Aleph-X builder's thread.

Hi Guys,

I`m finishing make upgrade to my Aleph X , now it is with JFET`s. The sound of it isent warm and full anymore. So I want to change the Resistors to old ones.
I`m wonder what resistors are you using guys and where can I buy them best in EU.
Mr. Nelsons personal preferences are Panasonics for the 1-3 watt
varieties, and Dale (Vishay) for the precision types.
I wonder where can I get those in EU?
 
Hi Guys,

I`m finishing make upgrade to my Aleph X , now it is with JFET`s. The sound of it isent warm and full anymore. So I want to change the Resistors to old ones.
I`m wonder what resistors are you using guys and where can I buy them best in EU.
Mr. Nelsons personal preferences are Panasonics for the 1-3 watt
varieties, and Dale (Vishay) for the precision types.
I wonder where can I get those in EU?

Hi,

are you sure the resistors are causing this? Did you change the McMillan resistors (R46/47) to fit the new front end bias?
How many JFets for how many output fets?

William
 
Hi,

are you sure the resistors are causing this? Did you change the McMillan resistors (R46/47) to fit the new front end bias?
How many JFets for how many output fets?

William

Hi William.

I didn'tupgrade by myself, a frend of mine.

He had changed those resistors from 390ohm to 820ohm. Brand: Vishay DALE

Does this have to be MCMILLAN?

I have 2 psc JFets 2SK74-BL and they drive 6MOS-Fets
I have total of 24 MOS-Fets in my monoblock.
 
Hi,

R46 and 47 were called McMillan resistor in honour of the guy who "invented" them. You need them to control the absolute DC-offset and they can be made a larger value if the front end bias (through the diff pair) is smaller. This also has an influence on the sound (in my eyperience the larger the better but you have to watch out, that absolute offset stays stable)


I also noticed a reduction in bandwidth going to JFets and driving only 12 Output fets so I think your amp is probably rolling off a lot earlier. Do you have some measurements?
The solution would be to parallel 2 or 3 pairs of input JFets.

William
 
Hi,

R46 and 47 were called McMillan resistor in honour of the guy who "invented" them. You need them to control the absolute DC-offset and they can be made a larger value if the front end bias (through the diff pair) is smaller. This also has an influence on the sound (in my eyperience the larger the better but you have to watch out, that absolute offset stays stable)


I also noticed a reduction in bandwidth going to JFets and driving only 12 Output fets so I think your amp is probably rolling off a lot earlier. Do you have some measurements?
The solution would be to parallel 2 or 3 pairs of input JFets.

William

affectionately I don't have any measurement on my Amplifier.
I I contacted mate who was upgraded my Aleph X and he told me that it will be a big problem for him to find the parts Matched JFETS and redesign the Amplifier to run on those.
So in other words I am a little bit worried about this and I have to reconsider to going back to previus stare with IRF9610. I veel love this amp on the IRF9610 and was hopeful to improve this with JFets upgrade.
Sad! :(
 
Hi,

those resistors are just plain 1/4 Watt metalfilm (1%). What is important is the value so if they were 4k7 change them to 12k and watch the absolute offset.

William
Hi William.

Tonight my mate has looked to the Aleph X shematic an we wäre talking about 2 different resistors. The 820ohm was for the feeding JFETS . The R45 and R46 wäre left untouched. I don't know the value till I'm coming back home.
So this is first thing we need to change.

Other things are that he was changed my little mosfets from BC550 to ZTX450.
Do you have any experience with those and also with MPSA18 ?
What you prefer?

KR.

Robert
 
Member
Joined 2002
Paid Member
I also noticed a reduction in bandwidth going to JFets and driving only 12 Output fets so I think your amp is probably rolling off a lot earlier. Do you have some measurements?
The solution would be to parallel 2 or 3 pairs of input JFets.

William

Hi William,

Are you using parallel jfets?
I'm wondering how much current do you have running through the
jfets to drive 3 mosfets per quadrant?

Thanks,
Dennis
 
Hi Dennis,

I´m using just one pair at a bit below 5mA per Jfet (910R) and disconnected 4 output fets for a total of 8 at around 4A total bias.
With 12 output fets bandwidth was quite reduced (details must be in the AlephJ-X thread)
Still planning to try a second pair of input JFets but it is not on top of my to do list......

William
 
it seems to me though, that the only way you can get a dead on
exact match is to have these devices in the circuit and running for
at leat 30 minutes before you can get an accurate reading. I don't
see how you can test them out of circuit and match them to within
less than .01 volt.

I supposed for the in circuit matching Q5/Q7 I need to put the in+/in- (amp input) to the ground, right?