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Old 16th May 2006, 06:01 AM   #1
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Default audio research LS25 preamp

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.
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Old 18th May 2006, 09:37 AM   #2
mirlo is offline mirlo  United States
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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
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Old 21st May 2006, 12:01 AM   #3
Luke is offline Luke  New Zealand
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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
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Old 21st May 2006, 06:59 AM   #4
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The schematic shows the tubes as 6H30 so this would make it an LS25 MKII

Scott
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Old 21st May 2006, 09:53 AM   #5
Luke is offline Luke  New Zealand
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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?
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Old 22nd May 2006, 12:56 AM   #6
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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
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Old 22nd May 2006, 11:03 AM   #7
sajti is offline sajti  Hungary
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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
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Old 22nd May 2006, 11:18 AM   #8
konst is offline konst  United Kingdom
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what is the fet, 6625? I apologize if i am being a bit slow here


EDIT: never mind i got it, doh
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Old 28th May 2007, 07:19 AM   #9
Luke is offline Luke  New Zealand
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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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Old 28th May 2007, 07:38 AM   #10
srh is online now srh  New Zealand
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Luke,

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

Steve.
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