Make a choice!Peter Daniel said:
either fit RF attenuation at the front end of every amplifier or suffer the consequences. Which sounds better?
For me there is no choice, I always fit RF attenuation to the front end of all power amps and all pre-amps.
AndrewT said:
Hi,
Sould I also put a cap in the from of my passive preamp?
Also how come I get radio? Some people try really hard to design radio, and I get one for free... where I don't want it.
Thanks
spacemen12 said:
This reminds me of living 1 block from "The 100,000 Watt Voice of the Four State Area!" as was frequently announced by my toaster, followed by agriculture news, whatever hats, boots and pickup trucks were on sale, and country-western music as well. The thing would NOT shut up!
Out of desperation (and a month of audio torture), I tried knots in the cord. That silenced it. 😉
Hi Andrew!
What would you suggest as values for rf blocking at:
1). Capacitor at Inverting to Non-inverting input?
2). RC at parallel with feedback resistor?
3). RC(s) at the secondary windings?
4). RC at the speaker output?
(when these are all used simultaneously and only for the purpose of blocking RF)
Caveat:
(Peter's response, seen as I would interpret it)
We, want to avoid even the slightest decrease of amplification at 8k (perceived upper treble) to 32k (octave above 16k contains harmonics for both 8k and 16k). The tolerances are near zero! A 300pF cap across the inputs is over the mark (too much), and that's why it isn't standard equipment on this style amplifier. What can we use to block RF without blocking/altering any audio?
Thank you!!!
What would you suggest as values for rf blocking at:
1). Capacitor at Inverting to Non-inverting input?
2). RC at parallel with feedback resistor?
3). RC(s) at the secondary windings?
4). RC at the speaker output?
(when these are all used simultaneously and only for the purpose of blocking RF)
Caveat:
(Peter's response, seen as I would interpret it)
We, want to avoid even the slightest decrease of amplification at 8k (perceived upper treble) to 32k (octave above 16k contains harmonics for both 8k and 16k). The tolerances are near zero! A 300pF cap across the inputs is over the mark (too much), and that's why it isn't standard equipment on this style amplifier. What can we use to block RF without blocking/altering any audio?
Thank you!!!
Hi,danielwritesbac said:
only 1). is RF attenuation.
You can add a cap in parallel to Zin resistor, making option 2).
300pF, or 330pF, depends for it's filtering effect on the source impedance. Simply quoting 300pF tells us nothing.
I recommend 0.5uS to 1.5uS, try listening to hear what you like. I use 0.68uS to 0.75uS.
Some recommend 0.3uS to 0.7uS, it's down to taste and equipment.
I quite like polystyrene 100pF across the input RCA AND 0.68uS at the PCB input. The 100pF interacts with source impedance and with cable inductance.
You can expect the input impedance to be variable between 4k and 22k in most of these cases. And, we need -0db at 32khz (no effect at 32k) for the Gainclone. Does that help?
My normal approach to dealing with RFI is to fit a common mode choke to every pair of connections passing into the box and to decouple to chassis ground immediately after the choke, remember that speaker leads are also possible routes for RF to get in, as is the power lead. The leads on the decoupling caps **MUST** be as short as possible, this is not somewhere where inch long leads (or traces) are acceptable.
PI section filters that go over at a few Mhz should have minimal effect on the audio while attenuating 100Mhz FM by enough to remove the problem (At least for FM, AM can be more of a pain).
There is something to be said for also having an active LPF at 70Khz or so downstream of the initial RFI protection to avoid running the amp into slew rate limiting if someone hooks up something with excessive ultrasonic energy in its output (Like say an unfiltered DAC), multiple feedback filters are generally better here then sallen & key as they make less demands on GBP.
IMHO Good design should be conservative in what it generates and liberal in what it accepts, and while appropriate RFI and protection circuits can increase the parts count by at least 50% for something simple, they are not optional if you want to have a product that works reliably under field conditions.
Regards, Dan.
PI section filters that go over at a few Mhz should have minimal effect on the audio while attenuating 100Mhz FM by enough to remove the problem (At least for FM, AM can be more of a pain).
There is something to be said for also having an active LPF at 70Khz or so downstream of the initial RFI protection to avoid running the amp into slew rate limiting if someone hooks up something with excessive ultrasonic energy in its output (Like say an unfiltered DAC), multiple feedback filters are generally better here then sallen & key as they make less demands on GBP.
IMHO Good design should be conservative in what it generates and liberal in what it accepts, and while appropriate RFI and protection circuits can increase the parts count by at least 50% for something simple, they are not optional if you want to have a product that works reliably under field conditions.
Regards, Dan.
I've also had radio pickup problems in my 47 Labs Gaincard.
I moved to Seattle two years ago and tried to set up my system with my Gaincard as the amp. The setup was pretty much the same as the one I used in New York, except that I was using Goertz ribbon speaker cable instead of single strand cable and also a different speaker.
I got a lot of hum from the local NPR station. I think it had to do with the length of the speaker cable - it's long enough that I have to coil it two or three times on each side, and I can reduce the pickup a bit by moving the cable around. I don't want to cut this cable because I might need the length at some point. But this keeps me from using the Gaincard.
Interestingly, I never had a problem with hum when I lived in Williamsburg in Brooklyn, or Providence, RI, or at the audio store where I used to work at in the NOHO area of Manhattan. Even with the Geortz cable in those areas, no hum. You would think the radio waves would be more prevalent in NYC.
It was only after I moved to Seattle that I had the issue. I do live about a mile and a half from the transmission tower, so that must be a contributor as well.
Anyhow, it does happen even with the real Gaincard.
Best,
KT
I moved to Seattle two years ago and tried to set up my system with my Gaincard as the amp. The setup was pretty much the same as the one I used in New York, except that I was using Goertz ribbon speaker cable instead of single strand cable and also a different speaker.
I got a lot of hum from the local NPR station. I think it had to do with the length of the speaker cable - it's long enough that I have to coil it two or three times on each side, and I can reduce the pickup a bit by moving the cable around. I don't want to cut this cable because I might need the length at some point. But this keeps me from using the Gaincard.
Interestingly, I never had a problem with hum when I lived in Williamsburg in Brooklyn, or Providence, RI, or at the audio store where I used to work at in the NOHO area of Manhattan. Even with the Geortz cable in those areas, no hum. You would think the radio waves would be more prevalent in NYC.
It was only after I moved to Seattle that I had the issue. I do live about a mile and a half from the transmission tower, so that must be a contributor as well.
Anyhow, it does happen even with the real Gaincard.
Best,
KT
Well inverse square law applies WRT distance to the tower, and an unfortunately resonant cable is entirely possible. Is this by any chance an AM tower, as these tend to be more readily demodulated?
Does that amp have a Zobel network at its output (The inductance helps to keep the RF out of the feedback network)?
I would be looking to add some clamp on ferrite common mode chokes, and maybe a few hundred pf from the speaker lines to chassis ground right where the cables come into the enclosure.
Too long speaker leads is fixed with a pair of side cutters, too short is easily fixed with a trip to the local electrical wholesaler.
What the hell is Goertz ribbon cable?
Regards, Dan.
Does that amp have a Zobel network at its output (The inductance helps to keep the RF out of the feedback network)?
I would be looking to add some clamp on ferrite common mode chokes, and maybe a few hundred pf from the speaker lines to chassis ground right where the cables come into the enclosure.
Too long speaker leads is fixed with a pair of side cutters, too short is easily fixed with a trip to the local electrical wholesaler.
What the hell is Goertz ribbon cable?
Regards, Dan.
The Gaincard uses Zobel.
I also had problem with Goertz speaker cables in customer's setup and adding Zobel didn't help, changing for different cables worked out fine.
I also had problem with Goertz speaker cables in customer's setup and adding Zobel didn't help, changing for different cables worked out fine.
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