# Pass' amps way too noisy?

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#### peranders

Paid Member
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?

#### 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.

#### peranders

Paid Member
djk said:
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 )

#### sernikus

peranders said:
[...]
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 ....

#### peranders

Paid Member
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.

#### sernikus

peranders said:
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

#### 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

#### vpharris

Noise Measurement

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.

#### peranders

Paid Member
Re: Noise Measurement

vpharris said:
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).

#### peranders

Paid Member
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.

#### 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

#### 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

#### cyclotronguy

Pass Amps

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

#### wayne

Paid Member
Noise

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.

#### swede

You can try mine...

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

#### 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

#### 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?

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

#### 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

#### 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

#### Nelson Pass

Paid Member
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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