EMRFD Message Archive 12190

Message Date From Subject
12190 2016-01-11 18:15:48 Bill Meara R2 audio amp question
The R2 has a nice complementary pair audio amplifier.  My question is about how to power it. The January 1993 QST article shows the collector of Q12 (PNP TIP30C) as connected to 12-Vsupply.  But it also shows a ground connection at this point.   The R1 article describes the same circuit. but the text says:  "Notice that the negative lead of the power supply must be connected at the collector of the PNP output transistor."  But the R1 schematic shows the collector of the PNP output transistor connected to chassis ground.

What do you think?  Is this circuit designed to be run via the standard +12V and negative ground configuration?  Or is it calling for some sort of split supply like we see in some op-amp circuits?

Thanks,   73  Bill N2CQR. 

12191 2016-01-11 18:37:45 kb1gmx Re: R2 audio amp question
In the R2 ground is negative side of power.  That version of the amp does not use a split supply.
In both cases ground is negative.  In some cases that's not true but in both of those it is.

FYI: those style amps can be built with a split supply typical is +-12v to as high as one can deal with 
(high power stereo).   The purpose of doing that is to eliminate the output capacitor for better low f
requency response in some cases well below a few Hertz.  For communications use a response 
below 50hz is likely not needed or desired.  Also a split supply is not convenient for portable work.

12192 2016-01-12 20:18:53 iq_rx Re: R2 audio amp question
Hi Bill,

I put those comments in those two places because audio engineers learn the hard way that there is a big difference between some casual ground symbol in a simulator and Real Ground.  It's important for the current from that PNP transistor to flow into the groundedest ground you've got, and it doesn't get any groundier than a big fat wire that goes from that exact point on the circuit board to the negative screw terminal of the power supply.  Play with it--it's cool!

So yes, it's all ground, no split supplies, but when you start pulling amps out of a complementary symmetry power transistor pair into an 8 ohm speaker, you need to get serious about fat wires, large capacitors connected directly between the collectors of the NPN and PNP transistors, and power supply leads connected right at those PNP and NPN collectors, not an inch away.  Milliohms matter.  The R1 and R2 output amplifiers are smaller than that, but they are part of a family of output amplifiers that go all the way up to 40 watts or so without changes to the layout.  Bigger amplifiers get even more serious about isolated stages, star ground connections bolted onto 40,000uF capacitor terminals, etc.  Look at the board in EMRFD on page 9.41 and the discussion in the adjacent paragraphs.

Split supplies are great for low frequency response, but connecting emitter followers that can supply amps of dc directly to expensive speakers involves some protection circuitry that I don't include on my experimental amplifiers.  1000uF in series with 8 ohms saves the speaker if the output goes to the rail, and the 3dB corner frequency is 20 Hz.  That sounds pretty good with the largest speakers I own.

Best Regards,

12193 2016-01-13 15:44:05 kb1gmx Re: R2 audio amp question
No kidding on that.

Back in the 70s I was prototyping a dual channel studio amp of a mere 500W per channel
or 1000W bridged into 4 ohms.  a plus and minus supply was used, and serious wire #10.
No small amount of circuit protection as well.  It was mean enough to destroy wires, screwdrivers
and anything that got in the way.  Also a 4 ohm 1000W dummy load or two along the way.

Yes, the peak currents are impressive and the typical bandwidth is in the hundreds of kilohertz
so keeping ground honest is critical.   

The general rule has been for years keep the high current stuff closest to the power supply.
In that cause its not the power source or battery its the point where the system has power 
applied to it.  The upstream stuff, where possible should use voltages that are decoupled 
or regulated as well.   A design that has the power stuff at one end  and the small signal 
stuff at the other usually satisfies that.  Its when you try to compact it then get weird.