Hi,
I bought this pre-amp and connected it to 12V 4A DC power-brick I already had. I changed the 4 grep film caps to Clarity Cap CSA 4.7uF 250V capacitors, changed the two JRC 55320 op-amps to JRC Muses02 and the valves to two matched Mullard M8100 NOS valves. I'm waiting for the two CSA 10uF 250V caps to change the two 10uF electrolytic capacitors between the two op-amps. I measured the potential difference between the anode & cathode on the valves and it's 70V. The valves are specced at 120V. How could I change the potential difference between the anode & cathode to get closer to 120V without changing the heater voltage?
I did notice the polarity was reversed so I just plugged the output red lead from the preamp to the white terminal on the Arcam rHead headphone etc...
Thanks
Haider
I bought this pre-amp and connected it to 12V 4A DC power-brick I already had. I changed the 4 grep film caps to Clarity Cap CSA 4.7uF 250V capacitors, changed the two JRC 55320 op-amps to JRC Muses02 and the valves to two matched Mullard M8100 NOS valves. I'm waiting for the two CSA 10uF 250V caps to change the two 10uF electrolytic capacitors between the two op-amps. I measured the potential difference between the anode & cathode on the valves and it's 70V. The valves are specced at 120V. How could I change the potential difference between the anode & cathode to get closer to 120V without changing the heater voltage?
I did notice the polarity was reversed so I just plugged the output red lead from the preamp to the white terminal on the Arcam rHead headphone etc...
Thanks
Haider
Changing Vak means you must change the tube operating point. It has nothing to do with the heater.
Note that if you do change the tube operating point, you have a different amplifier. It may sound better to you, or worse.
Reading between the lines it appears you think higher Vak means 'better' amplifier. This is not automatically the case.
Apart from the fact that it isn't recommended to operate a tube close or at its limits.
A competently designed amp has a well-thought out operating point.
At any rate you need a schematic to do the change.
Jan
Note that if you do change the tube operating point, you have a different amplifier. It may sound better to you, or worse.
Reading between the lines it appears you think higher Vak means 'better' amplifier. This is not automatically the case.
Apart from the fact that it isn't recommended to operate a tube close or at its limits.
A competently designed amp has a well-thought out operating point.
At any rate you need a schematic to do the change.
Jan
To be honest I saw the spec sheet on the Mullards and it said 120V, I measured the 70V so I was trying to bring it in-spec. I'm not sure if it would sound better but I was hoping to find out. It could well be that 70V sounds better. I'll ask Aiyima for the schematic...
The 120V is not given as 'that's what it should run at'. There are two example operating points, at 120 and 180V. The abs max is 200V.
Which operating point you chose is a design decision, and is part of the overall design of the amplifier. You can't just fiddle with a single parameter and expect it to improve; it might as well fall apart.
You really need to understand the whole design, and know what you are doing, to do things like this.
That means educating yourself.
I know; it's just like studying . 😎
Jan
Which operating point you chose is a design decision, and is part of the overall design of the amplifier. You can't just fiddle with a single parameter and expect it to improve; it might as well fall apart.
You really need to understand the whole design, and know what you are doing, to do things like this.
That means educating yourself.
I know; it's just like studying . 😎
Jan
Hi Jan,
That does make sense as the one made by FX Audio runs the tubes at 25V. Since the changes that were made to the caps and the op-amps I am really enjoying it. Laptop/iPhone->E1DA 9038d dac->Tube-A3->rHead->Sundara makes a very good setup. Admittedly it’s no longer a cheap as chips pre-amp but it performance has moved on in leaps and bounds...It up there with my main Marantz/Arcam separates...
That does make sense as the one made by FX Audio runs the tubes at 25V. Since the changes that were made to the caps and the op-amps I am really enjoying it. Laptop/iPhone->E1DA 9038d dac->Tube-A3->rHead->Sundara makes a very good setup. Admittedly it’s no longer a cheap as chips pre-amp but it performance has moved on in leaps and bounds...It up there with my main Marantz/Arcam separates...
Hmmm just wondering I know this is a pre-amp but if you don't ask you won't learn; headphones like the Sennheiser HD6XX/650 have a sensitivity of 105db+ & an impedance value of 300ohm. I believe the pre-amp can put output using the Jan GE 5654W tubes can output 9dB gain in 20K load at 0.5% THD can it power them? What about the HiFiMan Sundara which are 37ohm impedance & 94dB sensitivity?
Some further details, the DAC feeding the pre-amp can provide 20mW into 300ohm and 155mW in to 33ohm load.
I'm not familiar with the Aiyima version but it seems to be very similar to the FX. If Aiyima sends you a schematic, please post it here.
There's also a massive thread over at AK about the various FX versions and their cousins sold under other brand names. I think it's likely that the more recent models, and maybe even the Aiyima version, use essentially the same circuit. Some may even use the same board?
https://www.audiokarma.org/forums/index.php?threads/fx-audio-6j1-tube-preamp-a-31-wonder.848535/
The early, more basic, versions seem to share the same schematic for the audio portion but the PS sections differ a bit in that some are powered by a wall wart that has a DC output and others have an AC output and the rectification is done internally.
BTW, the FX doesn't run the tubes at 25v.
I don't own an FX but a while back, when I was experimenting with various tubes on a breadboard in the process of building a tube preamp, I got curious. An FX-01 owner was kind enough to measure the voltages for me using the stock wall wart and an adjustable linear PS. Many FX owners recommend a 13.8v linear supply.
The stock wall wart supplied 63.3v, the P-K voltage was 47.1v and the tube was running at 3.25mA. With his adjustable supply set to the point where the heater voltages were 6.3v, the supply voltage was 68.9v, the P-K voltage was 50.9v and the tube was running 3.56mA. With the power supply adjusted to 13.8v, the heaters were at their limit, the supply voltage was 74v and the P-K voltage was 54.7v with the tube running 3.95mA.
Obviously, running the heaters at their maximum will reduce the tube life considerably but the more concerning issue was that an input of 13.8v resulted in over 35v on their caps which were rated at 35v. Perhaps the caps in the newer versions are rated for higher voltages but it's something you should watch out for.
Since I had the breadboard set up and I was using the same basic audio circuit (which is called an anode follower) I tried the 6AK5 at the voltages and operating points used in the FX and then at more normal operating points, as suggested in the tube data sheet.
To me, the tube sounded much better at "normal" voltages than it did using the really low voltages found in the FX. I was, however, surprised that it sounded as good as it did at those low voltages. I really expected much worse.
That said, of the maybe 10 or 12 different types of tubes I tried, the 6AK5 was near the bottom. Even at normal voltages. I wasn't really considering using the 6AK5 family tubes because I knew they would have too much gain for my use but, as I said, I was curious.
If you're interested, here's a thread about the preamp I ended up building:
https://www.audiokarma.org/forums/index.php?threads/1626-preamp-build.931276/
Owners seem to go to great lengths with tube rolling and swapping op amps and a variety of other parts, which I'm sure is all fun stuff. And I don't doubt that they offer improvements. Of course, all these mods increase the cost considerably, which lessens the primary appeal of the original product. After the cost of the mods is factored in, there may be other options that sound better.
In my experience, running the tube at operating points that aren't restricted by a low voltage PS makes a huge difference in sound quality. At more reasonable voltages and current levels the sound is much more extended at both the low and high end and dynamics are also considerably better.
Apparently the Aiyima is running them slightly higher than the original FX did. Perhaps the newer versions of the FX and their cousins are also doing this.
Unfortunately, you can't just mod these in order to run the tubes at even higher voltages. If you're in love with these particular tube types for some reason, that's the way to get the most out of them.
Please also note that the operating points shown in the data sheets refer to pentode operation, while the tube is operated in triode mode in this preamp. That's because it would have way too much gain in pentode. As I recall, one manufacturer does show triode plate curves in their 6AK5 data but all of the other data they list refers to pentode mode.
I experimented with several higher voltage operating points and I posted a couple of schematics in the thread on AK. I used a bench supply when I breadboarded them because the power transformer I was using with other tubes wasn't well suited for the job. A traditional PS that provides a higher B+ would be easy to design but this would require building something from scratch in a different chassis instead of just parts swapping.
I should note that the plate resistance of this tube is not particularly low so its output impedance is better suited for use with a tube amp, which is what I used on the breadboard. Most people who use these little preamps seem to use them in order to add some "tube flavor" to systems that use SS or Class D amps.
I took a closer look at the Aiyima and looks like it's exactly the same as the FX-03, which is the more recent version that has tone controls. From what I've read on the AK thread, the op amps in this are used in the volume control and tone control circuits. They can be removed and the preamp still functions but it seems like you lose the volume control and tone controls.
Several posters in the AK thread have said that, after trying a variety of different op amps, they actually preferred the sound with no op amps at all.
Several posters in the AK thread have said that, after trying a variety of different op amps, they actually preferred the sound with no op amps at all.
All these vendors are just rebranding the TUBE-T1, there are different versions, mine V2-2. If you look at the photo at the top I posted you can see the PCB is marked accordingly. You can also see the marking on Suca & FX Audio. V1 PCBs were dated 2018/11/10...
To be honest whilst I studied A'Level physics and read BEng (Batchelor of Engineering) in Software Engineering. I'm not an audio engineer.
For the output coupling capacitors https://farmedia.com/tube-audio-line-level-buffers/ &
My problem was without a Parametric EQ on iPhone I need a way to tame the top end and bass on the HiFiMan Sundaras I listen to. I have changed the volume op-amp to a Burson V6 Classics and use the Muses02 on the tone controls. I had good results from this. I dial down the treble and up the bass and run it at 2/3 volume with some Siemens 5654W tubes. Thankfully I'm happy with the sound. I also found that unplugging both op-amps and using it on my laptop with Peace/Equaliser APO using Amir's settings from Audio Sciences I prefer the sound with the tube-t1 san op-amps in the chain...It's a richer, lush, warmer fat sound...
To be honest, at some point I will build your amp. Seems like a better fit for a solid state amplifier. My main sound system is
laptop->Beresford USB/SPDIF convertor->Marantz PM6006->Arcam Muso (compact bookshelf) speakers & Logo subwoofer
Meant to write that it is in PoC (proof-of-concept) mode. It allowed me to swap the caps out to see how the change modified the sound.
I measure the voltages of the pre-amp with the Mullards in and on 12v linear regulated PSU: -
I'm borrowing a variable power supply to see how much voltage it take to get the heater up 6.3v and see what the anode voltage hits whilst keeping it below the 16v of the power cap though I have my eye on a Panasonic 1000uF 105C 2caps rated to 24v
| Component | Measured | Typical | Max |
| g1 | 11.33v | 12v | |
| k | 10.74v | ||
| h1 | 5.7v | 6.3v | 6.3v |
| h2 | 5.7v | 6.3v | 6.3v |
| a | 72.3v | 120-180v | 200v |
| g2 | 72.3v | 120v | 155v |
| k | 10.74v |
| Power Caps | Label Voltage | Label Rating | Label Temp | Measured v |
| Cap1 | 16v | 1000uF | 105c | 11.4 |
| Cap2 | 250v | 105.3 | ||
| Cap3 | 250v | 100.2 |
| Tube Caps | Label Value | Measured v |
| Cap1 | 250v | 89.2 |
| Cap2 | 250v | 88.7 |
I'm borrowing a variable power supply to see how much voltage it take to get the heater up 6.3v and see what the anode voltage hits whilst keeping it below the 16v of the power cap though I have my eye on a Panasonic 1000uF 105C 2caps rated to 24v
I upped the voltage of the PSU to 13.1 DC output, the heaters showed 6.3v but the anode to cathode voltage remained the same at 71v. I checked the input voltage on the pre-amp power terminal was 13v. The voltages across the power smoothing circuit was also higher? Anybody have idea what stopping the anode to cathode potential difference increasing. I measure the PD between the cathode and the power + terminal and it was 10v, if that's the case then the cathode must be -3v (+ve power terminal is at +13v DC) and the anode must +74v...