New Amplifier Project

[The Peasant]

Hello everyone,

My current project is building a pair of amplifiers in a couple of empty Bryston 4B cases. These cases, along with a 3B case, were originally purchased from Bryston as display props for a dealer, and from the outside look like a complete working amplifier, complete with heatsinks, etc.

I have Acoustat model 2 electrostatic speakers, and have always been impressed by the way that PS Audio amplifiers have seemed to work well driving these speakers. For many years I have used a pair of PS model 2 power amplifiers which I heavily modified with new power supplies and upgraded parts, and have been quite satisfied with them, other than needing a bit more power. So I decided to build a better version in the 4B cases.

The circuit that I have been planning to use is essentially the same as a PS Audio model 2CPLUS amplifier, the last type in that series. I will be using MJ15024/25 for outputs, and will be paralleling 4 of each per side. Each 4B amplifier will be about 100 watts/8 ohms stereo, bridgeable to 400W mono. The amplifiers will also use a completely separate power supply for the driver section, as PS audio originally did. Resista resistors, wondercaps, and 4% silver solder will also be used throughout the amps. I have a prototype channel up and running, and have a few questions:


What is the function of the diode and resistor feedback network to the 2N3405 bias transistor? Is this good thing?

Can the gain of the amplifier be increased slightly? The prototype required well over 1 volt rms input for full output.

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?

Any recommendations for driver transistors? I am currently using TIP41/42.

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?

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?

I currently do not have a spice program, so if somebody would like to check the design that way I would certainly be interested in the results.

Any other recommendations for improving this circuit without getting too radical?

All suggestions and input are very much appreciated!

Thanks,
Doug

[tiagor]

Hi,

I'm not an expert or anything coming near it so, don't take my word for granted and someone please correct me if i'm wrong.

Can the gain of the amplifier be increased slightly? The prototype required well over 1 volt rms input for full output.

With a current mirror on the differential pair you will increase de open loop gain of the amplifier, and also increase the voltage swing.

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?

Not that i know of except the 6.3V cap being cheaper. I'd stick with the 35V one.

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?

You probably have an oscillation somewhere, even without the zobel. Use a scope and you'll probably see it clearly even with a purely resistive load. Though it may work without the zobel I'd try and correct the problem first.
By the way, what is the speaker's impedance you're using? if it's 8ohm try replacing the zobel components with 100pf and 10ohm.

Regards,

Tiago

[The Peasant]

Thanks for your reply!

You probably have an oscillation somewhere, even without the zobel. Use a scope and you'll probably see it clearly even with a purely resistive load. Though it may work without the zobel I'd try and correct the problem first.

The scope shows no oscillations anywhere until the network is connected.

Thanks,
Doug

[tiagor]

If you really have no oscillation without the zobel you can probably live without it.
Also, please try connecting with a R of 10ohm and C of 100pf, R connected to output and C to ground(I am still assuming a load of 8ohm). At what frequency does it oscillate? How is your grounding set up?

[JensRasmussen]

Hello Doug,

Snip<
With a current mirror on the differential pair you will increase de open loop gain of the amplifier, and also increase the voltage swing.
>Snip

Yes you will increase the open loop gain (and maybe make a nice oscillator), however it will *NOT* change the closed loop gain of the amp.

To change the gain of the amp, just think of it like it's an opamp in a non inverting configuration, and change the feedback circuit to get the gain you need.

\Jens

[tiagor]

Hello Jens,

the idea was having more open loop gain and then change the feedback network to have the same amount(or a little more) of feedback and still have more closed loop gain.
Increasing gain by reducing the feedback doesn't seem a very good idea.

Then again, i might just be wrong

Regards,

Tiago

[mcp]

Hi Doug

Basing on the schematics, one of the most obvious feature is an amplifier with quite a restricted bandwidth.

330pF + 23.1K at the input diff already sets the upper limit at 20KHz. Coupled with 750pF at the drivers, I would not be surprised if it measures -3dB at 20KHz.

The other prominent point is the larger than normal amount of negative feedback used. The voltage gain is set at 21.5x which works out to about 27dB. That accounts for "The prototype required well over 1 volt rms input for full output."

Going into the details, it would be much easier to set the diff current with a fixed resistor at the current source (sink actually) rather than to try to adjust it with a 5K preset.

[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.

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.

No real advantage in using a 35V cap in the feedback network, 6.3V will do. If you want to make it better: put two caps of 1000u in series with minus to minus and use the plusses as the actual connections. I.e. make it bipolar.

Is your Zobel going to the star earth point? The oscillations suggest it is lifting the ground somewhere.


Steven

[Steven]

Originally posted by mcp
Hi Doug

Basing on the schematics, one of the most obvious feature is an amplifier with quite a restricted bandwidth.

330pF + 23.1K at the input diff already sets the upper limit at 20KHz. Coupled with 750pF at the drivers, I would not be surprised if it measures -3dB at 20KHz.

The other prominent point is the larger than normal amount of negative feedback used. The voltage gain is set at 21.5x which works out to about 27dB. That accounts for "The prototype required well over 1 volt rms input for full output."

Going into the details, it would be much easier to set the diff current with a fixed resistor at the current source (sink actually) rather than to try to adjust it with a 5K preset.

The 330p + 23k1 do not create a pole at 20kHz. It's just a gentle lead lag network in combination with the 3k01 collector resistors. Also the 750p caps are not decreasing the bandwidth to a sub 20kHz. Remember, they mainly affect the open loop gain. The closed loop gain can still be far above 20kHz, depending on the gain margin.

Just be careful in adjusting the 5k preset. Turning to zero may blow some transistors. It is used to adjust the tail current of the diff stage in order to make the output offset voltage close to zero. The tail current should be such that the voltage across both the 3k01 collector resistors is around 4 diode drops, like in the BD912 emitter.

Steven

[Nelson Pass]

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

Sliding Bias, that's a good one, I recall the Audio Critic blasting
my products for it while touting the Bryston.