I know there are various arguments on whether an amp should have a characteristic sound or not, but putting those aside for a moment and considering a simple fixed topology (say 2 stage driver-power single ended amp in grounded cathode configuration), ...
how does changing various operating points and / or component values affect the "sound", assuming an ideal power supply?
i know my question is a bit naive but i'm looking for general guidelines, rather than use brand x tube or brand y capacitor.
thanks,
🙂
how does changing various operating points and / or component values affect the "sound", assuming an ideal power supply?
i know my question is a bit naive but i'm looking for general guidelines, rather than use brand x tube or brand y capacitor.
thanks,
🙂
This is something not often done. You select resistors and capacitors based on engineering principles, to make the circuit electrically happy, then tweak the easy but useless things, like $20 coupling capacitors. (Excuse me for a moment while I go laugh...)
You instead propose to adjust each resistor and capacitor's value to find the best sounding arrangement. Well, good luck...
Remember that lower plate resistor values will reduce gain and headroom, while increasing distortion. This requires more signal drive, which produces more distortion. For a significant change, the stage will go into asymmetrical clipping beacause headroom runs out (that is to say, distortion increases dramatically if you go too far). Cathode bias resistors will have similar effects. Grid leaks will have little effect until below twice the output impedance of the preceeding stage, where distortion will increase and gain and headroom will decrease (harder load). Too large will work until the tube's small grid current alters bias and the plate melts. Etc.
Tim
You instead propose to adjust each resistor and capacitor's value to find the best sounding arrangement. Well, good luck...
Remember that lower plate resistor values will reduce gain and headroom, while increasing distortion. This requires more signal drive, which produces more distortion. For a significant change, the stage will go into asymmetrical clipping beacause headroom runs out (that is to say, distortion increases dramatically if you go too far). Cathode bias resistors will have similar effects. Grid leaks will have little effect until below twice the output impedance of the preceeding stage, where distortion will increase and gain and headroom will decrease (harder load). Too large will work until the tube's small grid current alters bias and the plate melts. Etc.
Tim
'Voicing' an amp, like adding some color to the music...WOW! That's like me mixing coke with Lagavulin single malt, what a sin! But I like it...hehe.
Basically the easiest way is to add tone controls, but now it's the filters doing the voicing, which is fine, but I think you meant voicing a given circuit by changing working conditions and loadings.
One way to get more second order harmonics is to lower the Ia of the triode. Get it to work where the lines on the datasheet are curved.
One way to get it extremely linear all the way from almost zero Ia to a lot of Ia, is to run it with a little grid current. Like almost zero grid volts. But now it will not handle large input swings without drawing current and thus loading the previous stage. But this can be done in a subtle manner in which the grid current is about like in a BJT and thus not such a big load. Many sources can handle input impedances below 10kohms and some even recommend less than 1kohm for their source. If the signal is small like turntables it usually has no bad effects. So it can be pretty good i many cases.
When you change a tube's Ia you also change it's gain, both µ, rp, and Gm. This is not always a problem since the total gain is not always changed considerably. But in some cases where you want the same gain you must compensate with different cathode and plate resistor values.
You can also 'voice' a given circuit somewhat with different components. Metalfilm and carboncomposition resistors for instance, but in many cases this will give a very benign change in the original amp's 'voice'.
The are so many ways, and that's why I for one keep at it!
Basically the easiest way is to add tone controls, but now it's the filters doing the voicing, which is fine, but I think you meant voicing a given circuit by changing working conditions and loadings.
One way to get more second order harmonics is to lower the Ia of the triode. Get it to work where the lines on the datasheet are curved.
One way to get it extremely linear all the way from almost zero Ia to a lot of Ia, is to run it with a little grid current. Like almost zero grid volts. But now it will not handle large input swings without drawing current and thus loading the previous stage. But this can be done in a subtle manner in which the grid current is about like in a BJT and thus not such a big load. Many sources can handle input impedances below 10kohms and some even recommend less than 1kohm for their source. If the signal is small like turntables it usually has no bad effects. So it can be pretty good i many cases.
When you change a tube's Ia you also change it's gain, both µ, rp, and Gm. This is not always a problem since the total gain is not always changed considerably. But in some cases where you want the same gain you must compensate with different cathode and plate resistor values.
You can also 'voice' a given circuit somewhat with different components. Metalfilm and carboncomposition resistors for instance, but in many cases this will give a very benign change in the original amp's 'voice'.
The are so many ways, and that's why I for one keep at it!
Assuming 'ideal' PS is too limiting in real life - the PS will allow for more voicing than fine tuning the op points. SS or tube; hexfred or mercury; chokes or resistors. All these will have major effect upon the sound.
As the effects of passive components are so personal and difficult to describe accurately (unless like Tim you suffer bouts of uncontrolled laughter at the sound of good caps) you should bite the bullet and see for yourself what you like. Before establishing what your preference is it's dangerous and counterproductive to collect random opinions.
Konnichiwa,
Right. General guidelines.
!) Changing a valves overtone spectrum changes the sound. Certain overtone spectrums sound "better" than others. Lower distortion does not allways sound better, but lower high order distortion does sound pretty reliably better.
2) How changing the valve operating point effects the overtone spectrum tends to depend a lot on the valve and no general "go this way - good - go this way bad" gudance can ever work in reality.
I find that a somewhat tentative link exists with transconductance, actual stage employed (meani9ng signal level - eg MC Phono Input vs. Power Output) and "direction. There are four general directions for Valves,
"Low-Hot" - meaning low anode voltage, loads of current, device near maximum Pa
"High-Hot" - meaning high anode voltage, loads of current, device near maximum Pa
"Low-Cold" - meaning low anode voltages and low current
"High-Cold" - maning high anode voltages and low current
The next item in the mix is the load impedance. A "Low" operating point usually requires a lower load than a "High" one, but the increase in current may push the output transformer closer to limtes.
The upshot of all of this is to either rely on a lot of experience and a few basic "trusts" into opposing directions to see what may work better or to really "try it all". The numbers of possible combinations are endless, so have fun.
I used to have an Amp with widely variable output stage operating points and used that to get a good deal of experience how different output valves react to different "directions".
I know that is not what you wanted to hear, but I hope it helps a little.
Sayonara
zobsky said:
Right. General guidelines.
!) Changing a valves overtone spectrum changes the sound. Certain overtone spectrums sound "better" than others. Lower distortion does not allways sound better, but lower high order distortion does sound pretty reliably better.
2) How changing the valve operating point effects the overtone spectrum tends to depend a lot on the valve and no general "go this way - good - go this way bad" gudance can ever work in reality.
I find that a somewhat tentative link exists with transconductance, actual stage employed (meani9ng signal level - eg MC Phono Input vs. Power Output) and "direction. There are four general directions for Valves,
"Low-Hot" - meaning low anode voltage, loads of current, device near maximum Pa
"High-Hot" - meaning high anode voltage, loads of current, device near maximum Pa
"Low-Cold" - meaning low anode voltages and low current
"High-Cold" - maning high anode voltages and low current
The next item in the mix is the load impedance. A "Low" operating point usually requires a lower load than a "High" one, but the increase in current may push the output transformer closer to limtes.
The upshot of all of this is to either rely on a lot of experience and a few basic "trusts" into opposing directions to see what may work better or to really "try it all". The numbers of possible combinations are endless, so have fun.
I used to have an Amp with widely variable output stage operating points and used that to get a good deal of experience how different output valves react to different "directions".
I know that is not what you wanted to hear, but I hope it helps a little.
Sayonara
I don't believe anyone's mentioned feedback yet. Tube circuits are far, far more stable than solid state and you can vary feedback in real time. Yes, your gain will change, so readjust your volume. In fact, there were commercial units in the past that had a control on the front to do this, although I'm not aware of any now.
Tube circuits use lots less negative feedback than solid state. If you're accustomed to thinking in terms of 30, 40, 50dB (or more) of feedback, set your sights lower. Think in terms of 10-15dB. For that matter, you can run the thing open loop if you want. For a power amp, I'd think in terms of 6dB or more, but give it a shot and see what you think.
There's no doubt that caps make a difference. People who claim otherwise...well, never mind. It's all quite easily demonstrated with numbers and distortion and such. Even more easily demonstrated with ears, but heaven forbid that one should actually...say it quietly...listen. Egad!
That said, the pricing is sometimes absurd. It's not whether they make a difference, it's whether you can justify the price. The price/benefit ratio can get extreme. The one consolation is that they don't wear out the way that tubes do. Buy 'em once and forget about it.
Both high voltage and high current tend to lower distortion--assuming that's your goal--but keep an eye on plate dissipation. It's not that difficult to rig things so that you can change bias in real time. As long as the stage is cap-coupled, you can get away with it; don't try it if your stages are direct coupled.
Which brings me to another point: Don't forget that you can direct couple stages, thus eliminating a cap along with the attendant cost and sonic penalty.
Grey
Tube circuits use lots less negative feedback than solid state. If you're accustomed to thinking in terms of 30, 40, 50dB (or more) of feedback, set your sights lower. Think in terms of 10-15dB. For that matter, you can run the thing open loop if you want. For a power amp, I'd think in terms of 6dB or more, but give it a shot and see what you think.
There's no doubt that caps make a difference. People who claim otherwise...well, never mind. It's all quite easily demonstrated with numbers and distortion and such. Even more easily demonstrated with ears, but heaven forbid that one should actually...say it quietly...listen. Egad!
That said, the pricing is sometimes absurd. It's not whether they make a difference, it's whether you can justify the price. The price/benefit ratio can get extreme. The one consolation is that they don't wear out the way that tubes do. Buy 'em once and forget about it.
Both high voltage and high current tend to lower distortion--assuming that's your goal--but keep an eye on plate dissipation. It's not that difficult to rig things so that you can change bias in real time. As long as the stage is cap-coupled, you can get away with it; don't try it if your stages are direct coupled.
Which brings me to another point: Don't forget that you can direct couple stages, thus eliminating a cap along with the attendant cost and sonic penalty.
Grey
Hi,
No luck involved, just a lot of time and a pair of good ears...
Calculated values are just a starting point, things in the real world are bound to deviate from the ideal model so should require some modicus of optimisation.
I suppose it's the asking price that causes the laughter??? 🙄
Cheers, 😉
No luck involved, just a lot of time and a pair of good ears...
Calculated values are just a starting point, things in the real world are bound to deviate from the ideal model so should require some modicus of optimisation.
I suppose it's the asking price that causes the laughter??? 🙄
Cheers, 😉
Noone to my knowledge. I only said it's unlikely in the context of tube amps. No extra gain to burn.
Sch3mat1c said:
I do this on my breadboard all the time 😉
But yeah, I do use a Spice model as a reference 😱
thank you all for the many ideas.
some of the advice presented is fairly abstract (as befitting the question) but it definately gives me an idea of what to consider tweaking 🙂
some of the advice presented is fairly abstract (as befitting the question) but it definately gives me an idea of what to consider tweaking 🙂
A few thoughts on 'voicing' :
Every amp I have built has sounded fairly disappointing until after a lengthy running in period of up to a month. I can't come up with any scientific explanation for this but the effect is not subtle. These days I try to avoid any modifications until the amp is well run in.
As mentioned previously in this thread, small power supply design changes tend to have a big effect on tonal balance and speed / timing / musicality. When it comes to smoothing capacitor values bigger isn't always better.
Problems with layout earthing and decoupling etc sometimes don't produce any obvious noise but can make your amp sound just wrong.
Sometimes things which don't really make any sense can work rather well. In one of my amps I tried putting a silicon rectifier in series with the ht supply to each channel. The improvement in the bass was immediately noticeable but I have no idea why.
If you have a tried and tested design I would leave component values of the basic circuit well alone. Chances are someone has spent a lot of time finding the best ones.
Good luck!
Every amp I have built has sounded fairly disappointing until after a lengthy running in period of up to a month. I can't come up with any scientific explanation for this but the effect is not subtle. These days I try to avoid any modifications until the amp is well run in.
As mentioned previously in this thread, small power supply design changes tend to have a big effect on tonal balance and speed / timing / musicality. When it comes to smoothing capacitor values bigger isn't always better.
Problems with layout earthing and decoupling etc sometimes don't produce any obvious noise but can make your amp sound just wrong.
Sometimes things which don't really make any sense can work rather well. In one of my amps I tried putting a silicon rectifier in series with the ht supply to each channel. The improvement in the bass was immediately noticeable but I have no idea why.
If you have a tried and tested design I would leave component values of the basic circuit well alone. Chances are someone has spent a lot of time finding the best ones.
Good luck!
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