• 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.

How should Dick Olsher Blue Velvet measure??

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Hi, I built a Blue Velvet Preamp that’s posted be Dick Olsher http://www.blackdahlia.com/html/tip_44.html
My rendition uses a SS Shunt regulation but otherwise uses the same Audio schematic.
It sounds very nice but I a question on how much THD it shows.
On My distortion analyzer I get 1% at 3.00 volts out with a 100K load. I think this is kind of high. Its distortion looks more like a tube power amp. It does this through the entire audio spectrum.
With its design configuration what should I expect? Or should I just enjoy it.
I have a Lazarus Basic cascade that looks like a piece of wire as far as the distortion analyzer is concerned.
Thanks for your feedback, No pun intended.
Fred
 
QUOTE]With its design configuration what should I expect?[/QUOTE]

Intuitively, with 4.5k plate resistors on a 7k Ri tube I would have expected the THD to be through the roof. However, looking at the chosen operating point on the 6SN7 plate curves, the result appears to be pretty linear if the output is kept within moderate limits. The main result of the low value plate resistor appears to be low gain.

I would expect the local feedback loop to reduce THD to a pretty low amount. Probably lower than you are measuring. However, if the distrotion is mainly 2H and you like the sound I wouldn't be too perturbed. Maybe some tube rolling is in order.

It's an interesting design - using the second triode as an anode follower with local feedback to reduce output impedance instead of the usual cathode follower is a pretty novel approach.

pm
 
freddymac406 said:
Hi, I built a Blue Velvet Preamp that’s posted be Dick Olsher http://www.blackdahlia.com/html/tip_44.html
My rendition uses a SS Shunt regulation but otherwise uses the same Audio schematic.
It sounds very nice but I a question on how much THD it shows.
On My distortion analyzer I get 1% at 3.00 volts out with a 100K load. I think this is kind of high. Its distortion looks more like a tube power amp. It does this through the entire audio spectrum.
With its design configuration what should I expect? Or should I just enjoy it.
I have a Lazarus Basic cascade that looks like a piece of wire as far as the distortion analyzer is concerned.
Thanks for your feedback, No pun intended.
Fred
I've made also the Blue Velvet following the original schematic. I only used VR105 instead of OA2. The BV gave me exactly the same THD measurements with 10k load.
1% at 3.00 V and 0.63% at 2.00 V.
THD looks huge but sonics are excellent. Tubes i used are NOS KEN-RAD. I'm waiting to check some RCA or SYLVANIA.
 
Looking for a great 6sn7 cascade with 30X gain

Did anyone ever try messing with the resistor values on the Blue Velvet pre-amp?

I like the topology - but not the puny plate loads and puny B+ voltage. Something like 22K plate load seems worth a try...

Also: one could mess with the feedback voltage splitter. There is little DC difference there since both tube have identical operation points. But one could shift that around and then perhaps a 1:5 or 1:3 relationship between the two resistors forming the feedback splitter. More gain - but potentially higher output impedance. Worth a try...

D.
 
Thanks Mosquito. I built a version yesterday. Here is what I changed:

320V B+
6SN7s with 33K plate loads.
15K feedback resistor from output tubes plate to a 100 ohm resistor to the input tubes plate.
Cathode resistors for both is at 390 ohm.
- So how does it sound? Very very nice! Great detail and bass slam. Think I like it better than the same 6SN7s in a grounded cathode and a cathode follower (which is such a standard) configuration. Also: although I do not like feedback, - here we have no loop feedback - just local feedback around the last triode. So it was worth a try. Never assume anything! - Build and listen :)
Finally, you have to love the fact that the cascade here results in a pre-amp that does not invert signal polarity.
A great little weekend project.

D.
 
hi all,
I also did it a few years ago,but never posted here.I dont have a measurement,but it is ded quiet and sound great.
Cheers,
 

Attachments

  • IMAG0059.jpg
    IMAG0059.jpg
    828.8 KB · Views: 346
  • IMAG0067.jpg
    IMAG0067.jpg
    757.1 KB · Views: 311
  • IMAG0063.jpg
    IMAG0063.jpg
    921.9 KB · Views: 297
Nice build misterno! Did you ever monkey with the values?

And yes, Mosquito: R3 is 100 ohm and R4 is 15K. Also tried 22K on the plates. 27K as R4 with 470 ohm on the cathodes. I am beginning to like this triode - no feedback + triode out with feedback design. Very versatile. Trick is to keep plate loads and cathode resistors the same it seems.

D.
 
I know this is an old thread but recently found it when looking for some information about the performance of the Blue Velvet Preamp.

I bought a Blue Velvet Preamp with an integrated 18db per octave active crossover from Carl Smith. The pre can use 6SN7s or 12SN7s. Currently it is using 12SN7s. I am very satisfied with its performance.

I was experimenting with some operating points to use for a 6SN7 as a driver stage using the triode/pentode loadline calculator and then testing on my bench. I was surprised when I plugged in the operating point for my blue velvet first stage. Was I listening to that much distortion?

The power supply was originally built to the Blue Velvet spec using zener diodes instead of VR tubes and lower wattage dropping resistors. After 3 years the zeners and one of the dropping resistors failed. I modified it and used an LR8N + TIP50 voltage regulator with larger wattage dropping resistors. Nothing gets hot to the touch and the voltage is rock stable.

Both of the power amps that it drives reach maximum output with 1 volt input.

I connected it to an HP8903A analyzer and was quite pleased with the results at the drive levels I need. At 1 volt output the distortion was 0.23%. From 200mv to 500mv it was 0.12% (probably the range I listen to all the time). 1% distortion was achieved at 4.4 volts output.

Later this year I will connect it to a spectrum analyzer and determine how much distortion is 2nd and 3rd order.

Steve
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.