New Amplifier Project

[sam9]

What is the objective of the parralleled caps at the input, 8uF + .001uF + .68uF ? I understand the 8uF all by itself but the others? Was the orginal anticipating the 8uF would be some kind of electrolytic that didn't have flat bandwidth all the way out to 20kHz? Then there is the 150pF cap that filters HF. So first you let the HF pass then filter it off? If my WAG that it is a babdwidth issue reflecting limitations of ancient component why not just replace all three with one good quality 10uF metal film cap?

[sam9]

As long as I'm asking questions: What is the intended benefit of a sliding bias? I tried stiking the diode and resistors in to an existing SPICE model to see what the effect was. Vusually, any before/after traces I plotted on any scale just sat on top of each other. The "after" subtracted .00006 from the THD figure. Not earth shattering if true and just as likely to be an artifact of the order in which the emgine does it's calculation. Or simple attrition of significant digits.

Assuming N.P. wouldn't put it in a design just for fun, is it a way of making optimum bias easier to set and to maintain under real conditions?

[The Peasant]

Thank you to everyone who has posted responses so far, they have been very helpful!

When I removed my ammeter from the negative rail and cleaned up the wiring a bit the oscillations stopped! Doh!

To increase the closed loop gain a little, just make the 2k21 resistor of the feedback network a little smaller. You have open loopgain enough.

OK, I reduced it to 1R5, and the gain is fine now. But, will this effect the gain when I use the inverting input on one channel when bridging?

What is the objective of the parralleled caps at the input, 8uF + .001uF + .68uF ? I understand the 8uF all by itself but the others? Was the orginal anticipating the 8uF would be some kind of electrolytic that didn't have flat bandwidth all the way out to 20kHz? Then there is the 150pF cap that filters HF. So first you let the HF pass then filter it off? If my WAG that it is a babdwidth issue reflecting limitations of ancient component why not just replace all three with one good quality 10uF metal film cap?

The original design used a 0.1uF film cap in parallel with a 10uF electrolytic. I am using original vintage IAR Wondercaps for these amplifiers and that is their recommended configuration for signal bypassing. I have always felt that multiple bypassing like this is best.

As long as I'm asking questions: What is the intended benefit of a sliding bias?

Yes, I'd like to know the answer to this as well. Should I take out the circuit or leave it in? My older PS Audio amplifiers do not have it.

Does anyone have any comments on the separate driver/output power supply topology?

So far it looks like I'm getting 125 watts into 8 ohms, with a frequency response extending reasonably flat to at least 50kHz.

Thanks again, everyone!!!

[mcp]

Originally posted by Steven

The function of the diode and the resistor in the bias part seems to be there to increase the bias current for positive or negative going signals. Sliding bias. If the output is at ground potential the 1M resistor does nothing. For positive outputs the base of the bias transistor goes up and the 1M and 221k resistors are pulling it down a bit, trying to decrease Vbe and as a consequence of feedback by the 6k81 resistor the Vce will increase increasing the output bias current. A similar thing happens if the output goes down. Then the 221k resistor would pull up the base of the bias transistor via 1M, but that is prevented by the diode and the 91k4 resistor. These are pulling down and create a voltage divider with the 221k resistor to drive the base via 1M. Again bias current increases.

Hello Steven

It did not occur to me at first that it is a Sliding Bias. My initial impression was that it shuts down the drivers in the event the outputs went full positive or negative rails.

You are correct about the 330p + 23k1. I have given the impression that it sets the dominent pole. It is indeed a phase lag compensation network set at 20KHz. Thank you for highlighting this.

As for the 750pF at the drivers, the slew rate of the amplifier will be quite severely affected, leading to my conclusion of a bandwidth limited, high negative feedback design.

[slowhands]

Originally posted by The Peasant
..... What is the function of the diode and resistor feedback network to the 2N3405 bias transistor? Is this good thing?

As shown, I don't understand the function. The diode appears to be backwards, if this is an auto bias circuit, ala Nelson Pass A40.

Originally posted by The Peasant
..... Can gain be increased? The prototype required well over 1 volt rms input for full output.

That is normal and customary. Most preamps will drive it to full output easily. The gain is set at about 22 by the ratio of the 47K5 and 2K21 resistors, but you can change the ratio to say 30 by changing the 2K21 resistor to 1K6 (if you don't mind additional distortion). Personally, I would leave it alone, it's fine as is.

Originally posted by The Peasant
..... I've noticed that the feedback capacitors on amplifiers tend to be low voltage electrolytics, is there some advantage to using a 6.3V 470uF here as opposed to a 35V 470uF?

Even a 3v rating is fine and keeps physical size down. The voltage here should be almost zero in normal operation. However, the cap polarity is backward for the NPN input pair shown, as this cap will always have a slight negative voltage on it (maybe -300mv).

Originally posted by The Peasant
..... Any recommendations for driver transistors? I am currently using TIP41/42.

MJE15030/MJE15031 is much better pair. You're on the hairy edge of needing a predriver stage, given 4 output pairs. By the way, you need at most 2 pair for 100 watts.

Originally posted by The Peasant
..... When the zobel network is connected, the amplifier will oscillate at a very high frequency. Without it it works fine, even when driving the electrostatics. Any suggestions as to what to look for here?

Probably your Zobel network is putting high frequency into your signal ground circuit. Put the Zobel off card, right on the output binding posts is ideal. Then the ground current returns back to the star ground, not modulating your input and feedback signals. (You do have a separate return path for them, right?) Try that first and see if it helps.

There might be stability problems also if it oscillates after connecting the Zobel net. Hmm.... You might try an 8-10 ohm resistor in that net, but if moving the Zobel net off card does not help, I suspect you need to add a compensation capacitor on the VAS in addition to moving the Zobel net off card.

The usual practice to ensure stability would be to place the main compensation cap from base to collector of the VAS, here the BD912 on the diode string. The typical value is 50-100pF.

Originally posted by The Peasant
..... I feel that a separate regulated power supply for the driver circuitry is very important and results in a more stable and better sounding amplifier. This may be why this type amplifier works well with the difficult impedances and current demands of the electrostatic speakers. How do others feel about this?

My general philosophy is to try to make the front end immune to power supply noise, and use an unregulated supply with a modest RC filter for the front end (10 ohm, 100mf paralleled with your favorite film cap). The exception would be in driving MOSFETs, where there is significant voltage loss due to bias and gate drive requirements. Then a supply about 10V higher is advantageous.

But in the EF BJT case here it is risky. As has been pointed out elsewhere, if you aren't careful you will overdrive and saturate the outputs on clipping, and they will cross-conduct and self destruct. Baker clamps will prevent this.

[Ultima Thule]

Doug,

maybe you should move away those 750 pF and see if you still have oscillation.
Those capacitors may give a too large phase lag before you have unity gain which is the same as oscillation.

If you still experience oscillation:
Afterwards you can, as pointed out by others, put a Miller cap between B & C ob the BD912 VAS stage, but start with a much smaller value than 750 pF!

Tell us how it developed if/when you have tried it.


SAM9, (Doug),

I think you know that every capacitor have it's on fo, the smallest one on the input, 1 nF, do have propably a lower fo than 150 pF.
But on the other hand, I think 1 nF in the input cap is overkill, by putting a good 100 nF the Fo is still very high, probably couple of MHz.
This would be a better choice if RF is a concerne and the 150 pF will see a higher input impedance and make the work better if the smallest input cap is larger, eg. 100 nF.


Regards

[sam9]

Another question: Does the schematic imply lower voltage rails for the IS and VAS? It looks like it might but doesn't say so expressly.

[sam9]

Oh yes, a 1uF cap across the 103R/2W resistor between the BD911 and BD912 in the output stage will probably reduce crossover distortion. The way I understand it is that when one driver turns "on" as the signal starts to swing in it's direction, it forces the other driver "off" by "stealing" current from it, thus making the transition between upper and lower halves of the output section cleaner.

[The Peasant]

Another question: Does the schematic imply lower voltage rails for the IS and VAS? It looks like it might but doesn't say so expressly.

Yes, they were originally at 38 volts and the output rails at 42 volts. I am running them both a bit higher than that now.

Thanks,
Doug

[sam9]

I think the MPS8099s will limit you to 38V in the IS, at least if you want to use devices that are safe even if expossed to the total rail-to-rail potential. MPSA42/92 would take care of that problem. In fact you could then simplify things by having a single set of rails.