Hi,
I just wonder is there a way to lower even only harmonics, keep odd (especially 2nd harmonic), like in SE.
I like SE sound, but the wattage is small vs Push-Pull. But in PP, odd there is much even harmonic than odd. How to reduce this even only, either local or global feedback?
Thanks,
Ervin L
I just wonder is there a way to lower even only harmonics, keep odd (especially 2nd harmonic), like in SE.
I like SE sound, but the wattage is small vs Push-Pull. But in PP, odd there is much even harmonic than odd. How to reduce this even only, either local or global feedback?
Thanks,
Ervin L
It is not clear from your message (English not primary language is not an insult, just difficult too understand). I think you have even and odd harmonics switched.
I think you are asking how to enhance 2nd, 4th, etc harmonics while suppressing 3rd, 5th, etc.
If you are running class AB1, you can increase the bias of one output tube slightly to increase even harmonic generation.
Likewise, you can either increase or decrease the bias on an input buffer to push it into a non-linear region where more even harmonic distortion will be prevalent.
I think you are asking how to enhance 2nd, 4th, etc harmonics while suppressing 3rd, 5th, etc.
If you are running class AB1, you can increase the bias of one output tube slightly to increase even harmonic generation.
Likewise, you can either increase or decrease the bias on an input buffer to push it into a non-linear region where more even harmonic distortion will be prevalent.
Yes, how to enhance 2nd, 4th, etc harmonics while suppressing 3rd, 5th, etc. Like what SE amplifier has. But I would like more power like in PP (anyway, I think this PP is already in Class A, depends on Rk and current calculated from the load line).
I heard that PP will loose all odd harmonic (due to cancelling effect in PP) but leave even harmonics, which is un-expected. Then how to make even harmonics lower but enhance odd one, I don't think a feedback (local/global) can.
Do you mean making "unmatched" bias (or using unmatched tube) will do this job?
Thx,
Ervin L
First part is correct, you got mixed up again in the end. Even harmonics are cancelled out because of the PP arrangement. You could get some more even harmonics by biasing differently or using unmatchd tubes 😉
Try these pages.
Noble Amplifier Company - Custom Vacuum Tube Amplification (Push Pull amp)
Stereophile: Boulder 500AE power amplifier
The best solution is to get speakers with higher efficiency (>95db) and use SET amp. If not, you may want to tweak SET amp to get higher power (online search) instead of trying to increase even harmonics from P-P amp.
Thanks for the info. Sorry that I rather mixed-up (swapped) between odd and even harmonics of PP vs SET 🙂
Ervin L
It is not the problem to add distortions;
the problem is how to lower them.
It is easy to compensate even order harmonics making a curve symmetrically bent. But to minimize odd order harmonics the only way is to make the curve straighter. Either by extra gain compensated by negative feedback, or by extra power consumption compensated by room cooler and money spent on electricity. The first method is preferred for Hi-fi, the second one is used for High-End.
However, what I personally call High-End, is a combination of both methods, but it is a different story... 😉
the problem is how to lower them.
It is easy to compensate even order harmonics making a curve symmetrically bent. But to minimize odd order harmonics the only way is to make the curve straighter. Either by extra gain compensated by negative feedback, or by extra power consumption compensated by room cooler and money spent on electricity. The first method is preferred for Hi-fi, the second one is used for High-End.
However, what I personally call High-End, is a combination of both methods, but it is a different story... 😉
Note that a P-P output stage only cancels even harmonics generated within itself. It will faithfully pass on any even harmonics in the input signal. There are two ways to deliberately add evens without adding too much odds as well:
1. Include an amplifying stage which has a square-law response - ECC82/12AU7 can do this if biassed correctly (for this application). Or use a variable-mu valve such as EF92 or 6BA6, as these will generate lots of evens. Then don't have any overall feedback, as this will mix the even with the original signal to create more odds.
2. Design a good linear amp, with negative feedback, but include in the negative feedback path a square-law stage. Achieving loop stability might be interesting, though!
Alternatively, just buy a badly-designed preamplifier or buffer. There are plenty of these around on the internet. Some are discussed on this forum, after the hapless new owner wonders why he has spent good money to degrade his sound.
I won't ask why you want to do this.
1. Include an amplifying stage which has a square-law response - ECC82/12AU7 can do this if biassed correctly (for this application). Or use a variable-mu valve such as EF92 or 6BA6, as these will generate lots of evens. Then don't have any overall feedback, as this will mix the even with the original signal to create more odds.
2. Design a good linear amp, with negative feedback, but include in the negative feedback path a square-law stage. Achieving loop stability might be interesting, though!
Alternatively, just buy a badly-designed preamplifier or buffer. There are plenty of these around on the internet. Some are discussed on this forum, after the hapless new owner wonders why he has spent good money to degrade his sound.
I won't ask why you want to do this.
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Let me just throw out something for the experts to bat around. How about using an output tube designed for lower 3rd harmonic like the 6L6 and then bias carefully and instead of using global feedback use ultra linear or triode mode or maybe a touch of plate to plate feedback.
The track I am shooting for is to get a favorable spectrum not so much by adding 2nd harmonic but my minimizing higher order distortion in the first place. Then if some masking is needed a little shift in the bias could be used I suppose.
The track I am shooting for is to get a favorable spectrum not so much by adding 2nd harmonic but my minimizing higher order distortion in the first place. Then if some masking is needed a little shift in the bias could be used I suppose.
By adding "a touch of plate to plate feedback" you actually can't minimize high order distortions. You will minimize a little bit the 2'nd order distortion, but introduce higher order distortions, even when they did not exist.
Feedback is like a rifle: if you grabbed it use it, otherwise it will be even worse. It is what I called Hi-Fi approach: take an extra gain, and trade it off using feedback.
For high - end you need to take an appropriate tube and use it only on the most linear part of it's curve, wasting the rest.
Feedback is like a rifle: if you grabbed it use it, otherwise it will be even worse. It is what I called Hi-Fi approach: take an extra gain, and trade it off using feedback.
For high - end you need to take an appropriate tube and use it only on the most linear part of it's curve, wasting the rest.
That's not necessarily the way to go. The 6L6 might produce less h3, but it also produces a lot of higher order harmonics. Running open loop, these can sound quite nasty as a result. I did a project with the RF version: the 807, and found this to be the case. These definitely need help from local NFB to sound nice.
Other types such as the 6BQ6 produce more h3, but it's almost all h3, and open loop these don't have that pentode nastiness, and need just enough gNFB to tighten up the bass and take the edge off.
The way to do that is by selecting types like the 6BQ6, and not over doing the gNFB (otherwise these begin to sound solid statey at around 12db(v) of gNFB).
How many DB depends on many factors, like currents VS capacitances, local feedbacks, headrooms in each stage, etc...
Why not design your PP amp, get the distortion down to a reasonable level, then add an additional input stage outside the gnfb loop?
Bias this stage to generate the second harmonic distortion you seek. Since it is outside the gnfb loop it won't be effected by feedback.
Bias this stage to generate the second harmonic distortion you seek. Since it is outside the gnfb loop it won't be effected by feedback.
This is what actually I ask, how to bias the stage to generate the second (even) harmonic distortion, to compete/compensate with odd one.
I have 5687s tube, not sure if it can be tweaked to do so as driver stage (e.g. for EL34, EL84 or 6AS7/6080 PP).
Thx,
Ervin L
I'll take a stab at it.
Take a look at this data sheet and look at the plate curves on sheet 3.
http://tubedata.milbert.com/sheets/127/5/5687.pdf
If you drop the current down low and use a low value of plate load resistor your load line will be in a very non-linear portion of the curve. This is where I'd look for 2nd harmonic distortion.
I'd start out with a supply of 300V, 200-250V at the plate, cathode bias of -12 to -14V and use a plate resistor around 15K ohms. This should put you with a plate current around 5mA.
To get there you need a 2600Ohm cathode resistor. As a first gain stage you can swing +/- 10V and not clip but get close.
After looking close at the plot, I'd try to drop the anode resistor down to 1K although I don't know if this is really practical. I'd want to build a test amp to be sure.
I'd bypass the cathode resistor to prevent it from providing negative feedback and reducing distortion.
Hopefully someone will check my numbers as I did this in my head.
You may need to add a gain control to reduce the drive to the next stage if you end up with too much gain.
Take a look at this data sheet and look at the plate curves on sheet 3.
http://tubedata.milbert.com/sheets/127/5/5687.pdf
If you drop the current down low and use a low value of plate load resistor your load line will be in a very non-linear portion of the curve. This is where I'd look for 2nd harmonic distortion.
I'd start out with a supply of 300V, 200-250V at the plate, cathode bias of -12 to -14V and use a plate resistor around 15K ohms. This should put you with a plate current around 5mA.
To get there you need a 2600Ohm cathode resistor. As a first gain stage you can swing +/- 10V and not clip but get close.
After looking close at the plot, I'd try to drop the anode resistor down to 1K although I don't know if this is really practical. I'd want to build a test amp to be sure.
I'd bypass the cathode resistor to prevent it from providing negative feedback and reducing distortion.
Hopefully someone will check my numbers as I did this in my head.
You may need to add a gain control to reduce the drive to the next stage if you end up with too much gain.
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Try a 12AU7 or 6C4 with a bypassed 3-5k cathode resistor and a 1k anode resistor. The idea is to bias into the square-law region, then avoid anode feedback (which normally linearises a triode).
Hello
I think that TheGimp nailed it.
If you want triode distortion effects and high power output build an “effects” line stage amplifier and use it to drive a Solid State low distortion amplifier. Not Too many will admit to this goal.
For the effects amplifier use a variable mu tube like a PCC189 and make the bias adjustable to dial the desired level of second order distortion.
DT
Just for fun!
I think that TheGimp nailed it.
If you want triode distortion effects and high power output build an “effects” line stage amplifier and use it to drive a Solid State low distortion amplifier. Not Too many will admit to this goal.
For the effects amplifier use a variable mu tube like a PCC189 and make the bias adjustable to dial the desired level of second order distortion.
DT
Just for fun!
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