| johnny1 |
Its a douglas Self design.
I beleive you know the schematic but i will try to upload it. |
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| johnny1 |
Its actually two monoblocs with 1KVA trans, and 2x100.000uF/80V (United chemicon)
Here is a pic of what it looks like. Sorry for quality.
One monoblock shown here.
The second one will be ready next week |
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| johnny1 |
| This is the Soft start |
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| TomWaits |
Looks a little lite on the Heatsink side of things but looking good.
Shawn. |
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| CBS240 |
Hmm
Somewhere I have seen this circuit but I can't quite place it....I just wonder what would happen to the bias of Q3 if Q6 were to saturate. Perhaps a seperate bias for Q3 and Q6 would make this a non-issue?
Just curious, what would be the difference if you replaced Q8 with a PNP type, use Q5 as the source and Q4 as the mirror, drive the emitter of Q7 with the emitter of Q8(PNP) as a common base VAS instead of a cascode VAS? I have done this and it sounds great, may reduce miller effect but I haven't built a casode VAS to compare. Any thoughts? |
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| johnny1 |
| quote: | Originally posted by CBS240
Hmm
Somewhere I have seen this circuit but I can't quite place it....I just wonder what would happen to the bias of Q3 if Q6 were to saturate. Perhaps a seperate bias for Q3 and Q6 would make this a non-issue? |
Q6 never saturates. That's the reason R1 and R12 have different values. The main idea is that Q3 and Q6 should have the same bias.
| quote: | Originally posted by CBS240
Hmm
Just curious, what would be the difference if you replaced Q8 with a PNP type, use Q5 as the source and Q4 as the mirror, drive the emitter of Q7 with the emitter of Q8(PNP) as a common base VAS instead of a cascode VAS? I have done this and it sounds great, may reduce miller effect but I haven't built a casode VAS to compare. Any thoughts? |
It would be a different design !
:rolleyes:
haven't tried that, so i can not comment.
But, you should always take into considaration the absolute phase of the signal (0 deg, or 180deg.) so, you won't end up with possitive feedback. |
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| johnny1 |
| quote: | Originally posted by TomWaits
Looks a little lite on the Heatsink side of things but looking good.
Shawn. |
As long as the quiescent current remains below 80mA per transistor it's fine (already tested....). |
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| johnny1 |
and some specs:
145W @8 Ohms (20Hz to 20KHz)
268W @4 Ohms (20Hz to 20KHz)
The transf. has a 2x40.5V secondary, and the voltage on the caps is 54.9V with the amp idling.
At full power the voltage on the caps drops to 53V @8 Ohms and to 52.4V @4 Ohms.
It's the benefits of Overengineering!:worship: |
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| Leolabs |
| Hi Johnny1!Have you match the output BJTs??? |
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| johnny1 |
| quote: | Originally posted by Leolabs
Hi Johnny1!Have you match the output BJTs??? |
Of course i have them matched!
Not only the output BJT's but also all the complementary pairs.
The differential pair and the current mirror BJT's are also matced pairs. |
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| Onra |
Hi,
are you sure about the position of the main switch in the soft start schematic?
How do you manage to switch off?
Regards Onra |
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| johnny1 |
| quote: | Originally posted by Onra
Hi,
are you sure about the position of the main switch in the soft start schematic?
How do you manage to switch off?
Regards Onra |
S3 and S4 switch on or off simultaneously.
The timer and the bypass relay WILL turn off, because the Neutral side is "open" by S3 |
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| jmateus |
Hi Johnny 1
Is there a layout for this amplifier?
It looks interesting, I'd like to build it.
Thanks. |
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| jaycee |
| I've simulated similar and found out that you can make Q6 saturate quite easily by driving the amp into clipping. This then makes Q6 steal enough current through the base to keep Q3 from working. A simple cure with little drawback is to put a resistor in the base of Q6 to limit it. Try 470R |
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| MikeB |
I can't resist... A few comments as the amp has some minor flaws.
1, as jaycee already said, a basestopper to q6 is mandatory, it's sad how many published schematics forgot this one. Leaving it out can destroy your speakers...
2, Connect r7 to the other side of r8, this reduces DC-offset as the amps gets better balanced.
3, c8 must be a typo, you can't really put a permanent capacitive load to the amp ? This will ask for resonances in combination with L1.
L1 = 7uH ? Very big ! Use a much smaller value here (<1uH) and place a Zobel on the other side of the coil, skip c8...
4, a 2 diode ccs is nearly the worst ccs possible, consider a zener or led here. (or replace r4 with another ccs)
Mike |
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| jmateus |
Hello gents
This is the very SAME amplifier published by Silicon Chip in 2002, called
by them as Ultra_D 100W amplifier...
Very minor differences, for example CCS is done with another BC556
instead of two diodes or LED. There is also no base stopper on Q6
and the cap at the output loading the speaker is the same. The coil
L1 is 6.8 uH not too distant from 7 on this schema.
The BIG difference (This is a laugh) is the upper output transistors
on Ultra-D schema are PNP (MJL1302) the lower ones NPN. How
is this for a difference?
Needless to say the collectors of both pairs of outputs are tied up
to (each two) 4 common 1.5 ohm resistors.
I asked for the layout to Johnny 1 but I don't need it, I have it
from the Ultra-D, thanks. |
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| MikeB |
Sounds like they used a Sziklai/CFP ? Can you show schematic ?
I don't think the amplifier shown is bad, i just think that resolving the 4 issues i mentioned makes it a very recommendable one. (unlike the C200, damn it, i coudn't resist again... :D)
Mike |
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| jmateus |
| Here is the schematic, Mike |
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| ilimzn |
| ...output transistors in parallel without emitter resistors? Oh dear... |
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| MikeB |
Ah, CFP ! But as ilimzn pointed out, you can't really paralell output devices without REs. You need to add REs, or use an extra cfp-driver for each output-device. Also, most CFPs need a smallish b-c cap on the drivers to be stable.
Mike |
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| johnny1 |
| quote: | Originally posted by MikeB
I can't resist... A few comments as the amp has some minor flaws.
1, as jaycee already said, a basestopper to q6 is mandatory, it's sad how many published schematics forgot this one. Leaving it out can destroy your speakers...Will check it out
2, Connect r7 to the other side of r8, this reduces DC-offset as the amps gets better balanced. You are right. This is the correct topology. Nevertheless, the amp has only 3mV ofsset
3, c8 must be a typo, you can't really put a permanent capacitive load to the amp ? This will ask for resonances in combination with L1. Well, it is not! This cap is usually in series with an 22 Ohm resistor
L1 = 7uH ? Very big ! Use a much smaller value here (<1uH) and place a Zobel on the other side of the coil, skip c8...
The actuall design had 6.8uH. I put 7uH cause i couldn't find Jensen coil with that value. With these values, the -3dB limit is at 70KHz with a 4 Ohm load. I know, it's quite low, haven't decided if this is the final design.
4, a 2 diode ccs is nearly the worst ccs possible, consider a zener or led here. (or replace r4 with another ccs)
Mike |
Mike and jaycee,
I really apriciate your comments.
Please find my answers in red.
Before I decide to built this amp I had auditioned it (made by a friend 2 years ago) and like it very much.
The only flaw i found on the bench, is that the negative side clips 1 Volt earlier than the positive. |
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| johnny1 |
| quote: | Originally posted by jmateus
I asked for the layout to Johnny 1 but I don't need it, I have it
from the Ultra-D, thanks. |
Glad you found it, cause i don't have it in electronic form. |
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| jmateus |
Here are some transcriptions from the description of the amplifier:
"The four paralleled 1.5 ohm emitter (?) resistors for each CFB transistor pair are there to help to stabilise the quiescent current.
They also slightly improve the frequency response of the output stage by adding local current feedback."
"Note however, that there are no intrinsic means in the circuit for ensuring even current sharing between Q13 and Q114 and between Q15 and Q16. What current current sharing there is will depend on the inherent matching (or lack of it) between the transistors"
And finally:
"By the way, we did try the effect of small emitter resistors for each of the power transistors but these had the effect of worsening the distortion performance - so we left them out. Note that the current
and power ratings of the output transistors are such that even if the current sharing is quite poor, it won't cause any problems."
There are some other considerations about the output coil and capacitor serving as a load, but I think they are of no interest. We
all agree (I think) that the resistor in series with the cap will be
the way to go. As per the size of the coil, that depends of the taste.
There are some discrepancies about the assertions on this description one being calling the 1.5ohm resistors "emitter" resistors
and then stating they tried emitter resistors but they worsened
the frequency response. A bit odd, don't you think?
In any case, if I decided to build this amp, I'm not sure if I should
consider some modifications proposed by this thread.
I mentioned these as a way to cause some reaction and clarify
what could be the modifications to implement.
One thing that comes to mind is to invert the position of the
outputs, placing the NPN's on the top part of the amp and the
PNP's on the lower part. This way they will have the emitter
resistors (4x 1.5 ohm) tyied to the emitters...Would this be a valid
way to do it? I think so..... |
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| ilimzn |
[/quote]There are some other considerations about the output coil and capacitor serving as a load, but I think they are of no interest. We all agree (I think) that the resistor in series with the cap will be the way to go.| quote: |
Definitely. A relatively low resistance (10 ohms or less) will do fine here.
[quote]
As per the size of the coil, that depends of the taste.
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Well, not only on taste - a large L will limit the slew rate for low impedance loads - in fact, it is even possible it gets as low enough to affect the audio band. This is often missed with current dumping configurations like Quad 405 and similar!
| quote: |
There are some discrepancies about the assertions on this description one being calling the 1.5ohm resistors "emitter" resistors and then stating they tried emitter resistors but they worsened the frequency response. A bit odd, don't you think?
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No, no discrepancy - excpt they mentioned it worsened distortion.
A CFP behaves like a BJT polarised according to the driver (first) transistor in the CFP, with much higher gain. In that sense, the emitter of the 'driver' is also the collective emitter as far as the circuit is concerned, regardless of the fact that it is acyually the collector of the output transistor that connects to that point. Hence, the term 'emitter resistor' is correct.
The second point could be debated. On one side, current sharing between multi-emitter (RET, LAPT etc) transistors is unexpectedly good even without outside emitter resistors, compared to regular paralleled transistors only. This is because these transistor topologies have a sort of built-in emitter resistor. This also means that you can get very good current sharing by using relatively small emitter resistors (0.1 ohm). In theory, this would reduce OLG of the CFP since it is emitter degeneration, but improve linearity, so it is difficult to use a blanket statement that it would increase distortion. WHat does change, is biasing: such a CFP will need a different bias current for minimum distortion, compared to the one without emitter resistors in the outputs.
Keep in mind, however, that this does NOT mean the collective CFP 'emitter' resistors can be removed!!! (the 4x1.5 ohms). From outside of the CFP, the emitters of the output transistors are 'invisible', hidden in the l100% ocal NFB loop of the CFP itself.
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One thing that comes to mind is to invert the position of the outputs, placing the NPN's on the top part of the amp and the PNP's on the lower part. This way they will have the emitter resistors (4x 1.5 ohm) tyied to the emitters...Would this be a valid way to do it? I think so..... |
If by that you think of modifying the output from CFB pairs to EF 'darlington' (which of course means a different topolofgy, not just switching BJTs around, as the latter will not work!), it is certainly possible. That being said, the bias servo transistor then needs to be mounted on the heatsink of the output transistrs, whereas with the CFP it does NOT, but needs to be monted on a common heatsink with the drivers - unless the drivers and outputs are on a common heatsink. You may also need to modify the values of the resistors in the Vbe multiplier around the bias servo transistor, because CFP needs 2xVbe plus voltage drop on the Re's, while EF needs 4xVbe plus drop on Re's. |
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| jaycee |
The CFP while good for an amp that only needs a single pair of output transistors, is a nightmare when you need more than one. I much prefer the triple darlington in that situation.
The usual output LC is 10 ohms 2W, and then wind your inductor around the body of the resistor with enamelled wire. I don't know what inductance that is exactly (around 2uH i think) but it's worked well enough for me.
C8 on it's own has to be wrong - it looks like it should have formed a Zobel with a 10 ohm 1W resistor in series. While you see a lot of amps where the Zobel is before the LC network, you do see them after it too (usually on the speaker terminals, as recommended by Leach).
(edit - i read that you worked that out.. need more sleep..)
This schematic reminds me of an Elektor amp.. I can't recall the name of it at the moment, but it seemed very similar. |
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| johnny1 |
You were right about Q6. It saturates when the amp is close to clipping.
A base stoper helped it.
After some simulations, I'm considering to run the earlier stages (up to the Vbe Multip.) with a different supply with +5V higher rails (around 58 to 60Volts).
What do you think? |
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| jaycee |
| If this was a MOSFET amp it would be worth it. For BJT's it's not really worth it, unless you were planning to use a seperated, regulated voltage. I personally wouldn't bother. |
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| johnny1 |
A separate 200VA toroid with 2x10.000uF per channel is what i have in mind.......:D
I believe that regulation is going to "slow down" the amp |
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