It's not really so much of a thermal issue, the current mirrors driving the base of the VAS transistors lack reference to the emitters of the VAS transistors. Sorry if I mis-led the point. I was just giving an example of one parameter that can change bias. A single ended amplifier w/mirror has the VAS base driving voltage reference through the feedback loop because the VAS is loaded with a constant (CCS or resistor) which means the VAS current is translated to a voltage, then sensed by the feedback loop. With the complementary VAS, this is not the case. Each VAS is loaded with a dependent current source (the opposing VAS transistor) which is a variable. The base of the VAS transistors must be referenced to the emitter of the VAS transistors in some way. The easiest way to do this is with a resistor. I doubt very strongly you can create a balanced steady state with this circuit as it is.
Thanks for all the replies!
Updated Main Schematics.
Main
AndrewT
I finally heard the voice of reason, and replaced 0.1 degeneration resistors with 0.33. This gives circa 50W quiecent dissipation. Hope this will be ok.
wg_ski
I have switched to 5 pairs of 21193/4 and have 4281/4302 driving them.
nigelwright7557
I'll concider you methods. As it comes to microcontroller, although I know my way around them, I don't want any in this design
G.Kleinschmidt
It is stated in the book, that all designs are fully functional, weird... Why should mr. Slone publish unworkable design...
Tony
Done
EWorkshop1708
It seems, that I'll have a chance to let the main transformer to be custom made. I am thinking of 2*60V 2*5A 600VA, which gives 84V rails.
I have selected other design for the input and VAS. Hope the component values are OK. Also changed the overload protection.
Regards,
Aq
Updated Main Schematics.
Main
AndrewT
I finally heard the voice of reason, and replaced 0.1 degeneration resistors with 0.33. This gives circa 50W quiecent dissipation. Hope this will be ok.
wg_ski
I have switched to 5 pairs of 21193/4 and have 4281/4302 driving them.
nigelwright7557
I'll concider you methods. As it comes to microcontroller, although I know my way around them, I don't want any in this design
G.Kleinschmidt
It is stated in the book, that all designs are fully functional, weird... Why should mr. Slone publish unworkable design...
Tony
Done
EWorkshop1708
It seems, that I'll have a chance to let the main transformer to be custom made. I am thinking of 2*60V 2*5A 600VA, which gives 84V rails.
I have selected other design for the input and VAS. Hope the component values are OK. Also changed the overload protection.
Regards,
Aq
raitraak said:
If you're worried about this thing getting expensive fast, you might want to back up a minute before you go blow a wad of cash on a custom trafo. 60-0-60's aren't as common on the surplus market as we would all like them to be, but they do show up as they are a standard size. So are 30-0-30's and similar sizes, so don't be afraid to put identical units in series when it suits you. Two 30-0-30's at 495VA each (that's 990 VA) can be had locally for $70. Try getting anything that big custom made or through distribution that cheap. Can't be done. It would be more than big enough for your project. Explore options like that to save dough (so you don't have to buy transistors from questionable sources).
On the subject of a 60-0-60 transformer if you have a 115v ac line you might be able to find a big 120-240 to 120-240 transformer. I picked one up for next to nothing that has
2 120v primaries and 2 120v secondarys. If I connect both
of them in seriese this gives me 120 to 60-0-60 @1kva cost
me a whole $1
2 120v primaries and 2 120v secondarys. If I connect both
of them in seriese this gives me 120 to 60-0-60 @1kva cost
me a whole $1
"I'll concider you methods. As it comes to microcontroller, although I know my way around them, I don't want any in this design."
I have found the microcontroller output protect absolutely vital especially while the design is in the testing stage.
If you cant use a microcontroller then at least consider fuses in the line to the speaker until the design is proved.
Whats the price of a fuse compared to a set of speakers ?
I have found the microcontroller output protect absolutely vital especially while the design is in the testing stage.
If you cant use a microcontroller then at least consider fuses in the line to the speaker until the design is proved.
Whats the price of a fuse compared to a set of speakers ?
nigelwright7557 said:
When it's in the testing phase, you usually use a light bulb limiter. If you don't, you'll need to buy your fuses in the largest bulk pack they offer because you'll go through them like water. When using the amp (before and after it's proven) an RC timing circuit, comparator or two, and a relay or crowbar works just as well as a microcontroller. More than 50 milliseconds above 15 volts and you switch it off, no muss no fuss. And you don't need a firmware development system to get it working, and its operation does not depend on a computer program to be running properly.
wg_ski said:
I also have a 3 second power up delay on the relay to stop huge thumps through the speakers.
A micro could open up a whole enhanced amp experience with an LCD display showing output levels, monitor PSU levels etc etc
Power up surge relay could be incorporated.
I am from a micro design background so the thought of a PIC cct and a program for it is a very menial task.
FYI: this design is very functional, wanna listen to mine? haha
yup this is very functional, BUT I don't really funned of having up to
more than 220ohm of input diff degeneration resistors
unless you like some lousy bass! keep it less than or equal to 220R
or start with 100R on each emmitters neglect being adjustable when
diff trans are matched.......
mine operates up to plus/minus 100VDC!!! but I lowered it to 96vdc
to be safe for the caps.......(trafos are customized)
yup this is very functional, BUT I don't really funned of having up to
more than 220ohm of input diff degeneration resistors
unless you like some lousy bass! keep it less than or equal to 220R
or start with 100R on each emmitters neglect being adjustable when
diff trans are matched.......
mine operates up to plus/minus 100VDC!!! but I lowered it to 96vdc
to be safe for the caps.......(trafos are customized)
Hey!
Thanks for all the replies!!!
wg_ski
I just called the transformer producer and found out, that in Estonia it's cheaper to let the transformer be custom made, than to buy it from local electronics store. They asked 90$ for 600VA transformer. I think I can afford a 800 VA then. This gives me a lot of spare power
I got transistors as samples from ON Semi, I hope they are not a questionable source
woody
We have 230VAC mains.
nigelwright7557
I have speaker DC protection in main circuit, is that not enough? I'm thinking using only one LED on the front panel, so minimalist design. Volume pot will also go to rear panel. Now I have a Kenwood receiver for central unit. My next project is a DAC with DSP and an ATMega uC. They will have a display.
Tony
Thanks for recommendation.
hienrich
I don't get your point. I have 4 input diff degeneration resistors 50 ohms a piece, which makes 200 combined. That is lower than 220. Did I get it?
I you like to see pictures of my subwoofer and other speakers:
Pictures
Regards,
Aq
Thanks for all the replies!!!
wg_ski
I just called the transformer producer and found out, that in Estonia it's cheaper to let the transformer be custom made, than to buy it from local electronics store. They asked 90$ for 600VA transformer. I think I can afford a 800 VA then. This gives me a lot of spare power
I got transistors as samples from ON Semi, I hope they are not a questionable source
woody
We have 230VAC mains.
nigelwright7557
I have speaker DC protection in main circuit, is that not enough? I'm thinking using only one LED on the front panel, so minimalist design. Volume pot will also go to rear panel. Now I have a Kenwood receiver for central unit. My next project is a DAC with DSP and an ATMega uC. They will have a display.
Tony
Thanks for recommendation.
hienrich
I don't get your point. I have 4 input diff degeneration resistors 50 ohms a piece, which makes 200 combined. That is lower than 220. Did I get it?
I you like to see pictures of my subwoofer and other speakers:
Pictures
Regards,
Aq
no, it does not add up like that, rather the 50ohm in series with the emiiter is there to reduce the transconductance of the long-tail transistors somewhat to reduce gain and increase linearity of the diiferential pair....
your input differential transistors operate at collector currents of 2mA each, i guess 50 ohms is minimum, fyi, in the leach amps, 330ohms were used, my guess is anywhere from 47 to 470 ohms can be used here....
Hey!
I'll switch to 100 ohms resistors and use 220R pot paralleled with 1k resistors in the upper side of the mirror.
Some questions:
Is 2 mA per leg enough to achieve 300W on 84 volt rails (slew rate).
Why does mr Slone use 33p caps from collector of the VAS cascode transistor to the middle of feedback voltage divider. And is 33p enough for Miller compensation capacitor?
Aq
I'll switch to 100 ohms resistors and use 220R pot paralleled with 1k resistors in the upper side of the mirror.
Some questions:
Is 2 mA per leg enough to achieve 300W on 84 volt rails (slew rate).
Why does mr Slone use 33p caps from collector of the VAS cascode transistor to the middle of feedback voltage divider. And is 33p enough for Miller compensation capacitor?
Aq
gain of a transistor stage is gm*Rl, 1/gm of the transistor, re, is roughly 25/Ie(in mA) at 25*C.......so here you can see that a resistor in series with re reduces the gain of the stage, consistent with bandwidth and noise considerations...
Leach likewise used 2mA in the input transistors of his leach amps..
is this enough? i think so....
well, the VAS stage provides all the voltage swing required by the speakers, the input stage need only to drive the base of the VAS transistors...
the compensation caps C22,C34,C33,C26 provide for amplifier stability at high frequencies...
Hey!
I''l stick with this main schematic.
But there is some questions about other parts, to refresh:
Power
I have separated the main supply into two: low current one for input and VAS stage and other for output stage. The goal is to minimize noise from class B currents.
Questions:
* Is the divided supply idea worth using?
Soft Start
Questions:
*Is it a good idea to fuse second transormer's primary and secondary? When one of these fuses blows, soft start will be without a supply and main transformer is powered through resistors, that will overheat very quickly.
Preamp
Questions:
*Are the values for input filtering suitable?
*Should i create separate ground for signal and power. This way the signal ground should unite with main ground in the main amp. Otherwise there will be a ground loop (preamp - (signal ground) - main amp (ground) - power). Is it worth the concern?
Aq
I''l stick with this main schematic.
But there is some questions about other parts, to refresh:
Power
I have separated the main supply into two: low current one for input and VAS stage and other for output stage. The goal is to minimize noise from class B currents.
Questions:
* Is the divided supply idea worth using?
Soft Start
Questions:
*Is it a good idea to fuse second transormer's primary and secondary? When one of these fuses blows, soft start will be without a supply and main transformer is powered through resistors, that will overheat very quickly.
Preamp
Questions:
*Are the values for input filtering suitable?
*Should i create separate ground for signal and power. This way the signal ground should unite with main ground in the main amp. Otherwise there will be a ground loop (preamp - (signal ground) - main amp (ground) - power). Is it worth the concern?
An externally hosted image should be here but it was not working when we last tested it.
Aq
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.