UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Op-Amp RIAA Preamp with Tone Controls

cathoderay57 · 7th Sep 2020, 2:53 pm

OK, here is the measured gain at the final output resistor (68R) of the Self circuit, using one chip of the 4 on the OPA4134, and a 15mv pk-pk input from a sig gen with 600R output:
100Hz 43.7dB; 200Hz 38.8dB; 300Hz 36.5dB; 400Hz 34.5dB; 500Hz 33.4dB; 600Hz 32.0dB; 700Hz 32.0dB; 800Hz 31.3dB; 900Hz 30dB; 1kHz 30dB.
I'm guessing that is not too bad. The one thing I've noticed is at the front end I have got a 68k resistor across the non-inverting input and a 150k across the cartridge. Received wisdom seems to be these 2 components should be transposed so I will try that and see if the change in cartridge loading makes any audible difference at the top end.

7th Sep 2020, 2:53 pm	#7
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,388	Re: Op-Amp RIAA Preamp with Tone Controls - Help! OK, here is the measured gain at the final output resistor (68R) of the Self circuit, using one chip of the 4 on the OPA4134, and a 15mv pk-pk input from a sig gen with 600R output: 100Hz 43.7dB; 200Hz 38.8dB; 300Hz 36.5dB; 400Hz 34.5dB; 500Hz 33.4dB; 600Hz 32.0dB; 700Hz 32.0dB; 800Hz 31.3dB; 900Hz 30dB; 1kHz 30dB. I'm guessing that is not too bad. The one thing I've noticed is at the front end I have got a 68k resistor across the non-inverting input and a 150k across the cartridge. Received wisdom seems to be these 2 components should be transposed so I will try that and see if the change in cartridge loading makes any audible difference at the top end.