| metalman |
Ladies and Gents, I present to you a stuffed perfboard for a zen headphone amplifier based closely upon tortello's design (Thank you very much Tortello for sharing your excellently put together project!). My aim is to run my amp off a regulated 30V supply at a bais of about 300mA. I based my approach not off the schematic posted at Headwize, but off the first schematic posted in this forum by Tortello.
Today, unable to wait for my transformer to arrive to complete the project, I hooked it up to a variable DC power supply, and slowly fed it voltage until it was running off 30V. The good news, no pops, sparks or smoke! The bad news, the bias is only ~250mA and the drain of Q2 is at 10.4V referenced to ground instead of ~15. I am posting the schematic with the measured voltage at various points labelled in blue.
My thought at this point is that I need to replace R3 and R13 with pots (200K?) to allow me to adjust the bias and the voltage of Q2's drain. I'd appreciate and feedback of help that can be offered. |
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| metalman |
| A quick shot of the assembled board for the record. |
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| metalman |
| Lastly a schematic of the power supply I am building. |
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| metalman |
| I forgot to mention that both channels are operating exactly the same (voltage readings at each point are within a few mV for both channels). |
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| BobEllis |
Shooting from the hip having never built a zen or aleph, but the Vbe of Q3 looks low - if it was the standard .66 volts (as shown in Zen V4) you'd end up with more current - perhaps 300 mA. What current were you expecting? Odd that both channels ended up this way, but maybe new ZTX450s would help. Lowering the current source resistor will increase the bias.
A pot for R13 probably would be a good idea - Q2 seems turned on too much. Z V4 has the ratios of your R10 and R13 at least 2:1. You could try 75K-100K for R13, too, which should not turn on Q2 so much.
Odd that R4 shows DC current - you'd expect no DC current with a good mosfet.
Hope that this helps a little. |
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| VoltSecond |
| I haven't built the exact circuit you've got there. However in very similar circuits, the power supply rejection and current source output impedance was much better if C1 was attached to the source of Q1 instead of the emitter of Q3. |
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| altor |
I like vary much Zen design, and have buid some samples of ZenV4 and hybrid - ZenV3 with tube (6N23P) SRPP input stage - all work very nice!
But for the headamp - I'm not clearly understand why You use so complicate schematics?
Idle current 250-300mA - the low impedance (40 Ohm) headphones works very well, and with the rail of 30v - it is quite enough to drive any 600 Ohm headphones, without controlable current source!
So, I use more simple schematic - LM317 as a current source at the top, and IRF510 at the bottom, powered from "electronic capacitor" - i.e. mosfet with capacitor from the gate to ground. Of course, it possible to put also a zenner in parallel, to obtain not only filtered but alse stabilozed voltage, but I havn't do this, think it is not nessesary.
http://altor.sytes.net/headamp.jpg |
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| metalman |
Did my homework last night and figured out at least part of the problem. I was on the right track, and Bob confirmed it for me. R13 is too large, and a correct value should be around 130K, so I will replace R13 with a 100K resistor plus a 100K trimpot in series later today and see what effect that has on my current source problem.
As to why I am using the Aleph current source? I built the original Zen a couple of years ago, and I want to build another power amp, maybe a Zen4 or an Aleph 5 or an Aleph-X - haven't decided yet, and since I need a headphone amp I thought this would be a good exercise in playing with a more complicated topology on a smaller scale before I sale up to a larger project.
Thanks everyone for the suggestions and insight, and I will update how the toubleshooting goes as I make some progress.
Metalman |
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| metalman |
Over my lunch break I replaced R13 on both channels with 500K trimpots, powered them up and Voila! After a little adjusting the drain of Q2 is sitting happily at 15.1V. One problem down, one to go!:cannotbe:
I am posting my revised schematic again with the new measured voltages reference to ground listed again in blue. Of particular concern to me is tha fact that the base of Q3 is at a lower voltage than the emitter. My suspicion is that the value of R3 is too high creating too much of a voltage drop at Q3's gate. My plan is to remove R3 altogether, check the voltages again, and depending on what happens, I may replace it with a 100K trimpot.
Once again, any comments or suggestions will be greatly appreciated. Fun stuff this DIY, and so far haven't hit a brick wall!:D |
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| metalman |
| OK! Now I'm really confused! I replaced R3 with a 100K trimpot on both channels, connected it to a 30VDC supply, set R3 to max resistance (100K) and took some measurements. The schematic below shows what I found, which is not what I expected. |
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| metalman |
So next, I set R3 to the minimum resistance value, waited a bit for the circuit to stabilize, took more measurements, and found these results. These combined results have me completely confused, and I still don't understand why the base voltage always stays lower then the emitter voltage. Is it possible that I have Q3 in backwards (i.e. emitter attached to R2)? I was pretty careful about that but at this point I'm willing to believe I could be wrong.
By the way, both channels are doing pretty much exactly the same thing.
Greater minds of the forum, please help!:bawling: |
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| metalman |
| ****, I just realized that in my schematic that I show R7 as the 500K trimpot. That is incorrect. R7 is still a 2.2 ohm resistor, and R13 has been changed out to a 500K trim pot. Sorry for the confusion. |
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| digi01 |
Hi metalman,you main board look very neat,it's let I remember my first one zen headamp ;)
Have you tried to adjust her voice? About DC point, I advise you match the R7 and R6.
In addition, feedback loop of the active side of the circuit (Q2) will exert a tremendous influence to the sound. Some my diy friends propose that C2 should use a good one.
another playing method,put a little film capacity between b and c electrode of Q3.
regards,digi01 |
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| metalman |
Well, I tried setting R3 to various different levels, and there was a smooth inverse correlation between the value of R3's resistance and the resulting bias current. Lowering the resistance increased the bias and vice versa. Is this the way it is supposed to work. I also double checked, and I definitely have Q3 installed correctly.
Frustrated, I went back and reviewed (for the thousandth time) Nelson's Zen articles, and decided that since Nelson doesn't include R3, I'd disconnect mine. Without R3 the bias is ~260mA with a 0.56V drop across R6. However, the base of Q3 is still 0.87V lower than the emitter, which doesn't look right to me, and certainly contradicts what is in Nelson's articles. Currently, these are the voltage measurements referenced to ground. |
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| metalman |
So after all that I scratched my head and decided what the hell, it's sort of working, so I wired up the input and output connectors, connected it to the audio out of my laptop, plugged in my headphones and fired it up from a variable DC power supply. First thing I noticed is that the DC supply I have been using is pretty noisy. Without any music you can hear a nice 60Hz hum and a good dollop of hiss and noise. Oh well, I figured that would dissappear when I get the amps own PS built once the transformer arrives.
Dropped a CD in, hit play and was quite alarmed, as it sounded muddy, flat and lifeless. Had a moment of panic for a bit thinking that this issue surrounding Q3 was really making for a crappy sounding amp. Then common sense set in and I realized that this amp had never had a signal put through it before, and maybe I should give it some time to break in. So I wired the input for the left channel out of phase with the right, hooked it back up, crancked the volume, put the headphones in a drawer and left it play for 2.5 hours.
Break in definitely makes a difference. Much more dynamic now, certainly not muddy anymore, and sounding pretty good now. Still doesn't seem to sound quite right though (Yes, I did correct the polarity on the left channel!:D).
This problem with Q3 is still driving me nuts though, especially considering that the exact same behaviour is happening in both channels. I'd kill for some seasoned feedback on what might be going on about now. I have exhausted my bag of tricks from what I learned back in my second year electrical engineering course, i.e. I can tell how it is working but I can't quite grasp how to make that ZTX450 behave the way design says it should.
Anyway, here is a pic of the amp jury rigged to run off my laptop. |
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| azira |
| quote: | Originally posted by metalman
Well, I tried setting R3 to various different levels, and there was a smooth inverse correlation between the value of R3's resistance and the resulting bias current. Lowering the resistance increased the bias and vice versa. Is this the way it is supposed to work. I also double checked, and I definitely have Q3 installed correctly.
Frustrated, I went back and reviewed (for the thousandth time) Nelson's Zen articles, and decided that since Nelson doesn't include R3, I'd disconnect mine. Without R3 the bias is ~260mA with a 0.56V drop across R6. However, the base of Q3 is still 0.87V lower than the emitter, which doesn't look right to me, and certainly contradicts what is in Nelson's articles. Currently, these are the voltage measurements referenced to ground. |
I like debugging stuff.... You've got a pretty interesting problem here.
I'm going to just point out some (hopefully) obvious stuff here and hope it points you in the right direction for finding a solution.
Firstly, assuming your measurements are accurate and confirming that because both channels operate the same, you have the same voltage drop on both 2.2R's so that means the current is definately flowing through Q2. Good news because if Q2 was cut-off then you'd be sol to begin with.
So, since that has been established, we'll start by assuming that the Caps works properly (if you measured the bottom of C2 you could confirm this) and aren't shorted. C1 is the only POSSIBLE suspect device but very unlikely.
Therefore, the current flowing into R6 can only come from 2 places, the MOSFET source terminal OR the BJT base. We can't rule anything out.
By observing your potentials, there is several worrisom issues to consider:
1) The current flowing through R1 is not the same as the current flowing through R1 by 1ma. I hope this is just a slight measurement error or else C1 is trickling current through.
2) There is also apparently a slight current flowing OUT of the gate of Q1. This is a real issue for a MOSFET.
3) There is current flowing into Q3s base. ~1ma which is a very significant base current.
4) Both junctions of Q3 are reversed biased which should cause no current to flow.
Conclusion:
There seems to be a necessary voltage Vgs for Q1 to allow current to flow through it. In fact, Q1 is the only possible source for the current to flow because all the other current flows the wrong way.
Unfortunately, you've got some contradictions: while current is flowing into Q3, there is no current flowing out of it according to the measurements. This is an impossibility especially considering that Q3 is reversed biased.
Technically, this circuit shouldn't be working as you described it... |
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| metalman |
Azira, thanks very, very much for the feedback!
As you suggested, let's start with the basics!
"assuming your measurements are accurate"
So far I have been making all measurements with a Fluke Model 79 Mk III digital multimeter, connecting the COM probe to ground, and the measurement probe to the points indicated on the schematic by the voltage reading. I think this is a correct way, but then again, I'm a metallurgist/corrosion engineer, not an EE, and I think of potential measurements in electrochemical terms. Please correct me if I am wrong.
"we'll start by assuming that the Caps works properly (if you measured the bottom of C2 you could confirm this)"
The measurements have shifted aroung a bit after the burn in period, but I think this should partly answer your question. The junction between C4 and R9 measures 14.91V and the junction between R9 and C2 measures 14.14V, which says current is flowing into the negative terminal of C2, but then I measure the junction between C2 and Q3 at 14.49V which doesn't make sense. Then I place the probes at either end of R9 and I get a reading of 0.00V.
I just noticed something different again as I was getting you those measurements. A quick clarification on terminology first. When I say I am measuring A referenced to B, I mean that I have the positive probe connected to A and COM probe connected to B for the measurement. Following that terminology, I get the base of Q3 reading +0.47V referenced to the emitter. Next the collector reads +4.54V reference to the emitter, and the collector reads +3.58V referenced to the base. The collector of Q3 reads 20.13V, the base 14.49, and the emitter 14.82, all reference to ground. What does this say about my measurements??? I am beginning to think that perhaps I should remeasure using a different (better?) instrument.
A last note, if I measure the emitter reference to the base I get -0.61V, not the -0.47 I would expectfrom the above measurement.
OK, anyone want to give me a tutorial on how to measure voltage??? |
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| azira |
Well, I didn't nearly mean to imply that you didn't know how to use a DMM. You might have a chosen various reference points that were inconsistent however.
There should be no DC voltage across R9. So 0.00 is good. I personally use a cheap DMM that I got at a good price from a local hobby store. It's got a thermal couple so I'm happy.
So, those numbers you quotes for Q3 are all in the right direction. So that's reassuring.
Here's a sim I just did about what you should expect.
I have a suggestion. Try measuring Vbe of Q3 while adjusting R13. Turn it up a little and see if you can get that .47 to be more like .65 or .7. I noticed in my sims while trying to find the right value of R13 that if it was too low, Q3 got cut off although bias current still was able to flow. This was because the Vgs potential across Q1 was more than the threshold. If this works, then you might have to bias Q2:D a little higher than 1/2 Vcc and live with it. But at 30volts and headphones I'd say you don't need that much headroom.
In principle, you should be able to run a lower rail voltage with exactly the same circuit and have no problems once you nail the values right. |
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| metalman |
Thanks Azira,
I know you didn't mean to suggest any lack of knowledge on my part, but this whole scenario has me questioning the problem from every perspective including user error.
Thanks for your new suggestion. What you are suggesting follows logically from my perspective. Unfortunately I left the amp at work where I have access to equipment, so I'll give it a try over lunch tomorrow. Thanks again for your feedback and suggestions. They definitely are helping me to move in the right direction.
The 30V rails are because I could get a transformer with 24 or 30 V output, and I really wanted to regulate the output, so I needed about an 8V overhead. with the 24 V transformer output it was going to drop the PS voltage a little too low, so I went with the 30V output transformer, which after rectification, a CRC filter and regulation stage leaves me with 30. I could lower that, but hey, isn't more power always better :devilr:
BTW, With a few more hours of burn-in under its belt the amp is actually getting to sound better than pretty good. |
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| azira |
I use a 24V powersupply on my Zen and it's damn loud... loud enough for my garage while I'm working anyway...
But actually the main difference between your zen and mine (well I used the original CCS) is the bias current. If you biased it up, you could use it for loudspeakers, although you'd need a bigger heatsink.
good luck, let me know how it goes.
--
Danny |
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| metalman |
OK Danny,
I tried shifting around the midpoint voltage between Q1 and Q2, and as with how things have been going so far, what happened is not what we hoped. Adjusting R13 so that Q2 Vds was 20V resulted in almost no change at all in Q3's Vbe. Just to gather more info, I readjusted so that Q2's Vds was 10V and Q3's Vbe increased slightly to 0.50V, and the Bias current increased to a little over 300 mA. Interestingly, I had a listen to the amp at both settings, and the sound was substantially better at the lower Q2 Vds.
Next, just to try to compare efects of rail voltage and see if a lower rail voltage made any difference, I reset the Vds evenly for Q1 and Q2 and listened to the circuit at 15V, 23V, 30V and 38V. There was a noticeable increase in the gain of the circuit at 23V rails, but also a large increase in the distortion of the circuit as well as a drop in PS noise rejection. At 15V, 30 and 38V the gain was quite a bit lower, but the sound was MUCH cleaner and the PS noise rejection was better. Overall, the 15V rails produced the best sound!
Overall, I think this is a pretty clear idication that I might have been a little heavy handed when I soldered in my BJT's and overcooked them a bit, as the more I analyze the behaviour, everything is behaving according to the datasheet information except the BJT's. According to the datasheet, below 0.6Vbe the Ice should be limited to around 1mA putting Q1's Vgs well above 5V causing it to conduct wildly. I have seen Q3's Ice at over 10mA at a Vbe of 0.50V, which should never happen according to the datasheet. Does this make sense?
So I think I'll swap them out (luckily I ordered extras when I started this project) and see if that makes any difference. I still haven't received my transformer from Plitron, and the wife is making me a list of household chores, so it may be a day or two before I get around to it. I'll keep posting my progress as I move along.
Terry (aka Metalman) |
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| azira |
One more thing to double check when you put it in there is if you have the terminals the right way. BJTs will work if you put them in backwards... sort of. They can act like really bad BJTs. The only other thing to make sure is that you have the pin assignments right, some transistors are made BCE instead of CBE. The datasheet I downloaded says that they are CBEs though.
good luck |
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| metalman |
| Took a break from troubleshooting, and my transformer won't be arriving for at least another week, so I decided to build my CRC filter and voltage regulator circuits. This way I can reduce the noise on my variable DC power supply (or find out that there is no noise on my DC supply!) when I do further testing on the map after I make the next set of changes. I'll post a photo or two of the voltage regulator board tomorrow (forgot to bring the digital camera home fom work!). I think it looks pretty neat and tidy, but then again, I'm rather partail to my own work.:D |
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| metalman |
Finally got around to switching out the BJT's, and now I am getting measurements from the current source that make sense. I'm still not getting the 300mA bias I designed for, but I am getting 261mA Bias with the Vbe at 0.565V, a current of a few microamps at the BJT base. It isn't exactly what I was shooting for, but seeing as all the measurements are in the ballpark of what they are supposed to be and behaving in the expected fashion, I figure the circuit is working correctly. :D
Seem to have misplaced my digital camera, so may be a few days before I can post any photos of my voltage regulator board. Besides, my transformer from Plitron won't be arriving until the beginning of next week anyway. Time to work on the container.
One question: I didn't want to go to tyhe full expense of separate power supplies for each channel, but I wanted to isolate the channels as much as possible. So I went with splitting the supply after the CRC filter but before the voltage regulation stage. Is separate voltage regulation on each channel likely to provide a significant benefit?:xeye: |
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| azira |
Congrats!!! That's good news. 0.57 Volts is -BARELY- enough to turn on a BJT... actually it seems almost too low, make sure you hvae current flowing through the 1k collector resistor to be positive (should be a voltage drop across it). I'm still a little wary that it might not be working right.
With respect to the powersupply, that is way beyond me....
My zen has 8000uF of filtering, the PS I took apart to use for this quotes up to 2.1Vpp of ripple. Personally I've never heard the difference... Then again, I'm using a boombox headphone output to drive it and there is all kinds of noise (static, CD spin up...) besides music coming out of that thing. Not exactly what you'd call an ideal or hi-fi set up. It works for my garage though to give me music while I'm working on bigger and better things.
Oh yeah, and I'm using an athlon heatsink / channel to cool it which I believe swamps out ripple hum :)
I think I'm going to get banned or shunned from diyaudio.com for telling everyone what I used for my Zen :) |
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| metalman |
Danny,
I'm agree with you about the Vbe reading, but the voltage drop across the collector resistor indicates roughly 4.5mA running through it. According to the ZTX450 data sheet this should correspond to a Vbe of ~0.64V. But I triple checked the orientation of the BJT Leads (even harassed the wife into scrutinizing the data sheet and squinting at the transistor through a magnifying glass for a second opinion :hot: ), and took extreme painstaking care to solder them in with an absolute minimum of thermal effect, and last but not least I'm out of spare BJT's :cannotbe: :smash: The only opther indication I have is that if I change the R13 pot to get a larger voltage drop across the CCS mosfet, the bias current goes up, which my homemade mathematic circuit model predicts should happen.
This project for me is also a trial run at bigger and better things as well, to make sure I can put my money where my mouth is before I spend the money. Definitely discovered that I'm not ready for an Aleph-X, but a Zen V4 or an Aleph 5 look within reasonable reach, and I've committed to the Canadian Heatsink Group Purchase towards that end. Don't worry about being strung up by the forum for your Zen amp usage. Once they figure out I'm building the chassis for my headphone amp out of 1/4" MDF they'll be too busy hunting me down. (No one will ever confuse my work with Peter Daniel's :no: ) |
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| metalman |
| The chassis under construction. |
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| metalman |
| Voltage Regulator Board |
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| azira |
| Looks very nice. You might want a bleeder resistor on your caps. They will hold charge for a very long time without them. I found that I could use standard 1/4-watt resistors just fine as long as I paralleled them. Some people recommend 1-watt resistors. |
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| digi01 |
Hi PTSOUNDLAB,congratulations again:D
I am headwize member digi01. |
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| PTSOUNDLAB |
Thank you Digi
I have thanked and give homage to Marcello on Headwize et Nelson here ;)
It is important to not forget what Nelson do for DIY
Cordially
Pascal. |
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| metalman |
Well, it is finally, mostly completed. Actually received my transformer two weeks ago, but have been too busy to follow up on it (Damn career gets in the way:rolleyes: ). Completed the power supply, finished building the basic chassis, mounted all the parts, wired it together, powered it up and viola ... no smoke!
Funny thing is that all the Aleph CS issues I was concerning myself over have dissappeared now that I have it powered off its P/S. Suddenly my steady state bias is pretty much spot on 300mA. Can't say for sure what that is all about, but I'm filing that one under "All's well that ends well".
The Sound: Well, it is pretty damn good. My Grado Labs SR225's now have a very solid bass region with great extension down low, and I now have enough power for ungodly volume levels. Akira was right, 30V rails with 0.3A bias per channel is definitely overkill. There is also a liquidity present that was never there before.
One last problem remains. I have a bit of hum present, which I suspect is being radiated from the power supply, or maybe from a groundloop problem. It isn't particularly loud, just enough to be audible over the music, it is louder in the left channel than the right and its volume increases when I turn up the volume control. My concern is that the AC wiring running to the front panel power switch runs parallel to the mosfets of the left channel. I'm going to be inserting some shielding bewteen the P/S section and the amplifier section to find out if that is the problem. On the grounding side all I have so far is a 10ohm power resistor bewteen circuit ground and IEC ground terminal. Guess I should beef that up too. But before that I really need to get my volume control shaft and knob made.
Here are some shots of the current progress. |
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| metalman |
| The power supply. Plitron 50VA transformer with dual 15V secondaries connected in series. Pi filter is 12,000uF - 4ohm - 12,000uF connected to separate voltage regulator circuits for each channel. I only have 470uF of filter capacitance per channel after the regulators, so I will probably look at increasing that as a future upgrade. |
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| metalman |
| Another view from the side. I am using an Alps Blue Velvet 100Kohm dual potentiamoter as a volume control. My wiring layout for the conntection from the regulators to the amplifier board is less than optimal, which could also be causing the hum problem. Unfortunate case of building the chassis in a rush and realizing I hadn't drilled all the wiring holes in the divider before I glued it in. Oh well, it's only MDF. |
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| metalman |
| Font side with the cover on. |
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| metalman |
| Rear panel showing the Cardas RCA jacks and Furutech IEC Power Connector. |
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| metalman |
| And yes, I know, I built the chassis like a drawer. My wife has way tto much fun pointing that out to me. BUt she at least agrees it sounds good. |
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| Brian Donaldson |
Metalman makes a MDF case. For shame. :D
Very nice. One question. What's the advantage of the Aleph CS? I think it a excelent compromise in a 100W amp, but an unneeded evil in a HP amp. I'm not picking, I'm a hack and seek to understand the use of a heat saving device when we're only talking a few watts in the first place. I'll likely make a Zen HP amp before I make a case for my CCS Aleph and use the one that pleases me the most.
Happy listening |
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| metalman |
Brian,
Yes, the MDF is quite shamefull, but I was in too much of a rush to wait for my next chance to get into a metalshop to build a proper chassis. Which actually brings me to the reason for the Aleph CS. Since I don't have a metal chassis to act as a heatsink for the Fets, I am using the individual sinks you can see in the pictures, which have quite a limited heat dissipation capacity :hot:. As it is, you can touch the heatsinks for a second or so but not much longer than that without crying ouch! To get a higher maximum current output I thought I needed for my Grado's, I went with the Aleph CS which roughly doubles it. Now that I have it up and running, I can say with some confidence that I didn't need the extra output current :rolleyes:, and a plain CS would probably have worked fine. But hey, you can never have too much bias!:devilr:
I've been following your progress on your headamp and I look forward to seeing how it comes out. Trust me its worth the effort. |
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| azira |
| quote: | Originally posted by metalman
Akira was right, 30V rails with 0.3A bias per channel is definitely overkill. There is also a liquidity present that was never there before.
One last problem remains. I have a bit of hum present, which I suspect is being radiated from the power supply, or maybe from a groundloop problem. It isn't particularly loud, just enough to be audible over the music, it is louder in the left channel than the right and its volume increases when I turn up the volume control. My concern is that the AC wiring running to the front panel power switch runs parallel to the mosfets of the left channel. I'm going to be inserting some shielding bewteen the P/S section and the amplifier section to find out if that is the problem. On the grounding side all I have so far is a 10ohm power resistor bewteen circuit ground and IEC ground terminal. Guess I should beef that up too. But before that I really need to get my volume control shaft and knob made.
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Well, those Grado's are like 32-ohms? With 30V into those 32-ohms, you're way way overkill. 98db spl at 1mV, so you're talking like upwards of 140 dB with those rails, enough to seriously harm your hearing :)
There isn't a chance that you've got a center tapped transformer and just floating the CT? Otherwise I'd say half your rails and if you want, double your bias. Same power dissipation, less distortion. Still capable of liquifying your ears.
Anyway, looking at your pics, I'd suggest that you consider using some shielded cables instead for the audio and for the power. As far as your signal grounding goes, one pretty common way to isolate the GNDs is to connect your power GND (gnds from your circuit and the IEC and the transformer) to one star point and your signal GND (input gnd and output gnd) to another star point and then connect between the two with a single wire or a 10-ohm resistor. I think this is mentioned
on the passdiy pages somewhere.
Nice work though. Now I'm thinking about putting one together :)
--
Danny |
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| johnferrier |
| quote: | Originally posted by azira
Well, those Grado's are like 32-ohms? With 30V into those 32-ohms, you're way way overkill. 98db spl at 1mV, so you're talking like upwards of 140 dB with those rails, enough to seriously harm your hearing :)
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Yep, I've got 300 ohm Sennheisers and I get by just fine with +/-9V rails.
JF |
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| metalman |
If my calcs & measurements are correct, I am getting ~1.44W(RMS) into my 32ohm headphones, due to the output current limit. That works out to just a hair over 138dB max volume (headphone output is rated at 98dB at 1mV).
I designed for max power output roughly at 32 ohms to coincide with my headphone impedance, and max power is ~1.6W(RMS) into 38ohms. If my thinking is right, this is a graph of output power vs. headphone impedance. |
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| azira |
| quote: | Originally posted by metalman
If my calcs & measurements are correct, I am getting ~1.44W(RMS) into my 32ohm headphones, due to the output current limit. That works out to just a hair over 138dB max volume (headphone output is rated at 98dB at 1mV).
I designed for max power output roughly at 32 ohms to coincide with my headphone impedance, and max power is ~1.6W(RMS) into 38ohms. If my thinking is right, this is a graph of output power vs. headphone impedance. |
My hat's off to you. I just ran some hand calcs to verify what you were saying. That is a very interesting way to approach a class-A amplifier design. Actually it's a lot like how a friend (more like a mentor) of mine went about designing his headphone amp as well. In this way, you are maximizing your power efficiency into your headphones, minimizing the bad effects of a class-A. Your numbers sound dead on.
--
Danny |
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