New Lineup IDEA - Power Follower/Output stage

[FdW]

Originally Posted by lineup

If you look at my latest schematic in this post: New Lineup IDEA - Power Follower/Output stage
you see one trimpotentiometer on the input.
This is the way to adjust for the output offset.

Isn’t there a problem with this? If the trimmer is not in the center, then the degeneration resistors for the transistors are different and thus the gain will be different. Could that be a problem?

[sakis]

OK ....first attempt for pcb is done
here is some detailed pictures and also large files can be downloaded from my server .
There is also detailed explanation and also someone should double check for errors
Users that also have S-Print layout 4 may also down load a complete file from my server
board size is 135X80

kind regards
sakis

[sakis]

www.eastelectronics.gr/LINE%20UP/CONNECTIONS.GIF

http://www.eastelectronics.gr/LINE%2...20complete.GIF

http://www.eastelectronics.gr/LINE%2...20location.GIF

http://www.eastelectronics.gr/LINE%2...top%20side.GIF

http://www.eastelectronics.gr/LINE%2...0numbering.GIF

[Struth]

Hi Guys

The drivers should never be on the main heat sink; they should always have their own board-level heat sink.

As LC stated, the diamond input devices need to be in contact with each other to thermal track, not only from one half of the circuit to the other, but within each half. To make this simpler mechanically, all four BJTs in the diamond should be 2SC/2SA, then all four can be bolted together or at least bolted to a common piece of flat aluminum supported by the four BJTs.

Obviously there needs to be thermal tracking for the usual bias generator forms. Two-fold tracking is usually best, where ambient is tracked by one device and the output transistor temp by another device. For a CFP output, the driver would be tracked instead of the outputs. (For a CFP with gain, both need tracking).

Again with the offset trim: adjusting input node resistance seems tenuous as it makes the input impedance for the circuit halves different. This could effect performance, especially at high frequencies. Adjusting a current source on a one-time basis using a trimpot may work, but as with any trimpot approach the question still remains as to long-term reliability of the setting's effectiveness.

An op-amp based servo introduces its own issues. The discrete servo options outlined earlier are simple and have an appeal because they are discrete and can operate directly from the supply rails.

There should be local filter caps on the PCB. This improves transient response and reduces the interaction of the amp with the supply. A small bypass cap should go rail-to-rail to make the rails look "more identical" at very high frequencies. Bypasses across each filter cap are not always a good idea and can inject noise into the ground path.

Does your board supplier charge more for double-sided versus single-sided? The standard these days is double-sided, so there should be no need for jumpers. Also, get 2-ounce copper as a minimum. This used to be standard but now the default is 1-ounce unless you specify it.

Traces for the supply feeds to the outputs, the connections to the speaker, ground paths between the filters and speaker, should all be fairly wide. How wide depdns on the current to be handled. There are charts available on the net that show treack width versus copper weight and temperature rise. For example, 100Wrms output at 8R represents 3A5 rms or 5Apk. To be conservative, take the peak value. For a temperature rise of the trace limited to 10C, and 2oz copper, the track width should be about 60mil - 0.060" (1mil=0.001" - a North American machining term).- or about 1.5mm.

The tracck width example assumes a purely resistive load and a higher actual current may be present with a real speaker load, so make the high-current tracks wider if possible.

The front-end track width should be narrow for best high-frequency performance and not have any right angles. Even orthogonal traces bent at 45-degrees can be problematic as the sharpness of the corners can act like antenna. Some PCB software has a feature to round these, otherwise it must be done manually. The compromise with skinny traces is that they may not take much resoldering as usually the pads around the holes are also skinny.

Supply connections (+,0,-) should accommodate #18 wire. So should the speaker connections (out,0). The input should accommodate shielded wire, which might require a large-ish hole for the shield connection depending on the type of coax you wish to allow for. Input and output grounds should be decoupled.

Have fun
Kevin O'Connor

[sakis]

I will agree with most of the remarks with Kevin ...

Still Kevin this is just a demo board to make it run ...so from all the above the only thing that is really important for the operation of the amplifier is the thermal matching issue .

Line up has to give me directions about that and about which devices should be coupled with which ...

As about a diamond with all TO220 devices i think is a bad approach ...

Kind regards
sakis

[lineup]

Originally Posted by sakis

I will agree with most of the remarks with Kevin ...

Still Kevin this is just a demo board to make it run ...so from all the above the only thing that is really important for the operation of the amplifier is the thermal matching issue .

Line up has to give me directions about that and about which devices should be coupled with which ...

As about a diamond with all TO220 devices i think is a bad approach ...

Kind regards
sakis

AS Lazy Cat said here,
those 4 transistors in the diamond should be together 2 and 2.
First the two 2SC4793+2SA1837
and also the two 2N5551+2N5401.
It is also good if those two pairs are very close to eachother.

Another thing that was wrong in the schematic I emailed you, sakis
is the output filter.
Inductor+resistor comes first and at the end comes capacitor+resistor(Zobel).

You can see it in this picture:

[Struth]

Hi Guys

I've used the tabbed BJT solution for thermal tracking and it works fine. It is a mechanical convenience. The IC forms of the diamond buffer, like the LT1010 and similar, rely on the thermal tracking of all four devices to provide both wide response and any semblance of acceptable DC performance. It helps that the devices are fabricated on the same die and hopefully have similar specs.

Without close tracking of gains with temperature, you have to take a hit on bandwidth and increase degeneration. This allows less accuracy of all types of matching to be less problematic. For a chip that is supposed to be a video driver, such a solution is unacceptable. For a discrete circuit that is to handle audio frequencies, it is purely an aesthetic affront to have performance diminished from the simulated theoretical ideal.

This buffer is a wideband circuit, so the details in my post are not just refinements for a later date, but may adversely effect initial results without them being in place from the beginning. Despite the compensation caps, oscillation due to layout error is a possibility with this circuit. If you are going to the trouble to etch your own boards, or to order them from someone else, why not lay out as much of the final solution as possible? These are aspects of board layout that apply to every audio PA design, not just to this one. Lineup asked for opinions and direction about thermal tracking and basic layout. You can take or leave any advice offered.

I take a conservative approach when testing and designing. I haven't blown up devices during an initial test since I was in high-school and knew much less than the little I know today.

Have fun
Kevin O'Connor

[dadod]

Originally Posted by lineup

AS Lazy Cat said here,
those 4 transistors in the diamond should be together 2 and 2.
First the two 2SC4793+2SA1837
and also the two 2N5551+2N5401.
It is also good if those two pairs are very close to eachother.

Another thing that was wrong in the schematic I emailed you, sakis
is the output filter.
Inductor+resistor comes first and at the end comes capacitor+resistor(Zobel).

You can see it in this picture:

Most of the power amp zobels use RC connected close to the output and inductance part could be close to the loudspeaker terminal. D. Self does it that way and many others.
dado

[FdW]

Originally Posted by Struth

Again with the offset trim: adjusting input node resistance seems tenuous as it makes the input impedance for the circuit halves different. This could effect performance, especially at high frequencies. Adjusting a current source on a one-time basis using a trimpot may work, but as with any trimpot approach the question still remains as to long-term reliability of the setting's effectiveness.

As I noted, I also think that it will change the gain on both transistors. Is there any one going to react to this?

[Struth]

Hi Guys

Convention is not necessarily the best way. Leach had output stages oscillate to death with the zobel on the amp side of the coil. If you are tired of Leach, look at Bryston: they place the zobel after the choke, too. Chris Russell and Stuart Taylor are both very smart and tuned-in designers, as is Marshall Leach.

I think even Archie Leach would agree.

The trimpot for DC offset definitely will be problematic in its present position. The concern about performance is real. A less crucial facet is the fact both signal and DC currents are effected by the pot's rotation, so definitely adjust it WITHOUT a speaker in place - unless you like earthquake noises.

Considering the effort so far to create a power buffer with stupendous performance - wide bandwidth, super-low THD - it seems like a major fumble to retain the input offset trimpot. FdW and I seem in agreement on this, but Sakis will only consider Lineup's input for the PCB. His position is understandable considering his effort to design the card, but he has already acknowledged that this is only a beginning: more board designs will be necessary.

Have fun
Kevin O'Connor