Could you explain why azimuth affects the respective output levels of the two channels of a stereo cartridge?
The balance between the L/R channels is done by adjusting the azimuth of the cartridge, or head-shell.
Reversing the inputs, with a 300Hz test tone I measured the following values:
Tone volume.........Right.....Left
+12dB.................0.61V.....0.68V
+14dB.................0.85V.....0.93V
+16dB.................1.05V.....1.13V
In this case I calculated a 0.9 ~0.6dB difference between channels.
Playing with the numbers I found a cart imbalance of:
12dB test tone --- L/R cart imbalance of 0.4dB
14dB test tone --- L/R cart imbalance of 0.5dB
16dB test tone --- L/R cart imbalance of 0.7dB
Subtracting these values from Andrew´s calc, I get an average 1.3dB difference due to the riaa gain only.
It is not very much and nobody would tell just by listening but I will do the trimmer mod on second stage and verify if the differences of 1.3dB in gain are responsible for the differences in hiss.
That's an old post, I changed my mind when I factored in noise contribution if upping first stage Rs or to choke the cascode transistor for voltage if needing go down. I match channels by varying the second stage using 1:200 attenuator, 1kHz & FFT. The channels don't start by more than 0.5dB difference in my experience when the JFETs are just IDSS matched. It still holds that varying the second stage enough, changes the THD also. So if a JFET is too strong or too weak better think of spotting and changing that to a better matching one and then trim accurately. Its not so easy with just a cart and a test disc, but in your case its good enough since the differences are substantial. 1.3dB is not good. Get it down to 0.5dB if you can. Its broad range and the stereo will snap in better. Hiss can be coming from low level circa 5MHz oscillation if something is not perfect with the regulator if it is 1.2R and double mono so to chance on the bad channel, or the wild JFET can be noisier. You need an Oscope to check the rails are tame. K170s range from 0.9nVrtHz to 1.2-1.3nVrtHz and can do that slight hiss difference if in the head-amp (1st stage) along with high gain. Try substituting a proper IDSS one in the hissy channel first.
Ricardo
how do you measure? With inverse RIAA at the input? Output impedance of your signal generator and which signal voltage in relation do 0dbV?
This is, in my opinion as an engineer, important to know that you can compare other measurements. In the audio industry you are dealing with relations and given test scenarios.
Musical regards
Joao
he is comparing the 0.61V to the 0.68V and etc.
In post6636, 0.58V to 0.71V and etc.
Can Q3 (BC550C) contibute in any way to the differences in gain and hiss ?
I had no problems before I did my last mod that consisted in changing the 15n cap by a FT-1 and also replacing R3 resistor bypass, so I may have overheated Q3.
Hi João
I am using a test record that produces 300Hz at +12dB +14dB and +16dB.
That is why I managed to determine cart imbalance and subtract the value from overall output differences. I am using a AC voltmeter.
I am now prepared to make some measurements in order to understand if the circuit is acording to your plan.
Would you point me the most important values I should be looking for ?
First measurement:
Q2 Drain to GND
R = 7.3v
L = 6.9V
These should be 10~11v... what must I change ? Rs on se3cond stage ?
PSU = 28V
R6 (Rs) = 33r in both channels.
Would you point me the most important values I should be looking for ?
First measurement:
Q2 Drain to GND
R = 7.3v
L = 6.9V
These should be 10~11v... what must I change ? Rs on se3cond stage ?
PSU = 28V
R6 (Rs) = 33r in both channels.
Indeed.... I overlooked it. I am working on an old build.
I also found another issue.... I am using 820r for R7 (buffer Rs)... As R7 Vdrop is 7.9V, current on Q5 is 9.6mA (Over Q5 8.5mA Idss)... I wonder why it works ?
It looks like you are having high enough pinch off to need 43R. Its not a problem. Having both at 43R it will keep them in same balance. The idea was to have control to differentiate them for gain. Try replace the Jfet in the hissier channel's' head-amp first its still a better tactic if the difference starts there..
I will do that. This is an old build that was already subjected to severall mods and I am not sure the head amp jfets are matched.
Looking carefully, I found the BC550C to be different also (different makes).
The hissier channel plays louder.... Can this be due to using a higher Idss jfet there ?
Looking carefully, I found the BC550C to be different also (different makes).
The hissier channel plays louder.... Can this be due to using a higher Idss jfet there ?