Has anyone implemented a Pass style power amp with bipolars?

[event horizon]

Hi everyone, i was just wondering if anyone had implemented a power amplifier in the style of the Aleph series but used bipolar output transistors rather than mosfets? Having posted on shaans thread recently this idea suddenly occured to me. Ok so i know it's not a Pass clone, but there are similarities that gave me food for thought

I'm pretty well swamped with mosfets but thought about the fact that bipolars have a much higher transconductance than mosfets. I also happen to have quite a few MJ11016 N channel darlingtons that could easily be put to use

These would be more than suitable on something like a +/- 24V supply to keep them well in the SOA.

Ok, so they won't be as fast as mosfets, but higher transconductance should lead to better linearity with low feedback i'd have thought...

Any thoughts on the subject? Thanks in advance...


Mods, if this is in the wrong area feel free to move it. I didn't want to post it in the Passlabs section as it's simply a query

[Zen Mod]

you must include at least one additional buffer stage , in between input LTP and output stage
then it became not so "Aleph-ish" amp ....

[event horizon]

LOL, yes don't worry i have already thought about that

I'm not planning a clone, just something vaguely resembling an Aleph. You see i quite like the way the Alephs work with the variable current

[ilimzn]

Have a look at the SOA graph for your darlington, also at the Ft. As you say, +-24V would just about be as far as you can push it.
That being said, like Zen Mod says, if you wanted to do this with bipolars only, you would need at least a buffer stage after or before the LTP, because of current demand issues.
You could use a 'hybrid' approach, with a MOSFET input like in the original. Input capacitance notwithstanding (this is really the only truly problematic thing in that amp), it's relatively easy to get driving current from MOSFETs if you can get rid of the heat - you just increase the tail current. Added gm and in many cases better linearity (depends on the MOSFET chosen, if you wish you can always use a bit of degeneration given added tail current and keep the same gm at better linearity) are always a plus.

[event horizon]

I was thinking all bipolar. I don't have a problem with fairly high currents through the LTP & yes i was still thinking of a buffer to drive the base of the lower darlingtons & another single transistor emitter follower to drive the upper set.

I'd obviously be paralleling a few up just to make sure i keep them on the safe side, i realise they are close to the edge with a decent current @ these voltages. Well, not the 24V but when approaching the rails might be problematic..

I just fancy something a little different.

Cheers for your input ilimzn

[Telstar]

Originally Posted by event horizon

Any thoughts on the subject? Thanks in advance...

I have had the same thoughts, but my reason is that i prefer the sound of BJTs especially on the HF.
I think that I can post here instead of opening a new thread.

The following is what i have in mind.
The circuit on which I want to base my design is the BA-1:


Yes, the bipolars needs stronger drivers and probably predrivers too (= more stages). The frontend may well include jfets and mosfets.
Low beta droop fast transistors for output devices and VERY fast drivers, such as Sanken's 2SA1859/2SC4883.

Opinions are welcome.

[ilimzn]

Originally Posted by event horizon

I was thinking all bipolar. I don't have a problem with fairly high currents through the LTP & yes i was still thinking of a buffer to drive the base of the lower darlingtons & another single transistor emitter follower to drive the upper set.

I'd obviously be paralleling a few up just to make sure i keep them on the safe side, i realise they are close to the edge with a decent current @ these voltages. Well, not the 24V but when approaching the rails might be problematic..

I just fancy something a little different.

Cheers for your input ilimzn

Increasing the input tail current with bipolars offers advantages of higher gm, or more linearity if degeneration is used (and I would strongly recomend that it IS used), but it also increases base current, which in turn increases noise current and thus noise, unless you want to make the input impedance quite low. Of course, noise current is not a linear function of base current but in general with typical transistors we'd be talking about here, we are talking about a few mA. With MOSFETs you could just as well use 100mA and except for gm and heat, not much would change (even though MOSFETs also have optimum Id for lowest noise, but again, with types in question here, it's something they were never characterized for, so it can vary quite a lot depending on the actual type and manufacturer).
I think that the classical approach of a relatively small (compared to MOSFETs) tail current and a follower after the LTP would be the way to go. Using followers in front of the LTP may end up still needing a follower after, only because of the limitation of BJTs to use in a high current LTP not being that easy to find.
Still, I don't see why it wouldn't be doable...

[event horizon]

Originally Posted by Telstar

I have had the same thoughts, but my reason is that i prefer the sound of BJTs especially on the HF.
I think that I can post here instead of opening a new thread.

Hi Telstar, you are more than welcome on here chap I intend to add an Aleph type output stage with bipolars simply to reduce the quiescent current a bit. I can't exactly say whether i prefer mosfets or bipolars for outputs simply because i have only ever heard only one mosfet amp (that i know of) in my whole life

Originally Posted by ilimzn

Increasing the input tail current with bipolars offers advantages of higher gm, or more linearity if degeneration is used (and I would strongly recomend that it IS used), but it also increases base current, which in turn increases noise current and thus noise, unless you want to make the input impedance quite low. Of course, noise current is not a linear function of base current but in general with typical transistors we'd be talking about here, we are talking about a few mA. With MOSFETs you could just as well use 100mA and except for gm and heat, not much would change (even though MOSFETs also have optimum Id for lowest noise, but again, with types in question here, it's something they were never characterized for, so it can vary quite a lot depending on the actual type and manufacturer).
I think that the classical approach of a relatively small (compared to MOSFETs) tail current and a follower after the LTP would be the way to go. Using followers in front of the LTP may end up still needing a follower after, only because of the limitation of BJTs to use in a high current LTP not being that easy to find.
Still, I don't see why it wouldn't be doable...

I'll be using something like BC560C transistors on the LTP & yes i'll be using fairly heavy emitter degeneration. I'm not too worried about a low input impedance so i can certainly use a low value resistor for feedback & on the ground resistor on the input. Some messing with a bootstrap capacitor as per Doug Self could be used to increase AC input impedance, though i'm not sure i'll bother. I'd even consider paralleling a few transistors (with seperate degeneration resistors) if tail current became an issue. Though on a few amps i designed previously a 5mA tail current wasn't a problem.. It's all up in the air right now & this isn't the most important project that i'm working towards

I do have an idea of how i'm going to go about this & i'll endevour to get up some kind of circuit diagram as soon as possible. I'd draw it with LTspice if & when i suss out exactly how to use the thing properly I can't seem to get the thing to put PNP transistor into a circuit the correct way up

So it might be a scrawl on a bit of paper.

Again, thanks for showing an interest guys - much appreciated!

[event horizon]

Well it looks like this project is definately going to go ahead as i have commited myself by buying a couple of toroidal transformers

I was going to use a load of 0 - 18V frame types to generate +/- 24V but that was a tad too close to the 25V caps i have floating about here (47,000uf cans). Instead i found some odd toroids that are dual 0 - 17V at supposedly 1KVA yet the specs don't fit. These things are 165 x 90mm & weigh in at 9.5Kg each, looking at Airlink Transformers site it would appear they could well be 1.5KVA each

So i think a pair of balanced monoblocs are going to be built...

We shall see I'll be back

[event horizon]

We have transformers Brand new pair delivered for £60.

Had to do a little research on these beasts. They look to be using the same size core as the 1.5KVA transformers made by Airlink, but checking up on Lintrons' site they are definately 1KVA. Interesting that the regulation is better than the Airlink 1KVA & the same as their 1.5KVA toroidals. These things have been over-engineered The wire exiting the toroid is 6mm^2 on the secondaries, again, over-engineered & they measure bang on the money at 17.5V AC, which with 3% regulation will be 17V at full load - just what i wanted.

Well i'm sure they'll each cope with a 100W RMS (8ohm) monoblock