ruffrecords
Well-known member
I am testing my new tube mic pre design and have been making some initial noise measurements. With a 400Hz HPF in I get about 0.6mV of noise at the output with a shorted input and 40dB gain; not a bad noise performance for a tube circuit but I have had better. So I had a look at the noise on the scope and I noticed there was what appears to be a spiky 2KHz component. I switched everything off; the PC, the modem my LED bench light but is still remains. Normal scope trigger does not lock it but setting TV-H coupling does lock it. However, the only TV in the house is two floors below.
So I connected up my HP2581A Wave analyser and measured a fundamental at 1733Hz at a level of about -90dBu - not much you might think but each an every successive harmonic is at pretty much the same level. The circuit uses half a 12AX7 and a 6922. I tried three different 12AX7 tubes and get identical results. I also changed the 6922 and still no change - always spot on 1733Hz so conclude it is not generated by the pre itself but it is getting in from somewhere.
I've got some other test to do to try to track it down but in the meantime I thought I would ask here in case anyone has seen something similar before.
Cheers
Ian
So I connected up my HP2581A Wave analyser and measured a fundamental at 1733Hz at a level of about -90dBu - not much you might think but each an every successive harmonic is at pretty much the same level. The circuit uses half a 12AX7 and a 6922. I tried three different 12AX7 tubes and get identical results. I also changed the 6922 and still no change - always spot on 1733Hz so conclude it is not generated by the pre itself but it is getting in from somewhere.
I've got some other test to do to try to track it down but in the meantime I thought I would ask here in case anyone has seen something similar before.
Cheers
Ian
