Hi Tony,
Actually, it's failsafe!
If VR1 fails, the connection is broken between base and emitter and the collector resistor, which is fixed, turns on the Vbe multiplier hard, pulling its Vce back to saturation. This actually turns off the amp output stage, protects your speakers, merely sounds ordinary since zero bias.
No, this is a well sorted design with LOTS of power but it's an ultra conventional circuit with all that implies. The pcb layout, from memory, is very good, neat and elegant.
Cheers,
Hugh
Actually, it's failsafe!
If VR1 fails, the connection is broken between base and emitter and the collector resistor, which is fixed, turns on the Vbe multiplier hard, pulling its Vce back to saturation. This actually turns off the amp output stage, protects your speakers, merely sounds ordinary since zero bias.
No, this is a well sorted design with LOTS of power but it's an ultra conventional circuit with all that implies. The pcb layout, from memory, is very good, neat and elegant.
Cheers,
Hugh
Hi Tony,
I don't believe so. If the Vbe goes into saturation it does not turn off the controlling LTP of the amp.
Even if there is no bias at the output there will always be some leakage current through the outputs - these are large devices - and the feedback transistor will always monitor the offset and keep it under control.
I have biased up plenty of amps from zero quiescent and always been surprised at how effectively the LTP maintains offset.
How's that big LPG project coming along? Mr President's next weapon?
Cheers,
Hugh
I don't believe so. If the Vbe goes into saturation it does not turn off the controlling LTP of the amp.
Even if there is no bias at the output there will always be some leakage current through the outputs - these are large devices - and the feedback transistor will always monitor the offset and keep it under control.
I have biased up plenty of amps from zero quiescent and always been surprised at how effectively the LTP maintains offset.
How's that big LPG project coming along? Mr President's next weapon?
Cheers,
Hugh
Hey all,
The only thing I had to add is a capacitor to cancel any oscillation. The amps work very well and what you can't see is Q1,2 and 4,5 are connected, thermally. They are in close proximity and joined with heatsink grease. Another thing I did was add wire to all of the power parts of the PCB. This gave me 80Watts extra power. I actually use 90V rails. The to-220 packs are mounted on larger heat sinks as they get hotter than I like. They got too hot even with lower voltage rails. The PCB's were bought from RCS Radio in Australia. I bought a pair for 50 US dollars and that included shipping to the US. Not bad, in my opinion. NO, these amps aren't cheap to build but they are extremely quiet at low volume levels (no hiss) I got my outputs and drivers, for one of them from ON Semiconductor for free. They were samples.
The power transformer was scrapped from a Peavey 1000 amplifier and I got it for 70 US dollars. I see folks talking about high price. My rebuttal to that is ( nothing good is free or cheap) I don't believe that you could buy an amp like this NEARLY as cheap as you can build one, especially if you are a good scavenger. I do use a fan to cool them but since I don't drive them all that hard, I don't worry about it much. It is a great design and a great performer.... period. I get 612W RMS into a 4 ohm resistive load with a sine wave input. That is with 90V rails!!! If I add 3 or 4 more outputs per rail, I surly will see 1000W into 4 ohm loads.
Otter
The only thing I had to add is a capacitor to cancel any oscillation. The amps work very well and what you can't see is Q1,2 and 4,5 are connected, thermally. They are in close proximity and joined with heatsink grease. Another thing I did was add wire to all of the power parts of the PCB. This gave me 80Watts extra power. I actually use 90V rails. The to-220 packs are mounted on larger heat sinks as they get hotter than I like. They got too hot even with lower voltage rails. The PCB's were bought from RCS Radio in Australia. I bought a pair for 50 US dollars and that included shipping to the US. Not bad, in my opinion. NO, these amps aren't cheap to build but they are extremely quiet at low volume levels (no hiss) I got my outputs and drivers, for one of them from ON Semiconductor for free. They were samples.
The power transformer was scrapped from a Peavey 1000 amplifier and I got it for 70 US dollars. I see folks talking about high price. My rebuttal to that is ( nothing good is free or cheap) I don't believe that you could buy an amp like this NEARLY as cheap as you can build one, especially if you are a good scavenger. I do use a fan to cool them but since I don't drive them all that hard, I don't worry about it much. It is a great design and a great performer.... period. I get 612W RMS into a 4 ohm resistive load with a sine wave input. That is with 90V rails!!! If I add 3 or 4 more outputs per rail, I surly will see 1000W into 4 ohm loads. Otter
no,Ole_Otter said:
612W into 4r0 sends 70Vpk to the load. That shows that supply droop and resistive losses add up to about 20V when delivering maximum power.
1000W into 4r0 requires a drive voltage of 89.4Vpk to the load.
How does adding 3 or 4 pair of transistors reduce the losses from 20V to 0.6V?
It does not equate.
What extra devices will allow is higher current into a demanding reactive load. Your 4ohm speakers may well sound better if extra devices were added, but the original & extra devices would all need to be matched.
Tony said:
Hi Tony, Hugh,
Just looking at the replies, I wonder if Hugh has missed the fact that you're talking about VR1 on the input LTP? His reply seems to suggest that he's refering to VR2 on the Vbe multiplyer (hence the comments about it being failsafe, which is certainly true for VR2).
Hugh, I could be wrong
Tony, without thinking too hard about it, I tend to agree with your analysis in post #24
Quite why the OP needs 500W is still a mystery to me. Perhaps he's swallowed Musical Fidelity's latest marketing!
Mark
yes, i think Hugh may have overlooked....
way i look at it, Q1 and Q2 are biased from CCS formed by Q3 sourcing 7mA to the LTP Q1 and Q2. Current mirror pair Q4 and Q5 therefore are at 3.5mA each.
VAS formed by cascoded Q7 and Q8 with a CCS load supplied by Q6 operating at a collector current of about 28mA, quiscient dissipation then is about 2.2watts, and the VAS surely needs to be heatsinked!
now, what will happen if the wiper arm of VR1 got corroded and lost contact? the LTP will lose current, Q7 will be cut-off and Q6 still supplies current to the base of Q10, and your speaker surely smokes as a result.
now wrt power, assuming that the 80 volts rails remained steady(without voltage drop) the VAS can swing about 140 volts peak to peak or 49volts rms which translates to 600watts into 4 ohms...
so now depending on the size and weight of your transformer, 500w into 4 ohm is a realistic expectation...
Hi AndrewT,
You may be right about the power supply droop and more filtering caps would help for surge loads such as an explosion on a movie BUT adding output devices allows for more power to be used as each transistor has emitter resistance and it DOES have noticable effects. Like I said before, I added extra wire to the power traces on the board and got an extra 80W, and this was measured on a scope! The power transformer that I am using came from a 2X1000W Peavey amplifier so I know it will deliver the power for such cases. I guess my question would be... If I add more output transistors, how much more quiescent current would I need? With respect,
Otter
You may be right about the power supply droop and more filtering caps would help for surge loads such as an explosion on a movie BUT adding output devices allows for more power to be used as each transistor has emitter resistance and it DOES have noticable effects. Like I said before, I added extra wire to the power traces on the board and got an extra 80W, and this was measured on a scope! The power transformer that I am using came from a 2X1000W Peavey amplifier so I know it will deliver the power for such cases. I guess my question would be... If I add more output transistors, how much more quiescent current would I need? With respect,
Otter
Hi Mark,
Why do I need 500W? Well, actually, I don't NEED 500W. Call me crazy, call me stupid, but I'm a believer in "head-room" I know that I am "wasting" these amps by using them as sub amps but moving air requires a lot of power. At the time I built these amps, I was working for a pro audio shop and repairing a lot of amps with many output transistors. I wanted a much better understanding about these amps so I found a project that suited my curiosity and desire to learn. I learned much during this project but I have so very much more to learn. I also wanted a very good sounding amp. Many commercial sub amps sound like crap, even with a good speaker/driver...The NEED for over 1000W of power is NOT there, however, I do not regret building or using these awesome amps. With respect,
Otter
Why do I need 500W? Well, actually, I don't NEED 500W. Call me crazy, call me stupid, but I'm a believer in "head-room" I know that I am "wasting" these amps by using them as sub amps but moving air requires a lot of power. At the time I built these amps, I was working for a pro audio shop and repairing a lot of amps with many output transistors. I wanted a much better understanding about these amps so I found a project that suited my curiosity and desire to learn. I learned much during this project but I have so very much more to learn.
I also wanted a very good sounding amp. Many commercial sub amps sound like crap, even with a good speaker/driver...The NEED for over 1000W of power is NOT there, however, I do not regret building or using these awesome amps. With respect,Otter
juneray,
naa paka diha bay?
what is the purpose of your amp?is it for PA or for subs?
try this one...
http://dx.master.akadns.de/projekte...p?folder=/ZETA/
naa paka diha bay?
what is the purpose of your amp?is it for PA or for subs?
try this one...
http://dx.master.akadns.de/projekte...p?folder=/ZETA/
Ole_Otter said:Hi Mark,
Why do I need 500W? Well, actually, I don't NEED 500W. Call me crazy, call me stupid, but I'm a believer in "head-room" I know that I am "wasting" these amps by using them as sub amps but moving air requires a lot of power. At the time I built these amps, I was working for a pro audio shop and repairing a lot of amps with many output transistors. I wanted a much better understanding about these amps so I found a project that suited my curiosity and desire to learn. I learned much during this project but I have so very much more to learn.
Otter
Let no one say those to you... you did right...
This is just one of the many iterations of the basic Lin topology, i have built lower rails type of this amp. This topology lends itself to many possibilities......
the mje340/350 can be replaced with better transistors...or else get bigger sinks as these can run hot...
myself is a believer in having higher "headroom".....
Hey Tony,
Thanks for your post. I agree with you 100%. The thing that REALLY impresses me about this amp is that it is so quiet (no hiss)!!! The other thing I like is that when they need to deliver instant power (an explosion in a movie) they deliver it at once and don't distort at all (head room)!!! I'm very happy with them, which is why I am still replying to this thread. My only change, later, will be more filtering of the main power supply but caps of this voltage and capacity are expensive... FOO! There are so many smart people on this group!!! It is very interesting to hear what all have to say. I have had very few criticize the design. I DO hear some really interesting thoughts on small improvements. If it was still on the bench, I'd try some, just to see if they work. I appreciate all of the thoughts given so thanks to all!!! I hope the original person, who asked for help, finds what they are looking for....
Also.... I did the 2SJ's and Japanese transistors, each type on each amp. (I built 2 amps) Both work the same as far as I could read on the scope I had!!! I use a 4 inch AC fan like in PA audio amps to cool the heat sinks. The ones in the pics you have are not the heat sinks I have in the unit right now. All works good so far!!! As, is, the unit weighs 70 pounds... 30KG? The fan has an 830 ohm 20W resistor in series with the fan. I have a thermal switch, on the HS that closes with temperature, which is in parallel with the resistor, that shorts the resistor when excessive heat condition occurs and the fan runs at it's rated speed. Anyway.....
Regards,
Otter
Thanks for your post. I agree with you 100%. The thing that REALLY impresses me about this amp is that it is so quiet (no hiss)!!! The other thing I like is that when they need to deliver instant power (an explosion in a movie) they deliver it at once and don't distort at all (head room)!!! I'm very happy with them, which is why I am still replying to this thread. My only change, later, will be more filtering of the main power supply but caps of this voltage and capacity are expensive... FOO! There are so many smart people on this group!!! It is very interesting to hear what all have to say. I have had very few criticize the design. I DO hear some really interesting thoughts on small improvements. If it was still on the bench, I'd try some, just to see if they work. I appreciate all of the thoughts given so thanks to all!!! I hope the original person, who asked for help, finds what they are looking for....
Also.... I did the 2SJ's and Japanese transistors, each type on each amp. (I built 2 amps) Both work the same as far as I could read on the scope I had!!! I use a 4 inch AC fan like in PA audio amps to cool the heat sinks. The ones in the pics you have are not the heat sinks I have in the unit right now. All works good so far!!! As, is, the unit weighs 70 pounds... 30KG? The fan has an 830 ohm 20W resistor in series with the fan. I have a thermal switch, on the HS that closes with temperature, which is in parallel with the resistor, that shorts the resistor when excessive heat condition occurs and the fan runs at it's rated speed. Anyway.....
Regards,
Otter
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.