I have no tried to dig it (for the same reason than you). But it seems that some transition's smoothing occurs when the input stage turn in class B near the ops point. I believe it is too acrobatic to be used.
About slew rate, there is different ways to measure-it. By habit (my way) we use to:
- Remove the input low pass filter if any.
- Bring a fast square wave with at least enough level to saturate the OPS.
- Measure the time between -10% and +10% of this max level. (the center 80% of the signal).
But some companies, like Interstil measure 2/3 of the signal.
So, in this thread, may-be we can agree on a common method in order our numbers keep coherency.
for "sound" the measurements that matter are audio band - not squarewaves - no audio signal and loudspeaker load calls for ridiculous slewing
if you really think EMI/RF rectification is an issue - then test with reasonable values for that - with input filtering, output isolating network
if you really think EMI/RF rectification is an issue - then test with reasonable values for that - with input filtering, output isolating network
Do you have any evidence that the simpler amp sounds better if there is no overload?
I love simplicity too, just wonder about the data.
Isn't this basically a stiffer current source?
At first look it seems this could be done reasonably easily.
Not so much a cheat but just a matter of what you want to include when refer to the "OPS" (or whatever).
Please not another interminable debate about definitional differences😉
I like to use the term "section" rather than "stage". A stage has a non-arbitrary unity, like on a rocket.
A section is just where you cut it, like a salami!
Some of this stuff is two perspectives on the same effect.
The famous Blackman paper showed the relation between feedback and impedance.
So you can consider the increased feedback, or consider the reduced load.
I would call MC "non-optimal". Bode worked out the optimal function about 1940 and I find it a useful perspective to analyse different compensation schemes.
This is another one I see like Andrew, have I missed some point?
Best wishes
David
kgrlee said:
Why? If the objective is speed to "skip ... quickly" then shouldn't the VAS be biased at peak Ft?
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You have to make your own opinion by your own experience on this subject. You will never able to make from this a general law, for obvious reasons. 😀
After 40+ years of daily professional work with so many various and prestigious amplifiers, i tend to feel the same than Kgrlee now.
Again and again, we measure things we don't listen to (sinus, continuous background noises, symmetrical waves), our brain care to things we don't measure: Transients, changes in backgrounds. It is made that way for our survival. And music is made of non symmetrical signals.
...
I have a very simple example for objectivists about square waves, audio band etc...
Listen to 20KHz sin. You will not ear anything (yes, some can 'feel' something disagreeable, like the impression of having ears plugged). Now, listen to spaced positive or negative pulses of 25µs (half a 20Kz square wave). 😀
Or try to explain the difference we all noticed between 44.1KHz and 192 KHz sample rates.
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It's not just from a speed point but ALSO more feedback AND that the remaining OPS/VAS interaction distortion is lower order cos HiZ drive.
If you choose an arbitrary ULGF, you can rank 'pure Cherry', TMC & TPC in terms of high order harmonics in that order .. adjust each one to have an identical THD spec.
I'm not sure, this is discussion is getting anywhere. I've proposed a heretical view which is sorta supported by Cherry but scorned by other Gurus. I show some sims which support this ... but more importantly ... have used this approach to get better performance in 'real life'.
'Pure Cherry' isn't without its problems. Baxandall & Edmond proposed evil TMC to solve some of them .. but sullying 'pure Cherry' also loses some of its advantages. 🙂
I believe I can get around the problems while keeping Cherry pure .. but only in VFAs 😡
I dunno how to do this with equal facility in CFAs so am quite happy to use lesser stuff like TMC, TPC, TPMIC, evil Miller or whatever works. 😀 [*]
But even with evil Miller, I like to minimize the compensation I take from the VAS output and provide as much as possible via Cherry. I show this in #500
You don't have to agree with me on religious grounds. But its a valid approach cos it gives good 'real life' results.
[*] The only one I haven't tried and got good results from in 'real life' is TMC. I've even used Two Pole Cherry successfully. This is what dadod uses in his 200W MOSFET CFA amp thread.
All the properly conducted Double Blind Listening Tests bla bla that I know of without overload eg Moir in Wireless World commissioned by QUAD, showed no preference.
There are a couple in which some of the contenders were really dodgy Golden Pinnae amps but in fact these were marked down by the VERY few people who reliably tell the difference.
My tests were of 50W amps (and a smaller series for a 100W example) for a commercial product. The listener chooses his music AND the levels. As some of the music was uncompressed PCM-F1 recordings, it is likely overload occurred. I certainly clipped the amps cos I made some of the recordings.
Excuse the vagueness but this was more than 20yrs ago.
Yes. If you show me how, I will grovel at your feet 😱
I think you'll find my standing VAS currents are at about peak Ft/hfe. Bonsai etc run somewhat more. I also like to keep Pd within sensible limits 🙂
I don't only bias for peak Ft but also to endure that the TIS current is where possible appreciably higher than the worst case OPS load.
No problem with dissipation. Just use adequate heat sinking.
🙂
No problem with dissipation. Just use adequate heat sinking.
🙂
I know that Bob Cordell's book typically shows 10 mA (pp 72, 143, 144, 176) which is quite a bit lower than 30mA current for max Ft of the 2N5551 or a 2SC3503 / 2SA1381.
So when Richard says he used even less than Bob I was surprised.
But I haven't checked many specific amps so perhaps this is more about different perceptions of what some people "tend" to do.
Do you have any numbers handy for your amps?
Best wishes
David
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The output stage should be on triple Darlington with dual output transistors or FETs with an additional follower stage on their inputs
regards
Petr
regards
Petr
L.C said VSSA VAS drift from 12 to15 mA.
My amp (2 paralleled pairs of Hex fets) 10-15mA. Distortion increase if i increase.
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Thanks, I notice that you use 2SC3503/2SA1381 in the E-Amp and the SX, so that's pretty much spot on for maximum Ft.
Excellent choice😉
Don't understand why Ric says he uses less than you and Bob, unless different transistor choice.
Best wishes
David
In simulation I change VSSA compensation like this. It give better 20kHz distortion and wider bandwidth. Is VSSA will stable with this compensation? Is value of the compensation's capacitors is right for real implementation?
I still learning how to use LTSPICE. If anyone have models for transistors that usually used in this forum (except Bob Cordell models), please sharing to me. You can send to my email: anistardi@gmail.com
Regards,
Anistardi
I still learning how to use LTSPICE. If anyone have models for transistors that usually used in this forum (except Bob Cordell models), please sharing to me. You can send to my email: anistardi@gmail.com
Regards,
Anistardi
OK. Please post the ASC of this amp.
Could you post a stripped down, version, ready to run a Tian probe?
Neither of the versions in posts #1 or #3 of your thread were wired correctly and I don't want to make any "little" alterations that could have unforeseen consequences.
So no THD analyser either.
The ULGF around the output transistors in that simulates as 19MHz.
I am somewhat sceptical, but it is of interest because it seems to use the load presented by the OPS as part of the compensation.
This is the possibility you alluded to in #1438 and I am on a quest to find the speed limit.
Best wishes
David
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If you are talking about normal loop gains (so 40-60dB at 20 kHz using advanced comp techniques, or 25 to 35 dB using MC) then you have to close your loop (ULGF) at 3 MHz - and this is the upper end on an EF2. Further, at ULGF = 3 MHz, you will need a Zobel and output inductor. If you want to close at higher ULGF . . . only solution is to drop the loop gain so that you ensure that the OPS pole falls well below 0 dB.
If you design a very fast CFA with WIDE bandwidth and keep the loop gain low out to MHz, I can see that you can make a ULGF up at 5 or 10 MHz work. Or, run open loop ZGNF
Earlier on in this thread I proposed that we stle on all distortion assessments at a fixed ULGF of 3 MHz because we saw some designs propositions with very low THD but with ULGF's that were not going to be stable in practice.
If you design a very fast CFA with WIDE bandwidth and keep the loop gain low out to MHz, I can see that you can make a ULGF up at 5 or 10 MHz work. Or, run open loop ZGNF
Earlier on in this thread I proposed that we stle on all distortion assessments at a fixed ULGF of 3 MHz because we saw some designs propositions with very low THD but with ULGF's that were not going to be stable in practice.
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I see Guru Zan dun trust beach bums .. especially from Queensland 🙂
Here's the .ASC of the version of #499 which gives the results in #823.
I've left the THD Analyser in but to run the Tian probe, just delete it. Everything else is set up.
LG.plt is for Tian probes. Go 'Plot Settings' 'Open Plot Settings File' after the .AC run
Out.plt is to look at the output & residual of the THD meter. Go 'Plot Settings' 'Open Plot Settings File' while running the THD analysis
You need to change the Spice Directive to
.inc Analyzer_Controls.txt
.tran 0 {AnalTime} {SettleTime} {MaxStep}
.param prb=0
;.tran 0 11m 0m 1u
;.ac oct 50 10k 100e6
;.step param prb list -1 1 ; prb=0 turns off probe
;.op
THD.plt is to look at the individual harmonics. After the THD run, follow the instructions in Analyzer_Controls.txt and then Go 'Plot Settings' 'Open Plot Settings File'
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The models are Cordell's except for the J111 FETs from fas42. Details in ricardo.txt
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The Tian probe is set up for Loop Gain of the complete amplifier.
To look ONLY at the OPS + VAS + pure Cherry loop, move the Tian probe to the base of Q12 as in OutputLGcir.gif. You need to make a link between R9 and Q3 base to replace the Tian probe which you've moved.
This should give the results in OutputLG.gif & OutputNyq.gif
The Tian probe analysis tie in with my Jurassic Loop Gain efforts using BIG inductors, breaking the loop and my own linear Circuit Analysis package.
________________
Dave, I'm still not sure how you do your OPS Loop Gain. I don't think putting the Tian probe between the OPS emitter resistors and the feedback take off does this. I think this looks at LG for the whole amp but there are some scaling factors involved.
Can you post a pic or .ASC with one of your Tian probes to do OPS LG? How do you do LG of the whole amp?
Attachments
On the contrary. I want it ready to run because I don't trust any alterations unless I can test before and after. The "trivial" ones are often the worst.
Thanks
Perhaps a miscommunication.
I will try to post some pics to explain.
Best wishes
David
Just had a look at your loop results.
Conditional (Nyquist) stability and about 14 MHz ULGF!
I don't think Andrew will be impressed.
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I am strangling with lateral MOSFET OPS and how to tame it. If I go for very low distortion, suffers slew rate, and vice versa. It’s quite easy to get very high SR, about 400 V/usec, but then distortion is more then 10 ppm at 20 kHz, and if I go for below 5 ppm I can’t get SR better then 150 V/usec. I tried modified TPC(with OPS included in one C branch) in combination with shunt compensation (it looks that whiteout it CFA is not very stable), I tried pure Cherry(TPC could lower distortion two to tree times compared to pure Cherry), always some nasty OPS speaks show its ugly head.
Now what distortion and SR level could be set as good compromise. I asked that Richard(RNMarsh) but no answer, and I realy don’t have experience enough to decide. We have a lot of audio experts here, and the books I’ve read said that more then 50 V/usec (not talking mastodon power amp) is not needed for audio.
It looks that we chase SR now as a low distortion chase in VFA amps.
BR Damir
ps. complicate schematic it's not a problem for me as small transistors are cheap, why to make simpler then it's posible(A. Einstein)
Now what distortion and SR level could be set as good compromise. I asked that Richard(RNMarsh) but no answer, and I realy don’t have experience enough to decide. We have a lot of audio experts here, and the books I’ve read said that more then 50 V/usec (not talking mastodon power amp) is not needed for audio.
It looks that we chase SR now as a low distortion chase in VFA amps.
BR Damir
ps. complicate schematic it's not a problem for me as small transistors are cheap, why to make simpler then it's posible(A. Einstein)
Ric's Output Inclusive Cherry has >100dB at 20 kHz around the output transistors!
So the ULGF around the OPS is 13 MHz.
About where I expected.
No wonder the THD is low😉
I do wonder where the practical limit is.
Best wishes
David