"The schematic is not final, it was roughly thrown together and is missing a few things. Anyone see a potential problem?"
When clipping the higher voltage rails for the front end may cause problems when output transistors saturate hard. The base current will be a bit uncontrolled. Have you tried simulating it? An alternative could be trying it with the base resistors but without the higher rails. If the oscillation is eliminated you could try parallelling the base resistors with inductors of the same kind usually used at the amplifier output or a bit larger to reduce DC drop or just using smaller resistors.
The clipping recovery from negative might be a bit strange, it could be bettered by connecting a fast diode with cathode to R40-R41 junction and anode to negative rail. Only needs to cope with the VAS current but it must be able to stand off the rail to rail voltage. This will prevent hard saturation of Q24 during negative clipping.
When clipping the higher voltage rails for the front end may cause problems when output transistors saturate hard. The base current will be a bit uncontrolled. Have you tried simulating it? An alternative could be trying it with the base resistors but without the higher rails. If the oscillation is eliminated you could try parallelling the base resistors with inductors of the same kind usually used at the amplifier output or a bit larger to reduce DC drop or just using smaller resistors.
The clipping recovery from negative might be a bit strange, it could be bettered by connecting a fast diode with cathode to R40-R41 junction and anode to negative rail. Only needs to cope with the VAS current but it must be able to stand off the rail to rail voltage. This will prevent hard saturation of Q24 during negative clipping.
blueskynis said:Hi John,
Have you tried feedback network just like Leach did in his amplifier (pic attached)? Maybe this will also help to stop oscillations...
Hi blueskynis,
Did Leach use that to battle oscillation or to reduce TIM? Distortion of any kind is not a big issue for this amp, considering its effective frequency range.
AndrewT said:
Thanks Andrew,
A lot of these are not available to me here, though a couple can be found at B&D, where I tried to register and it keeps telling me to try later. In the meantime, I need to retype a shipload of info.
You didn't give an opinion - what do you think of the 2SB1011? Also, I already had these Zetex ones on order: ZTX458. Same problem with this ne is I'll need to create a heatsink for the E-line package.
megajocke said:
Hi,
I have simulated it (I sim everything, that's the only way I know it will work or not
). I thought the extra voltage could replace what is dropped across the base stoppers. In the sim, it works fine. For 4 ohm load, it allows the amp to produce over 500 watts before clipping. Without it, the max to 4 ohms is ~450-460 watts.Clipping behavior in the sim looks fine. When I put a diode in place as you suggest, it cuts off the top of the positive going wave. I used a 1N914 in the sim, but I haven't had real world performance from diodes in this simulator, so that might not mean anything.
I like the idea of an inductor parallel with the base stoppers.
"Hi,
I have simulated it (I sim everything, that's the only way I know it will work or not ). I thought the extra voltage could replace what is dropped across the base stoppers. In the sim, it works fine. For 4 ohm load, it allows the amp to produce over 500 watts before clipping. Without it, the max to 4 ohms is ~450-460 watts."
What happens without load and/or with a reactive load? That is the situation when it may cause problems.
"Clipping behavior in the sim looks fine. When I put a diode in place as you suggest, it cuts off the top of the positive going wave. I used a 1N914 in the sim, but I haven't had real world performance from diodes in this simulator, so that might not mean anything."
Looks like the simulator is doing its job then and is simulating the reverse breakdown of the diode. 1N914 is just a 100V diode from what I can find. It needs to stand the rail to rail voltage so a 200V part will be needed.
"I like the idea of an inductor parallel with the base stoppers."
I've seen Crown use them:
http://www.crownaudio.com/pdf/legacy/mt600_1200_schematic_j0275-8_b.pdf
Here they are before the driver stage though, which explains the rather large value.
I have simulated it (I sim everything, that's the only way I know it will work or not ). I thought the extra voltage could replace what is dropped across the base stoppers. In the sim, it works fine. For 4 ohm load, it allows the amp to produce over 500 watts before clipping. Without it, the max to 4 ohms is ~450-460 watts."
What happens without load and/or with a reactive load? That is the situation when it may cause problems.
"Clipping behavior in the sim looks fine. When I put a diode in place as you suggest, it cuts off the top of the positive going wave. I used a 1N914 in the sim, but I haven't had real world performance from diodes in this simulator, so that might not mean anything."
Looks like the simulator is doing its job then and is simulating the reverse breakdown of the diode. 1N914 is just a 100V diode from what I can find. It needs to stand the rail to rail voltage so a 200V part will be needed.
"I like the idea of an inductor parallel with the base stoppers."
I've seen Crown use them:
http://www.crownaudio.com/pdf/legacy/mt600_1200_schematic_j0275-8_b.pdf
Here they are before the driver stage though, which explains the rather large value.
I see you've moved to a standard darlington triple - you've learned your lesson like the rest of us.
A small lead compensation capacitor is usually required with triples. Try about 10 pF. For chip-amp circuits it normally goes in parallel with the feedback resistor. For discretes, you have the option of tapping off the VAS instead. That provides the separate HF feedback path that Leach was attempting to describe. I end up doing it this way in ALL my amps, but they usually have a boatload of NFB.
In your latest schem, you've gone to higher voltage supplies for the front end. Put the baker clamps from the output of the VAS back to the lower voltage rail so they forward bias when you try to overdrive, and use something like an HER-103. That will make it clip clean.
A small lead compensation capacitor is usually required with triples. Try about 10 pF. For chip-amp circuits it normally goes in parallel with the feedback resistor. For discretes, you have the option of tapping off the VAS instead. That provides the separate HF feedback path that Leach was attempting to describe. I end up doing it this way in ALL my amps, but they usually have a boatload of NFB.
In your latest schem, you've gone to higher voltage supplies for the front end. Put the baker clamps from the output of the VAS back to the lower voltage rail so they forward bias when you try to overdrive, and use something like an HER-103. That will make it clip clean.
megajocke said:
The pictures below is the simulated amp handling a slightly clipping squarewave at 30Hz. The scope is connected to the base of the VAS (This is where I look for instability that does not show on the output).
The two on the top are into a 4 ohm resistor, with the second one a magnification of the first.
The bottom two are into a 4 ohms in series with an 11mH inductor, paralleled with a 100uF cap. Like the top, the second is a magnification of the first.
I'm showing this because there's no visible difference between the two on output, and clipping behavior is also the same - clean, symmetrical clipping.
I put another 1N914 in series with the first and the top of the wave doesn't cut off anymore.
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A few questions and ideas...................
Would a double output stage be much more stable than a triple?
I was thinking he could try a lower gain double output, and drive it with a higher current VAS.
I wonder if the mutiple pairs affect stability............
Also, I notice how the outputs all share the same copper bus, even though they are parallel, the copper goes from series to each transistor. Would a "star" voltage rail hookup with each transistor with it's own connection (collector) to the +/- voltage rail make a difference?
Would a double output stage be much more stable than a triple?
I was thinking he could try a lower gain double output, and drive it with a higher current VAS.
I wonder if the mutiple pairs affect stability............
Also, I notice how the outputs all share the same copper bus, even though they are parallel, the copper goes from series to each transistor. Would a "star" voltage rail hookup with each transistor with it's own connection (collector) to the +/- voltage rail make a difference?
Hi,
I have been distracted away from this project with work and business related concerns, so apologies for not replying.
I haven't tried the darlington output yet, so I don't know how stable it will be. It is expected to be more stable than the CFP, which was not bad into the 4 ohm load, so I really don't anticipate problems.
I will be starting a new front end on the weekend which will include all of the compensation components suggested here. I will hopefully have it ready to try by early next week.
I did spend a few moments last night looking at the power supply, not the schematic so much as the actual physical layout of the smoothing caps inside the chassis. I am trying to make enough space for everything to fit well. If I add the extra voltage at the front end (pretty much a given) then I need to find space for that transformer, caps and rectifier bridge also. Sounds like not so big a deal but for a case that is close to the typical computer tower in size, there's not much free real estate available. I may need to redo some of my earlier PS circuit boards to combine functions on fewer boards to conserve space.
I have been distracted away from this project with work and business related concerns, so apologies for not replying.
I haven't tried the darlington output yet, so I don't know how stable it will be. It is expected to be more stable than the CFP, which was not bad into the 4 ohm load, so I really don't anticipate problems.
I will be starting a new front end on the weekend which will include all of the compensation components suggested here. I will hopefully have it ready to try by early next week.
I did spend a few moments last night looking at the power supply, not the schematic so much as the actual physical layout of the smoothing caps inside the chassis. I am trying to make enough space for everything to fit well. If I add the extra voltage at the front end (pretty much a given) then I need to find space for that transformer, caps and rectifier bridge also. Sounds like not so big a deal but for a case that is close to the typical computer tower in size, there's not much free real estate available. I may need to redo some of my earlier PS circuit boards to combine functions on fewer boards to conserve space.
wg_ski said:I see you've moved to a standard darlington triple - you've learned your lesson like the rest of us.
A small lead compensation capacitor is usually required with triples.
Hi wg_ski,
Yes, I'm learning, but that was my intention. This is hands-on learning.
Fortunately, the change to the darlington output is easily accomplished.As I've mentioned above, I'll be including every possible compensation component in the the scheme and (hopefully) the final board layout. That way, I can add these as I go to make the amp stable without making the board a mess.
megajocke said:
Here it is. The top is the output with the input 1Vpp over the input sensitivity. the red trace underneath is the collector of the VAS.
NOTE: This is the simulator, not the real amp.
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A nice pair of 4700uf caps for your input stage power won't take up too much room. Being low current, you could have the bridge just mounted to the case with a screw.
Input stage as a whole uses only 20-30mA tops, if even that much, so a small transformer is more than good enough, but I've never seen a small one at such a high voltage. Maybe you could make some higher voltage taps off your main transformer if it's a toroid.
Input stage as a whole uses only 20-30mA tops, if even that much, so a small transformer is more than good enough, but I've never seen a small one at such a high voltage. Maybe you could make some higher voltage taps off your main transformer if it's a toroid.
EWorkshop1708 said:
Hi EW,
The extra voltage for the front end will be supplied from a 6.3-0-6.3 VAC transformer. This rectified voltage will be added in series to the main power supply for the front end (as shown in the schematic in post#158.)
I have played around with the placement of components on the chassis, and I think I can get it all to fit. Originally I had a 12-0-12 transformer for the relays and control circuits. I will replace that with two 6-0-6, 3A transformers to supply the extra voltage for the front end, the 12V power for the controls and the 24V for the relays.
Here's how it looks. Everything will still fit under the cover shield. I have some new 4700uF caps coming to use in the low voltage supply.
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Hi There MJL21193,
what is the capacitance value / ripple current rating for your test supply?
The reason I ask is:
Recently I tested an amplifier with 8000uF/100V per rail (standard centre tapped linear supply) and there was some instability problems,
I increased the supply capacitance along with ripple current capability added some smaller faster caps across the large electrolytics and the normal 0.1uF caps at the lead ends and the instability (at clipping) was greatly reduced.
You probably already know all this but i thought perhaps this could help solve some of the the ringing problem if it is related to the supply in some way.
Cap quality played a very significant role.
-Dan
what is the capacitance value / ripple current rating for your test supply?
The reason I ask is:
Recently I tested an amplifier with 8000uF/100V per rail (standard centre tapped linear supply) and there was some instability problems,
I increased the supply capacitance along with ripple current capability added some smaller faster caps across the large electrolytics and the normal 0.1uF caps at the lead ends and the instability (at clipping) was greatly reduced.
You probably already know all this but i thought perhaps this could help solve some of the the ringing problem if it is related to the supply in some way.
Cap quality played a very significant role.
-Dan
Hi Dan,
My test supply used 2 Cornell Dubilier 10,000uF caps, one per rail.
The final supply will have 5 - 8200uF per rail (plus the two 10,000uf above, if I can make them fit).
I do see what you mean, that some of the "ringing" could be from the power supply not being able to keep the high current demand up.
I'm going to take the time to try to get the real power supply finished before I run anymore tests.
I think it's more the load I was trying to drive. My homemade resistors may be too inductive. I will make a point of measuring their inductance tomorrow with Speaker Workshop.
Shown earlier, here are my dummy load "resistors":
My test supply used 2 Cornell Dubilier 10,000uF caps, one per rail.
The final supply will have 5 - 8200uF per rail (plus the two 10,000uf above, if I can make them fit).
I do see what you mean, that some of the "ringing" could be from the power supply not being able to keep the high current demand up.
I'm going to take the time to try to get the real power supply finished before I run anymore tests.
I think it's more the load I was trying to drive. My homemade resistors may be too inductive. I will make a point of measuring their inductance tomorrow with Speaker Workshop.
Shown earlier, here are my dummy load "resistors":
Attachments
EWorkshop1708 said:
A double is always more stable. But they have a much harder time driving 2 ohms. You just run out of beta. The VAS would likely have to be increased to 30mA or more, and changed back to a darlington. If you're driving with an op-amp (like the 5532, in the QSC transnova amps), you have a lot of open loop gain which this design does not afford. It really needs a triple if he intends to run 2 ohm loads with any authority.
Absolutely multiple pairs affects stability. The more capacitance you drive the worse it gets. Not only that, the stage could potentially go into oscillations between parallel devices. It's worse with FETs than bipolars, too. That's why you need a base stopper on *each individual device* when paralleling. Try it without them on a FET output stage and it's almost guaranteed to oscillate.
wg_ski said:
Hi wg_ski,
Do you really think it's absolutely necessary to have base resistors on all of my output stage devices? I have decided to have one to supply all, not wanting to tear down the work I've done already unless it's needed.
This amps predecessor, the P68 500 watt version had no base stoppers at all, not even on the drivers. I guess Rod doesn't believe in them. Anyhow, that amp ran stable, and it had 4 paralleled outputs per side plus the CFP.
I just got lucky, I think.
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