Audio Research VT100 MK1 and LS15 HELP!!!


I recently bought this set VT100MK1 and preamp LS15 and while I was at this seller house he demonstrated the sound from his online Tidal on TV connect optical to the DAC Luxman DA - 06 then this go to the preamp LS15 and out into the VT100. This set up sounds amazing at his house. However, when I came home play CD from the Sony X1100ES Blu-ray player RCA output into the LS15 and out to the VT100 and it sound horrible - no bass at all in any type of music.  The sound sometime muffle and not clear and most importantly the bass missing tremendously. Im using a pair of KEF R900 Speakers. What is wrong here? can anyone help me correct the sounds? it is because I didn’t have the Luxman DA - 06 DAC? or it is because I don’t have those fancy hundred - thousands of dollars cable?  or the CD source is playin at 41khz /16 bit not high enough signal?  

This is my first system and I don’t know why I can’t get that high fidelity sound from the ARC equipments.  I'm waiting for the Audioengine B1 to arrive so I can stream Spotify into the LS15.   What is wrong with my set up?  Thank you in advance for helping me solve this.
kentrent

Showing 26 responses by lowrider57

OK, that's a good start to troubleshooting, but since the Mac is a receiver you've removed both the amp and preamp from the system.
Have you re-seated all the tubes in both components? Gently wiggle them out (not all the way) and push them back into the tube sockets. There's also a good chance you've got a bad tube in one of the critical stages.

Can you call the seller and tell him what you are experiencing?



You should be getting quality sound using your CD player.
What RCA interconnects are you using? Have you tried using a different input on the preamp?

Check to see if the tape monitor is set to off.



There’s nothing wrong with your speakers. I’ve heard the R900 with a 30W tube amp and it was a very pleasant listening experience. The R series is the entry-level for KEF, so there is some cost cutting to the cabinets and bracing, but not the drivers and crossovers. These speakers are revealing, dynamic and throw a huge soundstage.
Although listed as an 8 ohm speaker, the impedance curve indicates a large part of the all-important low and mid-bass frequencies are at about 4 ohms. So the 4 ohm taps should be tried first, as stated in the above comments.

Do you have any other sources? TV, turntable, tuner. 
Maybe the Mac 1500 has a preamp output you can plug into the amp.
No, I don't believe it would. I'll assume he was using balanced connections from dac to ARC preamp. That would provide higher gain to the preamp and then the amp.

RCA out of your CD into the proper RCA preamp input should result in good sonics at a lower level on the volume control.
Have you tried all the inputs to the preamp and made sure the tape monitor is off?
 
The only difference is the dac most likely has a higher gain than your CDP.
That doesn't explain why you have poor bass response. 
Jumpers are used in the amp XLR inputs when you are using RCA cables. But you're using XLR between amp and preamp, correct?

Where did the Gold Phantom come from? What kind of inputs does it have?

Can you repeat Jea's test but this time use the 4 ohm taps. The bass on your speakers is about 4 ohms.

For all we know, all the preamp tubes could be weak. But start by changing the 3 and 4 tubes.
Yes. Do you have a spare of the same tube type? If you replace the tube and it sparks again, you need service.
Have you been interrupting power to the preamp, eg, pulling out the AC cable, powering the preamp or amp on, off quickly?
There is a safety mode on the preamp if power is interrupted.


http://vpiforum.com/viewtopic.php?f=7&t=8590




No need to bias the preamp tubes. They're set at the factory.

If there's a short or other issue in the circuit the tube could blow again.  
It would be great if you only need a new tube. 

Preamp tubes are replaced in pairs. Two in the front, two in back. If the pair that supplies gain to the preamp are bad or weak, then there's not enough juice to drive the amp.

Look at the manual and identify which tube is bad...
V1 V2 V3 V4

You could, but the other tube could go bad at any time since you don't know it's age. When you replace the pair they will both be strong tubes.
They are left and right channel. Replace both since you don't know their age.
Then, hopefully the tube will not blow.
Hopefully it helps. But, there may be more than one bad tube. I hope I'm wrong.

A reminder...
Use the 4 ohm taps, re-seat all the tubes in the amp. Make sure they sit tight in the sockets.
  
@alphaguy65
The Op is using balanced cables from preamp to amp. Maybe he should try RCA unbalanced.

I've also listened to the R900's with only 30W; very large cabinets with deep bass, huge soundstage.

@kentrent  Something we haven't discussed is your speaker setup and your room.

Btw, one tube wouldn't cause such a serious problem. It would be the pair or could be tubes in the amp.


Also speakers are rated at 8ohms though
As I stated before, KEF may rate the speaker as a 8 ohm nominal load, but you look at the impedance curve to determine which tap to use (to start with).  The frequencies in the bass and mid-bass range are at approximately 4 ohms. This is the critical area of the frequency spectrum because it is harder to drive the speaker to reproduce good low-end response.


The EQ is a good temporary fix. Please keep an eye on the preamp for any shorting out or shutting down due to the tube.
:-)