Got the 4P1L "Siberian" preamp hooked in my 45 SE. Of course as expected something always goes wrong and I damage the Salas HV shunt regulator so had to remove it and power the pre-amp with the raw supply. Not ideal, but it's well filtered though. Will need to repair the Salas shunt regulator now.
The sad thing is that I can't do a side by side comparison against the 26, but let me tell you my first impressions of this preamp:
1) Ringing is not audible at all. With no signal I tap the table where the preamp sits and nothing. You need to tap the valves hardly enough to get them ringing. In my opinion, this has no contribution to my preamp performance or at least I can't notice it whilst listening to music.
2) Sound is really nice. Exceeded my expectations coming from a 26 preamp!. It has significantly more gain (Av=9). Played "Sonny Rollins' Way Out West" which has a lot of dynamics and superb bass phrases. Also played "Schumann's string quartets" which gave me the opportunity to appreciate the preamp in more detail. Overall the clarity is really good, great bass and warm tone.
3) Noise floor is very good, bearing in mind that the salas shunt regulator is not present. The 50 and 100Hz harmonics are below -100dB when the filament boards are running without HT applied. thanks to Rod Coleman's regulators! With HT noise comes up to over -90dB, which is not that bad
The quiescent point still is Ia=20mA and Va=91V. The THD is about 0.04% - fantastic if you compared to the rest of the DHTs I have been trying. You could get better THD if you run the 4P1L hotter!
Still need to play around to find the sweet spot of this preamp....
Now back to the workshop to fix the Salas Shunt Regulator and do the belated side by side test with the 26
The sad thing is that I can't do a side by side comparison against the 26, but let me tell you my first impressions of this preamp:
1) Ringing is not audible at all. With no signal I tap the table where the preamp sits and nothing. You need to tap the valves hardly enough to get them ringing. In my opinion, this has no contribution to my preamp performance or at least I can't notice it whilst listening to music.
2) Sound is really nice. Exceeded my expectations coming from a 26 preamp!. It has significantly more gain (Av=9). Played "Sonny Rollins' Way Out West" which has a lot of dynamics and superb bass phrases. Also played "Schumann's string quartets" which gave me the opportunity to appreciate the preamp in more detail. Overall the clarity is really good, great bass and warm tone.
3) Noise floor is very good, bearing in mind that the salas shunt regulator is not present. The 50 and 100Hz harmonics are below -100dB when the filament boards are running without HT applied. thanks to Rod Coleman's regulators! With HT noise comes up to over -90dB, which is not that bad
The quiescent point still is Ia=20mA and Va=91V. The THD is about 0.04% - fantastic if you compared to the rest of the DHTs I have been trying. You could get better THD if you run the 4P1L hotter!
Still need to play around to find the sweet spot of this preamp....
Now back to the workshop to fix the Salas Shunt Regulator and do the belated side by side test with the 26
Attachments
Looks quite similar.
No, the cruise missile.
http://www.audioportal.su/attachment.php?attachmentid=23765&d=1231268158
Have been running for 30 min or so. Now playing Beethoven's violin sonatas. It's getting better now! Any idea of any burn in time for these valves?
They don't glow too much, especially if you run them at low filament current like in this case. A dim glow from the filaments. See attached
They don't glow too much, especially if you run them at low filament current like in this case. A dim glow from the filaments. See attached
Attachments
THD response and updated schematic attached.
Looking at increasing the value of R3 to improve the gyrator frequency response. Haven't updated the LTspice simulations, but before doing so, any recommendations around the practical value of this resistor? 1M or even higher? I've seen this to be around 4M7, but then Rod recommended a lower value. Bearing in mind the capacitor is already 1uF
Thanks for the help
Ale
Looking at increasing the value of R3 to improve the gyrator frequency response. Haven't updated the LTspice simulations, but before doing so, any recommendations around the practical value of this resistor? 1M or even higher? I've seen this to be around 4M7, but then Rod recommended a lower value. Bearing in mind the capacitor is already 1uF
Thanks for the help
Ale
Attachments
Iko,
Yes. Gyrator loaded. See previous post. Added a 2uF capacitor on the preamp chassis. Blew up the salas HT shunt regulator so I'm using the HT raw supply. Is a CLCL stage using a hybrid AZ1 rectifier.
See attached
Attachments
THD response and updated schematic attached.
Looking at increasing the value of R3 to improve the gyrator frequency response. Haven't updated the LTspice simulations, but before doing so, any recommendations around the practical value of this resistor? 1M or even higher? I've seen this to be around 4M7, but then Rod recommended a lower value. Bearing in mind the capacitor is already 1uF
Thanks for the help
Ale
Amazing FFT, is this without the Salas? Espect the 60hz hump will go away with the salas or is this a groundloop. You mentioned measuring without the B+ only the coleman supplies, can you post this FFT ?
Awesom job BTW, it sounds like the cord you found for suspension is doing the job. Only thing is you said average gain of 9, I thought that with a ccs/gyrator this tube should be close to a mu of 15 ?
Hi Regal,
Doesn't seem to be a ground loop to me. I need to turn the volume up really high to listen to the hum. See attached the filament noise FFT - highly recommend Rod Coleman's supply boards!
The suspension seems to do the job BTW. Gain is 9, where did you get that the mu=15 triode strapped? As I didn't have the data at hand, I had to trace the curves. See attached....
This is a fantastic pre-amp, just bear in mind it costs! Really happy so far!!!
Doesn't seem to be a ground loop to me. I need to turn the volume up really high to listen to the hum. See attached the filament noise FFT - highly recommend Rod Coleman's supply boards!
The suspension seems to do the job BTW. Gain is 9, where did you get that the mu=15 triode strapped? As I didn't have the data at hand, I had to trace the curves. See attached....
This is a fantastic pre-amp, just bear in mind it costs! Really happy so far!!!
Attachments
You are right I was thinking of a different tube, the file trace I downloaded from somewhere shows a gain of 8.
For a headamp the 60hz bump at 90 may be an issue with these sensitive headphones, but I doubt it.
What was your output Vrms with the previous FFT? The complete absense of anything above 2H is really cool.
I guess I need to let you get back to restoring that Salas reg, if its any help I've found that the MJE350 is the one that usually goes (but I have protection diodes on the mofsets.)
For a headamp the 60hz bump at 90 may be an issue with these sensitive headphones, but I doubt it.
What was your output Vrms with the previous FFT? The complete absense of anything above 2H is really cool.
I guess I need to let you get back to restoring that Salas reg, if its any help I've found that the MJE350 is the one that usually goes (but I have protection diodes on the mofsets.)
Input signal was 1Vrms and output 9Vrmsvc
haven't tried it yet with my GRADO headphones. A lot of work this weekend so far, so will leave the Salas regulator for next one. So far so good
Given the cost of this preamp, it blew me out so far. I started playing with a breadboard using Thomas Mayer's aluminum rails, but liked its look when it evolves.
It has a minimalist look, very simple. Have shielded all connectors so left it without fitting it inside a proper aluminum box. So far so good
I'm a happy man
haven't tried it yet with my GRADO headphones. A lot of work this weekend so far, so will leave the Salas regulator for next one. So far so good
Given the cost of this preamp, it blew me out so far. I started playing with a breadboard using Thomas Mayer's aluminum rails, but liked its look when it evolves.
It has a minimalist look, very simple. Have shielded all connectors so left it without fitting it inside a proper aluminum box. So far so good
I'm a happy man
Ok, just tweaked the Salas shunt regulator to get about 67mA across the pass mosfet. Not sure if measured correctly the internal resistance of my HT raw supply but must be around the 1-2K based on previous measures.
Adjusted R8 to set the shunt current. Will 67mA suffice?
Adjusted R8 to set the shunt current. Will 67mA suffice?
