You are getting good advice from several sources.
The 12AX7 SRPP Would be a lot lower distortion driving a 150K load, rather than a 37K.
The topology you posted second with the cathode coupled amp and cathode follower has a lot more promise.
However, if you are going to run a 12AX7 with 150V, it really needs an active load (Read Constant Current Source) to be low distortion.
The Cathode follower needs to be a medium mu tube.
12AU7, 6DJ8, ECC99 12BX7, 6SN7 are some of the choices that will work better than a 12AX7 as a cathode follower.
It would also be best with an active load.
There is no reason to not dc couple the Grounded cathode to the Cathode follower.
HTH
Doug
The 12AX7 SRPP Would be a lot lower distortion driving a 150K load, rather than a 37K.
The topology you posted second with the cathode coupled amp and cathode follower has a lot more promise.
However, if you are going to run a 12AX7 with 150V, it really needs an active load (Read Constant Current Source) to be low distortion.
The Cathode follower needs to be a medium mu tube.
12AU7, 6DJ8, ECC99 12BX7, 6SN7 are some of the choices that will work better than a 12AX7 as a cathode follower.
It would also be best with an active load.
There is no reason to not dc couple the Grounded cathode to the Cathode follower.
HTH
Doug
Hello
Active load ?
I understand that 12AX7 tube and 12AX7 SRPP circuit are not the best to drive a 37k load.
I have 12AX7 tubes because I still have old equipments using those tubes. And ordering other tubes from here in mid-north of Quebec are a bit risky and costly.
So I will try to do my best with my 12AX7 tubes.
I have found another schematic like my second one but it need much higher voltage, I include the image.
Thank a lot guys.
Bye
Gaetan
Active load ?
I understand that 12AX7 tube and 12AX7 SRPP circuit are not the best to drive a 37k load.
I have 12AX7 tubes because I still have old equipments using those tubes. And ordering other tubes from here in mid-north of Quebec are a bit risky and costly.
So I will try to do my best with my 12AX7 tubes.
I have found another schematic like my second one but it need much higher voltage, I include the image.
Thank a lot guys.
Bye
Gaetan
Attachments
I could be wrong here, but the circuit and plate voltage makes me think it was designed for a ECC88/6DJ8.
In fact, I have tried something similar with a PCM56 DAC and 5687 tubes inside a Yamaha CD player once.
12AX7's sound much better with a much higher plate voltage, for that SRPP you could double it (change the resistors accordingly) and make the sound more transparent.
In fact, I have tried something similar with a PCM56 DAC and 5687 tubes inside a Yamaha CD player once.
12AX7's sound much better with a much higher plate voltage, for that SRPP you could double it (change the resistors accordingly) and make the sound more transparent.
If you have 4 12AX7's then paralleling them is probably the best way to go about it. I'll throw this schematic in as a starting point, you are just gonna have to build and listen to find what sounds good to you. I do agree with others about needing a fairly high B+ supply, an AX7 should be ran with 150V+ on the plate and 500uA+ cathode current and a minimum 100kohm plate load to be fairly linear, at least that has been my experience with them.
Attachments
Of course, once you run a 12AX7 at a decent plate voltage, you start complicating things since the upper tube will need its own heater supply. Besides the nonlinearity of the SRPP, there's also the noise issue, where the bottom several bits will be thrown away.
If memory serves, the TDA1541 swing a milliamp from zero to full signal. That means peak signal voltage at the 12AX7 grid is 20mV (15mV RMS). 16 bits down is 96dB, or 0.2uV. What's the noise of a 12AX7? Do they make good noise-free MC amps?
No, I'm sorry, the basic concept here should be scrapped and re-thought. It can't be easily fixed.
If memory serves, the TDA1541 swing a milliamp from zero to full signal. That means peak signal voltage at the 12AX7 grid is 20mV (15mV RMS). 16 bits down is 96dB, or 0.2uV. What's the noise of a 12AX7? Do they make good noise-free MC amps?
No, I'm sorry, the basic concept here should be scrapped and re-thought. It can't be easily fixed.
Hello Sy
Tube do not seem to have the op-amps problems wen used as a I/V amp. But as you said it can be more noisyer than op-amp or discrete I/V amp.
R2R DAC like the TDA1541A need very low Settling time Op-Amps, but as Jan Didden wrote in an old thread about using op-amps for an I/V amp "the problems with feedback is that the error signals being fed back are amplified by the full amp open loop gain, which normally is quite non-linear as well. That causes all kinds of intermodulation and harmonic distortion products."
That is why I was thinking to use a tube I/V amp, and with a NOS Dac the tube I/V amp are less prone than the op-amps to do IM artefacts cause by the 44 khz of the Dac.
Thank
Bye
Gaetan
Tube do not seem to have the op-amps problems wen used as a I/V amp. But as you said it can be more noisyer than op-amp or discrete I/V amp.
R2R DAC like the TDA1541A need very low Settling time Op-Amps, but as Jan Didden wrote in an old thread about using op-amps for an I/V amp "the problems with feedback is that the error signals being fed back are amplified by the full amp open loop gain, which normally is quite non-linear as well. That causes all kinds of intermodulation and harmonic distortion products."
That is why I was thinking to use a tube I/V amp, and with a NOS Dac the tube I/V amp are less prone than the op-amps to do IM artefacts cause by the 44 khz of the Dac.
Thank
Bye
Gaetan
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.