audio research LS25 preamp

Adams_Leo · 16th May 2006, 06:01 AM

I found the LS25 circuit diagram from the product service manual.I want to clone this amp.From this circuit,only remark the part number for FET input device.Which transistor device can substitute it should be better.Pls give you advise,I am so appreciate your response.

mirlo · 18th May 2006, 09:37 AM

2SK170 should work, but you might have to select them for a particular Idss or adjust some of the resistor values.

You would also have to match the two input devices very well, preferably at more than one gate-to-source voltage, to make sure of both DC and AC symmetry.

J309 might work but you might need to reduce the power supply somewhat, to make sure the 25 V rating isn't exceeded.

Please double check the current ratings before taking any of my suggestions; I admit I haven't done any exact calculations here.

Another one to look at: 2N5459. Again, check voltage and current ratings. But that one may not have high enough transconductance.

Basically you want a high transconductance FET with low noise and sufficient voltage and current handling capability.

Please post or email me the rest of the schematic: I'll look into it further and will do some calculations.

Do you have the schematic for the first version of this line stage? This one appears to be for the Mk II LS-25.

-- mirlo

Luke · 21st May 2006, 12:01 AM

Hi Mirlo,

could you not get around the jfet matching by using 2sk389?
I read a review about the LS25 and it claims that it uses 4 X 6922 tubes?
Is this the circuit for the MKII?
Adams_Leo, wheres the PSU

andirocks · 21st May 2006, 06:59 AM

The schematic shows the tubes as 6H30 so this would make it an LS25 MKII

Scott

Luke · 21st May 2006, 09:53 AM

Amazing such a simple preamp costs US$5K. Reviews are very favourable. This looks like a good diy clone candidate, what does the pot RV1 do?

Adams_Leo · 22nd May 2006, 12:56 AM

Quote:

Originally posted by Luke
Hi Mirlo,

could you not get around the jfet matching by using 2sk389?
I read a review about the LS25 and it claims that it uses 4 X 6922 tubes?
Is this the circuit for the MKII?
Adams_Leo, wheres the PSU

Yes,this circuit for MKII,sorry,i cannt found the PSU

sajti · 22nd May 2006, 11:03 AM

Quote:

Originally posted by Luke
Amazing such a simple preamp costs US$5K. Reviews are very favourable. This looks like a good diy clone candidate, what does the pot RV1 do?

RV1 set the bias for the input stage, with the Q3 current generator.

I'm working to build something like this, but with valves in the input stage, and resistor to set the bias

sajti

konst · 22nd May 2006, 11:18 AM

what is the fet, 6625? I apologize if i am being a bit slow here

EDIT: never mind i got it, doh

Luke · 28th May 2007, 07:19 AM

I calculate the power dissipated in the top (40K) resistor to be 109 watts

Am I right, is the shematic wrong or does p=I2r not apply in vintage circuits

BTW Adams_Leo,

you ever build this?

srh · 28th May 2007, 07:38 AM

Luke,

According to my calculator the 40k resistor dissipates just over 0.3w.

Steve.

22nd May 2006, 11:18 AM	#8
konst diyAudio Member Join Date: Apr 2006 Location: london (sout' of the river)	what is the fet, 6625? I apologize if i am being a bit slow here EDIT: never mind i got it, doh __________________ are you sure?

28th May 2007, 07:38 AM	#10
srh diyAudio Member Join Date: Mar 2004 Location: Whangarei. NZ	Luke, According to my calculator the 40k resistor dissipates just over 0.3w. Steve. __________________ The older I get, the more simplicity appeals.

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18th May 2006, 09:37 AM	#2
mirlo diyAudio Member Join Date: Jul 2002 Location: San Diego	2SK170 should work, but you might have to select them for a particular Idss or adjust some of the resistor values. You would also have to match the two input devices very well, preferably at more than one gate-to-source voltage, to make sure of both DC and AC symmetry. J309 might work but you might need to reduce the power supply somewhat, to make sure the 25 V rating isn't exceeded. Please double check the current ratings before taking any of my suggestions; I admit I haven't done any exact calculations here. Another one to look at: 2N5459. Again, check voltage and current ratings. But that one may not have high enough transconductance. Basically you want a high transconductance FET with low noise and sufficient voltage and current handling capability. Please post or email me the rest of the schematic: I'll look into it further and will do some calculations. Do you have the schematic for the first version of this line stage? This one appears to be for the Mk II LS-25. -- mirlo

21st May 2006, 12:01 AM	#3
Luke diyAudio Member Join Date: Sep 2001 Location: Wellington NZ	Hi Mirlo, could you not get around the jfet matching by using 2sk389? I read a review about the LS25 and it claims that it uses 4 X 6922 tubes? Is this the circuit for the MKII? Adams_Leo, wheres the PSU

21st May 2006, 06:59 AM	#4
andirocks diyAudio Member Join Date: Aug 2004 Location: Milwaukee, Wi	The schematic shows the tubes as 6H30 so this would make it an LS25 MKII Scott

21st May 2006, 09:53 AM	#5
Luke diyAudio Member Join Date: Sep 2001 Location: Wellington NZ	Amazing such a simple preamp costs US$5K. Reviews are very favourable. This looks like a good diy clone candidate, what does the pot RV1 do?

28th May 2007, 07:19 AM	#9
Luke diyAudio Member Join Date: Sep 2001 Location: Wellington NZ	I calculate the power dissipated in the top (40K) resistor to be 109 watts Am I right, is the shematic wrong or does p=I2r not apply in vintage circuits BTW Adams_Leo, you ever build this?

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