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]

Originally posted by 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 ).

[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.

Originally posted by 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.

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