X-preamp volume control

[Hans B]

Hi all

I thought I would try to start this thread up again.

I am designing an X-preamp and was hoping for some help on this volume control (maybe a hint ).

First of all the circuit:

The basic concept of the design is taken from the passlabs D1 service manual, except I use JFET 2sk389 as the input differential and the fact that I have X-ed the circuit.

Actually come to think of it - it looks like a mix of an XBOSOZ and the D1 circuit.

Specs are (according to spice -I havent build it yet, need the volume control still):

Vin max balanced : 7V peak
Vout max balanced : 28V peak
gain (balanced) : 4 (12dB)
Input impedance : around 20k balanced
output impedance : around 300 balanced
Frequency range at full scale (7v peak) : 0.03Hz-500kHz
THD @1kHz @ full scale 28V balanced out: 0.18%
THD @1kHz @ 4V balanced out: 0.0023%
Minimum load impedance (dependent on IL2/IR2) : 800 Ohm


I have tried to use shunt of bipolar switches (blue square area) and it seems to work quite well. As you can see a resistor (they are not really pots) is coupled to the collector of a bipolar transistor, and is turned on by voltage applied to the base. For simplicity I have only drawn 1 of these bipolar switches, but the idea is to have a number for bjt's each connected to their own resistor, to form the full volume control.


When using this method it is not possible to get 0V out with a direct shunt, because of the internal Rce, but with the used transistors I can get 800uV balanced out @7V balanced in, which is low enough - if you want lower, increase the input resistance. These BJTs might not be the same in the final design, if they are very expensive - I just picked some with low Rce.


Sofar so good, but initially I wanted to place the volume control after the amplification, that takes place in the differential pair. Either at the output or at the MOSFET gates.

If placed at the output, I would need a resistor in series with the signal (after the cap) and then shunt to ground, but this changes the output impedance quite a bit.

If placed at the MOSFET gates they should be shunt to a "virtual ground", which in this case should be 16V, but I have no idea how to do so.


My questions are:

Is there a way to place the bipolar shunt before the MOSFETs and in this case - how?

Is shunting between phases a good idea (Petters mentioned the problems with using BJTs to shunt phase)??

Am I (at all) in the right direction when thinking of "Waynes balanced passive masterpiece" - A hint perhaps Mr. Pass or Mr. Colburn??


Any thoughts, comments would be appriciated.


/Hans

PS. unfortunately I can't make orcad put out a decent picture - you will have to settle for this zipped version - sorry.

 

[UrSv]

And what does that shunt attenuator say when the input signal is negative? Seems like swithing something with an NPN where the collector is at a lower potential than the emitter would be hard.

 

[Hans B]

Hi UrSv

That is the reason why the BJT transistor is placed after the resistor (with emitter close to ground or virtual ground) and not before. When connected as on the diagram, the voltage over the BJT transistor will always be very small (in the range uV). If the BJT was placed before the resistor - the situation you are referring to occurs.

At least - thats how I view it

Hans

 

[MBK]

hmmmm... I still wonder if that is the famous Wayne circuit... I mean is this implementation a concept that's patentable? The shunt to ground thing alone can't be it, that's not novel in itself...

 

[Hans B]

Hi again

I think I've got progress!!!

I have found as section in the aleph L owners manual about a patent pending volume control. Could it be possible that passlabs has 2 volume control patents pending. I would assume that they only have 1.

As I read it, the volume control is a passive resistive network, when the gain of the preamp is less or equal to 0dB. When the preamp has to amplify (from 0dB to max gain) a switch determines that the signal goes through the active preamp circuitry. Simple (in principle) and very very smart (as always).

Think about it - for preamp with a 70dB volume control range of 1dB steps and 6dB of gain, the active circuitry are only used in 6 of the 70 steps - the rest (64 steps) is pure passive resistive network. And I don't know about you guys, but I usually don't have my volume control above the 3'O clock position (refer to Aleph L manual, where 3'O clock is 0dB gain - I'm not saying that all preamps has 0dB at 3'O clock!).


Could it be that the great Pass and his crew have been smiling this whole time, when Wayne Colburns famous attenuator has been brought up, knowing that some of this information already existed in some form on the passlabs web page???

The unbalanced version of the volume control is shown on page 10 in the aleph L manual. But how does bipolar shunt to ground come into play in this circuit???? If you choose to shunt to ground, your going to mess with the input/output impedance, and I can't see how it is possible to achieve smalle output resistance and high input resistance in this case. So I am a bit lost on this one - any ideas??

Do you think that this could be the patented solution - having passive attenuation below 0dB and active above 0dB????

Anyway - gotta get some sleep, I will look at it more tomorrow.


/Hans

 

[Petter]

I looked into this topic extensively about a year ago. I reverse-engineered said volume control and did measurements on it based on BJT, JFET, MOSFET and mechanical Relays.

When push comes to shove, the result was that even cheap relays significantly outperformed the only real other alternative (BJT).

The other advantage of relays are that they do not have to be referenced to ground.

The downside is power consumption, switching speed (potential for glitches), size, weight and longevity.

Petter

 

[macka]

Hans,

I like your design,

Have you gone any further at this stage and listened to the circuit?

Ian

 

[till]

As the dipchips solution was to expensive for me, and the ADC0804 relay volume control i made before not really satisfied me, i built this

http://home.tu-clausthal.de/~tpa/relais/index.html

 

[Petter]

Hi all

I thought I would try to start this thread up again.

I am designing an X-preamp and was hoping for some help on this volume control (maybe a hint ).

[snip]

My questions are:

(1) Is there a way to place the bipolar shunt before the MOSFETs and in this case - how?

(2) Is shunting between phases a good idea (Petters mentioned the problems with using BJTs to shunt phase)??

(3) Am I (at all) in the right direction when thinking of "Waynes balanced passive masterpiece" - A hint perhaps Mr. Pass or Mr. Colburn??

1. Yes you can do this. It will be similar to a potentiometer on the input.
2. I think phase shunting is a good idea for a number of reasons . The main issue is you have to be near ground level (or an alternative rail). There is also yet another alternative - shunt (or "deshunt") between sources of input transistors. When the resistance there is infinite you get zero gain. It is also even possible to change the feedback resistors. In principle the "de-shunting" seems promising but I have only seen it used (by the master) as a gain range setting. Not sure how well it will work in X mode though, depends on how much open loop gain you have and whether you are kicking into that before the feedback resistors take effect.
(3) The last item requires connection to ground and cannot be used phase to phase easily.

And what does that shunt attenuator say when the input signal is negative? Seems like swithing something with an NPN where the collector is at a lower potential than the emitter would be hard.

This is what baffles most of us, but has been replied to before. discrete BJT's provide the "best" performance after JFET's (terrible asymmetry) and MOSFET's (don't even think about it). There are of course quite expensive options which may be very good (such as semiconductor relays for the telecommunication industry manufactured by companies such as International Rectifier and Vishay.

Petter

 

[Petter]

You can also shunt between R4R and R4L is using relays, mechanical swithces or potentiometer (or through capacitor if going for ground).

Also, there is not all that much cost (albeit some work and complexity) to add to make this a real X.

Petter

 

[Hans B]

Macka:

I have not yet made the design and can't comment on sound quality. I have been waiting for the wayne's PRBLM, and I posted
here to see if I was on the right track with the shunt to ground method. Right now I don't know what I am going to do. I am down to 3 choices:

1: wait for the PRBLM description.
2: use X-pro's: Constant impedance relay-resistor logarithmic attenuator (balanced)
3: use part of the APOX circuit (relay shunt between phases)



Petter:

Thank you for your inputs. I have 3 comments.

1)
You mention that the resistor between the input JFETs (R1) could be used as a volume control, and I thought of this, but the resistor have to be really big before you hit 0V (infinity). The simulation shows that a 100k resistor gives you 0.4V at the output, which is too much. Of cause you can increase to 1M and decrease the gain further, but I don't like the idea of using very big resistors. Also, as I understand it, the resistor between the JFETs control the "amount" of X in the circuit (the distortion is cancelled through this resistor), and if you increase R1 you decrease the X-effect (although I could be wrong).

2)
I mentioned that I would like to shunt before the MOSFETs to ensure a constant input constant output impedance, however shunting before the MOSFETs doesn't seem to be a good solution in this circuit. I don't have enough current running through the differential to shunt between R4L and R4R, so I kinda scratched that idea now. I would now prefer to do it at the output


3)
you say:
Also, there is not all that much cost (albeit some work and complexity) to add to make this a real X.

Does this mean that you don't consider this an X circuit??
If so I please explain why - As I mentioned it is pretty much a cross between the XBOSOZ and the D1 circuit, and it should be an X circuit. The principle is basically the same as Henriks XBOSOZ.
http://www.diyaudio.com/forums/showthread.php?s=&threadid=3949&perpage=15&highlight=&pagenumber=17

Thanks again for your comments

/Hans

 

[Petter]

Macka:
1)
You mention that the resistor between the input JFETs (R1) could be used as a volume control, and I thought of this, but the resistor have to be really big before you hit 0V (infinity). The simulation shows that a 100k resistor gives you 0.4V at the output, which is too much. Of cause you can increase to 1M and decrease the gain further, but I don't like the idea of using very big resistors. Also, as I understand it, the resistor between the JFETs control the "amount" of X in the circuit (the distortion is cancelled through this resistor), and if you increase R1 you decrease the X-effect (although I could be wrong).
/Hans

It is true that you need an infinite resistance to get to zero volume this way. However, the "X'ing" resistor is basically equivalent to 2 source degeneration resistors in an ordinary long tailed pair. The larger these source degeneration resistors, the more linear (before feedback) and lower gain do you get. My personal opinion (no flames, please) is that too much is made of the "X'ing" resistor. Of course as your gain is lowered, the effect of the other feedback is reduced.

I suspect some of the benefit of the X unit is that it likely has high open loop gain with significant reduction through the feedback network. The way it is set up now, you will have low gain before feedback.

Macka:
2)
I mentioned that I would like to shunt before the MOSFETs to ensure a constant input constant output impedance, however shunting before the MOSFETs doesn't seem to be a good solution in this circuit. I don't have enough current running through the differential to shunt between R4L and R4R, so I kinda scratched that idea now. I would now prefer to do it at the output
/Hans

Yeah, it is painful to have a single gainstage - you have to do the volume either before or after the stage, not between. I have a fix for that too, but it is non-X.

Macka:
3)
you say:
Also, there is not all that much cost (albeit some work and complexity) to add to make this a real X.

Does this mean that you don't consider this an X circuit??
If so I please explain why - As I mentioned it is pretty much a cross between the XBOSOZ and the D1 circuit, and it should be an X circuit. The principle is basically the same as Henriks XBOSOZ.
http://www.diyaudio.com/forums/showthread.php?s=&threadid=3949&perpage=15&highlight=&pagenumber=17
/Hans

I missed IL1 and IL2 due to their non-standard placement . However, I don't like it all that much because M1 and M2 are not acting as cascodes - they need to be p-channel to do that. If they are p-channel, however, you need to turn them upside down and fix the source potential problem which means it does not draw from top rail, but from R4L,R. Then again, the output capacitors ... Check out the X100 thread, first posting .... Remember the top cascode does not get driven at it's gate ...

Now if you put the volume control at the output, you can still add class A emitter followers (if BJT, emitter to emitter with emitter degeneration) at the output.

Hope I am not discouraging you with what is not intended to be a negative posting.

Petter

 

[macka]

Thanks Hans,

I have asked about Wayne's remote pre amp also.

I am unaware of the delivery on that project

Ian

 

[Hans B]

Hi Petter

I am glad somebody is responding , and you have a lot of good points so I appriciate your response - If nobody replied I would consider this a sign, that I was on the wrong path.


I like your comments, and they are all true, and you have got me to look at the circuitry differently now. I should have stated that I was not trying to make a replica of the patent, but a JAXP (just another X preamp), and that the design wasn't a two stage with a folded cascode approach, but a more simple JFET input differential followed by a "source follower" mosfet, and that the design principle was low open loop gain and modest feedback.


What I think you are suggesting is to go for the folded cascode (like the patent and your x100 backengineered - great work btw) and have more open loop gain. This might be a good idea, I have to look at it some more, but you got me interested in going that way, so I will pursue it more in the following days.


Now this X-thing is really pusseling me.

For instance - the open loop gain:
I thought the idea of X circuits was that it cancelled the distortion through the differential, and thereby removed the idea of using a lot of feedback to remove distortion. And Nelson Pass states in the XSOZ thread that:
"Just about any amplifier with an input diff pair can be turned into an X, but it only seems to work well with simple circuits and low open loop gain. (too bad, huh?)"

And you say that: "I suspect some of the benefit of the X unit is that it likely has high open loop gain with significant reduction through the feedback network".

So my question is, why you think that more feedback is good for the X'ing - As I see it more feedback will lead to less distortion in the "traditional" feedback way, not by X. Anyway it might not be that much of a problem when using a folded cascode or my attemp, since the open loop gain is relatively small.


Another thing is the differentail pair. You mention that the change in the source resistor in the differential will lead to less distortion but also lower gain, and since I think I read somewhere that the distortion in the differential does not get cancelled by Xing, then you have a very valid point. In the next version (will be a folded cascode) I will ensure that the differential in itself has low distortion, and that the common gate FETs are in charge of most of the amplification. Anyway I might be getting ahead of myself here - I have to look at it more before stating such things.


So thanks for the comments

/Hans

 

[Petter]

Thanks for your detailed reply. I don't agree you had source follower at stage 2 - that would not work well into a current source IMO.

Hi Petter

And you say that: "I suspect some of the benefit of the X unit is that it likely has high open loop gain with significant reduction through the feedback network".

So my question is, why you think that more feedback is good for the X'ing - As I see it more feedback will lead to less distortion in the "traditional" feedback way, not by X. Anyway it might not be that much of a problem when using a folded cascode or my attemp, since the open loop gain is relatively small.
/Hans

If you look at the X100 thread, the input gain devices are IRF610's which have a pretty huge current gain. Now that those are loaded against the outer bottom current sources, you have very high gain.

The gain is attenuated by two things: First the "X'ing" resistor. This is more or less equivalent to having a single central bottom current source with source degeneration resistors on the input FET's.

The second item that sets up gain is the combination of input resistors and feedback resistors. If the fundamental gain is high enough, the total gain is exactly Rfb/Rin. (Rin includes output impedance of prior stage if you are getting picky)

So, why is this setup supposed to sound so well? Well, the input transistors are shielded and operate more or less optimally. The output is more or less at the same level as the input, and there is only one gain stage around which the feedback is placed.

Feedback is good. Feedback around several gain stages is normally shunned in audio circuits. Feeback compensates for the fact that there are cascode devices through which the signal has to pass + other adverse effects.

Hi Petter
Another thing is the differentail pair. You mention that the change in the source resistor in the differential will lead to less distortion but also lower gain, and since I think I read somewhere that the distortion in the differential does not get cancelled by Xing, then you have a very valid point.
/Hans

There may well be an Xing effect that the horizontal resistor has over ordinary degeneration. Also, when the Xing resistor is zero ....

Also, if you want a low gain unit, select JFET's which I seem to recall you had already done, and you are pretty much there.

Let me know how you get on.

 

[Hans B]

Hi

Petter:
I still don't understand why you don't consider the output MOSFETs of my first attempt a source follower (common drain). To me, the output is just a single ended class A follower driven by the voltage that appears from the differential. Erno Borbely uses followers with JFETs in one of his articles. If I am fundamentally wrong about this I would appriciate an explanation as to why this is


To the new design:
The design is now based more on your X100. It is now an all JFET amplifier, and uses the folded cascode like the X-circuit and a follower output (follower is taken from Erno Borbelys article 2 about JFETs), and I included the follower in the feedback loop. This made me wonder why you did not include the output stage in the feedback loop in your x100 - is there any particular reason?



Also does anyone know if the real passlabs X-preamps uses folded cascode??
(well, I know of at least 1 who does )


Any comments would be appriciated!


/Hans

 

[Petter]

I still don't understand why you don't consider the output MOSFETs of my first attempt a source follower (common drain). To me, the output is just a single ended class A follower driven by the voltage that appears from the differential. Erno Borbely uses followers with JFETs in one of his articles. If I am fundamentally wrong about this I would appriciate an explanation as to why this is
/Hans

They are source followers. What I don't like is the ac coupled output + driving the source of the follower through a current source.

The topology is a single gain stage with source follower with feedback around the source follower + a capacitor to level shift. I am not sure how the current source will work in this application.

One of the main reasons for using followers is to lower output impedance. The capacitor in series with the output is not something I am particularly fond of.

This made me wonder why you did not include the output stage in the feedback loop in your x100 - is there any particular reason?
/Hans

No reason not to, apart from the fact that NP and others suggest it is a bad idea. In a power amp in particular, the load is typically complex which can pass (no pun intended) various types of signal back. NP uses very low impedance output stages to control the load regardless of what it is. In a less expensive output stage, people do the same with feedback. Then they also often use an output inductor to reduce the load's effect on the input stage through the feedback.

Try both, pick what fits you best. You will have less distortion if you put the feedback around the output stage.

Hi
Also does anyone know if the real passlabs X-preamps uses folded cascode??
(well, I know of at least 1 who does )
/Hans

Many of Pass' recent designs that have been in the public domain use folded cascode. All X series units by definition need to use it (with the possible exception of the XA range). If you think about it, how else can you make a fully DC coupled single gain stage where the input DC level is the same as the output DC level?


I also assume you want comments on current design which I assume to be X oriented. If that is the case consider:

1. RL1 is not necessary per se but the value is so high it will likely have little impact.
2. Interesting output circutiry albeit unconventional. I would have put a resistor between J3A and IL3 to set up a direct representation of the same signal at different DC levels. The value of this resistance and IL3 would set the current. You might even want a capacitor across it. Then, you could drive the output buffer directly and control the current at the output stage by setting up source resistors (and varying the first resistor in the normal way). You could also use a Vbe multiplier in this position if you like but you are not likely to need negative tempco in a preamp.
3. The beauty of this design means that you don't need to have (nor want to have?) output capacitors CL1.
4. Note your gain will only be 2 per phase with said input/feedback resistors. You may also check if you really need two current sources at the bottom, but hey - this is totally getting somewhere.

Petter

 

[Nelson Pass]

I'm in the midst of writing up the Wayne PRBLM project.
Does anyone remember what that acronym stood for?

 

[Nelson Pass]

Many of Pass' recent designs that have been in the public domain use folded cascode. All X series units by definition need to use it (with the possible exception of the XA range). If you think about it, how else can you make a fully DC coupled single gain stage where the input DC level is the same as the output DC level?

Actually not true. Folded cascode is not essential, and appears in
in two commercial Pass Designs, the X600 and X1000 (which have
exactly the same circuit board. The lesser X designs are seen to
use ordinary cascoding on the front end.

In the XA designs we have no cascoding, and that would be consistent with the Aleph circuitry. That does not mean it
might not eventually make an appearance somewhere, folded
or otherwise.

 

[moe29]

PRBLM: Wayne's Passive Remote Balanced Line Masterpiece.

(and that's a quote) from the DIY progress report thread