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Old 11th March 2007, 09:59 PM   #1
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Default Power Amplifier Output Topologies

A few weeks ago, I asked for suggestions for output devices. Perhaps some of you recall that I once participated in the high end audio market in the late 1970s.

My current company manufactures SHARC based DSP boards. We are currently developing high performance DSP based crossovers.

This has got me thinking again about power amplifiers. I am not saying that Danville Signal is going into the power amplifier business (its possible but not likely). I only bring this up so that you all know I may have a commercial interest at some point.

VA Systems (aka Van Alstine Audio Systems) was founded by myself, Frank Van Alstine and two others. We manufactured power amplifiers and preamplifiers. Perhaps some of you remember these products. I can remember having a conversation with John Curl in our demo room at CES in Chicago. He was working with Levinson in those days.

--- Enough disclosure !!! - On to something interesting....

My current idea is to build a front end module that I can use to drive some arbitrary output stage with.

The VA Model 3 used a basic common collector (emitter follower) darlington configuration using what I remember as the JBL T drive circuit. I have attached a pdf of the output stage. We experimented with other output stages. I remember that we did not like the basic common emitter output since it suffers from lack of supply rejection that is intrincically good with a follower. Our version was very hard sounding as I recall.

I always admired Bryston to those days, but their amplifiers always sounded a little hard to me then. I believed they used a common emitter circuit. I found a schematic in Ben Duncan's book, that suggested they used a clever Quad variant that is kind of a common collector - common emitter hybrid. I have also attached a drawing of this circuit.

So here are the discussion points for starters:

1. What do you guys think of the various output topologies and why? I have been thinking of primarily BJT versions but of course, FET and FET BJT hybrids are also popular.

2. Where should a power amplifier clip? In the output stage or someone upstream? If you power the front end with higher supply rails than the output stage, the output stage can saturate.

Al Clark
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File Type: pdf va3_output.pdf (7.0 KB, 1098 views)
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Old 11th March 2007, 10:01 PM   #2
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Here is the Bryston Amplifier Stage attachment
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Old 11th March 2007, 11:12 PM   #3
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There is an error in the first attachment. Please correct the schematic.
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Old 12th March 2007, 12:59 AM   #4
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Could you elaborate?
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Old 12th March 2007, 01:53 AM   #5
AKSA is offline AKSA  Australia
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Al,

Nice to see you here, thanks for visiting.

Sixtek's comment: takeoff of the bottom outputs should be off the emitter of the driver, not the collector.

I'm with you on the common emitter/source output topologies.

#1 Poor PSRR
#2 Phase shift excacerbated by reactive loads
#3 Offset control limitations
#4 Stability at switch off is questionable.

The good ol' double emitter follower remains the standard in my view.

The Type II EF documented by Doug Self is very good, particularly if the charge suckout across the driver emitters is carefully tuned for the components chosen. This almost completely eliminates crossover artefacts due to switching.

Base stoppers are important, however, as you duly note.

For max power and thus best headroom, it should clip first at the outputs. I favor larger (0.47R) emitter resistors for better quiescent control and current sharing. Smaller, 0.22R, is favored, but current variation is not so good with multiple pairs, and since the resistor is in the fb loop it's size is not too important to damping factor.

High supply rails for the front end is only really beneficial in terms of S/N, because all output stage noise can now be thoroughly decoupled. But it's another pair of windings, more complexity and cost, a lot of people don't bother.

Cheers,

Hugh
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Old 12th March 2007, 02:14 AM   #6
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Quote:
Originally posted by AKSA
Al,

Sixtek's comment: takeoff of the bottom outputs should be off the emitter of the driver, not the collector.


Hugh

It amazing how blind you can get to really stupid errors. I have attached the obvious corrections.
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Old 12th March 2007, 02:32 AM   #7
jaycee is offline jaycee  United Kingdom
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Consider the Triple Darlington, as shown in The Leach Amp. This is my personal favorite.
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Old 12th March 2007, 02:33 AM   #8
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Default Re: Power Amplifier Output Topologies

Quote:
Originally posted by danville

My current company manufactures SHARC based DSP boards. We are currently developing high performance DSP based crossovers.
Al Clark

I'm sure you also considered a digital amp solution, especially with your DSP front-end and the world focus on efficiency. If you read the threads on "open system" digital and TI's higher frequency digital TAS5261 you might want to keep the analog days behind and develop an all digital solution which DIY'ers could test before a large firm customized for products.
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Old 12th March 2007, 02:54 AM   #9
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Default Re: Re: Power Amplifier Output Topologies

Quote:
Originally posted by LineSource



I'm sure you also considered a digital amp solution, especially with your DSP front-end and the world focus on efficiency. If you read the threads on "open system" digital and TI's higher frequency digital TAS5261 you might want to keep the analog days behind and develop an all digital solution which DIY'ers could test before a large firm customized for products.
Many of our customers are using ICEPOWER switching amplifiers. I think switching amplifiers may be a good practical solution for powered monitors where heat is a big issue.

I am not convinced that switchers make good sense for midrange and tweeter amplifiers. OTOH, you don't need a large power amplifier for the tweeter and midrange. It may be prudent to create hybrid systems of switchers for sub woofers and woofers and linear amplifiers for the higher frequency components.

I have also considered using a switching power supply for the rails of a linear power amplifier. I am not talking about Class G or H. I like the idea of refreshing the supply faster than 100 or 120 Hz. The ideal power supply is a voltage source. You still need to have a very low supply impedance, so this doesn't eliminate good quality capacitors located at the right place, but the capacitors don't need to be nearly as large.
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Old 12th March 2007, 05:21 AM   #10
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you asked about clipping an amp without saturation. tom holman used a baker clamp in the apt-1 which clipped the output stage within 0.3 or so volts of the supply rails without saturating the output devices. there was no "hangover" at clipping, and the amp actually clipped so cleanly that it was difficult to tell by hearing that the amp had clipped. once the amp was clipped by 2 to 5 percent, then you could begin to hear it. it was not a hard or harsh sounding amp, and it used triple darlington output topology with a baker clamp on the predrivers. the VAS was operated from +/- 75V rails and the predrivers, drivers, and output stage operated from +/- 65V in 8 ohm mode, and +/- 55V in 4 ohm mode. shown below is the "business end" of the apt-1.
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File Type: jpg apt1output.jpg (25.1 KB, 2674 views)
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