Just finished a build of this circuit/pcb:
6SN7 SRPP Preamplifier PCB (Stereo) - Analog Metric - DIY Audio Kit
at the output there is between, say, -50mV and +50mV of dc present at any given time. what could be causing this?
6SN7 SRPP Preamplifier PCB (Stereo) - Analog Metric - DIY Audio Kit
at the output there is between, say, -50mV and +50mV of dc present at any given time. what could be causing this?
You may be seeing supply rail variations due to your mains voltage varying. Your PSU is a low pass filter. The output cap and ground resistor is a high pass filter. In the subsonic overlap region where neither is attenuating too much you can get mains variations coming through.
This assumes that what you are seeing on your DMM is a varying 'DC' voltage, not a constant DC which happens to be different from time to time.
This assumes that what you are seeing on your DMM is a varying 'DC' voltage, not a constant DC which happens to be different from time to time.
OK excellent, thanks for the info
This may, or may not be, related, but there is a background humming sound which is present when the pot is at minimum, then disappears as the pot gets to 9 o'clock, then comes back and gets louder as the pot is turned up. The humming also disappears when I have my fingers on the pot itself - is this some kind of field interference or grounding issue?
This may, or may not be, related, but there is a background humming sound which is present when the pot is at minimum, then disappears as the pot gets to 9 o'clock, then comes back and gets louder as the pot is turned up. The humming also disappears when I have my fingers on the pot itself - is this some kind of field interference or grounding issue?
Get yourself an oscilloscope. Rigol 1052E DSO is below $400, old analog Tektronix'es are in that range too. You can't effectively diagnose circuit problems without it.
DF96 is spot on about the variations in mains voltage causing this apparent wondering DC bipolar offset, you can also look at it as ridiculously low frequency AC.
Should be only one connection point between the electronics and the chassis. RCA jacks should be isolated and run back to this point as pointed out in another post.
A 1K grid stopper right at the input grid of the 6SN7 should help with RF susceptibility and a tendency to oscillate.
A scope is highly recommended - it will make diagnosing these sorts of issues much easier.
Should be only one connection point between the electronics and the chassis. RCA jacks should be isolated and run back to this point as pointed out in another post.
A 1K grid stopper right at the input grid of the 6SN7 should help with RF susceptibility and a tendency to oscillate.
A scope is highly recommended - it will make diagnosing these sorts of issues much easier.
Update - (1) increased the grid stopper to 1.2K, (2) connected the signal ground to chassis ground.
She is clean as a whistle, no hum or buzz! You people are fantastic, thanks for the help.
The only behaviour which is still unaccounted for is a buzzing noise that happens when the volume is, say, at about 3 o'clock or higher and is wave my hand between the tubes and the volume pot. I tried changing the tubes, but it still happens - is this normal?
She is clean as a whistle, no hum or buzz! You people are fantastic, thanks for the help.
The only behaviour which is still unaccounted for is a buzzing noise that happens when the volume is, say, at about 3 o'clock or higher and is wave my hand between the tubes and the volume pot. I tried changing the tubes, but it still happens - is this normal?
They are not coupling caps, but bypass caps. They are at a low impedance point so won't pick up hum, except possibly through poor grounding if the PCB design is wrong.
By 'large' I meant physically large. Many audiophiles now use coupling caps which are huge in physical terms because they believe they sound better. The downside of this is that their external capacitance is large so they easily pick up electrostatic hum fields.
By 'large' I meant physically large. Many audiophiles now use coupling caps which are huge in physical terms because they believe they sound better. The downside of this is that their external capacitance is large so they easily pick up electrostatic hum fields.
3cm is a large cap by normal electronics standards, although perhaps small for audiophiles. 1uF seems rather large for feeding a 470K load - the input LF rolloff will be at 0.34Hz!! As a result, the LF rolloff (actually a shelf) will be set by the 220uF cathode bypass caps at around 3Hz. This breaks two of the rules of good design: set the LF rolloff early on (before subsonics get a chance to cause IM), and never set it with an electrolytic or other nonlinear cap. I would reduce the 1uF to 0.1uF, and consider doubling the 220uF to 470uF. You must remember that most Chinese seem to design to impress, not to get good sound.
MelB - confirm R13 and R14 are exactly matched.
DF96 - good idea, I'll drop the coupling caps to 0.1uF and change 220uF bypass caps to 470uF. My 220uF are 100V rated, but the DC measurement across the caps is about 3v, AC is about 6v, so some 470uF 35V rated electro caps I have on hand should be fine.
Will report back
DF96 - good idea, I'll drop the coupling caps to 0.1uF and change 220uF bypass caps to 470uF. My 220uF are 100V rated, but the DC measurement across the caps is about 3v, AC is about 6v, so some 470uF 35V rated electro caps I have on hand should be fine.
Will report back
Gain of the SRPP at the moment is about 22dB - which is not bad considering I am using an AD1865 dac with a passive i/v stage (220R) - so the output of the DAC is only a few mV.
If I dropped R6 and R15 to, say, 180K - it still might serve its purpose as a stage with a lot of gain (servicing a push pull power amp with 100k input impedance and 3v sensitivity) - but what changes would this entail for CX1, CX2 and the coupling caps?
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