In hindsight, perhaps you shouldn't have used the word "fired" in the title for the thread. I know I say the same thing, but when you stop and think about it, it's really quite an unfortunate turn of phrase, at least for electronics folks. Perhaps more appropriate for a potter preparing to start his kiln.
Grey
Grey
Geez, my condolences! 
I've been reading this with interest, I'm going to build a Leech or an Opti-MOS amp. I want to build a set of Electrostatic speakers, and both of these seem to do OK with ESL's.
Just as a matter of interest, why did you build your own board as opposed to buying the ones available?
I've got some of that "Iron On" PCB stuff that I've used before, although the traces weren't perfect, but I think I'll just buy a board. I'm working 70+hrs a week (no over time either, joys of 'management') and THAT part I don't mind having done for me.
I've been reading this with interest, I'm going to build a Leech or an Opti-MOS amp. I want to build a set of Electrostatic speakers, and both of these seem to do OK with ESL's.
Just as a matter of interest, why did you build your own board as opposed to buying the ones available?
I've got some of that "Iron On" PCB stuff that I've used before, although the traces weren't perfect, but I think I'll just buy a board. I'm working 70+hrs a week (no over time either, joys of 'management') and THAT part I don't mind having done for me.
My two cents of after-the-fact wisdom...
I have now built six perfectly running Leach Amp channels. I always completed everything before running a single test.
When powering up for the first time, having all stages in place, but with no load (and also no bias) I placed two 100 Ohm 0.25W resistors in place of fuses. Once (with my third Leach project) the resistors went up in smoke and I soon discovered I had the bias diodes connected the wrong way.
After the resistors survived and there appeared about 25mA flowing in both (that is, 2.5V over them), I unpowered, inserted one fuse and one current meter, powered again and set the bias as professor Leach recommended, waiting more than 10 minutes each time after it had stabilized to see if it really had.
Then I unpowered, inserted the second fuse and turned power back on. Always directly, with no variac, no bulbs. I have every time tested the power supply part before installing the circuit board and it's heatsink. Then I quickly tried if there was excess DC in output. Had every time less than 15mV. Connected an input from my preamp, then an ordinary speaker and... beautiful!
-Kimmo Sundqvist
I have now built six perfectly running Leach Amp channels. I always completed everything before running a single test.
When powering up for the first time, having all stages in place, but with no load (and also no bias) I placed two 100 Ohm 0.25W resistors in place of fuses. Once (with my third Leach project) the resistors went up in smoke and I soon discovered I had the bias diodes connected the wrong way.
After the resistors survived and there appeared about 25mA flowing in both (that is, 2.5V over them), I unpowered, inserted one fuse and one current meter, powered again and set the bias as professor Leach recommended, waiting more than 10 minutes each time after it had stabilized to see if it really had.
Then I unpowered, inserted the second fuse and turned power back on. Always directly, with no variac, no bulbs. I have every time tested the power supply part before installing the circuit board and it's heatsink. Then I quickly tried if there was excess DC in output. Had every time less than 15mV. Connected an input from my preamp, then an ordinary speaker and... beautiful!
-Kimmo Sundqvist
I make my own because it is cheaper than buying ready-made ones, and is more satisfying too. I use photoresist boards from MG Chemicals. The traces turn out perfectly nearly every time. Prof. Leach has PCB patterns available on the website, so if I have blank PCBs, developer, and etchant on hand, I can make the boards much quicker than ordering them too. It only takes about 45 min. to make a board once you get the hang of it. I wouldn't think of buying boards if the pattern is available for free.
Musher, I too had the board completed before running it, and it did work to some degree. I followed a procedure similar to what you do, but with a bulb and no resistors. When the bulb did not remain lit after the filter caps were charged, I knew it was safe to run the amp normally, which it was until I accidently toasted it with my probe. I had not yet measured DC offset, but I did set the bias current.
Musher, I too had the board completed before running it, and it did work to some degree. I followed a procedure similar to what you do, but with a bulb and no resistors. When the bulb did not remain lit after the filter caps were charged, I knew it was safe to run the amp normally, which it was until I accidently toasted it with my probe. I had not yet measured DC offset, but I did set the bias current.
Fried Leach II !!
Like Kilowatt, I put my Leach on fire last week. Now, after cleaning up the mess, I try to investigate what happened. The primary reason was a bad preamp that put abt. 3V DC level into the input.
But: The safety circuit in the amp should have avoided any troble. Actually, it did not, and I hope someone of us can help me find out why!
The "after-accident-check" revealed:
* R36 burnt to dust
* Q10: emitter burnt away
* Q11: fully interrupted
* C17, C17: puffed up
* D7, D8 working ! (???)
* Q16, Q17: completely shorted
* R41..R44: abt 450 Ohms (instead of 10)
* Q16..Q20: shorted
Just to shorten the discussion: I used fuses, and in one channel the fuse for the positive rail blew, in the other channel the fuse for the negative rail!
Hope for some hints for further investigation!!! Any ideas are highly welcome!
Greetings
Herman
Like Kilowatt, I put my Leach on fire last week. Now, after cleaning up the mess, I try to investigate what happened. The primary reason was a bad preamp that put abt. 3V DC level into the input.
But: The safety circuit in the amp should have avoided any troble. Actually, it did not, and I hope someone of us can help me find out why!
The "after-accident-check" revealed:
* R36 burnt to dust
* Q10: emitter burnt away
* Q11: fully interrupted
* C17, C17: puffed up
* D7, D8 working ! (???)
* Q16, Q17: completely shorted
* R41..R44: abt 450 Ohms (instead of 10)
* Q16..Q20: shorted
Just to shorten the discussion: I used fuses, and in one channel the fuse for the positive rail blew, in the other channel the fuse for the negative rail!
Hope for some hints for further investigation!!! Any ideas are highly welcome!
Greetings
Herman
Rail Fuses
Sorry to hear about your shorted amp ... there is a little discussion about rail fuses here http://www.diyaudio.com/forums/showthread.php?threadid=4748&pagenumber=2
Sorry to hear about your shorted amp ... there is a little discussion about rail fuses here http://www.diyaudio.com/forums/showthread.php?threadid=4748&pagenumber=2
Kilowatt
I have a decent Tek 442 for sale. It has 2mV resolution, 35MHz bandwidth and a manual. The going price for this scope is around $750 but I'll deliver it to you for $200. The first channel has a little jitter on the 1V and 0.5V setting when you jostle the pot but other than that it's in full functioning condition. My email is listed in the members section if you want to get in touch with me.
I have a decent Tek 442 for sale. It has 2mV resolution, 35MHz bandwidth and a manual. The going price for this scope is around $750 but I'll deliver it to you for $200. The first channel has a little jitter on the 1V and 0.5V setting when you jostle the pot but other than that it's in full functioning condition. My email is listed in the members section if you want to get in touch with me.
Herman:
The current limiter circuit won't protect the amplifier from an
excessive DC input; it will be necessary to put in a blocking
capacitor on the input. 4.7 to 10uF will do, polypropylene or
high quality back-to-back tantalums are recommended
depending on what you can afford and make fit.
Kilowatt:
I've got an extra Heathkit IG-18 audio generator here that's
not going to be used; you want? Distortion isn't the lowest,
but good enough for general work.
The current limiter circuit won't protect the amplifier from an
excessive DC input; it will be necessary to put in a blocking
capacitor on the input. 4.7 to 10uF will do, polypropylene or
high quality back-to-back tantalums are recommended
depending on what you can afford and make fit.
Kilowatt:
I've got an extra Heathkit IG-18 audio generator here that's
not going to be used; you want? Distortion isn't the lowest,
but good enough for general work.
Damon,
thanks for your reply.
I agree with you in that a dc blocking capacitor could easily rule out the problem, but in my opinion there is absolutely no evidence that the protection circuit can't do anything against excessive DC. The circuit simply measures the current through the power stages and should do that regardless of the reason for the excessive current flow!
I meanwhile found out that I accidentally used BC550/BC560 for the prot. circuit - which have an V_ceo of 50V. Now I suppose these transistors might have been destroyed before (by the 56V rail voltage), but nothing happened before dc came in...
I will replace them with 2N5401/2N5551 (which I have readily available), and maybe I "sacifice" another set of transistors at testing my amps response to excessive dc input.
How about that idea?
Greetings
Herman
thanks for your reply.
I agree with you in that a dc blocking capacitor could easily rule out the problem, but in my opinion there is absolutely no evidence that the protection circuit can't do anything against excessive DC. The circuit simply measures the current through the power stages and should do that regardless of the reason for the excessive current flow!
I meanwhile found out that I accidentally used BC550/BC560 for the prot. circuit - which have an V_ceo of 50V. Now I suppose these transistors might have been destroyed before (by the 56V rail voltage), but nothing happened before dc came in...
I will replace them with 2N5401/2N5551 (which I have readily available), and maybe I "sacifice" another set of transistors at testing my amps response to excessive dc input.
How about that idea?
Greetings
Herman
You have my heart felt sympathy, but I do have to be honest I'm sitting here with a wry smile ..... the last amp I had a catastrophic failure with was the very same beast (or v3 anyway)!
I was about 15yo at the time, this was my years savings and I was so keen to complete it that I made some fatal error .... I damn near cried at the time
The pain will fade ..... eventually!
cheers, mark
I was about 15yo at the time, this was my years savings and I was so keen to complete it that I made some fatal error .... I damn near cried at the time
The pain will fade ..... eventually!
cheers, mark
Herman:
Sorry, no. The current limiters are effectively only against low
impedance >output< loads; a high DC offset at the input is
another thing altogether. The amplifier will not tolerate a DC
offset without severe problems because it cannot compensate
for this bias. It is after all a fully DC-coupled design. If your
source has a DC offset then a blocking capacitor MUST be used!
Yes, you'll have to use higher voltage transistors even in the
protection circuits; the ones you mentioned should work well.
It's not easy building a DIY amplifier without test equipment; I
spent years acquiring a full test bench (mostly Heathkit). It gives
me some peace of mind knowing that I can objectively verfify
proper performance (well, most of the time...)
Anytime you feel rushed to complete a project, stop. Take a break and come back to it later. I've learned this lesson the hard way, sometimes discovering obvious mistakes (wiring errors, wrong value parts, etc.)
Sorry, no. The current limiters are effectively only against low
impedance >output< loads; a high DC offset at the input is
another thing altogether. The amplifier will not tolerate a DC
offset without severe problems because it cannot compensate
for this bias. It is after all a fully DC-coupled design. If your
source has a DC offset then a blocking capacitor MUST be used!
Yes, you'll have to use higher voltage transistors even in the
protection circuits; the ones you mentioned should work well.
It's not easy building a DIY amplifier without test equipment; I
spent years acquiring a full test bench (mostly Heathkit). It gives
me some peace of mind knowing that I can objectively verfify
proper performance (well, most of the time...)
Anytime you feel rushed to complete a project, stop. Take a break and come back to it later. I've learned this lesson the hard way, sometimes discovering obvious mistakes (wiring errors, wrong value parts, etc.)
Hi Killowatt,
My very first DIY electronic project was a tube 50W 10 meter Ham radio transmitter I built in a high school electronics class (1974). Everything was built from scratch with point to point wiring. I spent many hours punching and bending the chassis, soldering and lacing the wiring harnesses. Looked like a million bucks when I was done (at least it did to me).
First time I powered it up, smoke started rolling out of the power supply transformer. Room filled up with foul smelling smoke and an entire lab of excitable high school students spilled out into the hall. Turned out I had the main filter caps wired backwards and they had failed with a hard short. I was not only saddened, but pretty red in the face as I'm sure you can imagine since I had 20+ witnesses to my debacle.
No fuses in the AC line. The electronics instructor was pretty lax too, he never even checked out my AC wiring before I powered it up, could have been an electrocution hazard if I'd gotten something wrong in the primary AC wiring.
Made me a whole lot more careful on how I construct and power up projects after that.
Phil
My very first DIY electronic project was a tube 50W 10 meter Ham radio transmitter I built in a high school electronics class (1974). Everything was built from scratch with point to point wiring. I spent many hours punching and bending the chassis, soldering and lacing the wiring harnesses. Looked like a million bucks when I was done (at least it did to me).
First time I powered it up, smoke started rolling out of the power supply transformer. Room filled up with foul smelling smoke and an entire lab of excitable high school students spilled out into the hall. Turned out I had the main filter caps wired backwards and they had failed with a hard short. I was not only saddened, but pretty red in the face as I'm sure you can imagine since I had 20+ witnesses to my debacle.
No fuses in the AC line. The electronics instructor was pretty lax too, he never even checked out my AC wiring before I powered it up, could have been an electrocution hazard if I'd gotten something wrong in the primary AC wiring.
Made me a whole lot more careful on how I construct and power up projects after that.
Phil
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