pooge said:
For more info on this topic see also:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=94676&perpage=25&pagenumber=18
starting at post #450, in particular the controverse between TPC and TMC.
Regards, Edmond.
Re: Re: Two-pole compensation
I wouldn't make the blanket statement that they are not at "AC ground". I would say that they are an inferior "AC ground". Although this would depend on the design somewhat.
Also, the base of the VAS transistor is referenced to the supply rail too (you can't avoid that), so I can see some merit in the choice of method A in some situations.
Cheers,
Glen
estuart said:
I wouldn't make the blanket statement that they are not at "AC ground". I would say that they are an inferior "AC ground". Although this would depend on the design somewhat.
Also, the base of the VAS transistor is referenced to the supply rail too (you can't avoid that), so I can see some merit in the choice of method A in some situations.
Cheers,
Glen
Re: Re: Re: Two-pole compensation
I was thinking about that, as a common emitter the output would also be referenced to the same. It would seem that referencing the resistor to the common reference couldn't be a bad thing. So we have a vote for A, a vote for B, and a vote for either.
G.Kleinschmidt said:
I was thinking about that, as a common emitter the output would also be referenced to the same. It would seem that referencing the resistor to the common reference couldn't be a bad thing. So we have a vote for A, a vote for B, and a vote for either.
Re: Re: Re: Two-pole compensation
That's exactly what I mean, "an inferior AC ground"
Hi Glen,
Indeed, the base of the VAS transistor is referenced to the supply rail. That's bad enough (because of a poor PSRR). But this is no reason to ty Rp also to the same supply rail, that is, an equally bad reference point. Doing things two times wrong doesn't make it better, only worse.
Cheers, Edmond.
PS: My apologies for not looking any further at your 500W amp. All my time is going to design a fully symmetrical amp with NDFL, EC, and a common control loop for stabilizing the VAS currents, something like the schematic I have dumped on the forum a couple of month ago, but that thing is far from optimal, actually wrong. So forget my first attempt (if not already done). It's a joint project with syn08, who is building the amp. Not an easy job, because it is getting an extreme complicated design.
G.Kleinschmidt said:
That's exactly what I mean, "an inferior AC ground"
G.Kleinschmidt said:
Hi Glen,
Indeed, the base of the VAS transistor is referenced to the supply rail. That's bad enough (because of a poor PSRR). But this is no reason to ty Rp also to the same supply rail, that is, an equally bad reference point. Doing things two times wrong doesn't make it better, only worse.
Cheers, Edmond.
PS: My apologies for not looking any further at your 500W amp. All my time is going to design a fully symmetrical amp with NDFL, EC, and a common control loop for stabilizing the VAS currents, something like the schematic I have dumped on the forum a couple of month ago, but that thing is far from optimal, actually wrong. So forget my first attempt (if not already done). It's a joint project with syn08, who is building the amp. Not an easy job, because it is getting an extreme complicated design.
Re: Re: Re: Re: Two-pole compensation
G'day Edmond.
Hmmm...... I can see some potential benefit in having Rp tied to the same reference point as the VAS base. For instance, with the end of Rp tied directly to ground, the AC current induced in Rp due to rail fluctuations will modulate the base current of the VAS just the same.
I haven't looked into it much deeper than this, but I reckon a comparison between the two connection methods would make for a worthwhile investigation in SPICE.
No worries about the 500W amp – it’s since been completely redesigned, with the power output spec raised to 1kW. Some of the schematics are on my webpage under “1000W Class A+ amp”, but a major update is planned very soon (the Class A output stage schematic is obsolete). I’m building this thing ATM, as well as trying to finish off my 12 watter (tone control board, reg. PSU) and another design.
Good luck with your NDFL super amp – sounds good!
Cheers,
Glen
estuart said:
G'day Edmond.
Hmmm...... I can see some potential benefit in having Rp tied to the same reference point as the VAS base. For instance, with the end of Rp tied directly to ground, the AC current induced in Rp due to rail fluctuations will modulate the base current of the VAS just the same.
I haven't looked into it much deeper than this, but I reckon a comparison between the two connection methods would make for a worthwhile investigation in SPICE.
No worries about the 500W amp – it’s since been completely redesigned, with the power output spec raised to 1kW. Some of the schematics are on my webpage under “1000W Class A+ amp”, but a major update is planned very soon (the Class A output stage schematic is obsolete). I’m building this thing ATM, as well as trying to finish off my 12 watter (tone control board, reg. PSU) and another design.
Good luck with your NDFL super amp – sounds good!
Cheers,
Glen
Re: Re: Re: Re: Re: Two-pole compensation
Hi Glen and Pooge,
You are right and I was 100% wrong. I spiced it a second time and method A is the clear winner. If Cp1 = 330pF, Cp2 = 120pF and Rp = 3k3, method A shows a PSRR of -57dB at 10kHz, while method B shows only -35dB (the same without TPC).
Apparently, I made a really stupid error during the first simulation by exchanging the results, sorry.
Thanks, good luck too with your super amp!
Cheers, Edmond.
G.Kleinschmidt said:
Hi Glen and Pooge,
You are right and I was 100% wrong. I spiced it a second time and method A is the clear winner. If Cp1 = 330pF, Cp2 = 120pF and Rp = 3k3, method A shows a PSRR of -57dB at 10kHz, while method B shows only -35dB (the same without TPC).
Apparently, I made a really stupid error during the first simulation by exchanging the results, sorry.
G.Kleinschmidt said:
Thanks, good luck too with your super amp!
Cheers, Edmond.
Hi all
the bias stabiliser circuit C is best I think.
It should have a PNP in the second stage.
If anyone has Self's book did he point out the most important aspect of (c) which was that the ratio of the multiplication which is set by R9, R8 can be exactly set for the number of Vbe's you have in the driver /output stage (usu. 4). The bias voltage should be set by adjusting the current in Q3 (use adjustable resistor for R7).
I've had very good results with b but the bias transistor has to match the drivers etc. For example BD139 bias, BD139/140 output pair and 2N3055/MJ2955 = good stability; BD139 bias, BD139/140 driver; MJ15003/4 output = not good use circuit C to fix.
cheers
John
the bias stabiliser circuit C is best I think.
It should have a PNP in the second stage.
If anyone has Self's book did he point out the most important aspect of (c) which was that the ratio of the multiplication which is set by R9, R8 can be exactly set for the number of Vbe's you have in the driver /output stage (usu. 4). The bias voltage should be set by adjusting the current in Q3 (use adjustable resistor for R7).
I've had very good results with b but the bias transistor has to match the drivers etc. For example BD139 bias, BD139/140 output pair and 2N3055/MJ2955 = good stability; BD139 bias, BD139/140 driver; MJ15003/4 output = not good use circuit C to fix.
cheers
John
Hi all
I don't think two pole solves the performance problem of an amp.
First point (I've made before) is that the additional loading increases distortion/drive requirements of the VAS. Think Self briefly mentioned this?
It doesn't affect the transient response, so the two-pole might improve the sound for any of you who prefer to listen to continuous sinewaves instead of music...
cheers
John
I don't think two pole solves the performance problem of an amp.
First point (I've made before) is that the additional loading increases distortion/drive requirements of the VAS. Think Self briefly mentioned this?
It doesn't affect the transient response, so the two-pole might improve the sound for any of you who prefer to listen to continuous sinewaves instead of music...
cheers
John
john_ellis said:
Self did mention the disadavantage of loading the VAS, but stated unequivically that there was a significant net gain in distortion reduction. He seemed rather enamered with it, but concerned about the OL FR peak at the pole. It would seem that there would be a way to eliminate this peak with proper component selection, though. I have a paper I'm still studying that seems to show the proper component selection. Further in this or another paper, it shows that the error voltage is always lower for the two-pole.
Hi John,john_ellis said:
Care to specify what kind of "performance problem"
john_ellis said:
Sure. One more reason to use TMC in stead of TPC.
john_ellis said:
I'm sorry to say so, but this doesn't make any sense to me whatsoever.
First, TPC does affect the transient response, see: http://www.diyaudio.com/forums/showthread.php?s=&threadid=94676&perpage=25&pagenumber=19 post #466.
Second, what the heck has the transient response to do with continuous sine waves?
Third, nobody prefers to listen to continuous sine waves, with or without TPC, although they are very useful for (objective!) THD and IMD measurements.
So, what's your point?
Regards, Edmond.
Hi Edmond
"Performance problem" is to do with transient performance!
What I was thinking of when I wrote that was that at very high frequencies, the two capacitors in series appear to be a singleton. This is where the 12 db slope roll-off falls back into the 6 db roll-off, but of course you are right in that there will be a difference in response because the 12 db (two-pole advantage) region is different from the standard singleton.
The 6db point is seen in the very first stages (sub-200 nS) of a response to a transient, in that the delay times are comparable between a singleton and TPC.
But the "performance problem" arises if, say, the VAS stage cuts off in the transient response because it is being driven harder i.e. faster.
You can see the effects of this "hard" slew rate induced cut-off if you hit a TPC amplifier with a fast transient and compare it with an identical amp where the roll-off is a singleton. In the circuits I have simulated, the loading of the VAS occurs in the fast response period even for an amplifier which has had input degeneration designed so that the input stages don't cut off under normal use.
Here's my illustration:
standard amp: input resistors 330 ohm, current 3 mA each side
compensation capacitor 47 pF. VAS current 12 mA.
TPC: 150 pF on VAS, shunted with 470 ohm (puts the 12dB to 6 dB join at around 3 MHz) and 68 pF to base, the 470 ohm is a pretty heavy load. Needs VAS current to be around 20-25 mA to prevent cut-off. But you can make a critically-damped TPC amp work under these conditions. To damp the response peak I used a 4.7 k in series with 30 pF across the 10k feedback resistor. The 30 pF is critical to obtain accurate damping, (at least in simulation, probably means this in practice too, but at least you can use two 15 pF's) but maybe this is cheating to "tailor" the response to solve a problem arising from internal phase shifts.
Yes the slew rate is faster, but the performance problem isn't just slew rate.
But I hope you can see my concerns with VAS's running around 6 mA, and just "bunging in" a TPC.
cheers
John
"Performance problem" is to do with transient performance!
What I was thinking of when I wrote that was that at very high frequencies, the two capacitors in series appear to be a singleton. This is where the 12 db slope roll-off falls back into the 6 db roll-off, but of course you are right in that there will be a difference in response because the 12 db (two-pole advantage) region is different from the standard singleton.
The 6db point is seen in the very first stages (sub-200 nS) of a response to a transient, in that the delay times are comparable between a singleton and TPC.
But the "performance problem" arises if, say, the VAS stage cuts off in the transient response because it is being driven harder i.e. faster.
You can see the effects of this "hard" slew rate induced cut-off if you hit a TPC amplifier with a fast transient and compare it with an identical amp where the roll-off is a singleton. In the circuits I have simulated, the loading of the VAS occurs in the fast response period even for an amplifier which has had input degeneration designed so that the input stages don't cut off under normal use.
Here's my illustration:
standard amp: input resistors 330 ohm, current 3 mA each side
compensation capacitor 47 pF. VAS current 12 mA.
TPC: 150 pF on VAS, shunted with 470 ohm (puts the 12dB to 6 dB join at around 3 MHz) and 68 pF to base, the 470 ohm is a pretty heavy load. Needs VAS current to be around 20-25 mA to prevent cut-off. But you can make a critically-damped TPC amp work under these conditions. To damp the response peak I used a 4.7 k in series with 30 pF across the 10k feedback resistor. The 30 pF is critical to obtain accurate damping, (at least in simulation, probably means this in practice too, but at least you can use two 15 pF's) but maybe this is cheating to "tailor" the response to solve a problem arising from internal phase shifts.
Yes the slew rate is faster, but the performance problem isn't just slew rate.
But I hope you can see my concerns with VAS's running around 6 mA, and just "bunging in" a TPC.
cheers
John
Hi Edmond
And another (very subtle) point...
that TPC design I mentioned has an extremely advantageous (IMHO) design advantage over the Miller ... the differential input stage signals remain completely flat to over the audio band at 20 kHz.
Now if single-polers can give rise to audible effects (if?) because the differential signal increases at around 1 kHz or lower (depends on open loop gain, transition frequency etc) then this particular TPC MIGHT be free from the effects ...
(ifs and mights don't prove anything but the simulation shows no differential quirks until > 20 kHz. That at least has to be worth something.)
might be worth repeating this recipe for a TPC amp:
the only mods from the standard Miller are:
input degen to avoid transient cut off in the input stage
330 ohms, 3 mA/side
68 pF, 150 pF TPC with 470 ohm to ground
VAS current raised to 20 mA (think this is enough for 50W amp).
cheers
John
And another (very subtle) point...
that TPC design I mentioned has an extremely advantageous (IMHO) design advantage over the Miller ... the differential input stage signals remain completely flat to over the audio band at 20 kHz.
Now if single-polers can give rise to audible effects (if?) because the differential signal increases at around 1 kHz or lower (depends on open loop gain, transition frequency etc) then this particular TPC MIGHT be free from the effects ...
(ifs and mights don't prove anything but the simulation shows no differential quirks until > 20 kHz. That at least has to be worth something.)
might be worth repeating this recipe for a TPC amp:
the only mods from the standard Miller are:
input degen to avoid transient cut off in the input stage
330 ohms, 3 mA/side
68 pF, 150 pF TPC with 470 ohm to ground
VAS current raised to 20 mA (think this is enough for 50W amp).
cheers
John
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