I got my SE 300B amp measured. This was the result. But I did not get a lot of explanation about it. (There were other in the queue behind me so I had to move on)
I understand the 50Hz and100Hz thing. But what do the other spikes mean? The guy who measured it said. It was mostly 2nd harmonic. Someone else posted at the audioasylum that he was suprised at the amount of 3rd harmonic distortion (he suspected the pentode driver)
So who can teach me how to look at this graph?
An externally hosted image should be here but it was not working when we last tested it.
I understand the 50Hz and100Hz thing. But what do the other spikes mean? The guy who measured it said. It was mostly 2nd harmonic. Someone else posted at the audioasylum that he was suprised at the amount of 3rd harmonic distortion (he suspected the pentode driver)
So who can teach me how to look at this graph?
The 50, 100, 150, 200 Hz, etc are your AC mains hum and its harmonics. Worst case being -70 db is not bad at all, and probably could reduced further with additional filtering and or shielding..
The concern I would have here is the 2k, 3k, 4k Hz, etc. That's harmonic distortion. Defenitively tweak your design to minimize that. Play a little with the biasing of your driver.
Now this is a SE amp after all, distortion will only go down so far after all. Your ears will be the final judge.
The concern I would have here is the 2k, 3k, 4k Hz, etc. That's harmonic distortion. Defenitively tweak your design to minimize that. Play a little with the biasing of your driver.
Now this is a SE amp after all, distortion will only go down so far after all. Your ears will be the final judge.
There's one essential specification missing: What was the level for the 1Khz fundamental?
I'll presume that the levels are such that the fundamental would have been at 0dB.
If that's the case then the 2nd harmonic distortion is about 1% (-40dB), for the THD you have to add up all the harmonics of 1Khz. My guess is that you'll end up with about 1.2%.
The 50Hz can be hum or common mode noise (harmonics are all odd, 50-150-250 etc.)
!00Hz, as you probably know is from the power supply(ripple).
The peaks around 1Khz look like intermodulation products of the test signal with the 100Hz ripple. It looks as if the same thing happens with the harmonics of 1KHz.
I'll presume that the levels are such that the fundamental would have been at 0dB.
If that's the case then the 2nd harmonic distortion is about 1% (-40dB), for the THD you have to add up all the harmonics of 1Khz. My guess is that you'll end up with about 1.2%.
The 50Hz can be hum or common mode noise (harmonics are all odd, 50-150-250 etc.)
!00Hz, as you probably know is from the power supply(ripple).
The peaks around 1Khz look like intermodulation products of the test signal with the 100Hz ripple. It looks as if the same thing happens with the harmonics of 1KHz.
An externally hosted image should be here but it was not working when we last tested it.
If it helps...here is the thd vs output power graph.
Correct, that's why it says THD+N on the graph. If you remove the fundamental before analysis, what you have left are the harmonic products of the the test signal and the noise of the amplifier under test.
So you have mostly 2nd, 3rd and 4th order distortion. The higher harmonics are 30-40dB below those (good thing). If you're not used to thinking in dB's: -40dB=1%, -60dB=0.01%, -80dB=0.001% etc.
Simple, even harmonics are 2KHz, 4KHz, 6KHz etc. Odd harmonics are 3KHz, 5KHz, 7KHz etc, It's all in the name.
Intermodulation products of the 1KHz test signal with the 100Hz ripple are the sum and the difference between the two (1000+100, 1000-100, 1000+200 etc.).
The Audio Precsion appears to have been set to notch out the fundamental test frequency of 1kHz for the harmonic graph. The 1 kHz residual may be related AC mains noise (AC filaments?) The spikes from 2 kHz and up are strong enough to be considered true distortion harmonics. Any harmonic at an even frequency is even (2, 4, 6, 8 kHz...), the rest are odd. That harmonic signature very much suggests to me a driver in distress, probably looking into a load it can't handle. This circuit doesn't use global negative feedback?
edit: eerrr, like everyone else said while I typed.
edit: eerrr, like everyone else said while I typed.
Hello Bas.
I think that RDF has hit the nail on the head. You mentioned a pentode driver? This means (without seeing the circuit of course) that the output resistance of the driver will be approximately equal to the load which means it would have a tough job driving the capacitance of your output valve.
In your place I would insert a cathode follower or perhaps a transformer.
7N7
I think that RDF has hit the nail on the head. You mentioned a pentode driver? This means (without seeing the circuit of course) that the output resistance of the driver will be approximately equal to the load which means it would have a tough job driving the capacitance of your output valve.
In your place I would insert a cathode follower or perhaps a transformer.
7N7
Hi Bas,
Many good answers in the above replies.. Some design issues can probably be resolved without much speculation with some detail on your amplifier design.
Details like the topology used, driver tube, output tube, and opt primary impedance as a minimum. Plate voltage, operating point and type of output tube bias would be useful as well. Do you use AC heating on the output tube filament? How about a schematic?
The overall thd actually seems a bit high for a 300B SE amplifier, 8W with under 5% thd is easily achievable without nfb, but what is of more concern here is the spectral content of the distortion. The overall levels are low above the 4th harmonic, but very extended. (I have run AP FFT and thd measurements on some of my SE amplifier designs in the past and they exhibited predominantly 2nd and a little 3rd harmonic with little else until the onset of clipping.)
The low frequency components are also very extended and are mains fundamental and harmonics. Some of them could be magnetically coupled into the output transformer or electrostatically coupled into the driver stage. I would suspect anything above 100Hz could originate by one of these means or in some cases via the output tube filaments if AC heated or heated with poorly filtered dc.
Given a little more data it is likely I would second the notion that a more robust driver stage is required, and in my experience the operating point of that pentode and the load it is driving is relatively critical to spectral behavior than in a comparable triode. (More current required?)
Many good answers in the above replies.. Some design issues can probably be resolved without much speculation with some detail on your amplifier design.
Details like the topology used, driver tube, output tube, and opt primary impedance as a minimum. Plate voltage, operating point and type of output tube bias would be useful as well. Do you use AC heating on the output tube filament? How about a schematic?
The overall thd actually seems a bit high for a 300B SE amplifier, 8W with under 5% thd is easily achievable without nfb, but what is of more concern here is the spectral content of the distortion. The overall levels are low above the 4th harmonic, but very extended. (I have run AP FFT and thd measurements on some of my SE amplifier designs in the past and they exhibited predominantly 2nd and a little 3rd harmonic with little else until the onset of clipping.)
The low frequency components are also very extended and are mains fundamental and harmonics. Some of them could be magnetically coupled into the output transformer or electrostatically coupled into the driver stage. I would suspect anything above 100Hz could originate by one of these means or in some cases via the output tube filaments if AC heated or heated with poorly filtered dc.
Given a little more data it is likely I would second the notion that a more robust driver stage is required, and in my experience the operating point of that pentode and the load it is driving is relatively critical to spectral behavior than in a comparable triode. (More current required?)
Thanks again to all for the great contributions. Here is the schematic of the amp. Although the distortion is relatively high. I must admit this pentode driver is the best (sounding) I've had so far to my ears.
An externally hosted image should be here but it was not working when we last tested it.
Hi Bas,
I'd stick with that topology for now, but I would crank up the plate current quite a bit. Reduce that 450 ohm EL84 cathode bias resistor and the 15K resistor on the supply and see where that gets you. Something in excess of 20mA would be the target.
To make your hum canceling circuit on the output the ratio of those caps really needs to be C1=C2/(Mu+1) where C2 is the cathode bypass cap and C1 is what some people refer to as the "ultrapath" cap. So change that 100uF to about 150uF or reduce the Obligato to about 20uF - 22uF - it's not that critical. (I use the same technique in one of my 2A3 designs and it works well.)
I guess while I was at it I would boost the B+ a bit so that I had something like 350 or even 400V across the 300B - this would necessitate a change from 820 to probably 1.1K or so to maintain a reasonable idle current.
How about zener or gas tube regulated screen voltage on the driver..
I'd stick with that topology for now, but I would crank up the plate current quite a bit. Reduce that 450 ohm EL84 cathode bias resistor and the 15K resistor on the supply and see where that gets you. Something in excess of 20mA would be the target.
To make your hum canceling circuit on the output the ratio of those caps really needs to be C1=C2/(Mu+1) where C2 is the cathode bypass cap and C1 is what some people refer to as the "ultrapath" cap. So change that 100uF to about 150uF or reduce the Obligato to about 20uF - 22uF - it's not that critical. (I use the same technique in one of my 2A3 designs and it works well.)
I guess while I was at it I would boost the B+ a bit so that I had something like 350 or even 400V across the 300B - this would necessitate a change from 820 to probably 1.1K or so to maintain a reasonable idle current.
How about zener or gas tube regulated screen voltage on the driver..
Keep one thing in mind if you decide to play around with driver op points. The ratio of 2nd, 3rd and 4th harmonics strongly suggest 2nd harmonic cancellation between stages. If correct reducing the first stage distortion will cause the total amp's 2nd harmonic to rise and everything above it to drop. Measured THD will go up so don't be shocked. That's my personal preference given the choice, your ears might say different.
Hi Kevin,
Just did a check. I actually have a 150uF cap in there. (Never updated the schematic)(did not think it important enough)
But also I thought I had read on tubecad that the ultrapath cap only needed to be 1/3rd of the bypass cap. So I thought I was safe there already.
Like Ashok said. The 1uF is low. But it was on purpose. With the higher B+ the amp sounded a little strained. With the lower b+ it all sounded a little more relaxed. But to get the 100Hz down and to listen again...I'll add a bigger cap. ...to up the b+ in the process as well. That should kill 2 birds with one stone. Smoother b+ and higher current.
I will also lower the cathode resistor of the driver tube.
Gerrit.
Stuart, Colt45 and rdf. Thanks for explaining what even and odd order is. It should have been self explanatory but one should not assume too much methinks. Weird huh that I should only find that out after 6 years in the hobby.
Just did a check. I actually have a 150uF cap in there. (Never updated the schematic)(did not think it important enough)
But also I thought I had read on tubecad that the ultrapath cap only needed to be 1/3rd of the bypass cap. So I thought I was safe there already.
Like Ashok said. The 1uF is low. But it was on purpose. With the higher B+ the amp sounded a little strained. With the lower b+ it all sounded a little more relaxed. But to get the 100Hz down and to listen again...I'll add a bigger cap. ...to up the b+ in the process as well. That should kill 2 birds with one stone. Smoother b+ and higher current.
I will also lower the cathode resistor of the driver tube.
Gerrit.
Oo that is very handy info! Did not know that.
Stuart, Colt45 and rdf. Thanks for explaining what even and odd order is. It should have been self explanatory but one should not assume too much methinks. Weird huh that I should only find that out after 6 years in the hobby.
Will do Fernando.
Hi Stuart,
That is correct. Here is the screen print of with my values (except for the caps's resistance that is) (And now there is 5uF because I wanted to see what it would look like with an added 4uF...and it looks pretty much the same.)
rdf, unfortunately I won't be able to measure it again. And maybe that is exactly what happened with the higher b+. It just did not sound as good but maybe the total distortion was lower.
Regards,
Bas
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Bas Horneman said:
Sure you can
I recon you already have a PC. If it also has a sound card, and can be located not to far from the amp (or vice versa), it only requires a simple adapter cable and dummy load + some free software.
My personal preference is the ARTA software. I know it is mostly intended for loudspeaker measurements, but works very nice for amp measurements as well.
SveinB.
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