SP 12 SE

[laatsch55]

Joe, what are the standard in and out levels for line amp (pre-amps) ?? I know there has to be separate for Phono, and the rest are ???

 

[Gepetto]

Generally in the range of 200mV in and 2V out are the specs you often see cited for line inputs.

Phono is generally 3.5mV on the MM type phono inputs. Don't know if your pre has a MC input but those are generally 0.9mV

 

[laatsch55]

I had(before you posted) input 10mv on the CD input and cranked the volume knob all the way and got 77.5mv out. Promptly blew the LR8 again. The traces were starting tpo look bad, and , not out of the woods yet, i broke 3 IC pin sockets out of a DIP 14 socket. The LR87 is now socketed.The TIP 50 is also socketed.

 

[speakerman1]

Pulling hair out yet?

Larry

 

[laatsch55]

No Larrt. Not yet. I think I allowed too much gain somehow. It's also occured to me and then just un-occured to me I think the physically larger caps were drawing more current at startup. BUT, if you look at the B+ volts when you flip the switch, it's like it's on a long delay soft start. So, don'y know yet . Hooking it back up to the AP now.

 

[speakerman1]

It is possible for to much gain. LOL Start low go little at a time. Should get 20k gain. That is a 100K attenuator. So as far as the amp is concerned. Alice right to the moon. LOL

Larry

 

[laatsch55]

I can say we are getiing ----A CLEAN SINE WAVE---YEEHAW!!!!!!!!!!

 

[laatsch55]

Frequency Response

 

[speakerman1]

I can say we are getiing ----A CLEAN SINE WAVE---YEEHAW!!!!!!!!!!

I'm glad you are; because I'm not. LOL

Larry

 

[laatsch55]

Larrt---IT WORKS!! I have some 60hz hum. But it appears to be ground loop. so it can be fixed. On the graph of the THD & N you will see a dip at 60hz. That indicates when the fundamental frequency is filtered out and it happens to be at the same freq as the interference the distortion drops. That also means the distortion figures are dominated by that same interference.


Wassamatteryou sine wave??

 

[speakerman1]

It was low. LOL If you read that thing I linked to. They were having hum problems also.

Larry

 

[Gepetto]

Hi Lee
If the LR8 blew while you were doing tests, you should try to scope the regulated 265VDC output (without blowing your scope input in the process) to see what is going on. The TIP50 is supposed to buffer the LR8 and on top of that, the LR8 has internal current limit and thermal protection so should have self protected. I have never used Supertex devices so have no way of gauging their relative ruggedness compared to a typical LM317 type regulator (which are bulletproof).

Something is funky in there I suspect.

What are the THD specs on this pre? You were measuring 0.8% if I am reading your graph properly.

 

[speakerman1]

Joe I haven't seen the specs on it. I have always been under the assumption that tubes are different than SS. I could be wrong.

Larry

 

[speakerman1]

Lee

I know it is a ground loop. Can we daisy chain the grds. Like faceplate, PCBs, and trans? Just thinking out load. Run them to the IEC grd.? Or what do you think?

Larry

 

[laatsch55]

Joe, if the LR8 was in self protect mode , that's agood way to get diagnosed as burnt and get swapped out around here. When it goes into protect mode does it do so "open" or will it shunt output?
Larry-, I don't know, specifically what it will take to solve it. One wire at a time. I still don't know if I have the selector, gain and volume controls wired to the Ground here, don't ground there aspect of the thing. I wanted to get it passing a sine wave then I'll chase down the hum.

Joe- that THD & N number is correct , but, also know that that number is dominated by the hum and will drop considerably when the hum is gone----but---- you knew that.

 

[laatsch55]

Also Joe, I had broke a leg on the 105K-400 volt cap and replaced it with a 2G 105K , but it was too much cap and blew a fuse on power up so I removed it . That balso could havwe led to the LR8's demise.

Another question--- Larry has upgraded 6 of the caps to large silver in oils from the stock MKP's-- would that affect turn on surge?? What i've seen while monitoring start up is there are no surges on the B+ voltage, just a very relaxed and orderly voltage climb to the 265. takes about 5-8 seconds.

 

[laatsch55]

Plenty of gain out of the phono section. Freq sweep of the phono section next.

 

[laatsch55]

Lee

I know it is a ground loop. Can we daisy chain the grds. Like faceplate, PCBs, and trans? Just thinking out load. Run them to the IEC grd.? Or what do you think?

Larry

You should really try thinking out "Loud" thinking out "Load" might hurt.

 

[jbeckva]

Is that like thinking out the side of yer.... ??? nevermind... I've done that one before too..

 

[orange]

Lee

I know it is a ground loop. Can we daisy chain the grds. Like faceplate, PCBs, and trans? Just thinking out load. Run them to the IEC grd.? Or what do you think?

Larry

Hum is good if it's Tubular Bells.