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Old 15th February 2007, 01:10 AM   #1
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Lightbulb How should Dick Olsher Blue Velvet measure??

Hi, I built a Blue Velvet Preamp thatís 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
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Old 15th February 2007, 02:13 AM   #2
mach1 is offline mach1  Australia
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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.

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Old 15th February 2007, 03:37 AM   #3
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I have 3 differerent types of tube to toll
1. old raythyon
2. new EH
3. new sovtek
I like them in that order and #1 And #2 are very close.
The Sovtek should have been a mike . I can say Dam this sucks and hear it through my speakers.
Fred
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Old 15th February 2007, 07:43 AM   #4
aroS3 is offline aroS3  Greece
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Default Re: How should Dick Olsher Blue Velvet measure??

Quote:
Originally posted by freddymac406
Hi, I built a Blue Velvet Preamp thatís 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.
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