Can substitute a 2N4125 and 2N5952 Transistor for any here http://www.bitsbox.co.uk/transistors.html ?
I assume what you meant to say was :-
Is there an equivalent part to the 2N4125 and 2N5952 listed on this site.
Well without knowing what you want to do with the transistors it is impossible to say.
The 2N4125 is a general purpose PNP transistor so I suspect the BC558B would probably do.
However, the 2N5952 is an RF N channel FET and while there are small signal FETs listed here I don't know what parameters of the FET you need. Do not however use a high current FET.
They are need for a guitar effects pedal, thats all i know........i doubt a schematic would help?
i doubt a schematic would help
Well yes it would because then you can see how it is being used.
If it is only a guitar effects then the RF part is not important so and small signal FET will do.
did i say its the 2N4125 and 2N5952 i need replacements for?
Yes that's better I can see what it is doing now. As I said before the 2N4125 is a general purpose PNP transistor so I suspect the BC558B would probably do. It is only in the output audio stage.
As to the FETs, what this is doing is a voltage controlled filter, however the filter sections are only first order and you have in effect a variable resistance from pin 3 of the op amp to signal ground. The matching ensures that the cut off frequency of each stage changes by the same amount. So what you want to match is the on resistance against the gate voltage. I assume that circuit in the other thread is supposed to measure that some how but it is not completely obvious how you use it.
I would just set up a circuit with a pot on the gate (ends to each rail) source to ground and drain through a 20K resistor to +ve.
Then plot a graph for each FET of pot position / gate voltage against voltage on the drain. You only need about four or five points on each graph. Then match up the four FETs that have the closest curves.
Best of luck.
my observation on electronic musical "things" is that the designs tend not to be "careful" about minimizing the effects of semiconductor differences. (In a normal audio amplifier, one does things to ensure that the actual gain of the amp is not dependent on the exact gain of the transistors being used, for example.) That means that you can probably get SOME interesting behavior with a wide range of similar transistors, but if you want your box to sound exactly like "the original Xyz TB-ABC effects pedal", you may be stuck hunting to the exact components specified. (witness the scramble for rare parts for Adafruit's x0xb0x kit, for example...)
Otherwise, Grumpy's advice sounds spot-on, as usual.
So I can switch up the 2N4125 for the BC558B, and the 2N5952?
If you have no better solution, just do it. If it works, you're good. If not, figure out why and make the changes. Otherwise you'll be paralyzed waiting for the 'right' parts! :
Keep in mind, that a lot of these designs were created with the "transistor du jour". Many transistors came and went over the years while some were so generically useful that they get used to replace ones that quickly became obsolete.
When looking for replacements... do a search on a site like NTE Electronics Cross Reference Search or http://www.datasheetcatalog.com/ to get the part specifications.
Then look to see what is readily available to you at the price you need.
Lets say you have a circuit that wants a 2N4124. It is basically a small signal switch that operates at 30V, and can carry 310ma load and has a GAIN (Hfe of 50-150). Now I compare my replacement. Checking my junkbox for the most COMMON NPN transistor I have... I see the 2N2222. So, will it work? Probably... are they different? SURE. But the 2N2222 is basically a 30V device and it can carry a 800ma load and has a GAIN(Hfe) of 75. Almost all these parameters are close enough for a generic replacement and will likely work unless the circuit is really depending on some odd characteristic of the original.
I actually recognize the phase shifter circuit. I bet you could use the 2SK772 from FARNELL for the N channel JFET.