matching them ODs
I placed an order today for two identical toroids, each 500Va with 2x28VAC and one extra 13V/600mA secondaries. The 13V sec. will be used for the protection circuitry/delay in case I can't sort something from the main supply voltage. I was also thinking about using 4 pairs of output devices as the 2SA1943/2SC5200 are plastic casing and it should be easier to spread them evenly on the heatsinks. At this time I'm thinking how critical is matching them pairs and a proper way to do it. Matching output transistors will not be properly done if I use the digital multimeter's hfe measuring option, but OTOH, I might not be able to do some "perfect" or at least nearly perfect matching since I have the feeling that at this time, no one else is interested to seriously invest into this project (money is quite serious for this sort of amps, by now, for 4 sinks, 8*25000uF/75V caps and two 500VA toroids, I already spent around 400€ and this is far from being close to finished
)
So, at this time I only can go with these 8 pairs (of O.T.) I have, can't afford to waste money and buy qts and leave them for later projects since I neither plan to, neither believe in need for other project involving power transistors and reselling might be a slim chance task...
There is still a problem I'm facing about thermally connecting the heatsink pairs which will probably require a seriously bulky piece of copper (can't get proper thickness of aluminium in reasonably small pieces), otherwise I think I'll be fine.
The next thing to do is to prepare the documentation for the PCBs and start finding the parts for the amp, and of course, a lot of reading...
I placed an order today for two identical toroids, each 500Va with 2x28VAC and one extra 13V/600mA secondaries. The 13V sec. will be used for the protection circuitry/delay in case I can't sort something from the main supply voltage. I was also thinking about using 4 pairs of output devices as the 2SA1943/2SC5200 are plastic casing and it should be easier to spread them evenly on the heatsinks. At this time I'm thinking how critical is matching them pairs and a proper way to do it. Matching output transistors will not be properly done if I use the digital multimeter's hfe measuring option, but OTOH, I might not be able to do some "perfect" or at least nearly perfect matching since I have the feeling that at this time, no one else is interested to seriously invest into this project (money is quite serious for this sort of amps, by now, for 4 sinks, 8*25000uF/75V caps and two 500VA toroids, I already spent around 400€ and this is far from being close to finished
) So, at this time I only can go with these 8 pairs (of O.T.) I have, can't afford to waste money and buy qts and leave them for later projects since I neither plan to, neither believe in need for other project involving power transistors and reselling might be a slim chance task...
There is still a problem I'm facing about thermally connecting the heatsink pairs which will probably require a seriously bulky piece of copper (can't get proper thickness of aluminium in reasonably small pieces), otherwise I think I'll be fine.
The next thing to do is to prepare the documentation for the PCBs and start finding the parts for the amp, and of course, a lot of reading...
The heatsinks (4), electrolytes (8x25mF) are on the table (the transformers are the proper size but for an other project) all fits on a 40x40cm base, now the waiting game for the output transistor. Decided finally to follow the original design as much as possible, will go with ON Semi MJ15003/MJ15004 OD, order from Farnell, 2 pairs per channel. Now it's time to start the work on PCBs
edit: Can't upload the image. small enough, jpg, dunno...
edit: Can't upload the image. small enough, jpg, dunno...
help!
Today the toroids arrived, 500VA, 2x28VAC (each). I found the original schematics that was used at first to build the Delta PCBs but it ended up with a completely different setup and I need some additional data on resistor values if the rails run at approx 36-38VDC (that is what I expect to get after the rectifier bridge with the amp attached to the PSU)
I need the amp to work with low impedance load but don't need 200W in Class A @2ohm load. I decided to go with the MJ15003/4, two pairs per channel, huge sinks without fans and an "acceptable" power consumption. Acceptable means I have a wife, a five year old daughter (she knows that dad's glowing stuff is not to be messed with) and all the usual stuff that comes with being a family man. So, nothing silly, need the amp to listen to music and not to impress friends from work with how hot it gets...
Found several recommendations but not completely sure...
Today the toroids arrived, 500VA, 2x28VAC (each). I found the original schematics that was used at first to build the Delta PCBs but it ended up with a completely different setup and I need some additional data on resistor values if the rails run at approx 36-38VDC (that is what I expect to get after the rectifier bridge with the amp attached to the PSU)
I need the amp to work with low impedance load but don't need 200W in Class A @2ohm load. I decided to go with the MJ15003/4, two pairs per channel, huge sinks without fans and an "acceptable" power consumption. Acceptable means I have a wife, a five year old daughter (she knows that dad's glowing stuff is not to be messed with) and all the usual stuff that comes with being a family man. So, nothing silly, need the amp to listen to music and not to impress friends from work with how hot it gets...
Found several recommendations but not completely sure...
the KSA50 is designed to give 50W of ClassA from 2pair output stage.
You can't get 200W of ClassA from 2pair.
You need to understand the limits of ClassA.
If the KSA50 is set up with 1.9A of output bias, then twice that is the ClassA limit.
i.e. 3.8Apk
Max ClassA power into the load is Ipk^2*Rload/2
8r0 - 58W
4r0 - 29W
2r0 - 14W (not 200W)
You can't get 200W of ClassA from 2pair.
You need to understand the limits of ClassA.
If the KSA50 is set up with 1.9A of output bias, then twice that is the ClassA limit.
i.e. 3.8Apk
Max ClassA power into the load is Ipk^2*Rload/2
8r0 - 58W
4r0 - 29W
2r0 - 14W (not 200W)
Thanks, that's all ok, I can live with that, I basically need more specific data, values of E resistors; and there seem to be a difference in the base resistor values between the original schematic here where actually doesn't seem to be any, and what was done by Delta Audio. So, even though I designed and built a couple of tube preamps and amps, that's a completely different story. I would prefer not to mess up this one. Call me stupid but I have no clue which values should I use at specific positions.
The biggest problem is that the more I read into the subject, I become more confused.
The biggest problem is that the more I read into the subject, I become more confused.
Doh!... After understanding how computers work, had to learn how recumbent bikes and trikes work, learned how tubes work... I'm getting too old for this 5hit... If I start learning now, I'll probably figure out everything by the time I'll become completely deaf, blind and incapable of holding anything in my hand without shaking it to self-destruction...
I just need a confirmation if the schematic on the drawing is correct...
I just need a confirmation if the schematic on the drawing is correct...
values...
The readers digest version:
No base resistors in the original, if you must have them try something between 2 and 10 ohms.
Re vs output count :-
2 pairs : 0r68
3 pairs : 1r0
4 pairs : 1r33
5 pairs : 1r66
6 pairs : 2r0
more pairs: you are probably insane and should seek professional help.
The Re shown should leave the driver operating point the same as original.
Using way more transformer than output transistor is pretty much guaranteeing the transistors will be destroyed in the event of a headlong collision with an output short.
Have fun.
Stuart
The readers digest version:
No base resistors in the original, if you must have them try something between 2 and 10 ohms.
Re vs output count :-
2 pairs : 0r68
3 pairs : 1r0
4 pairs : 1r33
5 pairs : 1r66
6 pairs : 2r0
more pairs: you are probably insane and should seek professional help.
The Re shown should leave the driver operating point the same as original.
Using way more transformer than output transistor is pretty much guaranteeing the transistors will be destroyed in the event of a headlong collision with an output short.
Have fun.
Stuart
Thanks. This was the info I needed. In the meantime, I got an offer to get the same OD as used with the Delta PCBs so I'll just stick to whatever was decided during the process here. 3 output pairs of MJL21193G/94G...
Two images of the bulk parts waiting for the pcbs...
Two images of the bulk parts waiting for the pcbs...
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Re: values...
Soon I'll get to the part where having fun means not calling the fire department...
One more question, this time about setting up the amp... (still far away but...)
Since I'm going to use the KSA50 clone for a sensitive but low impedance load, should I have a different current setting that would not heat up the amp too much? Meaning (to rephrase the question), should I go down with the current setting? The amplifiers in our house often get turned on and keep working for weeks without being turned off and since the design of the Class A means all the heat going into the heatsinks when the amp is in idle state, it might get pretty hot if the amp is left on. For heating, we have better and cheaper solutions, so I don't really want the audio equipment to be the alternative for heating. Would it be advisable or possible to reduce the current from say 1.95 or 1.75 Amps to say 1.2 or maybe even 1 Amp or less. Except for the benefit from lesser power consumption, what would be the advantage if any or disadvantage? Currently I'm still use the primary system consisting of a single ended tube amp (EL34 pch, approx 6W in Class A) and a pair of Fostex FE126E horns and that amp consumes irrelevant amounts of power when idle. With about ten times that much power consumption, I'm not too happy (I know, I can always turn the amp off when not usig it...) So... What should I do to acommodate the amplifier to low impedance load?
Or should I just shut up and work on building it as it is?
Stuart Easson said:
Soon I'll get to the part where having fun means not calling the fire department...
One more question, this time about setting up the amp... (still far away but...)
Since I'm going to use the KSA50 clone for a sensitive but low impedance load, should I have a different current setting that would not heat up the amp too much? Meaning (to rephrase the question), should I go down with the current setting? The amplifiers in our house often get turned on and keep working for weeks without being turned off and since the design of the Class A means all the heat going into the heatsinks when the amp is in idle state, it might get pretty hot if the amp is left on. For heating, we have better and cheaper solutions, so I don't really want the audio equipment to be the alternative for heating. Would it be advisable or possible to reduce the current from say 1.95 or 1.75 Amps to say 1.2 or maybe even 1 Amp or less. Except for the benefit from lesser power consumption, what would be the advantage if any or disadvantage? Currently I'm still use the primary system consisting of a single ended tube amp (EL34 pch, approx 6W in Class A) and a pair of Fostex FE126E horns and that amp consumes irrelevant amounts of power when idle. With about ten times that much power consumption, I'm not too happy (I know, I can always turn the amp off when not usig it...) So... What should I do to acommodate the amplifier to low impedance load?
Or should I just shut up and work on building it as it is?
If this is your concern, maybe a Class AB instead? With reducing the BIAS, you are giving up a lot of the benefit of Class A.
There is a neat Class A spreadsheet on this thread somewhere that will show you what Class A and Class AB watts to be expected with your rail voltage and bias setting... you should be able to reduce your bias/temp in Class A to 25 wpc and have higher reserves for Class AB operation.
There is a neat Class A spreadsheet on this thread somewhere that will show you what Class A and Class AB watts to be expected with your rail voltage and bias setting... you should be able to reduce your bias/temp in Class A to 25 wpc and have higher reserves for Class AB operation.
Thanks, found that spreadsheet over Wiki, will continue to investigate and calculate. I asked because it's very unlikely that I'll connect anything like 8 ohms to this Krell clone and setting it to, say, 35W in Class A at 4 ohms would definitely sound more pleasing to my feeling of heading into right direction. I need some self confidence to be able to gather the courage to turn the power switch on for the first time... So, that is my main concern, getting the amp to work properly at lower impedances. I can supply enough of core, heatsink, capacitors and output devices but want to do it right from the very beginning. So, it's basically completely irrelevant for me how much power and in what class is the amplifier able to deliver at 8 ohm impedance... At 90dB sensitivity, I don't need too much output power but do need reserve for a low impedance load...
Power...
...is a product of current and voltage, you can reduce the idle dissipation by reducing the voltage rails or the idle current.
If you are satisfied you do not need huge voltage output from the amp (and with a 4r sensitive speaker you probably don't), you can reduce the voltage used for the power supply rails.
With a nominal 50w of class A output into 8 ohms, you need a minimum of 1.9A bias and 28v peak, the rails I use were about 10v higher than this at 37v, giving about 140w idle dissipation.
Had I decided I could live with 20W into 4 ohms, I'd recalculate the necessary idle current and voltage rails. This turns out to be 1.6A bias, with about 14v peak output. This can be fulfilled with ~23v rails, leading to idle dissipation in the 65-70W range, a much better deal...plus at this output level, (living a little dangerously perhaps) you don't need multiple sets of output transistors.
The zeners in the front end and the resistors that feed them would need to be recalculated, but thats no big deal.
HTH
Stuart
...is a product of current and voltage, you can reduce the idle dissipation by reducing the voltage rails or the idle current.
If you are satisfied you do not need huge voltage output from the amp (and with a 4r sensitive speaker you probably don't), you can reduce the voltage used for the power supply rails.
With a nominal 50w of class A output into 8 ohms, you need a minimum of 1.9A bias and 28v peak, the rails I use were about 10v higher than this at 37v, giving about 140w idle dissipation.
Had I decided I could live with 20W into 4 ohms, I'd recalculate the necessary idle current and voltage rails. This turns out to be 1.6A bias, with about 14v peak output. This can be fulfilled with ~23v rails, leading to idle dissipation in the 65-70W range, a much better deal...plus at this output level, (living a little dangerously perhaps) you don't need multiple sets of output transistors.
The zeners in the front end and the resistors that feed them would need to be recalculated, but thats no big deal.
HTH
Stuart
Re: values...
Stuart! You are STILL kicking around here? I pop in every now and again... But I haven't seen my amp in months!!!
I'm living in the UK now... Not permanently though! My amp was still going VERY strong when I packed it away! My only task left is to build a case (Yes, still after ALL these years... i've made the templates, but not got as far as getting the alloy plates cut), it's still going VERY well with 5 pairs per channel Never over heated, never cut out, never missed a beat! I think I built one of the biggest ones from memory... Unless someone wants to outdo:
5 pairs of outputs per channel
136,000uf/channel
800va torroid per channel...
4 (12v) fans running off an internal switchmode power supply (to be replaced with silent fans!)...
My only problem i've encountered is that the amp REALLY has to be in a separate room to the listener, purely for the heat... I can get quiet fans that will push enough air to keep it happily cool, but the heat is just unbelievable! I know everyone said it would be, so I stuck it in the next room! It still sounds lovely and when I get home, i've got a nice new Cambridge Audio preamp to stick on it!
Hope your's is still going well!
Aaron
Stuart Easson said:
Stuart! You are STILL kicking around here? I pop in every now and again... But I haven't seen my amp in months!!!
I'm living in the UK now... Not permanently though! My amp was still going VERY strong when I packed it away! My only task left is to build a case (Yes, still after ALL these years... i've made the templates, but not got as far as getting the alloy plates cut), it's still going VERY well with 5 pairs per channel Never over heated, never cut out, never missed a beat! I think I built one of the biggest ones from memory... Unless someone wants to outdo:5 pairs of outputs per channel
136,000uf/channel
800va torroid per channel...
4 (12v) fans running off an internal switchmode power supply (to be replaced with silent fans!)...
My only problem i've encountered is that the amp REALLY has to be in a separate room to the listener, purely for the heat... I can get quiet fans that will push enough air to keep it happily cool, but the heat is just unbelievable! I know everyone said it would be, so I stuck it in the next room! It still sounds lovely and when I get home, i've got a nice new Cambridge Audio preamp to stick on it!
Hope your's is still going well!
Aaron
3rd times a charm...
Hi Aaron,
Good to hear from you, what brings you to good ol' Blighty for such a long stay? I'm a regular visitor, but not thinking of moving back anytime soon.
Nowadays I'm an irregular visitor here, trying to help where I can.
If you are in the UK at the correct time of year you should track down the get together, we had a great one here BAF2007. Pinkmouse was organizing it last year not sure about after that.
I'm actually on my third version of the KSA50, this one being built with infinitely more WAF. The others, while doing everything one could expect of amps, were not cosmetically acceptable to SWMBO. To help I have teamed up with someone more capable in the chassis department, results to be displayed at BAF in October.
Stuart
Hi Aaron,
Good to hear from you, what brings you to good ol' Blighty for such a long stay? I'm a regular visitor, but not thinking of moving back anytime soon.
Nowadays I'm an irregular visitor here, trying to help where I can.
If you are in the UK at the correct time of year you should track down the get together, we had a great one here BAF2007. Pinkmouse was organizing it last year not sure about after that.
I'm actually on my third version of the KSA50, this one being built with infinitely more WAF. The others, while doing everything one could expect of amps, were not cosmetically acceptable to SWMBO. To help I have teamed up with someone more capable in the chassis department, results to be displayed at BAF in October.
Stuart