Either way, inserting a common-cathode 6V6-triode does nothing objectively good for your signal chain. Perhaps the goal is adding the THD from the 6V6 with volume control on the output. Fair enough.
Do the powered speakers have balanced inputs? That could make things interesting. I have a pair of JBL LSR305 powered speakers, which I think are quite good. They have balanced inputs, 10k ohm input impedance unbalanced, 20k ohm input impedance balanced. They can be set to a lower 'pro' input sensitivity of +4dBm, so perhaps a preamp with balanced output would be an actual improvement for unbalanced signal sources (like a phono preamp) into these powered speakers.
Just a thought...
Let's say your source puts out an average of 200mV of signal, and you will end up using the volume control to return the output from your preamp to 200mV (unity gain). Let's also say you use a 6V6-triode with no feedback, so you get a gain of about 8X.
0.2V(8) = 1.6V
Your 6V6-triode will be putting out 1.6V avg signal at its plate. We then use the potentiometer on the output of the preamp to attenuate that 1.6V back down to 200mV average signal out, which is attenuation of -18dB.
The preamp amplifying device is running full tilt with 1.6V output, with the inevitably higher distortion level that implies, but the output of the preamp is set to unity (200mV in, 200mV out). You've just added the distortion and noise from the amplifying device, with no gain.
What if the volume control is on the input of the preamp?
The gain of the preamp is 8X, and we want 200mV out from the preamp. That means at the input of the preamp we want 0.2/8 volts, which will be 0.025V or 25mV. We use the volume control at the input of the preamp to attentuate the 200mV input signal by 18dB. The difference is that this time the 6V6-triode sees 25mV at its input, which it amplifies to 200mV. At these lower signal levels the THD will be much lower. The penalty will be that all the noise from the 6V6-triode passes unattenuated.
So, your choice with this 6V6-triode preamp is:
- Volume control on the input = lowest THD, highest noise, or
- Volume control on the output = highest THD, lowest noise
Either way, inserting a common-cathode 6V6-triode does nothing objectively good for your signal chain. Perhaps the goal is adding the THD from the 6V6 with volume control on the output. Fair enough.
Do the powered speakers have balanced inputs? That could make things interesting. I have a pair of JBL LSR305 powered speakers, which I think are quite good. They have balanced inputs, 10k ohm input impedance unbalanced, 20k ohm input impedance balanced. They can be set to a lower 'pro' input sensitivity of +4dBm, so perhaps a preamp with balanced output would be an actual improvement for unbalanced signal sources (like a phono preamp) into these powered speakers.
Just a thought...
Aren't there some mistakes in that schematic?
The output resistor is 414 ohms? (with the 1uF output cap, it's guaranteed there will be no bass)
The plate cathode load resistor on the top triode is 1.2 ohms?
The input grid leak resistor is 194 ohms?
Those have to be mistakes. That just can't be right.
6N1P mu is about 33.
With unbypassed Rk to add degeneration figure the gain will be at least 15X (23.5dB).
That's a lot of gain for a line stage going into contemporary powered speakers that can be driven to full power with as little as 1V RMS input signal.
However, that problem can be solved with a bit of (gasp!!!) negative feedback. Perhaps something like this:
If the green LED D1 offends, it can be replaced by a 430 ohm resistor with a 330uF bypass cap, or just the 430R and leave out the bypass cap for some degenerative feedback to U1.
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Me i am looking to build a line preamplifier who use two 6sn7 and can drive my electronic crossover who have a input impedance of 20k. No high gain needed.
How much gain?
Impedance is not the whole story.
A 12AX7 with no NFB will give gain of about 35X to 50X. That's a LOT.
All that gain will undoubtedly add noise, possible RF pickup problems, etc.
And again the volume control is on the output of the preamp, so your 12AX7 will be amplifying full tilt, all the time, with 200mV average input so something like a whopping 8V continuous output (probably well over 1% THD at that point).
Your volume control will end up down at 7 o'clock most of the time, and be very twitchy to adjust.
Why is all that gain necessary, or even desirable?
I guess if the goal is to add a good dose of 'tube sound' (meaning, triode-generated harmonic distortion) then these line stage ideas like the 4S and 6V6-triode with volume control on the output make some sense.
Impedance is not the whole story.
A 12AX7 with no NFB will give gain of about 35X to 50X. That's a LOT.
All that gain will undoubtedly add noise, possible RF pickup problems, etc.
And again the volume control is on the output of the preamp, so your 12AX7 will be amplifying full tilt, all the time, with 200mV average input so something like a whopping 8V continuous output (probably well over 1% THD at that point).
Your volume control will end up down at 7 o'clock most of the time, and be very twitchy to adjust.
Why is all that gain necessary, or even desirable?
I guess if the goal is to add a good dose of 'tube sound' (meaning, triode-generated harmonic distortion) then these line stage ideas like the 4S and 6V6-triode with volume control on the output make some sense.
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There is light at the end of the tunnel when tube guys start to ask the right questions. Now when they choose the tube after having specified the required gain, drive capability etc. happiness will follow.
I use the 4S with cathode follower, still have a lot of gain. Maybe i must use 2 x 12au7.
Great explanation. My technical understanding is limited so I tend to approach things differently. On my Boogie Factor 1626 preamp I have volume controls on both the input and output. This was done for purely practical reasons.
A pair of mono pots allows me to alter the L/R balance because my listening position is often not centered between the speakers. A stereo stepped attenuator allows me to adjust the overall volume without altering the L/R balance once it's been set.
I have no idea about the technical implications. Care to comment?
As I said earlier, if you don't need much gain, why start with a tube that has much more gain than you need and then dump most of the signal to ground?
You will be a lonely person when thinking and speaking out that truth.
In the op shoes I 'd just add a sandcast R before the tweeter to drop the power response of the loudspeaker or change the placement.
Member KoiFarm has a thread on a preamp that can be configured as a Mu Follower or SRPP with adjustable gain and use several types of tubes.
https://www.diyaudio.com/community/threads/universal-pcb-for-purifi-1et400a-amps.379211/
In this thread he provides the gerber files required to have a PCB made for the amp.
https://www.diyaudio.com/community/...c85-6922-6dj8-etc-mu-follower-or-srpp.388295/
Steve
https://www.diyaudio.com/community/threads/universal-pcb-for-purifi-1et400a-amps.379211/
In this thread he provides the gerber files required to have a PCB made for the amp.
https://www.diyaudio.com/community/...c85-6922-6dj8-etc-mu-follower-or-srpp.388295/
Steve
Yep, it might be all messed up. After having the pre for a couple years and John Hogan passing away I thought I’d document the pre. I took all the marking of the bits of the pre.
Well, the problem seems to be a quasi-religious aversion to any form of negative feedback imposed on a circuit (while forgetting the internal feedback mechanism in all triodes, or the degenerative feedback produced by leaving a cathode load resistor unbypassed), along with the fact that there are practically no suitable triodes with mu = 2 or 3.
As for the original topic of a simple tube preamp build, how about this one? It just popped up right here on diyAudio:
https://www.diyaudio.com/community/...c85-6922-6dj8-etc-mu-follower-or-srpp.388295/
Perfectly suitable and you won't even have to build your own power supply. But it does use shunt NFB (ooooohhhhhh).