It works
Yup,
Even a blind squirrel finds a nut now and then. I matched the output fets on the board as you suggested and kept only 6 pairs. It's a damn good thing too because there would have been no way the heat sinks were big enough for 12 pairs at 170-200mV each. As is, I am able to bias to almost 250mA on the source resistors and still keep the temperature well in check. I still have a few details like hooking up these cool old VU meters. I know they serve no purpose other than to look cool but that's sometimes purpose enough.
I do have more ripple on the power supply than I'd like and need to take care of that somehow because I hear it in the speakers. I might need to use shielded mic cable on the inputs. I also made the switch that selects between 100%, 50% and 0% feedback. This was not such a hot idea for one reason. You can only adjust the DC offset for one feedback setting. As soon as I flip the switch to another feedback setting, the DC is way off again. I'll have to pick one and stick eith it.
I want to thank you all very much. This thing would have been flung off the deck long ago if you all weren't there to reel me back in. I'll be sure to follow up with some pics as soon as I'm done and hopefully add it to Nelson's gallery. Hope it makes the cut.
Yup,
Even a blind squirrel finds a nut now and then. I matched the output fets on the board as you suggested and kept only 6 pairs. It's a damn good thing too because there would have been no way the heat sinks were big enough for 12 pairs at 170-200mV each. As is, I am able to bias to almost 250mA on the source resistors and still keep the temperature well in check. I still have a few details like hooking up these cool old VU meters. I know they serve no purpose other than to look cool but that's sometimes purpose enough.
I do have more ripple on the power supply than I'd like and need to take care of that somehow because I hear it in the speakers. I might need to use shielded mic cable on the inputs. I also made the switch that selects between 100%, 50% and 0% feedback. This was not such a hot idea for one reason. You can only adjust the DC offset for one feedback setting. As soon as I flip the switch to another feedback setting, the DC is way off again. I'll have to pick one and stick eith it.
I want to thank you all very much. This thing would have been flung off the deck long ago if you all weren't there to reel me back in. I'll be sure to follow up with some pics as soon as I'm done and hopefully add it to Nelson's gallery. Hope it makes the cut.
Congratulations. I'm glad we have another member of the A75 club. Other than the hum, how does it sound?
Have you measured the rail ripple? This amp should be relatively immune, you may be picking up hum from AC mains routing. Is it 60 Hz or 120 Hz? For reference I used only 45,000 uf per rail, and have no hum issue. (at least once I got my grounding squared away)
Have you measured the rail ripple? This amp should be relatively immune, you may be picking up hum from AC mains routing. Is it 60 Hz or 120 Hz? For reference I used only 45,000 uf per rail, and have no hum issue. (at least once I got my grounding squared away)
I have 120000uF on each channel and about 200mV of ripple on the unregulated supplies. Off the top of my head 120Hz sounds right. I also see a real lot of gain (40dB). R 21 and 22 are 100 ohm each as listed in the article. My assumption was that any ripple on the input may be greatly amplified. Have you tried different values for R21 and 22? In the article, Nelson mentions that at 10K Ohm you'd see about 20dB gain. That sounds much better to me. I should be able to get about 4V out of a preamp and that would clip the amp anyway so I don't really see needing more than 20dB. I also assumed that with higher values for R81 I would have less gain. That was not the case. Anyway, I'm having fun experimenting now. I still am very grateful for any tips you think may help me.
If you are getting a gain of 40 db you are missing something in the feedback circuit - you aren't getting any. I think Nelson meant to say open loop gain "of" this stage rather than "at". Not that it really matters, closed loop gain should be around 20V/v (26 DB) single ended. Check all resistors in the feedback loop, including R2 and R4.
The ripple on your unregulated supply should be no problem. Check that your ground connections are tight (voice of experience), and that the star ground is not the junction between positive and negative rail caps. A lot of current flows there and could induce noise in the signal grounds. I did a simulation of the AX supply I intend to build and each cap in the CRC sees at least 24A peak current.
Another thing to check is the mains wiring to the power switch/triac. My first A75 was sloppy and the leads were not tightly twisted together and against the chassis. The triac switching noise sounded like 120 Hz, with an edge, until I dressed the leads properly.
The ripple on your unregulated supply should be no problem. Check that your ground connections are tight (voice of experience), and that the star ground is not the junction between positive and negative rail caps. A lot of current flows there and could induce noise in the signal grounds. I did a simulation of the AX supply I intend to build and each cap in the CRC sees at least 24A peak current.
Another thing to check is the mains wiring to the power switch/triac. My first A75 was sloppy and the leads were not tightly twisted together and against the chassis. The triac switching noise sounded like 120 Hz, with an edge, until I dressed the leads properly.
If I may, please allow me to throw in my 2 cents...
Bob is right: your gain is high. With R21 and R22 open (the amp run stock), I measured 20 dB. With folded cascode (R21 and R22 at 100 ohms) the gain goes down (to 6 dB if I remember correctly).
If you are using the PCBs from the Old Colony Sound Lab, you will have noticed mistakes on the silk screen. My notes show that R24, R26, R80 and R81 on the schematic respectively correspond on the PCB to R22 (next to Q11), R24, R21 (next to P3 and Q11) and R26 (next to C8).
As for R81, it does not affect the gain. It does, however, affect the DC offset. As R81 is reduced from infinity down to 0, the output stage is gradually included in the feedback loop, which in turn corrects DC offset at the output. This does not change the gain because the output stage does not provide voltage gain.
Hope this helps. Let us know...
Pierre
Bob is right: your gain is high. With R21 and R22 open (the amp run stock), I measured 20 dB. With folded cascode (R21 and R22 at 100 ohms) the gain goes down (to 6 dB if I remember correctly).
If you are using the PCBs from the Old Colony Sound Lab, you will have noticed mistakes on the silk screen. My notes show that R24, R26, R80 and R81 on the schematic respectively correspond on the PCB to R22 (next to Q11), R24, R21 (next to P3 and Q11) and R26 (next to C8).
As for R81, it does not affect the gain. It does, however, affect the DC offset. As R81 is reduced from infinity down to 0, the output stage is gradually included in the feedback loop, which in turn corrects DC offset at the output. This does not change the gain because the output stage does not provide voltage gain.
Hope this helps. Let us know...
Pierre
Hi Pierre
I always appreciate any help I can get. I did use the audioxpress boards but am getting some strange results. I am getting the 40dB gain with R21 and 22 set at 100 Ohm. In the original article, Nelson suggests that you could achieve 20dB of gain with a 10K Ohm resistor for R21 and 22. Have any of you tried this? and, if so, to what end?
Despite all else I think I need to check the component placement again. I am not sure that I identified all the silkscreen problems correctly. As a side note, I'm surprized that they don't include a little note with the boards warning people of these mistakes.
I always appreciate any help I can get. I did use the audioxpress boards but am getting some strange results. I am getting the 40dB gain with R21 and 22 set at 100 Ohm. In the original article, Nelson suggests that you could achieve 20dB of gain with a 10K Ohm resistor for R21 and 22. Have any of you tried this? and, if so, to what end?
Despite all else I think I need to check the component placement again. I am not sure that I identified all the silkscreen problems correctly. As a side note, I'm surprized that they don't include a little note with the boards warning people of these mistakes.
Dinu,
I'm sure the last thing you want to hear is pull the boards, but you should trace the feedback paths to ensure that you have the proper resistors in place.
The amp has two gains - open loop and closed loop. Open loop is what it would do without feedback. Closed loop is what it does with feedback. As long as the open loop gain exceeds the closed loop gain, then closed loop is what you see in operation. As I said earlier, I think Nelson meant to say that the VAS stage would have 20 db of gain with R21/22 approaching infinity. This would mean that there is more gain to throw away with feedback, resulting in lower measured distortion. You should have the same 26db of gain whether or not you use R21/22. therefore cehck the feedback resistors.
This amp does have 6 db less gain if you use single ended inputs and don't flip the switch, but that's something different.
I'm sure the last thing you want to hear is pull the boards, but you should trace the feedback paths to ensure that you have the proper resistors in place.
The amp has two gains - open loop and closed loop. Open loop is what it would do without feedback. Closed loop is what it does with feedback. As long as the open loop gain exceeds the closed loop gain, then closed loop is what you see in operation. As I said earlier, I think Nelson meant to say that the VAS stage would have 20 db of gain with R21/22 approaching infinity. This would mean that there is more gain to throw away with feedback, resulting in lower measured distortion. You should have the same 26db of gain whether or not you use R21/22. therefore cehck the feedback resistors.
This amp does have 6 db less gain if you use single ended inputs and don't flip the switch, but that's something different.
Hi BobEllis,
I noticed that pcb layout for A75 power supply posted from passDiy shows R18 connected accross GND and +U where it should be accross GND and base of Q1 from the schematic diagram.
Also, the schematic shows R3 and R4 wired to secondary taps A&D while the layout shows directly to C4&C5.
can you pls kindly help ?
regards,
roland
I noticed that pcb layout for A75 power supply posted from passDiy shows R18 connected accross GND and +U where it should be accross GND and base of Q1 from the schematic diagram.
Also, the schematic shows R3 and R4 wired to secondary taps A&D while the layout shows directly to C4&C5.
can you pls kindly help ?
regards,
roland
Roland,
You are correct, the layout is in error. If you are not using a fan you can leave out Q1 and the associated components. If you are using the audioexpress boards, there is an unmarked hole in the trace leading to Q1's base. Use that hole and the ground hole to connect R18 or you will have a fan operating on full output rail voltage. A 12V fan working on 60V really runs fast, for about 30 seconds
I didnt notice the R3-4 connection issue - the board works fine as is, but connecting per the schematic would reduce the peak currents in C4-5. As is it reduces the heat load for the regs.
You are correct, the layout is in error. If you are not using a fan you can leave out Q1 and the associated components. If you are using the audioexpress boards, there is an unmarked hole in the trace leading to Q1's base. Use that hole and the ground hole to connect R18 or you will have a fan operating on full output rail voltage. A 12V fan working on 60V really runs fast, for about 30 seconds
I didnt notice the R3-4 connection issue - the board works fine as is, but connecting per the schematic would reduce the peak currents in C4-5. As is it reduces the heat load for the regs.
I've had a bit of time this week and have been able to get it working, with one exception. I get a 60Hz hum on the output. If I ground the pos input pin the hum just gets louder. I'm guessing I have a grounding issue. All but the earth and chassis ground on the power supply are connected to 2 star grounds that are connected together with 12 gauge wire. The earth ground and chassis ground from the power supply are both connected to the chassis, not in the same point. I don't want to start guessing at my problem so I hope one of you may be able to help.
Dinu
It is good to hear that you are making progress. There is good information on Aleph 30 grounding in this thread, including two posts by Nelson Pass. In a nutshell, one wants grounds that are subject to heavy currents and thus voltage ripple (main caps, bridges and transfo) to exchange current between themselves, and not through signal grounds.
Pierre
It is good to hear that you are making progress. There is good information on Aleph 30 grounding in this thread, including two posts by Nelson Pass. In a nutshell, one wants grounds that are subject to heavy currents and thus voltage ripple (main caps, bridges and transfo) to exchange current between themselves, and not through signal grounds.
Pierre
I replaced the input wires with a pair of cabls I got from a really great electronics supply place in brooklyn. They have conductive plastic covers and a drain conductor. It helped but I still hear a hum. Would it be a good idea to isolate power ground and signal ground through a bridge rectifier? If so, Any ideas?
dinu said:
That's not quite the way NP had the grounding scheme. The mains earth ground connects to the chassis - always. However, the amplifier's grounds connect to the chassis through the diodes (D7, D8) and resistor (R17) on the PS board. Lift your star gound from the chassis and let the connection to the chassis be through the ground leads on your PSU board. The mounting hole near D7 and D8 is connected to the chassis grougn side of the circuit, so if you use condictive standoffs you should be OK. I mounted all teh potentially high current grounds at one end of the bolt used as my star and the signal grounds at the other.
Check your lead dress. Are all AC mains leads twisted together and close to the chassis? Are the rail leads twisted tightly together? Are the input connections as far as possible from the mains leads? How about the feedback and output drive connections to the main board?
When you say grounding the positive input makes the hum louder, is the negative input grounded and the amp set for single ended operation? Are you sure it is really grounded and you aren't acting as an antenna?
I don't think a diode would give you much change. You might try a negative temperature coefficient themistor for R19 if you can find one.
The question is why would you want to vary the fan speed? In class A the heat load is highest at idle and drops until you get into class AB. Chances are you won't ever make enough power to increase the heat load further.
The question is why would you want to vary the fan speed? In class A the heat load is highest at idle and drops until you get into class AB. Chances are you won't ever make enough power to increase the heat load further.
It has been awhile
Hello all.
The A75 is done and now at my buddy's house in Florida. It sat there for a very long time until I finished putting the output stage back together with some new Irf240's and complement. Fire it up and it works beautifully. I also included that feedback swith. I found a nice 3 position and made a selector for 0, 50%, and 100% feedback. I definitely preferred the no feedback setting for most music but it seemed like rock and other music with a decent amount of distortion sounded very harsh without any correction. It was almost grating. The other problem was that switching the feedback setting also switched the DC across the output. You would lose the carefully adjusted setting. All in all, I woul not make the selector again. For my listening style, I would set R81 to 500R and be very happy. My buddy's a different story. He listens to his German punk as well as alot of Classic Rock. As he was playing the Puhdys on my Joseph Audio RM25's I wanted to stab my ear with a fork. He loved it though. The ultimate insult was that he bought a pair of, in my opinion, totally inadequate speakers for such a beautiful amp. They are hooked up to a pair of Vienna Accoustic Schonbergs. He thought they looked cool. I was there when he bought them and despite my every effort to get him to stretch the wallet for a pair of Beethoven Grands....well the picture says it all. He had to buy me a few drinks afterwards to calm my nerves. I'm getting bent out of shape just thinking about it.
Anyway, it's new project time. I'm building an A75 for myself this time and intend to go all out. A full complement of Riken resistors are on their way as I live and breathe. They are being accompanied by a real nice pair if IXYS rectifiers and Mills resistors for the sources.
My intention is to drive 60V output rails off of a pair of 45V 1KW LoNo Plitron transformers.
A separate 60V 120W Plitron will provide the power for the front end.
I want to use the same regulator setup as the original design minus the voltage doubler. I figure I can get 77V if I just change R13 to 15K and R10 to 2K.
The output stage will consist of 8 pairs of IRF9240's and IRF240's.
Heat sinking will be substantial since I want nothing to do with a fan.
I don't want to use the Old Colony boards but would rather not have to go lay the whole thing out myself. Have any of you gone through this exercise already or have any ideas where I can get some really nice boards. I want to really tart this thing up. (Gold plated traces?).
BOB ELLIS WHERE ARE YOU?
GAME ON!!
Here's the Pic.
Hello all.
The A75 is done and now at my buddy's house in Florida. It sat there for a very long time until I finished putting the output stage back together with some new Irf240's and complement. Fire it up and it works beautifully. I also included that feedback swith. I found a nice 3 position and made a selector for 0, 50%, and 100% feedback. I definitely preferred the no feedback setting for most music but it seemed like rock and other music with a decent amount of distortion sounded very harsh without any correction. It was almost grating. The other problem was that switching the feedback setting also switched the DC across the output. You would lose the carefully adjusted setting. All in all, I woul not make the selector again. For my listening style, I would set R81 to 500R and be very happy. My buddy's a different story. He listens to his German punk as well as alot of Classic Rock. As he was playing the Puhdys on my Joseph Audio RM25's I wanted to stab my ear with a fork. He loved it though. The ultimate insult was that he bought a pair of, in my opinion, totally inadequate speakers for such a beautiful amp. They are hooked up to a pair of Vienna Accoustic Schonbergs. He thought they looked cool. I was there when he bought them and despite my every effort to get him to stretch the wallet for a pair of Beethoven Grands....well the picture says it all. He had to buy me a few drinks afterwards to calm my nerves. I'm getting bent out of shape just thinking about it.
Anyway, it's new project time. I'm building an A75 for myself this time and intend to go all out. A full complement of Riken resistors are on their way as I live and breathe. They are being accompanied by a real nice pair if IXYS rectifiers and Mills resistors for the sources.
My intention is to drive 60V output rails off of a pair of 45V 1KW LoNo Plitron transformers.
A separate 60V 120W Plitron will provide the power for the front end.
I want to use the same regulator setup as the original design minus the voltage doubler. I figure I can get 77V if I just change R13 to 15K and R10 to 2K.
The output stage will consist of 8 pairs of IRF9240's and IRF240's.
Heat sinking will be substantial since I want nothing to do with a fan.
I don't want to use the Old Colony boards but would rather not have to go lay the whole thing out myself. Have any of you gone through this exercise already or have any ideas where I can get some really nice boards. I want to really tart this thing up. (Gold plated traces?).
BOB ELLIS WHERE ARE YOU?
GAME ON!!
Here's the Pic.
dinu,
The only ones I know having built a A7 Is Peter Daniel, Bob Ellis and me...
The others can be seen on: http://www.passdiy.com/gallery-misc.htm
You'll have to ask.
Good luck with your A75 project.
PS: Pict missing?
Regards.
Alain.
The only ones I know having built a A7 Is Peter Daniel, Bob Ellis and me...
The others can be seen on: http://www.passdiy.com/gallery-misc.htm
You'll have to ask.
Good luck with your A75 project.
PS: Pict missing?
Regards.
Alain.
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