Checking final transistors
Hello MIKE,
I' nearly finish 10 pcs of symasym amplifier .
Now i'check beta of the output transistors and drivers i' have some disparate ratio .
Does i' need to combinate low ratio driver with high ratio output transitor together?
To keep the same balance of total gain between positive and negative supply ?
Many Thanks...
JC BALTHAZARD
Hello MIKE,
I' nearly finish 10 pcs of symasym amplifier .
Now i'check beta of the output transistors and drivers i' have some disparate ratio .
Does i' need to combinate low ratio driver with high ratio output transitor together?
To keep the same balance of total gain between positive and negative supply ?
Many Thanks...
JC BALTHAZARD
Hi all,
I'm interresting by the explendid amplifier so that's why i'm here.
I see Pavel Macura use two transformer of 300 VA but for me that's too enough 'cause this amplifier is a class AB so his efficiency is around 75 % therefore the suitable transformer, to my mind, is around 80 VA and 120 VA by channel. I think the transformer are bigger for unknown reasons for me. Could you explain to me why choose a so gigantisc power stage ?
Tnanks a lot.
I'm interresting by the explendid amplifier so that's why i'm here.
I see Pavel Macura use two transformer of 300 VA but for me that's too enough 'cause this amplifier is a class AB so his efficiency is around 75 % therefore the suitable transformer, to my mind, is around 80 VA and 120 VA by channel. I think the transformer are bigger for unknown reasons for me. Could you explain to me why choose a so gigantisc power stage ?
Tnanks a lot.
Hi ECS21, it's not a simple change to increase powerhandling of the symasym, it's possible to get it operational at these voltages, but the quality will be reduced because of the quite low vas current which can't be easily increased as symasym uses to92 transistors here...
I had plans for a higher power version, but didn't get to do it yet...
MaxS, for the quality it's always better to size the PSU slightly bigger, but if you drive 8ohms 2x120VA is enough. For 4 ohms it might be bigger, depending on your listening levels. If you typically listen at "normal" levels, the 2x120va is also suitable for 4ohms.
I use one 250VA (2x25v AC) plus 2x22000uF for both channels to drive my 4ohm speakers, it does not sound weak in any way and brings up my neighbors against me...
Mike
I had plans for a higher power version, but didn't get to do it yet...
MaxS, for the quality it's always better to size the PSU slightly bigger, but if you drive 8ohms 2x120VA is enough. For 4 ohms it might be bigger, depending on your listening levels. If you typically listen at "normal" levels, the 2x120va is also suitable for 4ohms.
I use one 250VA (2x25v AC) plus 2x22000uF for both channels to drive my 4ohm speakers, it does not sound weak in any way and brings up my neighbors against me...
Mike
MikeB said:
Hi ECS21,
Looks like Mike designed it for the above specs.
The amp would probably work OK with slightly higher rails but a designer has to allow for a margin of error for:
- 4, 6 or 8 ohm speakers
- dips in speaker impedance
- unusal speaker cables
- mains fluctuations of +-10%
- transformer regulation
- other things ???
Also, it is nice to design an amp around popular, easy to get components like a 25-0-25 transformer.
So the answer of your question really depends on your particular system. To be safe I'd use the recommended voltages of the designer.
BTW: You will probably not notice any improvement if you do actually increase the voltage by a few volts.
regards
MikeB said:
Follow up on this issue as posted a few pages back.. I replaced the bridges with 100v 25amp units, which should be fine. Still got some faint radio reception. Then I put 0.01uf ceramics across each leg of the bridge. No more radio. I still pick up some buzz on a cheap AM radio if I hold the antenna right above the amp. I don't get this with my previous two Syms, but I don't think it's a problem. The buzz stops immediately when I switch off, so It's not oscillation (which would continue for a while). Maybe this toroid is a little leaky compared to the others. Anyway, the amp sounds just fine. All systems go. I'll put together the second stereo amp this week and compare.
Sheldon
Oscillating 5.5Mhz
Hello Mike,
I finish the 10 pcs of symasym.
Two pcs was not working at the first time du to soldering bridge.
Now the ten are working well.
The more bad one in dc offset is 2.8mv the best 0.1mv very good result .
I' bought 150 pcs of smal signal transitor to make the selection in hfe.
I' also mix driver hfe and output device hfe to get average hfe for the couple of these power transistors.
I'put relay to change polarisation resistor between 40ma and 500ma. This function work well without important noise when actionning the relay.It's a very good point.
I'put DC coil in place of resistor R31 R32
I use fostex and td 2001 driver with sensibility of 106 and 109 db/w and noise is very low we can hear at 0,6 meter only from speaker.
The amplifier noise is very good compare to the other amp that i' can hear at 7 meter ! (luxman m03) .
In my configuration active crossover filter,each amplifier have different input an feedback capacitor to put the best choice thechnology and reduce the price.
I'take 10 times the frequency cut low pass frequence.
example the medium is working from 800 hz to 8 khz the frequency cut is 80hz in input cap and feedback cap .
The power suply is set to 37 v in stand by condition ( 40ma * 5 pcs )fall to 34 v at 500ma * 5pcs .
Iuse 3*300va transformer by chanel in tree phases star /delta configuration .
Tree phases bridge rectifier 25A /200V and then 2*47000 µf /40v decoupling by 3.3µf on each polarity.
The ondulation is 40mv at 7 A and 3mv at 40ma very good also!
The voltage droop to 28v at 7A.
I'have noticed an oscillation about 5.5Mhz on 3 amplifiers if the current is setting to 500 ma.
I know that Pavel also had this problem. I' will change the capacitor from 10pf to 4.7pf or 3.3pf according to his conclusion.
May be next week i will report the result ....
I'have also noticed that if current is setting to 500ma if i put small modulation the current of tha amplifier is changing according to the speaker current.
My believe was that the current in class A operation must be stable if speaker current is under amplifier current...
I' have to study it !
JC BALTHAZARD
Hello Mike,
I finish the 10 pcs of symasym.
Two pcs was not working at the first time du to soldering bridge.
Now the ten are working well.
The more bad one in dc offset is 2.8mv the best 0.1mv very good result .
I' bought 150 pcs of smal signal transitor to make the selection in hfe.
I' also mix driver hfe and output device hfe to get average hfe for the couple of these power transistors.
I'put relay to change polarisation resistor between 40ma and 500ma. This function work well without important noise when actionning the relay.It's a very good point.
I'put DC coil in place of resistor R31 R32
I use fostex and td 2001 driver with sensibility of 106 and 109 db/w and noise is very low we can hear at 0,6 meter only from speaker.
The amplifier noise is very good compare to the other amp that i' can hear at 7 meter ! (luxman m03) .
In my configuration active crossover filter,each amplifier have different input an feedback capacitor to put the best choice thechnology and reduce the price.
I'take 10 times the frequency cut low pass frequence.
example the medium is working from 800 hz to 8 khz the frequency cut is 80hz in input cap and feedback cap .
The power suply is set to 37 v in stand by condition ( 40ma * 5 pcs )fall to 34 v at 500ma * 5pcs .
Iuse 3*300va transformer by chanel in tree phases star /delta configuration .
Tree phases bridge rectifier 25A /200V and then 2*47000 µf /40v decoupling by 3.3µf on each polarity.
The ondulation is 40mv at 7 A and 3mv at 40ma very good also!
The voltage droop to 28v at 7A.
I'have noticed an oscillation about 5.5Mhz on 3 amplifiers if the current is setting to 500 ma.
I know that Pavel also had this problem. I' will change the capacitor from 10pf to 4.7pf or 3.3pf according to his conclusion.
May be next week i will report the result ....
I'have also noticed that if current is setting to 500ma if i put small modulation the current of tha amplifier is changing according to the speaker current.
My believe was that the current in class A operation must be stable if speaker current is under amplifier current...
I' have to study it !
JC BALTHAZARD
Hi Balthazard, have you tried changing r31/32 back to 47ohms instead of the DC-coils to check if these could cause the oscillations ?
Are you using the Toshibas as output devices ? Which devices are the drivers ?
About the ClassA current, that is normal, only a H-Bridge-ClassA has constant powerconsumption...
Mike
Are you using the Toshibas as output devices ? Which devices are the drivers ?
About the ClassA current, that is normal, only a H-Bridge-ClassA has constant powerconsumption...
Mike
Hi Pierre, i have not forgotten the high power symasym, i did not get into it yet. I know i have promised it... 
I can't give any definitive schedule, it's hobby not work...
How much power would you need ?
Pavel, do you use several symasyms or a mixture of different amps ?
balthazard, about the stability problems, you can also simply reduce openloopgain of the symasym by adding 22ohms REs to the inputdiffamp, these were reserved on the 5.3 PCB.
Mike
I can't give any definitive schedule, it's hobby not work...
How much power would you need ?
Pavel, do you use several symasyms or a mixture of different amps ?
balthazard, about the stability problems, you can also simply reduce openloopgain of the symasym by adding 22ohms REs to the inputdiffamp, these were reserved on the 5.3 PCB.
Mike