Hello
Any opinions about the Preamp Dayton Wright SPA mk1, how it sound compare to more modern high-end preamp ?
It use few ua749 op amp in line and output, it would need to change them for better opamp..
It can be found at low price used on Ebay.
Thank
Bye
Gaetan
Any opinions about the Preamp Dayton Wright SPA mk1, how it sound compare to more modern high-end preamp ?
It use few ua749 op amp in line and output, it would need to change them for better opamp..
It can be found at low price used on Ebay.
Thank
Bye
Gaetan
Attachments
Last edited:
You can’t change the 749 because the design uses a feed forward transistor from the compensation point which is why the 749 is a 14 pin IC.
The drawing you posted is for the SPL preamp.( the web site says its for the SPA , but its not. The SPA line stage is the same as the SPL, but the SPA phono stage has a cascode bipolar input stage before the ua749.
You can change all the small ceramic capacitors to polystyrene
( but do not change or remove the .01uf ceramics power supply decoupling the phono stage IC or it will oscillate!
Add the 2 RIAA hf correction circuitS at the output of the phono stage.
The values in the series network are a 4n7 capacitor and a hundred ohm resistor.
The values for the shunt RC network are a 7n3 capacitor in parallel with a 47,000 ohm resistor. These can be seen in the service manual for the SPA MK 2
The SPA phono stage is the same as the SPL mk 2, and when they did the board layout for the new SPA they missed this part of the circuit.
Changing regulators to Burson or Dexa for the 15 volt supplies works.
.Change the coupling 22uf /tantalum Interstage caps to bipolar electrolytics (Muse) with a .1uf polypropylene bypass
Replace the power capacitors to larger value and use Quasimodo diode bridge snubbing
The drawing you posted is for the SPL preamp.( the web site says its for the SPA , but its not. The SPA line stage is the same as the SPL, but the SPA phono stage has a cascode bipolar input stage before the ua749.
You can change all the small ceramic capacitors to polystyrene
( but do not change or remove the .01uf ceramics power supply decoupling the phono stage IC or it will oscillate!
Add the 2 RIAA hf correction circuitS at the output of the phono stage.
The values in the series network are a 4n7 capacitor and a hundred ohm resistor.
The values for the shunt RC network are a 7n3 capacitor in parallel with a 47,000 ohm resistor. These can be seen in the service manual for the SPA MK 2
The SPA phono stage is the same as the SPL mk 2, and when they did the board layout for the new SPA they missed this part of the circuit.
Changing regulators to Burson or Dexa for the 15 volt supplies works.
.Change the coupling 22uf /tantalum Interstage caps to bipolar electrolytics (Muse) with a .1uf polypropylene bypass
Replace the power capacitors to larger value and use Quasimodo diode bridge snubbing
Last edited: