Looking at this in real world it isn’t half the problem you might imagine it is.
1. Modern audio op amps have a low enough offset voltage not to be intrusive to the headroom of the signal path. Fet and Jfet opamps have virtually no offset for potentiometers.
An op amp is used in the Pearl 3 phono preamp. Bias it into class A and buffer it if it will help your sleep better.
2. Dc blocking of inputs can be achieved with PP capacitors by scaling up the input impedance.
Output coupling capacitors can be put inside the feedback loop to render the output impedance zero. ( Douglas Self)
A properly designed dc servo can be used at the output as a dc strategy.
This is a well documented approach for the input in small signal amplification (mc phono stages) to protect the mc cartridge.
In the diy situation to keep the design as simple as possible non polarised capacitors are okay to avoid complexity and cost. Simple designs are much easier for diy folks to implement.
Bypassing a NP coupling capacitor with a low value PP capacitor as Pass Labs has done for decades is acceptable. PCB space and cost are real considerations.
Tip. Oversize the NP capacitor value so that the -3 db point is 0.05 hertz or lower to avoid any THD caused by a voltage drop across the NP coupling capacitor (Douglas Self). Then bypass with a small value PP ie. Wima. The NP capacitor is a dead short in the audible audio band.
Try measuring the THD. There are numerous reasons why sight bias and other biases will colour our thoughts to the point we will believe anything our minds tell us. Fancy parts vendors have made a fortune out of this.
As Np told me once stop overthinking it!
Fancy capacitors can be a huge cost blow out for your diy project.
It’s in the output stage of power amplifiers that real problems exist. The diyer can avoid crossover distortion by build them hotter and bigger.
Remember that many of the most important recordings have been done at mixing consoles where teams of technicians are employed to change out electro coupling capacitors ( re capping) Abby Road studios is a case in point.
These analogue consoles are regarded as the holy grail by recording engineers and producers. Some employ audio transformers. Sometimes to create a particular artistic effect that would otherwise render a recording flat and un interesting.
1. Modern audio op amps have a low enough offset voltage not to be intrusive to the headroom of the signal path. Fet and Jfet opamps have virtually no offset for potentiometers.
An op amp is used in the Pearl 3 phono preamp. Bias it into class A and buffer it if it will help your sleep better.
2. Dc blocking of inputs can be achieved with PP capacitors by scaling up the input impedance.
Output coupling capacitors can be put inside the feedback loop to render the output impedance zero. ( Douglas Self)
A properly designed dc servo can be used at the output as a dc strategy.
This is a well documented approach for the input in small signal amplification (mc phono stages) to protect the mc cartridge.
In the diy situation to keep the design as simple as possible non polarised capacitors are okay to avoid complexity and cost. Simple designs are much easier for diy folks to implement.
Bypassing a NP coupling capacitor with a low value PP capacitor as Pass Labs has done for decades is acceptable. PCB space and cost are real considerations.
Tip. Oversize the NP capacitor value so that the -3 db point is 0.05 hertz or lower to avoid any THD caused by a voltage drop across the NP coupling capacitor (Douglas Self). Then bypass with a small value PP ie. Wima. The NP capacitor is a dead short in the audible audio band.
Try measuring the THD. There are numerous reasons why sight bias and other biases will colour our thoughts to the point we will believe anything our minds tell us. Fancy parts vendors have made a fortune out of this.
As Np told me once stop overthinking it!
Fancy capacitors can be a huge cost blow out for your diy project.
It’s in the output stage of power amplifiers that real problems exist. The diyer can avoid crossover distortion by build them hotter and bigger.
Remember that many of the most important recordings have been done at mixing consoles where teams of technicians are employed to change out electro coupling capacitors ( re capping) Abby Road studios is a case in point.
These analogue consoles are regarded as the holy grail by recording engineers and producers. Some employ audio transformers. Sometimes to create a particular artistic effect that would otherwise render a recording flat and un interesting.
I made the question above about what electrolytics can be bought from Aliexpress implying the following question.
China is fighting a semiconductors war with the USA, so what new passive and active semiconductors are being made there that replace or clone mad in the US components, considering that many (if not all of them) were being made in Asia, including China.
So why there are no high quality Chinese manufacturers and brands that are similar, or best, or totally new to Western semiconductors.
Isn't it time for such manufacturers to show themselves and offer their products, which will quite possibly be cheaper and perhaps better than the ones we know?
China is fighting a semiconductors war with the USA, so what new passive and active semiconductors are being made there that replace or clone mad in the US components, considering that many (if not all of them) were being made in Asia, including China.
So why there are no high quality Chinese manufacturers and brands that are similar, or best, or totally new to Western semiconductors.
Isn't it time for such manufacturers to show themselves and offer their products, which will quite possibly be cheaper and perhaps better than the ones we know?
Hi macka,
Dealing with Neve and SSL. Opposite ends of the spectrum. The best console has to do with the "sound" they want. SSL is cleaner by far and doesn't require a "recap". A Neve also benefits from new transistors with better characteristics just due to manufacturing improvements over the years. No, that does not affect the sound unless the old part was leaky or noisy.
Running stuff hot beyond a certain point has no benefit at all. None. If your output stage is pretty linear and low distortion you can get by with lower bias. A poor design needs high bias to make that stage more linear. A good front end with high transconductance also helps. This is not the direction Nelson designs in, but that's okay.
Dealing with Neve and SSL. Opposite ends of the spectrum. The best console has to do with the "sound" they want. SSL is cleaner by far and doesn't require a "recap". A Neve also benefits from new transistors with better characteristics just due to manufacturing improvements over the years. No, that does not affect the sound unless the old part was leaky or noisy.
Running stuff hot beyond a certain point has no benefit at all. None. If your output stage is pretty linear and low distortion you can get by with lower bias. A poor design needs high bias to make that stage more linear. A good front end with high transconductance also helps. This is not the direction Nelson designs in, but that's okay.
On the subject of coupling capacitors, I have a question. I use a tube preamp, which has an output coupling capacitor. This goes to a Pass 6-24 crossover, which has an input coupling capacitor and an output coupling capacitor. Then both low and high outputs go to amplifiers, an ACA Mini and a tube amp, that have input coupling capacitors. Aren't the capacitors in series the equivalent of a single cap of lower value? Should I increase the size (in microfarads) of all these caps? I have thought about bypassing the input and output caps of the crossover, which would give me DC on the interconnects. (I don't have kids or cats) But I have never heard of anyone doing this, so I am thinking this is not a good idea. Am I overthinking this? What are your thoughts.
Hi Decker,
Yes, capacitors in series reduce the total value. So a pair of 1 uF in series would be 0.5 uF total. Keep in mind the circuit impedance which should be very high. You may not have any concerns. It also depends on the capacitor type as to what effect it may have. Electrolytic types will have much higher distortion than a Polypropylene type if signal voltage is developed across it.
I would never remove and short coupling capacitors for any reason. I have seen op amps fail with large DC offset - and that burned out woofers in very expensive speakers because the manufacturer didn't think they needed DC blocking capacitors anywhere. It was a well respected brand, the client now believes the manufacturer was careless and I agree completely. Irresponsible even. You never know what component will be paired with another, so don't count on anything.
Distortion in bass frequencies is overblown. Compare it to the distortion in the bass driver itself at those frequencies. That's even if the speaker responds that far down. Look at the entire picture before getting worried, and certainly before taking action.
Yes, capacitors in series reduce the total value. So a pair of 1 uF in series would be 0.5 uF total. Keep in mind the circuit impedance which should be very high. You may not have any concerns. It also depends on the capacitor type as to what effect it may have. Electrolytic types will have much higher distortion than a Polypropylene type if signal voltage is developed across it.
I would never remove and short coupling capacitors for any reason. I have seen op amps fail with large DC offset - and that burned out woofers in very expensive speakers because the manufacturer didn't think they needed DC blocking capacitors anywhere. It was a well respected brand, the client now believes the manufacturer was careless and I agree completely. Irresponsible even. You never know what component will be paired with another, so don't count on anything.
Distortion in bass frequencies is overblown. Compare it to the distortion in the bass driver itself at those frequencies. That's even if the speaker responds that far down. Look at the entire picture before getting worried, and certainly before taking action.
I think you’re missing the point. It was meant tongue in cheek to get away from the discussion being so analytical and serious over on something that’s isn’t that important.
Come on...there has to be one Class A amplifier out there that you think is amazing.
I guess not, but that's okay.
I have two class a amps I'd like to bring to your house someday. Love to see the results from the fancy analyzer you have set up.
Someday...
Hi STOXX,
They aren't bad just because they are class A. They just are not automatically better than A-B just because they are class A.
The public was sold a bill of goods with "class A" that has no bearing on sound quality. You're welcome by at some point.
They aren't bad just because they are class A. They just are not automatically better than A-B just because they are class A.
The public was sold a bill of goods with "class A" that has no bearing on sound quality. You're welcome by at some point.
Without de railing the thread many, many years ago l was invited around to Hugh Dean’s house to look at an idea he had for power amplifier kits.
He said his biggest challenge was getting his customers to buy the right parts (you know caps, resistors ect)
I said Hugh give them the parts numbers of the bits you need them to buy. If they by something else you can’t really help them. He said lan that’s a great idea……Lol. The rest is history.
He said his biggest challenge was getting his customers to buy the right parts (you know caps, resistors ect)
I said Hugh give them the parts numbers of the bits you need them to buy. If they by something else you can’t really help them. He said lan that’s a great idea……Lol. The rest is history.
On subject, I have never had problems with electrolytic capacitors. The quality/price in the past 20+ years has been great in North America.
I am always surprised to test caps from the late 1970s early 80s and so many times they are damn near spot on. They get replaced when they are off the board, when in Rome. 1990s audio gear full of bad caps and cold solder joints.
I am always surprised to test caps from the late 1970s early 80s and so many times they are damn near spot on. They get replaced when they are off the board, when in Rome. 1990s audio gear full of bad caps and cold solder joints.
That is soooo true macka! How many people are determined to use parts that don't fit, or have technical issues.
capacitors = religion
capacitors = religion
Electrolytic capacitors have experienced improved technology just like everything else. Most that are not in stressful applications remain in good condition, so I don't replace them "just 'cause". Of course some manufacturers used poor quality parts and those are often degraded (need replacement). You'll find the odd "good" ones have failed, not the majority.
Friend has a saying, if it doesn't have 2 watts, doesn't glow and can't kill you, whats the point 😅
Well, a Carver M1.5 will kill you just fine, without even trying. Mind you, it is over 2 watts and only glows if you connect it to the wrong mains voltage. Then you want to run.
2 watts? What good is that? Not unless it is into headphones, I could respect that! lol! 🙂
2 watts? What good is that? Not unless it is into headphones, I could respect that! lol! 🙂
Hi Anatech,
I like your logic.
Bringing these different trains of thought together in the link below Borbely clearly explains the design process. From simple circuits to more comprehensive involving DC coupling and other challenges including load linearity.
Therefore, coupling capacitors are best looked at in the context of how and why they are used. Then you have choices on how to proceed.
In the diy world you do not need to make commercial promises to customers so in some instances you can get away with DC coupling with manageable offsets. In other situations not.
One of the themes that emerged from
this era was that bi amp or use of two different power amplifier classes A-B on woofers and A on mid and tweeters can be used to an advantage.
I have done this myself and the results are very good. In fact Kent English at Pass Labs noted this trend when the X series was introduced back in the early 2000’s. People preferred then the Aleph on the mid range and top end. Tastes and designs have to an extent moved on with time along with circuit refinements.
https://audioxpress.com/article/JFETs-The-New-Frontier-Part-2
Quote “There are two issues to consider when talking about the SE and balanced amplifiers. First of all, the output does not sit at 0V DC, but at some 10–20V above ground. If you wish to connect it to, say, a DC-coupled power amplifier, you must block this DC voltage from reaching the power-amp input. This is easily done using a capacitor, and this problem is well known to all SE fans, whether of tube or semiconductor variety.
A much more important question is whether these circuits can drive the input impedance of a power amplifier.”
The discussion then moves on to the use of followers
Quote “The circuits in 15a and b have a DC offset voltage at the output—the gatesource voltage at the given drain current. For the JFETs I used in the test setup, I measured a 0.2V offset. If you need zero DC output, you can use the circuit in Fig. 15c. Here the constant-current source is made with the same type of JFET as the follower.
If the two JFETs are matched and the two source resistors are equal, then the DC offset will be very small. With two matched K170BLs, I measured less than 1mV offset.”
In figure 15C the offset is small but can be further trimmed if desired.
A more comprehensive preamp using the above techniques is described here.
https://audioxpress.com/article/EB-604-410-All-JFET-Line-Amp-Starter-Kit
I like your logic.
Bringing these different trains of thought together in the link below Borbely clearly explains the design process. From simple circuits to more comprehensive involving DC coupling and other challenges including load linearity.
Therefore, coupling capacitors are best looked at in the context of how and why they are used. Then you have choices on how to proceed.
In the diy world you do not need to make commercial promises to customers so in some instances you can get away with DC coupling with manageable offsets. In other situations not.
One of the themes that emerged from
this era was that bi amp or use of two different power amplifier classes A-B on woofers and A on mid and tweeters can be used to an advantage.
I have done this myself and the results are very good. In fact Kent English at Pass Labs noted this trend when the X series was introduced back in the early 2000’s. People preferred then the Aleph on the mid range and top end. Tastes and designs have to an extent moved on with time along with circuit refinements.
https://audioxpress.com/article/JFETs-The-New-Frontier-Part-2
Quote “There are two issues to consider when talking about the SE and balanced amplifiers. First of all, the output does not sit at 0V DC, but at some 10–20V above ground. If you wish to connect it to, say, a DC-coupled power amplifier, you must block this DC voltage from reaching the power-amp input. This is easily done using a capacitor, and this problem is well known to all SE fans, whether of tube or semiconductor variety.
A much more important question is whether these circuits can drive the input impedance of a power amplifier.”
The discussion then moves on to the use of followers
Quote “The circuits in 15a and b have a DC offset voltage at the output—the gatesource voltage at the given drain current. For the JFETs I used in the test setup, I measured a 0.2V offset. If you need zero DC output, you can use the circuit in Fig. 15c. Here the constant-current source is made with the same type of JFET as the follower.
If the two JFETs are matched and the two source resistors are equal, then the DC offset will be very small. With two matched K170BLs, I measured less than 1mV offset.”
In figure 15C the offset is small but can be further trimmed if desired.
A more comprehensive preamp using the above techniques is described here.
https://audioxpress.com/article/EB-604-410-All-JFET-Line-Amp-Starter-Kit
EB-604/410 All-JFET Line Amp Starter Kit
I've found over many decades that the lowest distortion wins every time. Not what a meter pointer says, but a spectrum display. We are assuming the same behavior over the output power range of course. For low frequencies the amplifier is expected to dump current when needed (low output impedance). I've seen every system type, apprenticed on tubes. Dealt with so much in the way of single-ended stuff both tube and solid state, wow. Good and bad push-pull and everything in between.
Mids and highs are normally an easier load at lower power levels. Of course you can have to deal with electrostats, ribbons and other evil things as far as a transistor is concerned. Because a woofer cone has typically high mass, it naturally attenuates higher frequencies (harmonic distortion products), so you can sometimes get away with less good amplifiers. With mids and tweeters, everything is out front and center. You want something good there.
If you get into musical taste, there can be no consensus. Maybe we should just make accurate amplifiers and such, then sell effects boxes to suit those who have particular tastes.
DC offsets are always bad. 20 mV or less on an amplifier output is fine, you shouldn't need servos to accomplish that. That does require more care in design and component matching. Nothing wrong with a trim pot for DC offset to nudge it to zero, but not to correct for 200 mV (pointing to problems). For preamp outputs and tone or buffers, you really want zero to avoid switch clicks and pot scratching sounds. There isn't anything wrong with coupling capacitors in the signal chain. I prefer not to have them between a speaker and amplifier.
Mids and highs are normally an easier load at lower power levels. Of course you can have to deal with electrostats, ribbons and other evil things as far as a transistor is concerned. Because a woofer cone has typically high mass, it naturally attenuates higher frequencies (harmonic distortion products), so you can sometimes get away with less good amplifiers. With mids and tweeters, everything is out front and center. You want something good there.
If you get into musical taste, there can be no consensus. Maybe we should just make accurate amplifiers and such, then sell effects boxes to suit those who have particular tastes.
DC offsets are always bad. 20 mV or less on an amplifier output is fine, you shouldn't need servos to accomplish that. That does require more care in design and component matching. Nothing wrong with a trim pot for DC offset to nudge it to zero, but not to correct for 200 mV (pointing to problems). For preamp outputs and tone or buffers, you really want zero to avoid switch clicks and pot scratching sounds. There isn't anything wrong with coupling capacitors in the signal chain. I prefer not to have them between a speaker and amplifier.