| peranders |
The subject is just to start a debate. Did I succeed?
I read 600 uV noise at the output of some Pass amp. Let's say 1W out = fairly high volume with a normal speaker. This meens a S/N of only 73 dB approx a Dolby B (maybe C) cassette recording qualilty (if we only talk noise). Isn't that rather noisy?
I have never come across a Pass amp so I don't know, just curious about the use of MOSFET's at the input which is a big no-no in textbooks. JFET's and BJT are wellsuited for audio and (dualgate) MOSFET's good in HF front-ends.
If we only speak noise, how noisy are Nelson Pass' designs really? Does it bother those of you who has any Pass amp? |
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| djk |
| A turntable with a moving magnet cartridge is only capable of a 66dB S/N ratio becase of the thermal noise associated with the source impedance. |
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| peranders |
| quote: | Originally posted by djk
A turntable with a moving magnet cartridge is only capable of a 66dB S/N ratio becase of the thermal noise associated with the source impedance. |
Yes, but we do have CD and the dynamics of a CD isn't enough! We need 24 bits resolution, don't we? (no human beeing can really take advatange of 24 bit if you don't want to loose your hearing :) ) |
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| sernikus |
| quote: | Originally posted by peranders
[...]
I read 600 uV noise at the output of some Pass amp. Let's say 1W out = fairly high volume with a normal speaker. This meens a S/N of only 73 dB approx a Dolby B (maybe C) cassette recording qualilty (if we only talk noise). Isn't that rather noisy?
so what ? measurements means (mostly) nothing.... you should
ask : how it sounds ....
If we only speak noise, how noisy are Nelson Pass' designs really? Does it bother those of you who has any Pass amp? [/B] |
No .
why don`t you build your own copy of aleph ? it`s so simply ....
and you can try any modifications you want .... :) |
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| peranders |
I'm not after answers like "build your own amp", I'm simply trying to ask you who own a Pass amp how noisy they are at normal listening levels. 73 dB (according to my calulations) seems to be low.
so what ? measurements means (mostly) nothing.... you should
ask : how it sounds ....
I would turn the point of view around: Measurements really do mean something but not always!
I think that even Pass Labs measure thier products before delivery. They can't only listen to every amp in order to judge if it's OK. I think they have a more advanced quality system than that.
There is no need to show contemt for us who uses measurment instruments.
There is no shame to admit that Pass' amps has noise but otherwise sounds good. Since I don't have one I simply ask you about it. |
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| sernikus |
[QUOTE]Originally posted by peranders
[B]I'm not after answers like "build your own amp", I'm simply trying to ask you who own a Pass amp how noisy they are at normal listening levels. 73 dB (according to my calulations) seems to be low.
OK. noises from my aleph5 in normal listening levels are just inaudible. for me. even in higher levels. listening different kind of music (classical,jazz,rock,folk) i didn`t notice this noise ......
regards ,
waldek |
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| PedroPO |
Well I have built a Son of Zen with about 5W per channel and without any kind of technical knowledge( I project bridges, not amps';)') I have to say that with my "ear meter" the SOZ is way less noisy that my Audio Note Kit One, that is a Class A, Single ended, no feedback, Triode (300B) valve amp.
When I say less noisy I mean that with no source playing the hiss coming from the speaker is louder in the valve amp than the SOZ.
I also feel that SOZ is great at the low frequencies, (better than the valve amp)
and in the midrange the old valves kick butt...
Hope I've helped something
Pedro Oliveira |
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| vpharris |
Per Anders:
You're not getting the response you want, but can I ask- How do you go about making that measurement?
I can only read down to 5mv/division on my oscilloscope, so I'm not sure how to make a measurement to the precision that you apparently can. I would like to make those measurements when my stuff is done. |
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| peranders |
| quote: | Originally posted by vpharris
Per Anders:
You're not getting the response you want, but can I ask- How do you go about making that measurement?
I can only read down to 5mv/division on my oscilloscope, so I'm not sure how to make a measurement to the precision that you apparently can. I would like to make those measurements when my stuff is done. |
I have used a voltmeter with 10µV AC (wideband, more than the audioband) full scale. The instrument has also a couple of weighting filters. I have also connected the output of the voltmeter to an oscilloscope in order to "see" the noise. Can be valuable if I have problems outside the aodioband. Also I have listened to the noise (or the adsence of it). |
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| peranders |
I appreciate real observations in this matter. My question was very pratical.
Let's say the S/N is 73 dB at 1 W.
Normal speaker produces 90-92 dB. 73 dB down gives us 19 dB and this is slightly above the hearing threshold.
If 73 dB is enough why is 98 dB from a CD not enough so we want >120 dB from DVD? I know that higher resolution gives other advantages besides noise. Just thinking.... as I said in an another place the industry must come up with something new. |
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| GRollins |
I've got two pairs of Aleph 2s, one on midrange and one on tweeters. Noise, at least on my system, simply doesn't exist. In my case, the drivers are about 88dB/1W, only moderate in efficiency.
Someone with horns posted elsewhere and said that he had some hiss with the MOSFETs in the front end. Nelson's reply was to consider JFETs. I don't remember which amp the poster was using--I vaguely recall mention of 10W, which might indicate that he'd built a Mini-A or something of that nature. However, seeing as how the Mini-A is basically a straight port of an Aleph 30, the observation might still be applicable to your question.
All of the above is a long-winded way of saying that you can't ask a question about the amplifier in isolation. You need to consider what speaker it's being used with, what other electronics, and the listening levels involved.
Grey |
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| wuffwaff |
Hello peranders,
I´ve build an Aleph5 and use 86dB/2,83V speakers. You can hear some noise with your ear close to the speaker but this really isn´t relevant.
I´ve heard a lot of amps in my system and some (especially the powerfull ones) where certainly more noisy than the Aleph.
william |
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| cyclotronguy |
You would be amazed at how many pieces actually find their way to one of our homes for a quiet evening listening, before shipping out to the customer.
My personal speakers are close to 100dB/ watt/ meter.
Kent
Pass Labs |
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| wayne |
We spec 500 uV max. With carefull layout and supply attention the Aleph series is about 200 uV. Xamps vary from 120 uV- 400uV
depending on the model. |
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| swede |
Hi,
Per-Anders: You can try my Aleph 30, as soon as I get heatsinks, I'll assemble the whole thing. I just got the PC boards made and drilled. The next couple of days I'll match the transistors.
I live in Gothenburg as well.
Best regards,
//magnus |
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| rljones |
I build an Aleph about a year ago, using 3 output pairs per channel, and measured the noise on my HP distortion analyzer at about 450 microvolts (input shorted). At the time, I was using Lowther's (104 dB efficiency) and the noise was excessive and annoying.
I ended up regulating the power supply (mosfet shunt style ala Aleph P, but for more current), and dropped the noise level to about 70 microvolts. The amp was then dead quiet with the Lowthers.
Regards, Robert |
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| roddyama |
OK Guys,
Now you've made me nervous. I have most of the parts for my Aleph 3/5 that I plan to use as the high frequency amp in a tri-amp system. It will be responsible for powering JBL horns above 1200Hz, but a s/n of 73db would be a real problem with effeciencies of about 105db/w/m.
According to Robert with a simular efficiency, the stock Aleph noise level would be "excessive and annoying":(
Are there alternatives to regulating the supply?:confused:
Does anyone have an idea of how much of the noise is above 1kHz?
Will a PI filter in the supply have a greater affect on the hi-frequency noise, or the lo-frequency noise?
Thanks,
Rodd Yamashita |
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| wuffwaff |
Hi Rodd,
I use a pi filter (40mF-1.8mH-66mF) for my Aleph5 and get 7mV of ripple for 2A of bias. I think even the not so good PSSR is enough with this kind of ripple.
william |
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| rljones |
Let me clarify my comments with the Lowthers.
Before I regulated the supplies, I had no pi filter, but used about 40,000 microfarads per rail per channel. I had a noticeable hum component that I could trace from the power supply. Due to space contraints, regulation was easier to implement over a pi filter supply since I had a chassis with dual mono (no room for more caps and 4 chokes), and I didn't want to have a single power supply.
Pi filtering may be technically simpler and get the noise level down to a reasonable level with your horns. The sound from the Alephs on high efficiency speakers is extremely good; build them, you'll be happy. See if your layout is OK without pi filtering, but allow space for it as a modification if the noise should prove to be too great.
Good luck, Robert |
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| Nelson Pass |
After you have regulated the supply, the next step to the
lowest possible noise for the Alephs would be P channel
JFETs for the differential pair, like maybe 2SJ109's and a
quieter bipolar type current source for them.
Also, if your driver is that efficient, you can lower the gain
by 6 dB or so, and with it, the noise. |
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| roddyama |
Thanks for the replies, it's always a pleasure to get the constructive feedbacks, particularly from Mr. Pass.
I seen a thread somewhere on this forum where someone used an A40 frontend configuration to drive a set of Aleph output devices. Is this trickier then it sounds on the surface to accomplish?:confused:
I have all of the devices for an A40 including the boards. I haven't look at it closely yet, but I will. If I remember correctly, the A40 had darlington differential inputs with a JFET current source. Would this be sufficient to drive the set of three 240's?:confused:
Thanks,
Rodd Yamashita |
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| djk |
| I have used the A40 board to drive a tripple darlington BJT output stage and Lateral MOS, but do not think it is suitable for the IRF devices. |
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| macka |
Rodd,
I would like to hush your concerns regards noise.
Hum
In my setup (see pic of JBL Large format monitor speaker in biamp post) there is only discernable hum via the woofers at ground zero (100 hz I suspect).
Hiss
With my ears literally in the throat of the horn I can only just hear faint hiss (I do this reluctantly you can asppreciate why!!!)
My Aleph 5 uses +- 60,000 uf filter bank.
Asimple means of further reducing any trace of ripple would be to use a choke as desribed else where my Mr Pass (ie C+2MH+C).
Added noise from the BOSOZ is minor and I would expect most of your noise in the bi amp system to come from the electronic crossover.
As Mr Pass says, with plenty of speaker sensitivity just shave of some with a pad.
regards
macka:p |
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| tortello |
In my Aleph 5, biased with about 2.5 Amperes, the main contribution to the output noise is a 100 Hz hum, hence a power supply noise.
The capacitor bank is made by 132000uF per channel.
I've noted a 6 dB PSU noise reduction using an RC net (220 Ohm + 220uF) to filter the B+ rail for the first stage.
The cold side of the capacitor must be connected to the B- rail, near the R14's cold pin (in the A5 schematic).
With those values, the power on bump is almost the same.
Marcello |
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| roddyama |
Thanks for the feedbacks,
My plan is for an Aleph 5 configuration with 6 output devices/channel operating at about 2amps of bias (per set of 3), but at a lower voltage (about +-28vdc). I have all of the parts for the power supply including the inductors for the PI filter. I will also place hi-voltage caps across each of the bridge diodes.
Marcello,
If you don't mind me asking, how did you implement the RC filter? Is it just a resistor in series with the capacitor across the + and - PS rails?:confused:
I seen another thread that spoke about zener noise. Mr. Pass had suggested to use 1/2W zeners instead of 1W'ers. There was also a suggestion of adding a cap across the zener. I'm assuming that Z5 in the Aleph5 would be the zener of that discussion. Would a .1uf cap be an appropriate value placed across this zener?:confused:
I believe I will try these tweaks first and see how it turns out.
Thanks All
Rodd Yamashita |
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| tortello |
Yes, Rodd, but I've made a mistake: the capacitor was 100uF 100V, with 220uF the power-on bump slightly increased.
I also bypassed the current source zener with 0.1 uF metal film capacitor, but I'm not sure if this is a proper value.
Anyway, you can see the RC filter in the dashed box.
I found some Motorola zeners, employed as input-clippers, that where quite noisy - the noise floor could be seen by an ocilloscope! - , but if this happens, I think that the bypass capacitor is not able to correct this failure.
Marcello |
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| HarryHaller |
If the voltage to the front end current source is filtered too much you will have a hell of a turn on thump as the current sources for the output connected to the positive supply will come up before the current from the input stage and the output Mosfets to the negative supply.
The same result will occur from putting a large cap across the 9.1V zener reference for the current source. The solution? Use a LM329 voltage reference with a 0.1uF cap in parallel with it. The voltage is 6.9 volts so you will have to change the 221 ohm source resistor the get the same bias current for the front end.
The LM329 is very quiet and the AC impedance is over twenty times less than a zener for much better supply rejection. You can also also change the resistor that biases the LM329 (R10 on Aleph 5)to provide 1mA of current to it to get even more supply rejection. R = (Vpos-6.9) divided by 0.001. It is also much cheaper than the woofers you are going to eat by filtering the front end supply or zener.
http://www.national.com/ds/LM/LM129.pdf
H.H. |
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| PRBS |
| quote: | | [i]You can also also change the resistor that biases the LM329 (R10 on Aleph 5)to provide 1mA of current to it to get even more supply rejection. R = (Vpos-6.9) divided by 0.001. |
It should be R13. The LM329 can work from 0.6~15mA. (45-6.9)/10k=3.8mA. The orignal design will be ok.:p |
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| PRBS |
| Sorry, the calculation is for Aleph 2. :p |
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| HarryHaller |
R13 (10K) for Aleph 5. I used 1mA since that is the range where the LM329 is specified for the parameters in the data sheet, the math is easier (no calculator required), and I never use a part at the bottom or top end of it's range. Some one will pick the nearest 5% resistor value or the line voltage will be low one day and the part will drop out of regulation. Always use a little margin for error for changes in operating condition and parts tolerance!
"The orignal design(resistor value?) will be ok."
The whole point was to pick the minimum current and therefore the largest bias resistor value. This gives the greatest rejection of supply noise for the voltage reference. I would think one would want to do this right if he was going to go through this much trouble and we are talking a threefold or greater rejection ratio for the same parts count. A no brainer as they say.......
H.H. |
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| tortello |
With the values as above (220R + 100uF) the power-on peak increased less than 5%, as I could see by mu storage scope.
Marcello |
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| HarryHaller |
Yep and someone will go stick a 1000uF in there and bottom thier woofers on power up. Just wait......
H.H. |
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| tortello |
Harry, 100uF, not 1000uF!!
Unless a 1000% tolerance is normal for you...
Marcello |
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| HarryHaller |
You have never heard of the if X uF is good 10X uF would be better theory of power supply design. It is very popular in the US where we tend toward excess in everyyhing we do. My woofer warning still stands for the less cautious out there.....
H.H. |
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| PRBS |
| Why you said 10X uF, why not 1000X uF or 1000000X uF. You only based on THEORY to make an audio machine?? There are so many tricks.... |
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| roddyama |
Harry,
| quote: | | ...The solution? Use a LM329 voltage reference with a 0.1uF cap in parallel with... |
What a cool device:cool:
Have you ever used it before in this configuration? Did it perform up to the spec's?:confused:
Marcello, Harry,
| quote: | | ...and bottom thier woofers on power up... |
I won't bottom my woofer out 'cause I promise not to go more then 2X... er... maybe 3X.:D
Actually, I will be driving a pair of JBL LE85 horn drivers directly from this amps outputs. 5% of 50W starts to seem a bit much at 2.5W into a pair of horns. Although they won't hurt the horn drivers that can take 10X 2.5W, I would get a very rude turn-on spike of well over 100db. I may be able live with it if I had to since I leave my system on 24/7. I'll have to think about that one.
Thanks,
Rodd Yamashita |
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