The pre is good and I am comparing it to my counterpoint sa2000 ( valve hybrid) which imo sounds very good ( despite some strange behaviour)
Those ceramic caps have me intrigued. If you have the top off the preamp try a quick measure on ohms from pin 8 to the cap. See if there is direct continuity. Preamp off of course for that.
OK although I'll only be here for another ten minutes or so but tomorrow is another day 🙂
Just very curious on those opamps.
Just very curious on those opamps.
Also I can’t find any info ( data sheet ) that says which is pin 8
The board layout suggests the blue ones are rail decoupling to pins 7 and pins 4 with a common ground... maybe...
No continuity between pin 8 and the cap leg next to it but there is continuity between pin 1 and the right hand cap leg
They are Sibatit capacitors, a half-cooked attempt from Siemens at multilayers caps (2 in fact).
The other cap could be anything, like an alternative compensation cap; the LF3xx series has no provision for an external compensation cap
Thanks, no connection would be expected result as the 356 has pin 8 not connected.
Data sheet:
LF356
The connection to pin 1 would be a problem if swapping opamps on a plug and play basis. Pin 1 along with pin 5 is marked as 'Bal' or balance is typically used for trimming DC conditions. It is possible to use these pins for other use but it would only apply to the exact chip type the designer used. The cap is most likely some kind of frequency compensation using pin 1 as an 'unofficial' way into the chip circuitry. That kind of thing is very occasionally done but is very chip specific and would not work as intended if other chips were used.
So swapping the opamps is going to be a 'no' unless someone was prepared to alter the configurations slightly to not use these pins.
That's super sketchy. The paths through the offset trim pins are likely not well characterized for anything other than offset adjustment. When using "undocumented features" like that you run the risk that future runs of the chip (such as the ones made today) won't work in your circuit.
I wonder if the designer tried to do something clever, like implementing feed-forward. In that case, the capacitor would likely connect to pins 1 and 3. The LM301 supports feed-forward compensation.
It sure would be nice with a schematic of this amp.
From the TI LF356 data sheet: https://www.ti.com/lit/gpn/lf356
Tom
The term we would use is 'a bit fringy' 🙂 Seeing the circuit diagrams would be very useful to see exactly what they are trying to do.
The section of apparently step and repeat X4 circuits I'll guess is the phono srage, with its X1mV and X5mV labels. Yet the phone stage isnt connected in the bigger picture. Perhaps your sound source isnt even going through that circuitry at present? That would mean maybe two op amp changes, versus 10.
Back when I could still hear everything in the higher registers, I had a preamp with the LF353. I thought it sounded good, through KEF concentrics and Dynaco mkIII amps. I'm sure more sound would go though that op amp unscathed than I could hear today. Nice looking preamp!
Yeah. It looks like four 10x gain stages followed by the RIAA compensation. The resistors that set the gain are by pins 2,3. They're 100 Ω and 1 kΩ, respectively, ±1 % tolerance, ±50 ppm/ºC TCR if I'm reading them right. Those are pretty nice resistors - especially for an amp that old.
Tom
I wonder if the low values + many op-amps resulted in lower noise, than higher amplification values and fewer amplifiers?