Hi Hugh,
---Actually, I don't recommend a triple EF, but rather a buffer AFTER the VAS, then a double EF.---
A buffer after the VAS, loaded by a constant current source or bootstrapped
using only one more resistor and one more transistor, should ensure a better output bias compensation than a triple EF.
Isnt'it possible to have both : an input VAS buffer (as in the standard Blameless) and an output VAS buffer ? Then the output stage would always see a very low impedance at its input.
---Actually, I don't recommend a triple EF, but rather a buffer AFTER the VAS, then a double EF.---
A buffer after the VAS, loaded by a constant current source or bootstrapped
using only one more resistor and one more transistor, should ensure a better output bias compensation than a triple EF.
Isnt'it possible to have both : an input VAS buffer (as in the standard Blameless) and an output VAS buffer ? Then the output stage would always see a very low impedance at its input.
Carl
I would be tempted to replace Q8 (BC639) with a 2SC2240 , and reduce it's 22K collector resistor to 10K or perhaps even 4.7K
I used similar in an EF with an earlier version of my 15W SC Class A amplifier, and found that reducing this value markedly improved performance, as the English designer Graham Slee suggested it would. I originally used a collector resistor of 22K there, just as SC has done, for added protection to the EF.
Alex.
I would be tempted to replace Q8 (BC639) with a 2SC2240 , and reduce it's 22K collector resistor to 10K or perhaps even 4.7K
I used similar in an EF with an earlier version of my 15W SC Class A amplifier, and found that reducing this value markedly improved performance, as the English designer Graham Slee suggested it would. I originally used a collector resistor of 22K there, just as SC has done, for added protection to the EF.
Alex.
I can't see any harm in this, in fact the transistor is fast, and with a smaller collector resistor (could it be reduced to a link?) it will slightly speed things up.
However, the speed of the VAS is at the common emitter device itself, not the EF preceeding it. But every little thing helps.
The VAS transistor could be replaced with a 2SC3423, a Toshiba 200MHz device, MUCH faster than the BF philips device (60MHz), and this should bring marked improvement. BUT it might be necessary to slightly amend the miller cap.
Hope this helps,
Hugh
However, the speed of the VAS is at the common emitter device itself, not the EF preceeding it. But every little thing helps.
The VAS transistor could be replaced with a 2SC3423, a Toshiba 200MHz device, MUCH faster than the BF philips device (60MHz), and this should bring marked improvement. BUT it might be necessary to slightly amend the miller cap.
Hope this helps,
Hugh
Hugh
I eventually replaced the 22K in my original Class A with a link. However, I am now a bit wary after our friend from Sydney who worries about the sudden stop after a fall, managed to cause some damage after a trimpot in the CM went O/C during adjustment.That is also the reason for the suggested 470R across the 200R trimpot.
Alex
I eventually replaced the 22K in my original Class A with a link. However, I am now a bit wary after our friend from Sydney who worries about the sudden stop after a fall, managed to cause some damage after a trimpot in the CM went O/C during adjustment.That is also the reason for the suggested 470R across the 200R trimpot.
Alex
Alex,
AFAIK, a load in the collector and the emitter causes the transistor output to concertina, in the manner of a tube phase splitter. Fortunately, the voltage swings here are not large, of the order of millivolts only, but the effect in absolute terms would increase the so-called memory effect because chip dissipation would swing much more.
If the collector load is removed, then about 55V appears across the collector/emitter, and there is about four times less Vce variation. The emitter resistor is, from memory, around 2.2K, and the base bias would be around 200uA, so total current at idle would be around 480uA, giving a dissipation of 26.4mW. This is a trivial figure, nothing to it at all, and the device could easily go up to 150mW with reliability.
The reason a collector load was inserted was very likely to trim phase performance, which is important to stability. You could mess with it, but care would be needed to ensure the stability was unaffected. A CRO is really needed for this.
Cheers,
Hugh
AFAIK, a load in the collector and the emitter causes the transistor output to concertina, in the manner of a tube phase splitter. Fortunately, the voltage swings here are not large, of the order of millivolts only, but the effect in absolute terms would increase the so-called memory effect because chip dissipation would swing much more.
If the collector load is removed, then about 55V appears across the collector/emitter, and there is about four times less Vce variation. The emitter resistor is, from memory, around 2.2K, and the base bias would be around 200uA, so total current at idle would be around 480uA, giving a dissipation of 26.4mW. This is a trivial figure, nothing to it at all, and the device could easily go up to 150mW with reliability.
The reason a collector load was inserted was very likely to trim phase performance, which is important to stability. You could mess with it, but care would be needed to ensure the stability was unaffected. A CRO is really needed for this.
Cheers,
Hugh
Latest Schematic:
#1) Fixes the labels for Q1 & Q2 that somewhere along the line changed when I was adjusting the drawing.
#2) Q8 is now a 2SC2240 device and the collector resistor adjusted to 4K7 ohms.
#3) The fuses have been adjusted to reflect the layout of the PCB. In an actual build those fuses would be on the Power Supply PCB and not the amplifier board.
And what about the tempco diode string? Is as shown sufficient? Speak now or forever hold your peace.
I soon will be posting a new PCB layout based on this schematic. Going, going ...
#1) Fixes the labels for Q1 & Q2 that somewhere along the line changed when I was adjusting the drawing.
#2) Q8 is now a 2SC2240 device and the collector resistor adjusted to 4K7 ohms.
#3) The fuses have been adjusted to reflect the layout of the PCB. In an actual build those fuses would be on the Power Supply PCB and not the amplifier board.
And what about the tempco diode string? Is as shown sufficient? Speak now or forever hold your peace.
I soon will be posting a new PCB layout based on this schematic. Going, going ...
I have dealt with this company b4. Just thought you may like to have another supplier. You would need to specify everything about the board.
"Hot offer and Very Good prices for Multi-Layer PCB
Welcome to contact with me if you need Multi-LayerPCB. with good quality and very competitive prices!
I believe you will be satisfied with our service. Email me gerber and qty and then we will do all for you.
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Victor
Elecsound Electronics Co., Ltd.
Tel No : 0086 755 83045964
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Main Products: Printed Circuit Board, Tantalum capacitors, Film capacitors. Ceramic capacitors and Mono Capacitors"
ELECSOUND Victor
E-mail Address(es):
es.victorxu@gmail.com
I have never used them for PC boards.
Terry
"Hot offer and Very Good prices for Multi-Layer PCB
Welcome to contact with me if you need Multi-LayerPCB. with good quality and very competitive prices!
I believe you will be satisfied with our service. Email me gerber and qty and then we will do all for you.
Don't hesitate now pls!
Best regards
Victor
Elecsound Electronics Co., Ltd.
Tel No : 0086 755 83045964
Fax No.: 0755-8294 8027
Main Products: Printed Circuit Board, Tantalum capacitors, Film capacitors. Ceramic capacitors and Mono Capacitors"
ELECSOUND Victor
E-mail Address(es):
es.victorxu@gmail.com
I have never used them for PC boards.
Terry
I'm not sure how this will go down, but:
All speakers need some protection from infra bass. It stops the bass driver(s) from being asked to respond to freq lower than it can handle. Could we put a passive network at the input that can be used or not as the builder wants. This amp as shown has a lower cut off of 8 Hz. Not practicle to any low end driver.
I would still like to see the provision on the side of the board for a 4th pair of output devices should someone want to put them in.
Other comments.
Terry
All speakers need some protection from infra bass. It stops the bass driver(s) from being asked to respond to freq lower than it can handle. Could we put a passive network at the input that can be used or not as the builder wants. This amp as shown has a lower cut off of 8 Hz. Not practicle to any low end driver.
I would still like to see the provision on the side of the board for a 4th pair of output devices should someone want to put them in.
Other comments.
Terry
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