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Old 28th May 2001, 11:15 AM   #1
lohk is offline lohk  Europe
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Hi folks,

meditating about a mic-preamp circuit ba Phil Allison, published by Rod Elliott http://www.sound.au.com/project66.htm , I had the idea that one could take the first part of this preamp to form a single ended balanced in/out line gain stage according the way Aleph P or BalZen is suggesting.
The use of differential compound pairs of conventional transistors is at least an interesting thought.
I know, besides the approach of Master Pass, the single ended differential pair is done by Borbely with JFETS for example, but has anybody experiences with "normal" transistors?

Klaus
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Old 28th May 2001, 12:04 PM   #2
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Pass uses JFETs himself for his later X-series amps + pre-amps. Check out some of the service manuals for preamps -- can't remember which ones.

The problem with JFETs is power dissipation. You are thus limited to a certain voltage and current range. Put complementaries butt to butt with perhaps a resistor between them.

The disadvantage of "normal" transistors (bipolar junction types) is the base current required which usually means you need to find some interesting way to set up base ("gate") bias manually. Since tubes, JFETs and MOSFETs don't carry steady state gate (or grid) current to speak of, they can normally be self-biased.

Petter
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Old 31st May 2001, 01:02 AM   #3
lohk is offline lohk  Europe
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Petter,

when you look at the circuit mentioned above, the first part - the differential input with compound pairs - is actually an amp itself and is not included in the NFB path together with the following Opamp. So - assuming the complete circuit works - the first part should also work alone, probably with a relativly high output impedance. (And if it works with microphones it should work with other input sources too).
Finding the right output potentiometer impedance could be a problem.
BTW, I am not biased against mosfets in any way, but it was just an idea...

Klaus
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Old 31st May 2001, 01:49 AM   #4
jam is offline jam  United States
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Klaus,

Have you considered that this might make a great front end for a phono pre-amp. Since a phono cartridge is inherently balanced anyway and you could insert the eq. between the diff. amp and the op-amp.

Grey,
Have any comments? You are probably busy working on your tube amplifier's negative supply.

Jam
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Old 31st May 2001, 02:30 AM   #5
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Jam,
Funny you should mention balanced phono, since that's what I'm building...
Actually, the idea of a single stage line is pretty seductive. Once I get the phono circuit laid to rest, and nail a few other coffins shut, I'm planning on doing a single stage balanced tube line preamp. (Probably 6SN7s, but leaving the door open to 6922/6DJ8s.)
*WHAT?* Grey, are you crazy? High output impedance, blah, blah, blah...
Well, yeah, actually I am crazy. Or else I wouldn't have so many people telling me so.
The card I've got up my sleeve (you knew there was a card up my sleeve, didn't you?) is that I'm going to combine the line stage with the crossover, which will save about $20 trillion in interconnects (quad-amp systems get unwieldy quickly), not to mention keep the back of my system from being any more of a rat's nest than it already is. (I'll have a full range bypass out, in addition, just in case I want one down the road.)
Anyway, the basic plan is that it will be a hybrid, thusly:
Selector->Vol->Line (prob. 6SN7s--about 15-20 dB gain)->2N5457 (JFET as follower/buffer)->Sallen-Key filters->MPSA18 (bipolar for low output impedence [as follower])
balanced all the way through.
So, as you can see, I'll end up with low Zout after all. Yes, I could use tubes as cathode followers and accomplish the same result. Eventually I will get around to trying it as an alternative just to see if it sounds better.
Back to the original question Klaus raised: Actually, I think MPSA18's might be a good candidate for a single stage balanced preamp (MPSA42/92s if you want high rails), but I tend to side with Petter, overall. Still, it shouldn't be too difficult to rig a voltage divider to bias the transistors, or...argh! don't get me thinking like that...I've got a phono stage to build!
Anyway, the idea has merit, as MPSA18/42/92s can be made to sound pretty decent. Give it a whack and let us know how it sounds.

Grey
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Old 31st May 2001, 03:15 AM   #6
jam is offline jam  United States
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Grey,

What you suggest will probably sound great but in keeping with a tube differential, why not a White cathode follower?
BAT uses multilple tubes in parellel to get a low output impedence in their tube pre-amps. They believe that followers hurt the sound. The problem is that when you are diving a low impedence the gain changes (as the cathode resistors are in parallel with the load). This could be a paticular problem with amplifiers that have balanced inputs with different impedences for the + and - legs(for example the Pass Aleph amplifiers).
I myself like Fet's and Mosfets,but what ever you choose the follower could make or break the circuit, I have found the follower is more critical to the sound of the circuit than most people suspect.

I could tell you about ways to improve followers using current sources and cascodes but that would probably make your hair curl......

Jam

PS. Is your phono stage anything like Allen Wright's RTP?

[Edited by jam on 05-30-2001 at 09:29 PM]
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Old 31st May 2001, 04:38 AM   #7
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Jam,
The CJ ART line stage is also paralleled (10x6922 sections per channel, but single-ended, I believe). I had considered doing something similar, but in a balanced configuration...until I took a look at what it would require in a power supply. (I still may yet do it after I get a few other things settled.) Then I realized that there wasn't any need to worry about low Zout from the line stage since I was going to be working into a known load and could do it without interconnects and all the attendant L&C that they entail.
As far as followers and criticisms thereof. They've got their tradoffs, but there've been some damned fine sounding pieces of equipment just chock full of followers, whether cathode, emitter, or source. (As I've said before, I like solid state & tube, each for what it does well. In a pinch, I somewhat prefer tubes, particularly up front where I don't have to worry about power tubes going flooey.) The reason I'm using followers is high Zin, low Zout, which is ideal for filter math.
Current sources & cascodes with followers work well for some applications, but I don't really need them, and would prefer to keep it simple. I've already got a version 4.0 or so of the crossover running (been working at it off and on since last November or December). It's breadboarded and ugly as the girl in my sixth grade math class, but it sounds simply lovely. I've got one or two things I want to do to fine tune it, but my system already stomps anything I've heard in a month of Sundays.
Can it be improved? You betcha! The two main weaknesses at the moment are the phono stage--which is only 40dB gain--not optimal for a moving coil. I'm working on that. And lower bass power and control (15-30 Hz). I've been working on that sporadically for a while, but it's a Herculean task, as I've got to build 12 amplifiers, one for each woofer, and outfit each one with a feedback loop off the driver cone. I've got several ideas I want to pursue there, but the one I like best is optical. It'll be a bit before I get that done. For the time being the drivers are simply running in standard Thiele-Small cabinetry, good down to about 25Hz or so. It'll have to do for the time being.
I'm not familiar with the RTP. Got a website I can look at?
My phono stage:
6922 differential->6922 differential->passive RIAA->6922 differential
Total gain is on the order of 60 dB, plus or minus a few--I don't remember exactly; my notes are at home. Note no followers at the end, as I'll be running straight into the switch & a known load, etc. The way it's set up I have the option of using a 12AX7 up front to increase the gain, but don't really anticipate needing to do so. (Lyra Helicon cartridge--decent output.)

Grey
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Old 31st May 2001, 05:20 AM   #8
jam is offline jam  United States
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Grey,
This should get you where you need to be

http://www.vacuumstate.com/images/RTP5_rev_1.GIF

It sounds like you have a plan. My main concern would be noise.
The circuit that you describe looks very much like the Bat phono pre-amp.They include the option of inserting a transformer in circuit to allow for more gain in the more expensive model.
I myself might consider using FET'S in the input stage to avoid noise problems, but as you say your cartridge has probaably a high enough output to avoid this.

Jam
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Old 31st May 2001, 05:39 AM   #9
jam is offline jam  United States
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Grey,
On the subject of paralleling that many tubes, this can cause the sound to grey out (no pun intended). This could be caused by different characteristics of each tube. What if one tube drifts or one section of a tube drifts or fails and what if...........
I have played with the CJ and it can be a pain in the butt keeping it sounding consistant let alone one channel sounding like the other. No thanks, I have better things to do.

Jam
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Old 31st May 2001, 11:48 AM   #10
Alex M is offline Alex M  United Kingdom
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Jam wrote:

"Have you considered that this might make a great front end
for a phono pre-amp. Since a phono cartridge is inherently
balanced anyway and you could insert the eq. between the
diff. amp and the op-amp."

This is similar to what Ben Duncan did in his ADEQ phono
stage, albeit using the SSM2016 chip, which is basically a
pair of op-amps with a differential pair topology at its
input. Duncan points out, as you do, that cartridges are
particularly suited to a balanced input, although I've seen
very few preamps which exploit this. Using a discrete
differential pair at the input would definitely help to
keep the noise even lower, particularly for low-impedance
cartridges; the 2016 actually has a good noise figure,
despite being an integrated circuit.

In the ADEQ the 75us filter is put between the 2016 and the
first op-amp, while the latter looks after the rest of the
EQ. I'm sure Grey would be horrified at the number of
active devices in this circuit, but it does work very well
all the same.

I've just rebuilt my ADEQ with a balanced input, but since
the arm lead is still unbalanced I get a little hum pickup.
Rebuilding the arm lead is yet another of the projects on my
list of jobs to do.

Alex
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