audio research LS25 preamp

[Darry]

+20V on Q1 and Q2 drain

gain : 24dB

 

[Luke]

what would be the best way to get the gain down on this preamp?

 

[SY]

Change the ratio of the voltage divider resistors used as drain loads for the FETs.

 

[Luke]

Change the ratio of the voltage divider resistors used as drain loads for the FETs.

Do you mean R17 and R19, doesnt that just change the voltage at the drain of the fets?

 

[SY]

R5/17 and R14/16.

 

[jackinnj]

Read Erno Borbely's article from 1999 -- Jung also had an article or two in AX last year on current sources.

The Borbely article is on his website:
http://www.borbelyaudio.com/adobe/ae699bor.pdf

 

[SY]

Taking a second look at the schematic, the DC voltages for the rail, the grid of the cathode follower, and the drain of the FET seem mutually inconsistent. Or am I missing something?

edit: Never mind, I just reread jlm's post. He saw the same thing.

 

[jackinnj]

I modeled it with the 2SK117 and 2SJ246 as well as the 2SK170 -- for the current source -- unfortunately I don't have a 6H30 SPICE model...

The values on the schematic are inconsistent as pointed out...

 

[jackinnj]

OK, I got a 6H30 model which works -- seems that the 2SK246 as the current source is optimal. Of course, YMMV, as oftentimes (but by no means always) the CAD program's THD analyzer is spot on.

 

[jackinnj]

An externally hosted image should be here but it was not working when we last tested it.

 

[mbosko]

Hi
Is there any replacement for FET Q4 "6765"
in power supply section?


Thanks

 

[jackinnj]

The whole of the AR LS25 is now up on the AR Database

 

[johnmarkp]

I am interested in building one of these as well. Looking at all the schematics I do not see where the 6.3 volt heaters are powered from.

John

 

[analog_sa]

You blind? From the 12.6v LM337 outputs in Mk1 and rectified, smothed, biased at 70v in Mk2.

To tell the truth i don't quite get all the enthusiasm for this pre. It's mediocre at best. A FET pre with a tube buffer?

Not to mention the DS1666. While being better than cheap pots or PGA3210s it is a far cry from a good stepped attenuator. And yes, i have compared it.

There are virtually hundreds of diy projects which have the potential to sound better.

 

[johnmarkp]

No I can see fine. The 6H30 uses 6.3 volt heaters not 12.6 volt, so either it is a mistake or not the place for the voltage. As far as how it sounds, to each his own.

John

 

[analog_sa]

If you could indeed see fine you would also noitice heaters are in series

 

[johnmarkp]

Now I am starting to question my sight. I do not see the heaters anywhere on the schematic so I do not see them in series?? Is the heater voltage from the "-6.9v Audio". The mk1 shows that as ~11v. Most of my projects have been in Solid State so these new fangled tube things confuse me.

 

[johnmarkp]

OK I finally figured out my mistake. I had never seen the heaters with a anything in between the two. Any idea why the +180v goes to this through the 110k resistor? Also what would be the value of the transformer secondaries 9 and 10?

 

[analog_sa]

If you look at the datasheet you will notice there is a limit on cathode to heater voltage of 130v. At lower supply voltages biasing the heaters at 0v works fine and at higher the hum is minimised when the heater is biased at a positive voltage, in this case 70v. The transformer output is around 18v ac.

 

[Bill Fuss]

I noticed the PS shows chokes in the microhenry range, surely they meant millihenries, or am I nuts.

Bill