You have the book with all the relevant discussion on bias and every other important issue about audio power amplifiers. It would be sensible to read it - at least the sections relevant to class AB biasing and thermal stability.
The setting of the pot does not change stability, stability depends on the bias controller circuit design and the temperature coefficient which may not be linear and hence liable to instability when set at a different current than factory setting. Anyway, it seems odd that the amplifier had been set so low...as in no bias. Now how could that have occurred?
Just remember that when your ears lead you to believe that highest possible bias is the best setting, that your amplifier was designed for minimal class AB bias. Like we say, if you are radically altering the bias, make sure it remains stable over hours and with much heavier than idle loading. If you don't check operation at extremes more thoroughly, it may easily go into rapid thermal runaway when you hit it with loud music and fry your output stage, taking out those obsolete and unobtanium Toshiba transistors and their drivers.
Won't that be nice to repair!
You mean the VT that is 26mV @25 deg C? I never even relate to that. I choose 25mV so I get 50mA per transistor. 50mA sounds like a good number!!! I don't listen that loud, 200mA into 4 ohm is 0.8V peak or 1.6V peak peak running as Class A. That's where I came up with 25mV. It did seems to improve the sound.
I finished both channels and put it back in. It is definitely an improvement. Yesterday, I only have one channel modded and I had to listen one side at a time, I heard slight improvement. I used it like that last night only with one modded channel.
Today, I finish modding the second channel and put it in. I compare with just one channel modded vs both channel modded. It's quite a big difference. You really have to have both match to tell. I went back and fore, definitely improved if I have both channel modded.
Over all, it's more than I expected from the beginning of this project. I did not expect this big an improvement. All I did are:
1) Beefed up the ground of the input stage.
2) Double up the output wires.
3) Separate the input and pre driver rail by 50ohm and add filtering caps of 100uF//10uF//0.1uF for each rail.
4) Add 10uF//0.1uF on the power transistor rail.
5) Add 10uF//0.1uF across D3 and D6.
6) Add 2uF across C12 to increase the capacitance.
7) Increase idle current from 5mA to 50mA per power transistor complementary pair.
I decided not to touch Q1 and Q8. I am super happy. It's a significant step better. I would say the bias current improve by 1/3 and the mod still the major part of the improvement. The amp does get warmer. But it's worth while. Just love it. I paid about $1000 for the amp in 97. I would say the mod worth $500!!!! I am stoked.
Thanks for all the help
Today, I finish modding the second channel and put it in. I compare with just one channel modded vs both channel modded. It's quite a big difference. You really have to have both match to tell. I went back and fore, definitely improved if I have both channel modded.
Over all, it's more than I expected from the beginning of this project. I did not expect this big an improvement. All I did are:
1) Beefed up the ground of the input stage.
2) Double up the output wires.
3) Separate the input and pre driver rail by 50ohm and add filtering caps of 100uF//10uF//0.1uF for each rail.
4) Add 10uF//0.1uF on the power transistor rail.
5) Add 10uF//0.1uF across D3 and D6.
6) Add 2uF across C12 to increase the capacitance.
7) Increase idle current from 5mA to 50mA per power transistor complementary pair.
I decided not to touch Q1 and Q8. I am super happy. It's a significant step better. I would say the bias current improve by 1/3 and the mod still the major part of the improvement. The amp does get warmer. But it's worth while. Just love it. I paid about $1000 for the amp in 97. I would say the mod worth $500!!!! I am stoked.
Thanks for all the help
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Thanks.
How about more filter caps on the front end? I have 100uF//10uF//0.1uF through a 50 ohm. the break freq is 30Hz. What if I double up the caps?
Another thing, both outputs has 8mV offset. That will force 0.008V/4ohm=2mA through the woofer. does that affect the sound like DC current through the primary of the output transformer of tube amp.
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Thanks
The rectifier in the amp is chassis mount, it's quite big. It should be something like 20A. I saw a 200V 40A bridge rectifier on ebay, you think that's big enough?
"I've never blown a rectifier myself and I've put upwards of 100,000uF into my latest amp. "
I've seen plenty blow, especially with caps over about 30,000µF. Depends upon the transformer VA as well.
Big bridge rectifiers just have better heatsink construction, they generally have the same die inside as a 6A axial diode, so the maximum surge currents max out around 400A or so.
"both outputs has 8mV offset"
I don't get excited until it goes over 100mV or so.
I've seen plenty blow, especially with caps over about 30,000µF. Depends upon the transformer VA as well.
Big bridge rectifiers just have better heatsink construction, they generally have the same die inside as a 6A axial diode, so the maximum surge currents max out around 400A or so.
"both outputs has 8mV offset"
I don't get excited until it goes over 100mV or so.
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Hi guys, I'm back again!!!
Now that I settled in with the mode, so far, I have been playing it very safe. I only beefed up the filtering and increase the bias, totally non invasive. I am curious on some not so "non invasive", but not major major surgery just yet!!! So I am coming back for advice again.
Hi DJK, I know you advocated for resistor for long tail pair, Can I just put a resistor from the emitter of the LTP straight to the rail? That would be very simple.
Also, I have been reading some old threads on Acurus, it was suggested to up the tail current of the LTP input stage. Does that help?
What other simpler mode you guys can suggest will be appreciated.
Last question, Acurus is a middle of the road amp, any potential to be transformed into a great amp? Please don't say it's not worth it to modify an Acurus just yet, I already own this, not buying one to mod. So any cheaper way of mod is worth it to me.
Thanks
Now that I settled in with the mode, so far, I have been playing it very safe. I only beefed up the filtering and increase the bias, totally non invasive. I am curious on some not so "non invasive", but not major major surgery just yet!!! So I am coming back for advice again.
Hi DJK, I know you advocated for resistor for long tail pair, Can I just put a resistor from the emitter of the LTP straight to the rail? That would be very simple.
Also, I have been reading some old threads on Acurus, it was suggested to up the tail current of the LTP input stage. Does that help?
What other simpler mode you guys can suggest will be appreciated.
Last question, Acurus is a middle of the road amp, any potential to be transformed into a great amp? Please don't say it's not worth it to modify an Acurus just yet, I already own this, not buying one to mod. So any cheaper way of mod is worth it to me.
Thanks
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