Class i and siblings

[dadod]

Here is quick simulation of an amp with Edmond non switching auto bias OPS.
I appreciate comments and suggestions, specially VAS improvements.
Thanks to Edmond Stuart who is author of this excellent output stage.
I will try to simulate the same IPS with Class i OPS next.
Damir

[dadod]

And now I simulated almost the same IPS connected to the Class i OPS.
Distortion is higher then with Edmond OPS, but it could be improved with better( I suppose) VAS as input resistance of the Class i OPS is highly non linear.
Damir

[Edmond Stuart]

Hi Damir,

Indeed, the input impedance is highly nonlinear. Most likely, that's the cause of much more distortion. See below for the test circuit and its Zi as of function of the output voltage (Actually, it's the output current that alters Zi). Zi varies from 60kOhms to tens of MOhms.

Why improve the VAS (i.e. lower Zo of the VAS) to compensate for deficiencies of the Class-I OPS? Isn't it a waste of time time, as the OPS itself can easily be improved (by means of my AB2 OPS, as shown in your previous post)?

In the next few days, I will show that the nonlinear Zi of class-I is not the only downside of this OPS.

Cheers,
E.

[Edmond Stuart]

And here for the sake of comparison, the input impedance of the AB2 OPS:
Now, Zi varies between say 1 to 10 MOhms. Needless to say that this variation has far less impact on the distortion, as a typical VAS has far less difficult to handle such a variable load.

edit: Zi simmed at the DC level. Above 10kHz it drops considerably (of course).
See for more info on geometric mean biasing: http://home.tiscali.nl/data.odyssey/AutoBias_II.html

Cheers,
E.

[dadod]

Lower distortion with Class-i OPS.
Separate emitter resistors in Class.i input transistors lowered distortion but it is still ten times worst then with Edmond AB2 OPS. Not sure yet how this change influences thermal stability.
Damir

[Edmond Stuart]

Hi Damir,

I see that you have incorporated the filter network (R63, R15 & C23) between the VAS output and the OPS input. However, this filter is only needed if you are using the OPS in a standalone environment, e.g. connected to your tube pre-amp. When embedded in a 'complete' amp, as shown by your post above, you may omit this filter and probably get better distortion figures.

Regarding the 0.33 Ohm emitter resistors (RE), be warned. Since the output impedance (without global NFB), equals RE, any mismatch between the top and bottom RE will affect the performance. Kendall has already warned us: A mismatch of 1% will increase the 2nd harmonic (of 1kHz) to -77dB. Therefore, he recommends the use of precision (0.1%?) high power metal film resistors. I don't think that enough. Also unequal PCB traces and imperfect solder joints may ruin the performance. So I also recommend a Kelvin connection to these resistors.

Cheers,
E.

[dadod]

Originally Posted by Edmond Stuart

Hi Damir,

I see that you have incorporated the filter network (R63, R15 & C23) between the VAS output and the OPS input. However, this filter is only needed if you are using the OPS in a standalone environment, e.g. connected to your tube pre-amp. When embedded in a 'complete' amp, as shown by your post above, you may omit this filter and probably get better distortion figures.

Regarding the 0.33 Ohm emitter resistors (RE), be warned. Since the output impedance (without global NFB), equals RE, any mismatch between the top and bottom RE will affect the performance. Kendall has already warned us: A mismatch of 1% will increase the 2nd harmonic (of 1kHz) to -77dB. Therefore, he recommends the use of precision (0.1%?) high power metal film resistors. I don't think that enough. Also unequal PCB traces and imperfect solder joints may ruin the performance. So I also recommend a Kelvin connection to these resistors.

Cheers,
E.

Hi Edmond,
Two questions:
1. How critical emitter resistors are in your AB2 OPS, and how to adjust question current for different emitter resistors value(.22R)?
2. I intended to put all in one box, tube pre and OPS. Do I need the filter network in this case(how about AB2 OPS)?
Damir

[Edmond Stuart]

Originally Posted by dadod

Hi Edmond,
Two questions:
1. How critical emitter resistors are in your AB2 OPS,

Hi Damir.

In the AB2-OPS tight matching of emitter resistors is not critical, as the output impedance depends not RE. The trouble with class-i is that the feedback is taken from the RE's. At idle from both RE's, when sourcing current from the top RE and when sinking current FB is taken from the bottom RE.

In case of the AB2-OPS however, FB is taken directly from the output (as it should be). Therefore, the output impedance is much lower. This is a distinct and important difference.

and how to adjust question current for different emitter resistors value(.22R)?

By increasing the currents through Q9 and Q12 (in post 4), i.e. set R13=R14=100R. BUT I wouldn't do that, as the minimum current through the OP devices comes close to zero. A zero current should be avoided at any time. Set for example R13=R14=91R and you will see the devastation effect.

2. I intended to put all in one box, tube pre and OPS. Do I need the filter network in this case(how about AB2 OPS)?
Damir

Much depends on the output impedance (also at HF) of the pre-amp. It also depends on whether some HF filtering has been done in the pre-amp. The point is that the OPS (both versions) likes to see a low Zo of the preceding stage (in order to avoid instability) and don't like extreme fast transients.

Another point of concern is the DC offset. If the pre-amp is only AC coupled to the OPS (most likely) you have to define the DC operating point of the OPS by means of a suitable resistor at the input to GND. However, if this resistor is too low, the pre-amp doesn't like it (more distortion) and if it is too high you will get more offset. Somewhere in between there must be an optimal value (I hope).

Cheers,
E.

[dadod]

Hi Edmond,
Thank you, I think I see now what I am going to do.
As AB2 looks mutch better I will use this one, with my tube preamp, and in all transistors version.
What alternative is for WHAT transistors, matching individual transistors and somehow thermaly connect them?
damir

[Edmond Stuart]

Hi Damir,

Probably you mean THAT transistors (never mind). In principle, you can use any decent low noise small signal tranny (beta > 100; Ft > 100MHz and low internal Rb and Re), but they should be matched, that is, PNP to PNP respectively NPN to NPN. I guess that a delta Vbe of about 3mV (or less) is sufficient.
Regarding thermal coupling, again PNP to PNP resp. NPN to NPN.
Remember, we are dealing here with only 33mV across the emitter sense resistors at idle condition. So any Vbe offset will have a large impact on the quiescent and minimum current of the OPS. This applies to the Class-I as well as to the AB2 OPS.

BTW, Farnell has the THAT300 and THAT320 in stock, though they are a bit expensive, about 9 euro.

Cheers,
E.